A 'cosmic glitch' in gravity

A group of researchers at the University of Waterloo and the University of British Columbia have discovered a potential "cosmic glitch" in the universe's gravity, explaining its strange behaviour on a cosmic scale.

For the last 100 years, physicists have relied upon Albert Einstein's theory of "general relativity" to explain how gravity works throughout the universe. General relativity, proven accurate by countless tests and observations, suggests that gravity impacts not simply three physical dimensions but also a fourth dimension: time.

"This model of gravity has been essential for everything from theorizing the Big Bang to photographing black holes," said Robin Wen, the lead author on the project and a recent Waterloo Mathematical Physics graduate.

"But when we try to understand gravity on a cosmic scale, at the scale of galaxy clusters and beyond, we encounter apparent inconsistencies with the predictions of general relativity. It's almost as if gravity itself stops perfectly matching Einstein's theory. We are calling this inconsistency a 'cosmic glitch': gravity becomes around one per cent weaker when dealing with distances in the billions of light years. "

For more than twenty years, physicists and astronomers have been trying to create a mathematical model that explains the apparent inconsistencies of the theory of general relativity. Many of those efforts have taken place at Waterloo, which has a long history of cutting-edge gravitational research resulting from ongoing interdisciplinary collaboration between applied mathematicians and astrophysicists.

"Almost a century ago, astronomers discovered that our universe is expanding," said Niayesh Afshordi, a professor of astrophysics at the University of Waterloo and researcher at the Perimeter Institute.

"The farther away galaxies are, the faster they are moving, to the point that they seem to be moving at nearly the speed of light, the maximum allowed by Einstein's theory. Our finding suggests that, on those very scales, Einstein's theory may also be insufficient."

The research team's new model of a "cosmic glitch" modifies and extends Einstein's mathematical formulas in a way that resolves the inconsistency of some of the cosmological measurements without affecting existing successful uses of general relativity.

"Think of it as being like a footnote to Einstein's theory," Wen said. "Once you reach a cosmic scale, terms and conditions apply."

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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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Astronomers discover supernova explosion through rare 'cosmic magnifying glasses'

According to Einstein's general theory of relativity, time and space are fused together in a quantity known as spacetime. The theory suggests that massive objects, like a galaxy or galaxy clusters, can cause spacetime to curve. Gravitational lensing is a rare yet observable example of Einstein's theory in action; the mass of a large celestial body can significantly bend light as it travels through spacetime, much like a magnifying lens. When light from a more distant light source passes by this lens, scientists can use the resulting visual distortions to view objects that would otherwise be too far away and too faint to be seen.

An international team of scientists, including University of Maryland astronomer Igor Andreoni, recently discovered an exceptionally rare gravitationally lensed supernova, which the team named "SN Zwicky." Located more than 4 billion light years away, the supernova was magnified nearly 25 times by a foreground galaxy acting as a lens. The discovery presents a unique opportunity for astronomers to learn more about the inner cores of galaxies, dark matter and the mechanics behind universe expansion. The researchers published their findings -- including a comprehensive analysis, spectroscopic data and imaging of SN Zwicky -- in the journal Nature Astronomy on June 12, 2023.

"The discovery of SN Zwicky not only showcases the remarkable capabilities of modern astronomical instruments but also represents a significant step forward in our quest to understand the fundamental forces shaping our universe," said the paper's lead author Ariel Goobar, who is also the director of the Oskar Klein Center at Stockholm University.

Initially detected at the Zwicky Transient Facility (ZTF), SN Zwicky was quickly flagged as an object of interest due to its unusual brightness. Then, using adaptive optics instruments on the W.M. Keck Observatory, the Very Large Telescopes and NASA's Hubble Space Telescope, the team observed four images of SN Zwicky taken from different positions in the sky and confirmed that gravitational lensing was behind the supernova's extraordinary radiance.

According to Andreoni, who is a postdoctoral associate in UMD's Department of Astronomy and NASA's Goddard Space Flight Center, supernovae like SN Zwicky play a crucial role in helping scientists measure cosmic distances.

"SN Zwicky not only is magnified by the gravitational lense, but it also belongs to a class of supernovae that we call 'standard candles' because we can use their well-known luminosities to determine distance in space," Andreoni explained. "When a source of light is farther away, the light is dimmer -- just like seeing candles in a dark room. We can compare two light sources in this way and gain an independent measure of distance without having to actually study the galaxy itself."

In addition to being useful as a metric for cosmic distance, SN Zwicky also opens new avenues of research for scientists exploring the properties of galaxies, including dark matter (which is matter that does not absorb, reflect or emit light but make up the majority of matter in the universe). Researchers also believe that lensed supernovae like SN Zwicky could prove to be very promising tools for examining dark energy (a mysterious force counteracting gravity and drives the accelerated expansion of the universe) and refining current models describing the universe's expansion, including the calculation of the Hubble constant -- a value that describes how fast the universe is expanding.

For Andreoni, who is preparing for the opening of the Vera Rubin Observatory in Chile, the team's success in identifying and analyzing SN Zwicky is only the beginning. Now still in its construction phase, the new observatory is expected to begin full operations in 2024 and build upon the team's findings as it takes multiple images of the entire visible sky to search for other supernovae and asteroids. Andreoni believes that the "big picture" tactic used to find SN Zwicky will continue to help scientists gather large volumes of data about celestial events in the sky.

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New findings that map the universe's cosmic growth support Einstein's theory of gravity

For millennia, humans have been fascinated by the mysteries of the cosmos.

Unlike ancient philosophers imagining the universe's origins, modern cosmologists use quantitative tools to gain insights into the universe's evolution and structure. Modern cosmology dates back to the early 20th century, with the development of Albert Einstein's theory of general relativity.

Now, researchers from the Atacama Cosmology Telescope (ACT) collaboration have created a groundbreaking new image that reveals the most detailed map of dark matter distributed across a quarter of the entire sky, extending deep into the cosmos. What's more, it confirms Einstein's theory of how massive structures grow and bend light, over the entire 14-billion-year life span of the universe.

"We have mapped the invisible dark matter across the sky to the largest distances, and clearly see features of this invisible world that are hundreds of millions of light-years across, says Blake Sherwin, professor of cosmology at the University of Cambridge, where he leads a group of ACT researchers. "It looks just as our theories predict."

Despite making up 85% of the universe and influencing its evolution, dark matter has been hard to detect because it doesn't interact with light or other forms of electromagnetic radiation. As far as we know dark matter only interacts with gravity.

To track it down, the more than 160 collaborators who have built and gathered data from the National Science Foundation's Atacama Cosmology Telescope in the high Chilean Andes observe light emanating following the dawn of the universe's formation, the Big Bang -- when the universe was only 380,000 years old. Cosmologists often refer to this diffuse light that fills our entire universe as the "baby picture of the universe," but formally, it is known as the cosmic microwave background radiation (CMB).

The team tracks how the gravitational pull of large, heavy structures including dark matter warps the CMB on its 14-billion-year journey to us, like how a magnifying glass bends light as it passes through its lens.

"We've made a new mass map using distortions of light left over from the Big Bang," says Mathew Madhavacheril, assistant professor in the Department of Physics and Astronomy at the University of Pennsylvania. "Remarkably, it provides measurements that show that both the 'lumpiness' of the universe, and the rate at which it is growing after 14 billion years of evolution, are just what you'd expect from our standard model of cosmology based on Einstein's theory of gravity."

Sherwin adds, "our results also provide new insights into an ongoing debate some have called 'The Crisis in Cosmology,'"explaining that this crisis stems from recent measurements that use a different background light, one emitted from stars in galaxies rather than the CMB. These have produced results that suggest the dark matter was not lumpy enough under the standard model of cosmology and led to concerns that the model may be broken. However, the team's latest results from ACT were able to precisely assess that the vast lumps seen in this image are the exact right size.

"When I first saw them, our measurements were in such good agreement with the underlying theory that it took me a moment to process the results," says Cambridge Ph.D. student Frank Qu, part of the research team. "It will be interesting to see how this possible discrepancy between different measurements will be resolved."

"The CMB lensing data rivals more conventional surveys of the visible light from galaxies in their ability to trace the sum of what is out there," says Suzanne Staggs, director of ACT and Henry DeWolf Smyth Professor of Physics at Princeton University. "Together, the CMB lensing and the best optical surveys are clarifying the evolution of all the mass in the universe."

"When we proposed this experiment in 2003, we had no idea the full extent of information that could be extracted from our telescope," says Mark Devlin, the Reese Flower Professor of Astronomy at the University of Pennsylvania and the deputy director of ACT. "We owe this to the cleverness of the theorists, the many people who built new instruments to make our telescope more sensitive, and the new analysis techniques our team came up with."

