Unprecedentedly Wide And Sharp Dark Matter Map

A research team of multiple institutes, including the National Astronomical Observatory of Japan and University of Tokyo, released an unprecedentedly wide and sharp dark matter map based on the newly obtained imaging data by Hyper Suprime-Cam on the Subaru Telescope. The dark matter distribution is estimated by the weak gravitational lensing technique. The team located the positions and lensing signals of the dark matter halos and found indications that the number of halos could be inconsistent with what the simplest cosmological model suggests. This could be a new clue to understanding why the expansion of the Universe is accelerating.

Mystery of the accelerated Universe

In the 1930’s, Edwin Hubble and his colleagues discovered the expansion of the Universe. This was a big surprise to most of the people who believed that the Universe stayed the same throughout eternity. A formula relating matter and the geometry of space-time was required in order to express the expansion of the Universe mathematically. Coincidentally, Einstein had already developed just such a formula. Modern cosmology is based on Einstein’s theory for gravity.

It had been thought that the expansion is decelerating over time because the contents of the Universe (matter) attract each other. But in the late 1990’s, it was found that the expansion has been accelerating since about 8 Giga years ago. This was another big surprise which earned the astronomers who found the expansion a Nobel Prize in 2011. To explain the acceleration, we have to consider something new in the Universe which repels the space.

The simplest resolution is to put the cosmological constant back into Einstein’s equation. The cosmological constant was originally introduced by Einstein to realize a static universe, but was abandoned after the discovery of the expansion of the Universe. The standard cosmological model (called LCDM) incorporates the cosmological constant. LCDM is supported by many observations, but the question of what causes the acceleration still remains. This is one of the biggest problems in modern cosmology.

Wide and deep imaging survey using Hyper Suprime-Cam

The team is leading a large scale imaging survey using Hyper Suprime-Cam (HSC) to probe the mystery of the accelerating Universe. The key here is to examine the expansion history of the Universe very carefully.

In the early Universe, matter was distributed almost but not quite uniformly. There were slight fluctuations in the density which can now be observed through the temperature fluctuations of the cosmic microwave background. These slight matter fluctuations evolved over cosmic time because of the mutual gravitational attraction of matter, and eventually the large scale structure of the present day Universe become visible. It is known that the growth rate of the structure strongly depends on how the Universe expands. For example, if the expansion rate is high, it is hard for matter to contract and the growth rate is suppressed. This means that the expansion history can be probed inversely through the observation of the growth rate.

It is important to note that growth rate cannot be probed well if we only observe visible matter (stars and galaxies). This is because we now know that nearly 80 % of the matter is an invisible substance called dark matter. The team adopted the ‘weak gravitation lensing technique.’ The images of distant galaxies are slightly distorted by the gravitational field generated by the foreground dark matter distribution. Analysis of the systematic distortion enables us to reconstruct the foreground dark matter distribution.

This technique is observationally very demanding because the distortion of each galaxy is generally very subtle. Precise shape measurements of faint and apparently small galaxies are required. This motivated the team to develop Hyper Suprime-Cam. They have been carrying out a wide field imaging survey using Hyper Suprime-Cam since March 2014. At this writing in February 2018, 60 % of the survey has been completed.

Unprecedentedly wide and sharp dark matter map

The team is now presenting the dark matter map based on the imaging data taken by April 2016. This is only 11 % of the planned final map, but it is already unprecedentedly wide. There has never been such a sharp dark matter map covering such a wide area.

Imaging observations are made through five different color filters. By combining these color data, it is possible to make a crude estimate of the distances to the faint background galaxies (called photometric redshift). At the same time, the lensing efficiency becomes most prominent when the lens is located directly between the distant galaxy and the observer. Using the photometric redshift information, galaxies are grouped into redshift bins. Using this grouped galaxy sample, dark matter distribution is reconstructed using tomographic methods and thus the 3D distribution can be obtained. Data for 30 square degrees are used to reconstruct the redshift range between 0.1 (~1.3 G light-years) and 1.0 (~8 G light-years). At the redshift of 1.0, the angular span corresponds to 1.0 G x 0.25 G light-years. This 3D dark matter mass map is also quite new. This is the first time the increase in the number of dark matter halos over time can be seen observationally.

