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  • richardmitnick 9:50 pm on June 22, 2021 Permalink | Reply
    Tags: , , , Erik Verlinde's theory of emergent gravity, , Gravity, , ,   

    From University of Amsterdam [Universiteit van Amsterdam] (NL): Women in STEM-Margot Brouwer “Dark matter: ‘real stuff’ or gravity misunderstood?” 

    22 June 2021

    For many years now, astronomers and physicists have been in a conflict. Is the mysterious Dark Matter that we observe deep in the Universe real, or is what we see the result of subtle deviations from the laws of gravity as we know them? In 2016, Dutch physicist Erik Verlinde proposed a theory of the second kind: emergent gravity. New research, published in Astronomy & Astrophysics this week, pushes the limits of dark matter observations to the unknown outer regions of galaxies, and in doing so re-evaluates several dark matter models and alternative theories of gravity. Measurements of the gravity of 259,000 isolated galaxies show a very close relation between the contributions of dark matter and those of ordinary matter, as predicted in Verlinde’s theory of emergent gravity and an alternative model called Modified Newtonian Dynamics. However, the results also appear to agree with a computer simulation of the Universe that assumes that dark matter is ‘real stuff’.

    In the centre of the image the elliptical galaxy NGC5982, and to the right the spiral galaxy NGC5985. These two types of galaxies turn out to behave very differently when it comes to the extra gravity – and therefore possibly the dark matter – in their outer regions. Images: Bart Delsaert (www.delsaert.com).

    The new research was carried out by an international team of astronomers, led by Margot Brouwer (University of Groningen [Rijksuniversiteit Groningen] (NL) and UvA). Further important roles were played by Kyle Oman (RUG and Durham University (UK)) and Edwin Valentijn (RUG). In 2016, Brouwer also performed a first test of Verlinde’s ideas [MNRAS]; this time, Verlinde himself also joined the research team.

    Matter or gravity?

    So far, dark matter has never been observed directly – hence the name. What astronomers observe in the night sky are the consequences of matter that is potentially present: bending of starlight, stars that move faster than expected, and even effects on the motion of entire galaxies. Without a doubt all of these effects are caused by gravity, but the question is: are we truly observing additional gravity, caused by invisible matter, or are the laws of gravity themselves the thing that we haven’t fully understood yet?

    To answer this question, the new research uses a similar method to the one used in the original test in 2016. Brouwer and her colleagues make use of an ongoing series of photographic measurements that started ten years ago: the KiloDegree Survey (KiDS), performed using ESO’s VLT Survey Telescope in Chili.

    In these observations one measures how starlight from far away galaxies is bent by gravity on its way to our telescopes. Whereas in 2016 the measurements of such ‘lens effects’ only covered an area of about 180 square degrees on the night sky, in the mean time this has been extended to about 1000 square degrees – allowing the researchers to measure the distribution of gravity in around a million different galaxies.

    Comparative testing

    Brouwer and her colleagues selected over 259,000 isolated galaxies, for which they were able to measure the so-called ‘Radial Acceleration Relation’ (RAR). This RAR compares the amount of gravity expected based on the visible matter in the galaxy, to the amount of gravity that is actually present – in other words: the result shows how much ‘extra’ gravity there is, in addition to that due to normal matter. Until now, the amount of extra gravity had only been determined in the outer regions of galaxies by observing the motions of stars, and in a region about five times larger by measuring the rotational velocity of cold gas. Using the lensing effects of gravity, the researchers were now able to determine the RAR at gravitational strengths which were one hundred times smaller, allowing them to penetrate much deeper into the regions far outside the individual galaxies.

    This made it possible to measure the extra gravity extremely precisely – but is this gravity the result of invisible dark matter, or do we need to improve our understanding of gravity itself? Author Kyle Oman indicates that the assumption of ‘real stuff’ at least partially appears to work: “In our research, we compare the measurements to four different theoretical models: two that assume the existence of dark matter and form the base of computer simulations of our universe, and two that modify the laws of gravity – Erik Verlinde’s model of emergent gravity and the so-called ‘Modified Newtonian Dynamics’ or MOND. One of the two dark matter simulations, MICE, makes predictions that match our measurements very nicely. It came as a surprise to us that the other simulation, BAHAMAS, led to very different predictions. That the predictions of the two models differed at all was already surprising, since the models are so similar. But moreover, we would have expected that if a difference would show up, BAHAMAS was going to perform best. BAHAMAS is a much more detailed model than MICE, approaching our current understanding of how galaxies form in a universe with dark matter much closer. Still, MICE performs better if we compare its predictions to our measurements. In the future, based on our findings, we want to further investigate what causes the differences between the simulations.”

    Young and old galaxies

    Thus it seems that, at least one dark matter model does appear to work. However, the alternative models of gravity also predict the measured RAR. A standoff, it seems – so how do we find out which model is correct? Margot Brouwer, who led the research team, continues: “Based on our tests, our original conclusion was that the two alternative gravity models and MICE matched the observations reasonably well. However, the most exciting part was yet to come: because we had access to over 259,000 galaxies, we could divide them into several types – relatively young, blue spiral galaxies versus relatively old, red elliptical galaxies.” Those two types of galaxies come about in very different ways: red elliptical galaxies form when different galaxies interact, for example when two blue spiral galaxies pass by each other closely, or even collide. As a result, the expectation within the particle theory of dark matter is that the ratio between regular and dark matter in the different types of galaxies can vary. Models such as Verlinde’s theory and MOND on the other hand do not make use of dark matter particles, and therefore predict a fixed ratio between the expected and measured gravity in the two types of galaxies – that is, independent of their type. Brouwer: “We discovered that the RARs for the two types of galaxies differed significantly. That would be a strong hint towards the existence of dark matter as a particle.”

    A plot showing the Radial Acceleration Relation (RAR). The background is an image of the elliptical galaxy M87, showing the distance to the centre of the galaxy. The plot shows how the measurements range from high gravitational acceleration in the centre of the galaxy, to low gravitational acceleration in the far outer regions. Image: Chris Mihos (Case Western Reserve University (US)) / European Southern Observatory [Observatoire européen austral][Europäische Südsternwarte] (EU) (CL).

    However, there is a caveat: gas. Many galaxies are probably surrounded by a diffuse cloud of hot gas, which is very difficult to observe. If it were the case that there is hardly any gas around young blue spiral galaxies, but that old red elliptical galaxies live in a large cloud of gas – of roughly the same mass as the stars themselves – then that could explain the difference in the RAR between the two types. To reach a final judgement on the measured difference, one would therefore also need to measure the amounts of diffuse gas – and this is exactly what is not possible using the KiDS telescopes. Other measurements have been done for a small group of around one hundred galaxies, and these measurements indeed found more gas around elliptical galaxies, but it is still unclear how representative those measurements are for the 259,000 galaxies that were studied in the current research.

    Dark matter for the win?

    If it turns out that extra gas cannot explain the difference between the two types of galaxies, then the results of the measurements are easier to understand in terms of dark matter particles than in terms of alternative models of gravity. But even then, the matter is not settled yet. While the measured differences are hard to explain using MOND, Erik Verlinde still sees a way out for his own model. Verlinde: “My current model only applies to static, isolated, spherical galaxies, so it cannot be expected to distinguish the different types of galaxies. I view these results as a challenge and inspiration to develop an asymmetric, dynamical version of my theory, in which galaxies with a different shape and history can have a different amount of ‘apparent dark matter’.”

    Therefore, even after the new measurements, the dispute between dark matter and alternative gravity theories is not settled yet. Still, the new results are a major step forward: if the measured difference in gravity between the two types of galaxies is correct, then the ultimate model, whichever one that is, will have to be precise enough to explain this difference. This means in particular that many existing models can be discarded, which considerably thins out the landscape of possible explanations. On top of that, the new research shows that systematic measurements of the hot gas around galaxies are necessary. Edwin Valentijn formulates is as follows: “As observational astronomers, we have reached the point where we are able to measure the extra gravity around galaxies more precisely than we can measure the amount of visible matter. The counterintuitive conclusion is that we must first measure the presence of ordinary matter in the form of hot gas around galaxies, before future telescopes such as Euclid can finally solve the mystery of dark matter.”


    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, some 30 years later, 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

    Dark Matter Research

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


    See the full article here.


    Please help promote STEM in your local schools.

    Stem Education Coalition

    University of Amsterdam [Universiteit van Amsterdam] (NL) is a public research university located in Amsterdam, Netherlands. The UvA is one of two large, publicly funded research universities in the city, the other being the Free University of Amsterdam [Vrije Universiteit Amsterdam] (NL). Established in 1632 by municipal authorities and later renamed for the city of Amsterdam, the University of Amsterdam is the third-oldest university in the Netherlands. It is one of the largest research universities in Europe with 31,186 students, 4,794 staff, 1,340 PhD students and an annual budget of €600 million. It is the largest university in the Netherlands by enrollment. The main campus is located in central Amsterdam, with a few faculties located in adjacent boroughs. The university is organised into seven faculties: Humanities, Social and Behavioural Sciences, Economics and Business, Science, Law, Medicine, Dentistry.

    The University of Amsterdam has produced six Nobel Laureates and five prime ministers of the Netherlands. The university has been placed in the top 100 universities in the world by five major ranking tables. By the QS World University Rankings it was ranked 55th in the world, 14th in Europe, and 1st in the Netherlands in 2022. The UvA was placed in the top 50 worldwide in seven fields in the 2011 QS World University Rankings in the fields of linguistics, sociology, philosophy, geography, science, Economics and econometrics, and accountancy and finance. In 2018 and 2019 the two departments of Media and Communication were commonly ranked 1st in the world by subject by QS Ranking.

