## From Lawrence Berkeley National Lab: “Seeing the Universe Through New Lenses”

May 14, 2020
Glenn Roberts Jr.
(510) 520-0843
geroberts@lbl.gov

Images collected for dark energy telescope project reveal hundreds of new gravitational lens candidates.

This Hubble Space Telescope image shows a gravitational lens (center) that was first identified as a lens candidate with the assistance of a neural network that processed ground-based space images. The lens is artificially colorized and circled in this image. (Credit: Hubble Space Telescope)

Like crystal balls for the universe’s deeper mysteries, galaxies and other massive space objects can serve as lenses to more distant objects and phenomena along the same path, bending light in revelatory ways.

Gravitational lensing was first theorized by Albert Einstein more than 100 years ago to describe how light bends when it travels past massive objects like galaxies and galaxy clusters.

These lensing effects are typically described as weak or strong, and the strength of a lens relates to an object’s position and mass and distance from the light source that is lensed. Strong lenses can have 100 billion times more mass than our sun, causing light from more distant objects in the same path to magnify and split, for example, into multiple images, or to appear as dramatic arcs or rings.

The major limitation of strong gravitational lenses has been their scarcity, with only several hundred confirmed since the first observation in 1979, but that’s changing … and fast.

A new study by an international team of scientists revealed 335 new strong lensing candidates based on a deep dive into data collected for a U.S. Department of Energy-supported telescope project in Arizona called the Dark Energy Spectroscopic Instrument (DESI).

LBNL/DESI spectroscopic instrument on the Mayall 4-meter telescope at Kitt Peak National Observatory started in 2018

NOAO/Mayall 4 m telescope at Kitt Peak, Arizona, USA, Altitude 2,120 m (6,960 ft)

Kitt Peak National Observatory of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono O’odham Nation, 88 kilometers 55 mi west-southwest of Tucson, Arizona, Altitude 2,096 m (6,877 ft)

The study, published May 7 in The Astrophysical Journal, benefited from the winning machine-learning algorithm in an international science competition.

“Finding these objects is like finding telescopes that are the size of a galaxy,” said David Schlegel, a senior scientist in Lawrence Berkeley National Laboratory’s (Berkeley Lab’s) Physics Division who participated in the study. “They’re powerful probes of dark matter and dark energy.”

These newly discovered gravitational lens candidates could provide specific markers for precisely measuring distances to galaxies in the ancient universe if supernovae are observed and precisely tracked and measured via these lenses, for example.

Strong lenses also provide a powerful window into the unseen universe of dark matter, which makes up about 85 percent of the matter in the universe, as most of the mass responsible for lensing effects is thought to be Dark Matter. Dark Matter and the accelerating expansion of the universe, driven by Dark Energy, are among the biggest mysteries that physicists are working to solve.

In the latest study, researchers enlisted Cori, a supercomputer at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC), to automatically compare imaging data from the Dark Energy Camera Legacy Survey (DECaLS) – one of three surveys conducted in preparation for DESI – with a training sample of 423 known lenses and 9,451 non-lenses.

NERSC Cray Cori II supercomputer at NERSC at LBNL, named after Gerty Cori, the first American woman to win a Nobel Prize in science

The researchers grouped the candidate strong lenses into three categories based on the likelihood that they are, in fact, lenses: Grade A for the 60 candidates that are most likely to be lenses, Grade B for the 105 candidates with less pronounced features, and Grade C for the 176 candidate lenses that have fainter and smaller lensing features than those in the other two categories.

Xiaosheng Huang, the study’s lead author, noted that the team already succeeded in winning time on the Hubble Space Telescope to confirm some of the most promising lensing candidates revealed in the study, with observing time on the Hubble that began in late 2019.

“The Hubble Space Telescope can see the fine details without the blurring effects of Earth’s atmosphere,” Huang said.

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Dark Matter Background
Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

Fritz Zwicky from http:// palomarskies.blogspot.com

Coma cluster via NASA/ESA Hubble

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

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Dark Energy Survey

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

Timeline of the Inflationary Universe WMAP

The Dark Energy Survey (DES) is an international, collaborative effort to map hundreds of millions of galaxies, detect thousands of supernovae, and find patterns of cosmic structure that will reveal the nature of the mysterious dark energy that is accelerating the expansion of our Universe. DES began searching the Southern skies on August 31, 2013.

According to Einstein’s theory of General Relativity, gravity should lead to a slowing of the cosmic expansion. Yet, in 1998, two teams of astronomers studying distant supernovae made the remarkable discovery that the expansion of the universe is speeding up. To explain cosmic acceleration, cosmologists are faced with two possibilities: either 70% of the universe exists in an exotic form, now called dark energy, that exhibits a gravitational force opposite to the attractive gravity of ordinary matter, or General Relativity must be replaced by a new theory of gravity on cosmic scales.

DES is designed to probe the origin of the accelerating universe and help uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion with high precision. More than 400 scientists from over 25 institutions in the United States, Spain, the United Kingdom, Brazil, Germany, Switzerland, and Australia are working on the project. The collaboration built and is using an extremely sensitive 570-Megapixel digital camera, DECam, mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, high in the Chilean Andes, to carry out the project.

Over six years (2013-2019), the DES collaboration used 758 nights of observation to carry out a deep, wide-area survey to record information from 300 million galaxies that are billions of light-years from Earth. The survey imaged 5000 square degrees of the southern sky in five optical filters to obtain detailed information about each galaxy. A fraction of the survey time is used to observe smaller patches of sky roughly once a week to discover and study thousands of supernovae and other astrophysical transients.

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Bringing Science Solutions to the World
In the world of science, Lawrence Berkeley National Laboratory (Berkeley Lab) is synonymous with “excellence.” Thirteen Nobel prizes are associated with Berkeley Lab. Seventy Lab scientists are members of the National Academy of Sciences (NAS), one of the highest honors for a scientist in the United States. Thirteen of our scientists have won the National Medal of Science, our nation’s highest award for lifetime achievement in fields of scientific research. Eighteen of our engineers have been elected to the National Academy of Engineering, and three of our scientists have been elected into the Institute of Medicine. In addition, Berkeley Lab has trained thousands of university science and engineering students who are advancing technological innovations across the nation and around the world.

Berkeley Lab is a member of the national laboratory system supported by the U.S. Department of Energy through its Office of Science. It is managed by the University of California (UC) and is charged with conducting unclassified research across a wide range of scientific disciplines. Located on a 202-acre site in the hills above the UC Berkeley campus that offers spectacular views of the San Francisco Bay, Berkeley Lab employs approximately 3,232 scientists, engineers and support staff. The Lab’s total costs for FY 2014 were $785 million. A recent study estimates the Laboratory’s overall economic impact through direct, indirect and induced spending on the nine counties that make up the San Francisco Bay Area to be nearly$700 million annually. The Lab was also responsible for creating 5,600 jobs locally and 12,000 nationally. The overall economic impact on the national economy is estimated at \$1.6 billion a year. Technologies developed at Berkeley Lab have generated billions of dollars in revenues, and thousands of jobs. Savings as a result of Berkeley Lab developments in lighting and windows, and other energy-efficient technologies, have also been in the billions of dollars.

Berkeley Lab was founded in 1931 by Ernest Orlando Lawrence, a UC Berkeley physicist who won the 1939 Nobel Prize in physics for his invention of the cyclotron, a circular particle accelerator that opened the door to high-energy physics. It was Lawrence’s belief that scientific research is best done through teams of individuals with different fields of expertise, working together. His teamwork concept is a Berkeley Lab legacy that continues today.

A U.S. Department of Energy National Laboratory Operated by the University of California.

• #### Barbarina Zwicky 6:10 pm on May 15, 2020 Permalink | Reply

Rubin has been a constant nuisance to my father’s legacy in regard to Dark Matter and often took false credit for its discovery, crowning herself as “Discoverer of Dark Matter.” The naming of LSST after Rubin, is an undeserved honor for this celebrated plagiarist.

