Cold antimatter for quantum state-resolved precision measurements

Why does the universe contain matter and (virtually) no antimatter? The BASE international research collaboration at the European Organisation for Nuclear Research (CERN) in Geneva, headed by Professor Dr Stefan Ulmer from Heinrich Heine University Düsseldorf (HHU), has achieved an experimental breakthrough in this context. It can contribute to measuring the mass and magnetic moment of antiprotons more precisely than ever before -- and thus identify possible matter-antimatter asymmetries. BASE has developed a trap, which can cool individual antiprotons much more rapidly than in the past, as the researchers now explain in the scientific journal Physical Review Letters.

After the Big Bang more than 13 billion years ago, the universe was full of high-energy radiation, which constantly generated pairs of matter and antimatter particles such as protons and antiprotons. When such a pair collides, the particles are annihilated and converted into pure energy again. So, all in all, exactly the same quantities of matter and antimatter should be generated and annihilated again, meaning that the universe should be largely matterless as a consequence.

However, there is clearly an imbalance -- an asymmetry -- as material objects do exist. A minuscule amount more matter than antimatter has been generated -- which contradicts the standard model of particle physics. Physicists have therefore been seeking to expand the standard model for decades. To this end, they also need extremely precise measurements of fundamental physical parameters.

This is the starting point for the BASE collaboration ("Baryon Antibaryon Symmetry Experiment"). It involves the universities in Düsseldorf, Hanover, Heidelberg, Mainz and Tokyo, the Swiss Federal Institute of Technology in Zurich and the research facilities at CERN in Geneva, the GSI Helmholtz Centre in Darmstadt, the Max Planck Institute for Nuclear Physics in Heidelberg, the National Metrology Institute of Germany (PTB) in Braunschweig and RIKEN in Wako/Japan.

"The central question we are seeking to answer is: Do matter particles and their corresponding antimatter particles weigh exactly the same and do they have exactly the same magnetic moments, or are there minuscule differences?" explains Professor Stefan Ulmer, spokesperson of BASE. He is a professor at the Institute for Experimental Physics at HHU and also conducts research at CERN and RIKEN.

The physicists want to take extremely high resolution measurements of the so-called spin-flip -- quantum transitions of the proton spin -- for individual, ultra-cold and thus extremely low-energy antiprotons; i.e. the change in orientation of the spin of the proton. "From the measured transition frequencies, we can, among other things, determine the magnetic moment of the antiprotons -- their minute internal bar magnets, so to speak," explains Ulmer, adding: "The aim is to see with an unprecedented level of accuracy whether these bar magnets in protons and antiprotons have the same strength."

Preparing individual antiprotons for the measurements in a way that enables such levels of accuracy to be achieved is an extremely time-consuming experimental task. The BASE collaboration has now taken a decisive step forward in this regard.

Dr Barbara Maria Latacz from CERN and lead author of the study that has now been published as an "editor's suggestion" in Physical Review Letters, says: "We need antiprotons with a maximum temperature of 200 mK, i.e. extremely cold particles. This is the only way to differentiate between various spin quantum states. With previous techniques, it took 15 hours to cool antiprotons, which we obtain from the CERN accelerator complex, to this temperature. Our new cooling method shortens this period to eight minutes."

The researchers achieved this by combining two so-called Penning traps into a single device, a "Maxwell's daemon cooling double trap." This trap makes it possible to prepare solely the coldest antiprotons on a targeted basis and use them for the subsequent spin-flip measurement; warmer particles are rejected. This eliminates the time needed to cool the warmer antiprotons.

The significantly shorter cooling time is needed to obtain the required measurement statistics in a significantly shorter period of time so that measuring uncertainties can be reduced further. Latacz: "We need at least 1,000 individual measurement cycles. With our new trap, we need a measurement time of around one month for this -- compared with almost ten years using the old technique, which would be impossible to realise experimentally."

Ulmer: "With the BASE trap, we have already been able to measure that the magnetic moments of protons and antiprotons differ by max. one billionth -- we are talking about 10-9. We have been able to improve the error rate of the spin identification by more than a factor of 1,000. In the next measurement campaign, we are hoping to improve magnetic moment accuracy to 10-10."

Professor Ulmer on plans for the future: "We want to construct a mobile particle trap, which we can use to transport antiprotons generated at CERN in Geneva to a new laboratory at HHU. This is set up in such a way that we can hope to improve the accuracy of measurements by at least a further factor of 10."

Background: Traps for fundamental particles

Traps can store individual electrically charged fundamental particles, their antiparticles or even atomic nuclei for long periods of time using magnetic and electric fields. Storage periods of over ten years are possible. Targeted particle measurements can then be made in the traps.

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Brightest and fastest-growing: Astronomers identify record-breaking quasar

Using the European Southern Observatory's (ESO) Very Large Telescope (VLT), astronomers have characterised a bright quasar, finding it to be not only the brightest of its kind, but also the most luminous object ever observed. Quasars are the bright cores of distant galaxies and they are powered by supermassive black holes. The black hole in this record-breaking quasar is growing in mass by the equivalent of one Sun per day, making it the fastest-growing black hole to date.

The black holes powering quasars collect matter from their surroundings in a process so energetic that it emits vast amounts of light.

So much so that quasars are some of the brightest objects in our sky, meaning even distant ones are visible from Earth.

As a general rule, the most luminous quasars indicate the fastest-growing supermassive black holes.

"We have discovered the fastest-growing black hole known to date. It has a mass of 17 billion Suns, and eats just over a Sun per day. This makes it the most luminous object in the known Universe," says Christian Wolf, an astronomer at the Australian National University (ANU) and lead author of the study published today in Nature Astronomy. The quasar, called J0529-4351, is so far away from Earth that its light took over 12 billion years to reach us.

The matter being pulled in toward this black hole, in the form of a disc, emits so much energy that J0529-4351 is over 500 trillion times more luminous than the Sun.

"All this light comes from a hot accretion disc that measures seven light-years in diameter -- this must be the largest accretion disc in the Universe," says ANU PhD student and co-author Samuel Lai.

Seven light-years is about 15,000 times the distance from the Sun to the orbit of Neptune.

And, remarkably, this record-breaking quasar was hiding in plain sight.

"It is a surprise that it has remained unknown until today, when we already know about a million less impressive quasars. It has literally been staring us in the face until now," says co-author Christopher Onken, an astronomer at ANU.

He added that this object showed up in images from the ESO Schmidt Southern Sky Survey dating back to 1980, but it was not recognised as a quasar until decades later.

Finding quasars requires precise observational data from large areas of the sky.

The resulting datasets are so large, researchers often use machine-learning models to analyse them and tell quasars apart from other celestial objects.

However, these models are trained on existing data, which limits the potential candidates to objects similar to those already known.

If a new quasar is more luminous than any other previously observed, the programme might reject it and classify it instead as a star not too distant from Earth.

An automated analysis of data from the European Space Agency's Gaia satellite passed over J0529-4351 for being too bright to be a quasar, suggesting it to be a star instead.

The researchers identified it as a distant quasar last year using observations from the ANU 2.3-metre telescope at the Siding Spring Observatory in Australia.

Discovering that it was the most luminous quasar ever observed, however, required a larger telescope and measurements from a more precise instrument.

The X-shooter spectrograph on ESO's VLT in the Chilean Atacama Desert provided the crucial data.

The fastest-growing black hole ever observed will also be a perfect target for the GRAVITY+ upgrade on ESO's VLT Interferometer (VLTI), which is designed to accurately measure the mass of black holes, including those far away from Earth.

Additionally, ESO's Extremely Large Telescope (ELT), a 39-metre telescope under construction in the Chilean Atacama Desert, will make identifying and characterising such elusive objects even more feasible.

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Glacier Loss Day indi­cates record break­ing glacier melt

In the summer of 2022, one of Tyrol's largest glaciers experienced its most significant loss of mass on record. Last year, the Hintereisferner in Tyrol, Austria, reached its Glacier Loss Day (GLD) earlier than ever before. The GLD serves as an indicator of a glacier's health throughout the year, similar to how the Earth Overshoot Day measures Earth's resource consumption. Annelies Voordendag, together with a team of glaciologists at the Department of Atmospheric and Cryospheric Sciences at the University of Innsbruck, employs cutting-edge laser scanning techniques to determine the GLD.

