Tag Archives: #cosmos

Candid Cosmos: eROSITA Cameras Set A Benchmark For Astronomical Imaging (Cosmology)

An overview and performance assessment of the seven cameras of eROSITA, a space x-ray telescope launched in 2019

Recently, the eROSITA (extended Roentgen Survey with an Imaging Telescope Array) x-ray telescope, an instrument developed by a team of scientists at Max-Planck-Institut für Extraterrestrische Physik (MPE), has gained attention among astronomers. The instrument performs an all-sky survey in the x-ray energy band of 0.2-8 kilo electron volts aboard the Spectrum-Roentgen-Gamma (SRG) satellite that was launched in 2019 from the Baikonur cosmodrome in Kazakhstan.

“The eROSITA has been designed to study the large-scale structure of the universe and test cosmological models, including dark energy, by detecting galaxy clusters with redshifts greater than 1, corresponding to a cosmological expansion faster than the speed of light,” said Dr. Norbert Meidinger from MPE, a part of the team that developed the instrument. “We expect eROSITA to revolutionize our understanding of the evolution of supermassive black holes.” The details of the developmental work have been published in SPIE’s Journal of Astronomical Telescopes, Instruments, and Systems (JATIS).

eROSITA is not one telescope, but an array of seven identical, co-aligned telescopes, with each one composed of a mirror system and a focal-plane camera. The camera assembly, in turn, consists of the camera head, camera electronics, and filter wheel. The camera head is made up of the detector and its housing, a proton shield, and a heat pipe for detector cooling. The camera electronics include supply, control, and data acquisition electronics for detector operation. The filter wheel is mounted above the camera head and has four positions including an optical and UV blocking filter to reduce signal noise, a radioactive x-ray source for calibration, and a closed position that allows instrumental background measurements.

“It’s exciting to read about these x-ray cameras that are in orbit and enabling a broad set of scientific investigations on a major astrophysics mission,” says Megan Eckart of Lawrence Livermore National Laboratory, USA, who is the deputy editor of JATIS. “Dr. Meidinger and his team provide a clear description of the hardware development and ground testing, and wrap up the paper with a treat: first-light images from eROSITA and an assessment of onboard performance. Astrophysicists around the world will analyze data from these cameras for years to come.”

The eROSITA telescope is well on its way to becoming a game changer for x-ray astronomy.

First light of the eROSITA X-ray telescope in space
First light of the eROSITA X-ray telescope in space.

Read the open-access article by Norbert Meidinger et al., “eROSITA camera array on the SRG satellite,” J. Astron. Telesc. Instrum. Syst7(2), 025004 (2021). doi 10.1117/1.JATIS.7.2.025004

Featured image: A team of scientists from the Max-Planck-Institut für Extraterrestrische Physik, Germany, built an x-ray telescope called eROSITA consisting of an array of co-aligned focal plane cameras with one in the center and six surrounding it. Image credit: P. Friedrich, doi 10.1117/1.JATIS.7.2.025004


Provided by SPIE

All the Music That Comes From the Cosmos (Astronomy)

By connecting a loudspeaker to a radio telescope it is possible to transform radio signals into sound. Now the new frontier is to sonify one of the most sensational and elusive phenomena: gravitational waves. Astrophysics Patrizia Caraveo writes about it in this article published yesterday in Sole24Ore, here reproduced with the author’s consent

In the cosmic void, sounds cannot propagate. Yet celestial objects sing or, more correctly, vibrate and we, after having registered these vibrations, can transform them into sounds. Inside the Sun, sound waves are reflected by the layers that make up our star. Each reflection leaves a small signature and this is how, by studying the vibrations at different frequencies, we can understand what happens in the depths of our star. It is a different way of penetrating the intimacy of the Sun, made possible by the space probes that keep it under constant control. In addition to having a great scientific value, the data can also have an unexpected artistic value. NASA created Solarium, a multimedia show where the high-resolution images of the ever-boiling surface of the Sun envelop the viewer while the sonification of the vibrations makes us feel its internal music.

The sonification process is easily applied to all cyclic phenomena, because the periodicities immediately transform into sound frequencies. The idea is certainly not original. Interpreting the rhythmic motions of the planets in music was, perhaps, a pastime of Pythagoras. Certainly Kepler applied to it and wrote a treatise entitled Harmonices Mundiwhere he makes each planet correspond to a geometric solid and a musical harmony. If you want to get an idea of ​​the procedure, I recommend watching the video at the bottom of this article: you will see that each planet is described by a sequence of notes with duration proportional to its orbit. We gradually move from the farthest, slowest and lowest sounds (Saturn and Jupiter) to the nearest, faster planets with the highest sounds.

Kepler wanted to make people appreciate the harmony of creation, but his idea is a beautiful example of the sonification of the Solar System at the basis of the concept of the music of the spheres. By connecting a loudspeaker to their telescopes, radio astronomers transform the signals they receive into sound. Let us consider the case of neutron stars. These are corpses of stars that rotate very quickly around their axis and produce radio signals thanks to a process that concentrates the emission inside a thin cone that is formed by virtue of their very high magnetic field. These characteristics make neutron stars similar to the coastal lighthouses that, since the dawn of time, have guided sailors. Each lighthouse has its own rotation frequency, just as each neutron star has its own periodicity. Each star is different from the others and is easily recognizable from its song. Those that go one revolution per second resemble the ticking of an old alarm clock. The neutron star in the Sails constellation has a period of 89 ms, so it makes 11 rotations per second and looks like a diesel engine. If the idea of ​​a celestial object with a mass similar to that of the Sun that rotates 11 times per second seems difficult to imagine, think that the most hasty neutron stars rotate 800 times per second, making both Formula engines pale. 1 and those of the Moto Gp.

NASA’s “Solarium” multimedia show. 
Credits: Nasa

These are interesting experiments applied in a very specific field, because they sonify a wave event. Is it possible to generalize the technique by applying it to images to allow even the blind to appreciate the beauty and richness of celestial objects? Indeed, NASA has also led the way in this field and, thanks to efforts to make astronomy more inclusive, a good choice of sonified astronomical images are now available. Much of the work was done or inspired by Wanda Diaz Merced, a blind girl from Puerto Rico who did not want an incurable maculopathy to take away her dream of becoming an astronomer. With great determination Wanda graduated and applied to participate in a NASA internship dedicated to people with disabilities, then earned a PhD in Computer Science from the University of Glasgow in 2013. Proving that she has splendidly overcome the limitations caused by her handicap, Wanda worked in Japan and South Africa. In 2016 she was invited to participate in the Frontiers conferenceat the White House. She collaborated with Harvard University and the International Astronomical Union and has now moved to the Virgo gravitational wave observatory, near Pisa. Here Wanda will try to sonify one of the most elusive and most sensational astronomical phenomena: gravitational waves.