Vera Rubin was celebrated in the press and by several institutions for her work in specific in regard to Dark Matter, my father’s discovery, as well as responsible for the roughshod over my father, his memory, and credit for his original work, by falsely assigning that credit to herself in numerous incidents involving the media and even nomenclature of her lecture: “I left Vassar and Found Dark Matter.” I consider Vera Rubin a person who attached herself to my father’s original work in parasitic forced credit, repeatedly advanced this unethical agenda and academic dishonesty, crowning herself as “Discoverer of Dark Matter,” the published achievement of another. Rubin’s dictates of conscience revealed a failed ethical compass as she assigned herself credit for my father’s methodology and that of others in the sciences in regard to the mathematical calculations in regard to the rotational speeds of galaxies, as well as claiming to be the “Discoverer of Dark Matter.” Vera Rubin was a constant unwanted barnacle that was attached to my father’s discovery, Dark Matter. The advancement of bringing the gravitational phenomena of Dark Matter to light and into the modern consciousness of physicists worldwide would have regardless been unsealed from the echoes of my father’s original work in 1933. Fritz Zwicky: “I consequently engaged in the application of certain simple general principles of morphological research, and in particular the method of Directed Intuition that would allow me to predict and visualize the existence of as yet unknown cosmic objects and phenomena.” Fritz Zwicky’s eidolon was realized from the results of his observations published in “Die Rotverschiebung von extragalaktischen Nebeln”, Helv. Phys. Acta 6, 110-127 (1933). English translation Johannes Nicolai Meyling – Barbarina Exita Zwicky (2013). Fritz Zwicky discovered Dark Matter and coined, dunkle (kalte) Materie (cold dark matter) in his 1933 article referenced above. The Mass-Radial Acceleration Discrepancy by measuring the speeds of galaxies in the Coma Cluster originated with Fritz Zwicky, not Rubin, as using the more challenging methodology of the virial theorem, by relating the total average kinetic energy and the total average potential energy of the galaxies of the Coma Cluster. He advanced that the virial for a pair of orbiting masses is zero, and used the principle of superposition to craft the argument to a system of interacting mass points. Zwicky then used the position and velocity measurements to determine the mass of the galaxy cluster. The LSST will endeavor to discover Dark Matter and should not be renamed at all, and certainly not after Vera Rubin, who plagiarized discovery in regard to Dark Matter, without acknowledgment of its provenance and pioneer, Fritz Zwicky, and deprives rightful illumination to the Father of Dark Matter. It will highlight this interloper and celebrate this forced credit from the rightful person due, Fritz Zwicky, by memorializing the name of LSST after this faux “pioneer” and self-proclaimed “Discoverer of Dark Matter.”

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## From physicsworld.com: “Ultracold atomic comagnetometer joins the search for dark matter”

From physicsworld.com

05 May 2020
Hamish Johnston

In a spin: illustration of a Bose–Einstein condensate of rubidium atoms in two different quantum states. (Courtesy: ICFO/ P Gomez and M Mitchell)

A new atomic comagnetometer that could be used to detect hypothetical dark matter particles called axions has been created by physicists in Spain. The sensor uses two different quantum states of ultracold rubidium atoms to cancel out the effect of ambient magnetic fields, allowing physicists to focus on exotic spin-dependent interactions that may involve axions.

Dark matter is a mysterious substance that appears to account for about 85% of the matter in the universe – the other 15% being normal matter such as atoms and molecules. While myriad astrophysical observations point to the existence of dark matter, physicists have very little understanding of its precise nature.

Some dark matter could comprise hypothetical particles called axions, which were first proposed in the 1970s to solve a problem in quantum chromodynamics. If dark matter axions do exist, they could mediate exotic interactions between quantum-mechanical spins – in analogy to how photons mediate conventional magnetic interactions between spins.

Two detectors

These exotic interactions would be weak, but in principle they could be measured using an atomic comagnetometer, which comprises two different magnetic-field detectors that are in the same place. The device is set so that the effects of ambient magnetic fields in the two detectors can be cancelled out. So, a residual signal in the comagnetometer could be the result of an exotic interaction between atomic spins within the detector itself.

The new comagnetometer was created at the Institute of Photonic Sciences in Barcelona by Pau Gomez, Ferran Martin, Chiara Mazzinghi, Daniel Benedicto Orenes, Silvana Palacios and Morgan Mitchell. The two different detectors are rubidium-87 atoms that are in two different spin states that respond in different ways to magnetic fields.

Near absolute zero

The atoms are in a gas that is chilled to near absolute zero to create a Bose-Einstein condensate (BEC). In this state the atoms are relatively immune to being jostled about by thermal interactions. This means that for several seconds the spins can respond in a coherent way to spin interactions. The BEC is also very small – just 10 microns in diameter – which boosts its performance as a comagnetometer and means that short-range axion interactions can be probed.

The response of the spins to a magnetic field is measured by firing a polarized of a beam of light at the BEC and measuring how its polarization is rotated. By comparing measurements on the two different spin states, the effect of ambient magnetic fields can be removed, allowing the team to look for any exotic interactions that are affecting the spins.

Although no evidence of axions has been found by the device so far, the team has shown that the comagnetometer is highly immune to noise from ambient magnetic fields. They say that it could be run at a sensitivity on par with other types of comagnetometers that are currently looking for axions. The device has already been used to measure conventional spin interactions between the ultracold atoms and the team says that other potential applications include spin amplification, which could be used to study quantum fluctuations.

The comagnetometer is described in Physical Review Letters.

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## From particlebites: “Three Birds with One Particle: The Possibilities of Axions”

From particlebites

May 1, 2020
Amara McCune

Title: “Axiogenesis”

Author: Raymond T. Co and Keisuke Harigaya

Reference: https://arxiv.org/pdf/1910.02080.pdf

On the laundry list of problems in particle physics, a rare three-for-one solution could come in the form of a theorized light scalar particle fittingly named after a detergent: the axion. Frank Wilczek coined this term in reference to its potential to “clean up” the Standard Model once he realized its applicability to multiple unsolved mysteries. Although Axion the dish soap has been somewhat phased out of our everyday consumer life (being now primarily sold in Latin America), axion particles remain as a key component of a physicist’s toolbox. While axions get a lot of hype as a promising Dark Matter candidate, and are now being considered as a solution to matter-antimatter asymmetry, they were originally proposed as a solution for a different Standard Model puzzle: the strong CP problem.

The strong CP problem refers to a peculiarity of quantum chromodynamics (QCD), our theory of quarks, gluons, and the strong force that mediates them: while the theory permits charge-parity (CP) symmetry violation, the ardent experimental search for CP-violating processes in QCD has so far come up empty-handed. What does this mean from a physical standpoint? Consider the neutron electric dipole moment (eDM), which roughly describes the distribution of the three quarks comprising a neutron. Naively, we might expect this orientation to be a triangular one. However, measurements of the neutron eDM, carried out by tracking changes in neutron spin precession, return a value orders of magnitude smaller than classically expected. In fact, the incredibly small value of this parameter corresponds to a neutron where the three quarks are found nearly in a line.

The classical picture of the neutron (left) looks markedly different from the picture necessitated by CP symmetry (right). The strong CP problem is essentially a question of why our mental image should look like the right picture instead of the left. Source: https://arxiv.org/pdf/1812.02669.pdf

This would not initially appear to be a problem. In fact, in the context of CP, this makes sense: a simultaneous charge conjugation (exchanging positive charges for negative ones and vice versa) and parity inversion (flipping the sign of spatial directions) when the quark arrangement is linear results in a symmetry. Yet there are a few subtleties that point to the existence of further physics. First, this tiny value requires an adjustment of parameters within the mathematics of QCD, carefully fitting some coefficients to cancel out others in order to arrive at the desired conclusion. Second, we do observe violation of CP symmetry in particle physics processes mediated by the weak interaction, such as kaon decay, which also involves quarks.

These arguments rest upon the idea of naturalness, a principle that has been invoked successfully several times throughout the development of particle theory as a hint toward the existence of a deeper, more underlying theory. Naturalness (in one of its forms) states that such minuscule values are only allowed if they increase the overall symmetry of the theory, something that cannot be true if weak processes exhibit CP-violation where strong processes do not. This puts the strong CP problem squarely within the realm of “fine-tuning” problems in physics; although there is no known reason for CP symmetry conservation to occur, the theory must be modified to fit this observation. We then seek one of two things: either an observation of CP-violation in QCD or a solution that sets the neutron eDM, and by extension any CP-violating phase within our theory, to zero.

This term in the QCD Lagrangian allows for CP symmetry violation. Current measurements place the value of \theta at no greater than 10^{-10}. In Peccei-Quinn symmetry, Θ is promoted to a field.