The Hintereisferner, located at the back of the Tyrolean Ötztal, has been closely monitored for more than 100 years, and there have been continuous records of its mass balance since 1952. This makes it one of the best-studied glaciers in the Alps and has been key to glacier and climate research at the University of Innsbruck for decades. Since 2016, the researchers have also been surveying the glacier with a worldwide unique system: the surface of the glacier is scanned daily with a terrestrial laser scanner returning the glacier surface elevation changes. This way, the change in the volume of the Hintereisferner is monitored in real time. Innsbruck glaciologist Annelies Voordendag led the measurement on site at the Hintereisferner, the results of the researchers' investigations have now been published as highlighted article in the journal The Cryosphere.

"Already in the early summer of 2022, it became clear that the day when the ice the glacier gained during the winter starts melting away would be reached very soon. We call this day the 'Glacier Loss Day' or GLD for short. It can be compared to the Earth Overshoot Day, which marks the date when we use up more natural resources than the Earth can renew in a year," explains Annelies Voordendag. Monitoring a glacier's volume and mass alterations on a daily basis provides a quick assessment of its condition in a given year.

Observing glaciers' health

When the GLD arrives, it means the glacier is no longer in balance with the natural conditions for that year. The earlier the GLD happens, the more time is left in the remaining summer that the glacier likely will lose volume and thus, mass. "We track the daily volume changes with the automated terrestrial laser scanninng setup overlooking the glacier and derive the day that the mass gained during winter has been lost," says Voordendag. In 2022 the GLD was measured on the 23rd of June. In the two previous years, Glacier Loss Day was reached only in the middle of August.

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Despite doubts from quantum physicists: Einstein's theory of relativity reaffirmed

One of the most basic assumptions of fundamental physics is that the different properties of mass -- weight, inertia and gravitation -- always remain the same in relation to each other. Without this equivalence, Einstein's theory of relativity would be contradicted and our current physics textbooks would have to be rewritten. Although all measurements to date confirm the equivalence principle, quantum theory postulates that there should be a violation. This inconsistency between Einstein's gravitational theory and modern quantum theory is the reason why ever more precise tests of the equivalence principle are particularly important.

A team from the Center of Applied Space Technology and Microgravity (ZARM) at University of Bremen, in collaboration with the Institute of Geodesy (IfE) at Leibniz University Hannover, has now succeeded in proving with 100 times greater accuracy that passive gravitational mass and active gravitational mass are always equivalent -- regardless of the particular composition of the respective masses.

The research was conducted within the framework of the Cluster of Excellence "QuantumFrontiers." Today, the team published their findings as a highlights article in the scientific journal Physical Review Letters.

Physical context

Inertial mass resists acceleration. For example, it causes you to be pushed backwards into your seat when the car starts. Passive gravitational mass reacts on gravity and results in our weight on Earth. Active gravitational mass refers to the force of gravitation exerted by an object, or more precisely, the size of its gravitational field. The equivalence of these properties is fundamental to general relativity. Therefore, both the equivalence of inertial and passive gravitational mass and the equivalence of passive and active gravitational mass are being tested with increasing precision.

What was the study about?

If we assume that passive and active gravitational mass are not equal -- that their ratio depends on the material -- then objects made of different materials with a different centre of mass would accelerate themselves. Since the Moon consists of an aluminium shell and an iron core, with centres of mass offset against each other, the Moon should accelerate. This hypothetical change in speed could be measured with high precision, via "Lunar Laser Ranging." This involves pointing lasers from Earth at reflectors on the Moon placed there by the Apollo missions and the Soviet Luna programme. Since then, round trip travel times of laser beams are recorded. The research team analysed "Lunar Laser Ranging" data collected over a period of 50 years, from 1970 to 2022, and investigated such mass difference effects. Since no effect was found, this means that the passive and active gravitational masses are equal to approximately 14 decimal places. This estimate is a hundred times more accurate than the best previous study, dating back to 1986.

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M87 in 3D: New view of galaxy helps pin down mass of the black hole at its core

Seen from Earth, the giant elliptical galaxy M87 is just a two-dimensional blob, though one that appears perfectly symmetrical and thus a favored target of amateur astronomers.

Yet, a new, highly detailed analysis of the motion of stars around its central supermassive black hole — the first black hole to be imaged by the Event Horizon Telescope (EHT) in 2019 — reveals that it's not as perfect as it looks.

In fact, M87 is highly asymmetrical, like a russet potato. The galaxy's shortest axis is about three-fourths (72.2%) the length of its long axis, while the intermediate axis is about seven-eighths (84.5%) that of the long axis.

Knowing this, University of California, Berkeley, astronomers were able to determine the mass of the supermassive black hole at the galaxy's core to a high precision, estimating it at 5.37 billion times the mass of the sun. By comparison, our own Milky Way has at its center a massive black hole only 4 million times the mass of the sun.

They also were able to measure the rotation of the galaxy, which is a relatively sedate 25 kilometers per second. Interestingly, it is not rotating around any of the galaxy's major axes, but instead about an axis that is 40 degrees away from the long axis of its 2D image as observed by the Hubble Space Telescope.

The stereo reconstruction of the M87 galaxy and the more precise figure for the mass of the central black hole could help astrophysicists learn about a characteristic of the black hole they've had no way to determine before for any black hole: its spin.

"Now that we know the direction of the net rotation of stars in M87 and have an updated mass of the black hole, we can combine this information with the amazing data from the EHT team to constrain the spin," said Chung-Pei Ma, a UC Berkeley professor of astronomy and of physics who led the research. "This may point toward a certain direction and range of spin for the black hole, which would be remarkable. We are working on this.”

Further analyses to determine the true shape of giant elliptical galaxies — the galaxies with the largest black holes at their cores — will help astronomers understand better how large galaxies and large black holes form and could help astronomers better interpret gravitational wave signals. Ma leads a long-term study of supermassive black holes that is dubbed MASSIVE.

The results were published online March 15 in The Astrophysical Journal Letters (ApJ Letters).

Determining a galaxy's 3D shape

While spiral galaxies tend to be small, rotate quickly and have a well-recognized pancake shape, giant elliptical galaxies rotate slowly and have a blobby appearance, their 3D shape difficult to discern. Like M87, the largest galaxy in the massive Virgo Cluster of galaxies, giant elliptical galaxies have grown from the merger of many other galaxies. That's likely the reason M87's central black hole is so large — it assimilated the central black holes of all the galaxies it swallowed. In all, the galaxy contains about 100 billion stars, 10 times larger than the Milky Way.

Ma, UC Berkeley graduate student and lead author Emily Liepold, and Jonelle Walsh at Texas A&M University in College Station were able to determine the 3D shape of M87 thanks to a relatively new precision instrument mounted on the Keck II Telescope, one of the twin 10-meter Keck telescopes atop Mauna Kea, a volcano in Hawai'i. Called the Keck Cosmic Web Imager (KCWI), the integral field spectrometer allowed Ma and her team to measure the spectra of stars in the center of the galaxy.

They pointed the telescope at 62 adjacent locations in the galaxy, completely covering a region about 70,000 light-years across, and recorded the spectra of stars within that region. The observations span the central region — about 3,000 light-years across — where gravity is largely dominated by the supermassive black hole, as well as the outer part dominated by dark matter. Though the telescope cannot resolve individual stars — M87 lies about 53 million light- years from Earth — the spectra can reveal the range of velocities within each pixel of each image, enough information to calculate the gravitational mass they're orbiting.

"It's sort of like looking at a swarm of 100 billion bees that are going around in their own happy orbits," said Ma, the Judy Chandler Webb Professor in the Physical Sciences. "Though we are looking at them from a distance and can’t discern individual bees, we are getting very detailed information about their collective velocities. It's really the superb sensitivity of this spectrograph that allowed us to map out M87 so comprehensively."