It is a tiny deformation of space-time that originates when two black holes, which orbit one another in a binary system, get closer and closer, until they interpenetrate. The gravitational signal that, in a fraction of a second, increases in intensity while becoming more acute, lends itself to being sonified. It is called chirp and contains information on the starting black holes and on the one that resulted from the merger, slightly smaller than the sum of the two starting objects. The difference turns into a gravitational wave, a phenomenon predicted by Einstein’s general relativity in 1915 and revealed just 100 years later, in September 2015.

A real revolution in astronomy: a chirp of 0.2 seconds was enough to understand that many of the commonly accepted theories on the formation of black holes were to be revised. It is one of the most fascinating discoveries of modern astrophysics which, rightly, gives the title to the beautiful book by Massimiliano Razzano, Listening to the cosmos.. In truth, the book is not limited to gravitational waves, Maximilian describes the new approach that characterizes modern research. To understand the behavior of any celestial object, one must learn to study it at different wavelengths by doing multi-wavelength astronomy, but that’s not enough. We must also be ready to exploit the information that comes to us from the new frontiers of astrophysics from neutrinos to gravitational waves. The new astronomy is multi-messenger. The total picture, which is objectively difficult to compose, is of extraordinary richness. I hope gravitational science will soon become an inclusive art installation

Watch the video on Kepler’s Harmonices Mundi :

Featured image: Harmony of the cosmos. Credits: X-Ray: Nasa / Cxc / Sao; Optical: Nasa / Stsci; Ir: Spitzer Nasa / Jpl-Caltech; Sonification: Nasa / Cxc / Sao / K. Arcand, M. Russo & A. Santaguida


Provided by INAF

Experts Recreate a Mechanical Cosmos For the World’s First Computer (Engineering)

Researchers at UCL have solved a major piece of the puzzle that makes up the ancient Greek astronomical calculator known as the Antikythera Mechanism, a hand-powered mechanical device that was used to predict astronomical events.

Known to many as the world’s first analogue computer, the Antikythera Mechanism is the most complex piece of engineering to have survived from the ancient world. The 2,000-year-old device was used to predict the positions of the Sun, Moon and the planets as well as lunar and solar eclipses.

Published in Scientific Reports, the paper from the multidisciplinary UCL Antikythera Research Team reveals a new display of the ancient Greek order of the Universe (Cosmos), within a complex gearing system at the front of the Mechanism.

Lead author Professor Tony Freeth (UCL Mechanical Engineering) explained: “Ours is the first model that conforms to all the physical evidence and matches the descriptions in the scientific inscriptions engraved on the Mechanism itself.

“The Sun, Moon and planets are displayed in an impressive tour de force of ancient Greek brilliance.”

The Antikythera Mechanism has generated both fascination and intense controversy since its discovery in a Roman-era shipwreck in 1901 by Greek sponge divers near the small Mediterranean island of Antikythera.

The astronomical calculator is a bronze device that consists of a complex combination of 30 surviving bronze gears used to predict astronomical events, including eclipses, phases of the moon, positions of the planets and even dates of the Olympics.

Whilst great progress has been made over the last century to understand how it worked, studies in 2005 using 3D X-rays and surface imaging enabled researchers to show how the Mechanism predicted eclipses and calculated the variable motion of the Moon.

However, until now, a full understanding of the gearing system at the front of the device has eluded the best efforts of researchers. Only about a third of the Mechanism has survived, and is split into 82 fragments – creating a daunting challenge for the UCL team.

The biggest surviving fragment, known as Fragment A, displays features of bearings, pillars and a block. Another, known as Fragment D, features an unexplained disk, 63-tooth gear and plate.

Previous research had used X-ray data from 2005 to reveal thousands of text characters hidden inside the fragments, unread for nearly 2,000 years. Inscriptions on the back cover include a description of the cosmos display, with the planets moving on rings and indicated by marker beads. It was this display that the team worked to reconstruct.

Two critical numbers in the X-rays of the front cover, of 462 years and 442 years, accurately represent cycles of Venus and Saturn respectively. When observed from Earth, the planets’ cycles sometimes reverse their motions against the stars. Experts must track these variable cycles over long time-periods in order to predict their positions.

“The classic astronomy of the first millennium BC originated in Babylon, but nothing in this astronomy suggested how the ancient Greeks found the highly accurate 462-year cycle for Venus and 442-year cycle for Saturn,” explained PhD candidate and UCL Antikythera Research Team member Aris Dacanalis.

Using an ancient Greek mathematical method described by the philosopher Parmenides, the UCL team not only explained how the cycles for Venus and Saturn were derived but also managed to recover the cycles of all the other planets, where the evidence was missing.

PhD candidate and team member David Higgon explained: “After considerable struggle, we managed to match the evidence in Fragments A and D to a mechanism for Venus, which exactly models its 462-year planetary period relation, with the 63-tooth gear playing a crucial role.”

Professor Freeth added: “The team then created innovative mechanisms for all of the planets that would calculate the new advanced astronomical cycles and minimize the number of gears in the whole system, so that they would fit into the tight spaces available.”

“This is a key theoretical advance on how the Cosmos was constructed in the Mechanism,” added co-author, Dr Adam Wojcik (UCL Mechanical Engineering). “Now we must prove its feasibility by making it with ancient techniques. A particular challenge will be the system of nested tubes that carried the astronomical outputs.”

The discovery brings the research team a step closer to understanding the full capabilities of the Antikythera Mechanism and how accurately it was able to predict astronomical events. The device is kept at the National Archaeological Museum in Athens.

The UCL Antikythera Research Team is supported by the A.G. Leventis Foundation, Charles Frodsham & Co. and the Worshipful Company of Clockmakers.

The team is led by Dr Adam Wojcik and made up of Professor Tony Freeth, Professor Lindsay MacDonald (UCL CEGE), Dr Myrto Georgakopoulou (UCL Qatar) and PhD candidates David Higgon and Aris Dacanalis (both UCL Mechanical Engineering).