When such an expected symmetry violation is nowhere to be found, where is a theoretician to look for such a solution? The most straightforward answer is to turn to a new symmetry. This is exactly what Roberto Peccei and Helen Quinn did in 1977, birthing the Peccei-Quinn symmetry, an extension of QCD which incorporates a CP-violating phase known as the Θ term. The main idea behind this theory is to promote Θ to a dynamical field, rather than keeping it a constant. Since quantum fields have associated particles, this also yields the particle we dub the axion. Looking back briefly to the neutron eDM picture of the strong CP problem, this means that the angular separation should also be dynamical, and hence be relegated to the minimum energy configuration: the quarks again all in a straight line. In the language of symmetries, the U(1) Peccei-Quinn symmetry is approximately spontaneously broken, giving us a non-zero vacuum expectation value and a nearly-massless Goldstone boson: our axion.

This is all great, but what does it have to do with dark matter? As it turns out, axions make for an especially intriguing dark matter candidate due to their low mass and potential to be produced in large quantities. For decades, this prowess was overshadowed by the leading WIMP candidate (weakly-interacting massive particles), whose parameter space has been slowly whittled down to the point where physicists are more seriously turning to alternatives. As there are several production-mechanisms in early universe cosmology for axions, and 100% of dark matter abundance could be explained through this generation, the axion is now stepping into the spotlight.

This increased focus is causing some theorists to turn to further avenues of physics as possible applications for the axion. In a recent paper, Co and Harigaya examined the connection between this versatile particle and matter-antimatter asymmetry (also called baryon asymmetry). This latter term refers to the simple observation that there appears to be more matter than antimatter in our universe, since we are predominantly composed of matter, yet matter and antimatter also seem to be produced in colliders in equal proportions. In order to explain this asymmetry, without which matter and antimatter would have annihilated and we would not exist, physicists look for any mechanism to trigger an imbalance in these two quantities in the early universe. This theorized process is known as baryogenesis.

Here’s where the axion might play a part. The \theta term, which settles to zero in its possible solution to the strong CP problem, could also have taken on any value from 0 to 360 degrees very early on in the universe. Analyzing the axion field through the conjectures of quantum gravity, if there are no global symmetries then the initial axion potential cannot be symmetric [4]. By falling from some initial value through an uneven potential, which the authors describe as a wine bottle potential with a wiggly top, \theta would cycle several times through the allowed values before settling at its minimum energy value of zero. This causes the axion field to rotate, an asymmetry which could generate a disproportionality between the amounts of produced matter and antimatter. If the field were to rotate in one direction, we would see more matter than antimatter, while a rotation in the opposite direction would result instead in excess antimatter.

The team’s findings can be summarized in the plot above. Regions in purple, red, and above the orange lines (dependent upon a particular constant X which is proportional to weak scale quantities) signify excluded portions of the parameter space. The remaining white space shows values of the axion decay constant and mass where the currently measured amount of baryon asymmetry could be generated. Source: https://arxiv.org/pdf/1910.02080.pdf

Introducing a third fundamental mystery into the realm of axions begets the question of whether all three problems (strong CP, dark matter, and matter-antimatter asymmetry) can be solved simultaneously with axions. And, of course, there are nuances that could make alternative solutions to the strong CP problem more favorable or other dark matter candidates more likely. Like most theorized particles, there are several formulations of axion in the works. It is then necessary to turn our attention to experiment to narrow down the possibilities for how axions could interact with other particles, determine what their mass could be, and answer the all-important question: if they exist at all. Consequently, there are a plethora of axion-focused experiments up and running, with more on the horizon, that use a variety of methods spanning several subfields of physics. While these results begin to roll in, we can continue to investigate just how many problems we might be able to solve with one adaptable, soapy particle.

A comprehensive introduction to the strong CP problem, the axion solution, and other potential solutions: https://arxiv.org/pdf/1812.02669.pdf
Axions as a dark matter candidate: https://www.symmetrymagazine.org/article/the-other-dark-matter-candidate
The quantum gravity conjectures that axiogenesis builds upon: https://arxiv.org/abs/1810.05338
An overview of current axion-focused experiments: https://www.annualreviews.org/doi/full/10.1146/annurev-nucl-102014-022120

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What is ParticleBites?
ParticleBites is an online particle physics journal club written by graduate students and postdocs. Each post presents an interesting paper in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.

The papers are accessible on the arXiv preprint server. Most of our posts are based on papers from hep-ph (high energy phenomenology) and hep-ex (high energy experiment).

Reading a technical paper from an unfamiliar subfield is intimidating. It may not be obvious how the techniques used by the researchers really work or what role the new research plays in answering the bigger questions motivating that field, not to mention the obscure jargon! For most people, it takes years for scientific papers to become meaningful.

Our goal is to solve this problem, one paper at a time. With each brief ParticleBite, you should not only learn about one interesting piece of current work, but also get a peek at the broader picture of research in particle physics.

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## From The Kavli Foundation: “Detailed Cosmic Map to Reveal Dark Energy’s Sway”

From The Kavli Foundation

03/25/2020 [Just now in social media.]

Media Contact

Katie McKissick
The Kavli Foundation
(424) 353-8800
kmckissick@kavlifoundation.org

LBNL/DESI spectroscopic instrument on the Mayall 4-meter telescope at Kitt Peak National Observatory started in 2018

.

DESI, a bold celestial mapmaking effort, will help unravel the mystery of the universe’s accelerating expansion rate. Here, star trails take shape around the 14-story Mayall Telescope dome in this long-exposure image. The Dark Energy Spectroscopic Instrument resides within this dome. (Credit: P. Marenfeld and NOAO/AURA/NSF)​

NOAO/Mayall 4 m telescope at Kitt Peak, Arizona, USA, Altitude 2,120 m (6,960 ft)

Kitt Peak National Observatory of the Quinlan Mountains in the Arizona-Sonoran Desert on the Tohono O’odham Nation, 88 kilometers 55 mi west-southwest of Tucson, Arizona, Altitude 2,096 m (6,877 ft)

Compared to cosmic mapmakers, terrestrial cartographers have it relatively easy. Map generation here on Earth generally deals with just two dimensions—the latitude and longitude of where portions of landmasses and oceans all stitch together, forming the surface of our planet.

Out in space, though, the third dimension reigns.

After all, space is not called “space” for nothing; the sheer amount of volume out there through which galaxies are strewn is staggering. In order to understand the universe’s development, researchers need to chart its great expanse by accurately gauging distances between clusters of galaxies. Like on Earth, where the location of landmasses today speaks to eons of geophysical evolution, continental drift, tectonics, and so on, so, too, do the locations of galaxies speak to the dynamic history of the cosmos.

The geo-analogy is only so apt, however, because looking deep into space is also looking deep into time; the farther away something is, the farther back in the universe’s chronology we are glimpsing it. As a result, not only does a cosmic map capture where things are in relation to each other, it also captures when things are in relation to each other.

Tackling this time-and-space challenge anew is the Dark Energy Spectroscopic Instrument (DESI). Now mounted on a telescope in Arizona, DESI will soon begin making the most detailed 3D map of the universe to date. More than 500 researchers, hailing from 75 institutions in 13 countries, are part of the DESI project, including researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint project of Stanford University and the SLAC National Accelerator Laboratory.

Numbers-wise, over a five-year observing run that starts in 2020, DESI will look at a third of the sky, accurately mapping the distances to approximately 35 million galaxies and 2.4 million quasars. (Quasars are ultra-bright galaxies whose central supermassive black holes are gorging on matter.) That tidy galactic sum represents about 20 times more objects than any previous such effort.

“We’ll know in 3D space where the galaxies are with DESI, and with that, we can look at how galaxies are arranged in the universe,” says Aaron Roodman, who worked on image analysis elements of the Guiding, Focusing and Alignment (GFA) subsystem for DESI. Roodman is a professor and chair of the Particle Physics & Astrophysics department at SLAC and a member of KIPAC.

DESI’s mapmaking is not for astronaut traveling purposes, of course. (Heck, we humans will be lucky if we get back to the Moon in the next decade, after a 50-year hiatus.) Instead, the goal for DESI is to help us better understand the history and eventual fate of the universe by nailing down the workings of Dark Energy. Right there in DESI’s name, “dark energy” refers to an enigmatic force that theoretically comprises most of reality.

“This thing we call dark energy makes up 70 of the matter-energy density of the universe, but we don’t know what it is,” says Roodman.