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Curved spacetime in the lab

In a laboratory experiment, researchers from Heidelberg University have succeeded in realising an effective spacetime that can be manipulated. In their research on ultracold quantum gases, they were able to simulate an entire family of curved universes to investigate different cosmological scenarios and compare them with the predictions of a quantum field theoretical model.

According to Einstein's Theory of Relativity, space and time are inextricably connected. In our Universe, whose curvature is barely measurable, the structure of this spacetime is fixed. In a laboratory experiment, researchers from Heidelberg University have succeeded in realising an effective spacetime that can be manipulated. In their research on ultracold quantum gases, they were able to simulate an entire family of curved universes to investigate different cosmological scenarios and compare them with the predictions of a quantum field theoretical model. The research results were published in Nature.

The emergence of space and time on cosmic time scales from the Big Bang to the present is the subject of current research that can only be based on the observation of our single Universe. The expansion and curvature of space are essential to cosmological models. In a flat space like our current Universe, the shortest distance between two points is always a straight line. "It is conceivable, however, that our Universe was curved in its early phase. Studying the consequences of a curved spacetime is therefore a pressing question in research," states Prof. Dr Markus Oberthaler, a researcher at the Kirchhoff Institute for Physics at Heidelberg University. With his "Synthetic Quantum Systems" research group, he developed a quantum field simulator for this purpose.

The quantum field simulator created in the lab consists of a cloud of potassium atoms cooled to just a few nanokelvins above absolute zero. This produces a Bose-Einstein condensate -- a special quantum mechanical state of the atomic gas that is reached at very cold temperatures. Prof. Oberthaler explains that the Bose-Einstein condensate is a perfect background against which the smallest excitations, i.e. changes in the energy state of the atoms, become visible. The form of the atomic cloud determines the dimensionality and the properties of spacetime on which these excitations ride like waves. In our Universe, there are three dimensions of space as well as a fourth: time.

In the experiment conducted by the Heidelberg physicists, the atoms are trapped in a thin layer. The excitations can therefore only propagate in two spatial directions -- the space is two-dimensional. At the same time, the atomic cloud in the remaining two dimensions can be shaped in almost any way, whereby it is also possible to realise curved spacetimes. The interaction between the atoms can be precisely adjusted by a magnetic field, changing the propagation speed of the wavelike excitations on the Bose-Einstein condensate.

"For the waves on the condensate, the propagation speed depends on the density and the interaction of the atoms. This gives us the opportunity to create conditions like those in an expanding universe," explains Prof. Dr Stefan Flörchinger. The researcher, who previously worked at Heidelberg University and joined the University of Jena at the beginning of this year, developed the quantum field theoretical model used to quantitatively compare the experimental results.

Using the quantum field simulator, cosmic phenomena, such as the production of particles based on the expansion of space, and even the spacetime curvature can be made measurable. "Cosmological problems normally take place on unimaginably large scales. To be able to specifically study them in the lab opens up entirely new possibilities in research by enabling us to experimentally test new theoretical models," states Celia Viermann, the primary author of the "Nature" article. "Studying the interplay of curved spacetime and quantum mechanical states in the lab will occupy us for some time to come," says Markus Oberthaler, whose research group is also part of the STRUCTURES Cluster of Excellence at Ruperto Carola.

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'Wobbling black hole' most extreme example ever detected

Researchers at Cardiff University have identified a peculiar twisting motion in the orbits of two colliding black holes, an exotic phenomenon predicted by Einstein's theory of gravity.

Their study, which is published in Nature and led by Professor Mark Hannam, Dr Charlie Hoy and Dr Jonathan Thompson, reports that this is the first time this effect, known as precession, has been seen in black holes, where the twisting is 10 billion times faster than in previous observations.

The binary black hole system was found through gravitational waves in early 2020 in the Advanced LIGO and Virgo detectors. One of the black holes, 40 times bigger than our Sun, is likely the fastest spinning black hole to be found through gravitational waves. And unlike all previous observations, the rapidly revolving black hole distorted space and time so much that the binary's entire orbit wobbled back and forth.

This form of precession is specific to Einstein's theory of general relativity. These results confirm its existence in the most extreme physical event we can observe, the collision of two black holes.

"We've always thought that binary black holes can do this," said Professor Mark Hannam of Cardiff University's Gravity Exploration Institute. "We have been hoping to spot an example ever since the first gravitational wave detections. We had to wait for five years and over 80 separate detections, but finally we have one!"

A more down-to-earth example of precession is the wobbling of a spinning top, which may wobble -- or precess -- once every few seconds. By contrast, precession in general relativity is usually such a weak effect that it is imperceptible. In the fastest example previously measured from orbiting neutron stars called binary pulsars, it took over 75 years for the orbit to precess. The black-hole binary in this study, colloquially known as GW200129 (named after the date it was observed, January 29, 2020), precesses several times every second -- an effect 10 billion times stronger than measured previously.

Dr Jonathan Thompson, also of Cardiff University, explained: "It's a very tricky effect to identify. Gravitational waves are extremely weak and to detect them requires the most sensitive measurement apparatus in history. The precession is an even weaker effect buried inside the already weak signal, so we had to do a careful analysis to uncover it."

Gravitational waves were predicted by Einstein in 1916. They were first directly detected from the merger of two black holes by the Advanced LIGO instruments in 2015, a breakthrough discovery that led to the 2017 Nobel Prize. Gravitational wave astronomy is now one of the most vibrant fields of science, with a network of the Advanced LIGO, Virgo and KAGRA detectors operating in the US, Europe and Japan. To date there have been over 80 detections, all of merging black holes or neutron stars.

"So far most black holes we've found with gravitational waves have been spinning fairly slowly," said Dr Charlie Hoy, a researcher at Cardiff University during this study, and now at the University of Portsmouth. "The larger black hole in this binary, which was about 40 times more massive than the Sun, was spinning almost as fast as physically possible. Our current models of how binaries form suggest this one was extremely rare, maybe a one in a thousand event. Or it could be a sign that our models need to change."

The international network of gravitational-wave detectors is currently being upgraded and will start its next search of the universe in 2023. They are likely to find hundreds more black holes colliding, and will tell scientists whether GW200129 was a rare exception, or a sign that our universe is even stranger than they thought.

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In Einstein's footsteps and beyond

In physics, as in life, it's always good to look at things from different perspectives.

Since the beginning of quantum physics, how light moves and interacts with matter around it has mostly been described and understood mathematically through the lens of its energy. In 1900, Max Planck used energy to explain how light is emitted by heated objects, a seminal study in the foundation of quantum mechanics. In 1905, Albert Einstein used energy when he introduced the concept of photon.

But light has another, equally important quality known as momentum. And, as it turns out, when you take momentum away, light starts behaving in really interesting ways.

An international team of physicists led by Michaël Lobet, a research associate at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and Eric Mazur, the Balkanski Professor of Physics and Applied Physics at SEAS, are re-examining the foundations of quantum physics from the perspective of momentum and exploring what happens when the momentum of light is reduced to zero.

The research is published in Nature Light Science & Applications.

Any object with mass and velocity has momentum -- from atoms to bullets to asteroids -- and momentum can be transferred from one object to another. A gun recoils when a bullet is fired because the momentum of the bullet is transferred to the gun. At the microscopic scale, an atom recoils when it emits light because of the acquired momentum of the photon. Atomic recoil, first described by Einstein when he was writing the quantum theory of radiation, is a fundamental phenomenon which governs light emission.

But a century after Planck and Einstein, a new class of metamaterials is raising questions regarding these fundamental phenomena. These metamaterials have a refractive index close to zero, meaning that when light travels through them, it doesn't travel like a wave in phases of crests and troughs. Instead, the wave is stretched out to infinity, creating a constant phase. When that happens, many of the typical processes of quantum mechanics disappear, including atomic recoil.

Why? It all goes back to momentum. In these so-called near-zero index materials, the wave momentum of light becomes zero and when the wave momentum is zero, odd things happen.

"Fundamental radiative processes are inhibited in three dimensional near-zero index materials," says Lobet, who is currently a lecturer at the University of Namur in Belgium. "We realized that the momentum recoil of an atom is forbidden in near-zero index materials and that no momentum transfer is allowed between the electromagnetic field and the atom."

If breaking one of Einstein's rules wasn't enough, the researchers also broke perhaps the most well-known experiment in quantum physics -- Young's double-slit experiment. This experiment is used in classrooms across the globe to demonstrate the particle-wave duality in quantum physics -- showing that light can display characteristics of both waves and particles.