    Close ties are harbored with other institutions internationally through its membership in the League of European Research Universities (LERU), the Institutional Network of the Universities from the Capitals of Europe (EU) (UNICA), European University Association (EUA) (EU), the International Student Exchange Programs (ISEP), and Universitas 21.


    The University of Amsterdam is one of Europe’s largest research universities, with over 7,900 scientific publications in 2010. The university spends about €100 million on research each year via direct funding. It receives an additional €23 million via indirect funding and about €49 million from commercial partners. Faculty members often receive research prizes and grants, such as those from the Dutch Research Council (NWO – Nederlandse Organisatie voor Wetenschappelijk Onderzoek)(NL). Research is organized into fifteen research priority areas and 28 research institutes within the faculties oversee this research.

    The University of Amsterdam has an extensive central University Library (UBA), with over four million volumes. In addition, a number of departments have their own libraries. The main university library is located in the city center. It contains over four million books, 70,000 manuscripts, 500,000 letters, and 125,000 maps, as well as special collections of the Department of Rare and Precious Works, the Manuscript and Writing Museum, the Bibliotheca Rosenthaliana on Jewish history and culture, and the Department of Documentation on Social Movements. Three reading rooms are available for students to study in quiet. In addition to the main University Library, there are approximately 70 departmental libraries spread throughout the center of Amsterdam. The university’s printing arm, the Amsterdam University Press, has a publishing list of over 1,400 titles in both Dutch and English.

  • richardmitnick 3:22 pm on January 25, 2020 Permalink | Reply
    Tags: "People plan their movements anticipating the force of gravity by 'seeing it' through visual cues rather than 'feeling it'"., , , Gravity, , Sheba Medical Center   

    From Sheba Medical Center via MedicalXpress: “People plan their movements, anticipate force of gravity by ‘seeing it’ through visual cues rather than ‘feeling it'” 


    From Sheba Medical Center, Israel


    Medicalxpress bloc


    January 24, 2020

    Credit: CC0 Public Domain

    Gravity is the unseen force that dominates our entire lives. It’s what makes walking uphill so difficult and what makes parts of our body eventually point downhill. It is unyielding, everywhere, and a force that we battle with every time we make a move. But exactly how do people account for this invisible influence while moving through the world?

    A new study in Frontiers in Neuroscience used virtual reality to determine how people plan their movements by “seeing” gravity using visual cues in the landscape around them, rather than “feeling it” through changes in weight and balance. Ph.D. Student Desiderio Cano Porras, who worked in Dr. Meir Plotnik’s laboratory at the Sheba Medical Center, Israel and colleagues found that our capability to anticipate the influence of gravity relies on visual cues in order for us to walk safely and effectively downhill and uphill.

    In order to determine the influence of vision and gravity on how we move, the researchers recruited a group of 16 young, healthy adults for a virtual reality (VR) experiment. The researchers designed a VR environment that simulated level, uphill, and downhill walking. Participants were immersed in a large-scale virtual reality system in which they walked on a real-life treadmill that was at an upward incline, at a downward decline, or remained flat. Throughout the experiment, the VR visual environment either matched or didn’t match the physical cues that the participants experienced on the treadmill.

    Using this setup, the researchers were able to disrupt the visual and physical cues we all experience when anticipating going uphill or downhill. So, when participants saw a downhill environment in the VR visual scenery, they positioned their bodies to begin “braking” to go downhill despite the treadmill actually remaining flat or at an upward incline. They also found the reverse—people prepared for more “exertion” to go uphill in the VR environment even though the treadmill remained flat or was pointing downhill.

    The researchers showed that purely visual cues caused people to adjust their movements to compensate for predicted gravity-based changes (i.e., braking in anticipation of a downhill gravity boost and exertion in anticipation of uphill gravitational resistance). However, while participants initially relied on their vision, they quickly adapted to the real-life treadmill conditions using something called a “sensory reweighting mechanism” that reprioritized body-based cues over visual ones. In this way the participants were able to overcome the sensory mismatch and keep walking.

    “Our findings highlight multisensory interactions: the human brain usually gets information about forces from “touch” senses; however, it generates behavior in response to gravity by “seeing” it first, without initially “feeling” it,” says Dr. Plotnik.

    Dr. Plotnik also states that the study is an exciting application of new and emerging VR tech as “many new digital technologies, in particular virtual reality, allow a high level of human-technology interactions and immersion. We leveraged this immersion to explore and start to disentangle the complex visual-locomotor integration achieved by human sensory systems.”

    The research is a step towards the broader goal of understanding the intricate pathways that people use to decide how and when to move their bodies, but there is still work to be done.

    Dr. Plotnik states that “This study is only a ‘snapshot’ of a specific task involving transitioning to uphill or downhill walking. In the future we will explore the neuronal mechanisms involved and potential clinical implications for diagnosis and treatment.”

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    MedicalXpress is a web-based medical and health news service that is part of the renowned Science X network. Medical Xpress features the most comprehensive coverage in medical research and health news in the fields of neuroscience, cardiology, cancer, HIV/AIDS, psychology, psychiatry, dentistry, genetics, diseases and conditions, medications and more.

    Sheba Medical Center is dedicated to providing exceptional healthcare to all patients from all over the world. We are the largest hospital in Israel, and we set the standard of excellence in patient-focused care. Our state-of-the-art facilities are located on a comprehensive, all-inclusive campus with a full range of medical divisions and specialties. We have two Medical Tourism Tracks, for individuals and government bids, and treat each patient with advanced and holistic care based on integrated research and clinical practice.

    Sheba’s core value is to constantly deliver personalized, expert medicine to everyone – with no limits or boundaries. We have treated thousands of patients from across the globe, including Russia, Ukraine, Cyprus, Kazakhstan, Georgia, Bulgaria, Romania, Switzerland, Hungary, the United States, and Nigeria. We offer an extensive range of custom-designed care in several languages, including advanced imaging technologies, laboratory testing, diagnostics, surgery, and progressive rehabilitation programs. To ensure the most optimized experience possible, each patient is assigned a dedicated medical coordinator to guide them through every step of their treatment at Sheba.

  • richardmitnick 7:00 am on September 6, 2019 Permalink | Reply
    Tags: "UN offers use of ESA’s hypergravity centrifuge to researchers worldwide", , ESA’s hypergravity-generating Large Diameter Centrifuge, Gravity   

    From European Space Agency: “UN offers use of ESA’s hypergravity centrifuge to researchers worldwide” 

    ESA Space For Europe Banner

    From European Space Agency

    5 September 2019

    ESA Large Diameter Centrifuge at full speed

    Imagine being able to increase the force of gravity simply by turning a dial. A United Nations fellowship is offering this opportunity to researchers all over the world, through access to ESA’s hypergravity-generating Large Diameter Centrifuge.

    Manipulate gravity and a lot of other factors shift too: bubbles in liquid alter their behaviour, convection currents accelerate and metal alloys form in unprecedented ways. Electrical plasmas alter and test animals lose fat – even fire burns differently.

    For more than a decade ESA’s Large Diameter Centrifuge (LDC) at its ESTEC technical centre in the Netherlands has been a place of pilgrimage for European gravity researchers, including student experimenters on regular Spin Your Thesis! campaigns.

    The LDC is popular with life and physical science teams, as well as for commercial experiments. Internal ESA teams use the centrifuge to see how spacecraft materials and components would respond to the violent accelerations involved in launching into space.

    Now the United Nations Office for Outer Space Affairs (UNOOSA) is widening access to the LDC still further, as part of its ‘Access to Space for All’ initiative. This fellowship programme aims to provide opportunities to research teams including student members, with particular attention paid to developing nations.

    Jack van Loon of the centrifuge team comments: “Over the years we have seen an increasing interest in the application of hypergravity for life and physical sciences as well as for spaceflight payloads and materials within Europe.

    “This activity caught the attention of UNOOSA resulting in this unique, first time collaboration between ESA and the United Nations.”

    Operating within a sci-fi style white dome, the LDC is an 8-m diameter four-arm centrifuge that gives researchers access to a range of hypergravity up to 20 times Earth gravity for weeks or months at a time.

    At its fastest, the centrifuge rotates at up to 67 revs per minute, with its six gondolas placed at different points along its arms weighing in at 130 kg, and each capable of accommodating 80 kg of payload.

    Loading gondola

    This new programme is formally known as the ‘United Nations / European Space Agency Fellowship Programme on the Large Diameter Centrifuge Hypergravity Experiment Series’, or HyperGES for short.

    To be eligible to apply, research teams should consist of one academic supervisor and several Bachelor, Masters and/or PhD students, with the proposed experiment being integral part of the students’ syllabuses or research. For full details go here.

    See the full article here .

    Please help promote STEM in your local schools.

    Stem Education Coalition

    The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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  • richardmitnick 9:03 am on July 31, 2019 Permalink | Reply
    Tags: A0620-00- a binary star system 3300 light-years away- holds a dark secret: One of its stars isn’t a star at all but rather a black hole., , Along Unruh’s imaginary river a waterfall plunges at a supersonic speed—faster than the speed of sound in water., , , , , Black holes were first theorized in 1784 by English clergyman and astronomer John Michell., Building black hole models, , Eight years later in 1980 Unruh realized that the equations of motion for sound in the waterfall analogy were identical to those describing light at the horizon of a black hole., Gravity, , In 1972 William “Bill” Unruh a physicist at the University of British Columbia Vancouver connected gravity to fluid dynamics in an analogy, Most physicists believe that black holes don’t truly destroy information and that information is preserved in Hawking radiation but that conjecture may be impossible to test directly., physicists use everything from water to exotic ultracold states of matter to mimic black holes, , Stephen Hawking revolutionized the field by proposing that that something could in fact escape from a black hole., Today, Unruh’s work was rediscovered as physicists began probing gravity theoretically and experimentally with analog models., What happens if a fellow fish goes over the falls? You- a blind fish- cannot know; you will never hear it scream because the waterfall will drag the sound down faster than it can travel up., William “Bill” Unruh: “Imagine that you are a blind fish and are also a physicist living in a river” Unruh wrote.   