Dark energy’s presence looms large, invoked to explain why the universe’s rate of acceleration is increasing over history. The overall situation is thus: Matter, the substance that comprises everything we deal with every day, registers at just 5% of the total contents of the cosmos. A second enigmatic ingredient, Dark Matter, takes up the remaining 25% percent (once dark energy’s dominance – clocking in at nearly 70% – is counted). Dark matter interacts with normal matter only through gravity, so far as we know, acting like a universe-spanning scaffold upon which normal matter is hung. That normal matter interacts with itself through the forces of nature to give us galaxies, stars, planets, elephants, paramecia, you name it.

Researchers want to bring this overview picture into sharper focus. “With DESI, we will be able to independently measure the universe’s expansion rate and how fast its structure of matter and dark matter grow, both of which are influenced by dark energy. Then when you compare those measurements, you get a precise test of the physics governing the universe,” said Risa Wechsler, the current Director of KIPAC and former spokesperson of the DESI collaboration, in comments to The Kavli Foundation in 2017.

The other part of DESI’s name, “spectroscopic,” refers to how DESI will get the all-important distance measurements to millions of galaxies. DESI contains 5,000 robotic “eyes,” made of human-hair-thin, cosmic-light-gathering fiber optic cables, held in particular orientations by robotic positioners. Each “eye” is pointed at a bright galaxy to collect enough of its stars’ collective light to perform spectroscopy—breaking apart the light into different wavelengths for study. “Whatever light comes down the fiber, we spread out the whole rainbow,” says Roodman. The signature astronomers look for in this light is called redshift, caused by the universe’s expansion, and which can be used to determine a galaxy’s distance.

With precise distance measurements to an unprecedentedly large swath of galaxies, the DESI researchers will plot the course of dark energy’s influence as never before, better constraining its properties. Roodman, for one, can’t wait.

“What is causing the expansion of the universe to accelerate is a question I’m very interested in,” says Roodman. “DESI will really teach us something about dark energy.”

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Dark Energy Survey

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

Timeline of the Inflationary Universe WMAP

The Dark Energy Survey (DES) is an international, collaborative effort to map hundreds of millions of galaxies, detect thousands of supernovae, and find patterns of cosmic structure that will reveal the nature of the mysterious dark energy that is accelerating the expansion of our Universe. DES began searching the Southern skies on August 31, 2013.

According to Einstein’s theory of General Relativity, gravity should lead to a slowing of the cosmic expansion. Yet, in 1998, two teams of astronomers studying distant supernovae made the remarkable discovery that the expansion of the universe is speeding up. To explain cosmic acceleration, cosmologists are faced with two possibilities: either 70% of the universe exists in an exotic form, now called dark energy, that exhibits a gravitational force opposite to the attractive gravity of ordinary matter, or General Relativity must be replaced by a new theory of gravity on cosmic scales.
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Dark Matter Background
Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

Fritz Zwicky from http:// palomarskies.blogspot.com

Coma cluster via NASA/ESA Hubble

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

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Stem Education Coalition

The Kavli Foundation, based in Oxnard, California, is dedicated to the goals of advancing science for the benefit of humanity and promoting increased public understanding and support for scientists and their work.

The Foundation’s mission is implemented through an international program of research institutes, professorships, and symposia in the fields of astrophysics, nanoscience, neuroscience, and theoretical physics as well as prizes in the fields of astrophysics, nanoscience, and neuroscience.

## From UC Riverside: “Satellite galaxies of the Milky Way help test dark matter theory”

From UC Riverside

April 15, 2020
Iqbal Pittalwala

UC Riverside physicists demonstrate “self-interacting dark matter” model can be tested using astronomical observations of Draco and Fornax.

Image shows Draco on the left (Hubble Space Telescope) and Fornax (ESO/Digitized Sky Survey 2).

A research team led by physicists at the University of California, Riverside, reports tiny satellite galaxies of the Milky Way can be used to test fundamental properties of “dark matter” — nonluminous material thought to constitute 85% of matter in the universe.

Using sophisticated simulations, the researchers show a theory called self-interacting dark matter, or SIDM, can compellingly explain diverse dark matter distributions in Draco and Fornax, two of the Milky Way’s more than 50 discovered satellite galaxies.

The prevailing dark matter theory, called Cold Dark Matter, or CDM, explains much of the universe, including how structures emerge in it. But a long-standing challenge for CDM has been to explain the diverse dark matter distributions in galaxies.

Lamda Cold Dark Matter Accerated Expansion of The universe http scinotions.com the-cosmic-inflation-suggests-the-existence-of-parallel-universes
Alex Mittelmann, Coldcreation

The researchers, led by UC Riverside’s Hai-Bo Yu and Laura V. Sales, studied the evolution of SIDM “subhalos” in the Milky Way “tidal field” — the gradient in the gravitational field of the Milky Way that a satellite galaxy feels in the form of a tidal force. Subhalos are dark matter clumps that host the satellite galaxies.

“We found SIDM can produce diverse dark matter distributions in the halos of Draco and Fornax, in agreement with observations,” said Yu, an associate professor of physics and astronomy and a theoretical physicist with expertise in particle properties of dark matter. “In SIDM, the interaction between the subhalos and the Milky Way’s tides leads to more diverse dark matter distributions in the inner regions of subhalos, compared to their CDM counterparts.”

Draco and Fornax have opposite extremes in their inner dark matter contents. Draco has the highest dark matter density among the nine bright Milky Way satellite galaxies; Fornax has the lowest. Using advanced astronomical measurements, astrophysicists recently reconstructed their orbital trajectories in the Milky Way’s tidal field.

“Our challenge was to understand the origin of Draco and Fornax’s diverse dark matter distributions in light of these newly measured orbital trajectories,” Yu said. “We found SIDM can provide an explanation after taking into both tidal effects and dark matter self-interactions.”

Study results appear in Physical Review Letters.

Dark matter’s nature remains largely unknown. Unlike normal matter, it does not absorb, reflect, or emit light, making it difficult to detect. Identifying the nature of dark matter is a central task in particle physics and astrophysics.

In CDM, dark matter particles are assumed to be collisionless, and every galaxy sits within a dark matter halo that forms the gravitational scaffolding holding it together. In SIDM, dark matter is proposed to self-interact through a new dark force. Dark matter particles are assumed to strongly collide with one another in the inner halo, close to the galaxy’s center — a process called dark matter self-interaction.

“Our work shows satellite galaxies of the Milky Way may provide important tests of different dark matter theories,” said Sales, an assistant professor of physics and astronomy and an astrophysicist with expertise in numerical simulations of galaxy formation. “We show the interplay between dark matter self-interactions and tidal interactions can produce novel signatures in SIDM that are not expected in the prevailing CDM theory.”

In their work, the researchers mainly used numerical simulations, called “N-body simulations,” and obtained valuable intuition through analytical modeling before running their simulations.

“Our simulations reveal novel dynamics when an SIDM subhalo evolves in the tidal field,” said Omid Sameie, a former UCR graduate student who worked with Yu and Sales and is now a postdoctoral researcher at the University of Texas at Austin working on numerical simulations of galaxy formation. “It was thought observations of Draco were inconsistent with SIDM predictions. But we found a subhalo in SIDM can produce a high dark matter density to explain Draco.”

Sales explained SIDM predicts a unique phenomenon named “core collapse.” In certain circumstances, the inner part of the halo collapses under the influence of gravity and produces a high density. This is contrary to the usual expectation that dark matter self-interactions lead to a low-density halo. Sales said the team’s simulations identify conditions for the core collapse to occur in subhalos.

“To explain Draco’s high dark matter density, its initial halo concentration needs to be high,” she said. “More dark matter mass needs to be distributed in the inner halo. While this is true for both CDM and SIDM, for SIDM the core-collapse phenomenon can only occur if the concentration is high so that the collapse timescale is less than the age of the universe. On the other hand, Fornax has a low-concentrated subhalo, and hence its density remains low.”

The researchers stressed their current work mainly focuses on SIDM and does not make a critical assessment on how well CDM can explain both Draco and Fornax.

After the team used numerical simulations to properly take into account the dynamical interplay between dark matter self-interactions and tidal interactions, the researchers observed a striking result.

“The central dark matter of an SIDM subhalo could be increasing, contrary to usual expectations,” Sameie said. “Importantly, our simulations identify conditions for this phenomenon to occur in SIDM, and we show it can explain observations of Draco.”

The research team plans to extend the study to other satellite galaxies, including ultrafaint galaxies.

