Unlocking the secrets of the universe

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.

This artist's rendering shows the universe's first, massive, blue stars embedded in gaseous filaments, with the cosmic
microwave background just visible at the edges. Using radio observations of the distant universe, NSF-funded researchers
Judd Bowman of Arizona State University, Alan Rogers of MIT and their colleagues discovered the influence of such early
stars on primordial gas. Although they can't directly see the light from the massive stars, Bowman's team was able to infer
their presence from dimming of the cosmic microwave background (CMB), a result of the gaseous filaments absorbing the
stars' UV light. The CMB is dimmer than expected, indicating that the filaments may have been colder than expected,
possibly from interactions with dark matter [Credit: N.R.Fuller, National Science Foundation]

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).

In each instrument, radio waves are collected by an antenna consisting of two rectangular metal panels mounted
horizontally on fiberglass legs above a metal mesh. The EDGES detection required the radio quietness at the Murchison
Radio-astronomy Observatory, as Australian national legislation limits the use of radio transmitters near the site. This
discovery sets the stage for follow-up observations with other powerful low-frequency facilities, including HERA
and the forthcoming SKA-low [Credit: CSIRO Australia]

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.

A timeline of the universe, updated to show when the first stars emerged. This updated timeline of the universe reflects the
recent discovery that the first stars emerged by 180 million years after the Big Bang. The research behind this timeline
was conducted by Judd Bowman of Arizona State University and his colleagues, with funding from
 the National Science Foundation [Credit: N.R.Fuller, National Science Foundation]

"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.

Source: Arizona State University [February 28, 2018]

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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 in the Journal of Geophysical Research - Planets.

This artist's rendering shows the hot, molten Moon emerging from a synestia, a giant spinning donut of vaporized rock
that formed when planet-sized objects collided. The synestia is in the process of condensing to form the Earth.
This new model for the Moon's origin answers outstanding questions about how the Moon's composition
compares to that of Earth [Credit: Sarah Stewart/UC Davis based on NASA rendering]

"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.

Source: University of California - Davis [February 28, 2018]

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Black hole blasts may transform 'mini-Neptunes' into rocky worlds

A team of astrophysicists and planetary scientists has predicted that Neptune-like planets located near the center of the Milky Way were transformed into rocky planets by outbursts generated by the nearby supermassive black hole.

Credit: Harvard University

These findings combine computer simulations with data from recent exoplanet findings, as well as X-ray and ultraviolet observations of stars and black holes.

"It's pretty wild to think of black holes shaping the evolutionary destiny of a planet, but that very well may be the case in the center of our galaxy," said Howard Chen of Northwestern University in Illinois, who led the study.

Chen and collaborators from the Harvard-Smithsonian Center for Astrophysics (CfA) examined the environment around the closest supermassive black hole to Earth: the 4 million solar mass black hole known as Sagittarius A*.

It is well known that material falling into the black hole in occasional feeding frenzies will generate bright flares of X-ray and ultraviolet radiation. Indeed, X-ray telescopes such as NASA's Chandra X-ray Observatory and ESA's XMM-Newton have seen evidence for bright outbursts generated in the past, ranging from about 6 million years to just over a century ago.

"We wondered what these outbursts from Sagittarius A* would do to any planets in its vicinity," said John Forbes, a co-author from the CfA. "Our work shows the black hole could dramatically change a planet's life."

The authors considered the effects of this high-energy radiation on planets within 70 light-years of the black hole that have masses in between Earth and Neptune's.

They found that the X-ray and ultraviolet radiation would blast away a large amount of the thick gas atmosphere of such planets near the black hole. In some cases this would leave behind a bare, rocky core. Such rocky planets would be heavier than the Earth and are what astronomers call super-Earths.

"These super-Earths are one of the most common types of planet that astronomers have discovered outside our solar system," said co-author Avi Loeb, also of CfA, "Our work shows that in the right environment they might form in exotic ways."

The researchers think that this black hole impact may be one of the most common ways for rocky super-Earths to form close to the center of our galaxy.

While some of these planets will be located in the habitable zone of stars like the sun, the environment in which they exist would make it challenging for life to arise. The super-Earths would be buffeted by supernova explosions and gamma ray bursts, which might damage the chemistry of any atmosphere remaining on the planets. Additional outbursts from the supermassive black hole could provide a knockout punch and completely erode a planet's atmosphere.

These planets would also be subjected to the gravitational disruptions of a passing star that could fling the planet away from its life-sustaining host. Such encounters might occur frequently near the Milky Way's supermassive black hole since the region is so packed with stars. How crowded is it in the Galactic Center? Within about 70 light-years of the center of the galaxy, astronomers think the average separation between rocky worlds is between about 75 and 750 billion kilometers. By comparison, the nearest star to the solar system is 40,000 billion kilometers away.

"It is generally accepted that the innermost regions of the Milky Way are not favorable for life. Indeed, even though the deck seems stacked against life in this region, the likelihood of panspermia, where life is transmitted via interplanetary or interstellar contact, would be much more common in such a dense environment." said Loeb. "This process might give life a fighting chance to arise and survive."

There are formidable challenges required to directly detect such planets. The distance to the Galactic Center (26,000 light years from Earth), the crowded region, and the blocking of light by intervening dust and gas all make it hard to observe such planets.

However, these challenges may be met by the next generation of extraordinarily large ground-based telescopes. For example, searches for transits with future observatories like the European Extremely Large Telescope might detect evidence for these planets. Another possibility is searching for stars with unusual patterns of elements in their atmosphere that have migrated away from the center of the galaxy.

The study is published in The Astrophysical Journal  and is available online.

Source: Harvard University [February 28, 2018]

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How does water change the moon's origin story?

It's amazing what a difference a little water can make. The Moon formed between about 4.4 and 4.5 billion years ago when an object collided with the still-forming proto-Earth. This impact created a hot and partially vaporized disk of material that rotated around the baby planet, eventually cooling and accreting into the Moon.

Screen shot from a video simulation of the canonical model of the Moon's formation, in which the proto-Earth was hit
by a Mars-sized object between 4.4. and 4.5 billion years ago [Credit: Miki Nakajima and Dave Stevenson]

For years, scientists thought that in the aftermath of the collision hydrogen dissociated from water molecules and it and other elements that have low boiling temperatures, so-called "volatile elements," escaped from the disk and were lost to space. This would lead to a dry and volatile element-depleted Moon, which seemed to be consistent with previous analyses of lunar samples.

But ongoing research about the Moon's chemistry is revealing that it may be wetter than initially thought, which raises questions about some aspects of this origin story.

"This is still very much an area of active research, so there is much that scientists, including our Department of Terrestrial Magnetism staff scientist Erik Hauri, as well as many other Carnegie colleagues and alumni, are figuring out about how much water exists in the Moon. This is a highly important and challenging question to answer given that we have limited knowledge on the history and distribution of lunar water," explained Carnegie's Miki Nakajima who, together with Caltech's Dave Stevenson, set out to determine whether prevailing Moon-formation theories needed to be adjusted to account for the more recent higher estimates of lunar water content.