    From Symmetry: “Gravity’s Waterfall” 

    Symmetry Mag
    From Symmetry

    Daniel Garisto

    Physicists are using analog black holes to better understand gravity.

    Illustration by Sandbox Studio, Chicago with Ariel Davis

    A0620-00, a binary star system 3300 light-years away, holds a dark secret: One of its stars isn’t a star at all, but a black hole. As far as we know, this is the black hole closest to our planet. Astronomers know it’s there only because its partner star appears to be dancing alone, pulled along by an invisible lead.

    In recent years, scientists have found ways to study black holes, listening to the gravitational waves they unleash when they collide, and even creating an image of one by combining information from radio telescopes around the world.

    MIT /Caltech Advanced aLigo

    VIRGO Collaboration

    But our knowledge of black holes remains limited. No one will ever be able to test a real one in a lab, and with current technology, it would take about 50 million years for a probe to reach A0620-00.

    So scientists are figuring out how to make do with substitutes—analogs to black holes that may hold answers to mysteries about gravity and quantum mechanics.

    Building black hole models

    In 1972, William “Bill” Unruh, a physicist at the University of British Columbia, Vancouver, connected gravity to fluid dynamics in an analogy: “Imagine that you are a blind fish, and are also a physicist, living in a river,” Unruh wrote.

    Along Unruh’s imaginary river, a waterfall plunges at a supersonic speed—faster than the speed of sound in water. What happens if a fellow fish goes over the falls? You, a blind fish, cannot know; you will never hear it scream because the waterfall will drag the sound down faster than it can travel up.

    Unruh used this piscine drama to explain a property of black holes: Like sound that passes over the edge of the supersonic waterfall, light that crosses the horizon of a black hole cannot escape.

    The analogy turned out to be more accurate than Unruh initially thought. Eight years later, in 1980, he realized that the equations of motion for sound in the waterfall analogy were identical to those describing light at the horizon of a black hole.

    At the time, his research drew little attention—it was cited just four times in the decade after it was published. But in the ’90s, Unruh’s work was rediscovered as physicists began probing gravity theoretically and experimentally with analog models.

    Today, physicists use everything from water to exotic ultracold states of matter to mimic black holes. Proponents of the analogs say that these models have confirmed theoretical predictions about black holes. But many physicists still doubt that analogs can predict what happens where gravity warps spacetime so violently.

    Black holes were first theorized in 1784, by English clergyman and astronomer John Michell, who calculated that for a large enough star, “all light emitted from such a body would be made to return towards it, by its own proper gravity.”

    The idea was mostly put aside until the 20th century, when Einstein’s general theory of relativity overturned the paradigm of Newtonian gravity. Luminaries like Karl Schwarzchild, Subrahmanyan Chandrasekhar and John Archibald Wheeler developed theory about these monsters from which nothing could escape. But in 1974, a young physicist named Stephen Hawking revolutionized the field by proposing that that something could, in fact, escape from a black hole.

    Due to random quantum fluctuations in the fabric of spacetime, pairs of virtual particles and antiparticles pop into existence all the time, throughout the universe. Most of the time, these pairs annihilate instantly, disappearing back into the void. But, Hawking theorized, the horizon of a black hole could separate a pair: One particle would be sucked in, while the other would zoom away as a now real particle.

    Because of a mathematical quirk in Hawking radiation, swallowed virtual particles effectively have negative energy. Black holes that gobble up these particles shrink. To an observer, Hawking radiation would look a lot like a black hole spitting up what it swallowed and getting smaller.

    However, Hawking radiation is random and carries no information about the inside of a black hole—remember that the emitted particle comes from just outside the horizon. This creates a paradox: Quantum mechanics rests on the premise that information is never destroyed, but if particles emitted as Hawking radiation are truly random, information would be lost forever.

    Most physicists believe that black holes don’t truly destroy information and that information is preserved in Hawking radiation, but that conjecture may be impossible to test directly. “The temperature of Hawking radiation is very small—it’s much smaller than the background radiation of the universe,” says Hai Son Nguyen, a physicist at the Institute of Nanotechnology of Lyon. “That’s why we will never be able to observe Hawking radiation from a real black hole.”

    What about something that behaved a lot like a black hole? In his 1980 paper, Unruh calculated that phonons—quantum units of sound analogous to photons, quantum units of light—would be the Hawking radiation emitted from his analog black hole.

    Unruh was initially bleak about the prospects of actually making such a measurement, calling it “an extremely slim possibility.” But as more physicists joined Unruh in theorizing about analogs to black holes in the ’90s, the possibility of measuring Hawking radiation became a difficult, but achievable goal.

    Over the waterfall

    There are many different analog models of black holes, but they all have one aspect in common: a horizon. Mathematically, horizons are defined as the boundary beyond which events cannot escape—like the edge of Unruh’s waterfall. Because they can separate pairs of particles, any horizon creates a form of Hawking radiation.

    “Understanding of the phenomenology associated with the presence of horizons in different analog systems provides hints about phenomena that might also be present in the gravitational realm,” writes Carlos Barceló, a theoretical physicist at the Astrophysical Institute of Andalucia.

    Often, it’s useful to start with a simple analog like water, says Silke Weinfurtner, a physicist at the University of Nottingham. It’s possible to create a horizon by running water quickly enough over an obstacle; if the conditions are just right, surface waves are thwarted at the obstacle.

    But to properly measure the smallest—quantum-level—effects of a black hole, you need a quantum analog. Bose-Einstein condensates, or BECs, are typically ultracold gases like rubidium that are ruled by quantum effects odd enough to qualify them as another state of matter. Subtle quantum effects like Hawking radiation hidden by the noise present in normal fluids become apparent in BECs.

    Analog black holes can even use light as a fluid. The fluid is made of quasiparticles called polaritons, which are the collective state of a photon that couples to an electric field. Enough polaritons behave as a quantum fluid of light. So when the flow of polaritons goes faster than the speed of sound in the polariton fluid, just like Unruh’s waterfall, a horizon forms. Hawking radiation from this fluid of light still comes in the form of phonons.

    Some black hole analogs are “optical” because their Hawking radiation comes in photons. In optical fibers—like the type we send data through—intense laser pulses can create a horizon. The pulse changes the physical properties of the fiber, slowing down the speed of light within the fiber. This makes the leading edge of the pulse a horizon: Slowed light cannot escape past the pulse any more than sound can escape up out of Unruh’s waterfall.

    To date, though, experimental evidence of Hawking radiation in any of these analogs has been lacking in support—with one exception.

    In May, Jeff Steinhauer published his latest paper, with the strongest evidence yet for Hawking radiation. Steinhauer, a physicist at the Technion in Haifa, Israel, has been working on the problem for over a decade, chipping away relentlessly at the extremely difficult experimental task, largely on his own.

    Focusing a laser on rubidium gas, a BEC, Steinhauer created a high-energy region. Particles move from high-energy regions to low-energy regions, so the rubidium gas wanted to escape the laser. The edge of the laser here functioned as the horizon for the rubidium gas, similar to a waterfall that it could go over but not come back up. Steinhauer used the set-up to study the Hawking radiation resulting from quantum fluctuations separated by the horizon.

    The temperature of Hawking radiation—how much energy the emitted phonons have, in this case—depends on the slope of the horizon, or waterfall. The steeper it is, the higher the energy of the radiation. This is why Hawking radiation is low temperature for a black hole: A weak force like gravity doesn’t make for a steep horizon.

    By measuring the slope, and then separately measuring the energy of radiated phonons, Steinhauer was able to get corroboration for his data.

    Previous experiments from Steinhauer and others have claimed to find Hawking radiation [PhysicsWorld], but have lacked the rigor of this latest result. This time, Steinhauer and some other physicists believe he has observed Hawking radiation.

    “I think we verified Hawking’s calculation,” Steinhauer says. “He had a calculation with certain assumptions and approximations, and we have the same approximations, and so mathematically it’s equivalent.”

    However, Steinhauer points out, it’s quite possible that Hawking radiation works differently for black holes because of quantum gravity. Critics also claim phonons may not be perfect analogs to photons.

    Many physicists who work on quantum gravity are dismissive of the latest results, according to reporting in Quanta.

    Weinfurtner acknowledges the criticism and agrees that analogs cannot strictly prove anything about black holes. But to physicists working on analogs, the facsimiles of black holes are already worthwhile. “What we’re doing is already on its own really interesting,” she says. “We’re deepening our understanding of the analog gravity systems, and the hope is that such experiments stimulate new theoretical black hole studies.”


    See the full article here .


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    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 7:05 am on May 22, 2019 Permalink | Reply
    Tags: , , Gravity, ,   

    From European Space Agency: “Zero-G Spiderman” 

    ESA Space For Europe Banner

    From European Space Agency



    Gravity: we can live with it, and it turns out we can live without it, for a little while anyway.