What the dark matter halo count suggests and future prospects

The team counted the number of dark matter halos whose lensing signal is above a certain threshold. This is one of the simplest measurements of the growth rate. It is suggested that the number count of the dark matter halos is less than what is expected from LCDM. This could indicate there is a flaw in LCDM and that we might have to consider an alternative rather than the simple cosmological constant (Note 1).

The statistical significance is, however, still limited as the large error bars suggest. There has been no conclusive evidence to reject LCDM, but many astronomers are interested in testing LCDM because discrepancies can be a useful probe to unlock the mystery of the accelerating Universe. Further observation and analysis are needed to confirm the discrepancy with higher significance. There are some other probes of the growth rate and such analysis are also underway (e.g. angular correlation of galaxy shapes) in the team to check the validity of standard LCDM.

Hubble Observes Exoplanet Atmosphere In More Detail Than Ever Before

An international team of scientists has used the NASA/ESA Hubble Space Telescope to study the atmosphere of the hot exoplanet WASP-39b. By combining this new data with older data they created the most complete study yet of an exoplanet atmosphere. The atmospheric composition of WASP-39b hints that the formation processes of exoplanets can be very different from those of our own Solar System giants.

Investigating exoplanet atmospheres can provide new insight into how and where planets form around a star. “We need to look outward to help us understand our own Solar System,” explains lead investigator Hannah Wakeford from the University of Exeter in the UK and the Space Telescope Science Institute in the USA.

Therefore the British-American team combined the capabilities of the NASA/ESA Hubble Space Telescope with those of other ground- and space-based telescopes for a detailed study of the exoplanet WASP-39b. They have produced the most complete spectrum of an exoplanet’s atmosphere possible with present-day technology [1].

WASP-39b is orbiting a Sun-like star, about 700 light-years from Earth. The exoplanet is classified as a “Hot-Saturn,” reflecting both its mass being similar to the planet Saturn in our own Solar System and its proximity to its parent star. This study found that the two planets, despite having a similar mass, are profoundly different in many ways. Not only is WASP-39b not known to have a ring system, it also has a puffy atmosphere that is free of high-altitude clouds. This characteristic allowed Hubble to peer deep into its atmosphere.

By dissecting starlight filtering through the planet’s atmosphere [2] the team found clear evidence for atmospheric water vapour. In fact, WASP-39b has three times as much water as Saturn does. Although the researchers had predicted they would see water vapour, they were surprised by the amount that they found. This surprise, combined with the water abundance allowed to infer the presence of large amount of heavier elements in the atmosphere. This in turn suggests that the planet was bombarded by a lot of icy material which gathered in its atmosphere. This kind of bombardment would only be possible if WASP-39b formed much further away from its host star than it is right now.

“WASP-39b shows exoplanets are full of surprises and can have very different compositions than those of our Solar System,” says co-author David Sing from the University of Exeter, UK.

The analysis of the atmospheric composition and the current position of the planet indicate that WASP-39b most likely underwent an interesting inward migration, making an epic journey across its planetary system. “Exoplanets are showing us that planet formation is more complicated and more confusing than we thought it was. And that’s fantastic!,” adds Wakeford.

Having made its incredible inward journey WASP-39b is now eight times closer to its parent star, WASP-39, than Mercury is to the Sun and it takes only four days to complete an orbit. The planet is also tidally locked, meaning it always shows the same side to its star. Wakeford and her team measured the temperature of WASP-39b to be a scorching 750 degrees Celsius. Although only one side of the planet faces its parent star, powerful winds transport heat from the bright side around the planet, keeping the dark side almost as hot.

“Hopefully this diversity we see in exoplanets will help us figure out all the different ways a planet can form and evolve,” explains David Sing.

Looking ahead, the team wants to use the NASA/ESA/CSA James Webb Space Telescope — scheduled to launch in 2019 — to capture an even more complete spectrum of the atmosphere of WASP-39b. James Webb will be able to collect data about the planet’s atmospheric carbon, which absorbs light of longer wavelengths than Hubble can see [3]. Wakeford concludes: “By calculating the amount of carbon and oxygen in the atmosphere, we can learn even more about where and how this planet formed.”

Search For First Stars Uncovers ‘Dark Matter’

A team of astronomers led by Prof. Judd Bowman of Arizona State University unexpectedly stumbled upon “dark matter,” the most mysterious building block of outer space, while attempting to detect the earliest stars in the universe through radio wave signals, according to a study published this week in Nature.