This is the first time KCWI has been used to reconstruct the geometry of a distant galaxy, and M87 is one of only a handful of giant elliptical galaxies whose 3D structure has been determined. Ma’s team had previously determined the 3D structure of two other giant elliptical galaxies, NGC 1453 and NGC 2693, both harboring smaller black holes than M87.

The researchers took the data obtained during four nights of Keck observations between 2020 and 2022, along with earlier photometric data for M87 from NASA's Hubble Space Telescope, and compared them to computer model predictions of how stars move around the center of a triaxial galaxy. The best fit to the data — axial ratios of 1 to 0.84 to 0.72 — then allowed them to calculate the black hole mass.

"The Keck data are so good that we can measure the intrinsic shape of M87 along with the black hole at the same time," Ma said. "We made the first measurement of the actual 3D shape of the galaxy. And since we allowed the swarm of bees to have a more general shape than just a sphere or disk, we have a more robust dynamical measurement of the mass of the central black hole that is governing the bees’ orbiting velocities."

The authors dedicated their manuscript to the late astronomer Wallace "Wal" Sargent, who first suggested that a supermassive black hole lurked at the center of M87 and calculated its mass to be about 5 billion solar masses.

"His number is a twiddle with our error bars, which is very interesting to see after decades of work," said Ma, who credits Sargent with being a mentor when she was a postdoctoral fellow at the California Institute of Technology.

The previous estimate of the mass of the supermassive black hole in M87, published in 2011, was based on a similar analysis of the dynamical movement of stars around the black hole, though that study assumed the galaxy was axisymmetric. The number, 6.14 billion solar masses, is within error bars of the new, more precise estimate. When imaging the black hole four years ago, the EHT scientists estimated the black hole mass to be 6.5 billion solar masses, 21% higher than the new number.

Interestingly, the dark matter within the volume of the galaxy they analyzed is much higher than that of the black hole — about 388 billion solar masses, or 67% of the entire mass of M87. Though the identity of dark matter is still a mystery, it makes up about 85% of the mass of the universe.

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Artificial intelligence discovers secret equation for 'weighing' galaxy clusters

Astrophysicists at the Institute for Advanced Study, the Flatiron Institute and their colleagues have leveraged artificial intelligence to uncover a better way to estimate the mass of colossal clusters of galaxies. The AI discovered that by just adding a simple term to an existing equation, scientists can produce far better mass estimates than they previously had.

The improved estimates will enable scientists to calculate the fundamental properties of the universe more accurately, the astrophysicists reported March 17, 2023, in the Proceedings of the National Academy of Sciences.

"It's such a simple thing; that's the beauty of this," says study co-author Francisco Villaescusa-Navarro, a research scientist at the Flatiron Institute's Center for Computational Astrophysics (CCA) in New York City. "Even though it's so simple, nobody before found this term. People have been working on this for decades, and still they were not able to find this."

The work was led by Digvijay Wadekar of the Institute for Advanced Study in Princeton, New Jersey, along with researchers from the CCA, Princeton University, Cornell University and the Center for Astrophysics | Harvard & Smithsonian.

Understanding the universe requires knowing where and how much stuff there is. Galaxy clusters are the most massive objects in the universe: A single cluster can contain anything from hundreds to thousands of galaxies, along with plasma, hot gas and dark matter. The cluster's gravity holds these components together. Understanding such galaxy clusters is crucial to pinning down the origin and continuing evolution of the universe.

Perhaps the most crucial quantity determining the properties of a galaxy cluster is its total mass. But measuring this quantity is difficult -- galaxies cannot be 'weighed' by placing them on a scale. The problem is further complicated because the dark matter that makes up much of a cluster's mass is invisible. Instead, scientists deduce the mass of a cluster from other observable quantities.

In the early 1970s, Rashid Sunyaev, current distinguished visiting professor at the Institute for Advanced Study's School of Natural Sciences, and his collaborator Yakov B. Zel'dovich developed a new way to estimate galaxy cluster masses. Their method relies on the fact that as gravity squashes matter together, the matter's electrons push back. That electron pressure alters how the electrons interact with particles of light called photons. As photons left over from the Big Bang's afterglow hit the squeezed material, the interaction creates new photons. The properties of those photons depend on how strongly gravity is compressing the material, which in turn depends on the galaxy cluster's heft. By measuring the photons, astrophysicists can estimate the cluster's mass.

However, this 'integrated electron pressure' is not a perfect proxy for mass, because the changes in the photon properties vary depending on the galaxy cluster. Wadekar and his colleagues thought an artificial intelligence tool called 'symbolic regression' might find a better approach. The tool essentially tries out different combinations of mathematical operators -- such as addition and subtraction -- with various variables, to see what equation best matches the data.

Wadekar and his collaborators 'fed' their AI program a state-of-the-art universe simulation containing many galaxy clusters. Next, their program, written by CCA research fellow Miles Cranmer, searched for and identified additional variables that might make the mass estimates more accurate.

AI is useful for identifying new parameter combinations that human analysts might overlook. For example, while it is easy for human analysts to identify two significant parameters in a dataset, AI can better parse through high volumes, often revealing unexpected influencing factors.

"Right now, a lot of the machine-learning community focuses on deep neural networks," Wadekar explained. "These are very powerful, but the drawback is that they are almost like a black box. We cannot understand what goes on in them. In physics, if something is giving good results, we want to know why it is doing so. Symbolic regression is beneficial because it searches a given dataset and generates simple mathematical expressions in the form of simple equations that you can understand. It provides an easily interpretable model."

The researchers' symbolic regression program handed them a new equation, which was able to better predict the mass of the galaxy cluster by adding a single new term to the existing equation. Wadekar and his collaborators then worked backward from this AI-generated equation and found a physical explanation. They realized that gas concentration correlates with the regions of galaxy clusters where mass inferences are less reliable, such as the cores of galaxies where supermassive black holes lurk. Their new equation improved mass inferences by downplaying the importance of those complex cores in the calculations. In a sense, the galaxy cluster is like a spherical doughnut. The new equation extracts the jelly at the center of the doughnut that can introduce larger errors, and instead concentrates on the doughy outskirts for more reliable mass inferences.

The researchers tested the AI-discovered equation on thousands of simulated universes from the CCA's CAMELS suite. They found that the equation reduced the variability in galaxy cluster mass estimates by around 20 to 30 percent for large clusters compared with the currently used equation.

The new equation can provide observational astronomers engaged in upcoming galaxy cluster surveys with better insights into the mass of the objects they observe. "There are quite a few surveys targeting galaxy clusters [that] are planned in the near future," Wadekar noted. "Examples include the Simons Observatory, the Stage 4 CMB experiment and an X-ray survey called eROSITA. The new equations can help us in maximizing the scientific return from these surveys."

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Hubble directly measures mass of a lone white dwarf

Astronomers using NASA's Hubble Space Telescope have for the first time directly measured the mass of a single, isolated white dwarf -- the surviving core of a burned-out, Sun-like star.

Researchers found that the white dwarf is 56 percent the mass of our Sun. This agrees with earlier theoretical predictions of the white dwarf's mass and corroborates current theories of how white dwarfs evolve as the end product of a typical star's evolution. The unique observation yields insights into theories of the structure and composition of white dwarfs.

Until now, previous white dwarf mass measurements have been gleaned from observing white dwarfs in binary star systems. By watching the motion of two co-orbiting stars, straightforward Newtonian physics can be used to measure their masses. However, these measurements can be uncertain if the white dwarf's companion star is in a long-period orbit of hundreds or thousands of years. Orbital motion can be measured by telescopes only over a brief slice of the dwarf's orbital motion.

For this companion-less white dwarf, researchers had to employ a trick of nature, called gravitational microlensing. The light from a background star was slightly deflected by the gravitational warping of space by the foreground dwarf star. As the white dwarf passed in front of the background star, microlensing caused the star to appear temporarily offset from its actual position on the sky.