    Under the elemental force of nature keeping all our parts and planet together, humans thrive. But in weightlessness and funny things begin to happen. Our muscles start to wear away, our bones decay, our balance shifts and our spatial perception falters.

    Astronauts living and working in space are helping researchers determine the acceptable limits of these changes. So too are subjects taking part in experiments here on Earth.

    In this image, a volunteer tries to get to the tennis ball as part of an experiment testing the influence of weightlessness on the perception of distance. He must first determine the distance of the ball from his person under normal gravity conditions by walking blindfolded to it.

    For the microgravity portion of the experiment, researchers set up a sled along which subjects can pull themselves to the ball. In this scenario, the body is reclined and the arms are helping, giving the brain some more signals to work with to estimate the distance.

    The experiment, developed by the Lyon Neuroscience Research Center in France, is taking place on this week’s parabolic flight campaign aboard a Novespace Zero-G aircraft. The special aircraft simulates different levels of gravity, from 2g to 0g, by flying in parabolas.

    Researchers will compare the results in normal gravity conditions (1g), nearly twice the force on the upward incline of the plane (1.8 g), and at freefall during the plane’s descent (0g).

    Astronauts have long reported spatial disorientation in orbit. Without a grip on where you are in space, it is hard to measure distance. This can affect astronauts’ performance when using the robotic arm or during a spacewalk. To solve the problem, researchers must first assess the full scope of it.

    Previous runs of this experiment had the subjects blind-pulling themselves up or down while sitting up and lying down. In the latest iteration, researchers will test lateral distance perception by having subjects blind-pull themselves to the left and right to the ball.

    The ultimate goals of the experiment are to better understand to what degree gravity or the lack of it affects the sensorimotor (what we see) and neurocognitive (what we think) systems.

    Deeper insights into these systems will help researchers fine tune the countermeasures that help keep astronauts living in space healthy during and after spaceflight.

    On Earth, we deal with gravity every day. We feel it, we fight it, and – more importantly – we investigate it. Space agencies such as ESA routinely launch spacecraft against our planet’s gravity, and sometimes these spacecraft borrow the gravity of Earth or other planets to reach interesting places in the Solar System. We study the gravity field of Earth from orbit, and fly experiments on parabolic flights, sounding rockets and the International Space Station to examine a variety of systems under different gravitational conditions. On the grandest scales, our space science missions explore how gravity affects planets, stars and galaxies across the cosmos and probe how matter behaves in the strong gravitational field created by some of the Universe’s most extreme objects like black holes. Join the conversation online this week following the hashtag #GravityRules

    See the full article here .

    Please help promote STEM in your local schools.

    Stem Education Coalition

    The European Space Agency (ESA), established in 1975, is an intergovernmental organization dedicated to the exploration of space, currently with 19 member states. Headquartered in Paris, ESA has a staff of more than 2,000. ESA’s space flight program includes human spaceflight, mainly through the participation in the International Space Station program, the launch and operations of unmanned exploration missions to other planets and the Moon, Earth observation, science, telecommunication as well as maintaining a major spaceport, the Guiana Space Centre at Kourou, French Guiana, and designing launch vehicles. ESA science missions are based at ESTEC in Noordwijk, Netherlands, Earth Observation missions at ESRIN in Frascati, Italy, ESA Mission Control (ESOC) is in Darmstadt, Germany, the European Astronaut Centre (EAC) that trains astronauts for future missions is situated in Cologne, Germany, and the European Space Astronomy Centre is located in Villanueva de la Cañada, Spain.

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  • richardmitnick 2:54 pm on February 24, 2019 Permalink | Reply
    Tags: "Ask Ethan: How Can We Measure The Curvature Of Spacetime?", A difference in the height of two atomic clocks of even ~1 foot (33 cm) can lead to a measurable difference in the speed at which those clocks run, A team of physicists working in Europe were able to conjugate three atom interferometers simultaneously, At every point you can infer the force of gravity or the amount of spacetime curvature, , Decades before Newton put forth his law of universal gravitation Italian scientists Francesco Grimaldi and Giovanni Riccioli made the first calculations of the gravitational constant G, , Gravity, In the future it may be possible to extend this technique to measure the curvature of spacetime not just on Earth but on any worlds we can put a lander on. This includes other planets moons asteroids , It’s been over 100 years since Einstein and over 300 since Newton. We’ve still got a long way to go, Making multiple measurements of the field gradient simultaneously allows you to measure G between multiple locations that eliminates a source of error: the error induced when you move the apparatus. B, Pound-Rebka experiment, , The same law of gravity governs the entire Universe, We can do even better than the Pound-Rebka experiment today by using the technology of atomic clocks, You can even infer G the gravitational constant of the Universe.   

    From Ethan Siegel: “Ask Ethan: How Can We Measure The Curvature Of Spacetime?” 

    From Ethan Siegel
    Feb 23, 2019

    Instead of an empty, blank, 3D grid, putting a mass down causes what would have been ‘straight’ lines to instead become curved by a specific amount. In General Relativity, we treat space and time as continuous, but all forms of energy, including but not limited to mass, contribute to spacetime curvature. For the first time, we can measure the curvature at Earth’s surface, as well as how that curvature changes with altitude. (CHRISTOPHER VITALE OF NETWORKOLOGIES AND THE PRATT INSTITUTE)

    It’s been over 100 years since Einstein, and over 300 since Newton. We’ve still got a long way to go.

    From measuring how objects fall on Earth to observing the motion of the Moon and planets, the same law of gravity governs the entire Universe. From Galileo to Newton to Einstein, our understanding of the most universal force of all still has some major holes in it. It’s the only force without a quantum description. The fundamental constant governing gravitation, G, is so poorly known that many find it embarrassing. And the curvature of the fabric of spacetime itself went unmeasured for a century after Einstein put forth the theory of General Relativity. But much of that has the potential to change dramatically, as our Patreon supporter Nick Delroy realized, asking:

    Can you please explain to us how awesome this is, and what you hope the future holds for gravity measurement. The instrument is obviously localized but my imagination can’t stop coming up with applications for this.

    The big news he’s excited about, of course, is a new experimental technique that measured the curvature of spacetime due to gravity for the first time [Physical Review Letters].

    The identical behavior of a ball falling to the floor in an accelerated rocket (left) and on Earth (right) is a demonstration of Einstein’s equivalence principle. Although you cannot tell whether an acceleration is due to gravity or any other acceleration from a single measurement, measuring differing accelerations at different points can show whether there’s a gravitational gradient along the direction of acceleration. (WIKIMEDIA COMMONS USER MARKUS POESSEL, RETOUCHED BY PBROKS13)

    Think about how you might design an experiment to measure the strength of the gravitational force at any location in space. Your first instinct might be something simple and straightforward: take an object at rest, release it so it’s in free-fall, and observe how it accelerates.

    By measuring the change in position over time, you can reconstruct what the acceleration at this location must be. If you know the rules governing the gravitational force — i.e., you have the correct law of physics, like Newton’s or Einstein’s theories — you can use this information to determine even more information. At every point, you can infer the force of gravity or the amount of spacetime curvature. Beyond that, if you know additional information (like the relevant matter distribution), you can even infer G, the gravitational constant of the Universe.

    Newton’s law of Universal Gravitation relied on the concept of an instantaneous action (force) at a distance, and is incredibly straightforward. The gravitational constant in this equation, G, along with the values of the two masses and the distance between them, are the only factors in determining a gravitational force. Although Newton’s theory has since been superseded by Einstein’s General Relativity, G also appears in Einstein’s theory. (WIKIMEDIA COMMONS USER DENNIS NILSSON)

    This simple approach was the first one taken to investigate the nature of gravity. Building on the work of others, Galileo determined the gravitational acceleration at Earth’s surface. Decades before Newton put forth his law of universal gravitation, Italian scientists Francesco Grimaldi and Giovanni Riccioli made the first calculations of the gravitational constant, G.

    But experiments like this, as valuable as they are, are limited. They can only give you information about gravitation along one dimension: towards the center of the Earth. Acceleration is based on either the sum of all the net forces (Newton) acting on an object, or the net curvature of spacetime (Einstein) at one particular location in the Universe. Since you’re observing an object in free-fall, you’re only getting a simplistic picture.

    According to legend, the first experiment to show that all objects fell at the same rate, irrespective of mass, was performed by Galileo Galilei atop the Leaning Tower of Pisa. Any two objects dropped in a gravitational field, in the absence of (or neglecting) air resistance, will accelerate down to the ground at the same rate. This was later codified as part of Newton’s investigations into the matter. (GETTY IMAGES)

    Thankfully, there’s a way to get a multidimensional picture as well: perform an experiment that’s sensitive to changes in the gravitational field/potential as an object changes its position. This was first accomplished, experimentally, in the 1950s by the Pound-Rebka experiment [ Explanation of the Pound-Rebka experiment http://vixra.org/pdf/1212.0035v1.pdf ].

    What the experiment did was cause a nuclear emission at a low elevation, and note that the corresponding nuclear absorption didn’t occur at a higher elevation, presumably due to gravitational redshift, as predicted by Einstein. Yet if you gave the low-elevation emitter a positive boost to its speed, through attaching it to a speaker cone, that extra energy would balance the loss of energy that traveling upwards in a gravitational field extracted. As a result, the arriving photon has the right energy, and absorption occurs. This was one of the classical tests of General Relativity, confirming Einstein where his theory’s predictions departed from Newton’s.

