Figure 1: Both the LUX and PandaX-II experiments look for dark matter particles (𝜒) by sensing their interaction with xenon atoms. The detector in each experiment consists of a large tank of ultrapure liquid xenon (dark purple) topped with xenon gas (light purple). An interaction produces two light signals, one from photons, S1, and another, S2, from electrons when they drift into the gas. The signals are detected by photomultiplier tubes at the top and bottom of the tank (yellow cylinders). 

Jodi A. Cooley, Department of Physics, Southern Methodist University, 3215 Daniel Ave., Dallas, TX 75205, USA

January 11, 2017• Physics 10, 3

No dark matter particles have been observed by two of the world’s most sensitive direct-detection experiments, casting doubt on a favored dark matter model.

Over 80 years ago astronomers and astrophysicists began to inventory the amount of matter in the Universe. In doing so, they stumbled into an incredible discovery: the motion of stars within galaxies, and of galaxies within galaxy clusters, could not be explained by the gravitational tug of visible matter alone [1]. So to rectify the situation, they suggested the presence of a large amount of invisible, or “dark,” matter. We now know that dark matter makes up 84% of the matter in the Universe [2], but its composition—the type of particle or particles it’s made from—remains a mystery. Researchers have pursued a myriad of theoretical candidates, but none of these “suspects” have been apprehended. The lack of detection has helped better define the parameters, such as masses and interaction strengths, that could characterize the particles. For the most compelling dark matter candidate, WIMPs, the viable parameter space has recently become smaller with the announcement in September 2016 by the PandaX-II Collaboration [3] and now by the Large Underground Xenon (LUX) Collaboration [4] that a search for the particles has come up empty.

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The dishes of the Karl G. Jansky Very Large Array are seen making thefirst-ever precision localization of a Fast Radio Burst, and therebypointing the way to the host galaxy of FRB121102. Credit: Danielle Futselaar (artsource.nl)

One of the rare and brief bursts of cosmic radio waves that have puzzled astronomers since they were first detected nearly 10 years ago has finally been tied to a source: an older dwarf galaxy more than 3 billion light years from Earth.

Fast radio bursts, which flash for just a few milliseconds, created a stir among astronomers because they seemed to be coming from outside our galaxy, which means they would have to be very powerful to be seen from Earth, and because none of those first observed were ever seen again.

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New research confirms the role Type Ia supernovae, like G299, play in measuring universe expansion. Credit: NASA

New research by cosmologists at the University of Chicago and Wayne State University confirms the accuracy of Type Ia supernovae in measuring the pace at which the universe expands. The findings support a widely held theory that the expansion of the universe is accelerating and such acceleration is attributable to a mysterious force known as dark energy. The findings counter recent headlines that Type Ia supernova cannot be relied upon to measure the expansion of the universe.

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Following their discovery, the US White House Committee on Science, Space, and Technology asked LIGO Scientific Collaboration members to testify on the discovery, its meaning for science and society, and what the future may hold. From left to right: assistant director of the NSF's Directorate of Mathematical and Physical Sciences, Fleming Crim; LIGO lab director David Reitze; LIGO spokesperson Gabriela Gonzalez; and LIGO MIT director David Shoemaker. (Courtesy: LIGO Collaboration)

The Physics World 2016 Breakthrough of the Year goes to "the LIGO Scientific Collaboration for its revolutionary, first-ever direct observations of gravitational waves". Nine other achievements are highly commended and cover topics ranging from nuclear physics to material science and more.

Almost exactly 100 years after they were first postulated by Albert Einstein in his general theory of relativity, gravitational waves hit the headlines in 2016 as the US-based LIGO collaboration detected two separate gravitational-wave events using the Advanced Laser Interferometer Gravitational-wave Observatory (aLIGO). The first observation was made on 14 September 2015 and was announced in February this year. A second set of gravitational waves rolled through LIGO's detectors on 26 December 2015, and this so-called "Boxing Day event" was announced in June this year. Gravitational waves are ripples in the fabric of space–time, and these observations mark the end of a decades-long hunt for these interstellar undulations.

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Nicolle R. Fuller/Science Photo Library

Black hole mergers captured by LIGO offer a chance to explore new physics.

Gravitational-wave data show tentative signs of firewalls or other exotic physics.

Zeeya Merali
09 December 2016

It was hailed as an elegant confirmation of Einstein’s general theory of relativity — but ironically the discovery of gravitational waves earlier this year could herald the first evidence that the theory breaks down at the edge of black holes. Physicists have analysed the publicly released data from the Laser Interferometer Gravitational-Wave Observatory (LIGO), and claim to have found “echoes” of the waves that seem to contradict general relativity’s predictions1.

The echoes could yet disappear with more data. If they persist, the finding would be extraordinary. Physicists have predicted that Einstein’s hugely successful theory could break down in extreme scenarios, such as at the centre of black holes. The echoes would indicate the even more dramatic possibility that relativity fails at the black hole’s edge, far from its core.

