Month: December 2017

The Missing Link Between Exploding Stars, Clouds, And Climate On Earth

The study reveals how atmospheric ions, produced by the energetic cosmic rays raining down through the atmosphere, helps the growth and formation of cloud condensation nuclei — the seeds necessary for forming clouds in the atmosphere. When the ionization in the atmosphere changes, the number of cloud condensation nuclei changes affecting the properties of clouds. More cloud condensation nuclei mean more clouds and a colder climate, and vice versa. Since clouds are essential for the amount of Solar energy reaching the surface of Earth the implications can be significant for our understanding of why climate has varied in the past and also for future climate changes.

Cloud condensation nuclei can be formed by the growth of small molecular clusters called aerosols. It has until now been assumed that additional small aerosols would not grow and become cloud condensation nuclei, since no mechanism was known to achieve this. The new results reveal, both theoretically and experimentally, how interactions between ions and aerosols can accelerate the growth by adding material to the small aerosols and thereby help them survive to become cloud condensation nuclei. It gives a physical foundation to the large body of empirical evidence showing that Solar activity plays a role in variations in Earth’s climate. For example, the Medieval Warm Period around year 1000 AD and the cold period in the Little Ice Age 1300-1900 AD both fits with changes in Solar activity.

“Finally we have the last piece of the puzzle explaining how particles from space affect climate on Earth. It gives an understanding of how changes caused by Solar activity or by super nova activity can change climate.” says Henrik Svensmark, from DTU Space at the Technical University of Denmark, lead author of the study. Co-authors are senior researcher Martin Bødker Enghoff (DTU Space), Professor Nir Shaviv (Hebrew University of Jerusalem), and Jacob Svensmark, (University of Copenhagen).

The new study

The fundamental new idea in the study is to include a contribution to growth of aerosols by the mass of the ions. Although the ions are not the most numerous constituents in the atmosphere the electro-magnetic interactions between ions and aerosols compensate for the scarcity and make fusion between ions and aerosols much more likely. Even at low ionization levels about 5% of the growth rate of aerosols is due to ions. In the case of a nearby super nova the effect can be more than 50% of the growth rate, which will have an impact on the clouds and the Earth’s temperature.

To achieve the results a theoretical description of the interactions between ions and aerosols was formulated along with an expression for the growth rate of the aerosols. The ideas were then tested experimentally in a large cloud chamber. Due to experimental constraints caused by the presence of chamber walls, the change in growth rate that had to be measured was of the order 1%, which poses a high demand on stability during the experiments, and experiments were repeated up to 100 times in order to obtain a good signal relative to unwanted fluctuations. Data was taken over a period of 2 years with total 3100 hours of data sampling. The results of the experiments agreed with the theoretical predictions.

The hypothesis in a nutshell

Cosmic rays, high-energy particles raining down from exploded stars, knock electrons out of air molecules. This produces ions, that is, positive and negative molecules in the atmosphere.

The ions help aerosols — clusters of mainly sulphuric acid and water molecules — to form and become stable against evaporation. This process is called nucleation. The small aerosols need to grow nearly a million times in mass in order to have an effect on clouds.

The second role of ions is that they accelerate the growth of the small aerosols into cloud condensation nuclei — seeds on which liquid water droplets form to make clouds. The more ions the more aerosols become cloud condensation nuclei. It is this second property of ions which is the new result published in Nature Communications.

Low clouds made with liquid water droplets cool the Earth’s surface.

Variations in the Sun’s magnetic activity alter the influx of cosmic rays to the Earth.

When the Sun is lazy, magnetically speaking, there are more cosmic rays and more low clouds, and the world is cooler.

When the Sun is active fewer cosmic rays reach the Earth and, with fewer low clouds, the world warms up.

The implications of the study suggests that the mechanism can have affected:

The climate changes observed during the 20th century

The coolings and warmings of around 2oC that have occurred repeatedly over the past 10,000 years, as the Sun’s activity and the cosmic ray influx have varied.