The idea that these signals implicate dark matter is based on a second Nature paper published this week, by Prof. Rennan Barkana of Tel Aviv University, which suggests that the signal is proof of interactions between normal matter and dark matter in the early universe. According to Prof. Barkana, the discovery offers the first direct proof that dark matter exists and that it is composed of low-mass particles.

The signal, recorded by a novel radio telescope called EDGES, dates to 180 million years after the Big Bang.

What the universe is made of

“Dark matter is the key to unlocking the mystery of what the universe is made of,” says Prof. Barkana, Head of the Department of Astrophysics at TAU’s School of Physics and Astronomy. “We know quite a bit about the chemical elements that make up the earth, the sun and other stars, but most of the matter in the universe is invisible and known as ‘dark matter.’ The existence of dark matter is inferred from its strong gravity, but we have no idea what kind of substance it is. Hence, dark matter remains one of the greatest mysteries in physics.

“To solve it, we must travel back in time. Astronomers can see back in time, since it takes light time to reach us. We see the sun as it was eight minutes ago, while the immensely distant first stars in the universe appear to us on earth as they were billions of years in the past.”

Prof. Bowman and colleagues reported the detection of a radio wave signal at a frequency of 78 megahertz. The width of the observed profile is largely consistent with expectations, but they also found it had a larger amplitude (corresponding to deeper absorption) than predicted, indicating that the primordial gas was colder than expected.

Prof. Barkana suggests that the gas cooled through the interaction of hydrogen with cold, dark matter.

“Tuning in” to the early universe

“I realized that this surprising signal indicates the presence of two actors: the first stars, and dark matter,” says Prof. Barkana. “The first stars in the universe turned on the radio signal, while the dark matter collided with the ordinary matter and cooled it down. Extra-cold material naturally explains the strong radio signal.”

Physicists expected that any such dark matter particles would be heavy, but the discovery indicates low-mass particles. Based on the radio signal, Prof. Barkana argues that the dark-matter particle is no heavier than several proton masses. “This insight alone has the potential to reorient the search for dark matter,” says Prof. Barkana.

Once stars formed in the early universe, their light was predicted to have penetrated the primordial hydrogen gas, altering its internal structure. This would cause the hydrogen gas to absorb photons from the cosmic microwave background, at the specific wavelength of 21 cm, imprinting a signature in the radio spectrum that should be observable today at radio frequencies below 200 megahertz. The observation matches this prediction except for the unexpected depth of the absorption.

Prof. Barkana predicts that the dark matter produced a very specific pattern of radio waves that can be detected with a large array of radio antennas. One such array is the SKA, the largest radio telescope in the world, now under construction. “Such an observation with the SKA would confirm that the first stars indeed revealed dark matter,” concludes Prof. Barkana.

Black Holes From Small Galaxies Might Emit Gamma Rays

As a general rule of thumb, if there is a puzzling phenomenon occurring somewhere deep in outer space, a black hole is often the culprit behind it.

This is according to postdoctoral researcher Vaidehi Paliya in the department of physics and astronomy, whose January 2018 publication in The Astrophysical Journal Letters details the discovery of seven galaxies that could potentially shake up what astrophysicists thought they knew about how the size of a galaxy — and the black hole at its center — can affect its behavior.

It has been widely believed that only massive galaxies contain enough energy to become blazars, which are stupendous jets of radiation powerful enough to stretch thousands of light years. But Paliya’s latest research might indicate that smaller galaxies can also do this, if the conditions are right.

There are three main types of galaxies: oval-shaped ellipticals, disk-like spirals and irregulars that don’t quite fit in with either of the former classes.

“Elliptical galaxies are the oldest, most massive galaxies in the universe,” Paliya said. “People propose that elliptical galaxies form when two smaller galaxies collide, merging into one big elliptical. Typically, ellipticals are found to host a black hole that is more than a billion times the mass of our sun.”

Through their inherent, inescapable gravitational force, black holes at the center of galaxies will grow larger by drawing in and “eating” the surrounding matter through a process called accretion.

“It’s like when you pour water in the kitchen sink, you see it forms a spiral, then it goes down the drain. In a similar way, matter forms an accretion disk around the black hole,” Paliya said. “The black hole then grows rapidly and becomes a monster.”