If the echoes go away, then general relativity will have withstood a test of its power — previously, it wasn’t clear that physicists would be able to test their non-standard predictions.

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December 7, 2016

This map of dark matter in the Universe was obtained from data from the KiDS survey, using the VLT Survey Telescope at ESO’s Paranal Observatory in Chile. It reveals an expansive web of dense (light) and empty (dark) regions. This image is one out of five patches of the sky observed by KiDS. Here the invisible dark matter is seen rendered in pink, covering an area of sky around 420 times the size of the full moon. This image reconstruction was made by analysing the light collected from over three million distant galaxies more than 6 billion light-years away. The observed galaxy images were warped by the gravitational pull of dark matter as the light travelled through the Universe. Some small dark regions, with sharp boundaries, appear in this image. They are the locations of bright stars and other nearby objects that get in the way of the observations of more distant galaxies and are hence masked out in these maps as no weak-lensing signal can be measured in these areas. Credit: Kilo-Degree Survey Collaboration/H. Hildebrandt & B. Giblin/ESO

Analysis of a giant new galaxy survey, made with ESO's VLT Survey Telescope in Chile, suggests that dark matter may be less dense and more smoothly distributed throughout space than previously thought. An international team used data from the Kilo Degree Survey (KiDS) to study how the light from about 15 million distant galaxies was affected by the gravitational influence of matter on the largest scales in the Universe. The results appear to be in disagreement with earlier results from the Planck satellite.

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Colour composite photo of the sky field around the lonely neutron star RX J1856.5-3754 in the constellation of Corona Australis and the related cone-shaped nebula. It is based on a series of exposures obtained with the multi-mode FORS2 instrument at VLT KUEYEN through three different optical filters. The trail of an asteroid is seen in the field with intermittent blue, green and red colours. RX J1856.5-3754 is exactly in the centre of the image. Image credit: ESO.

By studying the light emitted from an extraordinarily dense and strongly magnetised neutron star using ESO’s Very Large Telescope, astronomers may have found the first observational indications of a strange quantum effect, first predicted in the 1930s. The polarisation of the observed light suggests that the empty space around the neutron star is subject to a quantum effect known as vacuum birefringence.
A team led by Roberto Mignani from INAF Milan (Italy) and from the University of Zielona Gora (Poland), used ESO’s Very Large Telescope (VLT) at the Paranal Observatory in Chile to observe the neutron star RX J1856.5-3754, about 400 light-years from Earth.

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The newborn universe may have glowed with light beams moving much faster than they do today, according to a theory that overturns Einstein’s century-old claim that the speed of light is a constant.

João Magueijo, of Imperial College London, and Niayesh Afshordi, of the University of Waterloo in Canada, propose that light tore along at infinite speed at the birth of the universe when the temperature of the cosmos was a staggering ten thousand trillion trillion celsius.

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A new theory of gravity might explain the curious motions of stars in galaxies. Emergent gravity, as the new theory is called, predicts the exact same deviation of motions that is usually explained by invoking dark matter. Prof. Erik Verlinde, renowned expert in string theory at the University of Amsterdam and the Delta Institute for Theoretical Physics, published a new research paper today in which he expands his groundbreaking views on the nature of gravity.

In 2010, Erik Verlinde surprised the world with a completely new theory of gravity. According to Verlinde, gravity is not a fundamental force of nature, but an emergent phenomenon. In the same way that temperature arises from the movement of microscopic particles, gravity emerges from the changes of fundamental bits of information, stored in the very structure of spacetime.

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Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA)

The observable Universe contains about two trillion galaxies—more than ten times as many as previously estimated, according to the first significant revision of the count in two decades.

Since the mid-1990s, the working estimate for the number of galaxies in the Universe has been around 120 billion. That number was based largely on a 1996 study called Hubble Deep Field. Researchers pointed the Hubble Space Telescope at a small region of space for a total of ten days so that the long exposures would reveal extremely faint objects.

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by Shane L. Larson

The seas of the Cosmos are vast and deep. From our vantage point here on the shores of Earth, we have seen much that is beautiful, awe-inspiring, frightening, humbling, confusing, and enigmatic. The simple truth of astronomy is that it is a spectator sport. The only thing we can do, is watch the skies and wait for the next Big Thing to happen. We collect events, like bottle-caps or flowers, and add them to our collection. Each new addition is a mystery, a new piece of a puzzle that takes shape ever-so-slowly over time.

On 14 September 2015, the LIGO-Virgo collaboration announced that they had detected the first gravitational waves ever, and that those waves had been created by a pair of merging black holes far across the Cosmos.

Today, we have some more news: LIGO has detected the second gravitational wave event ever, and those waves were also created by a pair of merging black holes far across the Cosmos. But as is often the case with astronomy, we know what we’ve observed, but we still don’t know what it means.

