Category: Dark Matter and Dark Energy

A New Twist In The Dark Matter Tale

An innovative interpretation of X-ray data from a cluster of galaxies could help scientists fulfill a quest they have been on for decades: determining the nature of dark matter.

The finding involves a new explanation for a set of results made with NASA’s Chandra X-ray Observatory, ESA’s XMM-Newton and Hitomi, a Japanese-led X-ray telescope. If confirmed with future observations, this may represent a major step forward in understanding the nature of the mysterious, invisible substance that makes up about 85% of matter in the universe.

“We expect that this result will either be hugely important or a total dud,” said Joseph Conlon of Oxford University who led the new study. “I don’t think there is a halfway point when you are looking for answers to one of the biggest questions in science.”

The story of this work started in 2014 when a team of astronomers led by Esra Bulbul (Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.) found a spike of intensity at a very specific energy in Chandra and XMM-Newton observations of the hot gas in the Perseus galaxy cluster.

This spike, or emission line, is at an energy of 3.5 kiloelectron volts (keV). The intensity of the 3.5 keV emission line is very difficult if not impossible to explain in terms of previously observed or predicted features from astronomical objects, and therefore a dark matter origin was suggested. Bulbul and colleagues also reported the existence of the 3.5 keV line in a study of 73 other galaxy clusters using XMM-Newton.

The plot of this dark matter tale thickened when only a week after Bulbul’s team submitted their paper a different group, led by Alexey Boyarsky of Leiden University in the Netherlands, reported evidence for an emission line at 3.5 keV in XMM-Newton observations of the galaxy M31 and the outskirts of the Perseus cluster, confirming the Bulbul et al. result.

However, these two results were controversial, with other astronomers later detecting the 3.5 keV line when observing other objects, and some failing to detect it.

The debate seemed to be resolved in 2016 when Hitomi especially designed to observe detailed features such as line emission in the X-ray spectra of cosmic sources, failed to detect the 3.5 keV line in the Perseus cluster.

“One might think that when Hitomi didn’t see the 3.5 keV line that we would have just thrown in the towel for this line of investigation,” said co-author Francesca Day, also from Oxford. “On the contrary, this is where, like in any good story, an interesting plot twist occurred.”

Conlon and colleagues noted that the Hitomi telescope had much fuzzier images than Chandra, so its data on the Perseus cluster are actually comprised of a mixture of the X-ray signals from two sources: a diffuse component of hot gas enveloping the large galaxy in the center of the cluster and X-ray emission from near the supermassive black hole in this galaxy. The sharper vision of Chandra can separate the contribution from the two regions. Capitalizing on this, Bulbul et al. isolated the X-ray signal from the hot gas by removing point sources from their analysis, including X-rays from material near the supermassive black hole.

In order to test whether this difference mattered, the Oxford team re-analyzed Chandra data from close to the black hole at the center of the Perseus cluster taken in 2009. They found something surprising: evidence for a deficit rather than a surplus of X-rays at 3.5 keV. This suggests that something in Perseus is absorbing X-rays at this exact energy. When the researchers simulated the Hitomi spectrum by adding this absorption line to the hot gas’ emission line seen with Chandra and XMM-Newton, they found no evidence in the summed spectrum for either absorption or emission of X-rays at 3.5 keV, consistent with the Hitomi observations.

The challenge is to explain this behavior: detecting absorption of X-ray light when observing the black hole and emission of X-ray light at the same energy when looking at the hot gas at larger angles away from the black hole.

In fact, such behavior is well known to astronomers who study stars and clouds of gas with optical telescopes. Light from a star surrounded by a cloud of gas often shows absorption lines produced when starlight of a specific energy is absorbed by atoms in the gas cloud. The absorption kicks the atoms from a low to a high energy state. The atom quickly drops back to the low energy state with the emission of light of a specific energy, but the light is re-emitted in all directions, producing a net loss of light at the specific energy—an absorption line—in the observed spectrum of the star. In contrast, an observation of a cloud in a direction away from the star would detect only the re-emitted, or fluorescent light at a specific energy, which would show up as an emission line.

The Oxford team suggests in their report that dark matter particles may be like atoms in having two energy states separated by 3.5 keV. If so, it could be possible to observe an absorption line at 3.5 keV when observing at angles close to the direction of the black hole, and an emission line when looking at the cluster hot gas at large angles away from the black hole.

“This is not a simple picture to paint, but it’s possible that we’ve found a way to both explain the unusual X-ray signals coming from Perseus and uncover a hint about what dark matter actually is,” said co-author Nicholas Jennings, also of Oxford.

To write the next chapter of this story, astronomers will need further observations of the Perseus cluster and others like it. For example, more data is needed to confirm the reality of the dip and to exclude a more mundane possibility, namely that we have a combination of an unexpected instrumental effect and a statistically unlikely dip in X-rays at an energy of 3.5 keV. Chandra, XMM-Newton and future X-ray missions will continue to observe clusters to address the dark matter mystery.

Star Mergers: A New Test Of Gravity, Dark Energy Theories

When scientists recorded a rippling in space-time, followed within two seconds by an associated burst of light observed by dozens of telescopes around the globe, they had witnessed, for the first time, the explosive collision and merger of two neutron stars.