The results are reported in the Monthly Notices of the Royal Astronomical Society. The lead author is Peter McGill, formerly of the University of Cambridge (now based at the University of California, Santa Cruz).

McGill used Hubble to precisely measure how light from a distant star bent around the white dwarf, known as LAWD 37, causing the background star to temporarily change its apparent position in the sky.

Kailash Sahu of the Space Telescope Science Institute in Baltimore, Maryland, the principal Hubble investigator on this latest observation, first used microlensing in 2017 to measure the mass of another white dwarf, Stein 2051 B. But that dwarf is in a widely separated binary system. "Our latest observation provides a new benchmark because LAWD 37 is all by itself," Sahu said.

The collapsed remains of a star that burned out 1 billion years ago, LAWD 37 has been extensively studied because it is only 15 light-years away in the constellation Musca. "Because this white dwarf is relatively close to us, we've got lots of data on it -- we've got information about its spectrum of light, but the missing piece of the puzzle has been a measurement of its mass," said McGill.

The team zeroed in on the white dwarf thanks to ESA's Gaia space observatory, which makes extraordinarily precise measurements of nearly 2 billion star positions. Multiple Gaia observations can be used to track a star's motion. Based on this data, astronomers were able to predict that LAWD 37 would briefly pass in front of a background star in November 2019.

Once this was known, Hubble was used to precisely measure over several years how the background star's apparent position in the sky was temporarily deflected during the white dwarf's passage.

"These events are rare, and the effects are tiny," said McGill. "For instance, the size of our measured offset is like measuring the length of a car on the Moon as seen from Earth."

Since the light from the background star was so faint, the main challenge for astronomers was extracting its image from the glare of the white dwarf, which is 400 times brighter than the background star. Only Hubble can make these kinds of high-contrast observations in visible light.

"The precision of LAWD 37's mass measurement allows us to test the mass-radius relationship for white dwarfs," said McGill. "This means testing the theory of degenerate matter (a gas so super-compressed under gravity it behaves more like solid matter) under the extreme conditions inside this dead star," he added.

The researchers say their results open the door for future event predictions with Gaia data. In addition to Hubble, these alignments can now be detected with NASA's James Webb Space Telescope. Because Webb works at infrared wavelengths, the blue glow of a foreground white dwarf looks dimmer in infrared light, and the background star looks brighter.

Based on Gaia's predictive powers, Sahu is observing another white dwarf, LAWD 66, with NASA's James Webb Space Telescope. The first observation was done in 2022. More observations will be taken as the deflection peaks in 2024 and then subsides.

"Gaia has really changed the game -- it's exciting to be able to use Gaia data to predict when events will happen, and then observe them happening," said McGill. "We want to continue measuring the gravitational microlensing effect and obtain mass measurements for many more types of stars."

In his 1915 theory of general relativity, Einstein predicted that when a massive compact object passes in front of a background star, the light from the star would bend around the foreground object due to the warping of space by its gravitational field.

Exactly a century before this latest Hubble observation, in 1919, two British-organized expeditions to the southern hemisphere first detected this lensing effect during a solar eclipse on May 19th. It was hailed as the first experimental proof of general relativity -- that gravity warps space. However, Einstein was pessimistic that the effect could ever be detected for stars outside our solar system because of the precision involved. "Our measurement is 625 times smaller than the effect measured at the 1919 solar eclipse," said McGill.

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Two out of three glaciers could be lost by 2100

Assistant Professor David Rounce of Civil and Environmental Engineering led an international effort to produce new projections of glacier mass loss through the century under different emissions scenarios. The projections were aggregated into global temperature change scenarios to support adaptation and mitigation discussions, such as those at the recent United Nations Conference of Parties (COP 27). His work showed that the world could lose as much as 41 percent of its total glacier mass this century -- or as little as 26 percent -- depending on today's climate change mitigation efforts.

Specifically, Rounce and his team found that in a future scenario with continued investment in fossil fuels, over 40 percent of the glacial mass will be gone within the century, and over 80 percent of glaciers by number could well disappear. Even in a best-case, low-emissions scenario, where the increase in global mean temperature is limited to +1.5° C relative to pre-industrial levels, over 25 percent of glacial mass will be gone and nearly 50 percent of glaciers by number are projected to disappear. A majority of these lost glaciers are small (less than one km2) by glacial standards, but their loss can negatively impact local hydrology, tourism, glacier hazards, and cultural values.

His work provides better context for regional glacier modeling, and he hopes it will spur climate policymakers to lower temperature change goals beyond the 2.7° C mark that pledges from COP-26 are projected to hit. Smaller glacial regions like Central Europe and Western Canada and the United States will be disproportionately affected by temperatures rising more than 2° C. At a 3° C rise, glaciers in these regions almost disappear completely.

Rounce noted that the way in which glaciers respond to changes in climate takes a long time. He describes the glaciers as extremely slow-moving rivers. Cutting emissions today will not remove previously emitted greenhouse gasses, nor can it instantly halt the inertia they contribute to climate change, meaning even a complete halt to emissions would still take between 30 and 100 years to be reflected in glacier mass loss rates.

Many processes govern how glaciers lose mass and Rounce's study advances how models account for different types of glaciers, including tidewater and debris-covered glaciers. Tidewater glaciers refer to glaciers that terminate in the ocean, which causes them to lose a lot of mass at this interface. Debris-covered glaciers refer to glaciers that are covered by sand, rocks, and boulders. Prior work by Rounce has shown that the thickness and distribution of debris cover can have a positive or negative effect on glacial melt rates across an entire region, depending on the debris thickness. In this newest work, he found that accounting for these processes had relatively little impact on the global glacier projections, but substantial differences in mass loss were found when analyzing individual glaciers.

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Cosmic chocolate pralines: General neutron star structure revealed

So far, little is known about the interior of neutron stars, those extremely compact objects that can form after the death of a star: the mass of our sun or even more is compressed into a sphere with the diameter of a large city. Since their discovery more than 60 years ago, scientists have been trying to decipher their structure. The greatest challenge is to simulate the extreme conditions inside neutron stars, as they can hardly be recreated on Earth in the laboratory. There are therefore many models in which various properties -- from density and temperature -- are described with the help of so-called equations of state. These equations attempt to describe the structure of neutron stars from the stellar surface to the inner core.

Now physicists at Goethe University Frankfurt have succeeded in adding further crucial pieces to the puzzle. The working group led by Prof. Luciano Rezzolla at the Institute of Theoretical Physics developed more than a million different equations of state that satisfy the constraints set by data obtained from theoretical nuclear physics on the one hand, and by astronomical observations on the other. When evaluating the equations of state, the working group made a surprising discovery: "Light" neutron stars (with masses smaller than about 1.7 solar masses) seem to have a soft mantle and a stiff core, whereas "heavy" neutron stars (with masses larger than 1.7 solar masses) instead have a stiff mantle and a soft core. "This result is very interesting because it gives us a direct measure of how compressible the centre of neutron stars can be," says Prof. Luciano Rezzolla, "Neutron stars apparently behave a bit like chocolate pralines: light stars resemble those chocolates that have a hazelnut in their centre surrounded by soft chocolate, whereas heavy stars can be considered more like those chocolates where a hard layer contains a soft filling."

Crucial to this insight was the speed of sound, a study focus of Bachelor's student Sinan Altiparmak. This quantity measure describes how fast sound waves propagate within an object and depends on how stiff or soft matter is. Here on Earth, the speed of sound is used to explore the interior of the planet and discover oil deposits.

By modelling the equations of state, the physicists were also able to uncover other previously unexplained properties of neutron stars. For example, regardless of their mass, they very probably have a radius of only 12 km. Thus, they are just as large in diameter as Goethe University's hometown Frankfurt. Author Dr. Christian Ecker explains: "Our extensive numerical study not only allows us to make predictions for the radii and maximum masses of neutron stars, but also to set new limits on their deformability in binary systems, that is, how strongly they distort each other through their gravitational fields. These insights will become particularly important to pinpoint the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars."