    Physicist Glen Rebka, at the lower end of the Jefferson Towers, Harvard University, calling Professor Pound on the phone during setup of the famed Pound-Rebka experiment. (CORBIS MEDIA / HARVARD UNIVERSITY)

    We can do even better than the Pound-Rebka experiment today, by using the technology of atomic clocks. These clocks are the best timekeepers in the Universe, having surpassed the best natural clocks — pulsars — decades ago. Now capable of monitoring time differences to some 18 significant features between clocks, Nobel Laureate David Wineland led a team that demonstrated that raising an atomic clock by barely a foot (about 33 cm in the experiment) above another one caused a measurable frequency shift in what the clock registered as a second.

    If we were to take these two clocks to any location on Earth, and adjust the heights as we saw fit, we could understand how the gravitational field changes as a function of elevation. Not only can we measure gravitational acceleration, but the changes in acceleration as we move away from Earth’s surface.

    A difference in the height of two atomic clocks of even ~1 foot (33 cm) can lead to a measurable difference in the speed at which those clocks run. This allows us to measure not only the strength of the gravitational field, but the gradient of the field as a function of altitude/elevation. (DAVID WINELAND AT PERIMETER INSTITUTE, 2015)

    But even these achievements cannot map out the true curvature of space. That next step wouldn’t be achieved until 2015: exactly 100 years after Einstein first put forth his theory of General Relativity. In addition, there was another problem that has cropped up in the interim, which is the fact that various methods of measuring the gravitational constant, G, appear to give different answers.

    Three different experimental techniques have been used to determine G: torsion balances, torsion pendulums, and atom interferometry experiments. Over the past 15 years, measured values of the gravitational constant have ranged from as high as 6.6757 × 10–11 N/kg2⋅m2 to as low as 6.6719 × 10–11 N/kg2⋅m2. This difference of 0.05%, for a fundamental constant, makes it one of the most poorly-determined constants in all of nature.

    In 1997, the team of Bagley and Luther performed a torsion balance experiment that yielded a result of 6.674 x 10^-11 N/kg²/m², which was taken seriously enough to cast doubt on the previously reported significance of the determination of G. Note the relatively large variations in the measured values, even since the year 2000.(DBACHMANN / WIKIMEDIA COMMONS)

    But that’s where the new study, first published in 2015 but refined many times over the past four years, comes in. A team of physicists, working in Europe, were able to conjugate three atom interferometers simultaneously. Instead of using just two locations at different heights, they were able to get the mutual differences between three different heights at a single location on the surface, which enables you to not simply get a single difference, or even the gradient of the gravitational field, but the change in the gradient as a function of distance.

    When you explore how the gravitational field changes as a function of distance, you can understand the shape of the change in spacetime curvature. When you measure the gravitational acceleration in a single location, you’re sensitive to everything around you, including what’s underground and how it’s moving. Measuring the gradient of the field is more informative than just a single value; measuring how that gradient changes gives you even more information.

    The scheme of the experiment that measures the three atomic groupings launched in rapid sequence and then excited by lasers to measure not only the gravitational acceleration, but showing the effects of the changes in curvature that had never been measured before. (G. ROSI ET AL., PHYS. REV. LETT. 114, 013001, 2015)

    That’s what makes this new technique so powerful. We’re not simply going to a single location and finding out what the gravitational force is. Nor are we going to a location and finding out what the force is and how that force is changing with elevation. Instead, we’re determining the gravitational force, how it changes with elevation, and how the change in the force is changing with elevation.

    “Big deal,” you might say, “we already know the laws of physics. We know what those laws predict. Why should I care that we’re measuring something that confirms to slightly better accuracy what we’ve known should be true all along?”

    Well, there are multiple reasons. One is that making multiple measurements of the field gradient simultaneously allows you to measure G between multiple locations that eliminates a source of error: the error induced when you move the apparatus. By making three measurements, rather than two, simultaneously, you get three differences (between 1 and 2, 2 and 3, and 1 and 3) rather than just 1 (between 1 and 2).

    The top of the Makkah royal clock tower runs a few quadrillionths of a second faster than the same clock would at the base, due to differences in the gravitational field. Measuring the changes in the gradient of the gravitational field provides even more information, enabling us to finally measure the curvature of space directly. (AL JAZEERA ENGLISH C/O: FADI EL BENNI)

    But another reason that’s perhaps even more important is to better understand the gravitational pull of the objects we’re measuring. The idea that we know the rules governing gravity is true, but we only know what the gravitational force should be if we know the magnitude and distribution of all the masses that are relevant to our measurement. The Earth, for example, is not a uniform structure at all. There are fluctuations in the gravitational strength we experience everywhere we go, dependent on factors like:

    the density of the crust beneath your feet,
    the location of the crust-mantle boundary,
    the extent of isostatic compensation that takes place at that boundary,
    the presence or absence of oil reservoirs or other density-varying deposits underground,

    and so on. If we can implement this technique of three-atom interferometry wherever we like on Earth, we can better understand our planet’s interior simply by making measurements at the surface.

    Various geologic zones in the Earth’s mantle create and move magma chambers, leading to a variety of geological phenomena. It’s possible that external intervention could trigger a catastrophic event. Improvements in geodesy could improve our understanding of what’s happening, existing, and changing beneath Earth’s surface. (KDS4444 / WIKIMEDIA COMMONS)

    In the future, it may be possible to extend this technique to measure the curvature of spacetime not just on Earth, but on any worlds we can put a lander on. This includes other planets, moons, asteroids and more. If we want to do asteroid mining, this could be the ultimate prospecting tool. We could improve our geodesy experiments significantly, and improve our ability to monitor the planet. We could better track internal changes in magma chambers, as just one example. If we applied this technology to upcoming spacecrafts, it could even help correct for Newtonian noise in next-generation gravitational wave observatories like LISA or beyond.

    ESA/NASA eLISA space based, the future of gravitational wave research

    The gold-platinum alloy cubes, of central importance to the upcoming LISA mission, have already been built and tested in the proof-of-concept LISA Pathfinder mission.

    ESA/LISA Pathfinder

    This image shows the assembly of one of the Inertial Sensor Heads for the LISA Technology Package (LTP). Improved techniques for accounting for Newtonian noise in the experiment might improve LISA’s sensitivity significantly. (CGS SPA)

    The Universe is not simply made of point masses, but of complex, intricate objects. If we ever hope to tease out the most sensitive signals of all and learn the details that elude us today, we need to become more precise than ever. Thanks to three-atom interferometry, we can, for the first time, directly measure the curvature of space.

    Understanding the Earth’s interior better than ever is the first thing we’re going to gain, but that’s just the beginning. Scientific discovery isn’t the end of the game; it’s the starting point for new applications and novel technologies. Come back in a few years; you might be surprised at what becomes possible based on what we’re learning for the first time today.

    See the full article here .


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    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

  • richardmitnick 11:52 am on December 19, 2018 Permalink | Reply
    Tags: AdS/CFT, Beyond Einstein: Physicists find surprising connections in the cosmos, , From tiny bits of string, Gravity, Our world when we get down to the level of particles is a quantum world, , , Relatability between gravity and subatomic particles provides a sort of Rosetta stone for physics, The idea that fundamental particles are actually tiny bits of vibrating string was taking off and by the mid-1980s “string theory” had lassoed the imaginations of many leading physicists,   

    From Princeton University: “Beyond Einstein: Physicists find surprising connections in the cosmos” 

    Princeton University
    From Princeton University

    Dec. 17, 2018
    Catherine Zandonella

    Gravity, the force that brings baseballs back to Earth and governs the growth of black holes, is mathematically relatable to the peculiar antics of the subatomic particles that make up all the matter around us. Illustration by J.F. Podevin

    Albert Einstein’s desk can still be found on the second floor of Princeton’s physics department. Positioned in front of a floor-to-ceiling blackboard covered with equations, the desk seems to embody the spirit of the frizzy-haired genius as he asks the department’s current occupants, “So, have you solved it yet?”

    Einstein never achieved his goal of a unified theory to explain the natural world in a single, coherent framework. Over the last century, researchers have pieced together links between three of the four known physical forces in a “standard model,” but the fourth force, gravity, has always stood alone.

    No longer. Thanks to insights made by Princeton faculty members and others who trained here, gravity is being brought in from the cold — although in a manner not remotely close to how Einstein had imagined it.

    Though not yet a “theory of everything,” this framework, laid down over 20 years ago and still being filled in, reveals surprising ways in which Einstein’s theory of gravity relates to other areas of physics, giving researchers new tools with which to tackle elusive questions.

    The key insight is that gravity, the force that brings baseballs back to Earth and governs the growth of black holes, is mathematically relatable to the peculiar antics of the subatomic particles that make up all the matter around us.

    This revelation allows scientists to use one branch of physics to understand other seemingly unrelated areas of physics. So far, this concept has been applied to topics ranging from why black holes run a temperature to how a butterfly’s beating wings can cause a storm on the other side of the world.

    This relatability between gravity and subatomic particles provides a sort of Rosetta stone for physics. Ask a question about gravity, and you’ll get an explanation couched in the terms of subatomic particles. And vice versa.

    “This has turned out to be an incredibly rich area,” said Igor Klebanov, Princeton’s Eugene Higgins Professor of Physics, who generated some of the initial inklings in this field in the 1990s. “It lies at the intersection of many fields of physics.”

    From tiny bits of string

    The seeds of this correspondence were sprinkled in the 1970s, when researchers were exploring tiny subatomic particles called quarks. These entities nest like Russian dolls inside protons, which in turn occupy the atoms that make up all matter. At the time, physicists found it odd that no matter how hard you smash two protons together, you cannot release the quarks — they stay confined inside the protons.

