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Astronomers Have Found the Universe’s Missing Matter

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.

(A) Images of one million galaxy pairs were aligned and added together.
(B) Astronomers mapped all the gas within the actual galaxies.
(C) By subtracting the galaxies (B) from the initial image (A), researchers revealed filamentary gas hiding in intergalactic space.Adapted by Quanta Magazine

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.

Lighthouse Effect

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.

Fabrizio Nicastro used light from a quasar to track the missing gas.Courtesy of Fabrizio Nicastro

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.

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Space

Voyager 2 has discovered something amazing: Denser space outside the solar system

In November 2018, after a 41-year voyage, Voyager 2 crossed the boundary beyond which the Sun’s influence ends, and entered interstellar space. But the mission of the little probe is not yet complete – it continues to make amazing discoveries

Perhaps the probes have found some kind of traffic jam at the edge of the solar system. The Voyager flight continues and we will soon find out what it was.

Voyager 2 discovered something amazing: as the distance from the Sun increases, the density of space increases.

Voyager 1, which entered interstellar space in 2012, transmitted similar indicators to Earth. New data have shown that the increase in density may be a feature of the interstellar medium.

The solar system has several boundaries, one of which, called the heliopause, is determined by the solar wind, or rather by its significant weakening. The space inside the heliopause is the heliosphere, and the space outside is the interstellar medium. But the heliosphere is not round. It looks more like an oval, in which the solar system is at the leading edge, and a kind of tail stretches behind it.

Both Voyagers crossed the heliopause at the leading edge, but within 67 degrees heliographic latitude and 43 degrees longitude apart.

Interstellar space is usually considered a vacuum, but this is not entirely true. The density of matter is extremely small, but it still exists. In the solar system, the solar wind has an average density of protons and electrons from 3 to 10 particles per cubic centimeter, but it is lower the further from the Sun.

The average concentration of electrons in the interstellar space of the Milky Way is estimated to be about 0.037 particles per cubic centimeter. And the plasma density in the outer heliosphere reaches approximately 0.002 electrons per cubic centimeter. When the Voyager probes crossed the heliopause, their instruments recorded the electron density of the plasma through plasma oscillations.

Voyager 1 crossed the heliopause on August 25, 2012 at a distance of 121.6 astronomical units from the Earth (121.6 times the distance from Earth to the Sun – about 18.1 billion km). When he first measured plasma oscillations after crossing the heliopause on October 23, 2013 at a distance of 122.6 astronomical units (18.3 billion km), he found a plasma density of 0.055 electrons per cubic centimeter.

After flying another 20 astronomical units (2.9 billion kilometers), Voyager 1 reported an increase in the density of interstellar space to 0.13 electrons per cubic centimeter.

Voyager 2 crossed the heliopause on November 5, 2018 at a distance of 119 astronomical units (17.8 billion kilometers. On January 30, 2019, it measured plasma oscillations at a distance of 119.7 astronomical units (17.9 billion kilometers), finding that the density plasma is 0.039 electrons per cubic centimeter.

In June 2019, Voyager 2’s Instruments showed a sharp increase in density to about 0.12 electrons per cubic centimeter at a distance of 124.2 astronomical units (18.5 billion kilometers).

What caused the increase in the density of space? One theory is that the lines of force of the interstellar magnetic field become stronger with distance from the heliopause. This can cause electromagnetic ion cyclotron instability. Voyager 2 did detect an increase in the magnetic field after crossing the heliopause.

Another theory is that the material carried away by the interstellar wind should slow down in the heliopause, forming a kind of plug, as evidenced by the weak ultraviolet glow detected by the New Horizons probe in 2018, caused by the accumulation of neutral hydrogen in the heliopause.

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Space

NASA has banned fighting and littering on the moon

New details of the agreement signed by representatives of a number of countries on the development of the moon and the extraction of minerals within the framework of the Artemis program have appeared. Reported by the National Aeronautics and Space Administration (NASA).