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Uncovering the massive quantum mysteries of black holes

Bizarre quantum properties of black holes -- including their mind-bending ability to have different masses simultaneously -- have been confirmed by University of Queensland physicists.

A UQ-led team of theoretical physicists, headed by PhD candidate Joshua Foo, ran calculations that reveal surprising black hole quantum phenomena.

"Black holes are an incredibly unique and fascinating feature of our universe," Mr Foo said.

"They're created when gravity squeezes a vast amount of matter incredibly densely into a tiny space, creating so much gravitational pull that even light cannot escape.

"It's a phenomenon that can be triggered by a dying star.

"But, until now, we haven't deeply investigated whether black holes display some of the weird and wonderful behaviours of quantum physics.

"One such behaviour is superposition, where particles on a quantum scale can exist in multiple states at the same time.

"This is most commonly illustrated by Schrödinger's cat, which can be both dead and alive simultaneously.

"But, for black holes, we wanted to see whether they could have wildly different masses at the same time, and it turns out they do.

"Imagine you're both broad and tall, as well as short and skinny at the same time -- it's a situation which is intuitively confusing since we're anchored in the world of traditional physics.

"But this is reality for quantum black holes."

To reveal this, the team developed a mathematical framework allowing us to "place" a particle outside a theoretical mass-superposed black hole.

Mass was looked at specifically, as it is a defining feature of a black hole, and as it is plausible that quantum black holes would naturally have mass superposition.

Research co-supervisor, Dr Magdalena Zych, said that the research in fact reinforces conjectures raised by pioneers of quantum physics.

"Our work shows that the very early theories of Jacob Bekenstein -- an American and Israeli theoretical physicist who made fundamental contributions to the foundation of black hole thermodynamics -- were on the money," she said.

"He postulated that black holes can only have masses that are of certain values, that is, they must fall within certain bands or ratios -- this is how energy levels of an atom works, for example.

"Our modelling showed that these superposed masses were, in fact, in certain determined bands or ratios -- as predicted by Bekenstein.

"We didn't assume any such pattern going in, so the fact we found this evidence was quite surprising.

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Stars determine their own masses

Last year, a team of astrophysicists including key members from Northwestern University launched STARFORGE, a project that produces the most realistic, highest-resolution 3D simulations of star formation to date. Now, the scientists have used the highly detailed simulations to uncover what determines the masses of stars, a mystery that has captivated astrophysicists for decades.

In a new study, the team discovered that star formation is a self-regulatory process. In other words, stars themselves set their own masses. This helps explain why stars formed in disparate environments still have similar masses. The new finding may enable researchers to better understand star formation within our own Milky Way and other galaxies.

The study was published last week in the Monthly Notices of the Royal Astronomical Society. The collaborative team included experts from Northwestern, University of Texas at Austin (UT Austin), Carnegie Observatories, Harvard University and the California Institute of Technology. The lead author of the new study is Dávid Guszejnov, a postdoctoral fellow at UT Austin.

"Understanding the stellar initial mass function is such an important problem because it impacts astrophysics across the board -- from nearby planets to distant galaxies," said Northwestern's Claude-André Faucher-Giguère, a study co-author. "This is because stars have relatively simple DNA. If you know the mass of a star, then you know most things about the star: how much light it emits, how long it will live and what will happen to it when it dies. The distribution of stellar masses is thus critical for whether planets that orbit stars can potentially sustain life, as well as what distant galaxies look like."

Faucher-Giguère is an associate professor of physics and astronomy in Northwestern's Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).

Outer space is filled with giant clouds, consisting of cold gas and dust. Slowly, gravity pulls far-flung specks of this gas and dust toward each other to form dense clumps. Materials in these clumps fall inward, crashing and sparking heat to create a newborn star.

Surrounding each of these "protostars" is a rotating disk of gas and dust. Every planet in our solar system was once specks in such a disk around our newborn sun. Whether or not planets orbiting a star could host life is dependent on the mass of the star and how it formed. Therefore, understanding star formation is crucial to determining where life can form in the universe.

"Stars are the atoms of the galaxy," said Stella Offner, associate professor of astronomy at UT Austin. "Their mass distribution dictates whether planets will be born and if life might develop."

Every subfield in astronomy depends on the mass distribution of stars -- or initial mass function (IMF) -- which has proved challenging for scientists to model correctly. Stars much bigger than our sun are rare, making up only 1% of newborn stars. And, for every one of these stars there are up to 10 sun-like stars and 30 dwarf stars. Observations found that no matter where we look in the Milky Way these ratios (i.e., the IMF) are the same, for both newly formed star clusters and for those that are billions of years old.

This is the mystery of the IMF. Every population of stars in our galaxy, and in all the dwarf galaxies that surround us, has this same balance -- even though their stars were born under wildly different conditions over billions of years. In theory, the IMF should vary dramatically, but it is virtually universal, which has puzzled astronomers for decades.

"For a long time, we have been asking why," Guszejnov said. "Our simulations followed stars from birth to the natural endpoint of their formation to solve this mystery."

The new simulations, however, showed that stellar feedback, in an effort to oppose gravity, pushes stellar masses toward the same mass distribution. These simulations are the first to follow the formation of individual stars in a collapsing giant cloud, while also capturing how these newly formed stars interact with their surroundings by giving off light and shedding mass via jets and winds -- a phenomenon referred to as "stellar feedback."

The STARFORGE project is a multi-institutional initiative, co-led by Guszejnov and Michael Grudić of Carnegie Observatories. Grudić was a CIERA postdoctoral fellow at Northwestern when the project was initiated. STARFORGE simulations are the first to simultaneously model star formation, evolution and dynamics while accounting for stellar feedback, including jets, radiation, wind and nearby supernovae activity. While other simulations have incorporated individual types of stellar feedback, STARFORGE puts them all together to simulate how these various processes interact to affect star formation.

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Cells take out the trash before they divide

MIT researchers have discovered that before cells start to divide, they do a little cleanup, tossing out molecules that they appear not to need anymore.

Using a new method they developed for measuring the dry mass of cells, the researchers found that cells lose about 4 percent of their mass as they enter cell division. The researchers believe that this emptying of trash helps cells to give their offspring a "fresh start," without the accumulated junk of the parent cell.

"Our hypothesis is that cells might be throwing out things that are building up, toxic components or just things that don't function properly that you don't want to have there. It could allow the newborn cells to be born with more functional contents," says Teemu Miettinen, an MIT research scientist and the lead author of the new study.

Scott Manalis, the David H. Koch Professor of Engineering in the departments of Biological Engineering and Mechanical Engineering, and a member of the Koch Institute for Integrative Cancer Research, is the senior author of the paper, which appears today in eLife. MIT biological engineering undergraduates Kevin Ly and Alice Lam are also authors of the paper.

Measuring mass

Measuring the dry mass of a cell -- the weight of its contents not including the water -- is commonly done using a microscopy technique called quantitative phase microscopy. This technique can measure cell growth, but it does not reveal information about the molecular content of the dry mass and it is difficult to use with cells that grow in suspension.

Manalis' lab has previously developed a technique for measuring the buoyant mass of cells, which is their mass as they float in a fluid such as water. This method measures buoyant mass by flowing cells through a channel embedded in a vibrating cantilever, which can be done repeatedly to track changes in a particular cell's mass over many hours or days.

For their new study, the researchers wanted to adapt the technique so that it could be used to calculate the dry mass of cells, as well as the density of the dry mass. About 10 years ago, they had discovered that they could calculate a cell's dry mass if they first measured the cell in normal water and then in heavy water (which contains deuterium instead of ordinary hydrogen). These two measurements can be used to calculate the cell's dry mass.

However, heavy water is toxic to cells, so they were only able to obtain a single measurement per cell. Last year, Miettinen set out to see if he could design a system in which cells could be measured repeatedly with minimal exposure to heavy water.

In the system he came up with, cells are exposed to heavy water very briefly as they flow through microfluidic channels. It takes only one second for a cell to completely exchange its water content, so the researchers could measure the cell's mass when it was full of heavy water, compare it to the mass in normal water, and then calculate the dry mass.