    One person working on quark confinement was Alexander Polyakov, Princeton’s Joseph Henry Professor of Physics. It turns out that quarks are “glued together” by other particles, called gluons. For a while, researchers thought gluons could assemble into strings that tie quarks to each other. Polyakov glimpsed a link between the theory of particles and the theory of strings, but the work was, in Polyakov’s words, “hand-wavy” and he didn’t have precise examples.

    Meanwhile, the idea that fundamental particles are actually tiny bits of vibrating string was taking off, and by the mid-1980s, “string theory” had lassoed the imaginations of many leading physicists. The idea is simple: just as a vibrating violin string gives rise to different notes, each string’s vibration foretells a particle’s mass and behavior. The mathematical beauty was irresistible and led to a swell of enthusiasm for string theory as a way to explain not only particles but the universe itself.

    One of Polyakov’s colleagues was Klebanov, who in 1996 was an associate professor at Princeton, having earned his Ph.D. at Princeton a decade earlier. That year, Klebanov, with graduate student Steven Gubser and postdoctoral research associate Amanda Peet, used string theory to make calculations about gluons, and then compared their findings to a string-theory approach to understanding a black hole. They were surprised to find that both approaches yielded a very similar answer. A year later, Klebanov studied absorption rates by black holes and found that this time they agreed exactly.

    That work was limited to the example of gluons and black holes. It took an insight by Juan Maldacena in 1997 to pull the pieces into a more general relationship. At that time, Maldacena, who had earned his Ph.D. at Princeton one year earlier, was an assistant professor at Harvard. He detected a correspondence between a special form of gravity and the theory that describes particles. Seeing the importance of Maldacena’s conjecture, a Princeton team consisting of Gubser, Klebanov and Polyakov followed up with a related paper formulating the idea in more precise terms.

    Another physicist who was immediately taken with the idea was Edward Witten of the Institute for Advanced Study (IAS), an independent research center located about a mile from the University campus. He wrote a paper that further formulated the idea, and the combination of the three papers in late 1997 and early 1998 opened the floodgates.

    “It was a fundamentally new kind of connection,” said Witten, a leader in the field of string theory who had earned his Ph.D. at Princeton in 1976 and is a visiting lecturer with the rank of professor in physics at Princeton. “Twenty years later, we haven’t fully come to grips with it.”


    Two sides of the same coin

    This relationship means that gravity and subatomic particle interactions are like two sides of the same coin. On one side is an extended version of gravity derived from Einstein’s 1915 theory of general relativity. On the other side is the theory that roughly describes the behavior of subatomic particles and their interactions.

    The latter theory includes the catalogue of particles and forces in the “standard model” (see sidebar), a framework to explain matter and its interactions that has survived rigorous testing in numerous experiments, including at the Large Hadron Collider.

    In the standard model, quantum behaviors are baked in. Our world, when we get down to the level of particles, is a quantum world.

    Notably absent from the standard model is gravity. Yet quantum behavior is at the basis of the other three forces, so why should gravity be immune?

    The new framework brings gravity into the discussion. It is not exactly the gravity we know, but a slightly warped version that includes an extra dimension. The universe we know has four dimensions, the three that pinpoint an object in space — the height, width and depth of Einstein’s desk, for example — plus the fourth dimension of time. The gravitational description adds a fifth dimension that causes spacetime to curve into a universe that includes copies of familiar four-dimensional flat space rescaled according to where they are found in the fifth dimension. This strange, curved spacetime is called anti-de Sitter (AdS) space after Einstein’s collaborator, Dutch astronomer Willem de Sitter.

    The breakthrough in the late 1990s was that mathematical calculations of the edge, or boundary, of this anti-de Sitter space can be applied to problems involving quantum behaviors of subatomic particles described by a mathematical relationship called conformal field theory (CFT). This relationship provides the link, which Polyakov had glimpsed earlier, between the theory of particles in four space-time dimensions and string theory in five dimensions. The relationship now goes by several names that relate gravity to particles, but most researchers call it the AdS/CFT (pronounced A-D-S-C-F-T) correspondence.


    Tackling the big questions

    This correspondence, it turns out, has many practical uses. Take black holes, for example. The late physicist Stephen Hawking startled the physics community by discovering that black holes have a temperature that arises because each particle that falls into a black hole has an entangled particle that can escape as heat.

    Using AdS/CFT, Tadashi Takayanagi and Shinsei Ryu, then at the University of California-Santa Barbara, discovered a new way to study
    entanglement in terms of geometry, extending Hawking’s insights in a fashion that experts consider quite remarkable.

    In another example, researchers are using AdS/CFT to pin down chaos theory, which says that a random and insignificant event such as the flapping of a butterfly’s wings could result in massive changes to a large-scale system such as a faraway hurricane. It is difficult to calculate chaos, but black holes — which are some of the most chaotic quantum systems possible — could help. Work by Stephen Shenker and Douglas Stanford at Stanford University, along with Maldacena, demonstrates how, through AdS/CFT, black holes can model quantum chaos.

    One open question Maldacena hopes the AdS/CFT correspondence will answer is the question of what it is like inside a black hole, where an infinitely dense region called a singularity resides. So far, the relationship gives us a picture of the black hole as seen from the outside, said Maldacena, who is now the Carl P. Feinberg Professor at IAS.

    “We hope to understand the singularity inside the black hole,” Maldacena said. “Understanding this would probably lead to interesting lessons for the Big Bang.”

    The relationship between gravity and strings has also shed new light on quark confinement, initially through work by Polyakov and Witten, and later by Klebanov and Matt Strassler, who was then at IAS.

    Those are just a few examples of how the relationship can be used. “It is a tremendously successful idea,” said Gubser, who today is a professor of physics at Princeton. “It compels one’s attention. It ropes you in, it ropes in other fields, and it gives you a vantage point on theoretical physics that is very compelling.”

    The relationship may even unlock the quantum nature of gravity. “It is among our best clues to understand gravity from a quantum perspective,” said Witten. “Since we don’t know what is still missing, I cannot tell you how big a piece of the picture it ultimately will be.”

    Still, the AdS/CFT correspondence, while powerful, relies on a simplified version of spacetime that is not exactly like the real universe. Researchers are working to find ways to make the theory more broadly applicable to the everyday world, including Gubser’s research on modeling the collisions of heavy ions, as well as high-temperature superconductors.

    Also on the to-do list is developing a proof of this correspondence that draws on underlying physical principles. It is unlikely that Einstein would be satisfied without a proof, said Herman Verlinde, Princeton’s Class of 1909 Professor of Physics, the chair of the Department of Physics and an expert in string theory, who shares office space with Einstein’s desk.

    “Sometimes I imagine he is still sitting there,” Verlinde said, “and I wonder what he would think of our progress.”

    See the full article here .


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    Stem Education Coalition

    Princeton University Campus

    About Princeton: Overview

    Princeton University is a vibrant community of scholarship and learning that stands in the nation’s service and in the service of all nations. Chartered in 1746, Princeton is the fourth-oldest college in the United States. Princeton is an independent, coeducational, nondenominational institution that provides undergraduate and graduate instruction in the humanities, social sciences, natural sciences and engineering.

    As a world-renowned research university, Princeton seeks to achieve the highest levels of distinction in the discovery and transmission of knowledge and understanding. At the same time, Princeton is distinctive among research universities in its commitment to undergraduate teaching.

    Today, more than 1,100 faculty members instruct approximately 5,200 undergraduate students and 2,600 graduate students. The University’s generous financial aid program ensures that talented students from all economic backgrounds can afford a Princeton education.

    Princeton Shield

  • richardmitnick 1:50 pm on October 18, 2018 Permalink | Reply
    Tags: , , , , , Gravity, , ,   

    From Symmetry: “Five mysteries the Standard Model can’t explain” 

    Symmetry Mag
    From Symmetry

    Oscar Miyamoto Gomez

    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Standard Model of Particle Physics from Symmetry Magazine

    Our best model of particle physics explains only about 5 percent of the universe.

    The Standard Model is a thing of beauty. It is the most rigorous theory of particle physics, incredibly precise and accurate in its predictions. It mathematically lays out the 17 building blocks of nature: six quarks, six leptons, four force-carrier particles, and the Higgs boson. These are ruled by the electromagnetic, weak and strong forces.

    “As for the question ‘What are we?’ the Standard Model has the answer,” says Saúl Ramos, a researcher at the National Autonomous University of Mexico (UNAM). “It tells us that every object in the universe is not independent, and that every particle is there for a reason.”

    For the past 50 years such a system has allowed scientists to incorporate particle physics into a single equation that explains most of what we can see in the world around us.

    Despite its great predictive power, however, the Standard Model fails to answer five crucial questions, which is why particle physicists know their work is far from done.

    Illustration by Sandbox Studio, Chicago with Ana Kova

    1. Why do neutrinos have mass?

    Three of the Standard Model’s particles are different types of neutrinos. The Standard Model predicts that, like photons, neutrinos should have no mass.

    However, scientists have found that the three neutrinos oscillate, or transform into one another, as they move. This feat is only possible because neutrinos are not massless after all.

    “If we use the theories that we have today, we get the wrong answer,” says André de Gouvêa, a professor at Northwestern University.

    The Standard Model got neutrinos wrong, but it remains to be seen just how wrong. After all, the masses neutrinos have are quite small.

    Is that all the Standard Model missed, or is there more that we don’t know about neutrinos? Some experimental results have suggested, for example, that there might be a fourth type of neutrino called a sterile neutrino that we have yet to discover.

