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Artist’s illustration of a binary star system consisting of two highly magnetized neutron stars. [John Rowe Animations]
Fast radio bursts (FRBs) are brief radio signals that last on the order of milliseconds. They appear to be extragalactic, coming from small, point-like areas on the sky. Some FRBs are one-off events, while others are periodic or “repeating”. The sources of FRBs are still unknown, but binary neutron star systems might be a piece of the puzzle.
Wanted: A Reliable Source of Repeating Fast Radio Bursts
Any proposed model for a repeating FRB must explain a number of observed behaviors. Among them are the following:
Repeating bursts from a given FRB source are consistent in frequency and overall intensity on the timescale of years.
Bursts exhibit small-scale variations in measures of the source’s magnetic environment.
FRBs seem to be preferentially hosted in massive, Milky-Way-like galaxies.
Example of an FRB from a repeating source, showing the intensity and various frequencies contained in a single burst (darker means more intense, lighter means less intense). The red lines just below and above 550 MHz and those near 450 MHz and 650 MHz indicate frequencies that were unused due to other radio signals interfering [adapted from the CHIME/FRB Collaboration, Andersen et al. 2019].
CHIME Canadian Hydrogen Intensity Mapping Experiment -A partnership between the University of British Columbia, the University of Toronto, McGill University, Yale and the National Research Council in British Columbia, at the Dominion Radio Astrophysical Observatory in Penticton, British Columbia, CA Altitude 545 m (1,788 ft)
Binary neutron stars (BNSs) have been considered as possible solutions to the repeating FRB puzzle. Specifically, binary neutron star mergers might produce FRBs, along with gamma-ray bursts and gravitational waves. But how could BNSs produce repeating, consistent FRBs?
In a recent study, Bing Zhang (University of Nevada Las Vegas; Kyoto University, Japan) attempts to explain repeating FRBs using BNSs in a novel way. Instead of considering the neutron-star merger itself, Zhang examined whether the years leading up to the merger could produce repeating FRBs.
A Magnetic Dance
Repeating FRBs put out an enormous amount of energy over a few milliseconds — at least as much energy as the Sun puts out over three days. To put constraints on the average FRB-producing BNS, Zhang used the double-pulsar system PSR J0737-3039A/B (pulsars are fast-rotating neutron stars with strong magnetic fields), which is very well characterized in terms of its component stars and overall structure.
Aside from having enormous amounts of rotational energy intrinsically and in their orbits, BNSs also have strong magnetic fields. These magnetic fields are key to the production of FRBs in Zhang’s scenario — as the neutron stars orbit each other, their magnetic fields interact, possibly triggering a flow of particles that would produce FRBs.
On the scale of centuries or even decades pre-merger, these triggers could occur repeatedly and consistently, satisfying a key requirement for repeating FRBs. This picture of interacting magnetic fields would also explain the small-scale variations in the magnetic environment measures, and there is an overlap between the sorts of galaxies that host FRBs and those that host the gamma-ray bursts that could be associated with BNS mergers.
By Way of Gravitational Waves
An observational test for this scenario is the detection of gravitational waves from an FRB source. Space-based gravitational wave detectors, such as the Laser Interferometer Space Antenna, would be well-suited for this.
Gravity is talking. Lisa will listen. Dialogos of EideESA/NASA eLISA space based, the future of gravitational wave research
Ground-based detectors would also play a role, picking up waves from the BNSs actually merging.
MIT /Caltech Advanced aLigo VIRGO Gravitational Wave interferometer, near Pisa, Italy
And of course, the more FRBs we observe, the more we can narrow down their properties and sources. Fortunately, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) is predicted to detect 2 to 50 FRBs per day, and other radio telescopes are hard at work as well. So maybe this FRB mystery will be solved sooner than we think!
Citation
“Fast Radio Bursts from Interacting Binary Neutron Star Systems,” Bing Zhang 2020 ApJL 890 L24.
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Matteo Guainazzi
Athena study scientist
European Space Agency
Email: matteo.guainazzi@esa.int
Paul McNamara
LISA study scientist
European Space Agency
Email: paul.mcnamara@esa.int
Markus Bauer
ESA Science Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
Black holes after a merger
What happens when two supermassive black holes collide? Combining the observing power of two future ESA missions, Athena and LISA, would allow us to study these cosmic clashes and their mysterious aftermath for the first time.
ESA/Athena spacecraft depiction
ESA/NASA eLISA space based, the future of gravitational wave research
Supermassive black holes, with masses ranging from millions to billions of Suns, sit at the core of most massive galaxies across the Universe. We don’t know exactly how these huge, enormously dense objects took shape, nor what triggers a fraction of them to start devouring the surrounding matter at extremely intense rates, radiating copiously across the electromagnetic spectrum and turning their host galaxies into ‘active galactic nuclei’.
Tackling these open questions in modern astrophysics is among the main goals of two future missions in ESA’s space science programme: Athena, the Advanced Telescope for High-ENergy Astrophysics, and LISA, the Laser Interferometer Space Antenna. Currently in the study phase, both missions are scheduled for launch in the early 2030s.
“Athena and LISA are both outstanding missions set to make breakthroughs in many areas of astrophysics,” says Günther Hasinger, ESA Director of Science.
“But there is one extremely exciting experiment that we could only perform if both missions are operational at the same time for at least a few years: bringing sound to the ‘cosmic movies’ by observing the merger of supermassive black holes both in X-rays and gravitational waves.
“With this unique opportunity to perform unprecedented observations of one of the most fascinating phenomena in the cosmos, the synergy between Athena and LISA would greatly increase the scientific return from both missions, ensuring European leadership in a key, novel area of research.”
Two missions to probe the extreme Universe
Athena will be the largest X-ray observatory ever built, investigating some of the hottest and most energetic phenomena in the cosmos with unprecedented accuracy and depth.
It is designed to answer two fundamental questions: how supermassive black holes at the centre of galaxies form and evolve, and how ‘ordinary’ matter assembles, along with the invisible dark matter, to form the wispy ‘cosmic web’ that pervades the Universe.
“Athena is going to measure several hundreds of thousands of black holes, from relatively nearby to far away, observing the X-ray emission from the million-degree-hot matter in their surroundings,” says Matteo Guainazzi, Athena study scientist at ESA.
“We are in particular interested in the most distant black holes, those that formed in the first few hundred million years of the Universe’s history, and we hope we’ll be able to finally understand how they formed.”
Meanwhile, LISA will be the first space-borne observatory of gravitational waves – fluctuations in the fabric of spacetime produced by the acceleration of cosmic objects with very strong gravity fields, like pairs of merging black holes.
Gravitational-wave astronomy, inaugurated only a few years ago, is currently limited to the high-frequency waves that can be probed by ground-based experiments like LIGO and Virgo. These experiments are sensitive to the mergers of relatively small black holes – a few times to a few tens of times more massive than the Sun.
Artist’s impression of the merger of two supermassive black holes during a galaxy collision.
LISA will expand these studies by detecting low-frequency gravitational waves, such as the ones released when two supermassive black holes collide during a merger of galaxies.
“LISA will be the first mission of its kind, looking primarily for gravitational waves coming from supermassive black holes smashing into one another,” explains Paul McNamara, LISA study scientist at ESA.
“This is one of the most energetic phenomena we know of, releasing more energy than all the quiescent Universe does at any time. If two supermassive black holes merge anywhere in the cosmos, LISA will see it.”
The first few gravitational wave events detected by LIGO and Virgo between 2015 and 2017 all originated from pairs of stellar-mass black holes, which are known to not radiate any light upon coalescence. Then, in August 2017, gravitational waves coming from a different source – the merger of two neutron stars – were discovered.
This time, the gravitational waves were accompanied by radiation across the electromagnetic spectrum, readily observed with a multitude of telescopes on Earth and in space. By combining information from the various types of observations in an approach known as multi-messenger astronomy, scientists could delve into the details of this never-before-observed phenomenon.
With Athena and LISA together, we would be able to apply multi-messenger astronomy to supermassive black holes for the first time. Simulations predict that their mergers, unlike those of their stellar-mass counterparts, emit both gravitational waves and radiation – the latter originating in the hot, interstellar gas of the two colliding galaxies stirred by the black holes pair when they fall towards one another.
LISA and Athena synergy
LISA will detect the gravitational waves emitted by the spiralling black holes about a month before their final coalescence, when they are still separated by a distance equivalent to several times their radii. Scientists expect that a fraction of the mergers found by LISA, especially those within distances of a few billion light years from us, will give rise to an X-ray signal that can be eventually seen by Athena.
“When LISA first detects a signal, we won’t know yet where exactly it’s coming from, because LISA is an all-sky sensor, so it works more like a microphone than a telescope,” explains Paul.
“However, as the black holes inspiral towards each other, the amplitude of their gravitational wave signal increases. This, coupled with the motion of the satellites along their orbits, will allow LISA to gradually improve the localisation of the source in the sky, up until the time when the black holes finally merge.”
A few days before the final phase of the merger, the gravitational wave data will constrain the position of the source to a patch on the sky measuring about 10 square degrees – roughly 50 times the area of the full Moon.
This is still pretty large, but would allow Athena to start scanning the sky to search for an X-ray signal from this titanic clash. Simulations indicate that the two spiralling black holes modulate the motion of the surrounding gas, so it is likely that the X-ray signature will have a frequency commensurate to that of the gravitational wave signal.
Then, just a few hours before the final coalescence of the black holes, LISA can provide a much more precise indication in the sky, roughly the size of the field of view of Athena’s Wide Field Imager (WFI), so the X-ray observatory can directly point towards the source.
“Catching the X-ray signal before the black holes become one will be very challenging, but we are pretty confident that we can make a detection during and after the merger,” explains Matteo.
“We could see the emergence of a new X-ray source, and perhaps witness the birth of an active galactic nucleus, with jets of high-energy particles being launched at close to the speed of light above and beyond the newly formed black hole.”
When supermassive black holes merge
We have never observed merging supermassive black holes – we do not yet have the facilities for such observations. First, we need LISA to detect the gravitational waves and tell us where to look in the sky; then we need Athena to observe it with high precision in X-rays to see how the mighty collision affects the gas surrounding the black holes. We can use theory and simulations to predict what might happen, but we need to combine these two great missions to find out.
One hundred years ago this month, on 29 May 1919, observations of the positions of stars during a total eclipse of the Sun provided the first empirical evidence of the gravitational bending of light predicted a few years earlier by Albert Einstein’s general theory of relativity.