"Our idea was that if we minimize the cells' exposure to the heavy water, we could engineer the system so that we could repeat this measurement over extended time periods without hurting the cell," Miettinen says. "That enabled us for the first time to track not just the dry mass of a cell, which is what others do using microscopic methods, but also the density of the dry mass, which informs us of the cell's biomolecular composition."

The researchers showed that their dry mass measurements qualitatively agreed with previous work using quantitative phase microscopy. And, in addition to providing density of the dry mass, the MIT team's method enables higher temporal resolution, which proved to be useful for revealing dynamics during mitosis (cell division).

Taking out the trash

In cells undergoing mitosis, the researchers used their new technique to study what happens to cell mass and composition during that process. In a 2019 paper, Miettinen and Manalis found that buoyant mass increases slightly as mitosis begins. However, other studies that used quantitative phase microscopy suggested that cells might retain or lose dry mass early in cell division.

In the new study, the MIT team measured three types of cancer cells, which are easier to study because they divide more frequently than healthy cells. To their surprise, the researchers found that the dry mass of cells actually decreases when they enter the cell division cycle. This mass is regained later on, before division is complete.

Further experiments revealed that as cells enter mitosis, they ramp up activity of a process called lysosomal exocytosis. Lysosomes are cell organelles that break down or recycle cellular waste products, and exocytosis is the process they use to jettison any molecules that aren't needed any more.

The researchers also found that the density of the dry mass increases as the cells lose dry mass, leading them to believe that the cells are losing low-density molecules such as lipids or lipoproteins. They hypothesize that cells use this process to clear out toxic molecules before dividing. "What we are seeing is that cells might be trying to throw out damaged components before dividing," Miettinen says.

The researchers speculate that their findings may help explain why neurons, which do not divide, are more likely to accumulate toxic proteins such as Tau or amyloid beta, which are linked to the development of Alzheimer's disease.

The findings could also be relevant to cancer: Cancer cells can expel some chemotherapy drugs using exocytosis, helping them to become resistant to the drugs. In theory, preventing exocytosis from occurring before cell division could help to make cancer cells more susceptible to such drugs.

"There are diseases where we might want upregulate exocytosis, for example in neurodegenerative diseases, but then there are diseases like cancer where maybe we want to dial it down," Miettinen says. "In the future, if we could better understand the molecular mechanism behind this, and find a way to trigger it outside of mitosis or prevent it during mitosis, we could really have a new toggle to use when treating disease."

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New simulations refine axion mass, refocusing dark matter search

Physicists searching -- unsuccessfully -- for today's most favored candidate for dark matter, the axion, have been looking in the wrong place, according to a new supercomputer simulation of how axions were produced shortly after the Big Bang 13.6 billion years ago.

Using new calculational techniques and one of the world's largest computers, Benjamin Safdi, assistant professor of physics at the University of California, Berkeley; Malte Buschmann, a postdoctoral research associate at Princeton University; and colleagues at MIT and Lawrence Berkeley National Laboratory simulated the era when axions would have been produced, approximately a billionth of a billionth of a billionth of a second after the universe came into existence and after the epoch of cosmic inflation.

The simulation at Berkeley Lab's National Research Scientific Computing Center (NERSC) found the axion's mass to be more than twice as big as theorists and experimenters have thought: between 40 and 180 microelectron volts (micro-eV, or ?eV), or about one 10-billionth the mass of the electron. There are indications, Safdi said, that the mass is close to 65 ?eV. Since physicists began looking for the axion 40 years ago, estimates of the mass have ranged widely, from a few ?eV to 500 ?eV.

"We provide over a thousandfold improvement in the dynamic range of our axion simulations relative to prior work and clear up a 40-year old question regarding the axion mass and axion cosmology," Safdi said.

The more definitive mass means that the most common type of experiment to detect these elusive particles -- a microwave resonance chamber containing a strong magnetic field, in which scientists hope to snag the conversion of an axion into a faint electromagnetic wave -- won't be able to detect them, no matter how much the experiment is tweaked. The chamber would have to be smaller than a few centimeters on a side to detect the higher-frequency wave from a higher-mass axion, Safdi said, and that volume would be too small to capture enough axions for the signal to rise above the noise.

"Our work provides the most precise estimate to date of the axion mass and points to a specific range of masses that is not currently being explored in the laboratory," he said. "I really do think it makes sense to focus experimental efforts on 40 to 180 ?eV axion masses, but there's a lot of work gearing up to go after that mass range."

One newer type of experiment, a plasma haloscope, which looks for axion excitations in a metamaterial -- a solid-state plasma -- should be sensitive to an axion particle of this mass, and could potentially detect one.

"The basic studies of these three-dimensional arrays of fine wires have worked out amazingly well, much better than we ever expected," said Karl van Bibber, a UC Berkeley professor of nuclear engineering who is building a prototype of the plasma haloscope while also participating in a microwave cavity axion search called the HAYSTAC experiment. "Ben's latest result is very exciting. If the post-inflation scenario is right, after four decades, discovery of the axion could be greatly accelerated."

If axions really exist.

The work will be published Feb. 25 in the journal Nature Communications.

Axion top candidate for dark matter

Dark matter is a mysterious substance that astronomers know exists -- it affects the movements of every star and galaxy -- but which interacts so weakly with the stuff of stars and galaxies that it has eluded detection. That doesn't mean dark matter can't be studied and even weighed. Astronomers know quite precisely how much dark matter exists in the Milky Way Galaxy and even in the entire universe: 85% of all matter in the cosmos.

To date, dark matter searches have focused on massive compact objects in the halo of our galaxy (called massive compact halo objects, or MACHOs), weakly interacting massive particles (WIMPs) and even unseen black holes. None turned up a likely candidate.

"Dark matter is most of the matter in the universe, and we have no idea what it is. One of the most outstanding questions in all of science is, 'What is dark matter?'" Safdi said. "We suspect it is a new particle we don't know about, and the axion could be that particle. It could be created in abundance in the Big Bang and be floating out there explaining observations that have been made in astrophysics."

Though not strictly a WIMP, the axion also interacts weakly with normal matter. It passes easily through the earth without disruption. It was proposed in 1978 as a new elementary particle that could explain why the neutron's spin does not precess or wobble in an electric field. The axion, according to theory, suppresses this precession in the neutron.

"Still to this day, the axion is the best idea we have about how to explain these weird observations about the neutron," Safdi said.

In the 1980s, the axion began to be seen also as a candidate for dark matter, and the first attempts to detect axions were launched. Using the equations of the well-vetted theory of fundamental particle interactions, the so-called Standard Model, in addition to the theory of the Big Bang, the Standard Cosmological Model, it is possible to calculate the axion's precise mass, but the equations are so difficult that to date we have only estimates, which have varied immensely. Since the mass is known so imprecisely, searches employing microwave cavities -- essentially elaborate radio receivers -- must tune through millions of frequency channels to try to find the one corresponding to the axion mass.

"With these axion experiments, they don't know what station they're supposed to be tuning to, so they have to scan over many different possibilities," Safdi said.

Safdi and his team produced the most recent, though incorrect, axion mass estimate that experimentalists are currently targeting. But as they worked on improved simulations, they approached a team from Berkeley Lab that had developed a specialized code for a better simulation technique called adaptive mesh refinement. During simulations, a small part of the expanding universe is represented by a three-dimensional grid over which the equations are solved. In adaptive mesh refinement, the grid is made more detailed around areas of interest and less detailed around areas of space where nothing much happens. This concentrates computing power on the most important parts of the simulation.

The technique allowed Safdi's simulation to see thousands of times more detail around the areas where axions are generated, allowing a more precise determination of the total number of axions produced and, given the total mass of dark matter in the universe, the axion mass. The simulation employed 69,632 physical computer processing unit (CPU) cores of the Cori supercomputer with nearly 100 terabytes of random access memory (RAM), making the simulation one of the largest dark matter simulations of any kind to date.

The simulation showed that after the inflationary epoch, little tornadoes, or vortices, form like ropey strings in the early universe and throw off axions like riders bucked from a bronco.