But when the accretion disk surrounding the black hole begins emitting extreme bursts of energy — in radio, infrared or X-ray bands — the galaxy is said to be “active,” opening the door to another galaxy classifier beyond shape.

“Blazars are one type of active galaxy,” said Marco Ajello, a professor of physics and astronomy and Paliya’s advisor. “These are galaxies that host a supermassive black hole, and this black hole — in some way — is able to accelerate particles to near the speed of light and keep them collimated in narrow beams, called jets, which become very bright sources of light when they are pointing toward us.”

These jets are some of the most extreme sources of gamma-ray radiation in outer space.

“These blazars have jets that are like fountains. If you wanted a huge fountain, you’d need to have a very powerful engine at the base. Blazars need to have very massive black holes at their centers to be able to launch jets,” Paliya said. “Generally, we don’t expect these powerful jets from sources that are small, like our galaxy.”

The Milky Way is a spiral galaxy with pinwheel-like arms made up of gas and dust that contain a bright center of older stars. Typically, spiral galaxies are less massive and much less active than their elliptical counterparts.

When the Fermi Gamma-Ray Space Telescope, launched in 2008 by NASA, detected gamma ray emission from four spiral galaxies in its first year of orbit, physicists were perplexed.

“It was unexpected — we have only seen that kind of gamma ray emission from blazars,” said Dieter Hartmann, a professor of physics and astronomy and co-author of the study. “When these four sources were discovered, people speculated that they could be blazars. But since there were so few sources, nobody was certain about it. Then the question became: are these really a new type of source, or are they just exceptions to the standard?”

The question was left up in the air, until Paliya’s collaborators in India released a catalog of active spiral galaxies in 2017. Known as Seyfert galaxies, these are another type of active galaxy with relatively low mass black holes residing at their centers. However, rather than emitting violent bursts of gamma-ray radiation, like blazars, Seyfert galaxies are known for their strong ultra-violet emissions.

The catalog provided the first chance for astrophysicists to address the question of the Fermi telescope’s 2008 discovery. Is it possible for a spiral galaxy to emit jetted gamma-ray radiation?

“I took this catalog of 11,101 Seyfert galaxies, and I studied them in the gamma ray band using the data from the Large Area Telescope onboard Fermi satellite,” Paliya said. “From that, I found four new gamma ray sources and three that were earlier known as blazars but we believe are actually Seyfert galaxies.”

This breakthrough is an indication that even smaller sources are capable of launching powerful gamma ray jets — a potential paradigm shift in the field of astrophysics.

“If the jet is similar to that of blazars, but its black hole is small, you can imagine it like a car. Say a smaller car is going the same speed as another car that has a much bigger engine. The engine in the smaller car would then need to be much more efficient,” Ajello said. “So, it could be that the black hole is working more efficiently in smaller, spiral systems than it is in larger objects like blazars.”

To understand the elliptical/spiral nature of the host galaxies of these seven gamma-ray detected sources, Ajello and Paliya intend to obtain deep images with the highest resolution — a challenge for ground-based optical telescopes due to the blurring effects of the atmosphere.

“The light-collecting power of a telescope is proportional to the square of its diameter. This means that with bigger telescopes, we can collect a lot more photons. More photons mean more information,” Paliya said.

The Gran Telescopio Canarias, or the “Great Canary Telescope,” is a 10.4-meter reflecting telescope that began gathering observations in 2007. Currently holding the title of the “world’s largest single-aperture optical telescope,” the Gran Telescopio Canarias is slated to be surpassed in the next decade with the unveiling of the Thirty Meter Telescope (TMT). When finished, TMT will have a 30-meter primary mirror and will allow researchers to see outer space with unprecedented clarity — at least 10 times better than the Hubble Space Telescope.

Ajello and Paliya intend to use the Hubble Space Telescope, and potentially upcoming facilities like TMT, to peer beyond the bright centers of the seven sources they uncovered to distinguish with certainty whether the galaxies are elliptical or spiral.

“If it is an elliptical, then it’s true that we are just looking at a normal blazar. It’s probably a smaller elliptical and a smaller black hole,” Ajello said. “But if it’s a spiral, then the jets can happen in any environment that is a black hole, within some newfound conditions.”