This historic eclipse inaugurated a century of gravity experiments on Earth and in space, setting the stage for inspiring missions like Athena and LISA, and more exciting discoveries.
Notes
Athena was selected as the second large (L2) mission in ESA’s Cosmic Vision programme in 2014, and LISA as the third large (L3) mission in 2017. The additional science that could be performed with both missions operating jointly is described in a 2019 white paper by the Athena-LISA synergy working group.
Athena is an ESA-led mission with important contributions from NASA and JAXA. The WFI instrument is provided by an international consortium led by the Max Planck Institute for extraterrestrial Physics in Germany, involving several ESA Member States and the US. Under the management of CNES, the X-IFU instrument is provided by an international consortium led by France, The Netherlands, and Italy, furthermore involving several ESA Member States, Japan and the US.
LISA is an ESA-led mission with important contributions from NASA. The LISA Consortium, led by the Max Planck Institute for Gravitational Physics in Germany, involves several ESA Member States and the US.
The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.
Researchers propose a device to verify the performance of the laser-based equipment that will fly on the Laser Interferometer Space Antenna.
ESA/NASA eLISA space based, the future of gravitational wave research The gold-platinum alloy cubes, of central importance to the upcoming LISA mission, have already been built and tested in the proof-of-concept LISA Pathfinder mission
The European Space Agency is moving ahead with plans to launch a gravitational-wave detector called “LISA” into space. LISA, which stands for Laser Interferometer Space Antenna, will listen for gravitational waves that are currently undetectable from ground-based facilities such as the Laser Interferometer Gravitational-Wave Observatory. Catching these subtle spacetime ripples will require instrumentation with phenomenally stringent precision. Now, researchers have developed a device to test a core piece of LISA’s laser-based technology and ensure that it meets the requisite performance requirements.
D. Penkert/Max Planck Institute for Gravitational Physics
Max Planck Institute for Gravitational Physics
The planned LISA detector consists of three spacecrafts flying in triangle formation. Incoming gravitational waves will change the 2.5 million kilometers between each spacecraft by a few trillionths of a meter. To track those changes, the spacecraft will look for phase shifts in the laser light they receive from their two companions, a feat requiring precise measuring instruments with exceptionally low noise and low distortion.
To test the precision of LISA’s phase-shift-measuring hardware, Thomas Schwarze and colleagues at the Max Planck Institute for Gravitational Physics in Germany built and trialed a calibration device. Their device consists of three lasers whose beams interfere with each other in such a way that their phases—after being extracted by a prototype of LISA’s phase-measuring hardware—should cancel each other out. Any nonzero value reported reflects noise or distortion introduced by the phase-measuring hardware.
Others have suggested using three-laser setups to test LISA’s detectors, but the team’s device introduces an order of magnitude less noise than other proposals. With further refinements to their setup—such as swapping out photodetectors for models with lower noise—the team envisions that they could reduce measurement noise further. Doing that, they say, could enable their setup to serve as a critical performance check for LISA’s hardware.
Physicists are drowning in a flood of research papers in their own fields and coping with an even larger deluge in other areas of physics. How can an active researcher stay informed about the most important developments in physics? Physics highlights a selection of papers from the Physical Review journals. In consultation with expert scientists, the editors choose these papers for their importance and/or intrinsic interest. To highlight these papers, Physics features three kinds of articles: Viewpoints are commentaries written by active researchers, who are asked to explain the results to physicists in other subfields. Focus stories are written by professional science writers in a journalistic style and are intended to be accessible to students and non-experts. Synopses are brief editor-written summaries. Physics provides a much-needed guide to the best in physics, and we welcome your comments (physics@aps.org).
LIGO, here on Earth, has exquisitely-precise distances its lasers travel. With three spacecrafts in motion, how could LISA work?
Since it began operating in 2015, advanced LIGO has ushered in an era of a new type of astronomy: using gravitational wave signals. The way we do it, however, is through a very special technique known as laser interferometry. By splitting a laser and sending each half of the beam down a perpendicular path, reflecting them back, and recombining them, we can create an interference pattern. If the lengths of those paths change, the interference pattern changes, enabling us to detect those waves. And that leads to the best question I got about science during my recent Astrotour in Iceland, courtesy of Ben Turner, who asked:
LIGO works by having these exquisitely precise lasers, reflected down perfectly length-calibrated paths, to detect these tiny changes in distance (less than the width of a proton) induced by a passing gravitational wave. With LISA, we plan on having three independent, untethered spacecrafts freely-floating in space. They’ll be affected by all sorts of phenomena, from gravity to radiation to the solar wind. How can we possibly get a gravitational wave signal out of this?
VIRGO Gravitational Wave interferometer, near Pisa, Italy
Gravitational waves. Credit: MPI for Gravitational Physics/W.BengerGravity is talking. Lisa will listen. Dialogos of EideESA/eLISA the future of gravitational wave research
Localizations of gravitational-wave signals detected by LIGO in 2015 (GW150914, LVT151012, GW151226, GW170104), more recently, by the LIGO-Virgo network (GW170814, GW170817). After Virgo came online in August 2018
Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)
It’s a great question, and the toughest one posed to me all year thus far. Let’s explore the answer.
3D rendering of the gravitational waves emitted from a binary neutron star system at merger. The central region (in density) is stretched by a factor of ~5 for better visibility. The orientation of the merger itself determines how the signal will be polarized. (AEI POTSDAM-GOLM)
Since the dawn of time, humanity has been practicing astronomy with light, which has progressed from naked-eye viewing to the use of telescopes, cameras, and wavelengths that go far beyond the limits of human vision. We’ve detected cosmic particles from space in a wide variety of flavors: electrons, protons, atomic nuclei, antimatter, and even neutrinos.
But gravitational waves are an entirely new way for humanity to view the Universe. Instead of some detectable, discrete quantum particle that interacts with another, leading to a detectable signal in some sort of electronic device, gravitational waves act as ripples in the fabric of space itself. With a certain set of properties, including:
propagation speed,
orientation,
polarization,
frequency, and
amplitude,
they affect everything occupying the space that they pass through.
Gravitational waves propagate in one direction, alternately expanding and compressing space in mutually perpendicular directions, defined by the gravitational wave’s polarization. Gravitational waves themselves, in a quantum theory of gravity, should be made of individual quanta of the gravitational field: gravitons. (M. PÖSSEL/EINSTEIN ONLINE)
When one of these gravitational waves passes through a LIGO-like detector, it does exactly what you might suspect. The gravitational wave, along the direction it propagates at the speed of gravity (which equals the speed of light), doesn’t affect space at all. Along the plane perpendicular to its propagation, however, it alternately causes space to expand and contract in mutually perpendicular directions. There are multiple types of polarization that are possible:
“plus” (+) polarization, where the up-down and left-right directions expand and contract,
“cross” (×) polarization, where the left-diagonal and right-diagonal directions expand and contract,
or “circularly” polarized waves, similar to way light can be circularly polarized; this is a different parameterization of plus and cross polarizations.
Whatever the physical case, the polarization is determined by the nature of the source.
When a wave enters a detector, any two perpendicular directions will be compelled to contract and expand, alternately and in-phase, relative to one another. The amount that they contract or expand is related to the amplitude of the wave. The period of the expansion and contraction is determined by the frequency of the wave, which a detector of a specific arm length (or effective arm length, where there are multiple reflections down the arms, as in the case of LIGO) will be sensitive to.
With multiple such detectors in a variety of orientations to one another in three-dimensional space, the location, orientation, and even polarization of the original source can be reconstructed. By using the predictive power of Einstein’s General Relativity and the effects of gravitational waves on the matter-and-energy occupying the space they pass through, we can learn about events happening all across the Universe.
LIGO and Virgo have discovered a new population of black holes with masses that are larger than what had been seen before with X-ray studies alone (purple). This plot shows the masses of all ten confident binary black hole mergers detected by LIGO/Virgo (blue), along with the one neutron star-neutron star merger seen (orange). LIGO/Virgo, with the upgrade in sensitivity, should detect multiple mergers every week. (LIGO/VIRGO/NORTHWESTERN UNIV./FRANK ELAVSKY)
But it’s only due to the extraordinary technical achievement of these interferometers that we can actually make these measurements. In a terrestrial, LIGO-like detector, the distances of the two perpendicular arms are fixed. Laser light, even if reflected back-and-forth along the arms thousands of times, will eventually see the two beams come back together and construct a very specific interference pattern.
If the noise can be minimized below a certain level, the pattern will hold absolutely steady, so long as no gravitational waves are present.
If, then, a gravitational wave passes through, and one arm contracts while the other expands, the pattern will shift.
When the two arms are of exactly equal length and there is no gravitational wave passing through, the signal is null and the interference pattern is constant. As the arm lengths change, the signal is real and oscillatory, and the interference pattern changes with time in a predictable fashion. (NASA’S SPACE PLACE)
By measuring the amplitude and frequency at which the pattern shifts, the properties of a gravitational wave can be reconstructed. By measuring a coincident signal in multiple such gravitational wave detectors, the source properties and location can be reconstructed as well. The more detectors with differing orientations and locations are present, the better-constrained the properties of the gravitational wave source will be.
This is why adding the Virgo detector to the twin LIGO detectors in Livingston and Hanford enabled a far superior reconstruction of the location of gravitational wave sources. In the future, additional LIGO-like detectors in Japan and India will allow scientists to pinpoint gravitational waves in an even superior fashion.
But there’s a limit to what we can do with detectors like this. Seismic noise from being located on the Earth itself limits how sensitive a ground-based detector can be. Signals below a certain amplitude can never be detected. Additionally, when light signals are reflected between mirrors, the noise generated by the Earth accumulates cumulatively.
The fact that the Earth itself exists in the Solar System, even if there were no plate tectonics, ensures that the most common type of gravitational wave events — binary stars, supermassive black holes, and other low-frequency sources (taking 100 seconds or more to oscillate) — cannot be seen from the ground. Earth’s gravitational field, human activity, and natural geological processes means that these low-frequency signals cannot be practically seen from Earth. For that, we need to go to space.
And that’s where LISA comes in.
The sensitivities of a variety of gravitational wave detectors, old, new, and proposed. Note, in particular, Advanced LIGO (in orange), LISA (in dark blue), and BBO (in light blue). LIGO can only detect low-mass and short-period events; longer-baseline, lower-noise observatories are needed for more massive black holes. (MINGLEI TONG, CLASS.QUANT.GRAV. 29 (2012) 155006)
LISA is the Laser Interferometer Space Antenna. In its current design, it consists of three dual-purpose spacecrafts, separated in an equilateral triangle configuration by roughly 5,000,000 kilometers along each laser arm.