The intense cosmological event observed on Aug. 17 also had other reverberations here on Earth: It ruled out a class of dark energy theories that modify gravity, and challenged a large class of theories.

Dark energy, which is driving the accelerating expansion of the universe, is one of the biggest mysteries in physics. It makes up about 68 percent of the total mass and energy of the universe and functions as a sort of antigravity, but we don’t yet have a good explanation for it. Simply put, dark energy acts to push matter away from each other, while gravity acts to pull matter together.

The neutron star merger created gravitational waves — a squiggly distortion in the fabric of space and time, like a tossed stone sending ripples across a pond — that traveled about 130 million light-years through space, and arrived at Earth at almost the same instant as the high-energy light that jetted out from this merger.

The gravity waves signature was detected by a network of Earth-based detectors called LIGO and Virgo, and the first intense burst of light was observed by the Fermi Gamma-ray Space Telescope.

That nearly simultaneous arrival time is a very important test for theories about dark energy and gravity.

“Our results make significant progress to elucidate the nature of dark energy,” said Miguel Zumalacárregui, a theoretical physicist who is part of the Berkeley Center for Cosmological Physics at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley.

“The simplest theories have survived,” he said. “It’s really about the timing.”

He and Jose María Ezquiaga, who was a visiting Ph.D. researcher in the Berkeley Center for Cosmological Physics, participated in this study, which was published Dec. 18 in the journal Physical Review Letters.

A 100-year-old “cosmological constant” theory introduced by Albert Einstein in relation to his work on general relativity and some other theories derived from this model remain as viable contenders because they propose that dark energy is a constant in both space and time: Gravitational waves and light waves are affected in the same way by dark energy, and thus travel at the same rate through space.

“The favorite explanation is this cosmological constant,” he said. “That’s as simple as it’s going to get.”

There are some complicated and exotic theories that also hold up to the test presented by the star-merger measurements. Massive gravity, for example — a theory of gravity that assigns a mass to a hypothetical elementary particle called a graviton — still holds a sliver of possibility if the graviton has a very slight mass.

Some other theories, though, which held that the arrival of gravitational waves would be separated in time from the arriving light signature of the star merger by far longer periods — stretching up to millions of years — don’t explain what was seen, and must be modified or scrapped.

The study notes that a class of theories known as scalar-tensor theories is particularly challenged by the neutron-star merger observations, including Einstein-Aether, MOND-like (relating to modified Newtonian dynamics), Galileon, and Horndeski theories, to name a few.

With tweaks, some of the challenged models can survive the latest test by the star merger, Zumalacárregui said, though they “lose some of their simplicity” in the process.

Zumalacárregui joined the cosmological center last year and is a Marie Sk?odowska-Curie global research fellow who specializes in studies of gravity and dark energy.

He began studying whether gravitational waves could provide a useful test of dark energy following the February 2016 announcement that the two sets of gravitational-wave detectors called LIGO (the Laser Interferometer Gravitational-Wave Observatory) captured the first confirmed measurement of gravitational waves. Scientists believe those waves were created in the merger of two black holes to create a larger black hole.

But those types of events do not produce an associated burst of light. “You need both — not just gravitational waves to help test theories of gravity and dark energy,” Zumalacárregui said.

Another study, which he published with Ezquiaga and others in April 2017, explored the theoretical conditions under which gravity waves could travel at a different velocity than light.

Another implication for this field of research is that, by collecting gravitational waves from these and possibly other cosmological events, it may be possible to use their characteristic signatures as “standard sirens” for measuring the universe’s expansion rate.

This is analogous to how researchers use the similar light signatures for objects — including a type of exploding stars known as Type Ia supernovae and pulsating stars known as cepheids — as “standard candles” to gauge their distance.

Cosmologists use a combination of such measurements to build a so-called distance ladder for gauging how far away a given object is from Earth, but there are some unresolved discrepancies that are likely due to the presence of space dust and imperfections in calculations.

Gathering more data from events that generate both gravitational waves and light could also help resolve different measurements of the Hubble constant — a popular gauge of the universe’s expansion rate.

The Hubble rate calibrated with supernovae distance measurements differs from the Hubble rate obtained from other cosmological observations, Zumalacárregui noted, so finding more standard sirens like neutron-star mergers could possibly improve the distance measurements.

The August neutron star merger event presented an unexpected but very welcome opportunity, he said.

“Gravitational waves are a very independent confirmation or refutation of the distance ladder measurements,” he said. “I’m really excited for the coming years. At least some of these nonstandard dark energy models could explain this Hubble rate discrepancy.

“Maybe we have underestimated some events, or something is unaccounted for that we’ll need to revise the standard cosmology of the universe,” he added. “If this standard holds, we will need radically new theoretical ideas that are difficult to verify experimentally, like multiple universes — the multiverse. However, if this standard fails, we will have more experimental avenues to test those ideas.”

New instruments and sky surveys are coming online that also aim to improve our understanding of dark energy, including the Berkeley Lab-led Dark Energy Spectroscopic Instrument project that is scheduled to begin operating in 2019. And scientists studying other phenomena, such as optical illusions in space caused by gravitational lensing — a gravity-induced effect that causes light from distant objects to bend and distort around closer objects — will also be useful in making more precise measurements.

“It could change the way we think about our universe and our place in it,” Zumalacárregui said. “It’s going to require new ideas.”