In a typical material, light passing through two slits produces two coherent sources of waves that interfere to form a bright spot in the center of the screen with a pattern of light and dark fringes on either side, known as diffraction fringes.

"When we modelled and numerically computed Young's double-slit experiment, it turned out that the diffraction fringes vanished when the refractive index was lowered," said co-author Larissa Vertchenko, of the Technical University of Denmark.

"As it can be seen, this work interrogates fundamental laws of quantum mechanics and probes the limits of wave-corpuscle duality," said co-author Iñigo Liberal, of the Public University of Navarre in Pamplona, Spain.

While some fundamental processes are inhibited in near-zero refractive index materials, others are enhanced. Take another famous quantum phenomenon -- Heisenberg's uncertainty principle, more accurately known in physics as the Heisenberg inequality. This principle states that you cannot know both the position and speed of a particle with perfect accuracy and the more you know about one, the less you know about the other. But, in near-zero index materials, you know with 100% certainty that the momentum of a particle is zero, which means you have absolutely no idea where in the material the particle is at any given moment.

"This material would make a really poor microscope, but it does enable to cloak objects quite perfectly," Lobet said. "In some way, objects become invisible."

"These new theoretical results shed new light on near-zero refractive index photonics from a momentum perspective," said Mazur. "It provides insights in the understanding of light-matter interactions in systems with a low- refraction index, which can be useful for lasing and quantum optics applications."

The research could also shed light on other applications, including quantum computing, light sources that emit a single photon at a time, the lossless propagation of light through a waveguide and more.

The team next aims to revisit other foundational quantum experiments in these materials from a momentum perspective. Afterall, even though Einstein didn't predict near-zero refractive index materials, he did stress the importance of momentum. In his seminal 1916 paper on fundamental radiative processes, Einstein insisted that, from a theoretical point of view, energy and momentum "should be considered on a completely equal footing since energy and momentum are linked in the closest possible way."

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Dark energy: Neutron stars will tell us if it's only an illusion

A huge amount of mysterious dark energy is necessary to explain cosmological phenomena, such as the accelerated expansion of the Universe, with Einstein's theory. But what if dark energy was just an illusion and general relativity itself had to be modified? A new SISSA study, published in Physical Review Letters, offers a new approach to answer this question. Thanks to huge computational and mathematical effort, scientists produced the first simulation ever of merging binary neutron stars in theories beyond general relativity that reproduce a dark- energy like behavior on cosmological scales. This allows the comparison of Einstein's theory and modified versions of it, and, with sufficiently accurate data, may solve the dark energy mystery.

For about 100 years now, general relativity has been very successful at describing gravity on a variety of regimes, passing all experimental tests on Earth and the solar system. However, to explain cosmological observations such as the observed accelerated expansion of the Universe, we need to introduce dark components, such as dark matter and dark energy, which still remain a mystery.

Enrico Barausse, astrophysicist at SISSA (Scuola Internazionale Superiore di Studi Avanzati) and principal investigator of the ERC grant GRAMS (GRavity from Astrophysical to Microscopic Scales) questions whether dark energy is real or, instead, it may be interpreted as a breakdown of our understanding of gravity. "The existence of dark energy could be just an illusion," he says, "the accelerated expansion of the Universe might be caused by some yet unknown modifications of general relativity, a sort of 'dark gravity'."

The merger of neutron stars offers a unique situation to test this hypothesis because gravity around them is pushed to the extreme. "Neutron stars are the densest stars that exist, typically only 10 kilometers in radius, but with a mass between one or two times the mass of our Sun," explains the scientist. "This makes gravity and the spacetime around them extreme, allowing for abundant production of gravitational waves when two of them collide. We can use the data acquired during such events to study the workings of gravity and test Einstein's theory in a new window."

In this study, published in Physical Review Letters, SISSA scientists in collaboration with physicists from Universitat de les Illes Balears in Palma de Mallorca, produced the first simulation of merging binary neutron stars in theories of modified gravity relevant for cosmology: "This type of simulations is extremely challenging," clarifies Miguel Bezares, first author of the paper, "because of the highly non-linear nature of the problem. It requires a huge computational effort -months of run in supercomputers -- that was made possible also by the agreement between SISSA and CINECA consortium as well as novel mathematical formulations that we developed. These represented major roadblocks for many years till our first simulation."

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Atomic clocks measure Einstein's general relativity at millimeter scale

JILA physicists have measured Albert Einstein's theory of general relativity, or more specifically, the effect called time dilation, at the smallest scale ever, showing that two tiny atomic clocks, separated by just a millimeter or the width of a sharp pencil tip, tick at different rates.

The experiments, described in the Feb. 17 issue of Nature, suggest how to make atomic clocks 50 times more precise than today's best designs and offer a route to perhaps revealing how relativity and gravity interact with quantum mechanics, a major quandary in physics.

JILA is jointly operated by the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder.

"The most important and exciting result is that we can potentially connect quantum physics with gravity, for example, probing complex physics when particles are distributed at different locations in the curved space-time," NIST/JILA Fellow Jun Ye said. "For timekeeping, it also shows that there is no roadblock to making clocks 50 times more precise than today -- which is fantastic news."

Einstein's 1915 theory of general relativity explains large-scale effects such as the gravitational effect on time and has important practical applications such as correcting GPS satellite measurements. Although the theory is more than a century old, physicists remain fascinated by it. NIST scientists have used atomic clocks as sensors to measure relativity more and more precisely, which may help finally explain how its effects interact with quantum mechanics, the rulebook for the subatomic world.

According to general relativity, atomic clocks at different elevations in a gravitational field tick at different rates. The frequency of the atoms' radiation is reduced -- shifted toward the red end of the electromagnetic spectrum -- when observed in stronger gravity, closer to Earth. That is, a clock ticks more slowly at lower elevations. This effect has been demonstrated repeatedly; for example, NIST physicists measured it in 2010 by comparing two independent atomic clocks, one positioned 33 centimeters (about 1 foot) above the other.

The JILA researchers have now measured frequency shifts between the top and bottom of a single sample of about 100,000 ultracold strontium atoms loaded into an optical lattice, a lab setup similar to the group's earlier atomic clocks. In this new case the lattice, which can be visualized as a stack of pancakes created by laser beams, has unusually large, flat, thin cakes, and they are formed by less intense light than normally used. This design reduces the distortions in the lattice ordinarily caused by the scattering of light and atoms, homogenizes the sample, and extends the atoms' matter waves, whose shapes indicate the probability of finding the atoms in certain locations. The atoms' energy states are so well controlled that they all ticked between two energy levels in exact unison for 37 seconds, a record for what is called quantum coherence.

Crucial to the new results were the Ye group's imaging innovation, which provided a microscopic map of frequency distributions across the sample, and their method of comparing two regions of an atom cloud rather than the traditional approach of using two separate clocks.

The measured redshift across the atom cloud was tiny, in the realm of 0.0000000000000000001, consistent with predictions. (While much too small for humans to perceive directly, the differences add up to major effects on the universe as well as technology such as GPS.) The research team resolved this difference quickly for this type of experiment, in about 30 minutes of averaging data. After 90 hours of data, their measurement precision was 50 times better than in any previous clock comparison.

"This a completely new ballgame, a new regime where quantum mechanics in curved space-time can be explored," Ye said. "If we could measure the redshift 10 times even better than this, we will be able to see the atoms' whole matter waves across the curvature of space-time. Being able to measure the time difference on such a minute scale could enable us to discover, for example, that gravity disrupts quantum coherence, which could be at the bottom of why our macroscale world is classical."

Better clocks have many possible applications beyond timekeeping and navigation. Ye suggests atomic clocks can serve as both microscopes to see minuscule links between quantum mechanics and gravity and as telescopes to observe the deepest corners of the universe. He is using clocks to look for mysterious dark matter, believed to constitute most matter in the universe. Atomic clocks are also poised to improve models and understanding of the shape of the Earth through the application of a measurement science called relativistic geodesy.

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Challenging Einstein’s greatest theory with extreme stars

Researchers at the University of East Anglia and the University of Manchester have helped conduct a 16-year long experiment to challenge Einstein's theory of general relativity.

The international team looked to the stars -- a pair of extreme stars called pulsars to be precise -- through seven radio telescopes across the globe.

And they used them to challenge Einstein's most famous theory with some of the most rigorous tests yet.

The study, published today in the journal Physical Review X, reveals new relativistic effects that, although expected, have now been observed for the first time.