Inside each spacecraft, there are two free-floating cubes that are shielded by the spacecraft itself from the effects of interplanetary space. They will remain at a constant temperature, pressure, and will be unaffected by the solar wind, radiation pressure, or the bombardment of micrometeorites.
By carefully measuring the distances between pairs of cubes on different spacecrafts, using the same laser interferometry technique, scientists can do everything that multiple LIGO detectors do, except for these long-period gravitational waves that only LISA is sensitive to. Without the Earth to create noise, it seems like an ideal setup.
The primary scientific goal of the Laser Interferometer Space Antenna (LISA) mission is to detect and observe gravitational waves from massive black holes and galactic binaries with periods in the range of a tens of seconds to a few hours. This low-frequency range is inaccessible to ground-based interferometers because of the unshieldable background of local gravitational noise arising from atmospheric effects and seismic activity. (ESA-C. VIJOUX)
But even without the terrestrial effects of human activity, seismic noise, and being deep within Earth’s gravitational field, there are still sources of noise that LISA must contend with. The solar wind will strike the detectors, and the LISA spacecrafts must be able to compensate for that. The gravitational influence of other planets and solar radiation pressure will induce tiny orbital changes relative to one another. Quite simply, there is no way to hold the spacecract at a fixed, constant distance of exactly 5 million km, relative to one another, in space. No amount of rocket fuel or electric thrusters will be able to maintain that exactly.
Remember: the goal is to detect gravitational waves — themselves a tiny, minuscule signal — over and above the background of all this noise.
The three LISA spacecraft will be placed in orbits that form a triangular formation with center 20° behind the Earth and side length 5 million km. This figure is not to scale. (NASA)
So how does LISA plan to do it?
The secret is in these gold-platinum alloy cubes. In the center of each optical system, a solid cube that’s 4 centimeters (about 1.6″) on each side floats freely in the weightless conditions of space. While external sensors monitor the solar wind and solar radiation pressure, with electronic sensors compensating for those extraneous forces, the gravitational forces from all the known bodies in the Solar System can be calculated and anticipated.
As the spacecrafts, and the cubes, move relative to one another, the lasers adjust in a predictable, well-known fashion. So long as they continue to reflect off of the cubes, the distances between them can be measured.
The gold-platinum alloy cubes, of central importance to the upcoming LISA mission, have already been built and tested in the proof-of-concept LISA Pathfinder missionESA/LISA Pathfinder
It’s not a matter of keeping the distances fixed and measuring a tiny change due to a passing wave; it’s a matter of understanding exactly how the distances will behave over time, accounting for them, and then looking for the periodic departures from those measurements to a high-enough precision. LISA won’t hold the three spacecrafts in a fixed position, but will allow them to adjust freely as Einstein’s laws dictate. It’s only because gravity is so well-understood that the additional signal of the gravitational waves, assuming the wind and radiation from the Sun is sufficiently compensated for, can be teased out.
The proposed ‘Big Bang Observer’ would take the design of LISA, the Laser Interferometer Space Antenna, and create a large equilateral triangle around Earth’s orbit to get the longest-baseline gravitational wave observatory ever. (GREGORY HARRY, MIT, FROM THE LIGO WORKSHOP OF 2009, LIGO-G0900426)
If we want to go even farther, we have dreams of putting three LISA-like detectors in an equilateral triangle around different points in Earth’s orbit: a proposed mission called Big Bang Observer (BBO). While LISA can detect binary systems with periods ranging from minutes to hours, BBO will be able to detect the grandest behemonths of all: supermassive binary black holes anywhere in the Universe, with periods of years.
If we’re willing to invest in it, space-based gravitational wave observatories could allow us to map out all of the most massive, densest objects located throughout the entire Universe. The key isn’t holding your laser arms fixed, but simply in knowing exactly how, in the absence of gravitational waves, they’d move relative to one another. The rest is simply a matter of extracting the signal of each gravitational wave out. Without the Earth’s noise to slow us down, the entire cosmos is within our reach.
“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan
Oct. 2, 2018
Jeanette Kazmierczak
jeanette.a.kazmierczak@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, Md.
This animation rotates 360 degrees around a frozen version of the simulation in the plane of the disk. Credit: NASA’s Goddard Space Flight Center
A new model is bringing scientists a step closer to understanding the kinds of light signals produced when two supermassive black holes, which are millions to billions of times the mass of the Sun, spiral toward a collision. For the first time, a new computer simulation that fully incorporates the physical effects of Einstein’s general theory of relativity shows that gas in such systems will glow predominantly in ultraviolet and X-ray light.
Just about every galaxy the size of our own Milky Way or larger contains a monster black hole at its center. Observations show galaxy mergers occur frequently in the universe, but so far no one has seen a merger of these giant black holes.
“We know galaxies with central supermassive black holes combine all the time in the universe, yet we only see a small fraction of galaxies with two of them near their centers,” said Scott Noble, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “The pairs we do see aren’t emitting strong gravitational-wave signals because they’re too far away from each other. Our goal is to identify — with light alone — even closer pairs from which gravitational-wave signals may be detected in the future.”
A paper describing the team’s analysis of the new simulation was published Tuesday, Oct. 2, in The Astrophysical Journal.
Gas glows brightly in this computer simulation of supermassive black holes only 40 orbits from merging. Models like this may eventually help scientists pinpoint real examples of these powerful binary systems. Credits: NASA’s Goddard Space Flight Center
Scientists have detected merging stellar-mass black holes — which range from around three to several dozen solar masses — using the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO).
Gravitational waves are space-time ripples traveling at the speed of light. They are created when massive orbiting objects like black holes and neutron stars spiral together and merge.
Black holes heading toward a merger. Precise laser interferometry can detect the ripples in space-time generated when two black holes collide. LIGO-Caltech-MIT-Sonoma State Aurore SimonnCornell SXS, the Simulating eXtreme Spacetimes (SXS) project
Supermassive mergers will be much more difficult to find than their stellar-mass cousins. One reason ground-based observatories can’t detect gravitational waves from these events is because Earth itself is too noisy, shaking from seismic vibrations and gravitational changes from atmospheric disturbances. The detectors must be in space, like the Laser Interferometer Space Antenna (LISA) led by ESA (the European Space Agency) and planned for launch in the 2030s.
ESA/NASA eLISA space based, the future of gravitational wave research
Observatories monitoring sets of rapidly spinning, superdense stars called pulsars may detect gravitational waves from monster mergers. Like lighthouses, pulsars emit regularly timed beams of light that flash in and out of view as they rotate. Gravitational waves could cause slight changes in the timing of those flashes, but so far studies haven’t yielded any detections.
But supermassive binaries nearing collision may have one thing stellar-mass binaries lack — a gas-rich environment. Scientists suspect the supernova explosion that creates a stellar black hole also blows away most of the surrounding gas. The black hole consumes what little remains so quickly there isn’t much left to glow when the merger happens.
Supermassive binaries, on the other hand, result from galaxy mergers. Each supersized black hole brings along an entourage of gas and dust clouds, stars and planets. Scientists think a galaxy collision propels much of this material toward the central black holes, which consume it on a time scale similar to that needed for the binary to merge. As the black holes near, magnetic and gravitational forces heat the remaining gas, producing light astronomers should be able to see.
“It’s very important to proceed on two tracks,” said co-author Manuela Campanelli, director of the Center for Computational Relativity and Gravitation at the Rochester Institute of Technology in New York, who initiated this project nine years ago. “Modeling these events requires sophisticated computational tools that include all the physical effects produced by two supermassive black holes orbiting each other at a fraction of the speed of light. Knowing what light signals to expect from these events will help modern observations identify them. Modeling and observations will then feed into each other, helping us better understand what is happening at the hearts of most galaxies.”
The new simulation shows three orbits of a pair of supermassive black holes only 40 orbits from merging. The models reveal the light emitted at this stage of the process may be dominated by UV light with some high-energy X-rays, similar to what’s seen in any galaxy with a well-fed supermassive black hole.
Three regions of light-emitting gas glow as the black holes merge, all connected by streams of hot gas: a large ring encircling the entire system, called the circumbinary disk, and two smaller ones around each black hole, called mini disks. All these objects emit predominantly UV light. When gas flows into a mini disk at a high rate, the disk’s UV light interacts with each black hole’s corona, a region of high-energy subatomic particles above and below the disk. This interaction produces X-rays. When the accretion rate is lower, UV light dims relative to the X-rays.
Based on the simulation, the researchers expect X-rays emitted by a near-merger will be brighter and more variable than X-rays seen from single supermassive black holes. The pace of the changes links to both the orbital speed of gas located at the inner edge of the circumbinary disk as well as that of the merging black holes.
This 360-degree video places the viewer in the middle of two circling supermassive black holes around 18.6 million miles (30 million kilometers) apart with an orbital period of 46 minutes. The simulation shows how the black holes distort the starry background and capture light, producing black hole silhouettes. A distinctive feature called a photon ring outlines the black holes. The entire system would have around 1 million times the Sun’s mass. Credits: NASA’s Goddard Space Flight Center; background, ESA/Gaia/DPAC
“The way both black holes deflect light gives rise to complex lensing effects, as seen in the movie when one black hole passes in front of the other,” said Stéphane d’Ascoli, a doctoral student at École Normale Supérieure in Paris and lead author of the paper. “Some exotic features came as a surprise, such as the eyebrow-shaped shadows one black hole occasionally creates near the horizon of the other.”
The simulation ran on the National Center for Supercomputing Applications’ Blue Waters supercomputer at the University of Illinois at Urbana-Champaign.
U Illinois Urbana-Champaign Blue Waters Cray Linux XE/XK hybrid machine supercomputer
Modeling three orbits of the system took 46 days on 9,600 computing cores. Campanelli said the collaboration was recently awarded additional time on Blue Waters to continue developing their models.
The original simulation estimated gas temperatures. The team plans to refine their code to model how changing parameters of the system, like temperature, distance, total mass and accretion rate, will affect the emitted light. They’re interested in seeing what happens to gas traveling between the two black holes as well as modeling longer time spans.
“We need to find signals in the light from supermassive black hole binaries distinctive enough that astronomers can find these rare systems among the throng of bright single supermassive black holes,” said co-author Julian Krolik, an astrophysicist at Johns Hopkins University in Baltimore. “If we can do that, we might be able to discover merging supermassive black holes before they’re seen by a space-based gravitational-wave observatory.”
NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.
Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.