"You can think of these strings as composed of axions hugging the vortices while these strings whip around forming loops, connecting, undergoing a lot of violent dynamical processes during the expansion of our universe, and the axions hugging the sides of these strings are trying to hold on for the ride," Safdi said. "But when something too violent happens, they just get thrown off and whip away from these strings. And those axions which get thrown off of the strings end up becoming the dark matter much later on."

By keeping track of the axions that are whipped off, researchers are able to predict the amount of dark matter that was created.

Adaptive mesh refinement allowed the researchers to simulate the universe much longer than previous simulations and over a much bigger patch of the universe than previous simulations.

"We solve for the axion mass both in a more clever way and also by throwing just as much computing power as we could possibly find onto this problem," Safdi said. "We could never simulate our entire universe because it's too big. But we don't need to stimulate our entire universe. We just need to simulate a big enough patch of the universe for a long enough period of time, such that we capture all of the dynamics that we know are contained within that box."

The team is working with a new supercomputing cluster now being built at Berkeley Lab that will enable simulations that will provide an even more precise mass. Called Perlmutter, after Saul Perlmutter, a UC Berkeley and Berkeley Lab physicist who won the 2011 Nobel Prize in Physics for discovering the accelerating expansion of the universe driven by so-called dark energy, the next-generation supercomputer will quadruple the computing power of NERSC.

"We want to make even bigger simulations at even higher resolution, which will allow us to shrink these error bars, hopefully down to the 10% level, so we can tell you a very precise number, like 65 plus or minus 2 micro-eV. That then really changes the game experimentally, because then it would become an easier experiment to verify or exclude the axion in such a narrow mass range," Safdi said.

For van Bibber, who was not a member of Safdi's simulation team, the new mass estimate tests the limits of microwave cavities, which work less well at high frequencies. So, while the lower limit of the mass range is still within the ability of the HAYSTAC experiment to detect, he is enthused about the plasma haloscope.

"Over the years, new theoretical understanding has loosened the constraints on the axion mass; it can be anywhere within 15 orders of magnitude, if you consider the possibility that axions formed before inflation. It's become an insane task for experimentalists," said van Bibber, who holds UC Berkeley's Shankar Sastry Chair of Leadership and Innovation. "But a recent paper by Frank Wilczek's Stockholm theory group may have resolved the conundrum in making a resonator which could be simultaneously both very large in volume and very high in frequency. An actual resonator for a real experiment is still some ways away, but this could be the way to go to get to Safdi's predicted mass."

Once simulations give an even more precise mass, the axion may, in fact, be easy to find.

"It was really crucial that we teamed up with this computer science team at Berkeley Lab," Safdi said. "We really expanded beyond the physics field and actually made this a computing science problem."

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Ancient dwarf galaxy reconstructed with MilkyWay@home volunteer computer

Astrophysicists for the first time have calculated the original mass and size of a dwarf galaxy that was shredded in a collision with the Milky Way billions of years ago. Reconstructing the original dwarf galaxy, whose stars today thread through the Milky Way in a stellar "tidal stream," will help scientists understand how galaxies like the Milky Way formed, and could aid in the search for dark matter in our galaxy.

"We've been running simulations that take this big stream of stars, back it up for a couple of billion years, and see what it looked like before it fell into the Milky Way," said Heidi Newberg, a professor of physics, astrophysics, and astronomy at Rensselaer Polytechnic Institute. "Now we have a measurement from data, and it's the first big step toward using the information to find dark matter in the Milky Way."

Billions of years ago, the dwarf galaxy and others like it near the Milky Way were pulled into the larger galaxy. As each dwarf galaxy coalesced with the Milky Way, its stars were pulled by "tidal forces," the same kind of differential forces that make tides on Earth. The tidal forces distorted and eventually ripped the dwarf galaxy apart, stretching its stars into a tidal stream flung across the Milky Way. Such tidal mergers are fairly common, and Newberg estimates that "immigrant" stars absorbed into the Milky Way make up most of the stars in the galactic halo, a roughly spherical cloud of stars that surrounds the spiral arms of the central disk.

Critically, the position and velocities of the tidal stream stars carry information about the Milky Way's gravitational field.

Reconstructing the dwarf galaxy is a research task that combines data from star surveys, physics, and Newberg's MilkyWay@Home distributed supercomputer, which harnesses 1.5 petaflops -a measure of computer processing speed- of home computer power donated by volunteers. This large amount of processing power makes it possible to simulate the destruction of a large number of dwarf galaxies with different shapes and sizes, and identify a model that best matches the tidal stream of stars that we see today.

"It's an enormous problem, and we solve it by running tens of thousands of different simulations until we get one that actually matches. And that takes a lot of computer power, which we get with the help of volunteers all over the world who are part of MilkyWay@Home," Newberg said "We're brute-forcing it, but given how complicated the problem is, I think this method has a lot of merit."

As published today in The Astrophysical Journal, Newberg's team estimates the total mass of the original galaxy whose stars today form the Orphan-Chenab Stream as 2x107 times the mass of our sun.

However, only a little more than 1% of that mass is estimated to be made up of ordinary matter like stars. The remainder is assumed to be a hypothetical substance called dark matter that exerts gravitational force, but that we cannot see because it does not absorb or give off light. The existence of dark matter would explain a discrepancy between the gravitational pull of the mass of the matter we can see, and the far larger pull needed to account for the formation and movement of galaxies. The gravitational pull from dark matter is estimated to make up as much as 85% of the matter in the universe, and tidal streams of stars that fell in with dwarf galaxies could be used to determine where dark matter is located in our galaxy.

"Tidal stream stars are the only stars in our galaxy for which it is possible to know their positions in the past," Dr. Newberg said. "By looking at the current speeds of stars along a tidal stream, and knowing they all used to be in about the same place and moving at the same speed, we can figure out how much the gravity changes along that stream. And that will tell us where the dark matter is in the Milky Way."

The research also finds that the progenitor of the Orphan-Chenab stream has less mass than the galaxies measured in the outskirts of our galaxy today, and if this small mass is confirmed it could change our understanding of how small stellar systems form and then merge together to make larger galaxies like our Milky Way.

Dr. Newberg, an expert in the galactic halo, is a pioneer in identifying stellar tidal streams in the Milky Way. One day, she hopes that MilkyWay@home will help her measure more than the properties of one disintegrated dwarf galaxy. Ideally, she would like to simultaneously fit many dwarf galaxies, their orbits, and the properties of the Milky Way galaxy itself. This goal is complicated by the fact that the properties of our galaxy change over the billions of years that it takes for a small galaxy to fall in and be ripped apart to make these tidal streams.

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Black hole size revealed by its eating pattern

The feeding patterns of black holes offer insight into their size, researchers report. A new study revealed that the flickering in the brightness observed in actively feeding supermassive black holes is related to their mass.

Supermassive black holes are millions to billions of times more massive than the sun and usually reside at the center of massive galaxies. When dormant and not feeding on the gas and stars surrounding them, SMBHs emit very little light; the only way astronomers can detect them is through their gravitational influences on stars and gas in their vicinity. However, in the early universe, when SMBHs were rapidly growing, they were actively feeding -- or accreting -- materials at intensive rates and emitting an enormous amount of radiation -- sometimes outshining the entire galaxy in which they reside, the researchers said.

The new study, led by the University of Illinois Urbana-Champaign astronomy graduate student Colin Burke and professor Yue Shen, uncovered a definitive relationship between the mass of actively feeding SMBHs and the characteristic timescale in the light-flickering pattern. The findings are published in the journal Science.

The observed light from an accreting SMBH is not constant. Due to physical processes that are not yet understood, it displays a ubiquitous flickering over timescales ranging from hours to decades. "There have been many studies that explored possible relations of the observed flickering and the mass of the SMBH, but the results have been inconclusive and sometimes controversial," Burke said.