“It is of great importance to better understand the environments of super-massive black holes that are able to launch jets in which particle acceleration takes place under extreme astrophysical conditions,” Hartmann added.

Paliya also intends to study whether the differences observed in gamma rays translate across the electromagnetic spectrum.

“This is all about optics,” Paliya said. “How do blazars behave at, say, radio frequencies? Then, how do these Seyferts compare? This discovery has indicated that yes, something different is occurring.”

The researchers said that discoveries such as these are important in helping us understand the evolution of the universe. These discoveries could represent some of the missing pieces of the puzzle of how galaxies and black holes have grown together throughout history.

Mineralogy Of Potential Lunar Exploration Site

A detailed study of a giant impact crater on the Moon’s far side could provide a roadmap for future lunar explorers.

The study, by planetary scientists from Brown University, maps the mineralogy of the South Pole-Aitken (SPA) basin, a gash in the lunar surface with a diameter of approximately 2,500 kilometers (1,550 miles). SPA is thought to be the oldest and largest impact basin on the Moon, and scientists have long had their eyes on it as a target for future lunar landers.

“This is a highly detailed look at the compositional structure of this huge impact basin using modern, cutting-edge data,” said Dan Moriarty, a postdoctoral researcher at NASA’s Goddard Space Flight Center who led the research while a doctoral student at Brown. “Given that it’s such an important target for future exploration and perhaps returning a sample to Earth, we hope this will serve as a framework for more detailed study and landing site selection.”

The study will be published in the Journal of Geophysical Research: Planets.

The impact that created SPA is thought to have blasted all the way through the Moon’s crust and into the mantle, which is part of the reason that scientists are so interested in it. Visiting SPA and grabbing a sample of that exposed mantle material could provide critical clues about the Moon’s origin and evolution. A sample could also help scientists put a firm date on the impact. SPA is thought to be the Moon’s oldest basin, so a firm date would be a key milestone in the timeline of lunar history as well as events affecting early Earth.

But in order to get the right samples, it’s important to know the best spots to find them. That’s what Moriarty and co-author Carlé Pieters, a professor in Brown’s Department of Earth, Environmental and Planetary Sciences, had in mind for this study. They used detailed data from Moon Mineralogy Mapper, a spectrometer that flew aboard India’s Chandrayaan-1 spacecraft for which Pieters is principal investigator.

“Having global access with modern imaging spectrometers from lunar orbit is the next best thing to having a geologist with a rock hammer doing the field work across the surface.” Pieters said. “Ideally, in the future we’ll have both working together.”

The research identified four distinct mineralogical regions that form a bullseye pattern within and around the basin. At the bulleye’s center is a region of what appears to be deposits of volcanic material, a sign that the center of the basin may have been covered by a volcanic flow sometime soon after the SPA impact. That central region is surrounded by a ring of material dominated by magnesium-rich pyroxene, a mineral thought to be plentiful in the lunar mantle. Outside of that is a ring in which pyroxene mixes with the standard crustal rocks of the lunar highlands. Outside of that ring is the basin exterior, where the signatures of impact-related material disappear.

The findings have some interesting implications for SPA exploration, the researchers say. The research suggests, for example, that finding pristine mantle material in the middle of the basin might be a bit tricky because of the large volcanic deposit.

“That’s a little bit counterintuitive,” Moriarty said. “Typically the deepest excavation would be in the middle of the crater. But we show that the middle of SPA has been covered over by what looks like a volcanic flow.”

So if you’re looking for mantle, it might be wise to land in the ring surrounding the center, where what appears to be mantle material is highly concentrated.

But an ideal landing site, Moriarty says, might be a spot that has both mantle and volcanic material, because those volcanics are interesting in their own right. Their composition is a little different than that of other volcanic rocks found on the Moon, which suggests they have a unique origin.

“If these rocks are indeed volcanic, it means that there was a really interesting kind of volcanism happening at SPA,” Moriarty said. “It could be related to the extreme geophysical environment that would have been in place during the formation of the basin. That would be really interesting to look at in more depth.”

With that in mind, Moriarty says a good spot to land might be near the border of the volcanic center and the pyroxene ring. Another strategy could be to look for a spot where the volcanic material has been pierced by a subsequent impact. Moriarty and Pieters found several such craters in the volcanic patch where the pyroxene material has been re-excavated.