This artist’s conception shows a pair of black holes heading toward a merger. Precise laser interferometry can detect the ripples in space-time generated when two black holes collide. [LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)]
The advent of gravitational-wave astrophysics has made possible the study of elusive cosmic phenomena — like the mysterious merging of stellar-mass black holes.
As of November 15, 2017, six black-hole mergers have been discovered via gravitational waves. [LSC/LIGO/Caltech/Sonoma State (Aurore Simonnet)]
The cataclysmic inspiraling of a pair of black holes doomed to merge sends ripples through space-time. Thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO), we have now detected a handful of instances of these ripples — enough to take a closer look at the broader population of binary black-hole mergers.
Beyond just collecting individual merger events, we can now explore whether or not the rate at which black hole mergers occur has evolved over the course of cosmic time. The merger rate reflects the underlying star formation rate as well as the particulars of stellar evolution. Ultimately, understanding how the merger rate has changed can help us learn how black-hole binaries form.
How can we tell whether or not the rate of binary black-hole mergers has evolved with redshift? A team led by Maya Fishbach (University of Chicago) aimed to extract this information from the first six binary black-hole detections from LIGO/Virgo.
The cumulative probability distribution of detected black-hole binaries depends on black hole mass, detector sensitivity (with the dashed lines indicating a more sensitive detector), and the underlying redshift distribution. Evolution of the merger rate with redshift would shift these curves — which demonstrate the case of a uniform redshift distribution — to the left or right. [Fishbach et al. 2018]
LIGO Provides a Listening Ear
VIRGO Gravitational Wave interferometer, near Pisa, Italy
Gravitational waves. Credit: MPI for Gravitational Physics/W.BengerESA/eLISA the future of gravitational wave research
Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)
One challenge is that the redshift distribution of black-hole binaries that we observe from LIGO/Virgo isn’t just a function of the underlying redshift distribution — it’s also a function of the mass distribution. Since mergers of more massive black holes generate “louder” signals and are more likely to be detected, a binary black-hole population with more massive members will generate more detections at high redshift than a population with fewer massive members.
To remedy this, Fishbach and collaborators used models of realistic redshift distributions to fit the redshift and the two component masses simultaneously. Based on the six available binary black-hole detections, Fishbach and collaborators find that the observations are consistent with a merger rate that is constant in redshift.
There does appear to be a slight decrease in the merger rate density with increasing redshift, but the authors caution that this could arise if the detections of the “quieter” mergers are published later; an artificially large proportion of “louder” events could skew the redshift distribution toward low-redshift events.
Looking Ahead to Future Detections
Merger rate density as a function of redshift for two redshift parameterizations. Both redshift models are consistent with a constant merger rate, which is indicated by the dotted line. Click to enlarge. [Fishbach et al. 2018]
What does the future hold for estimating the black hole merger rate as a function of redshift? To explore this question, Fishbach and collaborators generated synthetic black hole populations and modeled the likely detections by LIGO/Virgo.
They find that with a few hundred binary black-hole detections per year — an estimate based off of the expected improvements to LIGO/Virgo sensitivity — any deviations from a constant merger rate should be detectable within a few years. Exciting developments to come!
The mission of the American Astronomical Society is to enhance and share humanity’s scientific understanding of the Universe.
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Next-generation gravitational wave detector in space will complement LIGO on Earth.
ESA/eLISA space based the future of gravitational wave research
The historic first detection of gravitational waves from colliding black holes far outside our galaxy opened a new window to understanding the universe. A string of detections — four more binary black holes and a pair of neutron stars — soon followed the Sept. 14, 2015, observation.
The first optical image of a gravitational wave source was taken by a team led by Ryan Foley of UC Santa Cruz using the Swope Telescope at the Carnegie Institution’s Las Campanas Observatory in Chile. This image of Swope Supernova Survey 2017a (SSS17a, indicated by arrow) shows the light emitted from the cataclysmic merger of two neutron stars. (Image credit: 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)
Carnegie Institution Swope telescope at Las Campanas, Chile, 100 kilometres (62 mi) northeast of the city of La Serena. near the north end of a 7 km (4.3 mi) long mountain ridge. Cerro Las Campanas, near the southern end and over 2,500 m (8,200 ft) high, at Las Campanas, Chile
A neutron star forms when a massive star runs out of fuel and explodes as a supernova, throwing off its outer layers and leaving behind a collapsed core composed almost entirely of neutrons. Neutrons are the uncharged particles in the nucleus of an atom, where they are bound together with positively charged protons. In a neutron star, they are packed together just as densely as in the nucleus of an atom, resulting in an object with one to three times the mass of our sun but only about 12 miles wide.
“Basically, a neutron star is a gigantic atom with the mass of the sun and the size of a city like San Francisco or Manhattan,” said Foley, an assistant professor of astronomy and astrophysics at UC Santa Cruz.
These objects are so dense, a cup of neutron star material would weigh as much as Mount Everest, and a teaspoon would weigh a billion tons. It’s as dense as matter can get without collapsing into a black hole.
THE MERGER
Like other stars, neutron stars sometimes occur in pairs, orbiting each other and gradually spiraling inward. Eventually, they come together in a catastrophic merger that distorts space and time (creating gravitational waves) and emits a brilliant flare of electromagnetic radiation, including visible, infrared, and ultraviolet light, x-rays, gamma rays, and radio waves. Merging black holes also create gravitational waves, but there’s nothing to be seen because no light can escape from a black hole.
Foley’s team was the first to observe the light from a neutron star merger that took place on August 17, 2017, and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).
VIRGO Gravitational Wave interferometer, near Pisa, Italy
Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-ZibESA/eLISA the future of gravitational wave research
Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)
Now, for the first time, scientists can study both the gravitational waves (ripples in the fabric of space-time), and the radiation emitted from the violent merger of the densest objects in the universe.
The UC Santa Cruz team found SSS17a by comparing a new image of the galaxy N4993 (right) with images taken four months earlier by the Hubble Space Telescope (left). The arrows indicate where SSS17a was absent from the Hubble image and visible in the new image from the Swope Telescope. (Image credits: Left, Hubble/STScI; Right, 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)
It’s that combination of data, and all that can be learned from it, that has astronomers and physicists so excited. The observations of this one event are keeping hundreds of scientists busy exploring its implications for everything from fundamental physics and cosmology to the origins of gold and other heavy elements.
A small team of UC Santa Cruz astronomers were the first team to observe light from two neutron stars merging in August. The implications are huge.
ALL THE GOLD IN THE UNIVERSE
It turns out that the origins of the heaviest elements, such as gold, platinum, uranium—pretty much everything heavier than iron—has been an enduring conundrum. All the lighter elements have well-explained origins in the nuclear fusion reactions that make stars shine or in the explosions of stars (supernovae). Initially, astrophysicists thought supernovae could account for the heavy elements, too, but there have always been problems with that theory, says Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz.
The violent merger of two neutron stars is thought to involve three main energy-transfer processes, shown in this diagram, that give rise to the different types of radiation seen by astronomers, including a gamma-ray burst and a kilonova explosion seen in visible light. (Image credit: Murguia-Berthier et al., Science)
A theoretical astrophysicist, Ramirez-Ruiz has been a leading proponent of the idea that neutron star mergers are the source of the heavy elements. Building a heavy atomic nucleus means adding a lot of neutrons to it. This process is called rapid neutron capture, or the r-process, and it requires some of the most extreme conditions in the universe: extreme temperatures, extreme densities, and a massive flow of neutrons. A neutron star merger fits the bill.
Ramirez-Ruiz and other theoretical astrophysicists use supercomputers to simulate the physics of extreme events like supernovae and neutron star mergers. This work always goes hand in hand with observational astronomy. Theoretical predictions tell observers what signatures to look for to identify these events, and observations tell theorists if they got the physics right or if they need to tweak their models. The observations by Foley and others of the neutron star merger now known as SSS17a are giving theorists, for the first time, a full set of observational data to compare with their theoretical models.
According to Ramirez-Ruiz, the observations support the theory that neutron star mergers can account for all the gold in the universe, as well as about half of all the other elements heavier than iron.
RIPPLES IN THE FABRIC OF SPACE-TIME
Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity, but until recently they were impossible to observe. LIGO’s extraordinarily sensitive detectors achieved the first direct detection of gravitational waves, from the collision of two black holes, in 2015. Gravitational waves are created by any massive accelerating object, but the strongest waves (and the only ones we have any chance of detecting) are produced by the most extreme phenomena.
Two massive compact objects—such as black holes, neutron stars, or white dwarfs—orbiting around each other faster and faster as they draw closer together are just the kind of system that should radiate strong gravitational waves. Like ripples spreading in a pond, the waves get smaller as they spread outward from the source. By the time they reached Earth, the ripples detected by LIGO caused distortions of space-time thousands of times smaller than the nucleus of an atom.
The rarefied signals recorded by LIGO’s detectors not only prove the existence of gravitational waves, they also provide crucial information about the events that produced them. Combined with the telescope observations of the neutron star merger, it’s an incredibly rich set of data.
LIGO can tell scientists the masses of the merging objects and the mass of the new object created in the merger, which reveals whether the merger produced another neutron star or a more massive object that collapsed into a black hole. To calculate how much mass was ejected in the explosion, and how much mass was converted to energy, scientists also need the optical observations from telescopes. That’s especially important for quantifying the nucleosynthesis of heavy elements during the merger.
LIGO can also provide a measure of the distance to the merging neutron stars, which can now be compared with the distance measurement based on the light from the merger. That’s important to cosmologists studying the expansion of the universe, because the two measurements are based on different fundamental forces (gravity and electromagnetism), giving completely independent results.
“This is a huge step forward in astronomy,” Foley said. “Having done it once, we now know we can do it again, and it opens up a whole new world of what we call ‘multi-messenger’ astronomy, viewing the universe through different fundamental forces.”
IN THIS REPORT
Neutron stars
A team from UC Santa Cruz was the first to observe the light from a neutron star merger that took place on August 17, 2017 and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)
Graduate students and post-doctoral scholars at UC Santa Cruz played key roles in the dramatic discovery and analysis of colliding neutron stars.Astronomer Ryan Foley leads a team of young graduate students and postdoctoral scholars who have pulled off an extraordinary coup. Following up on the detection of gravitational waves from the violent merger of two neutron stars, Foley’s team was the first to find the source with a telescope and take images of the light from this cataclysmic event. In so doing, they beat much larger and more senior teams with much more powerful telescopes at their disposal.