The much larger variations of up to 10oC occuring as the Sun and Earth travel through the Galaxy visiting regions with varying numbers of exploding stars.

Radio Observations Point To Likely Explanation For Neutron-Star Merger Phenomena

Three months of observations with the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) have allowed astronomers to zero in on the most likely explanation for what happened in the aftermath of the violent collision of a pair of neutron stars in a galaxy 130 million light-years from Earth. What they learned means that astronomers will be able to see and study many more such collisions.

On August 17, 2017, the LIGO and VIRGO gravitational-wave observatories combined to locate the faint ripples in spacetime caused by the merger of two superdense neutron stars. It was the first confirmed detection of such a merger and only the fifth direct detection ever of gravitational waves, predicted more than a century ago by Albert Einstein.

The gravitational waves were followed by outbursts of gamma rays, X-rays, and visible light from the event. The VLA detected the first radio waves coming from the event on September 2. This was the first time any astronomical object had been seen with both gravitational waves and electromagnetic waves.

The timing and strength of the electromagnetic radiation at different wavelengths provided scientists with clues about the nature of the phenomena created by the initial neutron-star collision. Prior to the August event, theorists had proposed several ideas — theoretical models — about these phenomena. As the first such collision to be positively identified, the August event provided the first opportunity to compare predictions of the models to actual observations.

Astronomers using the VLA, along with the Australia Telescope Compact Array and the Giant Metrewave Radio Telescope in India, regularly observed the object from September onward. The radio telescopes showed the radio emission steadily gaining strength. Based on this, the astronomers identified the most likely scenario for the merger’s aftermath.

“The gradual brightening of the radio signal indicates we are seeing a wide-angle outflow of material, traveling at speeds comparable to the speed of light, from the neutron star merger,” said Kunal Mooley, now a National Radio Astronomy Observatory (NRAO) Jansky Postdoctoral Fellow hosted by Caltech.

The observed measurements are helping the astronomers figure out the sequence of events triggered by the collision of the neutron stars.

The initial merger of the two superdense objects caused an explosion, called a kilonova, that propelled a spherical shell of debris outward. The neutron stars collapsed into a remnant, possibly a black hole, whose powerful gravity began pulling material toward it. That material formed a rapidly-spinning disk that generated a pair of narrow, superfast jets of material flowing outward from its poles.

If one of the jets were pointed directly toward Earth, we would have seen a short-duration gamma-ray burst, like many seen before, the scientists said.

“That clearly was not the case,” Mooley said.

Some of the early measurements of the August event suggested instead that one of the jets may have been pointed slightly away from Earth. This model would explain the fact that the radio and X-ray emission were seen only some time after the collision.

“That simple model — of a jet with no structure (a so-called top-hat jet) seen off-axis — would have the radio and X-ray emission slowly getting weaker. As we watched the radio emission strengthening, we realized that the explanation required a different model,” said Alessandra Corsi, of Texas Tech University.

The astronomers looked to a model published in October by Mansi Kasliwal of Caltech, and colleagues, and further developed by Ore Gottlieb, of Tel Aviv University, and his colleagues. In that model, the jet does not make its way out of the sphere of explosion debris. Instead, it gathers up surrounding material as it moves outward, producing a broad “cocoon” that absorbs the jet’s energy.

The astronomers favored this scenario based on the information they gathered from using the radio telescopes. Soon after the initial observations of the merger site, the Earth’s annual trip around the Sun placed the object too close to the Sun in the sky for X-ray and visible-light telescopes to observe. For weeks, the radio telescopes were the only way to continue gathering data about the event.

“If the radio waves and X-rays both are coming from an expanding cocoon, we realized that our radio measurements meant that, when NASA’s Chandra X-ray Observatory could observe once again, it would find the X-rays, like the radio waves, had increased in strength,” Corsi said.