Video

Links

Featured image: Exploded model of the Cosmos gearing of the Antikythera Mechanism. ©2020 Tony Freeth


Provided by University College London

Searching for Dark Matter Through the Fifth Dimension (Astronomy)

A discovery in theoretical physics could help to unravel the mysteries of dark matter

Theoretical physicists of the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU) are working on a theory that goes beyond the Standard Model of particle physics and can answer questions where the Standard Model has to pass – for example, with respect to the hierarchies of the masses of elementary particles or the existence of dark matter. The central element of the theory is an extra dimension in spacetime. Until now, scientists have faced the problem that the predictions of their theory could not be tested experimentally. They have now overcome this problem in a publication in the current issue of the European Physical Journal C.

Already in the 1920s, in an attempt to unify the forces of gravity and electromagnetism, Theodor Kaluza and Oskar Klein speculated about the existence of an extra dimension beyond the familiar three space dimensions and time – which in physics are combined into 4-dimensional spacetime. If it exists, such a new dimension would have to be incredible tiny and unnoticeable to the human eye. In the late 1990s, this idea has seen a remarkable renaissance when it was realized that the existence of a fifth dimension could resolve some of the profound open questions of particle physics. In particular, Yuval Grossman of Stanford University and Matthias Neubert, then a professor at Cornell University in the US, showed in a highly cited publication that the embedding of the Standard Model of particle physics in a 5-dimensional spacetime could explain the so far mysterious patterns seen in the masses of elementary particles.

Another 20 years later, the group of Professor Matthias Neubert – since 2006 on the faculty of Johannes Gutenberg University Mainz and spokesperson of the PRISMA+ Cluster of Excellence – made another unexpected discovery: they found that the 5-dimensional field equations predicted the existence of a new heavy particle with similar properties as the famous Higgs boson but a much heavier mass – so heavy, in fact, that it cannot be produced even at the highest-energy particle collider in the world, the Large Hadron Collider (LHC) at the European Center for Nuclear Research CERN near Geneva in Switzerland. “It was a nightmare,” recalled Javier Castellano Ruiz, a PhD student involved in the research. “We were excited by the idea that our theory predicts a new particle, but it appeared to be impossible to confirm this prediction in any foreseeable experiment.”

The detour through the fifth dimension

In a recent paper published in the European Physical Journal C, the researchers found a spectacular resolution to this dilemma. They discovered that their proposed particle would necessarily mediate a new force between the known elementary particles of our visible universe and the mysterious dark matter, the dark sector. Even the abundance of dark matter in the cosmos, as observed in astrophysical experiments, can be explained by their theory. This offers exciting new ways to search for the constituents of the dark matter – literally via a detour through the extra dimension – and obtain clues about the physics at a very early stage in the history of our universe, when dark matter was produced. “After years of searching for possible confirmations of our theoretical predictions, we are now confident that the mechanism we have discovered would make dark matter accessible to forthcoming experiments, because the properties of the new interaction between ordinary matter and dark matter – which is mediated by our proposed particle – can be calculated accurately within our theory,” said Professor Matthias Neubert, head of the research team. “In the end – so our hope – the new particle may be discovered first through its interactions with the dark sector.” This example nicely illustrates the fruitful interplay between experimental and theoretical basic science – a hallmark of the PRISMA+ Cluster of Excellence.

Featured image: Prof. Dr. Matthias Neubert © JGU


Reference:
A. Carmona, J. Castellano, M. Neubert, A warped scalar portal to fermionic dark matter, The European Physical Journal C 81, 20 January 2021,
DOI:10.1140/epjc/s10052-021-08851-0


Provided by JGU

Did We Find First Evidence of Cosmic Strings? (Cosmology / Quantum)

Summary:

Recently, NANOGrav Collaboration presented its results of a search for an isotropic SGWB based on its 12.5-year data set. Remarkably enough, this study yields strong evidence for the presence of a stochastic process across the 45 pulsars included in the analysis. They claimed they found 1st evidence of cosmic strings.

○ Now, Simone and colleagues shown their results in favor of NANOGrav Collaboration, they demonstrated that stochastic gravitational-wave background (SGWB) could be emitted by a cosmic-string network in the early Universe.

○ They showed that NANOGrav signal points to symmetry breaking scales in the range v ∼ 10¹⁴⋯10^16 GeV.

They also argued that the current gravitational wave (GW) experiments could not able to detect the signal at the sufficient signal to noise ratio, but entire viable parameter region will be probed in future GW experiments.

According to Simone, if confirmed in the future, the NANOGrav signal will mark the beginning of a new era in GW astronomy and revolutionize our understanding of the cosmos.


Many models of new physics beyond the Standard Model predict cosmological phase transitions in the early Universe that lead to the spontaneous breaking of an Abelian symmetry. An exciting phenomenological consequence of such phase transitions is the generation of a network of cosmic strings, vortex-like topological defects that restore the broken symmetry at their core. Cosmic strings can form closed loops that lose energy and shrink via the emission of gravitational waves (GWs). Indeed, numerical simulations of cosmic strings based on the Nambu–Goto action showed that this is the dominant energy loss mechanism of cosmic-string loops, if the underlying broken symmetry corresponds to a local gauge symmetry. The primordial GW signal from a cosmic-string network, which encodes crucial information on ultraviolet physics far beyond the reach of terrestrial experiments, is therefore a major target of ongoing and upcoming searches for a stochastic GW background (SGWB).

FIG. 1: Scan over the cosmic-string tension Gµ and loop size α projected onto the γ – A plane, where −γ represents the spectral index of the pulsar timing-residual cross-power spectrum and A is the characteristic GW strain amplitude at f = fyr. The black contours denote the 1 σ and 2 σ posteriors in the NANOGrav analysis that allow to describe the observed stochastic process.. Here, they use the contours based on the five lowest frequency bins in the NANOGrav data. The gray vertical line indicates the theoretical prediction for a population of SMBHBs, γ = 13/3. The parameter values of the benchmark points (⋆, ⬩, •) are listed in Tab. I. For γ < 5 (γ > 5), the GW spectrum is rising (decreasing) as a function of frequency. In this case, NANOGrav observes GWs at frequencies below (above) the radiation–matter-equality peak in the spectrum. At the same time, most of the points clustering around γ ≃ 5 belong to the flat plateau in the spectrum at frequencies above the peak. © Simone et al.