“We’re sort of the scrappy young upstarts who worked hard and got the job done,” said Foley, an untenured assistant professor of astronomy and astrophysics at UC Santa Cruz.
David Coulter, graduate student
The discovery on August 17, 2017, has been a scientific bonanza, yielding over 100 scientific papers from numerous teams investigating the new observations. Foley’s team is publishing seven papers, each of which has a graduate student or postdoc as the first author.
“I think it speaks to Ryan’s generosity and how seriously he takes his role as a mentor that he is not putting himself front and center, but has gone out of his way to highlight the roles played by his students and postdocs,” said Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz and the most senior member of Foley’s team.
“Our team is by far the youngest and most diverse of all of the teams involved in the follow-up observations of this neutron star merger,” Ramirez-Ruiz added.
Charles Kilpatrick, postdoctoral scholar
Charles Kilpatrick, a 29-year-old postdoctoral scholar, was the first person in the world to see an image of the light from colliding neutron stars. He was sitting in an office at UC Santa Cruz, working with first-year graduate student Cesar Rojas-Bravo to process image data as it came in from the Swope Telescope in Chile. To see if the Swope images showed anything new, he had also downloaded “template” images taken in the past of the same galaxies the team was searching.
Ariadna Murguia-Berthier, graduate student
“In one image I saw something there that was not in the template image,” Kilpatrick said. “It took me a while to realize the ramifications of what I was seeing. This opens up so much new science, it really marks the beginning of something that will continue to be studied for years down the road.”
At the time, Foley and most of the others in his team were at a meeting in Copenhagen. When they found out about the gravitational wave detection, they quickly got together to plan their search strategy. From Copenhagen, the team sent instructions to the telescope operators in Chile telling them where to point the telescope. Graduate student David Coulter played a key role in prioritizing the galaxies they would search to find the source, and he is the first author of the discovery paper published in Science.
Matthew Siebert, graduate student
“It’s still a little unreal when I think about what we’ve accomplished,” Coulter said. “For me, despite the euphoria of recognizing what we were seeing at the moment, we were all incredibly focused on the task at hand. Only afterward did the significance really sink in.”
Just as Coulter finished writing his paper about the discovery, his wife went into labor, giving birth to a baby girl on September 30. “I was doing revisions to the paper at the hospital,” he said.
It’s been a wild ride for the whole team, first in the rush to find the source, and then under pressure to quickly analyze the data and write up their findings for publication. “It was really an all-hands-on-deck moment when we all had to pull together and work quickly to exploit this opportunity,” said Kilpatrick, who is first author of a paper comparing the observations with theoretical models.
César Rojas Bravo, graduate student
Graduate student Matthew Siebert led a paper analyzing the unusual properties of the light emitted by the merger. Astronomers have observed thousands of supernovae (exploding stars) and other “transients” that appear suddenly in the sky and then fade away, but never before have they observed anything that looks like this neutron star merger. Siebert’s paper concluded that there is only a one in 100,000 chance that the transient they observed is not related to the gravitational waves.
Ariadna Murguia-Berthier, a graduate student working with Ramirez-Ruiz, is first author of a paper synthesizing data from a range of sources to provide a coherent theoretical framework for understanding the observations.
Another aspect of the discovery of great interest to astronomers is the nature of the galaxy and the galactic environment in which the merger occurred. Postdoctoral scholar Yen-Chen Pan led a paper analyzing the properties of the host galaxy. Enia Xhakaj, a new graduate student who had just joined the group in August, got the opportunity to help with the analysis and be a coauthor on the paper.
Yen-Chen Pan, postdoctoral scholar
“There are so many interesting things to learn from this,” Foley said. “It’s a great experience for all of us to be part of such an important discovery.”
Abbott et al., Nature, A gravitational-wave standard siren measurement of the Hubble constant (The LIGO Scientific Collaboration and The Virgo Collaboration, The 1M2H Collaboration, The Dark Energy Camera GW-EM Collaboration and the DES Collaboration, The DLT40 Collaboration, The Las Cumbres Observatory Collaboration, The VINROUGE Collaboration & The MASTER Collaboration)
Writing: Tim Stephens
Video: Nick Gonzales
Photos: Carolyn Lagattuta
Header image: Illustration by Robin Dienel courtesy of the Carnegie Institution for Science
Design and development: Rob Knight
Project managers: Sherry Main, Scott Hernandez-Jason, Tim Stephens
Dark Energy Camera [DECam], built at FNAL NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet
Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile, at an altitude of 7200 feet
Noted in the vdeo but not in te article:
NASA/Chandra TelescopeNASA/SWIFT TelescopeNRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USAPrompt telescope CTIO ChileNASA NuSTAR X-ray telescope
Now, another detector is being built to crack this window wider open. This next-generation observatory, called LISA, is expected to be in space in 2034, and it will be sensitive to gravitational waves of a lower frequency than those detected by the Earth-bound Laser Interferometer Gravitational-Wave Observatory (LIGO).
A new Northwestern University study predicts dozens of binaries (pairs of orbiting compact objects) in the globular clusters of the Milky Way will be detectable by LISA (Laser Interferometer Space Antenna). These binary sources would contain all combinations of black hole, neutron star and white dwarf components. Binaries formed from these star-dense clusters will have many different features from those binaries that formed in isolation, far from other stars.
The study is the first to use realistic globular cluster models to make detailed predictions of LISA sources. “LISA Sources in Milky-Way Globular Clusters” was published today, May 11, by the journal Physical Review Letters.
“LISA is sensitive to Milky Way systems and will expand the breadth of the gravitational wave spectrum, allowing us to explore different types of objects that aren’t observable with LIGO,” said Kyle Kremer, the paper’s first author, a Ph.D. student in physics and astronomy in Northwestern’s Weinberg College of Arts and Sciences and a member of a computational astrophysics research collaboration based in Northwestern’s Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).
In the Milky Way, 150 globular clusters have been observed so far. The Northwestern research team predicts one out of every three clusters will produce a LISA source. The study also predicts that approximately eight black hole binaries will be detectable by LISA in our neighboring galaxy of Andromeda and another 80 in nearby Virgo.
Before the first detection of gravitational waves by LIGO, as the twin detectors were being built in the United States, astrophysicists around the world worked for decades on theoretical predictions of what astrophysical phenomena LIGO would observe. That is what the Northwestern theoretical astrophysicists are doing in this new study, but this time for LISA, which is being built by the European Space Agency with contributions from NASA.
“We do our computer simulations and analysis at the same time our colleagues are bending metal and building spaceships, so that when LISA finally flies, we’re all ready at the same time,” said Shane L. Larson, associate director of CIERA and an author of the study. “This study is helping us understand what science is going to be contained in the LISA data.”
A globular cluster is a spherical structure of hundreds of thousands to millions of stars, gravitationally bound together. The clusters are some of the oldest populations of stars in the galaxy and are efficient factories of compact object binaries.
The Northwestern research team had numerous advantages in conducting this study. Over the past two decades, Frederic A. Rasio and his group have developed a powerful computational tool — one of the best in the world — to realistically model globular clusters. Rasio, the Joseph Cummings Professor in Northwestern’s department of physics and astronomy, is the senior author of the study.
The researchers used more than a hundred fully evolved globular cluster models with properties similar to those of the observed globular clusters in the Milky Way. The models, which were all created at CIERA, were run on Quest, Northwestern’s supercomputer cluster. This powerful resource can evolve the full 12 billion years of a globular cluster’s life in a matter of days.
NASA (ATP grant NNX14AP92G) and the National Science Foundation (grant AST-1716762) supported the research.
Other authors of the paper include Sourav Chatterjee and Katelyn Breivik, both of Northwestern and CIERA, and Carl L. Rodriguez, of the MIT-Kavli Institute for Astrophysics and Space Research.
On May 31, 1850, nine men gathered to begin planning a university that would serve the Northwest Territory.
Given that they had little money, no land and limited higher education experience, their vision was ambitious. But through a combination of creative financing, shrewd politicking, religious inspiration and an abundance of hard work, the founders of Northwestern University were able to make that dream a reality.
In 1853, the founders purchased a 379-acre tract of land on the shore of Lake Michigan 12 miles north of Chicago. They established a campus and developed the land near it, naming the surrounding town Evanston in honor of one of the University’s founders, John Evans. After completing its first building in 1855, Northwestern began classes that fall with two faculty members and 10 students.
Twenty-one presidents have presided over Northwestern in the years since. The University has grown to include 12 schools and colleges, with additional campuses in Chicago and Doha, Qatar.
Northwestern is recognized nationally and internationally for its educational programs.
Although we’ve seen black holes directly merging three separate times in the Universe, we know many more exist. When supermassive black holes merge together, LISA will allow us to predict, up to years in advance, exactly when the critical event will occur.
LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)
ESA/NASA eLISA space based the future of gravitational wave research
Across the Universe, innumerable masses are locked in an inevitable death spiral. As white dwarfs, neutron stars, and black holes orbit each other, they travel through the curved spacetime that the other one’s mass creates. Accelerating through this has an inevitable consequence in General Relativity: the emission of gravitational radiation, also known as gravitational waves. Since these waves carry energy away, these orbits eventually decay, leading to an inspiral and merger. Over the past 2-3 years, LIGO has directly detected the very first mergers of black holes and neutron stars, with many more to come. But even with optimal technology, we’ll never get a signal more than seconds in advance of the actual merger.
The first optical image of a gravitational wave source was taken by a team led by Ryan Foley of UC Santa Cruz using the Swope Telescope at the Carnegie Institution’s Las Campanas Observatory in Chile. This image of Swope Supernova Survey 2017a (SSS17a, indicated by arrow) shows the light emitted from the cataclysmic merger of two neutron stars. (Image credit: 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)
Carnegie Institution Swope telescope at Las Campanas, Chile, 100 kilometres (62 mi) northeast of the city of La Serena. near the north end of a 7 km (4.3 mi) long mountain ridge. Cerro Las Campanas, near the southern end and over 2,500 m (8,200 ft) high, at Las Campanas, Chile
A neutron star forms when a massive star runs out of fuel and explodes as a supernova, throwing off its outer layers and leaving behind a collapsed core composed almost entirely of neutrons. Neutrons are the uncharged particles in the nucleus of an atom, where they are bound together with positively charged protons. In a neutron star, they are packed together just as densely as in the nucleus of an atom, resulting in an object with one to three times the mass of our sun but only about 12 miles wide.
“Basically, a neutron star is a gigantic atom with the mass of the sun and the size of a city like San Francisco or Manhattan,” said Foley, an assistant professor of astronomy and astrophysics at UC Santa Cruz.