    Illustration by Sandbox Studio, Chicago with Ana Kova

    2. What is dark matter?

    Scientists realized they were missing something when they noticed that galaxies were spinning much faster than they should be, based on the gravitational pull of their visible matter. They were spinning so fast that they should have torn themselves apart. Something we can’t see, which scientists have dubbed “dark matter,” must be giving additional mass—and hence gravitional pull—to these galaxies.

    Dark matter is thought to make up 27 percent of the contents of the universe. But it is not included in the Standard Model.

    Scientists are looking for ways to study this mysterious matter and identify its building blocks. If scientists could show that dark matter interacts in some way with normal matter, “we still would need a new model, but it would mean that new model and the Standard Model are connected,” says Andrea Albert, a researcher at the US Department of Energy’s SLAC National Laboratory who studies dark matter, among other things, at the High-Altitude Water Cherenkov Observatory in Mexico. “That would be a huge game changer.”

    HAWC High Altitude Cherenkov Experiment, located on the flanks of the Sierra Negra volcano in the Mexican state of Puebla at an altitude of 4100 meters(13,500ft), at WikiMiniAtlas 18°59′41″N 97°18′30.6″W. searches for cosmic rays

    Illustration by Sandbox Studio, Chicago with Ana Kova

    3. Why is there so much matter in the universe?

    Whenever a particle of matter comes into being—for example, in a particle collision in the Large Hadron Collider or in the decay of another particle—normally its antimatter counterpart comes along for the ride. When equal matter and antimatter particles meet, they annihilate one another.

    Scientists suppose that when the universe was formed in the Big Bang, matter and antimatter should have been produced in equal parts. However, some mechanism kept the matter and antimatter from their usual pattern of total destruction, and the universe around us is dominated by matter.

    The Standard Model cannot explain the imbalance. Many different experiments are studying matter and antimatter in search of clues as to what tipped the scales.

    Illustration by Sandbox Studio, Chicago with Ana Kova

    4. Why is the expansion of the universe accelerating?

    Before scientists were able to measure the expansion of our universe, they guessed that it had started out quickly after the Big Bang and then, over time, had begun to slow. So it came as a shock that, not only was the universe’s expansion not slowing down—it was actually speeding up.

    The latest measurements by the Hubble Space Telescope and the European Space Agency observatory Gaia indicate that galaxies are moving away from us at 45 miles per second. That speed multiplies for each additional megaparsec, a distance of 3.2 million light years, relative to our position.

    This rate is believed to come from an unexplained property of space-time called dark energy, which is pushing the universe apart. It is thought to make up around 68 percent of the energy in the universe. “That is something very fundamental that nobody could have anticipated just by looking at the Standard Model,” de Gouvêa says.

    Illustration by Sandbox Studio, Chicago with Ana Kova

    5. Is there a particle associated with the force of gravity?

    The Standard Model was not designed to explain gravity. This fourth and weakest force of nature does not seem to have any impact on the subatomic interactions the Standard Model explains.

    But theoretical physicists think a subatomic particle called a graviton might transmit gravity the same way particles called photons carry the electromagnetic force.

    “After the existence of gravitational waves was confirmed by LIGO, we now ask: What is the smallest gravitational wave possible? This is pretty much like asking what a graviton is,” says Alberto Güijosa, a professor at the Institute of Nuclear Sciences at UNAM.

    More to explore

    These five mysteries are the big questions of physics in the 21st century, Ramos says. Yet, there are even more fundamental enigmas, he says: What is the source of space-time geometry? Where do particles get their spin? Why is the strong force so strong while the weak force is so weak?

    There’s much left to explore, Güijosa says. “Even if we end up with a final and perfect theory of everything in our hands, we would still perform experiments in different situations in order to push its limits.”

    “It is a very classic example of the scientific method in action,” Albert says. “With each answer come more questions; nothing is ever done.”

    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Symmetry is a joint Fermilab/SLAC publication.

  • richardmitnick 11:08 am on September 8, 2018 Permalink | Reply
    Tags: , , , , Dark matter is our leading theory for a reason, Gravity, Modified Gravity, Modified Gravity Could Soon Be Ruled Out Says New Research On Dwarf Galaxies, New detailed studies of the smallest galaxies could kill off the most studied alternative, Newton’s law of gravity   

    From Ethan Siegel: “Modified Gravity Could Soon Be Ruled Out, Says New Research On Dwarf Galaxies” 

    From Ethan Siegel
    Sep 7, 2018

    Dark matter is our leading theory for a reason. New, detailed studies of the smallest galaxies could kill off the most studied alternative.

    Only approximately 1000 stars are present in the entirety of dwarf galaxies Segue 1 and Segue 3, which has a gravitational mass of 600,000 Suns. The stars making up the dwarf satellite Segue 1 are circled here. If new research is correct, then dark matter will obey a different distribution depending on how star formation, over the galaxy’s history, has heated it. (MARLA GEHA AND KECK OBSERVATORIES)

    Keck Observatory, Maunakea, Hawaii, USA.4,207 m (13,802 ft), above sea level, showing also NASA’s IRTF and NAOJ Subaru

    When you look out at the Universe, there are a few things you’d rationally expect. You’d expect that the same things that made up everything we saw — like atoms and light — made up everything there was. You’d expect that the fundamental laws would apply equally well everywhere you looked, from small scales to large scales. And you’d expect that if you had multiple ways of measuring the same physical quantity, they’d give you the same answer.

    Which is why the dark matter problem is such a puzzle. There are a huge variety of measurements we can make that indicate that about 5/6ths of the Universe, by mass, isn’t made up of any of the known particles. It doesn’t interact with normal matter or light. And if you measure the mass of a galaxy directly, from its light, it doesn’t match the mass you infer from gravity.

    According to models and simulations, all galaxies should be embedded in dark matter halos, whose densities peak at the galactic centers. On long enough timescales, of perhaps a billion years, a single dark matter particle from the outskirts of the halo will complete one orbit. The effects of gas, feedback, star formation, supernovae, and radiation all complicate this environment, making it extremely difficult to extract universal dark matter predictions. (NASA, ESA, AND T. BROWN AND J. TUMLINSON (STSCI))

    NASA/ESA Hubble Telescope

    Traditionally, the way to approach this problem has been to add a single ingredient: dark matter.

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

    Caterpillar Project A Milky-Way-size dark-matter halo and its subhalos circled, an enormous suite of simulations . Griffen et al. 2016

    If you assume that the Universe isn’t simply made up of the matter we can directly detect, but that there’s an additional component, you wouldn’t expect that those two mass measurements would line up. If there’s something besides protons, neutrons, and electrons making up the Universe, their gravitational effects would show themselves without necessarily leaving a visible light signature.

    But another option would be to modify the law of gravity. If you simply add in an additional term to Newton’s law of gravity that defines a minimum acceleration scale, you can explain how galaxies rotate to a superior degree to the dark matter idea. The great hope of modified gravity is to reproduce the entire observable Universe without adding in dark matter.

    Individual galaxies could, in principle, be explained by either dark matter or a modification to gravity, but they are not the best evidence we have for what the Universe is made of, or how it got to be the way it is today. (STEFANIA.DELUCA OF WIKIMEDIA COMMONS)

    While attempts to make a modification to gravity that explain all the cosmic observations have proved elusive thus far, this remains the best option to explain how galaxies (and smaller objects) behave. Without a direct detection of a theoretical particle that could be responsible for dark matter, the door must remained open for alternatives. Despite the overwhelming cosmological evidence pointing to dark matter, other options deserve consideration, too.

    Our galaxy is thought to be embedded in an enormous, diffuse dark matter halo, indicating that there must be dark matter flowing through the solar system. But it isn’t very much, density-wise, and that makes it extremely difficult to detect locally. (ROBERT CALDWELL & MARC KAMIONKOWSKI NATURE 458, 587–589 (2009))[Not made available]

    In science, the way you decide which ideas are admissible versus which ones are no longer possible is to put them to the test against one another. Dark matter and modified gravity have a hard time going head-to-head on galactic scales because there are a number of confounding elements involved. For galaxies, star formation, feedback between gas, radiation, and dark matter, as well as stellar winds and complicated merger scenarios make universal predictions difficult on these small scales. Modified gravity might give you much cleaner predictions on these small scales, but fail catastrophically when you attempt to extend these modifications to larger ones, where dark matter achieves its greatest successes.

    The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. Alternative theories now need to be so contrived that they are considered by many to be quite ridiculous. But dark matter and modified gravity are both contenders for explaining the Universe on small (galactic) scales. (X-RAY: NASA/CXC/ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND/D.HARVEY NASA/CXC/DURHAM UNIV/R.MASSEY; OPTICAL/LENSING MAP: NASA, ESA, D. HARVEY (ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND) AND R. MASSEY (DURHAM UNIVERSITY, UK))

    NASA/Chandra X-ray Telescope

    But there’s a new paper out [MNRAS] that has devised a brilliant, head-to-head test for dark matter against modified gravity. If the law of gravity is truly different from Einstein’s General Relativity, then it should apply equally well to all galaxies under all conditions.

    If we can find two galaxies with the same mass profiles — where they’re not only the same overall mass, but have the same mass-as-a-function-of-radius as one another — we’d expect them to exhibit the same internal motions as one another. If there’s no dark matter, but just the matter we observe, the force of gravity, even if it’s a modified force of gravity, would have to be the same.