A cosmic-string-induced SGWB is expected to stretch across a vast range of GW frequencies, making it an ideal signal for multifrequency GW astronomy. At high frequencies in the milli- to kilohertz range, the signal can be searched for in space- and ground-based GW interferometers, while at low frequencies in the nanohertz range, pulsar timing array (PTA) experiments are sensitive to the signal. Now, Simon Blassi and colleagues investigated the latter possibility, a cosmic-string-induced GW signal at Nanohertz frequencies, in light of the recent results reported by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) PTA experiment.

FIG. 2: NANOGrav 1 σ and 2 σ posterior contours projected onto the α – Gµ plane. They do not consider α values larger than α = 0.1, which is the maximal value found in simulations. At α < 10^−8 , they quickly cease to find viable points because α > lk/t in Eq. (5) given in paper is no longer satisfied when evaluated in the first NANOGrav frequency bin, i.e., f ≃ 2 × 10−9 Hz. The ⋆ benchmark point saturates the CMB limit on Gµ (see Tab. I). The diagonal black line labeled α =Γ Gµ distinguishes between the small-loop and the large-loop regime © Simone et al.

Recently in Dec 2020, the NANOGrav Collaboration presented its results of a search for an isotropic SGWB based on its 12.5-year data set. Remarkably enough, this study yields strong evidence for the presence of a stochastic process across the 45 pulsars included in the analysis. The interpretation of the observed signal in terms of a common-spectrum process is strongly preferred over independent red-noise processes (a Bayesian model comparison yields a log10 Bayes factor larger than 4); however, a conclusive statement on the physical origin of the signal is currently not yet feasible.

FIG. 3: GW spectra for the benchmark points (⋆, ⬩, •) in Tab. I alongside the power-law-integrated sensitivity curves of various present (solid boundaries) and future (dashed boundaries) GW experiments. The EPTA, PPTA, and NANOGrav curves at low frequencies represent the status of PTA constraints on the GW spectrum prior to the new NANOGrav result! Their benchmark spectra therefore illustrate that the NANOGrav signal exceeds previous PTA constraints © Simone et al.

Now, Simone Blasi and colleagues investigated the results of the NANOGrav analysis based on the assumption that this stochastic process corresponds to a primordial SGWB emitted by cosmic strings in the early Universe.

They studied stable Nambu–Goto strings in dependence of their tension Gµ and loop size α and identified the viable cosmic-string parameter space and argued that the current GW experiments could not able to detect the signal at the sufficient signal to noise ratio, but entire viable parameter region will be probed in future GW experiments.

©Simone et al.

They noted that the height of the flat plateau in the GW spectrum roughly scales as follows in dependence of α and Gµ,

where ¯α  max {α, 9/4 Γ Gµ}. © Simone et al.

According to this relation, all viable points in Fig. 2 predict a plateau that is at most suppressed by a factor of O(10−³) compared to their benchmark spectrum. As evident from Fig. 3, all viable points will therefore be probed in future experiments.


PTA experiments will be able to improve our understanding on the current NANOGrav analysis and confirm (or refute) the presence of the signal at increasingly higher significance.

— said Simone Blasi, lead author of the study.

They also commented on the relation between the cosmic-string tension Gµ and the underlying energy scale v of spontaneous U(1) symmetry breaking,

implying that the NANOGrav signal points to symmetry breaking scales in the range v ∼ 10¹⁴⋯10^16 GeV.


This is an exciting result that may indicate a connection between the observed signal and spontaneous symmetry breaking close to the energy scale of grand unification.

— said Kai Schmitz, co-author of the study

If confirmed in the future, the NANOGrav signal will mark the beginning of a new era in GW astronomy and revolutionize our understanding of the cosmos.


References: (1) Zaven Arzoumanian, Paul T. Baker et al., “The NANOGrav 12.5 yr Data Set: Search for an Isotropic Stochastic Gravitational-wave Background”, The Astrophysical Journal Letters, Volume 905, Number 2, Dec 2020. https://iopscience.iop.org/article/10.3847/2041-8213/abd401/meta (2) Simone Blasi, Vedran Brdar, and Kai Schmitz, “Has NANOGrav Found First Evidence for Cosmic Strings?”, Phys. Rev. Lett. 126, 041305 – Published 28 January 2021. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.041305


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Mira’s Last Journey: Exploring the Dark Universe (Astronomy)

A massive simulation of the cosmos and a nod to the next generation of computing

A team of physicists and computer scientists from the U.S. Department of Energy’s (DOE) Argonne National Laboratory performed one of the five largest cosmological simulations ever. Data from the simulation will inform sky maps to aid leading large-scale cosmological experiments.

The simulation, called the Last Journey, follows the distribution of mass across the universe over time — in other words, how gravity causes a mysterious invisible substance called ​“dark matter” to clump together to form larger-scale structures called halos, within which galaxies form and evolve.

“We’ve learned and adapted a lot during the lifespan of Mira, and this is an interesting opportunity to look back and look forward at the same time.”

— Adrian Pope, Argonne physicist

The scientists performed the simulation on Argonne’s supercomputer Mira. The same team of scientists ran a previous cosmological simulation called the Outer Rim in 2013, just days after Mira turned on. After running simulations on the machine throughout its seven-year lifetime, the team marked Mira’s retirement with the Last Journey simulation.

Argonne’s Mira supercomputer was recently retired after seven years of enabling groundbreaking science. (Image by Argonne National Laboratory.)

The Last Journey demonstrates how far observational and computational technology has come in just seven years, and it will contribute data and insight to experiments such as the Stage-4 ground-based cosmic microwave background experiment (CMB-S4), the Legacy Survey of Space and Time (carried out by the Rubin Observatory in Chile), the Dark Energy Spectroscopic Instrument and two NASA missions, the Roman Space Telescope and SPHEREx.

“We worked with a tremendous volume of the universe, and we were interested in large-scale structures, like regions of thousands or millions of galaxies, but we also considered dynamics at smaller scales,” said Katrin Heitmann, deputy division director for Argonne’s High Energy Physics (HEP) division.

The code that constructed the cosmos

The six-month span for the Last Journey simulation and major analysis tasks presented unique challenges for software development and workflow. The team adapted some of the same code used for the 2013 Outer Rim simulation with some significant updates to make efficient use of Mira, an IBM Blue Gene/Q system that was housed at the Argonne Leadership Computing Facility (ALCF), a DOE Office of Science User Facility.