These objects are so dense, a cup of neutron star material would weigh as much as Mount Everest, and a teaspoon would weigh a billion tons. It’s as dense as matter can get without collapsing into a black hole.
THE MERGER
Like other stars, neutron stars sometimes occur in pairs, orbiting each other and gradually spiraling inward. Eventually, they come together in a catastrophic merger that distorts space and time (creating gravitational waves) and emits a brilliant flare of electromagnetic radiation, including visible, infrared, and ultraviolet light, x-rays, gamma rays, and radio waves. Merging black holes also create gravitational waves, but there’s nothing to be seen because no light can escape from a black hole.
Foley’s team was the first to observe the light from a neutron star merger that took place on August 17, 2017, and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).
VIRGO Gravitational Wave interferometer, near Pisa, Italy
Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-ZibESA/eLISA the future of gravitational wave research
Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)
Now, for the first time, scientists can study both the gravitational waves (ripples in the fabric of space-time), and the radiation emitted from the violent merger of the densest objects in the universe.
The UC Santa Cruz team found SSS17a by comparing a new image of the galaxy N4993 (right) with images taken four months earlier by the Hubble Space Telescope (left). The arrows indicate where SSS17a was absent from the Hubble image and visible in the new image from the Swope Telescope. (Image credits: Left, Hubble/STScI; Right, 1M2H Team/UC Santa Cruz & Carnegie Observatories/Ryan Foley)
It’s that combination of data, and all that can be learned from it, that has astronomers and physicists so excited. The observations of this one event are keeping hundreds of scientists busy exploring its implications for everything from fundamental physics and cosmology to the origins of gold and other heavy elements.
A small team of UC Santa Cruz astronomers were the first team to observe light from two neutron stars merging in August. The implications are huge.
ALL THE GOLD IN THE UNIVERSE
It turns out that the origins of the heaviest elements, such as gold, platinum, uranium—pretty much everything heavier than iron—has been an enduring conundrum. All the lighter elements have well-explained origins in the nuclear fusion reactions that make stars shine or in the explosions of stars (supernovae). Initially, astrophysicists thought supernovae could account for the heavy elements, too, but there have always been problems with that theory, says Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz.
The violent merger of two neutron stars is thought to involve three main energy-transfer processes, shown in this diagram, that give rise to the different types of radiation seen by astronomers, including a gamma-ray burst and a kilonova explosion seen in visible light. (Image credit: Murguia-Berthier et al., Science)
A theoretical astrophysicist, Ramirez-Ruiz has been a leading proponent of the idea that neutron star mergers are the source of the heavy elements. Building a heavy atomic nucleus means adding a lot of neutrons to it. This process is called rapid neutron capture, or the r-process, and it requires some of the most extreme conditions in the universe: extreme temperatures, extreme densities, and a massive flow of neutrons. A neutron star merger fits the bill.
Ramirez-Ruiz and other theoretical astrophysicists use supercomputers to simulate the physics of extreme events like supernovae and neutron star mergers. This work always goes hand in hand with observational astronomy. Theoretical predictions tell observers what signatures to look for to identify these events, and observations tell theorists if they got the physics right or if they need to tweak their models. The observations by Foley and others of the neutron star merger now known as SSS17a are giving theorists, for the first time, a full set of observational data to compare with their theoretical models.
According to Ramirez-Ruiz, the observations support the theory that neutron star mergers can account for all the gold in the universe, as well as about half of all the other elements heavier than iron.
RIPPLES IN THE FABRIC OF SPACE-TIME
Einstein predicted the existence of gravitational waves in 1916 in his general theory of relativity, but until recently they were impossible to observe. LIGO’s extraordinarily sensitive detectors achieved the first direct detection of gravitational waves, from the collision of two black holes, in 2015. Gravitational waves are created by any massive accelerating object, but the strongest waves (and the only ones we have any chance of detecting) are produced by the most extreme phenomena.
Two massive compact objects—such as black holes, neutron stars, or white dwarfs—orbiting around each other faster and faster as they draw closer together are just the kind of system that should radiate strong gravitational waves. Like ripples spreading in a pond, the waves get smaller as they spread outward from the source. By the time they reached Earth, the ripples detected by LIGO caused distortions of space-time thousands of times smaller than the nucleus of an atom.
The rarefied signals recorded by LIGO’s detectors not only prove the existence of gravitational waves, they also provide crucial information about the events that produced them. Combined with the telescope observations of the neutron star merger, it’s an incredibly rich set of data.
LIGO can tell scientists the masses of the merging objects and the mass of the new object created in the merger, which reveals whether the merger produced another neutron star or a more massive object that collapsed into a black hole. To calculate how much mass was ejected in the explosion, and how much mass was converted to energy, scientists also need the optical observations from telescopes. That’s especially important for quantifying the nucleosynthesis of heavy elements during the merger.
LIGO can also provide a measure of the distance to the merging neutron stars, which can now be compared with the distance measurement based on the light from the merger. That’s important to cosmologists studying the expansion of the universe, because the two measurements are based on different fundamental forces (gravity and electromagnetism), giving completely independent results.
“This is a huge step forward in astronomy,” Foley said. “Having done it once, we now know we can do it again, and it opens up a whole new world of what we call ‘multi-messenger’ astronomy, viewing the universe through different fundamental forces.”
IN THIS REPORT
Neutron stars
A team from UC Santa Cruz was the first to observe the light from a neutron star merger that took place on August 17, 2017 and was detected by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO)
Graduate students and post-doctoral scholars at UC Santa Cruz played key roles in the dramatic discovery and analysis of colliding neutron stars.Astronomer Ryan Foley leads a team of young graduate students and postdoctoral scholars who have pulled off an extraordinary coup. Following up on the detection of gravitational waves from the violent merger of two neutron stars, Foley’s team was the first to find the source with a telescope and take images of the light from this cataclysmic event. In so doing, they beat much larger and more senior teams with much more powerful telescopes at their disposal.
“We’re sort of the scrappy young upstarts who worked hard and got the job done,” said Foley, an untenured assistant professor of astronomy and astrophysics at UC Santa Cruz.
David Coulter, graduate student
The discovery on August 17, 2017, has been a scientific bonanza, yielding over 100 scientific papers from numerous teams investigating the new observations. Foley’s team is publishing seven papers, each of which has a graduate student or postdoc as the first author.
“I think it speaks to Ryan’s generosity and how seriously he takes his role as a mentor that he is not putting himself front and center, but has gone out of his way to highlight the roles played by his students and postdocs,” said Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz and the most senior member of Foley’s team.
“Our team is by far the youngest and most diverse of all of the teams involved in the follow-up observations of this neutron star merger,” Ramirez-Ruiz added.
Charles Kilpatrick, postdoctoral scholar
Charles Kilpatrick, a 29-year-old postdoctoral scholar, was the first person in the world to see an image of the light from colliding neutron stars. He was sitting in an office at UC Santa Cruz, working with first-year graduate student Cesar Rojas-Bravo to process image data as it came in from the Swope Telescope in Chile. To see if the Swope images showed anything new, he had also downloaded “template” images taken in the past of the same galaxies the team was searching.
Ariadna Murguia-Berthier, graduate student
“In one image I saw something there that was not in the template image,” Kilpatrick said. “It took me a while to realize the ramifications of what I was seeing. This opens up so much new science, it really marks the beginning of something that will continue to be studied for years down the road.”
At the time, Foley and most of the others in his team were at a meeting in Copenhagen. When they found out about the gravitational wave detection, they quickly got together to plan their search strategy. From Copenhagen, the team sent instructions to the telescope operators in Chile telling them where to point the telescope. Graduate student David Coulter played a key role in prioritizing the galaxies they would search to find the source, and he is the first author of the discovery paper published in Science.
Matthew Siebert, graduate student
“It’s still a little unreal when I think about what we’ve accomplished,” Coulter said. “For me, despite the euphoria of recognizing what we were seeing at the moment, we were all incredibly focused on the task at hand. Only afterward did the significance really sink in.”
Just as Coulter finished writing his paper about the discovery, his wife went into labor, giving birth to a baby girl on September 30. “I was doing revisions to the paper at the hospital,” he said.
It’s been a wild ride for the whole team, first in the rush to find the source, and then under pressure to quickly analyze the data and write up their findings for publication. “It was really an all-hands-on-deck moment when we all had to pull together and work quickly to exploit this opportunity,” said Kilpatrick, who is first author of a paper comparing the observations with theoretical models.
César Rojas Bravo, graduate student
Graduate student Matthew Siebert led a paper analyzing the unusual properties of the light emitted by the merger. Astronomers have observed thousands of supernovae (exploding stars) and other “transients” that appear suddenly in the sky and then fade away, but never before have they observed anything that looks like this neutron star merger. Siebert’s paper concluded that there is only a one in 100,000 chance that the transient they observed is not related to the gravitational waves.
Ariadna Murguia-Berthier, a graduate student working with Ramirez-Ruiz, is first author of a paper synthesizing data from a range of sources to provide a coherent theoretical framework for understanding the observations.
Another aspect of the discovery of great interest to astronomers is the nature of the galaxy and the galactic environment in which the merger occurred. Postdoctoral scholar Yen-Chen Pan led a paper analyzing the properties of the host galaxy. Enia Xhakaj, a new graduate student who had just joined the group in August, got the opportunity to help with the analysis and be a coauthor on the paper.
Yen-Chen Pan, postdoctoral scholar
“There are so many interesting things to learn from this,” Foley said. “It’s a great experience for all of us to be part of such an important discovery.”
Abbott et al., Nature, A gravitational-wave standard siren measurement of the Hubble constant (The LIGO Scientific Collaboration and The Virgo Collaboration, The 1M2H Collaboration, The Dark Energy Camera GW-EM Collaboration and the DES Collaboration, The DLT40 Collaboration, The Las Cumbres Observatory Collaboration, The VINROUGE Collaboration & The MASTER Collaboration)
Writing: Tim Stephens
Video: Nick Gonzales
Photos: Carolyn Lagattuta
Header image: Illustration by Robin Dienel courtesy of the Carnegie Institution for Science
Design and development: Rob Knight
Project managers: Sherry Main, Scott Hernandez-Jason, Tim Stephens
Dark Energy Camera [DECam], built at FNAL NOAO/CTIO Victor M Blanco 4m Telescope which houses the DECam at Cerro Tololo, Chile, housing DECam at an altitude of 7200 feet
Gemini South telescope, Cerro Tololo Inter-American Observatory (CTIO) campus near La Serena, Chile, at an altitude of 7200 feet
Noted in the vdeo but not in te article:
NASA/Chandra TelescopeNASA/SWIFT TelescopeNRAO/Karl V Jansky VLA, on the Plains of San Agustin fifty miles west of Socorro, NM, USAPrompt telescope CTIO ChileNASA NuSTAR X-ray telescope
This figure shows reconstructions of the four confident and one candidate (LVT151012) gravitational wave signals detected by LIGO and Virgo to date for black holes, including the most recent black hole detection GW170814 (which was observed in all three detectors). Note the duration of the merger is paltry: from hundreds of milliseconds up to approximately 2 seconds at the greatest. LIGO/Virgo/B. Farr (University of Oregon)
With the launch of LISA, the Laser Interferometer Space Antenna, scheduled for the 2030s, however, all of that is set to change. For the first time, we’ll be able to know exactly when and where to point our telescopes to watch the fireworks from the very start. Here’s the story of how.