Yu, Sales, and Sameie were joined in the study by Mark Vogelsberger of the Massachusetts Institute of Technology and Jesús Zavala of the University of Iceland. Sameie is the first author of the research paper.

The research was supported by grants from the U.S. Department of Energy, National Aeronautics and Space Administration, NASA MIRO FIELDS Fellowship, National Science Foundation, the Hell

five-ways-keep-your-child-safe-school-shootings

Stem Education Coalition

The University of California, Riverside is one of 10 universities within the prestigious University of California system, and the only UC located in Inland Southern California.

Widely recognized as one of the most ethnically diverse research universities in the nation, UCR’s current enrollment is more than 21,000 students, with a goal of 25,000 students by 2020. The campus is in the midst of a tremendous growth spurt with new and remodeled facilities coming on-line on a regular basis.

We are located approximately 50 miles east of downtown Los Angeles. UCR is also within easy driving distance of dozens of major cultural and recreational sites, as well as desert, mountain and coastal destinations.

## From Symmetry: “Dark matter decoys”

04/07/20
Evelyn Lamb

Inside the ADMX experiment hall at the University of Washington Credit Mark Stone U. of Washington. Axion Dark Matter Experiment

The ADMX experiment trains scientists to deal with real signals—by creating fake ones.

The Axion Dark Matter Experiment searches for dark matter the way you might search for a radio station in an unfamiliar location. In a process that takes quite a bit longer than simply turning the dial, it scans across frequency bands that correspond to the possible masses of the particle they’re looking for. If they get a hint on their first pass—metaphorically, a few notes that sound like the kind of music they’d like to hear—they conduct a more thorough analysis of that frequency.

They usually do get a few hints on each pass, says University of Washington physicist Gray Rybka, co-spokesperson of ADMX. Some of this is due to random signal fluctuation. Some of it is due to leaky radio signals. (At one point it was a local religious broadcaster. “We received a message from God,” Rybka jokes.)

And some of it is actually a test: A small subset of ADMX scientists are responsible for injecting synthetic signals into the data.

A tricky signal

Dark matter, so called because it does not interact with light or other electromagnetic radiation, explains many observations about the distribution and movements of stars and galaxies. Astrophysicists estimate that it makes up 85% of the total matter of the universe, but they don’t know what it is. “Everything in our zoo of particle physics—every particle we know of—does not fit the bill,” Rybka says

The axion is one of several dark matter candidates. The particle was originally proposed in the 1970s as a potential solution to the strong CP problem in particle physics. Later, researchers saw that the particle could also explain dark matter.

“This is two for one,” says ADMX analysis team member Leanne Duffy of the US Department of Energy’s Los Alamos National Laboratory. “Not only do you solve this existing problem with the Standard Model, but you also get an excellent dark matter candidate out of it.”

Assuming dark matter axions exist, the Earth and everyone on it is traveling through a “galactic halo” that is thick with them. To touch an axion, we don’t need to do anything.

ADMX is the only one of DOE’s flagship dark matter searches looking for axions. The question is how to detect them. ADMX scientists hope to do it by converting them into particles that are much easier to detect: photons, quanta of light.

In the presence of a strong magnetic field, axions should convert into photons. ADMX creates a magnetic field and isolates waves of specific frequencies in a microwave cavity where they can record any axions-turned-photons they come across.

Passing the test

Keeping the experiment cold (less than 100 millikelvins above absolute 0) helps separate the signal from the noise by decreasing the number of background photons coming from other sources. But some still do sneak in.

To make sure the scientists are up to the task of eliminating those background signals, ADMX scientists do something that other experiments do as well—they regularly inject false signals into their data.

“There is always a part of us that is excited to see a signal because you don’t know if it’s an axion signal or an injected signal.” says Rakshya Khatiwada, a physicist at Fermilab.

When they inject synthetic signals, the team members responsible for injecting them usually reveal them after the second pass. One time in late 2018, the test proceeded further than that. Only Noah Oblath, a researcher at the Pacific Northwest National Laboratory, and one other colleague knew. “It was a little bit strange,” Oblath says. “I like generally being honest with people.”

The team proceeded with the next steps of the analysis. When the signal persisted, they had a meeting to discuss how to proceed. “Fortunately this was a teleconference, and I didn’t have any video on, so I didn’t have to worry about covering my grin or anything,” Oblath says.

They kept up the ruse this time in order to test the scientists’ reactions.

Rybka says he was doubtful. “There was nothing strange about it,” he says.

And that was the problem. The signal had been perfectly clear, and its shape was exactly what they had predicted. “When I looked at it, I said, ‘This might be too good to be true.’”

Duffy had her suspicions as well. And unlike Rybka, she had the tools to test them.

The high-resolution analysis would have exposed the injections as false immediately. But going to the high-resolution channel wasn’t part of the analysis protocol. Still, she admits, “If I hadn’t been so busy, I probably would have gone and looked at it and just not told anyone.”

On the call, the doubtful scientists couldn’t let their suspicions guide their actions. If it was a test, it was a test of their process. They began to discuss the next step: Turning the detector’s magnet off to see whether changing the magnetic field affected the signal, as they would expect if it came from a real axion.

“At that point, Gray paused and gave me a chance to reveal whether it was an injection or not,” Oblath says.

Powering the magnet down would delay the rest of the experiment, so it was time for Oblath to confess. The test had gone according to plan.

“It was a great way to test that our axion detection procedure works,” Duffy says. “But it would be nice to actually detect a real axion at some point.”

Dark Matter Research

Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

Scientists studying the cosmic microwave background hope to learn about more than just how the universe grew—it could also offer insight into dark matter, dark energy and the mass of the neutrino.

Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al

Dark Matter Particle Explorer China

DEAP Dark Matter detector, The DEAP-3600, suspended in the SNOLAB deep in Sudbury’s Creighton Mine

LBNL LZ Dark Matter project at SURF, Lead, SD, USA

Dark Matter Background

Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

Fritz Zwicky from http:// palomarskies.blogspot.com

Coma cluster via NASA/ESA Hubble

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

Symmetry is a joint Fermilab/SLAC publication.

## From Fermi National Accelerator Lab: “Quantum and accelerator science enable mysterious dark sector searches at Fermilab”

FNAL Art Image by Angela Gonzales

From Fermi National Accelerator Lab , an enduring source of strength for the US contribution to scientific research world wide.

March 16, 2020
Bailey Bedford

Photons are the fundamental particles of light. They illuminate our world, letting us see the universe we live in. But light has failed to show us an extraordinary 85% of the matter in the universe, called Dark Matter. Scientists hope that an as yet unseen cousin of the photon, called a dark photon, will provide a clue about the nature of this mysterious dark matter.

A dark photon may sound like a contradiction in terms, but physicists use it to describe the hypothesized, photon-like particles that simply pass through ordinary matter. These invisible particles are part of the hypothetical dark sector — less ominously known as the hidden sector — of quantum fields and particles.

Scientists Anna Grassellino, Roni Harnik and Alexander Romanenko lead the Dark SRF experiment at the U.S. Department of Energy’s Fermilab. To search for the elusive particles, the team has repurposed technology developed for particle accelerators. The researchers hope to generate dark photons and then spot signs of them traveling through solid metal.

The theorized dark photon has the same properties as a photon, but a different mass — that is, if it has any mass. (Photons are massless.) Also, dark photons and photons are inextricably linked: One type can morph into the other, and dark photons interact with matter only through this transforming act.

Fundamental particles are already known to come in varied copies of each other. For example, the familiar electron has two similar, heavier cousins — the muon and tau. That pattern is an important motivation in looking for dark photons, explained Harnik, the main theorist on the project.

“The fact that the muon exists as a copy of the electron makes one wonder whether nature tends to have several copied of each particle. If so, perhaps there is a similar replication of the photon,” Harnik said. “That’s the dark photon.”

Although theorists can propose the dark photon, theory alone cannot predict its mass or its interaction probability. Experimenters and observers have been able to eliminate broad swaths of possible properties. But the search in unexplored territory is on.

Grassellino, who is the Fermilab deputy chief technology officer and led the organization of the experiment, explained that theorists are pushing experimentalists “in all possible directions to fill this map and say, ‘Oh, nothing here, nothing here, nothing here. Where else should we look?’”

During experimental runs of Dark SRF, two cavities (shown here) are bathed in liquid helium to keep them cold. One is held perfectly still, tuned to a specific frequency and kept as empty of photons as possible. The other cavity, which is intended to generate dark photons, has a special setup to adjust it to match the frequency of the first cavity in real time. The experiment aims to catch photons produced in the adjustable cavity as they make their ghostly journey, through metal walls, into the empty cavity. Photo: Reidar Hahn, Fermilab.