Specifically, the scientists used the Hardware/Hybrid Accelerated Cosmology Code (HACC) and its analysis framework, CosmoTools, to enable incremental extraction of relevant information at the same time as the simulation was running.

“Running the full machine is challenging because reading the massive amount of data produced by the simulation is computationally expensive, so you have to do a lot of analysis on the fly,” said Heitmann. ​“That’s daunting, because if you make a mistake with analysis settings, you don’t have time to redo it.”

The team took an integrated approach to carrying out the workflow during the simulation. HACC would run the simulation forward in time, determining the effect of gravity on matter during large portions of the history of the universe. Once HACC determined the positions of trillions of computational particles representing the overall distribution of matter, CosmoTools would step in to record relevant information — such as finding the billions of halos that host galaxies — to use for analysis during post-processing.

“When we know where the particles are at a certain point in time, we characterize the structures that have formed by using CosmoTools and store a subset of data to make further use down the line,” said Adrian Pope, physicist and core HACC and CosmoTools developer in Argonne’s Computational Science (CPS) division. ​“If we find a dense clump of particles, that indicates the location of a dark matter halo, and galaxies can form inside these dark matter halos.”

The scientists repeated this interwoven process — where HACC moves particles and CosmoTools analyzes and records specific data — until the end of the simulation. The team then used features of CosmoTools to determine which clumps of particles were likely to host galaxies. For reference, around 100 to 1,000 particles represent single galaxies in the simulation.

“We would move particles, do analysis, move particles, do analysis,” said Pope. ​“At the end, we would go back through the subsets of data that we had carefully chosen to store and run additional analysis to gain more insight into the dynamics of structure formation, such as which halos merged together and which ended up orbiting each other.”

Using the optimized workflow with HACC and CosmoTools, the team ran the simulation in half the expected time.

Community contribution

The Last Journey simulation will provide data necessary for other major cosmological experiments to use when comparing observations or drawing conclusions about a host of topics. These insights could shed light on topics ranging from cosmological mysteries, such as the role of dark matter and dark energy in the evolution of the universe, to the astrophysics of galaxy formation across the universe.

“This huge data set they are building will feed into many different efforts,” said Katherine Riley, director of science at the ALCF. ​“In the end, that’s our primary mission — to help high-impact science get done. When you’re able to not only do something cool, but to feed an entire community, that’s a huge contribution that will have an impact for many years.”

The team’s simulation will address numerous fundamental questions in cosmology and is essential for enabling the refinement of existing models and the development of new ones, impacting both ongoing and upcoming cosmological surveys.

“We are not trying to match any specific structures in the actual universe,” said Pope. ​“Rather, we are making statistically equivalent structures, meaning that if we looked through our data, we could find locations where galaxies the size of the Milky Way would live. But we can also use a simulated universe as a comparison tool to find tensions between our current theoretical understanding of cosmology and what we’ve observed.”

Looking to exascale

“Thinking back to when we ran the Outer Rim simulation, you can really see how far these scientific applications have come,” said Heitmann, who performed Outer Rim in 2013 with the HACC team and Salman Habib, CPS division director and Argonne Distinguished Fellow. ​“It was awesome to run something substantially bigger and more complex that will bring so much to the community.”

As Argonne works towards the arrival of Aurora, the ALCF’s upcoming exascale supercomputer, the scientists are preparing for even more extensive cosmological simulations. Exascale computing systems will be able to perform a billion billion calculations per second — 50 times faster than many of the most powerful supercomputers operating today.

“We’ve learned and adapted a lot during the lifespan of Mira, and this is an interesting opportunity to look back and look forward at the same time,” said Pope. ​“When preparing for simulations on exascale machines and a new decade of progress, we are refining our code and analysis tools, and we get to ask ourselves what we weren’t doing because of the limitations we have had until now.”

The Last Journey was a gravity-only simulation, meaning it did not consider interactions such as gas dynamics and the physics of star formation. Gravity is the major player in large-scale cosmology, but the scientists hope to incorporate other physics in future simulations to observe the differences they make in how matter moves and distributes itself through the universe over time.

“More and more, we find tightly coupled relationships in the physical world, and to simulate these interactions, scientists have to develop creative workflows for processing and analyzing,” said Riley. ​“With these iterations, you’re able to arrive at your answers — and your breakthroughs — even faster.”

A paper on the simulation, titled ​“The Last Journey. I. An extreme-scale simulation on the Mira supercomputer,” was published on Jan. 27 in the Astrophysical Journal Supplement Series. The scientists are currently preparing follow-up papers to generate detailed synthetic sky catalogs.

The work was a multidisciplinary collaboration between high energy physicists and computer scientists from across Argonne and researchers from Los Alamos National Laboratory.

Funding for the simulation is provided by DOE’s Office of Science.

Featured image: Visualization of the Last Journey simulation. Shown is the large-scale structure of the universe as a thin slice through the full simulation (lower left) and zoom-ins at different levels. The lower right panel shows one of the largest structures in the simulation. (Image by Argonne National Laboratory.)


Provided by Argonne National Laboratory


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One in All: Double Branes with Radion Field Can Solve all Cosmological Problems (Astronomy / Physics)

Sumanta and Soumitra using a two brane setup have shown that, along with gauge hierarchy and cosmological constant problem, this model is also capable of addressing the missing mass problem through the kinematics of the brane separation, i.e., radion field.

Recent astrophysical observations strongly suggest existence of non-baryonic dark matter at the galactic as well as extra-galactic scales (if the dark matter is baryonic in nature, the third peak in the Cosmic Microwave Background power spectrum would have been lower compared to the observed height of the spectrum). These observations can be divided into two branches —(a) behavior of galactic rotation curves and (b) mass discrepancy in clusters of galaxies.

The first one, i.e., rotation curves of spiral galaxies, show clear evidences of problems associated with Newtonian and general relativity prescriptions. In these galaxies neutral hydrogen clouds are observed much beyond the extent of luminous Baryonic matter. In Newtonian description, the equilibrium of these clouds moving in a circular orbit of radius r is obtained through equality of centrifugal and gravitational force. For cloud velocity v(r), the centrifugal force is given by v²/r and the gravitational force by GM(r)/r², where M(r) stands for total gravitational mass within radius r. Equating these two will lead to the mass profile of the galaxy as, M(r) = rv²/G. This immediately posed serious problem, for at large distances from the center of the galaxy, the velocity remains nearly constant v ∼ 200 km/s, which suggests that mass inside radius r should increase monotonically with r, even though at large distance very little luminous matter can be detected.