In our Universe, all sorts of astrophysical phenomena take place that generate gravitational waves. Whenever there’s a large mass that either:
accelerates through a strongly curved region of space,
rapidly rearranges its shape,
causes another enormous mass to accelerate-and-fall onto it,
or otherwise alters the fabric of spacetime from its pre-existing state, gravitational energy is radiated away. These ripples travel through space at the speed of light, carrying energy away. The way that energy gets conserved is that the original masses must wind up more tightly bound than they were before: gravitational potential energy gets converted into these gravitational waves.
Any object or shape, physical or non-physical, would be distorted as gravitational waves passed through it. Whenever one large mass is accelerated through a region of curved spacetime, gravitational wave emission is an inevitable consequence. NASA/Ames Research Center/C. Henze.
The strongest amplitude signals come from the strongest changes in gravitational fields. This means that large masses accelerating at extremely short distances are the best candidates. Things like neutron star pairs, black hole binaries, supernovae, glitching pulsars, or neutron star-black hole systems are the best candidate systems for a detector like LIGO. These aren’t, however, the strongest signals in the entire Universe; they’re simply the strongest signals at the frequencies LIGO is sensitive to. These gravitational wave signals truly are waves: they have a wavelength and a frequency, depending on, for example, the orbital period of a binary system.
LIGO, with its 4-kilometer arms that reflects light back-and-forth around a few thousand times, is sensitive to phenomena that generate waves with periods of milliseconds. The reason is that light travels thousands of kilometers in just a few milliseconds, so anything with a longer-period orbit will generate waves that are simply too large for LIGO to detect. Supernovae, merging neutron stars, and inspiraling black holes are processes that take minuscule fractions-of-a-second to complete, and hence they’re ideally suited for these relatively small gravitational wave detectors. However, there are plenty of other massive systems — in some cases, far more massive than the ones LIGO can see — that take far longer to complete a period.
The five black hole-black hole mergers discovered by LIGO (and Virgo), along with a sixth, insufficiently significant signal. The most massive black hole seen by LIGO, thus far, was 36 solar masses, pre-merger. However, galaxies contain supermassive black holes millions or even billions of times the mass of the Sun, and while LIGO isn’t sensitive to them, LISA will be. LIGO/Caltech/Sonoma State (Aurore Simonnet)
The black holes we’ve seen are only a few tens of times the mass of the Sun; we know there are black holes out there with millions or even billions of times the Sun’s mass. At the centers of practically every galaxy are these supermassive behemoths, and they routinely devour asteroids, planets, stars, or even other massive black holes. However, with such large masses, they have enormous event horizons, so large that even an object revolving at the very edge would take many seconds or even minutes to complete a revolution. LIGO could never be sensitive to such a long-period gravitational wave, as its arms are too short. To see that, we’d need a gravitational wave detector in space: exactly what LISA is going to be.
With three spacecraft orbiting one another far away from the Earth, LISA will be sensitive to inspirals and mergers of objects around supermassive black holes: the most reliable and expected source of gravitational waves out there. Mergers or collisions involving two supermassive black holes, as well as smaller objects merging or inspiraling into a lone supermassive black hole, are guaranteed to create gravitational waves with wavelengths many millions of kilometers in size. With an orbiting space antenna and comparably-sized laser arms, however, LISA will be able to see these objects. All of a sudden, objects with periods of minutes-to-hours are within reach.
The core of galaxy NGC 4261, like the core of a great many galaxies, show signs of a supermassive black hole in both infrared and X-ray observations. When a planet, star, black hole, or other massive object spirals into the central supermassive black hole, gravitational waves will be emitted, and the electromagnetic counterpart should be visible to our other great observatories, if we know where and when to look. NASA / Hubble and ESA
When we detect black hole-black hole events with LIGO, it’s only the last few orbits that have a large enough amplitude to be seen above the background noise. The entirety of the signal’s duration lasts from a few hundred milliseconds to only a couple of seconds. By time a signal is collected, identified, processed, and localized, the critical merger event has already passed. There’s no way to point your telescopes — the ones that could find an electromagnetic counterpart to the signal — quickly enough to catch them from birth. Even inspiraling and merging neutron stars could only last tens of seconds before the critical “chirp” moment arrives. Processing time, even under ideal conditions, makes predicting the particular when-and-where a signal will occur a practical impossibility. But all of this will change with LISA.
For the past 2+ years, gravitational waves have been detected on Earth, from merging neutron stars and merging black holes. By building a gravitational wave observatory in space, we may be able to reach the sensitivities necessary to predict when a merger involving a supermassive black hole will occur.
ESA / NASA and the LISA collaboration
These extreme masses can generate signals of a much greater amplitude at a much lower frequency, meaning that they’ll be detectable in an instrument like LISA not seconds, but weeks, months, or even years in advance. Rather than looking at your data after-the-fact and concluding, “hey, we had a gravitational wave event here a few minutes ago,” you could look at your data and know, “in 2 years, 1 month, 21 days, 4 hours, 13 minutes and 56 seconds, we should point our telescopes at this location on the sky.” It will mean we can make these predictions way in advance, and the era of real-time, predictive, multi-messenger astronomy will have truly arrived.
Active galaxies both devour, as well as accelerate and eject infalling matter, that gets close to their central, supermassive black hole. With the localization and timing capabilities of LISA, we should know exactly when and where to point our telescopes to see the action unfold from the outset.
Gravitational wave astronomy, as a science, is still only in its infancy, but it provides a whole new way to look at and study the entire Universe. While LIGO may only be sensitive to millisecond-period events, LISA will extend that to minutes-and-hours, while other techniques like pulsar timing and polarization measurements of the Big Bang’s leftover glow could capture events that take years or decades, or even billions of years, respectively. With LIGO, we have no realistic hope of collecting, processing, and analyzing the data fast enough to tell our telescopes where to point in advance of the critical event; optical astronomy is destined to remain a follow-up only. But with the advent of LISA, we’ll be able to know exact when and where to point our telescopes to get the ultimate cosmic show from the moment an event begins. For the first time, we won’t be reacting to the Universe; we’ll have a bona fide way to predict its most spectacular events ahead of time.
“Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan
Nov. 3, 2017
David Salisbury
david.salisbury@vanderbilt.edu
ESA/LISA PathfinderESA/eLISA space based the future of gravitational wave research
Illustration of one of the three satellites that will form the Laser Interferometer Space Antenna (NASA)
Kelly Holley-Bockelmann, associate professor of astrophysics at Vanderbilt University, has been appointed by NASA’s Astrophysics Directorate to be chair of the U.S. Laser Interferometer Space Antenna (LISA) Study Team, a group of 18 scientists who will advise NASA on science issues related to the proposed space observatory.
Kelly Holley-Bockelmann
LISA, which is designed to take the fledgling field of gravitational wave astronomy to the next level, is an international scientific effort led by the European Space Agency in collaboration with NASA. The $1 billion-plus project consists of three satellites linked by laser beams, all orbiting the sun in an equilateral triangle 2.5 million kilometers on a side, tentatively scheduled for launch in 2030.
Only two years ago, a land-based gravitational wave observatory confirmed Einstein’s prediction that gravitational fluctuations from moving matter excite infinitesimal ripples in space—this first detection of gravitational waves earned the 2017 Nobel Prize in Physics. Just last month, the collision of a pair of neutron stars was observed in both light and gravity through a joint effort involving thousands of astronomers on every continent in the world. These achievements demonstrated that gravitational waves open a new window on the cosmos, one that can provide important new insights into the nature of some of the most violent phenomena in the universe.
VIRGO Gravitational Wave interferometer, near Pisa, Italy
Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-ZibESA/eLISA the future of gravitational wave research
Skymap showing how adding Virgo to LIGO helps in reducing the size of the source-likely region in the sky. (Credit: Giuseppe Greco (Virgo Urbino group)
Ground-based gravitational wave detectors, such as the Laser Interferometer Gravitational Wave Observatory in the U.S. and the European Gravitational Observatory in Italy, are tuned to detect higher frequency, shorter wavelength ripples in space produced by rotating neutron stars, mergers between neutron stars and stellar mass black holes and stellar explosions. LISA is tuned to detect lower frequencies and longer wavelengths produced by mergers between black holes millions of times more massive than the sun. The space-based system is also designed to track neutron stars and stellar mass black holes in orbit around the massive black hole at heart of the Milky Way, and will map tens of millions of tightly bound binary star systems throughout the galaxy.
As chair of the study team, Holley-Bockelmann will also represent U.S. interests within the international LISA Consortium.
“Our team will lead the U.S. effort to build the new field of gravitational wave astronomy,” said Holley-Bockelmann. “The choices we make will help dictate the pace of discovery, the health and the culture of this new field. Taking a step back, this is the first time we’ve had a chance to build an entirely new field in physics since quantum mechanics about 100 years ago. It’s an incredible time to be an astrophysicist.”
In her new position, Holley-Bockelmann’s goals are to help develop the case for LISA science for the National Academy of Science’s 2020 Decadal Survey, which contains the academy’s recommendations for astronomical research in the next decade. She also intends to act as a representative and advocate for LISA science through invited talks, workshops, town hall meetings, social media and other communication channels.
In addition, Holley-Bockelmann—co-director of the Fisk-Vanderbilt Masters-to-PhD Bridge Program, which assists under-represented minorities obtain doctoral degrees in science, math and engineering—intends to help prepare traditional astronomers and gravitational wave scientists to join forces and combine data from gravitational and conventional astronomical observatories. This “multimessenger astronomy” promises a more comprehensive picture of the titanic collisions, explosions and other cosmic events that generate powerful ripples in space time.