Mooley and his colleagues posted a paper with their radio measurements, their favored scenario for the event, and this prediction online on November 30. Chandra was scheduled to observe the object on December 2 and 6.

“On December 7, the Chandra results came out, and the X-ray emission had brightened just as we predicted,” said Gregg Hallinan, of Caltech.

“The agreement between the radio and X-ray data suggests that the X-rays are originating from the same outflow that’s producing the radio waves,” Mooley said.

“It was very exciting to see our prediction confirmed,” Hallinan said. He added, “An important implication of the cocoon model is that we should be able to see many more of these collisions by detecting their electromagnetic, not just their gravitational, waves.”

Mooley, Hallinan, Corsi, and their colleagues reported their findings in the scientific journal Nature.

Giant Bubbles On Red Giant Star’s Surface

Astronomers using ESO’s Very Large Telescope have for the first time directly observed granulation patterns on the surface of a star outside the Solar System — the ageing red giant π1 Gruis. This remarkable new image from the PIONIER instrument reveals the convective cells that make up the surface of this huge star, which has 350 times the diameter of the Sun. Each cell covers more than a quarter of the star’s diameter and measures about 120 million kilometres across. These new results are being published this week in the journal Nature.

Located 530 light-years from Earth in the constellation of Grus (The Crane), π1 Gruis is a cool red giant. It has about the same mass as our Sun, but is 350 times larger and several thousand times as bright [1]. Our Sun will swell to become a similar red giant star in about five billion years.

An international team of astronomers led by Claudia Paladini (ESO) used the PIONIER instrument on ESO’s Very Large Telescope to observe π1 Gruis in greater detail than ever before. They found that the surface of this red giant has just a few convective cells, or granules, that are each about 120 million kilometres across — about a quarter of the star’s diameter [2]. Just one of these granules would extend from the Sun to beyond Venus. The surfaces — known as photospheres — of many giant stars are obscured by dust, which hinders observations. However, in the case of π1 Gruis, although dust is present far from the star, it does not have a significant effect on the new infrared observations [3].

When π1 Gruis ran out of hydrogen to burn long ago, this ancient star ceased the first stage of its nuclear fusion programme. It shrank as it ran out of energy, causing it to heat up to over 100 million degrees. These extreme temperatures fueled the star’s next phase as it began to fuse helium into heavier atoms such as carbon and oxygen. This intensely hot core then expelled the star’s outer layers, causing it to balloon to hundreds of times larger than its original size. The star we see today is a variable red giant. Until now, the surface of one of these stars has never before been imaged in detail.

By comparison, the Sun’s photosphere contains about two million convective cells, with typical diameters of just 1500 kilometres. The vast size differences in the convective cells of these two stars can be explained in part by their varying surface gravities. π1 Gruis is just 1.5 times the mass of the Sun but much larger, resulting in a much lower surface gravity and just a few, extremely large, granules.

While stars more massive than eight solar masses end their lives in dramatic supernovae explosions, less massive stars like this one gradually expel their outer layers, resulting in beautiful planetary nebulae. Previous studies of π1 Gruis found a shell of material 0.9 light-years away from the central star, thought to have been ejected around 20,000 years ago. This relatively short period in a star’s life lasts just a few tens of thousands of years — compared to the overall lifetime of several billion — and these observations reveal a new method for probing this fleeting red giant phase.

Mars: Not As Dry As It Seems

When searching for life, scientists first look for an element key to sustaining it: fresh water.

Although today’s Martian surface is barren, frozen and inhabitable, a trail of evidence points to a once warmer, wetter planet, where water flowed freely. The conundrum of what happened to this water is long standing and unsolved. However, new research published in Nature suggests that this water is now locked in the Martian rocks.

Scientists at Oxford’s Department of Earth Sciences, propose that the Martian surface reacted with the water and then absorbed it, increasing the rocks oxidation in the process, making the planet uninhabitable.