The mass discrepancy of galaxy clusters also provides direct hint for existence of dark matter. The mass of galaxy clusters, which are the largest virialized structures in the universe, can be determined in two possible ways — (i) from the knowledge about motion of the member galaxies one can estimate the virial mass MV, secondly, (ii) estimating mass of individual galaxies and then summing over them in order to obtain total baryonic mass M. Almost without any exception MV turns out to be much large compared to M, typically one has MV/M ∼ 20-30. Recently, new methods have been developed to determine the mass of galaxy clusters, these are — (i) dynamical analysis of hot X-ray emitting gas and (ii) gravitational lensing of background galaxies — these methods also lead to similar results. Thus dynamical mass of galaxy clusters are always found to be in excess compared to their visible or baryonic mass. This missing mass issue can be explained through postulating that, every galaxy and galaxy cluster is embedded in a halo made up of dark matter. Thus the difference MV – M is originating from the mass of the dark matter halo, the galaxy cluster is embedded in.

The physical properties and possible candidates for dark matter can be summarized as follows: dark matter is assumed to be non-relativistic (hence cold and pressure-less), interacting only through gravity. Among many others, the most popular choice being weakly interacting massive particles. Among different models, the one with sterile neutrinos (with masses of several keV) has attracted much attention. Despite of few successes it comes with its own limitations. In the sterile neutrino scenario, the X-ray produced from their decay can enhance production of molecular hydrogen & thereby speeding up cooling of gas and early star formation. Even after a decade long experimental and observational efforts no non-gravitational signature for the dark matter has ever been found. Thus a priori the possibility of breaking down of gravitational theories at galactic scale cannot be excluded.

“A possible and viable way to modify the behaviour of gravity in our four dimensional space-time is by introducing extra spatial dimensions.”, said Soumitra SenGupta.

The extra dimensions were first introduced to explain the hierarchy problem (i.e., observed large difference between the weak and Planck energy scales). However the initial works did not incorporate gravity, but used large extra dimensions (and hence large volume factor) to reduce the Planck scale to TeV scale. Introduction of gravity, i.e., warped extra dimensions drastically altered the situation. Lisa Randall et al. was first to show that anti-de Sitter solution in higher dimensional spacetime (henceforth referred to as bulk) leads to exponential suppression of the energy scales on the visible four dimensional embedded sub-manifold (called as brane) thereby solving the hierarchy problem. Even though this scenario of warped geometry model solves the hierarchy problem, it also introduces additional correction terms to the gravitational field equations, leading to deviations from Einstein’s theory at high energy, with interesting cosmological and black hole physics applications. This conclusion is not bound to Einstein’s gravity alone but holds in higher curvature gravity theories¹ as well. Since the gravitational field equations get modified due to introduction of extra dimensions it is legitimate to ask, whether it can solve the problem of missing mass in galaxy clusters. Several works in this direction exist and can explain the velocity profile of galaxy clusters. However they emerge through the following setup:

• Obtaining effective gravitational field equations on a lower dimensional hypersurface, starting from the full bulk spacetime, which involves additional contributions from the bulk Weyl tensor. The bulk Weyl tensor in spherically symmetric systems leads to a component behaving as mass and is known as “dark mass” (Sumanta & Soumitra emphasize that this notion extends beyond Einstein’s gravity and holds for any arbitrary dimensional reduction). It has been shown by Harko and Cheng that introduction of the “dark mass” term is capable to yield an effect similar to the dark matter. Some related aspects were also explored by Troisi et al., Marek Szydlowski et al., Supratik Pal et al. & Boehmer and Harko in their paper, keeping the conclusions unchanged.
• In the second approach, the bulk spacetime is always taken to be anti-De Sitter such that bulk Weyl tensor vanishes. Unlike the previous case, which required S¹/Z2 orbifold symmetry, arbitrary embedding has been considered by Sepangi et al. following Maia and Monte. This again introduces additional corrections to the gravitational field equations. These additional correction terms in turn lead to the observed virial mass for galaxy clusters.

However all these approaches are valid for a single brane system. So, Sumanta and Soumitra in their work generalize previous results for a two brane system. This approach not only gives a handle on the hierarchy problem at the level of Planck scale but is also capable of explaining the missing mass problem at the scale of galaxy clusters. Moreover, in this setup the additional corrections will depend on the radion field, which represents the separation between the two branes. Hence in their setup the missing mass problem for galaxy clusters can also shed some light on the kinematics of the separation between the two branes.

Further the same setup is also shown to explain the observed rotation curves of galaxies as well. Hence both the problems associated with dark matter, namely, the missing mass problem for galaxy clusters and the rotation curves for galaxies can be explained by the two brane system introduced in this work via the kinematics of the radion field. Lets have a closer look.

Figure 1: Best fit curves for four chosen low surface brightness galaxies, NGC 959, NGC 7137, UGC 11820, UGC 477, respectively. On the vertical axis they have plotted observed velocity in km/s and the horizontal axis illustrates the radius measured in arc second. Good fit shows that the assumption of spherical symmetry is a good one, also the fact that baryonic matter plus radion field explains the galactic rotation curves fairly well. © Sumanta et al.

Friends, brane world models can address some of the long standing puzzles in theoretical physics, namely— (a) the hierarchy problem and (b) the cosmological constant problem. To solve the hierarchy problem there is a need of two branes, (that’s what Sumanta and Soumitra considered in their present work), with warped five dimensional geometry such that energy scale on the visible brane gets suppressed exponentially leading to TeV scale physics. For the cosmological constant, brane tension plays a crucial role. Two brane models naturally inherit an additional field, the separation between the branes (known as the radion field). Radion field is also very important in both macroscopic and microscopic physics, for it can have possible signatures in inflationary scenario, black hole physics, collider searches, etc. Along with the gauge hierarchy and cosmological constant problem, another very important problem in physics, is the missing mass problem. This appears since baryonic and virial mass of a galaxy cluster do not coincide.