A technological case of adaptive reuse.

The Dark SRF experiment searches for dark photons in a region that researchers have yet to probe. The region might be said to be low-hanging fruit for Fermilab, but it would be more accurate to say that Fermilab had already designed and built a tall enough ladder to easily harvest new dark photon fruit.

This metaphorical ladder is the superconducting accelerator cavity. Cavities are hollow, metal, resonating structures in particle accelerators that push particles to near the speed of light. Over the last decade or so, Fermilab has made sweeping strides in increasing their efficiency, getting particles to higher energies over shorter distances.

Romanenko was the first to realize that Fermilab’s cavities are seemingly tailor-made for a different use and could be instrumental in the search for dark photons.

“At Fermilab, we have unique technologies that we’re pushing to unprecedented levels of sensitivity or efficiency,” Grassellino said. “We recognize that we need to also make the effort to either think of what we can do with it for unique experiments like Dark SRF or make them available to the other experimenters.”

In Dark SRF, the superconducting cavity is designed so that photons of a specific microwave energy oscillate together, bouncing back and forth inside it about a 100 billion times before being lost. The cavity maintains the microwaves in much the way a bell or tuning fork maintains sound vibrations.

When kept filled, a cavity can contain around 10 septillion photons — a 1 followed by 25 zeroes. That’s about the number grains of sand in all the deserts and beaches in one thousand Earths (based on a high estimate of how sandy Earth is). The cavity’s astronomically high photon capacity makes it perfect for coaxing hypothetical dark photons out of their hiding place: Each of those regular photons has some chance of being converted into its dark counterpart — an alluring bet that dark photons will be seen, if they exist.

“If a dark photon indeed exists, the filled superconducting cavity acts as a transmitting antenna of dark photons,” Harnik said.

In addition to their excellence in storing photons, the cavities can also keep out stray light, creating a perfect place to hunt for photons arriving unexpectedly. In Dark SRF, a second empty cavity would pick up a dark-photon signal that originated from the 10 septillion photons vibrating inside the first.

“The beauty of this experiment is it’s so simple, given that we have all this technology in hand,” Romanenko said. “We’re starting to push these cavities into the quantum regime, pairing a cavity bursting with photons with another almost completely devoid of them and being able to detect a single one.”

A dark photon’s journey.

If dark photons exist, it is their ability to travel through walls that the Dark SRF team will use to identify them. The Dark SRF experiment brings significantly improved technology to a type of undertaking called a “light shining through a wall” experiment, which looks for light making a seemingly impossible journey through an opaque barrier.

The experiment uses two cavities, one above the other, that are bathed in liquid helium to keep them cold. One is held perfectly still, tuned to a specific frequency and kept as empty of photons as possible. The other cavity, which is intended to generate dark photons, has a special setup to adjust it to match the frequency of the other cavity in real time.

The experiment aims to catch photons produced in the adjustable cavity as they make their ghostly journey, through metal walls, into the empty cavity.

The journey begins when some of the 10 septillion photons that are bouncing around the tunable cavity convert into dark photons, which then pass through the wall of that cavity. Dark photons’ lack of interaction with mundane matter makes them invisible to us. It also renders the walls of the cavities, and everything else, intangible to them. Some of these dark photons will travel into the other cavity, and some fraction of those will revert into regular photons.

The appearance of these seemingly teleported photons signals the existence of their dark cousins. Sighting them would be the eureka moment.

The Dark SRF team at Fermilab is advancing the search for dark photons. Photo: Reidar Hahn, Fermilab.

The Dark SRF difference.

The success of Dark SRF’s simple design hinges on the extraordinarily fine calibration of the two chambers. The second chamber acts as a trap, capturing the reverted photons. But it will build up a noticeable number only if the two chambers’ frequencies precisely match. Otherwise, the photons’ journeys end with them being quickly absorbed by the second chamber’s walls, never to be seen, and the dark photon will continue to fly under the radar.

How fine must the calibration be? The required alignment is unforgiving: The roughly quarter-meter-long cavities must be perfectly positioned to within a billionth of a meter. That’s like correctly plotting the length of a regulation soccer field to within the length of a chromosome.

And once in harmony, these chambers become magnificently sensitive antennas. This is thanks to their high quality factor, a measure of how efficiently they retain energy. The higher the quality factor, the more photons the generating cavity produces, and the more sensitive to dark photons the receiving cavity becomes. The Dark SRF cavities have a quality factor of 1011 when chilled to 1.4 kelvins — the highest-efficiency engineered resonators in the world [Physical Review Applied]. Their quality factor leads to both a flood of potential progenitor photons and heightened sensitivity — both of which give scientists a fighting chance of plucking a dark photon from the vacuum.

Fermilab’s expertise in cryogenic cooling also contributes to Dark SRF’s ability to explore new ground in the search for dark photons. Any photons converted from dark photons must be picked out from a background crowd of other, normal photons generated by the cavity’s heat. In Dark SRF, the cavities’ cold 1.4-kelvin temperature helps reduce the background to a mere 1,000 in the receiving cavity. Researchers plan to modify the system in the future to operate at about 6 millikelvins, winnowing the number to less than one on average and providing the opportunity to search for a more elusive version of the dark photon.

“If you want to hunt for one photon, we are the only place in the world where you can do that with a cavity,” Grassellino said.

Blue sky science for the dark world.

The Dark SRF experiment is an example of how technology and expertise developed for a particular purpose — designing efficient particle accelerators — finds use in another pursuit — searching for hidden particles.

Since it began operation in 2019, scientists on the Dark SRF Experiment have already made significant progress in their search. They’ve experimentally ruled out values for a particular quantity, called a kinetic mixing, that would point to the existence of a dark photon of a certain mass range (between 10 billionths and hundreds of millionths of an electronvolt). This narrows the possible values of the kinetic mixing by a factor of 1,000 compared to previous searches. Now they are pushing at the boundary of the parameter measurement by repeating the experiment using advanced quantum techniques.

If the experiment does find evidence of dark photons, it will introduce a whole world of new questions to be explored: How common are dark photons in the universe? Are dark photons the dark matter scientists have been eagerly searching for? Do dark photons interact with other dark matter similar to how photons do with regular matter? If so, do they reveal another part of our universe as complex as our own but previously invisible?

Whether the Dark SRF experiment discovers the dark photon or not, it will contribute to our understanding of the dark matter that we know is there but have yet to see.

“The path forward follows what we all learnt at school: In science we make a hypothesis, we test it.” Harnik said. “If we find it, ‘Hooray!’ If we don’t find it — science progressed.”

This work is supported by the DOE Office of Science.

See the full here.

Stem Education Coalition

Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
collaborate at Fermilab on experiments at the frontiers of discovery.

## From Harvard-Smithsonian Center for Astrophysics: “Dark Matter and Massive Galaxies”

From Harvard-Smithsonian Center for Astrophysics

March 6, 2020

A dark matter map, created by Japanese astronomers using weak lensing.
The background image of a wide field of galaxies was analyzed for weak lensing effects and the inferred dark matter distribution is indicated with the contours. Satoshi Miyazaki.

About eighty-five percent of the matter in the universe is in the form of Dark Matter, whose nature remains a mystery, and the rest is of the kind found in atoms. Dark matter exhibits gravity but otherwise does not interact with normal matter, nor does it emit light. Astronomers studying the evolution of galaxies find that because it is so abundant dark matter does, however, dominate the formation in the universe of large-scale structures like clusters of galaxies.

Despite being hard to detect directly, dark matter can be traced by modeling sensitive observations of the distributions of galaxies across a range of scales. Galaxies generally reside at the centers of vast clumps of dark matter called haloes because they surround the galaxies. Gravitational lensing of more distant galaxies by foreground dark matter haloes offers a particularly unique and powerful probe of the detailed distribution of dark matter.

Gravitational Lensing NASA/ESA

“Weak lensing” results in modestly yet systematically deforming shapes of background galaxies and can provide robust constraints on the distribution of dark matter within the clusters; “strong lensing,” in contrast, creates highly distorted, magnified and occasionally multiple images of a single source.