Previous research has suggested that the majority of the water was lost to space as a result of the collapse of the planet’s magnetic field, when it was either swept away by high intensity solar winds or locked up as sub-surface ice. However, these theories do not explain where all of the water has gone.

Convinced that the planet’s minerology held the answer to this puzzling question, a team led by Dr Jon Wade, NERC Research Fellow in Oxford’s Department of Earth Sciences, applied modelling methods used to understand the composition of Earth rocks to calculate how much water could be removed from the Martian surface through reactions with rock. The team assessed the role that rock temperature, sub-surface pressure and general Martian make-up, have on the planetary surfaces.

The results revealed that the basalt rocks on Mars can hold approximately 25 per cent more water than those on Earth, and as a result drew the water from the Martian surface into its interior.

Dr Wade said: ‘People have thought about this question for a long time, but never tested the theory of the water being absorbed as a result of simple rock reactions. There are pockets of evidence that together, leads us to believe that a different reaction is needed to oxidise the Martian mantle. For instance, Martian meteorites are chemically reduced compared to the surface rocks, and compositionally look very different. One reason for this, and why Mars lost all of its water, could be in its minerology.

‘The Earth’s current system of plate tectonics prevents drastic changes in surface water levels, with wet rocks efficiently dehydrating before they enter the Earth’s relatively dry mantle. But neither early Earth nor Mars had this system of recycling water. On Mars, (water reacting with the freshly erupted lavas’ that form its basaltic crust, resulted in a sponge-like effect. The planet’s water then reacted with the rocks to form a variety of water bearing minerals. This water-rock reaction changed the rock mineralogy and caused the planetary surface to dry and become inhospitable to life.’

As to the question of why Earth has never experienced these changes, he said: ‘Mars is much smaller than Earth, with a different temperature profile and higher iron content of its silicate mantle. These are only subtle distinctions but they cause significant effects that, over time, add up. They made the surface of Mars more prone to reaction with surface water and able to form minerals that contain water. Because of these factors the planet’s geological chemistry naturally drags water down into the mantle, whereas on early Earth hydrated rocks tended to float until they dehydrate.’

The overarching message of Dr Wade’s paper, that planetary composition sets the tone for future habitability, is echoed in new research also published in Nature, examining the Earth’s salt levels. Co-written by Professor Chris Ballentine of Oxford’s Department of Earth Sciences, the research reveals that for life to form and be sustainable, the Earth’s halogen levels (Chlorine, Bromine and Iodine) have to be just right. Too much or too little could cause sterilisation. Previous studies have suggested that halogen level estimates in meteorites were too high. Compared to samples of the meteorites that formed the Earth, the ratio of salt to Earth is just too high.

Many theories have been put forward to explain the mystery of how this variation occurred, however, the two studies combined elevate the evidence and support a case for further investigation. Dr Wade said ‘Broadly speaking the inner planets in the solar system have similar composition, but subtle differences can cause dramatic differences — for example, rock chemistry. The biggest difference being, that Mars has more iron in its mantle rocks, as the planet formed under marginally more oxidising conditions.’

We know that Mars once had water, and the potential to sustain life, but by comparison little is known about the other planets, and the team are keen to change that.

Dr Wade, said: ‘To build on this work we want to test the effects of other sensitivities across the planets — very little is known about Venus for example. Questions like: what if the Earth had more or less iron in the mantle, how would that change the environment? What if the Earth was bigger or smaller? These answers will help us to understand how much of a role rock chemistry determines a planet’s future fate.

When looking for life on other planets it is not just about having the right bulk chemistry, but also very subtle things like the way the planet is put together, which may have big effects on whether water stays on the surface. These effects and their implications for other planets have not really been explored.’

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.

NASA Solves How A Jupiter Jet Stream Shifts Into Reverse

Speeding through the atmosphere high above Jupiter’s equator is an east-west jet stream that reverses course on a schedule almost as predictable as a Tokyo train’s. Now, a NASA-led team has identified which type of wave forces this jet to change direction.