Dr Robert Ferdman, from UEA's School of Physics, said: "As spectacularly successful as Einstein's theory of general relativity has proven to be, we know that is not the final word in gravitational theory.

"More than 100 years later, scientists around the world continue their efforts to find flaws in his theory.

"General relativity is not compatible with the other fundamental forces, described by quantum mechanics. It is therefore important to continue to place the most stringent tests upon general relativity as possible, to discover how and when the theory breaks down.

"Finding any deviation from general relativity would constitute a major discovery that would open a window on new physics beyond our current theoretical understanding of the Universe.

"And it may help us toward eventually discovering a unified theory of the fundamental forces of nature."

Led by Michael Kramer from the Max Planck Institute for Radio Astronomy in Bonn, Germany, the international team of researchers from ten countries, put Einstein's theory to the most rigorous tests yet.

Dr Ferdman said: "A pulsar is a highly magnetised rotating compact star that emits beams of electromagnetic radiation out of its magnetic poles.

"They weigh more than our sun but they are only about 15 miles across -- so they are incredibly dense objects that produce radio beams that sweep the sky like a lighthouse.

"We studied a double pulsar, which was discovered by members of the team in 2003 and presents the most precise laboratory we currently have to test Einstein's theory. Of course, his theory was conceived when neither these types of extreme stars, nor the techniques used to study them, could be imagined."

The double pulsar consists of two pulsars which orbit each other in just 147 minutes with velocities of about 1 million km/h. One pulsar is spinning very fast, about 44 times a second. The companion is young and has a rotation period of 2.8 seconds. It is their motion around each other which can be used as a near perfect gravity laboratory.

Seven sensitive radio telescopes were used to observe this double pulsar -- in Australia, the US, France, Germany, the Netherlands and in the UK (the Lovell Radio Telescope).

Prof Kramer said: "We studied a system of compact stars that is an unrivalled laboratory to test gravity theories in the presence of very strong gravitational fields.

"To our delight we were able to test a cornerstone of Einstein's theory, the energy carried by gravitational waves, with a precision that is 25 times better than with the Nobel-Prize winning Hulse-Taylor pulsar, and 1000 times better than currently possible with gravitational wave detectors."

He explained that the observations are not only in agreement with the theory, "but we were also able to see effects that could not be studied before''.

Prof Benjamin Stappers, from the University of Manchester, said: "The discovery of the double pulsar system was made as part of a survey co-led from the University of Manchester and presented us with the only known instance of two cosmic clocks which allow precise measurement of the structure and evolution of an intense gravitational field.

"The Lovell Telescope at the Jodrell Bank Observatory has been monitoring it every couple of weeks since then. This long baseline of high quality and frequent observations provided an excellent data set to be combined with those from observatories around the world."

Prof Ingrid Stairs from the University of British Columbia at Vancouver, said: "We follow the propagation of radio photons emitted from a cosmic lighthouse, a pulsar, and track their motion in the strong gravitational field of a companion pulsar.

"We see for the first time how the light is not only delayed due to a strong curvature of spacetime around the companion, but also that the light is deflected by a small angle of 0.04 degrees that we can detect. Never before has such an experiment been conducted at such a high spacetime curvature."

Prof Dick Manchester from Australia's national science agency, CSIRO, said: "Such fast orbital motion of compact objects like these -- they are about 30 per cent more massive than the Sun but only about 24 km across -- allows us to test many different predictions of general relativity -- seven in total!

"Apart from gravitational waves and light propagation, our precision allows us also to measure the effect of "time dilation" that makes clocks run slower in gravitational fields.

"We even need to take Einstein's famous equation E = mc2 into account when considering the effect of the electromagnetic radiation emitted by the fast-spinning pulsar on the orbital motion.

"This radiation corresponds to a mass loss of 8 million tonnes per second! While this seems a lot, it is only a tiny fraction -- 3 parts in a thousand billion billion(!) -- of the mass of the pulsar per second."

The researchers also measured -- with a precision of 1 part in a million(!) -- that the orbit changes its orientation, a relativistic effect also well known from the orbit of Mercury, but here 140,000 times stronger.

They realised that at this level of precision they also need to consider the impact of the pulsar's rotation on the surrounding spacetime, which is "dragged along" with the spinning pulsar.

Dr Norbert Wex from the MPIfR, another main author of the study, said: "Physicists call this the Lense-Thirring effect or frame-dragging. In our experiment it means that we need to consider the internal structure of a pulsar as a neutron star.

"Hence, our measurements allow us for the first time to use the precision tracking of the rotations of the neutron star, a technique that we call pulsar timing to provide constraints on the extension of a neutron star."

The technique of pulsar timing was combined with careful interferometric measurements of the system to determine its distance with high resolution imaging, resulting in a value of 2400 light years with only 8 per cent error margin.

Team member Prof Adam Deller, from Swinburne University in Australia and responsible for this part of the experiment, said: "It is the combination of different complementary observing techniques that adds to the extreme value of the experiment. In the past similar studies were often hampered by the limited knowledge of the distance of such systems."

This is not the case here, where in addition to pulsar timing and interferometry also the information gained from effects due to the interstellar medium were carefully taken into account.

Prof Bill Coles from the University of California San Diego agrees: "We gathered all possible information on the system and we derived a perfectly consistent picture, involving physics from many different areas, such as nuclear physics, gravity, interstellar medium, plasma physics and more. This is quite extraordinary."

Paulo Freire, also from MPIfR, said: "Our results are nicely complementary to other experimental studies which test gravity in other conditions or see different effects, like gravitational wave detectors or the Event Horizon Telescope.

"They also complement other pulsar experiments, like our timing experiment with the pulsar in a stellar triple system, which has provided an independent and superb test of the universality of free fall."

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First detection of light from behind a black hole

Watching X-rays flung out into the universe by the supermassive black hole at the center of a galaxy 800 million light-years away, Stanford University astrophysicist Dan Wilkins noticed an intriguing pattern. He observed a series of bright flares of X-rays -- exciting, but not unprecedented -- and then, the telescopes recorded something unexpected: additional flashes of X-rays that were smaller, later and of different "colors" than the bright flares.

According to theory, these luminous echoes were consistent with X-rays reflected from behind the black hole -- but even a basic understanding of black holes tells us that is a strange place for light to come from.

"Any light that goes into that black hole doesn't come out, so we shouldn't be able to see anything that's behind the black hole," said Wilkins, who is a research scientist at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford and SLAC National Accelerator Laboratory. It is another strange characteristic of the black hole, however, that makes this observation possible. "The reason we can see that is because that black hole is warping space, bending light and twisting magnetic fields around itself," Wilkins explained.

The strange discovery, detailed in a paper published July 28 in Nature, is the first direct observation of light from behind a black hole -- a scenario that was predicted by Einstein's theory of general relativity but never confirmed, until now.

"Fifty years ago, when astrophysicists starting speculating about how the magnetic field might behave close to a black hole, they had no idea that one day we might have the techniques to observe this directly and see Einstein's general theory of relativity in action," said Roger Blandford, a co-author of the paper who is the Luke Blossom Professor in the School of Humanities and Sciences and Stanford and SLAC professor of physics and particle physics.

How to see a black hole

The original motivation behind this research was to learn more about a mysterious feature of certain black holes, called a corona. Material falling into a supermassive black hole powers the brightest continuous sources of light in the universe, and as it does so, forms a corona around the black hole. This light -- which is X-ray light -- can be analyzed to map and characterize a black hole.

The leading theory for what a corona is starts with gas sliding into the black hole where it superheats to millions of degrees. At that temperature, electrons separate from atoms, creating a magnetized plasma. Caught up in the powerful spin of the black hole, the magnetic field arcs so high above the black hole, and twirls about itself so much, that it eventually breaks altogether -- a situation so reminiscent of what happens around our own Sun that it borrowed the name "corona."

"This magnetic field getting tied up and then snapping close to the black hole heats everything around it and produces these high energy electrons that then go on to produce the X-rays," said Wilkins.

As Wilkins took a closer look to investigate the origin of the flares, he saw a series of smaller flashes. These, the researchers determined, are the same X-ray flares but reflected from the back of the disk -- a first glimpse at the far side of a black hole.

"I've been building theoretical predictions of how these echoes appear to us for a few years," said Wilkins. "I'd already seen them in the theory I've been developing, so once I saw them in the telescope observations, I could figure out the connection."