In the past decade, observations and hydrodynamic simulations have significantly furthered our understanding of how massive galaxies develop, with a two-phase scenario now favored. In the first step, the massive cores of today’s galaxies form at cosmological times from the gravitational collapse of matter into a galaxy, together with their surrounding dark matter halo. Star-formation then boosts the stellar mass of the galaxy. The most massive galaxies, however, have a second phase in which they capture stars from the outer regions of other galaxies, and once their own star formation subsides this phase dominates their assembly. Computer models and some observational results appear to confirm this scenario.

CfA astronomer Joshua Speagle was a member of a team that used ultra-sensitive, wide-field-of-view imaging at optical and near infrared wavelength on the Subaru telescope to study massive galaxy assembly.

NAOJ/Subaru Telescope at Mauna Kea Hawaii, USA,4,207 m (13,802 ft) above sea level

Their technique took advantage of weak lensing effects because massive galaxies also tend to have more massive, dark matter haloes that distort light. The astronomers studied about 3200 galaxies whose stellar masses are more than that of the Milky Way (roughly about four hundred billion solar masses). Using weak lensing analyses, they found that information about the assembly history of massive dark matter halos is encoded in the stellar mass distributions of massive central galaxies. Among other implications, the scientists show that for galaxies of the same mass, those with more extended shapes tend to have more massive dark matter halos. The results open a new window for exploring how massive galaxies form and evolve over cosmic time.

Science paper:
Weak Lensing Reveals a Tight Connection between Dark Matter Halo Mass and the Distribution of Stellar Mass in Massive Galaxies
MNRAS

____________________________________________________

Dark Matter Background

Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

Fritz Zwicky from http:// palomarskies.blogspot.com

Coma cluster via NASA/ESA Hubble

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

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The Center for Astrophysics combines the resources and research facilities of the Harvard College Observatory and the Smithsonian Astrophysical Observatory under a single director to pursue studies of those basic physical processes that determine the nature and evolution of the universe. The Smithsonian Astrophysical Observatory (SAO) is a bureau of the Smithsonian Institution, founded in 1890. The Harvard College Observatory (HCO), founded in 1839, is a research institution of the Faculty of Arts and Sciences, Harvard University, and provides facilities and substantial other support for teaching activities of the Department of Astronomy.

## From University of York via Science Alert: “Physicists Think We Might Have a New, Exciting Dark Matter Candidate”

From University of York, UK

via

4 MAR 2020
MICHELLE STARR

(Mark Garlick/Science Photo Library/Getty Images)

Something in the Universe is creating more mass than we can detect directly. We know it’s there because of its gravitational effect on the stuff we can detect; but we don’t know what it is, or how it got here.

We call that invisible mass Dark Matter [see Dark Matter Background below], and physicists have just identified a particle that could be behind it.

The candidate culprit is a recently discovered subatomic particle called a d-star hexaquark. And in the primordial darkness following the Big Bang, it could have come together to create dark matter.

For almost a century, dark matter has perplexed astronomers. It was first noticed in the vertical motions of stars, which hinted that there was more mass around them than what we could see.

We can now see the effect of dark matter in other dynamics, too – in gravitational lensing, for instance, wherein light bends around massive objects such as galaxy clusters; and the outer rotation of galactic discs, which is too fast to be explained by visible mass.

Dark matter has, so far, proven impossible to detect directly, as it neither absorbs, emits, nor reflects any kind of electromagnetic radiation. But its gravitational effect is strong – so strong that as much as 85 percent of the matter in our Universe could be dark matter.

Scientists would very much like to get to the bottom of the dark matter mystery, though. It’s not just because they’re very nosy – figuring out what dark matter is could tell us a lot about how our Universe formed, and how it works.

If dark matter doesn’t actually exist, that would mean there’s something very wrong with the standard model of particle physics we use to describe and understand the Universe.

There have been a number of dark matter candidates put forward over the years, but we still don’t seem to be much closer to finding an answer. This is where the d-star hexaquark – more formally, d*(2380) – enters the picture.

“The origin of dark matter in the Universe is one of the biggest questions in science and one that, until now, has drawn a blank,” explained nuclear physicist Daniel Watts of the University of York in the UK.

“Our first calculations indicate that condensates of d-stars are a feasible new candidate for dark matter. This new result is particularly exciting since it doesn’t require any concepts that are new to physics.”

Quarks are fundamental particles that usually combine in groups of three to make up protons and neutrons. Collectively, these three-quark particles are called baryons, and most of the observable matter in the Universe is made of them. You’re baryonic. So’s the Sun. And the planets, and space dust.

When six quarks combine, this creates a type of particle called a dibaryon, or hexaquark. We haven’t actually observed many of these at all. The d-star hexaquark, described in 2014 [Physical Review Letters], was the first non-trivial detection.

D-star hexaquarks are interesting because they’re bosons, a type of particle that obeys Bose-Einstein statistics, a framework for describing how particles behave. In this case, it means that collection of d-star hexaquarks can form something called a Bose-Einstein condensate.

Also known as the fifth state of matter, these condensates form when a low-density gas of bosons is cooled to just above absolute zero. At that stage, the atoms in the gas go from their regular wiggling and jiggling to quite still – the lowest quantum state possible.

If such a gas of d-star hexaquarks was floating around in the early Universe as it cooled in the wake of the Big Bang, according to the team’s modelling, it could come together to form Bose-Einstein condensates. And those condensates could be what we now call dark matter.

Obviously this is all highly theoretical, but the more dark matter candidates we find – and confirm or rule out – the closer we are to identifying what dark matter is. And aren’t you just dying to know?

So, there’s more work to be done here. The team is planning to search for d-star hexaquarks out there in space, and to test their current work to see if they can break it. They’re also planning to conduct more work on d-star hexaquarks in the lab.

“The next step to establish this new dark matter candidate will be to obtain a better understanding of how the d-stars interact – when do they attract and when do they repel each other,” said University of York physicist Mikhail Bashkanov.

“We are leading new measurements to create d-stars inside an atomic nucleus and see if their properties are different to when they are in free space.”

The research has been published in the Journal of Physics G: Nuclear and Particle Physics.

________________________________________
Dark Matter Background

Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

Fritz Zwicky from http:// palomarskies.blogspot.com

Coma cluster via NASA/ESA Hubble

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

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## From EarthSky: “What is dark matter?”

From EarthSky

February 23, 2020
Andy Briggs

Dark Matter doesn’t emit light. It can’t be directly observed with any of the existing tools of astronomers. Yet astrophysicists believe it and Dark Energy make up most of the mass of the cosmos. What dark matter is, and what it isn’t. here.

Since the 1930s, astrophysicists have been trying to explain why the visible material in galaxies can’t account for how galaxies are shaped, or how they behave. They believe a form of dark or invisible matter pervades our universe, but they still don’t know what this dark matter might be. Image via ScienceAlert.

Dark matter is a mysterious substance thought to compose perhaps about 27% of the makeup of the universe. What is it? It’s a bit easier to say what it isn’t.

It isn’t ordinary atoms – the building blocks of our own bodies and all we see around us – because atoms make up only somewhere around 5% of the universe, according to a cosmological model called the Lambda Cold Dark Matter Model (aka the Lambda-CDM model, or sometimes just the Standard Model).

Lamda Cold Dark Matter Accerated Expansion of The universe http scinotions.com the-cosmic-inflation-suggests-the-existence-of-parallel-universes
Alex Mittelmann, Coldcreation

Dark Matter isn’t the same thing as Dark Energy, which makes up some 68% of the universe, according to the Standard Model.

Dark Energy Survey

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

Timeline of the Inflationary Universe WMAP

The Dark Energy Survey (DES) is an international, collaborative effort to map hundreds of millions of galaxies, detect thousands of supernovae, and find patterns of cosmic structure that will reveal the nature of the mysterious dark energy that is accelerating the expansion of our Universe. DES began searching the Southern skies on August 31, 2013.

According to Einstein’s theory of General Relativity, gravity should lead to a slowing of the cosmic expansion. Yet, in 1998, two teams of astronomers studying distant supernovae made the remarkable discovery that the expansion of the universe is speeding up. To explain cosmic acceleration, cosmologists are faced with two possibilities: either 70% of the universe exists in an exotic form, now called dark energy, that exhibits a gravitational force opposite to the attractive gravity of ordinary matter, or General Relativity must be replaced by a new theory of gravity on cosmic scales.