“We think going after both mantle and volcanics would make for a richer science return,” Moriarty said.

Moriarty is hopeful that these findings will give mission planners something to think about. China is currently in the process of planning for a mission to SPA. The region has appeared repeatedly on NASA’s “decadal survey” of planetary scientists, which is used to inform the agency’s mission priorities.

“Impacts are the dominant process that drove solar system creation and evolution, and SPA is the largest confirmed impact structure on the Moon, if not the entire solar system,” Moriarty said. “That makes it an important end member in understanding impact processes. We think this work could provide a roadmap for exploring SPA in more detail.”

The Moon Formed Inside A Vaporized Earth Synestia

A new explanation for the Moon’s origin has it forming inside the Earth when our planet was a seething, spinning cloud of vaporized rock, called a synestia. The new model led by researchers at the University of California, Davis and Harvard University resolves several problems in lunar formation and is published Feb. 28 in the Journal of Geophysical Research — Planets.

“The new work explains features of the Moon that are hard to resolve with current ideas,” said Sarah Stewart, professor of Earth and Planetary Sciences at UC Davis. “The Moon is chemically almost the same as the Earth, but with some differences,” she said. “This is the first model that can match the pattern of the Moon’s composition.”

Current models of lunar formation suggest that the Moon formed as a result of a glancing blow between the early Earth and a Mars-size body, commonly called Theia. According to the model, the collision between Earth and Theia threw molten rock and metal into orbit that collided together to make the Moon.

The new theory relies instead on a synestia, a new type of planetary object proposed by Stewart and Simon Lock, graduate student at Harvard and visiting student at UC Davis, in 2017. A synestia forms when a collision between planet-sized objects results in a rapidly spinning mass of molten and vaporized rock with part of the body in orbit around itself. The whole object puffs out into a giant donut of vaporized rock.

Synestias likely don’t last long — perhaps only hundreds of years. They shrink rapidly as they radiate heat, causing rock vapor to condense into liquid, finally collapsing into a molten planet.

“Our model starts with a collision that forms a synestia,” Lock said. “The Moon forms inside the vaporized Earth at temperatures of four to six thousand degrees Fahrenheit and pressures of tens of atmospheres.”

An advantage of the new model, Lock said, is that there are multiple ways to form a suitable synestia — it doesn’t have to rely on a collision with the right sized object happening in exactly the right way.

Once the Earth-synestia formed, chunks of molten rock injected into orbit during the impact formed the seed for the Moon. Vaporized silicate rock condensed at the surface of the synestia and rained onto the proto-Moon, while the Earth-synestia itself gradually shrank. Eventually, the Moon would have emerged from the clouds of the synestia trailing its own atmosphere of rock vapor. The Moon inherited its composition from the Earth, but because it formed at high temperatures it lost the easily vaporized elements, explaining the Moon’s distinct composition.

Additional authors on the paper are Michail Petaev and Stein Jacobsen at Harvard University, Zoe Leinhardt and Mia Mace at the University of Bristol, England and Matija Cuk, SETI Institute, Mountain View, Calif. The work was supported by grants from NASA, the U.S. Department of Energy and the UK’s Natural Environment Research Council.

Within 180 Million Years Of The Big Bang, Stars Were Born

Long ago, about 400,000 years after the beginning of the universe (the Big Bang), the universe was dark. There were no stars or galaxies, and the universe was filled primarily with neutral hydrogen gas.

Then, for the next 50-100 million years, gravity slowly pulled the densest regions of gas together until ultimately the gas collapsed in some places to form the first stars.

What were those first stars like and when did they form? How did they affect the rest of the universe? These are questions astronomers and astrophysicists have long pondered.

Now, after 12 years of experimental effort, a team of scientists, led by ASU School of Earth and Space Exploration astronomer Judd Bowman, has detected the fingerprints of the earliest stars in the universe. Using radio signals, the detection provides the first evidence for the oldest ancestors in our cosmic family tree, born by a mere 180 million years after the universe began.

“There was a great technical challenge to making this detection, as sources of noise can be a thousand times brighter than the signal — it’s like being in the middle of a hurricane and trying to hear the flap of a hummingbird’s wing.” says Peter Kurczynski, the National Science Foundation program officer who supported this study. “These researchers with a small radio antenna in the desert have seen farther than the most powerful space telescopes, opening a new window on the early universe.”