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One hundred years ago, Albert Einstein predicted the existence of ripples in the fabric of space called gravitational waves. He didn't believe we could ever hear them; he thought they'd be too quiet.

But scientists have just proven Einstein wrong a second time.

More than 1,000 physicists recorded the first gravitational waves on September 14, 2015, yet spent months confirming the unprecedented signal, officially announcing their discovery on February 11, 2016.

On Wednesday, the international collaboration announced its second-ever detection of gravitational waves - and the alluring signs of a third.

The researchers behind the huge experiment that found these events, called the Laser Interferometer Gravitational-Wave Observatory (LIGO), think two black holes collided to create the cosmic reverberations heard trillions of miles away on Earth.

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 An artist’s conception of the LISA Pathfinder spacecraft in orbit at Lagrange Point 1. Photovoltaic solar cells on the top of the spacecraft provide power. Micronewton thrusters can be seen on the sides of the spacecraft. The test masses and laser interferometer readout system are located inside the spacecraft

David Reitze, LIGO Laboratory, California Institute of Technology, Pasadena, CA 91125, USA
June 7, 2016• Physics 9, 63

The first results from the LISA Pathfinder mission demonstrate that two test masses can be put in free fall with a relative acceleration sufficiently free of noise to meet the requirements needed for space-based gravitational-wave detection.

The announcement in February 2016 that the Laser Interferometer Gravitational-wave Observatory (LIGO) had detected gravitational waves from the merger of two black holes stunned and electrified much of the physics and astronomy communities [1]. However, while all eyes were turned toward LIGO, the LISA Pathfinder (LPF)—a technology demonstration mission for the Laser Interferometer Space Antenna (LISA) gravitational-wave detector [2]—was quietly but convincingly paving the way toward the next revolution in gravitational-wave astronomy more than 1.5 million kilometers away from Earth. After a six-month program that began with the launch of the spacecraft in early December 2015, the team behind LPF has now announced the first results from the mission [3]. Following a 50-day journey to Lagrange Point 1 of the Sun-Earth system, LPF settled into orbit to begin a series of spacecraft acceptance tests and an observing campaign to measure the limits with which two test masses can achieve free fall.

LPF was designed to test many of the key technologies needed by LISA. LISA will target a much lower gravitational-wave frequency band than LIGO, from about 100 mHz to 1 Hz. This regime is sensitive to gravitational waves from mergers of intermediate to massive black holes in the range of 10^4 to 10^7 solar masses, as well as from mergers of black holes that have an extreme mass ratio (in which one black hole is much more massive than the other). But it necessitates a space-based platform to avoid low-frequency noise sources arising on Earth, which easily overwhelm the signal from such waves. These mergers will provide the most stringent tests of General Relativity in the strong-gravity regime.

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By collecting gravitational wave data at facilities such as LIGO in the US, researchers can better understand the constantly changing nature of the universe. Image credit: LIGO Caltech

The sooner-than-expected discovery of gravitational waves, announced in February, has given a new impetus to scientists in the field, who are now working to make sense of what it means not only for their research but also for our understanding of Einstein’s theory of general relativity.

Imagine two figure skaters on the ice, spiralling in closer and closer toward one another, until they finally form one spinning clump. But you can’t see them. Your only clue to their motion is by listening to how the ice subtly cracks and contorts under their movement over the roar of a packed crowd.

That’s the sort of challenge scientists are up against when they look for gravitational waves – ripples in the fabric of space which are given off during the merger of pairs of black holes or neutron stars, predicted by Einstein’s theory of general relativity.

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By Gabriel Popkin

APS April Meeting 2016 — Members of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Scientific Collaboration took a victory lap of sorts at the APS April Meeting 2016 in Salt Lake City, Utah. Talk after talk began with slides showing the now-famous signal from GW150914, the formal name for the September 14, 2015 detection of gravitational waves from two black holes that merged 1.3 billion years ago.

“For the first time when I present this talk, I can start with a discovery, not just upper limits,” said Alessandra Corsi, an astrophysicist at Texas Tech University.

But speakers quickly pivoted to new astrophysics emerging from GW150914 and LVT151012, a second candidate event that appeared in LIGO data but did not reach the critical “5-sigma” statistical threshold needed to claim a true detection. Researchers also shared new ideas for peering deeper into the universe and increasing the frequency spectrum that gravitational-wave detectors can probe.

For astrophysics, GW150914 heralded a series of firsts — not just the first detection of a gravitational wave, but also the the first proof that black holes form merging pairs (only inspiraling neutron stars had been previously seen), and the first evidence of black holes more than 25 times the mass of the sun. The large sizes of the merging black holes also revealed that their source stars were low in heavy elements, and that their spins were substantially lower than the maximum possible value allowed under general relativity.

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