DES is designed to probe the origin of the accelerating universe and help uncover the nature of dark energy by measuring the 14-billion-year history of cosmic expansion with high precision. More than 400 scientists from over 25 institutions in the United States, Spain, the United Kingdom, Brazil, Germany, Switzerland, and Australia are working on the project. The collaboration built and is using an extremely sensitive 570-Megapixel digital camera, DECam, mounted on the Blanco 4-meter telescope at Cerro Tololo Inter-American Observatory, high in the Chilean Andes, to carry out the project.

Over six years (2013-2019), the DES collaboration used 758 nights of observation to carry out a deep, wide-area survey to record information from 300 million galaxies that are billions of light-years from Earth. The survey imaged 5000 square degrees of the southern sky in five optical filters to obtain detailed information about each galaxy. A fraction of the survey time is used to observe smaller patches of sky roughly once a week to discover and study thousands of supernovae and other astrophysical transients.

Dark matter is invisible; it doesn’t emit, reflect or absorb light or any type of electromagnetic radiation such as X-rays or radio waves. Thus, dark matter is undetectable directly, as all of our observations of the universe, apart from the detection of gravitational waves, involve capturing electromagnetic radiation in our telescopes.

Gravitational waves Werner Benger-ZIB-AEI-CCT-LSU

Yet dark matter does interact with ordinary matter. It exhibits measurable gravitational effects on large structures in the universe such as galaxies and galaxy clusters. Because of this, astronomers are able to make maps of the distribution of dark matter in the universe, even though they cannot see it directly.

They do this by measuring the effect dark matter has on ordinary matter, through gravity.

This all-sky image – released in 2013 – shows the distribution of dark matter across the entire history of the universe as seen projected on the sky. It’s based on data collected with the European Space Agency’s Planck satellite.

ESA/Planck 2009 to 2013

Dark blue areas represent regions that are denser than their surroundings. Bright areas represent less dense regions. The gray portions of the image correspond to patches of the sky where foreground emission, mainly from the Milky Way but also from nearby galaxies, prevents cosmologists from seeing clearly. Image via ESA.

There is currently a huge international effort to identify the nature of dark matter. Bringing an armory of advanced technology to bear on the problem, astronomers have designed ever-more complex and sensitive detectors to tease out the identity of this mysterious substance.

Dark Matter Research

Universe map Sloan Digital Sky Survey (SDSS) 2dF Galaxy Redshift Survey

Scientists studying the cosmic microwave background hope to learn about more than just how the universe grew—it could also offer insight into dark matter, dark energy and the mass of the neutrino.

Dark matter cosmic web and the large-scale structure it forms The Millenium Simulation, V. Springel et al

Dark Matter Particle Explorer China

DEAP Dark Matter detector, The DEAP-3600, suspended in the SNOLAB deep in Sudbury’s Creighton Mine

LBNL LZ Dark Matter project at SURF, Lead, SD, USA

Inside the ADMX experiment hall at the University of Washington Credit Mark Stone U. of Washington. Axion Dark Matter Experiment

Dark matter might consist of an as yet unidentified subatomic particle of a type completely different from what scientists call baryonic matter – that’s just ordinary matter, the stuff we see all around us – which is made of ordinary atoms built of protons and neutrons.

The list of candidate subatomic particles breaks down into a few groups: there are the WIMPs (Weakly Interacting Massive Particles), a class of particles thought to have been produced in the early universe. Astronomers believe that WIMPs might self-annihilate when colliding with each other, so they have searched the skies for telltale traces of events such as the release of neutrinos or gamma rays. So far, they’ve found nothing. In addition, although a theory called supersymmetry predicts the existence of particles with the same properties as WIMPs, repeated searches to find the particles directly have also found nothing, and experiments at the Large Hadron Collider to detect the expected presence of supersymmetry have completely failed to find it.

Standard Model of Supersymmetry via DESY

CERN/LHC Map

CERN LHC Maximilien Brice and Julien Marius Ordan

SixTRack CERN LHC particles

Several different types of detector have been used to detect WIMPs. The general idea is that very occasionally, a WIMP might collide with an ordinary atom and release a faint flash of light, which can be detected. The most sensitive detector built to date is XENON1T, which consists of a 10-meter cylinder containing 3.2 tons of liquid xenon, surrounded by photomultipliers to detect and amplify the incredibly faint flashes from these rare interactions. As of July 2019, when the detector was decommissioned to pave the way for a more sensitive instrument, the XENONnT, no collisions between WIMPs and the xenon atoms had been seen.

XENON1T at Gran Sasso LABORATORI NAZIONALI del GRAN SASSO, located in the Abruzzo region of central Italy

Gran Sasso LABORATORI NAZIONALI del GRAN SASSO, located in the Abruzzo region of central Italy

At the moment, a hypothetical particle called the Axion is receiving much attention.

CERN CAST Axion Solar Telescope

As well as being a strong candidate for dark matter, the existence of axions is also thought to provide the answers to a few other persistent questions in physics such as the Strong CP Problem.

Fritz Zwicky discovered Dark Matter in the 1930s when observing the movement of the Coma Cluster., Vera Rubin a Woman in STEM denied the Nobel, did most of the work on Dark Matter.

Fritz Zwicky from http:// palomarskies.blogspot.com

Coma cluster via NASA/ESA Hubble

In modern times, it was astronomer Fritz Zwicky, in the 1930s, who made the first observations of what we now call dark matter. His 1933 observations of the Coma Cluster of galaxies seemed to indicated it has a mass 500 times more than that previously calculated by Edwin Hubble. Furthermore, this extra mass seemed to be completely invisible. Although Zwicky’s observations were initially met with much skepticism, they were later confirmed by other groups of astronomers.

Thirty years later, astronomer Vera Rubin provided a huge piece of evidence for the existence of dark matter. She discovered that the centers of galaxies rotate at the same speed as their extremities, whereas, of course, they should rotate faster. Think of a vinyl LP on a record deck: its center rotates faster than its edge. That’s what logic dictates we should see in galaxies too. But we do not. The only way to explain this is if the whole galaxy is only the center of some much larger structure, as if it is only the label on the LP so to speak, causing the galaxy to have a consistent rotation speed from center to edge.

Vera Rubin, following Zwicky, postulated that the missing structure in galaxies is dark matter. Her ideas were met with much resistance from the astronomical community, but her observations have been confirmed and are seen today as pivotal proof of the existence of dark matter.

Astronomer Vera Rubin at the Lowell Observatory in 1965, worked on Dark Matter (The Carnegie Institution for Science)

Vera Rubin measuring spectra, worked on Dark Matter (Emilio Segre Visual Archives AIP SPL)

Vera Rubin, with Department of Terrestrial Magnetism (DTM) image tube spectrograph attached to the Kitt Peak 84-inch telescope, 1970. https://home.dtm.ciw.edu

The Vera C. Rubin Observatory currently under construction on the El Peñón peak at Cerro Pachón Chile, a 2,682-meter-high mountain in Coquimbo Region, in northern Chile, alongside the existing Gemini South and Southern Astrophysical Research Telescopes.

LSST Data Journey, Illustration by Sandbox Studio, Chicago with Ana Kova

Some astronomers have tried to negate the need the existence of dark matter altogether by postulating something called Modified Newtonian dynamics (MOND).

Mordehai Milgrom, MOND theorist, is an Israeli physicist and professor in the department of Condensed Matter Physics at the Weizmann Institute in Rehovot, Israel http://cosmos.nautil.us

MOND Modified Newtonian Dynamics a Humble Introduction Marcus Nielbock

The idea behind this is that gravity behaves differently over long distances to what it does locally, and this difference of behavior explains phenomena such as galaxy rotation curves which we attribute to dark matter. Although MOND has its supporters, while it can account for the rotation curve of an individual galaxy, current versions of MOND simply cannot account for the behavior and movement of matter in large structures such as galaxy clusters and, in its current form, is thought unable to completely account for the existence of dark matter. That is to say, gravity does behave in the same way at all scales of distance. Most versions of MOND, on the other hand, have two versions of gravity, the weaker one occurring in regions of low mass concentration such as in the outskirts of galaxies. However, it is not inconceivable that some new version of MOND in the future might yet account for dark matter.

Although some astronomers believe we will establish the nature of dark matter in the near future, the search so far has proved fruitless, and we know that the universe often springs surprises on us so that nothing can be taken for granted.

The approach astronomers are taking is to eliminate those particles which cannot be dark matter, in the hope we will be left with the one which is.

It remains to be seen if this approach is the correct one.

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Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. “Being an EarthSky editor is like hosting a big global party for cool nature-lovers,” she says.

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