“Oddly enough, I think my work with the Bridge program will be useful here. I know what techniques can help people transition from one type of expertise to another, and hope to implement some of these practices to build a bridge between electromagnetic and gravitational wave astronomers,” she observed.
Commodore Cornelius Vanderbilt was in his 79th year when he decided to make the gift that founded Vanderbilt University in the spring of 1873.
The $1 million that he gave to endow and build the university was the commodore’s only major philanthropy. Methodist Bishop Holland N. McTyeire of Nashville, husband of Amelia Townsend who was a cousin of the commodore’s young second wife Frank Crawford, went to New York for medical treatment early in 1873 and spent time recovering in the Vanderbilt mansion. He won the commodore’s admiration and support for the project of building a university in the South that would “contribute to strengthening the ties which should exist between all sections of our common country.”
McTyeire chose the site for the campus, supervised the construction of buildings and personally planted many of the trees that today make Vanderbilt a national arboretum. At the outset, the university consisted of one Main Building (now Kirkland Hall), an astronomical observatory and houses for professors. Landon C. Garland was Vanderbilt’s first chancellor, serving from 1875 to 1893. He advised McTyeire in selecting the faculty, arranged the curriculum and set the policies of the university.
For the first 40 years of its existence, Vanderbilt was under the auspices of the Methodist Episcopal Church, South. The Vanderbilt Board of Trust severed its ties with the church in June 1914 as a result of a dispute with the bishops over who would appoint university trustees.
kirkland hallFrom the outset, Vanderbilt met two definitions of a university: It offered work in the liberal arts and sciences beyond the baccalaureate degree and it embraced several professional schools in addition to its college. James H. Kirkland, the longest serving chancellor in university history (1893-1937), followed Chancellor Garland. He guided Vanderbilt to rebuild after a fire in 1905 that consumed the main building, which was renamed in Kirkland’s honor, and all its contents. He also navigated the university through the separation from the Methodist Church. Notable advances in graduate studies were made under the third chancellor, Oliver Cromwell Carmichael (1937-46). He also created the Joint University Library, brought about by a coalition of Vanderbilt, Peabody College and Scarritt College.
Remarkable continuity has characterized the government of Vanderbilt. The original charter, issued in 1872, was amended in 1873 to make the legal name of the corporation “The Vanderbilt University.” The charter has not been altered since.
The university is self-governing under a Board of Trust that, since the beginning, has elected its own members and officers. The university’s general government is vested in the Board of Trust. The immediate government of the university is committed to the chancellor, who is elected by the Board of Trust.
The original Vanderbilt campus consisted of 75 acres. By 1960, the campus had spread to about 260 acres of land. When George Peabody College for Teachers merged with Vanderbilt in 1979, about 53 acres were added.
wyatt centerVanderbilt’s student enrollment tended to double itself each 25 years during the first century of the university’s history: 307 in the fall of 1875; 754 in 1900; 1,377 in 1925; 3,529 in 1950; 7,034 in 1975. In the fall of 1999 the enrollment was 10,127.
In the planning of Vanderbilt, the assumption seemed to be that it would be an all-male institution. Yet the board never enacted rules prohibiting women. At least one woman attended Vanderbilt classes every year from 1875 on. Most came to classes by courtesy of professors or as special or irregular (non-degree) students. From 1892 to 1901 women at Vanderbilt gained full legal equality except in one respect — access to dorms. In 1894 the faculty and board allowed women to compete for academic prizes. By 1897, four or five women entered with each freshman class. By 1913 the student body contained 78 women, or just more than 20 percent of the academic enrollment.
National recognition of the university’s status came in 1949 with election of Vanderbilt to membership in the select Association of American Universities. In the 1950s Vanderbilt began to outgrow its provincial roots and to measure its achievements by national standards under the leadership of Chancellor Harvie Branscomb. By its 90th anniversary in 1963, Vanderbilt for the first time ranked in the top 20 private universities in the United States.
Vanderbilt continued to excel in research, and the number of university buildings more than doubled under the leadership of Chancellors Alexander Heard (1963-1982) and Joe B. Wyatt (1982-2000), only the fifth and sixth chancellors in Vanderbilt’s long and distinguished history. Heard added three schools (Blair, the Owen Graduate School of Management and Peabody College) to the seven already existing and constructed three dozen buildings. During Wyatt’s tenure, Vanderbilt acquired or built one-third of the campus buildings and made great strides in diversity, volunteerism and technology.
The university grew and changed significantly under its seventh chancellor, Gordon Gee, who served from 2000 to 2007. Vanderbilt led the country in the rate of growth for academic research funding, which increased to more than $450 million and became one of the most selective undergraduate institutions in the country.
On March 1, 2008, Nicholas S. Zeppos was named Vanderbilt’s eighth chancellor after serving as interim chancellor beginning Aug. 1, 2007. Prior to that, he spent 2002-2008 as Vanderbilt’s provost, overseeing undergraduate, graduate and professional education programs as well as development, alumni relations and research efforts in liberal arts and sciences, engineering, music, education, business, law and divinity. He first came to Vanderbilt in 1987 as an assistant professor in the law school. In his first five years, Zeppos led the university through the most challenging economic times since the Great Depression, while continuing to attract the best students and faculty from across the country and around the world. Vanderbilt got through the economic crisis notably less scathed than many of its peers and began and remained committed to its much-praised enhanced financial aid policy for all undergraduates during the same timespan. The Martha Rivers Ingram Commons for first-year students opened in 2008 and College Halls, the next phase in the residential education system at Vanderbilt, is on track to open in the fall of 2014. During Zeppos’ first five years, Vanderbilt has drawn robust support from federal funding agencies, and the Medical Center entered into agreements with regional hospitals and health care systems in middle and east Tennessee that will bring Vanderbilt care to patients across the state.
studentsToday, Vanderbilt University is a private research university of about 6,500 undergraduates and 5,300 graduate and professional students. The university comprises 10 schools, a public policy center and The Freedom Forum First Amendment Center. Vanderbilt offers undergraduate programs in the liberal arts and sciences, engineering, music, education and human development as well as a full range of graduate and professional degrees. The university is consistently ranked as one of the nation’s top 20 universities by publications such as U.S. News & World Report, with several programs and disciplines ranking in the top 10.
Cutting-edge research and liberal arts, combined with strong ties to a distinguished medical center, creates an invigorating atmosphere where students tailor their education to meet their goals and researchers collaborate to solve complex questions affecting our health, culture and society.
Vanderbilt, an independent, privately supported university, and the separate, non-profit Vanderbilt University Medical Center share a respected name and enjoy close collaboration through education and research. Together, the number of people employed by these two organizations exceeds that of the largest private employer in the Middle Tennessee region.
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June 22, 2017
Francis Reddy
NASA’s Goddard Space Flight Center, Greenbelt, Md.
ESA (the European Space Agency) has selected the Laser Interferometer Space Antenna (LISA) for its third large-class mission in the agency’s Cosmic Vision science program. The three-spacecraft constellation is designed to study gravitational waves in space and is a concept long studied by both ESA and NASA.
ESA’s Science Program Committee announced the selection at a meeting on June 20. The mission will now be designed, budgeted and proposed for adoption before construction begins. LISA is expected to launch in 2034. NASA will be a partner with ESA in the design, development, operations and data analysis of the mission.
ESA/eLISA the future of gravitational wave research
Gravitational radiation was predicted a century ago by Albert Einstein’s general theory of relativity. Massive accelerating objects such as merging black holes produce waves of energy that ripple through the fabric of space and time. Indirect proof of the existence of these waves came in 1978, when subtle changes observed in the motion of a pair of orbiting neutron stars showed energy was leaving the system in an amount matching predictions of energy carried away by gravitational waves.
In September 2015, these waves were first directly detected by the National Science Foundation’s ground-based Laser Interferometer Gravitational-Wave Observatory (LIGO).
Caltech/MIT Advanced aLigo Hanford, WA, USA installation Caltech/MIT Advanced aLigo detector installation Livingston, LA, USACornell SXS, the Simulating eXtreme Spacetimes (SXS) project Gravitational waves. Credit: MPI for Gravitational Physics/W.Benger-Zib
ESA/eLISA the future of gravitational wave research
The signal arose from the merger of two stellar-mass black holes located some 1.3 billion light-years away. Similar signals from other black hole mergers have since been detected.
Seismic, thermal and other noise sources limit LIGO to higher-frequency gravitational waves around 100 cycles per second (hertz). But finding signals from more powerful events, such as mergers of supermassive black holes in colliding galaxies, requires the ability to detect frequencies much lower than 1 hertz, a sensitivity level only possible from space.
LISA consists of three spacecraft separated by 1.6 million miles (2.5 million kilometers) in a triangular formation that follows Earth in its orbit around the sun. Each spacecraft carries test masses that are shielded in such a way that the only force they respond to is gravity. Lasers measure the distances to test masses in all three spacecraft. Tiny changes in the lengths of each two-spacecraft arm signals the passage of gravitational waves through the formation.
For example, LISA will be sensitive to gravitational waves produced by mergers of supermassive black holes, each with millions or more times the mass of the sun. It will also be able to detect gravitational waves emanating from binary systems containing neutron stars or black holes, causing their orbits to shrink. And LISA may detect a background of gravitational waves produced during the universe’s earliest moments.
For decades, NASA has worked to develop many technologies needed for LISA, including measurement, micropropulsion and control systems, as well as support for the development of data analysis techniques.
For instance, the GRACE Follow-On mission, a U.S. and German collaboration to replace the aging GRACE satellites scheduled for launch late this year, will carry a laser measuring system that inherits some of the technologies originally developed for LISA.
NASA/DLR Grace
The mission’s Laser Ranging Interferometer will track distance changes between the two satellites with unprecedented precision, providing the first demonstration of the technology in space.
In 2016, ESA’s LISA Pathfinder successfully demonstrated key technologies needed to build LISA.
ESA/LISA Pathfinder
Each of LISA’s three spacecraft must gently fly around its test masses without disturbing them, a process called drag-free flight. In its first two months of operations, LISA Pathfinder demonstrated this process with a precision some five times better than its mission requirements and later reached the sensitivity needed for the full multi-spacecraft observatory. U.S. researchers collaborated on aspects of LISA Pathfinder for years, and the mission carries a NASA-supplied experiment called the ST7 Disturbance Reduction System, which is managed by NASA’s Jet Propulsion Laboratory in Pasadena, California.
For more information about the LISA project, visit:
NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.
Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.
sciencesprings 