Similar equatorial jet streams have been identified on Saturn and on Earth, where a rare disruption of the usual wind pattern complicated weather forecasts in early 2016. The new study combines modeling of Jupiter’s atmosphere with detailed observations made over the course of five years from NASA’s Infrared Telescope Facility, or IRTF, in Hawai’i. The findings could help scientists better understand the dynamic atmosphere of Jupiter and other planets, including those beyond our solar system.

“Jupiter is much bigger than Earth, much farther from the Sun, rotates much faster, and has a very different composition, but it turns out to be an excellent laboratory for understanding this equatorial phenomenon,” said Rick Cosentino, a postdoctoral fellow at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and lead author of the paper published in the Journal of Geophysical Research-Planets.

Earth’s equatorial jet stream was discovered after observers saw debris from the 1883 eruption of the Krakatoa volcano being carried by a westward wind in the stratosphere, the region of the atmosphere where modern airplanes achieve cruising altitude. Later, weather balloons documented an eastward wind in the stratosphere. Scientists eventually determined that these winds reversed course regularly and that both cases were part of the same phenomenon.

The alternating pattern starts in the lower stratosphere and propagates down to the boundary with the troposphere, or lowest layer of the atmosphere. In its eastward phase, it’s associated with warmer temperatures. The westward phase is associated with cooler temperatures. The pattern is called Earth’s quasi-biennial oscillation, or QBO, and one cycle lasts about 28 months. The phase of the QBO seems to influence the transport of ozone, water vapor and pollution in the upper atmosphere as well as the production of hurricanes.

Jupiter’s cycle is called the quasi-quadrennial oscillation, or QQO, and it lasts about four Earth years. Saturn has its own version of the phenomenon, the quasi-periodic oscillation, with a duration of about 15 Earth years. Researchers have a general understanding of these patterns but are still working out how much various types of atmospheric waves contribute to driving the oscillations and how similar the phenomena are to each other.

Previous studies of Jupiter had identified the QQO by measuring temperatures in the stratosphere to infer wind speed and direction. The new set of measurements is the first to span one full cycle of the QQO and covers a much larger area of Jupiter. Observations extended over a large vertical range and spanned latitudes from about 40 degrees north to about 40 degrees south. The team achieved this by mounting a high-resolution instrument called TEXES, short for Texas Echelon Cross Echelle Spectrograph, on the IRTF.

“These measurements were able to probe thin vertical slices of Jupiter’s atmosphere,” said co-author Amy Simon, a Goddard scientist who specializes in planetary atmospheres. “Previous data sets had lower resolution, so the signals were essentially smeared out over a large section of the atmosphere.”

The team found that the equatorial jet extends quite high into Jupiter’s stratosphere. Because the measurements covered such a large region, the researchers could eliminate several kinds of atmospheric waves from being major contributors to the QQO, leaving gravity waves as the primary driver. Their model assumes gravity waves are produced by convection in the lower atmosphere and travel up into the stratosphere, where they force the QQO to change direction.

The results of simulations were an excellent match to the new set of observations, indicating that they correctly identified the mechanism. On Earth, gravity waves are considered most likely to be responsible for forcing the QBO to change direction, though they don’t appear to be strong enough to do the job alone.

“Through this study we gained a better understanding of the physical mechanisms coupling the lower and upper atmosphere in Jupiter, and thus a better understanding of the atmosphere as a whole,” said Raúl Morales-Juberías, the second author on the paper and an associate professor at the New Mexico Institute of Mining and Technology in Socorro. “Despite the many differences between Earth and Jupiter, the coupling mechanisms between the lower and upper atmospheres in both planets are similar and have similar effects. Our model could be applied to study the effects of these mechanisms in other planets of the solar system and in exoplanets.”

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.”