Radio Astronomy

To find these fingerprints, Bowman’s team used a ground-based instrument called a radio spectrometer, located at the Australia’s national science agency (CSIRO) Murchison Radio-astronomy Observatory (MRO) in Western Australia. Through their Experiment to Detect the Global EoR Signature (EDGES), the team measured the average radio spectrum of all the astronomical signals received across most of the southern-hemisphere sky and looked for small changes in power as a function of wavelength (or frequency).

As radio waves enter the ground-based antenna, they are amplified by a receiver, and then digitized and recorded by computer, similar to how FM radio receivers and TV receivers work. The difference is that the instrument is very precisely calibrated and designed to perform as uniformly as possible across many radio wavelengths.

The signals detected by the radio spectrometer in this study came from primordial hydrogen gas that filled the young universe and existed between all the stars and galaxies. These signals hold a wealth of information that opens a new window on how early stars — and later, black holes, and galaxies — formed and evolved.

“It is unlikely that we’ll be able to see any earlier into the history of stars in our lifetimes,” says Bowman. “This project shows that a promising new technique can work and has paved the way for decades of new astrophysical discoveries.”

This detection highlights the exceptional radio quietness of the MRO, particularly as the feature found by EDGES overlaps the frequency range used by FM radio stations. Australian national legislation limits the use of radio transmitters within 161.5 miles (260 km) of the site, substantially reducing interference which could otherwise drown out sensitive astronomy observations.

The results of this study have been recently published in Nature by Bowman, with co-authors Alan Rogers of the Massachusetts Institute of Technology’s Haystack Observatory, Raul Monsalve of the University of Colorado, and Thomas Mozdzen and Nivedita Mahesh also of ASU’s School of Earth and Space Exploration.

Unexpected results

The results of this experiment confirm the general theoretical expectations of when the first stars formed and the most basic properties of early stars.

“What’s happening in this period,” says co-author Rogers of MIT’s Haystack Observatory, “is that some of the radiation from the very first stars is starting to allow hydrogen to be seen. It’s causing hydrogen to start absorbing the background radiation, so you start seeing it in silhouette, at particular radio frequencies. This is the first real signal that stars are starting to form, and starting to affect the medium around them.”

The team originally tuned their instrument to look later in cosmic time, but in 2015 decided to extend their search. “As soon as we switched our system to this lower range, we started seeing things that we felt might be a real signature,” Rogers says. “We see this dip most strongly at about 78 megahertz, and that frequency corresponds to roughly 180 million years after the Big Bang,” Rogers says. “In terms of a direct detection of a signal from the hydrogen gas itself, this has got to be the earliest.”

The study also revealed that gas in the universe was probably much colder than expected (less than half the expected temperature). This suggests that either astrophysicists’ theoretical efforts have overlooked something significant or that this may be the first evidence of non-standard physics: Specifically, that baryons (normal matter) may have interacted with dark matter and slowly lost energy to dark matter in the early universe, a concept that was originally proposed by Rennan Barkana of Tel Aviv University.

“If Barkana’s idea is confirmed,” says Bowman, “then we’ve learned something new and fundamental about the mysterious dark matter that makes up 85 percent of the matter in the universe, providing the first glimpse of physics beyond the standard model.”

The next steps in this line of research are for another instrument to confirm this team’s detection and to keep improving the performance of the instruments, so that more can be learned about the properties of early stars. “We worked very hard over the last two years to validate the detection,” says Bowman, “but having another group confirm it independently is a critical part of the scientific process.”

Bowman would also like to see an acceleration of efforts to bring on new radio telescopes like the Hydrogen Epoch of Reionization Array (HERA) and the Owens Valley Long Wavelength Array (OVRO-LWA).

“Now that we know this signal exists,” says Bowman, “we need to rapidly bring online new radio telescopes that will be able to mine the signal much more deeply.”

The antennas and portions of the receiver used in this experiment were designed and constructed by Rogers and the MIT Haystack Observatory team. The ASU team and Monsalve added the automated antenna reflection measurement system to the receiver, outfitted the control hut with the electronics, constructed the ground plane and conducted the field work for the project. The current version of EDGES is the result of years of design iteration and ongoing detailed technical refinement of the calibration instrumentation to reach the levels of precision necessary for successfully achieving this difficult measurement.