    Some galaxies are observed, if we try to fit them with dark matter, to have a ‘core’ in the center where the density is low, while others have a ‘cusp’ where the density is high. If dark matter gets heated based on the galaxy’s star formation history, this mystery could at last be solved. (J. I. READ, M. G. WALKER, P. STEGER; ARXIV:1808.06634 [above])

    So if we look at two galaxies and see that they don’t match, either at least one of the galaxies must be out of equilibrium, meaning it’s in a state of change, or modified gravity can’t explain it.

    On the other hand, there is a tremendously powerful explanation that dark matter offers that could explain it all, even if both galaxies are in equilibrium. The reason? Because galaxies could have formed stars at different times or different rates, and star-formation history affects not just the normal matter, but the dark matter as well.

    The Illustris Simulation

    While the web of dark matter (purple) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red) can severely impact galactic scales. Even small galaxies are subject to these effects, and if dark matter heats up from star formation, the effect can be quite severe. (ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

    While it’s true that only normal matter interacts (i.e., scatters) with photons, both normal matter and dark matter should respond to radiation pressure. If a galaxy formed stars only a very long time ago, and not for many billions of years, there should be plenty of dark matter that now populates the inner reaches of a galaxy. But if there has been a lot of recent star formation occurring in multiple bursts, it should evacuate the mass from the galactic center. With less mass there, the orbits of the dark matter particles changes, lowering the inner density of dark matter in the innermost regions. (There was a nice review of this back in 2014 [Nature].) As Justin Read explained in a conversation with him:

    “…radiation pressure, stellar winds and supernovae push the gas (via the usual electromagnetic interaction) and dark matter then responds to the altered central gravitational potential.”

    The best laboratory to test this is with small, dwarf galaxies, where these effects should be the largest.

    Dwarf galaxy NGC 5477 is one of many irregular dwarf galaxies. The blue regions are indicative of new star formation, but many such galaxies have formed no new stars in many billions of years. Even with the same light profiles, their mass profiles appear to be different, a challenge for modified theories of gravity. (NASA/ESA Hubble)

    If the galaxies all demonstrate the same gravitational behavior, it would be a victory for modified gravity. But if we can trace out the star-formation histories of these galaxies — which we can do by examining the stellar populations found inside them — and if these galaxies exhibit different gravitational behaviors because of them, that would be a victory for dark matter, and a blow to the theories of modified gravity that make contrary predictions.

    The number of galaxies we’ve found and examined to test this is small, but in a new paper [https://arxiv.org/abs/1808.06634v1 (above)] led by Justin Read, they look at 16 such galaxies, and find that the dark matter “heating” explanation appears to work!

    The dwarf ‘twins’ Carina and Draco: a challenge for alternative gravity explanations for DM. The solid and dashed black and purple lines show predictions for Draco and Carina in MOND, which clearly fare poorly. Despite their similarities in terms of light, stellar kinematics imply that Draco is substantially denser than Carina. (FIG 7. FROM J. I. READ, M. G. WALKER, P. STEGER; ARXIV:1808.06634 [above])

    They looked at 8 dwarf spheroidal and 8 dwarf irregular galaxies, and found that there were two populations: one where star formation hasn’t occurred for the past 6 billion years, and one where it has. The ones where star formation didn’t occur recently are consistent with lots of dark mass in the center (no recent heating), and the ones where it did occur recently show far less dark matter in their centers (evidence for recent heating). It’s an indication that there is dark matter, it is cold and collisionless, and that it can be heated up by recent star formation.

    The Draco dwarf spheroidal galaxy is one of the 16 galaxies examined in the Read et al. paper, and displays extremely different mass profiles from its gravitational effects than the Carina galaxy, which otherwise appears extremely similar except for a different star formation history. (BERNHARD HUBL / ASTROPHOTON.COM)

    Carina Nebula. 1.5-m Danish telescope at ESO’s La Silla Observatory

    ESO Danish 1.54 meter telescope at La Silla, 600 km north of Santiago de Chile at an altitude of 2400 metres.

    In particular, two of the galaxies (Draco and Carina) have almost the same masses and normal mass profiles, but widely different gravitational effects.

    The Carina dwarf galaxy, very similar in size, star distribution, and morphology to the Draco dwarf galaxy, exhibits a very different gravitational profile from Draco. This can be cleanly explained with dark matter if it can be heated up by star formation, but not by modified gravity. (ESO/G. BONO & CTIO)

    The authors note:

    These two galaxies require different dynamical mass profiles for almost the same radial light profile. This is a challenge not only for MOND, but for any weak-field gravity theory that seeks to fully explain DM.

    Mordehai Milgrom, MOND theorist, is an Israeli physicist and professor in the department of Condensed Matter Physics at the Weizmann Institute in Rehovot, Israel

    The fact that these two galaxies exhibit such different gravitational effects tell us that either something is very funny with one of them (something must be out-of-equilibrium), or that dark matter gets heated up by star formation and modified gravity cannot explain this. As always, more data, additional galaxies, and further research will be required to solve this mystery, but at long last, we’re looking at a viable way to prove modified gravity wrong on galaxy scales. Even without directly detecting a particle, dark matter might just achieve a knockout blow over its greatest competing alternative.



    Please help promote STEM in your local schools.

    Stem Education Coalition

    See the full article here .

    “Starts With A Bang! is a blog/video blog about cosmology, physics, astronomy, and anything else I find interesting enough to write about. I am a firm believer that the highest good in life is learning, and the greatest evil is willful ignorance. The goal of everything on this site is to help inform you about our world, how we came to be here, and to understand how it all works. As I write these pages for you, I hope to not only explain to you what we know, think, and believe, but how we know it, and why we draw the conclusions we do. It is my hope that you find this interesting, informative, and accessible,” says Ethan

  • richardmitnick 9:05 am on August 23, 2018 Permalink | Reply
    Tags: , , , Bouncing barrier, , , Gravity, , NASA Researchers Find Evidence of Planet-Building Clumps, Planetesimal formation   

    From NASA Ames: “NASA Researchers Find Evidence of Planet-Building Clumps” 

    NASA Ames Icon

    From NASA AMES

    Aug. 21, 2018
    Darryl Waller
    NASA Ames Research Center, Silicon Valley

    Noah Michelsohn
    NASA Johnson Space Center, Houston

    False-color image of Allendale meteorite showing the apparent golf ball size clumps. Credits: NASA/J. Simon and J. Cuzzi

    NASA scientists have found the first evidence supporting a theory that golf ball-size clumps of space dust formed the building blocks of our terrestrial planets.

    A new paper from planetary scientists at the Astromaterials Research and Exploration Science Division (ARES) at NASA’s Johnson Space Center in Houston, Texas, and NASA’s Ames Research Center in Silicon Valley, California, provides evidence for an astrophysical theory called “pebble accretion” where golf ball-sized clumps of space dust came together to form tiny planets, called planetesimals, during the early stages of planetary formation.

    “This is very exciting because our research provides the first direct evidence supporting this theory,” said Justin Simon, a planetary researcher in ARES. “There have been a lot of theories about planetesimal formation, but many have been stymied by a factor called the ‘bouncing barrier.’”

    “The bouncing barrier principle stipulates that planets cannot form directly through the accumulation of small dust particles colliding in space because the impact would knock off previously attached aggregates, stalling growth. Astrophysicists had hypothesized that once the clumps grew to the size of a golf ball, any small particle colliding with the clump would knock other material off. Yet, if the colliding objects were not the size of a particle, but much larger – for example, clumps of dust the size of a golf ball – that they could exhibit enough gravity to hold themselves together in clusters to form larger bodies.”

    Mosaic photograph of the ancient Northwest Africa 5717 ordinary chondrite with clusters of particles. Credits: NASA/J. Simon and J. Cuzzi

    The research provides evidence of a common, possibly universal, dust sticking process from studying two ancient meteorites – Allende and Northwest Africa 5717 – that formed in the pre-planetary period of the Solar System and have remained largely unaltered since that time. Scientists know through dating methods that these meteorites are older than Earth, Moon, and Mars, which means they have remained unaltered since the birth of the Solar System. The meteorites studied for this research are so old that they are often used to date the Solar System itself.

    The meteorites were analyzed using electron microscope images and high-resolution photomicrographs that showed particles within the meteorite slices appeared to concentrate together in three to four-centimeter clumps. The existence of the clumps demonstrates that the meteorites themselves were produced by the clustering of golf ball-sized objects, providing strong evidence that the process was possible for other bodies as well.

    The research, titled “Particle size distributions in chondritic meteorites: Evidence for pre-planetesimal histories,” was published in the journal Earth and Planetary Science Letters in July. The publication culminated six years of research that was led by planetary scientists Simon at Johnson and Jeffrey Cuzzi at Ames.

    Dig up more about how NASA studies meteorites, visit:


    See the full article here .


    Please help promote STEM in your local schools.

    Stem Education Coalition

    Ames Research Center, one of 10 NASA field Centers, is located in the heart of California’s Silicon Valley. For over 60 years, Ames has led NASA in conducting world-class research and development. With 2500 employees and an annual budget of $900 million, Ames provides NASA with advancements in:
    Entry systems: Safely delivering spacecraft to Earth & other celestial bodies
    Supercomputing: Enabling NASA’s advanced modeling and simulation
    NextGen air transportation: Transforming the way we fly
    Airborne science: Examining our own world & beyond from the sky
    Low-cost missions: Enabling high value science to low Earth orbit & the moon
    Biology & astrobiology: Understanding life on Earth — and in space
    Exoplanets: Finding worlds beyond our own
    Autonomy & robotics: Complementing humans in space
    Lunar science: Rediscovering our moon
    Human factors: Advancing human-technology interaction for NASA missions
    Wind tunnels: Testing on the ground before you take to the sky

    NASA image

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