Future observations

The mission to characterize and understand coronas continues and will require more observation. Part of that future will be the European Space Agency's X-ray observatory, Athena (Advanced Telescope for High-ENergy Astrophysics). As a member of the lab of Steve Allen, professor of physics at Stanford and of particle physics and astrophysics at SLAC, Wilkins is helping to develop part of the Wide Field Imager detector for Athena.

"It's got a much bigger mirror than we've ever had on an X-ray telescope and it's going to let us get higher resolution looks in much shorter observation times," said Wilkins. "So, the picture we are starting to get from the data at the moment is going to become much clearer with these new observatories."

Co-authors of this research are from Saint Mary's University (Canada), Netherlands Institute for Space Research (SRON), University of Amsterdam and The Pennsylvania State University.

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Timekeeping theory combines quantum clocks and Einstein's relativity

 A phenomenon of quantum mechanics known as superposition can impact timekeeping in high-precision clocks, according to a theoretical study from Dartmouth College, Saint Anselm College and Santa Clara University.

Research describing the effect shows that superposition -- the ability of an atom to exist in more than one state at the same time -- leads to a correction in atomic clocks known as "quantum time dilation."

The research, published in the journal Nature Communications, takes into account quantum effects beyond Albert Einstein's theory of relativity to make a new prediction about the nature of time.

"Whenever we have developed better clocks, we've learned something new about the world," said Alexander Smith, an assistant professor of physics at Saint Anselm College and adjunct assistant professor at Dartmouth College, who led the research as a junior fellow in Dartmouth's Society of Fellows. "Quantum time dilation is a consequence of both quantum mechanics and Einstein's relativity, and thus offers a new possibility to test fundamental physics at their intersection."

In the early 1900s, Albert Einstein presented a revolutionary picture of space and time by showing that the time experienced by a clock depends on how fast it is moving -- as the speed of a clock increases, the rate at which it ticks decreases. This was a radical departure from Sir Isaac Newton's absolute notion of time.

Quantum mechanics, the theory of motion governing the atomic realm, allows for a clock to move as if it were simultaneously traveling at two different speeds: a quantum "superposition" of speeds. The research paper takes this possibility into account and provides a probabilistic theory of timekeeping, which led to the prediction of quantum time dilation.

To develop the new theory, the team combined modern techniques from quantum information science with a theory developed in the 1980s that explains how time might emerge out of a quantum theory of gravity.

"Physicists have sought to accommodate the dynamical nature of time in quantum theory for decades," said Mehdi Ahmadi, a lecturer at Santa Clara University who co-authored the study. "In our work, we predict corrections to relativistic time dilation which stem from the fact that the clocks used to measure this effect are quantum mechanical in nature."

In the same way that carbon dating relies on decaying atoms to determine the age of organic objects, the lifetime of an excited atom acts as a clock. If such an atom moves in a superposition of different speeds, then its lifetime will either increase or decrease depending on the nature of the superposition relative to an atom moving at a definite speed.

The correction to the atom's lifetime is so small that it would be impossible to measure in terms that make sense at the human scale. But the ability to account for this effect could enable a test of quantum time dilation using the most advanced atomic clocks.

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Einstein's description of gravity just got much harder to beat

 Einstein's theory of general relativity -- the idea that gravity is matter warping spacetime -- has withstood over 100 years of scrutiny and testing, including the newest test from the Event Horizon Telescope collaboration, published today in the latest issue of Physical Review Letters.

According to the findings, Einstein's theory just got 500 times harder to beat.

Despite its successes, Einstein's robust theory remains mathematically irreconcilable with quantum mechanics, the scientific understanding of the subatomic world. Testing general relativity is important because the ultimate theory of the universe must encompass both gravity and quantum mechanics.

"We expect a complete theory of gravity to be different from general relativity, but there are many ways one can modify it. We found that whatever the correct theory is, it can't be significantly different from general relativity when it comes to black holes. We really squeezed down the space of possible modifications," said UArizona astrophysics professor Dimitrios Psaltis, who until recently was the project scientist of the Event Horizon Telescope collaboration. Psaltis is lead author of a new paper that details the researchers' findings.

"This is a brand-new way to test general relativity using supermassive black holes," said Keiichi Asada, an EHT science council member and an expert on radio observations of black holes for Academia Sinica Institute of Astronomy and Astrophysics.

To perform the test, the team used the first image ever taken of the supermassive black hole at the center of nearby galaxy M87 obtained with the EHT last year. The first results had shown that the size of the black-hole shadow was consistent with the size predicted by general relativity.

"At that time, we were not able to ask the opposite question: How different can a gravity theory be from general relativity and still be consistent with the shadow size?" said UArizona Steward Theory Fellow Pierre Christian. "We wondered if there was anything we could do with these observations in order to cull some of the alternatives."

The team did a very broad analysis of many modifications to the theory of general relativity to identify the unique characteristic of a theory of gravity that determines the size of a black hole shadow.

"In this way, we can now pinpoint whether some alternative to general relativity is in agreement with the Event Horizon Telescope observations, without worrying about any other details," said Lia Medeiros, a postdoctoral fellow at the Institute for Advanced Study who has been part of the EHT collaboration since her time as a UArizona graduate student.

The team focused on the range of alternatives that had passed all the previous tests in the solar system.

"Using the gauge we developed, we showed that the measured size of the black hole shadow in M87 tightens the wiggle room for modifications to Einstein's theory of general relativity by almost a factor of 500, compared to previous tests in the solar system," said UArizona astrophysics professor Feryal Özel, a senior member of the EHT collaboration. "Many ways to modify general relativity fail at this new and tighter black hole shadow test."

"Black hole images provide a completely new angle for testing Einstein's theory of general relativity," said Michael Kramer, director of the Max Planck Institute for Radio Astronomy and EHT collaboration member.

"Together with gravitational wave observations, this marks the beginning of a new era in black hole astrophysics," Psaltis said.

Testing the theory of gravity is an ongoing quest: Are the general relativity predictions for various astrophysical objects good enough for astrophysicists to not worry about any potential differences or modifications to general relativity?

"We always say general relativity passed all tests with flying colors -- if I had a dime for every time I heard that," Özel said. "But it is true, when you do certain tests, you don't see that the results deviate from what general relativity predicts. What we're saying is that while all of that is correct, for the first time we have a different gauge by which we can do a test that's 500 times better, and that gauge is the shadow size of a black hole."

Next, the EHT team expects higher fidelity images that will be captured by the expanded array of telescopes, which includes the Greenland Telescope, the 12-meter Telescope on Kitt Peak near Tucson, and the Northern Extended Millimeter Array Observatory in France.

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ESO telescope sees star dance around supermassive black hole, proves Einstein right

Observations made with ESO's Very Large Telescope (VLT) have revealed for the first time that a star orbiting the supermassive black hole at the centre of the Milky Way moves just as predicted by Einstein's general theory of relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton's theory of gravity. This long-sought-after result was made possible by increasingly precise measurements over nearly 30 years, which have enabled scientists to unlock the mysteries of the behemoth lurking at the heart of our galaxy.

"Einstein's General Relativity predicts that bound orbits of one object around another are not closed, as in Newtonian Gravity, but precess forwards in the plane of motion. This famous effect -- first seen in the orbit of the planet Mercury around the Sun -- was the first evidence in favour of General Relativity. One hundred years later we have now detected the same effect in the motion of a star orbiting the compact radio source Sagittarius A* at the centre of the Milky Way. This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the Sun," says Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany and the architect of the 30-year-long programme that led to this result.

Located 26,000 light-years from the Sun, Sagittarius A* and the dense cluster of stars around it provide a unique laboratory for testing physics in an otherwise unexplored and extreme regime of gravity. One of these stars, S2, sweeps in towards the supermassive black hole to a closest distance less than 20 billion kilometres (one hundred and twenty times the distance between the Sun and Earth), making it one of the closest stars ever found in orbit around the massive giant. At its closest approach to the black hole, S2 is hurtling through space at almost three percent of the speed of light, completing an orbit once every 16 years. "After following the star in its orbit for over two and a half decades, our exquisite measurements robustly detect S2's Schwarzschild precession in its path around Sagittarius A*," says Stefan Gillessen of the MPE, who led the analysis of the measurements published today in the journal Astronomy & Astrophysics.

Most stars and planets have a non-circular orbit and therefore move closer to and further away from the object they are rotating around. S2's orbit precesses, meaning that the location of its closest point to the supermassive black hole changes with each turn, such that the next orbit is rotated with regard to the previous one, creating a rosette shape. General Relativity provides a precise prediction of how much its orbit changes and the latest measurements from this research exactly match the theory. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole.