In their work using a two brane setup they have shown that, along with gauge hierarchy and cosmological constant problem, this model is also capable of addressing the missing mass problem through the kinematics of the brane separation, i.e., radion field. Due to the presence of this additional field, the gravitational field equations on the brane gets modified and yields additional correction terms on top of Einstein’s field equations. By considering relativistic Boltzmann equation they have derived the virial mass of galaxy clusters, which depends on an effective additional mass constructed out of radion field. Moreover these correction terms modifies the structure of gravity and hence the motion under its influence at large distance, thereby producing a linear increase in the virial mass of the galaxy clusters. This in turn leads to the appropriate velocity law for galaxies within a galaxy cluster, solving the missing mass problem.

You will get further details of their work in below reference paper. If you are interested you can refer it. Thanks..

Reference: Chakraborty, S., SenGupta, S. Kinematics of radion field: a possible source of dark matter. Eur. Phys. J. C 76, 648 (2016). https://link.springer.com/article/10.1140/epjc/s10052-016-4512-z https://doi.org/10.1140/epjc/s10052-016-4512-z

Copyright of this article totally belongs to our author S. Aman.. One is allowed to reuse it only by giving proper credit either to him or to us

New Horizons Spacecraft Answers Question: How Dark Is Space? (Astronomy)

New measurements of the sky’s blackness show galaxies only number in the hundreds of billions.

How dark is the sky, and what does that tell us about the number of galaxies in the visible universe? Astronomers can estimate the total number of galaxies by counting everything visible in a Hubble deep field and then multiplying them by the total area of the sky. But other galaxies are too faint and distant to directly detect. Yet while we can’t count them, their light suffuses space with a feeble glow.

This artist’s illustration shows NASA’s New Horizons spacecraft in the outer solar system. In the background lies the Sun and a glowing band representing zodiacal light, caused by sunlight reflecting off of dust. Credits: Joe Olmsted/STScI

To measure that glow, astronomical satellites have to escape the inner solar system and its light pollution, caused by sunlight reflecting off dust. A team of scientists has used observations by NASA’s New Horizons mission to Pluto and the Kuiper Belt to determine the brightness of this cosmic optical background. Their result sets an upper limit to the abundance of faint, unresolved galaxies, showing that they only number in the hundreds of billions, not 2 trillion galaxies as previously believed.

How dark does space get? If you get away from city lights and look up, the sky between the stars appears very dark indeed. Above the Earth’s atmosphere outer space dims even further, fading to an inky pitch-black. And yet even there, space isn’t absolutely black. The universe has a suffused feeble glimmer from innumerable distant stars and galaxies.

New measurements of that weak background glow show that the unseen galaxies are less plentiful than some theoretical studies suggested, numbering only in the hundreds of billions rather than the previously reported two trillion galaxies.

“It’s an important number to know – how many galaxies are there?” said Marc Postman of the Space Telescope Science Institute in Baltimore, Maryland, a lead author on the study. “We simply don’t see the light from two trillion galaxies.”

The earlier estimate was extrapolated from very deep sky observations by NASA’s Hubble Space Telescope. It relied on mathematical models to estimate how many galaxies were too small and faint for Hubble to see. That team concluded that 90% of the galaxies in the universe were beyond Hubble’s ability to detect in visible light. The new findings, which relied on measurements from NASA’s distant New Horizons mission, suggest a much more modest number.

“Take all the galaxies Hubble can see, double that number, and that’s what we see – but nothing more,” said Tod Lauer of NSF’s NOIRLab, a lead author on the study.

These results will be presented on Wednesday, Jan. 13th at a meeting of the American Astronomical Society, which is open to registered participants.

The cosmic optical background that the team sought to measure is the visible-light equivalent of the more well-known cosmic microwave background – the weak afterglow of the big bang itself, before stars ever existed.

“While the cosmic microwave background tells us about the first 450,000 years after the big bang, the cosmic optical background tells us something about the sum total of all the stars that have ever formed since then,” explained Postman. “It puts a constraint on the total number of galaxies that have been created, and where they might be in time.”

As powerful as Hubble is, the team couldn’t use it to make these observations. Although located in space, Hubble orbits Earth and still suffers from light pollution. The inner solar system is filled with tiny dust particles from disintegrated asteroids and comets. Sunlight reflects off those particles, creating a glow called the zodiacal light that can be observed even by skywatchers on the ground.

To escape the zodiacal light, the team had to use an observatory that has escaped the inner solar system. Fortunately the New Horizons spacecraft, which has delivered the closest ever images of Pluto and the Kuiper Belt object Arrokoth, is far enough to make these measurements. At its distance (more than 4 billion miles away when these observations were taken), New Horizons experiences an ambient sky 10 times darker than the darkest sky accessible to Hubble.

“These kinds of measurements are exceedingly difficult. A lot of people have tried to do this for a long time,” said Lauer. “New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do it.”

The team analyzed existing images from the New Horizons archives. To tease out the feeble background glow, they had to correct for a number of other factors. For example, they subtracted the light from the galaxies expected to exist that are too faint to be identifiable. The most challenging correction was removing light from Milky Way stars that was reflected off interstellar dust and into the camera.

The remaining signal, though extremely faint, was still measurable. Postman compared it to living in a remote area far from city lights, lying in your bedroom at night with the curtains open. If a neighbor a mile down the road opened their refrigerator looking for a midnight snack, and the light from their refrigerator reflected off the bedroom walls, it would be as bright as the background New Horizons detected.

So, what could be the source of this leftover glow? It’s possible that an abundance of dwarf galaxies in the relatively nearby universe lie just beyond detectability. Or the diffuse halos of stars that surround galaxies might be brighter than expected. There might be a population of rogue, intergalactic stars spread throughout the cosmos. Perhaps most intriguing, there may be many more faint, distant galaxies than theories suggest. This would mean that the smooth distribution of galaxy sizes measured to date rises steeply just beyond the faintest systems we can see – just as there are many more pebbles on a beach than rocks.

NASA’s upcoming James Webb Space Telescope may be able to help solve the mystery. If faint, individual galaxies are the cause, then Webb ultra-deep field observations should be able to detect them.

This study is accepted for publication in the Astrophysical Journal.

