Astronomers have finally found the last of the missing universe. It’s been hiding since the mid-1990s, when researchers decided to inventory all the “ordinary” matter in the cosmos—stars and planets and gas, anything made out of atomic parts. (This isn’t “dark matter,” which remains a wholly separate enigma.) They had a pretty good idea of how much should be out there, based on theoretical studies of how matter was created during the Big Bang. Studies of the cosmic microwave background (CMB)—the leftover light from the Big Bang—would confirm these initial estimates.
So they added up all the matter they could see—stars and gas clouds and the like, all the so-called baryons. They were able to account for only about 10 percent of what there should be. And when they considered that ordinary matter makes up only 15 percent of all matter in the universe—dark matter makes up the rest—they had only inventoried a mere 1.5 percent of all matter in the universe.
Now, in a series of three recent papers, astronomers have identified the final chunks of all the ordinary matter in the universe. (They are still deeply perplexed as to what makes up dark matter.) And despite the fact that it took so long to identify it all, researchers spotted it right where they had expected it to be all along: in extensive tendrils of hot gas that span the otherwise empty chasms between galaxies, more properly known as the warm-hot intergalactic medium, or WHIM.
Early indications that there might be extensive spans of effectively invisible gas between galaxies came from computer simulations done in 1998. “We wanted to see what was happening to all the gas in the universe,” said Jeremiah Ostriker, a cosmologist at Princeton University who constructed one of those simulations along with his colleague Renyue Cen. The two ran simulations of gas movements in the universe acted on by gravity, light, supernova explosions and all the forces that move matter in space. “We concluded that the gas will accumulate in filaments that should be detectable,” he said.
Except they weren’t — not yet.
“It was clear from the early days of cosmological simulations that many of the baryons would be in a hot, diffuse form — not in galaxies,” said Ian McCarthy, an astrophysicist at Liverpool John Moores University. Astronomers expected these hot baryons to conform to a cosmic superstructure, one made of invisible dark matter, that spanned the immense voids between galaxies. The gravitational force of the dark matter would pull gas toward it and heat the gas up to millions of degrees. Unfortunately, hot, diffuse gas is extremely difficult to find.
To spot the hidden filaments, two independent teams of researchers searched for precise distortions in the CMB, the afterglow of the Big Bang. As that light from the early universe streams across the cosmos, it can be affected by the regions that it’s passing through. In particular, the electrons in hot, ionized gas (such as the WHIM) should interact with photons from the CMB in a way that imparts some additional energy to those photons. The CMB’s spectrum should get distorted.
Unfortunately the best maps of the CMB (provided by the Planck satellite) showed no such distortions. Either the gas wasn’t there, or the effect was too subtle to show up.
But the two teams of researchers were determined to make them visible. From increasingly detailed computer simulations of the universe, they knew that gas should stretch between massive galaxies like cobwebs across a windowsill. Planck wasn’t able to see the gas between any single pair of galaxies. So the researchers figured out a way to multiply the faint signal by a million.
First, the scientists looked through catalogs of known galaxies to find appropriate galaxy pairs — galaxies that were sufficiently massive, and that were at the right distance apart, to produce a relatively thick cobweb of gas between them. Then the astrophysicists went back to the Planck data, identified where each pair of galaxies was located, and then essentially cut out that region of the sky using digital scissors. With over a million clippings in hand (in the case of the study led by Anna de Graaff, a Ph.D. student at the University of Edinburgh), they rotated each one and zoomed it in or out so that all the pairs of galaxies appeared to be in the same position. They then stacked a million galaxy pairs on top of one another. (A group led by Hideki Tanimura at the Institute of Space Astrophysics in Orsay, France, combined 260,000 pairs of galaxies.) At last, the individual threads — ghostly filaments of diffuse hot gas — suddenly became visible.
The technique has its pitfalls. The interpretation of the results, said Michael Shull, an astronomer at the University of Colorado at Boulder, requires assumptions about the temperature and spatial distribution of the hot gas. And because of the stacking of signals, “one always worries about ‘weak signals’ that are the result of combining large numbers of data,” he said. “As is sometimes found in opinion polls, one can get erroneous results when one has outliers or biases in the distribution that skew the statistics.”