The study with ESO's VLT also helps scientists learn more about the vicinity of the supermassive black hole at the centre of our galaxy. "Because the S2 measurements follow General Relativity so well, we can set stringent limits on how much invisible material, such as distributed dark matter or possible smaller black holes, is present around Sagittarius A*. This is of great interest for understanding the formation and evolution of supermassive black holes," say Guy Perrin and Karine Perraut, the French lead scientists of the project.

This result is the culmination of 27 years of observations of the S2 star using, for the best part of this time, a fleet of instruments at ESO's VLT, located in the Atacama Desert in Chile. The number of data points marking the star's position and velocity attests to the thoroughness and accuracy of the new research: the team made over 330 measurements in total, using the GRAVITY, SINFONI and NACO instruments. Because S2 takes years to orbit the supermassive black hole, it was crucial to follow the star for close to three decades, to unravel the intricacies of its orbital movement.

The research was conducted by an international team led by Frank Eisenhauer of the MPE with collaborators from France, Portugal, Germany and ESO. The team make up the GRAVITY collaboration, named after the instrument they developed for the VLT Interferometer, which combines the light of all four 8-metre VLT telescopes into a super-telescope (with a resolution equivalent to that of a telescope 130 metres in diameter). The[ same team reported in 2018] -- another effect predicted by General Relativity: they saw the light received from S2 being stretched to longer wavelengths as the star passed close to Sagittarius A*. "Our previous result has shown that the light emitted from the star experiences General Relativity. Now we have shown that the star itself senses the effects of General Relativity," says Paulo Garcia, a researcher at Portugal's Centre for Astrophysics and Gravitation and one of the lead scientists of the GRAVITY project.

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Scientists detect the ringing of a newborn black hole for the first time

If Albert Einstein's theory of general relativity holds true, then a black hole, born from the cosmically quaking collisions of two massive black holes, should itself "ring" in the aftermath, producing gravitational waves much like a struck bell reverbates sound waves. Einstein predicted that the particular pitch and decay of these gravitational waves should be a direct signature of the newly formed black hole's mass and spin.

Now, physicists from MIT and elsewhere have "heard" the ringing of an infant black hole for the first time, and found that the pattern of this ringing does, in fact, predict the black hole's mass and spin -- more evidence that Einstein was right all along.

The findings, published today in Physical Review Letters, also favor the idea that black holes lack any sort of "hair" -- a metaphor referring to the idea that black holes, according to Einstein's theory, should exhibit just three observable properties: mass, spin, and electric charge. All other characteristics, which the physicist John Wheeler termed "hair," should be swallowed up by the black hole itself, and would therefore be unobservable.

The team's findings today support the idea that black holes are, in fact, hairless. The researchers were able to identify the pattern of a black hole's ringing, and, using Einstein's equations, calculated the mass and spin that the black hole should have, given its ringing pattern. These calculations matched measurements of the black hole's mass and spin made previously by others.

If the team's calculations deviated significantly from the measurements, it would have suggested that the black hole's ringing encodes properties other than mass, spin, and electric charge -- tantalizing evidence of physics beyond what Einstein's theory can explain. But as it turns out, the black hole's ringing pattern is a direct signature of its mass and spin, giving support to the notion that black holes are bald-faced giants, lacking any extraneous, hair-like properties.

"We all expect general relativity to be correct, but this is the first time we have confirmed it in this way," says the study's lead author, Maximiliano Isi, a NASA Einstein Fellow in MIT's Kavli Institute for Astrophysics and Space Research. "This is the first experimental measurement that succeeds in directly testing the no-hair theorem. It doesn't mean black holes couldn't have hair. It means the picture of black holes with no hair lives for one more day."

A chirp, decoded

On Sept. 9, 2015, scientists made the first-ever detection of gravitational waves -- infinitesimal ripples in space-time, emanating from distant, violent cosmic phenomena. The detection, named GW150914, was made by LIGO, the Laser Interferometer Gravitational-wave Observatory. Once scientists cleared away the noise and zoomed in on the signal, they observed a waveform that quickly crescendoed before fading away. When they translated the signal into sound, they heard something resembling a "chirp."

Scientists determined that the gravitational waves were set off by the rapid inspiraling of two massive black holes. The peak of the signal -- the loudest part of the chirp -- linked to the very moment when the black holes collided, merging into a single, new black hole. While this infant black hole likely gave off gravitational waves of its own, its signature ringing, physicists assumed, would be too faint to decipher amid the clamor of the initial collision.

Isi and his colleagues, however, found a way to extract the black hole's reverberation from the moments immediately after the signal's peak. In previous work led by Isi's co-author, Matthew Giesler, the team showed through simulations that such a signal, and particularly the portion right after the peak, contains "overtones" -- a family of loud, short-lived tones. When they reanalyzed the signal, taking overtones into account, the researchers discovered that they could successfully isolate a ringing pattern that was specific to a newly formed black hole.

In the team's new paper, the researchers applied this technique to actual data from the GW150914 detection, concentrating on the last few milliseconds of the signal, immediately following the chirp's peak. Taking into account the signal's overtones, they were able to discern a ringing coming from the new, infant black hole. Specifically, they identified two distinct tones, each with a pitch and decay rate that they were able to measure.

"We detect an overall gravitational wave signal that's made up of multiple frequencies, which fade away at different rates, like the different pitches that make up a sound," Isi says. "Each frequency or tone corresponds to a vibrational frequency of the new black hole."

Listening beyond Einstein

Einstein's theory of general relativity predicts that the pitch and decay of a black hole's gravitational waves should be a direct product of its mass and spin. That is, a black hole of a given mass and spin can only produce tones of a certain pitch and decay. As a test of Einstein's theory, the team used the equations of general relativity to calculate the newly formed black hole's mass and spin, given the pitch and decay of the two tones they detected.

They found their calculations matched with measurements of the black hole's mass and spin previously made by others. Isi says the results demonstrate that researchers can, in fact, use the very loudest, most detectable parts of a gravitational wave signal to discern a new black hole's ringing, where before, scientists assumed that this ringing could only be detected within the much fainter end of the gravitational wave signal, and only with much more sensitive instruments than what currently exist.

"This is exciting for the community because it shows these kinds of studies are possible now, not in 20 years," Isi says.

As LIGO improves its resolution, and more sensitive instruments come online in the future, researchers will be able to use the group's methods to "hear" the ringing of other newly born black holes. And if they happen to pick up tones that don't quite match up with Einstein's predictions, that could be an even more exciting prospect.

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Einstein's general relativity theory is questioned but still stands for now

Black hole illustration

More than 100 years after Albert Einstein published his iconic theory of general relativity, it is beginning to fray at the edges, said Andrea Ghez, UCLA professor of physics and astronomy. Now, in the most comprehensive test of general relativity near the monstrous black hole at the center of our galaxy, Ghez and her research team report July 25 in the journal Science that Einstein's theory of general relativity holds up.

"Einstein's right, at least for now," said Ghez, a co-lead author of the research. "We can absolutely rule out Newton's law of gravity. Our observations are consistent with Einstein's theory of general relativity. However, his theory is definitely showing vulnerability. It cannot fully explain gravity inside a black hole, and at some point we will need to move beyond Einstein's theory to a more comprehensive theory of gravity that explains what a black hole is."

Einstein's 1915 theory of general relativity holds that what we perceive as the force of gravity arises from the curvature of space and time. The scientist proposed that objects such as the sun and the Earth change this geometry. Einstein's theory is the best description of how gravity works, said Ghez, whose UCLA-led team of astronomers has made direct measurements of the phenomenon near a supermassive black hole -- research Ghez describes as "extreme astrophysics."

The laws of physics, including gravity, should be valid everywhere in the universe, said Ghez, who added that her research team is one of only two groups in the world to watch a star known as S0-2 make a complete orbit in three dimensions around the supermassive black hole at the center of the Milky Way. The full orbit takes 16 years, and the black hole's mass is about four million times that of the sun.

The researchers say their work is the most detailed study ever conducted into the supermassive black hole and Einstein's theory of general relativity.

The key data in the research were spectra that Ghez's team analyzed this April, May and September as her "favorite star" made its closest approach to the enormous black hole. Spectra, which Ghez described as the "rainbow of light" from stars, show the intensity of light and offer important information about the star from which the light travels. Spectra also show the composition of the star. These data were combined with measurements Ghez and her team have made over the last 24 years.

Spectra -- collected at the W.M. Keck Observatory in Hawaii using a spectrograph built at UCLA by a team led by colleague James Larkin -- provide the third dimension, revealing the star's motion at a level of precision not previously attained. (Images of the star the researchers took at the Keck Observatory provide the two other dimensions.) Larkin's instrument takes light from a star and disperses it, similar to the way raindrops disperse light from the sun to create a rainbow, Ghez said.