Reference: Tod R. Lauer, Marc Postman, Harold A. Weaver, John R. Spencer, S. Alan Stern, Marc W. Buie, Daniel D. Durda, Carey M. Lisse, A. R. Poppe, Richard P. Binzel, Daniel T. Britt, Bonnie J. Buratti, Andrew F. Cheng, W.M. Grundy, Mihaly Horanyi J.J. Kavelaars, Ivan R. Linscott, William B. McKinnon, Jeffrey M. Moore, J. I. Nuñez, Catherine B. Olkin, Joel W. Parker, Simon B. Porter, Dennis C. Reuter, Stuart J. Robbins, Paul Schenk, Mark R. Showalter, Kelsi N. Singer, Anne. J. Verbiscer, Leslie A. Young, “New Horizons Observations of the Cosmic Optical Background”, ArXiv, pp. 1-32, 2021. https://arxiv.org/abs/2011.03052v2 https://imgsrc.hubblesite.org/hvi/uploads/science_paper/file_attachment/622/2011.03052.pdf

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Astronomers Document the Rise and Fall of a Rarely Observed Stellar Dance (Planetary Science)

The sun is the only star in our system. But many of the points of light in our night sky are not as lonely. By some estimates, more than three-quarters of all stars exist as binaries — with one companion — or in even more complex relationships. Stars in close quarters can have dramatic impacts on their neighbors. They can strip material from one another, merge or twist each other’s movements through the cosmos.

And sometimes those changes unfold over the course of a few generations.

That is what a team of astronomers from the University of Washington, Western Washington University and the University of California, Irvine discovered when they analyzed more than 125 years of astronomical observations of a nearby stellar binary called HS Hydrae. This system is what’s known as an eclipsing binary: From Earth, the two stars appear to pass over one another — or eclipse one another — as they orbit a shared center of gravity. The eclipses cause the amount of light emitted by the binary to dim periodically.

An image from the Digitized Sky Survey showing HS Hydrae in the center. Space Telescope Science Institute

On Jan. 11 at the 237th meeting of the American Astronomical Society, the team reported more than a century’s worth of changes to the eclipses by the stars in HS Hydrae. The two stars began to eclipse in small amounts starting around a century ago, increasing to almost full eclipses by the 1960s. The degree of eclipsing then plummeted over the course of just a half century, and will cease around February 2021.

“There is a historical record of observations of HS Hydrae that essentially spans modern astronomy — starting with photographic plates in the late 19th century up through satellite images taken in 2019. By diving into those records, we documented the complete rise and fall of this rare type of eclipsing binary,” said team leader James Davenport, a research assistant professor of astronomy at the UW and associate director of the UW’s DIRAC Institute.

The eclipses of the two stars that make up HS Hydrae are changing because another body — most likely a third, unobserved companion star — is turning the orientation of the binary with respect to Earth. Systems like this, which are called evolving eclipsing binaries, are rare, with only about a dozen known to date, according to Davenport. Identifying this type of binary requires multiple observations to look for long-term changes in the degree of dimming, which would indicate that the orientation of the binary is changing over time.

HS Hydrae has such an observational record because, at 342 light- years away, it is a relatively close and bright system and the two stars orbit each other every 1.5 days. Scientists first reported that HS Hydrae was an eclipsing binary in 1965. In a 2012 paper, astronomers based in Switzerland and the Czech Republic reported that the amount of dimming from HS Hydrae decreased from 1975 through 2008, indicating that the two stars were eclipsing smaller and smaller portions of one another over time. That team also predicted that the eclipses would end around 2022.

Davenport and his team checked in on HS Hydrae using observations of the system in 2019 by the NASA’s Transiting Exoplanet Survey Satellite, or TESS. They saw only a 0.0075-magnitude drop in light from HS Hydrae, a sign that the two stars were barely covering one another during eclipses. For comparison, eclipses in 1975 saw a more than 0.5-magnitude drop.

“Fifty years ago, these two stars were almost completely eclipsing each other. By the early 21st century, the degree of eclipse was around 10%, and in the most recent observations from 2019, they barely overlapped,” said Davenport.

With these new data, the team now predicts that HS Hydrae eclipses will cease around February 2021.

Image of a photographic plate from 1945, which was digitized for the Digital Access to a Sky Century at Harvard, or DASCH, catalog.DASCH/Harvard University

The observations from the 1960s through 2019 catalogue the decline of HS Hydrae as an evolving eclipsing binary. But Davenport and his team also uncovered evidence for its rise. The Digital Access to a Sky Century at Harvard, or DASCH, is a digital catalog of photometric data taken from more than a century’s worth of astro-photographic plates at Harvard University. The team mined this record and found observations of HS Hydrae from 1893 through 1955 that they could analyze to search for signs of dimming.

The researchers broke down DASCH observations of HS Hydrae by decade. From the late 19th century through the roaring ’20s, HS Hydrae showed no measurable dimming. But things began to change in the 1930s, where they measured a modest 0.1-magnitude drop in brightness. The degree of dimming rose through the 1940s and peaked in the 1950s with a 0.5-magnitude drop in brightness.

Based off this 126-year history of HS Hydrae observations, the team predicts that the system will start eclipsing again around the year 2195. But, that assumes that the third companion — which other teams have predicted is a small, dim M-dwarf star — continues to behave as it has to date.

Image of an astronomical log book from 1945. These observations are now part of the Digital Access to a Sky Century at Harvard, or DASCH, catalog.DASCH/Harvard University

“We won’t know for sure unless we keep looking,” said Davenport. “The best we can say right now is that HS Hydrae has been changing constantly over the course of modern astronomy.”

Missions like TESS will likely identify more evolving eclipsing binaries in the coming years. This should open new opportunities for astronomers to understand how star systems are built, as well as how they change over time — whether they are busy, dynamic systems like HS Hydrae, or more quiet systems, like ours.

Co-authors on the paper are UW graduate students Diana Windemuth and Jessica Birky; UW researcher Karen Warmbein; Erin Howard at Western Washington University; and Courtney Klein at UC Irvine. The research was funded by NASA, the National Science Foundation, the Heising-Simons Foundation, the Research Corporation for Science Advancement, the DIRAC Institute, the UW Department of Astronomy, the Charles and Lisa Simonyi Fund for Arts and Sciences and the Washington Research Foundation.

References: (1) “The Rise and Fall of a Remarkable Eclipsing Binary Star,” presented by James Davenport (University of Washington), AAS 237, Jan. 11, 2021, Session 133.06 (Binary Stellar Systems I) (2) AAS press conference “Evolving Stars & Nebulae I,” Thursday, Jan. 14, 2021 at 4:30 p.m. U.S. Eastern Time (1:30 p.m. U.S. Pacific Time)

Provided by University of Washington