In part because of these concerns, the cosmological community didn’t consider the case settled. What was needed was an independent way of measuring the hot gas. This summer, one arrived.
While the first two teams of researchers were stacking signals together, a third team followed a different approach. They observed a distant quasar — a bright beacon from billions of light-years away — and used it to detect gas in the seemingly empty intergalactic spaces through which the light traveled. It was like examining the beam of a faraway lighthouse in order to study the fog around it.
Usually when astronomers do this, they try to look for light that has been absorbed by atomic hydrogen, since it is the most abundant element in the universe. Unfortunately, this option was out. The WHIM is so hot that it ionizes hydrogen, stripping its single electron away. The result is a plasma of free protons and electrons that don’t absorb any light.
So the group decided to look for another element instead: oxygen. While there’s not nearly as much oxygen as hydrogen in the WHIM, atomic oxygen has eight electrons, as opposed to hydrogen’s one. The heat from the WHIM strips most of those electrons away, but not all. The team, led by Fabrizio Nicastro of the National Institute for Astrophysics in Rome, tracked the light that was absorbed by oxygen that had lost all but two of its electrons. They found two pockets of hot intergalactic gas. The oxygen “provides a tracer of the much larger reservoir of hydrogen and helium gas,” said Shull, who is a member of Nicastro’s team. The researchers then extrapolated the amount of gas they found between Earth and this particular quasar to the universe as a whole. The result suggested that they had located the missing 30 percent.
The number also agrees nicely with the findings from the CMB studies. “The groups are looking at different pieces of the same puzzle and are coming up with the same answer, which is reassuring, given the differences in their methods,” said Mike Boylan-Kolchin, an astronomer at the University of Texas, Austin.
The next step, said Shull, is to observe more quasars with next-generation X-ray and ultraviolet telescopes with greater sensitivity. “The quasar we observed was the best and brightest lighthouse that we could find. Other ones will be fainter, and the observations will take longer,” he said. But for now, the takeaway is clear. “We conclude that the missing baryons have been found,” their team wrote.
Russia and America’s Long Space Partnership Could Soon Fall Apart
It’s Not You
During the 1960s, the United States and Russia were engaged in a bitter space race. But starting in the 1970s, their rival space agencies started to collaborate. Nowadays, both countries help run the International Space Station.
But it’s starting to look, Ars Technica reports, as though international rivalries could tear that mutually beneficial relationship apart. If it does, it’ll be a blow not just to space research but to the prospects of a friendly, demilitarized international space community.
I Just Need Some Space
One key issue driving the split is that after NASA decommissioned its Space Shuttle program, it started relying on Russia to launch its astronauts and equipment into orbit. Increasingly, though, NASA has inked contracts with American companies like SpaceX, cutting Russia out of the loop.
“I think we are going through a long transition in the relationship,” space historian John Logsdon told Ars. “When Russia joined the station partnership, it demanded and got, on the basis of its human spaceflight experience, treatment as first among US partners. Now, 25 years later, it is no longer a space superpower, but one among several second-tier countries.”
What does China want to do on the Moon’s far side?
What will China’s Chang’e-4 mission learn about the far side of the Moon? Here are a few things the mission is designed to do.
Learn about the Moon’s history
No space mission has ever explored the far side from the surface. As such, it’s the first chance to explore a mysterious region of Earth’s natural satellite.
The “face” that’s never seen from Earth has some key differences to the more familiar “near side”. The far side has a thicker, older crust that is pocked with more craters. There are also very few of the “maria” (dark basaltic “seas” created by lava flows) that are evident on the near side.
Chang’e-4 has reportedly landed at a site known as Von Kármán crater, a 180km depression located in the far side’s southern hemisphere. But Von Kármán lies within a much bigger hole punched in the Moon – the South Pole-Aitken basin.