"What's so special about S0-2 is we have its complete orbit in three dimensions," said Ghez, who holds the Lauren B. Leichtman and Arthur E. Levine Chair in Astrophysics. "That's what gives us the entry ticket into the tests of general relativity. We asked how gravity behaves near a supermassive black hole and whether Einstein's theory is telling us the full story. Seeing stars go through their complete orbit provides the first opportunity to test fundamental physics using the motions of these stars."

Ghez's research team was able to see the co-mingling of space and time near the supermassive black hole. "In Newton's version of gravity, space and time are separate, and do not co-mingle; under Einstein, they get completely co-mingled near a black hole," she said.

"Making a measurement of such fundamental importance has required years of patient observing, enabled by state-of-the-art technology," said Richard Green, director of the National Science Foundation's division of astronomical sciences. For more than two decades, the division has supported Ghez, along with several of the technical elements critical to the research team's discovery. "Through their rigorous efforts, Ghez and her collaborators have produced a high-significance validation of Einstein's idea about strong gravity."

Keck Observatory Director Hilton Lewis called Ghez "one of our most passionate and tenacious Keck users." "Her latest groundbreaking research," he said, "is the culmination of unwavering commitment over the past two decades to unlock the mysteries of the supermassive black hole at the center of our Milky Way galaxy."

The researchers studied photons -- particles of light -- as they traveled from S0-2 to Earth. S0-2 moves around the black hole at blistering speeds of more than 16 million miles per hour at its closest approach. Einstein had reported that in this region close to the black hole, photons have to do extra work. Their wavelength as they leave the star depends not only on how fast the star is moving, but also on how much energy the photons expend to escape the black hole's powerful gravitational field. Near a black hole, gravity is much stronger than on Earth.

Ghez was given the opportunity to present partial data last summer, but chose not to so that her team could thoroughly analyze the data first. "We're learning how gravity works. It's one of four fundamental forces and the one we have tested the least," she said. "There are many regions where we just haven't asked, how does gravity work here? It's easy to be overconfident and there are many ways to misinterpret the data, many ways that small errors can accumulate into significant mistakes, which is why we did not rush our analysis."

Ghez, a 2008 recipient of the MacArthur "Genius" Fellowship, studies more than 3,000 stars that orbit the supermassive black hole. Hundreds of them are young, she said, in a region where astronomers did not expect to see them.

It takes 26,000 years for the photons from S0-2 to reach Earth. "We're so excited, and have been preparing for years to make these measurements," said Ghez, who directs the UCLA Galactic Center Group. "For us, it's visceral, it's now -- but it actually happened 26,000 years ago!"

This is the first of many tests of general relativity Ghez's research team will conduct on stars near the supermassive black hole. Among the stars that most interest her is S0-102, which has the shortest orbit, taking 11 1/2 years to complete a full orbit around the black hole. Most of the stars Ghez studies have orbits of much longer than a human lifespan.

Ghez's team took measurements about every four nights during crucial periods in 2018 using the Keck Observatory -- which sits atop Hawaii's dormant Mauna Kea volcano and houses one of the world's largest and premier optical and infrared telescopes. Measurements are also taken with an optical-infrared telescope at Gemini Observatory and Subaru Telescope, also in Hawaii. She and her team have used these telescopes both on site in Hawaii and remotely from an observation room in UCLA's department of physics and astronomy.

Black holes have such high density that nothing can escape their gravitational pull, not even light. (They cannot be seen directly, but their influence on nearby stars is visible and provides a signature. Once something crosses the "event horizon" of a black hole, it will not be able to escape. However, the star S0-2 is still rather far from the event horizon, even at its closest approach, so its photons do not get pulled in.)

Ghez's co-authors include Tuan Do, lead author of the Science paper, a UCLA research scientist and deputy director of the UCLA Galactic Center Group; Aurelien Hees, a former UCLA postdoctoral scholar, now a researcher at the Paris Observatory; Mark Morris, UCLA professor of physics and astronomy; Eric Becklin, UCLA professor emeritus of physics and astronomy; Smadar Naoz, UCLA assistant professor of physics and astronomy; Jessica Lu, a former UCLA graduate student who is now a UC Berkeley assistant professor of astronomy; UCLA graduate student Devin Chu; Greg Martinez, UCLA project scientist; Shoko Sakai, a UCLA research scientist; Shogo Nishiyama, associate professor with Japan's Miyagi University of Education; and Rainer Schoedel, a researcher with Spain's Instituto de Astrofsica de Andalucia.

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Einstein's general relativity confirmed near black hole

Observations made with ESO's Very Large Telescope have for the first time revealed the effects predicted by Einstein's general relativity on the motion of a star passing through the extreme gravitational field near the supermassive black hole in the center of the Milky Way. This long-sought result represents the climax of a 26-year-long observation campaign using ESO's telescopes in Chile.

Obscured by thick clouds of absorbing dust, the closest supermassive black hole to the Earth lies 26,000 light years away at the centre of the Milky Way. This gravity monster, which has a mass four million times that of the Sun, is surrounded by a small group of stars orbiting at high speed. This extreme environment -- the strongest gravitational field in our galaxy -- makes it the perfect place to test gravitational physics, particularly Einstein's general theory of relativity.

New infrared observations from the exquisitely sensitive GRAVITY, NACO and SINFONI instruments on ESO's Very Large Telescope (VLT) have now allowed astronomers to follow one of these stars, called S2, as it passed very close to the black hole during May 2018 at a speed in excess of 25 million kilometres per hour -- three percent of the speed of light -- and at a distance of less than 20 billion kilometres.

These extremely delicate measurements were made by an international team led by Reinhard Genzel of the Max Planck Institute for extraterrestrial physics (MPE) in Garching, Germany, in conjunction with collaborators around the world. The observations form the culmination of a 26-year series of ever more precise observations of the centre of the Milky Way using ESO instruments. 'This is the second time that we have observed the close passage of S2 around the black hole in our galactic centre. But this time, because of much improved instrumentation, we were able to observe the star with unprecedented resolution', explains Genzel. 'We have been preparing intensely for this event over several years, as we wanted to make the most of this unique opportunity to observe general relativistic effects.'

The new measurements clearly reveal an effect called gravitational redshift. Light from the star is stretched to longer wavelengths by the very strong gravitational field of the black hole. And the stretch in wavelength of light from S2 agrees precisely with that predicted by Einstein's theory of general relativity. This is the first time that this deviation from the predictions of simpler Newtonian gravity has been observed in the motion of a star around a supermassive black hole. The team used SINFONI to measure the motion of S2 towards and away from Earth and the GRAVITY interferometric instrument to make extraordinarily precise measurements of the position of S2 in order to define the shape of its orbit. GRAVITY creates such sharp images that it can reveal the motion of the star from night to night as it passes close to the black hole -- 26,000 light years from Earth.

'Our first observations of S2, about two years ago, already showed that we would have the ideal black hole laboratory', adds Frank Eisenhauer (MPE), Co-Principal Investigator of the GRAVITY instrument. 'During the close passage, we managed not only to precisely follow the star on its orbit, we could even detect the faint glow around the black hole on most of the images.' By combining the position and velocity measurements from SINFONI and GRAVITY, as well as previous observations using other instruments, the team could compare them to the predictions of Newtonian gravity, general relativity and other theories of gravity. As expected, the new results are inconsistent with Newtonian predictions and in excellent agreement with the predictions of general relativity. More than one hundred years after he published his paper setting out the equations of general relativity, Einstein has been proved right once more.

The hardware contribution of the Institute of Physics I of the University of Cologne was the development and construction of the two spectrometers of GRAVITY. The spectrometers analyse the wavelength of the observed stellar light and convert the received photons into electronic signals. 'GRAVITY is a technological challenge. However, after more than two decades of astrophysical research on the high velocity stars in the Galactic Centre and on the development of astronomical instrumentation, the effort has been rewarded with an excellent result in experimental physics', says Andreas Eckhart from the University of Cologne.

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Einstein proved right in another galaxy

The gravitational lens from LRG 3-757 galaxy taken with the Hubble Space Telescope's Wide Field Camera 3.

An international team of astronomers have made the most precise test of gravity outside our own solar system.

By combining data taken with NASA's Hubble Space Telescope and the European Southern Observatory's Very Large Telescope, the researchers show that gravity in this galaxy behaves as predicted by Albert Einstein's general theory of relativity, confirming the theory's validity on galactic scales.