The team compiled a large data set of actively feeding SMBHs to study the variability pattern of flickering. They identified a characteristic timescale, over which the pattern changes, that tightly correlates with the mass of the SMBH. The researchers then compared the results with accreting white dwarfs, the remnants of stars like our sun, and found that the same timescale-mass relation holds, even though white dwarfs are millions to billions times less massive than SMBHs.

The light flickers are random fluctuations in a black hole's feeding process, the researchers said. Astronomers can quantify this flickering pattern by measuring the power of the variability as a function of timescales. For accreting SMBHs, the variability pattern changes from short timescales to long timescales. This transition of variability pattern happens at a characteristic timescale that is longer for more massive black holes.

The team compared black hole feeding to our eating or drinking activity by equating this transition to a human belch. Babies frequently burp while drinking milk, while adults can hold in the burp for a more extended amount of time. Black holes kind of do the same thing while feeding, they said.

"These results suggest that the processes driving the flickering during accretion are universal, whether the central object is a supermassive black hole or a much more lightweight white dwarf," Shen said.

"The firm establishment of a connection between the observed light flicker and fundamental properties of the accretor will certainly help us better understand accretion processes," said Yan-Fei Jiang, a researcher at the Flatiron Institute and study co-author.

Astrophysical black holes come in a broad spectrum of mass and size. In between the population of stellar-mass black holes, which weigh less than several tens of times the mass of the sun, and SMBHs, there is a population of black holes called intermediate-mass black holes that weigh between about 100 and 100,000 times the mass of the sun.

IMBHs are expected to form in large numbers through the history of the universe, and they may provide the seeds necessary to grow into SMBHs later. However, observationally this population of IMBHs is surprisingly elusive. There is only one indisputably confirmed IMBH that weighs about 150 times the mass of the sun. But that IMBH was serendipitously discovered by the gravitational wave radiation from the coalescence of two less-massive black holes.

"Now that there is a correlation between the flickering pattern and the mass of the central accreting object, we can use it to predict what the flickering signal from an IMBH might look like," Burke said.

Astronomers worldwide are waiting for the official kickoff of an era of massive surveys that monitor the dynamic and variable sky. The Vera C. Rubin Observatory in Chile's Legacy Survey of Space and Time will survey the sky over a decade and collect light flickering data for billions of objects, starting in late 2023.

"Mining the LSST data set to search for flickering patterns that are consistent with accreting IMBHs has the potential to discover and fully understand this long-sought mysterious population of black holes," said co-author Xin Liu, an astronomy professor at the U. of I.

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Ocean world: Rocky exoplanet has just half the mass of Venus

A team of astronomers have used the European Southern Observatory's Very Large Telescope (ESO's VLT) in Chile to shed new light on planets around a nearby star, L 98-59, that resemble those in the inner Solar System. Amongst the findings are a planet with half the mass of Venus -- the lightest exoplanet ever to be measured using the radial velocity technique -- an ocean world, and a possible planet in the habitable zone.

"The planet in the habitable zone may have an atmosphere that could protect and support life," says María Rosa Zapatero Osorio, an astronomer at the Centre for Astrobiology in Madrid, Spain, and one of the authors of the study published today in Astronomy & Astrophysics.

The results are an important step in the quest to find life on Earth-sized planets outside the Solar System. The detection of biosignatures on an exoplanet depends on the ability to study its atmosphere, but current telescopes are not large enough to achieve the resolution needed to do this for small, rocky planets. The newly studied planetary system, called L 98-59 after its star, is an attractive target for future observations of exoplanet atmospheres. Its orbits a star only 35 light-years away and has now been found to host rocky planets, like Earth or Venus, which are close enough to the star to be warm.

With the contribution of ESO's VLT, the team was able to infer that three of the planets may contain water in their interiors or atmospheres. The two planets closest to the star in the L 98-59 system are probably dry, but might have small amounts of water, while up to 30% of the third planet's mass could be water, making it an ocean world.

Furthermore, the team found "hidden" exoplanets that had not previously been spotted in this planetary system. They discovered a fourth planet and suspect there is a fifth, in a zone at the right distance from the star for liquid water to exist on its surface. "We have hints of the presence of a terrestrial planet in the habitable zone of this system," explains Olivier Demangeon, a researcher at the Instituto de Astrofísica e Ciências do Espaço, University of Porto in Portugal and lead author of the new study.

The study represents a technical breakthrough, as astronomers were able to determine, using the radial velocity method, that the innermost planet in the system has just half the mass of Venus. This makes it the lightest exoplanet ever measured using this technique, which calculates the wobble of the star caused by the tiny gravitational tug of its orbiting planets.

The team used the Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO) instrument on ESO's VLT to study L 98-59. "Without the precision and stability provided by ESPRESSO this measurement would have not been possible," says Zapatero Osorio. "This is a step forward in our ability to measure the masses of the smallest planets beyond the Solar System."

The astronomers first spotted three of L 98-59's planets in 2019, using NASA's Transiting Exoplanet Survey Satellite (TESS). This satellite relies on a technique called the transit method -- where the dip in the light coming from the star caused by a planet passing in front of it is used to infer the properties of the planet -- to find the planets and measure their sizes. However, it was only with the addition of radial velocity measurements made with ESPRESSO and its predecessor, the High Accuracy Radial velocity Planet Searcher (HARPS) at the ESO La Silla 3.6-metre telescope, that Demangeon and his team were able to find extra planets and measure the masses and radii of the first three. "If we want to know what a planet is made of, the minimum that we need is its mass and its radius," Demangeon explains.

The team hopes to continue to study the system with the forthcoming NASA/ESA/CSA James Webb Space Telescope (JWST) , while ESO's Extremely Large Telescope (ELT), under construction in the Chilean Atacama Desert and set to start observations in 2027, will also be ideal for studying these planets. "The HIRES instrument on the ELT may have the power to study the atmospheres of some of the planets in the L 98-59 system, thus complementing the JWST from the ground," says Zapatero Osorio.

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Mass of human chromosomes measured

Mass of human chromosomes have been measured for the first time.

The mass of human chromosomes, which contain the instructions for life in nearly every cell of our bodies, has been measured with X-rays for the first time in a new study led by UCL researchers.

For the study, published in Chromosome Research, researchers used a powerful X-ray beam at the UK's national synchrotron facility, Diamond Light Source, to determine the number of electrons in a spread of 46 chromosomes which they used to calculate mass.

They found that the chromosomes were about 20 times heavier than the DNA they contained -- a much larger mass than previously expected, suggesting there might be missing components yet to be discovered.

As well as DNA, chromosomes consist of proteins that serve a variety of functions, from reading the DNA to regulating processes of cell division to tightly packaging two-metre strands of DNA into our cells.

Senior author Professor Ian Robinson (London Centre for Nanotechnology at UCL) said: "Chromosomes have been investigated by scientists for 130 years but there are still parts of these complex structures that are poorly understood.

"The mass of DNA we know from the Human Genome Project, but this is the first time we have been able to precisely measure the masses of chromosomes that include this DNA.

"Our measurement suggests the 46 chromosomes in each of our cells weigh 242 picograms (trillionths of a gram). This is heavier than we would expect, and, if replicated, points to unexplained excess mass in chromosomes."

In the study, researchers used a method called X-ray ptychography, which involves stitching together the diffraction patterns that occur as the X-ray beam passes through the chromosomes, to create a highly sensitive 3D reconstruction. The fine resolution was possible as the beam deployed at Diamond Light Source was billions of times brighter than the Sun (ie, there was a very large number of photons passing through at a given time).

The chromosomes were imaged in metaphase, just before they were about to divide into two daughter cells. This is when packaging proteins wind up the DNA into very compact, precise structures.

Archana Bhartiya, a PhD student at the London Centre for Nanotechnology at UCL and lead author of the paper, said: "A better understanding of chromosomes may have important implications for human health.

"A vast amount of study of chromosomes is undertaken in medical labs to diagnose cancer from patient samples. Any improvements in our abilities to image chromosomes would therefore be highly valuable."

Each human cell, at metaphase, normally contains 23 pairs of chromosomes, or 46 in total. Within these are four copies of 3.5 billion base pairs of DNA.

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