It’s the oldest, largest and deepest such basin on the Moon and formed when an asteroid – perhaps 500km across, or more – collided with it billions of years ago.
This event was so powerful that it is thought to have ploughed through the Moon’s outer crust layer and through into the zone known as the mantle.
One of the mission’s objectives is to study any exposed material from the mantle present at the landing site. This would provide insights into the internal structure and history of the Moon.
Indeed, data from orbiting spacecraft show that the composition of the basin is different from the surrounding lunar highlands. But exposed mantle material on the surface is just one possibility among several to explain this observation.
The rover will use its panoramic camera to identify interesting locations and its Visible and Near-Infrared Imaging Spectrometer (VNIS) to study minerals in the floor of the crater (as well as of ejecta – rocks thrown out by nearby space impacts).
Additionally, the Lunar Penetrating Radar (LPR) instrument will be able to look into the shallow subsurface of the Moon, down to a depth of about 100m. It could probe the thickness of the lunar regolith – the broken up rocks and dust that make up the surface – and shed light on the structure of the upper lunar crust.
After the huge impact that created the South Pole-Aitken basin, a large amount of melted rock would have filled the depression. The science team wants to use Chang’e-4 to identify and study variations in its composition.
Filling an astronomy gap
The far side of the Moon has long been regarded as an ideal spot for conducting a particular kind of radio astronomy – in the low-frequency band – because it’s shielded from the radio noise of Earth.
There’s a frequency band (below about 10MHz) where radio astronomy observations can’t be conducted from Earth, because of manmade radio interference and other, natural factors.
Chang’e-4’s lander is carrying an instrument called the Low Frequency Spectrometer (LFS) which can make low frequency radio observations. It will be used in concert with a similar experiment on the Queqiao orbiting satellite.
The objectives include making a map of the radio sky at low frequencies and studying the behaviour of the Sun.
Speaking in 2016, Liu Tongjie, from the Chinese space agency (CNSA), said: “Since the far side of the Moon is shielded from electromagnetic interference from the Earth, it’s an ideal place to research the space environment and solar bursts, and the probe can ‘listen’ to the deeper reaches of the cosmos.”
Thus, the mission will fill a gap in astronomical observation, allowing scientists to study cosmic phenomena in a way that has never been possible from our planet.
Radiation on the Moon
Several space agencies want to land humans on the Moon in the not-too-distant future, and might send astronauts there for longer than we’ve ever stayed before. So understanding the potential risks from radiation are vital.
Earth’s thick atmosphere and strong magnetic field provide adequate shielding against galactic cosmic rays and energetic charged particles travelling from the Sun.
But astronauts on the Moon will be outside this protective bubble and exposed to particles travelling through open space at near the speed of light – with potentially damaging consequences for their health.
The Lunar Lander Neutrons and Dosimetry (LND) experiment, supplied by researchers in Germany, will aim to fill in some gaps in our understanding about the lunar radiation environment.
It will provide dosimetry (measure the ionising radiation dose that could be absorbed by the human body) with a view to future exploration, and contribute to understanding of particles originating from the Sun.
Mysterious signals are coming from deep space
Image Credit: CC BY 4.0 ESO / S. Brunier
Astronomers have picked up a very unusual repeating signal from a distant galaxy and nobody knows what it is.
Known as a fast radio burst – the signal is a powerful burst of radio waves that, despite lasting mere milliseconds, generates as much energy as the Sun does in an entire day.
While several of these bursts have been picked up over the last few years, this one – which is coming from a source 1.5 billion light years away – is particularly unusual because it appears to be repeating.
It is only the second time a repeating fast radio burst has ever been detected by scientists and as things stand, its exact nature and origins remain a complete mystery.
It has even been suggested that these repeating signals could be evidence of intelligent aliens.
“Knowing that there is another suggests that there could be more out there,” said astrophysicist Ingrid Stairs from the University of British Columbia.
“And with more repeaters and more sources available for study, we may be able to understand these cosmic puzzles – where they’re from and what causes them.”
Source: BBC News |
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