In 1915 Albert Einstein proposed his general theory of relativity (GR) to explain how gravity works. Since then GR has passed a series of high precision tests within the solar system, but there have been no precise tests of GR on large astronomical scales.

It has been known since 1929 that the Universe is expanding, but in 1998 two teams of astronomers showed that the Universe is expanding faster now than it was in the past. This surprising discovery -- which won the Nobel Prize in 2011 -- cannot be explained unless the Universe is mostly made of an exotic component called dark energy. However, this interpretation relies on GR being the correct theory of gravity on cosmological scales. Testing the long distance properties of gravity is important to validate our cosmological model.

A team of astronomers, led by Dr Thomas Collett of the Institute of Cosmology and Gravitation at the University of Portsmouth, used a nearby galaxy as a gravitational lens to make a precise test of gravity on astronomical length scales.

Dr Collett said: "General Relativity predicts that massive objects deform space-time, this means that when light passes near another galaxy the light's path is deflected. If two galaxies are aligned along our line of sight this can give rise to a phenomenon, called strong gravitational lensing, where we see multiple images of the background galaxy. If we know the mass of the foreground galaxy, then the amount of separation between the multiple images tells us if General Relativity is the correct theory of gravity on galactic scales."

A few hundred strong gravitational lenses are known, but most are too distant to precisely measure their mass, so they can't be used to accurately test GR. However, the galaxy ESO325-G004 is amongst the closest lenses, at 500 million light years from Earth.

Dr Collett continues: "We used data from the Very Large Telescope in Chile to measure how fast the stars were moving in E325 -- this let us infer how much mass there must be in E325 to hold these stars in orbit. We then compared this mass to the strong lensing image separations that we observed with the Hubble Space telescope and the result was just what GR predicts with 9 per cent precision. This is the most precise extrasolar test of GR to date, from just one galaxy."

"The Universe is an amazing place providing such lenses which we can then use as our laboratories," adds team member Professor Bob Nichol, Director of the Institute of Cosmology and Gravitation. "It is so satisfying to use the best telescopes in the world to challenge Einstein, only to find out how right he was."

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Atomic clock comparison confirms key assumptions of 'Einstein's elevator'

Einstein's elevator: As the Earth falls like an elevator in the sun's gravity, atomic clocks based on hydrogen and cesium maintain the same synchronicity in their ticking properties. NIST physicists just measured this phenomenon better than ever before.

By comparing different types of remote atomic clocks, physicists at the National Institute of Standards and Technology (NIST) have performed the most accurate test ever of a key principle underlying Albert Einstein's famous theory of general relativity, which describes how gravity relates to space and time.

The NIST result, made possible by continual improvements in the world's most accurate atomic clocks, yields a record-low, exceedingly small value for a quantity that Einstein predicted to be zero.

As described in a Nature Physics paper posted online June 4, NIST researchers used the solar system as a laboratory for testing Einstein's thought experiment involving Earth as a freefalling elevator. Einstein theorized that all objects located in such an elevator would accelerate at the same rate, as if they were in a uniform gravitational field -- or no gravity at all. Moreover, he predicted, these objects' properties relative to each other would remain constant during the elevator's free-fall.

In their experiment, the NIST team regarded Earth as an elevator falling through the Sun's gravitational field. They compared recorded data on the "ticks" of two types of atomic clocks located around the world to show they remained in sync over 14 years, even as the gravitational pull on the elevator varied during the Earth's slightly off-kilter orbit around the sun. Researchers compared data from 1999 to 2014 for a total of 12 clocks -- four hydrogen masers (microwave lasers) in the NIST time scale with eight of the most accurate cesium fountain atomic clocks operated by metrology laboratories in the United States, the United Kingdom, France, Germany and Italy.

The experiment was designed to test a prediction of general relativity, the principle of local position invariance (LPI), which holds that in a falling elevator, measures of nongravitational effects are independent of time and place. One such measurement compares the frequencies of electromagnetic radiation from atomic clocks at different locations. The researchers constrained the violation of LPI to a value of 0.00000022 plus or minus 0.00000025 -- the most miniscule number yet, consistent with general relativity's predicted result of zero, and corresponding to no violation. This means the ratio of hydrogen to cesium frequencies remained the same as the clocks moved together in the falling elevator.

This result has five times less uncertainty than NIST's best previous measurement of the LPI violation, translating to five times greater sensitivity. That earlier 2007 result, from a 7-year comparison of cesium and hydrogen atomic clocks, was 20 times more sensitive than the previous tests.

The latest measurement advance is due to improvements in several areas, namely more accurate cesium fountain atomic clocks, better time transfer processes (which enable devices at different locations to compare their time signals), and the latest data for computing the position and velocity of Earth in space, NIST's Bijunath Patla said.

"But the main reason we did this work was to highlight how atomic clocks are used to test fundamental physics; in particular, the foundations of general relativity," Patla said. "This is the claim made most often when clockmakers strive for better stability and accuracy. We tie together tests of general relativity with atomic clocks, note the limitations of the current generation of clocks, and present a future outlook for how next-generation clocks will become very relevant."

Further limits on LPI are unlikely to be obtained using hydrogen and cesium clocks, the researchers say, but experimental next-generation clocks based on optical frequencies, which are much higher than the frequencies of hydrogen and cesium clocks, could offer much more sensitive results. NIST already operates a variety of these clocks based on atoms such as ytterbium and strontium.

Because so many scientific theories and calculations are intertwined, NIST researchers used their new value for the LPI violation to calculate variations in several fundamental "constants" of nature, physical quantities thought to be universal and widely used in physics. Their results for the light quark mass were the best ever, while results for the fine structure constant agreed with previously reported values for any pair of atoms.

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Can we tell black holes apart?

Can we tell black holes apart? Astrophysicists at Goethe University Frankfurt answer this question by computing images of feeding non-Einsteinian black holes: at present it is hard to tell them apart from standard black holes.

One of the most fundamental predictions of Einstein's theory of relativity is the existence of black holes. In spite of the recent detection of gravitational waves from binary black holes by LIGO, direct evidence using electromagnetic waves remains elusive and astronomers are searching for it with radio telescopes. Astrophysicists at Goethe University Frankfurt, and collaborators in the ERC-funded project BlackHoleCam in Bonn and Nijmegen have created and compared self-consistent and realistic images of the shadow of an accreting supermassive black hole -- such as the black-hole candidate Sagittarius A* (Sgr A*) in the heart of our galaxy -- both in general relativity and in a different theory of gravity. The goal was to test if Einsteinian black holes can be distinguished from those in alternative theories of gravity.

Not all of the light rays (or photons) produced by matter falling into a black hole are trapped by the event horizon, a region of spacetime from which nothing can escape. Some of these photons will reach distant observers, so that when a black hole is observed directly a "shadow" is expected against the background sky. The size and shape of this shadow will depend on the black-hole's properties but also on the theory of gravity.

Because the largest deviations from Einstein's theory of relativity are expected very close to the event horizon and since alternative theories of gravity make different predictions on the properties of the shadow, direct observations of Sgr A* represent a very promising approach for testing gravity in the strongest regime. Making such images of the black-hole shadow is the primary goal of the international Event Horizon Telescope Collaboration (EHTC), which combines radio data from telescopes around the world.

Scientists from the BlackHoleCam team in Europe, who are part of the EHTC, have now gone a step further and investigated whether it is possible to distinguish between a "Kerr" black hole from Einstein's gravity and a "dilaton" black hole, which is a possible solution of an alternative theory of gravity.

The researchers studied the evolution of matter falling into the two very different types of black holes and calculated the radiation emitted to construct the images. Furthermore, real-life physical conditions in the telescopes and interstellar medium were used to create physically realistic images. "To capture the effects of different black holes we used realistic simulations of accretion disks with near-identical initial setups. These expensive numerical simulations used state-of-the-art codes and took several months on the Institute's supercomputer LOEWE," says Dr. Yosuke Mizuno, lead author of the study.

Moreover, expected radio images obviously have a limited resolution and image fidelity. When using realistic image resolutions, the scientists found, to their surprise, that even highly non-Einsteinian black holes could disguise themselves as normal black holes.

"Our results show that there are theories of gravity in which black holes can masquerade as Einsteinian, so new techniques of analyzing EHT data may be needed to tell them apart," remarks Luciano Rezzolla, professor at Goethe University and leader of the Frankfurt team. "While we believe general relativity is correct, as scientists we need to be open-minded. Luckily, future observations and more advanced techniques will eventually settle these doubts," concludes Rezzolla.

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