So, astronauts involved in the mission are prohibited from littering and fighting on the territory of a natural satellite of the Earth.

So, we present to you the new rules for being on the Moon:

Everyone comes in peace;

Confidentiality is prohibited, all launched objects must be identified and registered;

All travel participants agree to help each other in case of emergencies;

All received data is transferred to the rest of the participants, and space systems must be universal;

Historic sites must be preserved and all rubbish must be disposed of;

Rovers and spacecraft should not interfere with other participants.

“”It is important not only to go to the moon with our astronauts, but also that we bring our values ​​with us,” said Mike Gold, acting head of NASA’s international and inter-agency relations.

According to him, violators of the above rules will be asked to “just leave” the territory of the moon.

The effect of these principles so far applies to eight signatory countries of the agreement: the USA, Australia, Canada, Italy, Japan, Luxembourg, the United Arab Emirates and the United Kingdom. Countries other than China can join if they wish.

 It should be noted that at the moment NASA is prohibited from signing any bilateral agreements with the PRC leadership.

The first NASA mission to the moon, known as “Artemis 1”, is scheduled for 2021 without astronauts, and “Artemis 2” will fly with a crew in 2023.

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Space

Methane snow found on the tops of Pluto’s equatorial mountains

Scientists believe that it arose as a result of the accumulation of large amounts of methane at an altitude of several kilometers above the surface of the planet.

In the images of the Cthulhu region – a dark region in the equatorial regions of Pluto – planetary scientists have found large reserves of methane snow that covers the peaks of local mountains and hills. It formed quite differently from how snow forms on Earth, astronomers write in the scientific journal Nature Communications.

“The white caps on the tops of Pluto’s mountains did not arise from the cooling of air currents that rise along the slopes into the upper atmosphere, as it happens on Earth, but from the accumulation of large amounts of methane at an altitude of several kilometers above Pluto’s surface. This gas condensed on the mountain tops, “the scientists write.

We owe almost everything we know about Pluto to the New Horizons interplanetary station. It was launched in January 2006, and in mid-July 2015 the station reached the Pluto system. New Horizons flew just 13 thousand km from the dwarf planet, taking many photographs of its surface. 

New Horizons data indicated an interesting feature of Pluto – in its depths, a giant subglacial ocean of liquid water can be hidden. It can be a kind of engine of those geological processes, traces of which can be seen on the surface of a dwarf planet. Because of this discovery of New Horizons, many discussions began among planetary scientists. Scientists are trying to understand how such a structure could have arisen, as well as to find out the appearance of Pluto in the distant past.

Members of the New Horizons science team and their colleagues from France, led by planetary scientist from NASA’s Ames Research Center (USA) Tanguy Bertrand, have discovered another unusual feature of Pluto. They studied the relief of one of the regions of the dwarf planet – the Cthulhu region. This is what astronomers call a large dark region at Pluto’s equator, which is whale-like in shape and is covered in many craters, mountains and hills.

Snow in Pluto’s mountains

By analyzing images of these structures taken by the LORRI camera installed on board New Horizons, astronomers have noticed many blank spots on the slopes of the highest mountain peaks. Having studied their composition, scientists have found that they consist mainly of methane.

Initially, planetary scientists assumed that these are deposits of methane ice. However, Bertrand and his colleagues found that the slopes and even the tops of Pluto’s equatorial mountains are actually covered not only with ice, but also with exotic methane snow that forms right on their surface.

Planetary scientists came to this conclusion by calculating how methane behaves in Pluto’s atmosphere. In doing so, they took into account how the molecules of its gases interact with the sun’s rays and other heat sources. It turned out that at the equator of Pluto, at an altitude of 2-3 km from its surface, due to the special nature of the movement of winds, unique conditions have formed, due to which snow is formed from methane vapor.

Unlike Earth, where such deposits are formed as a result of the rise of warm air into the upper atmosphere, on Pluto this process goes in the opposite direction – as a result of contact of the cold surface of the peaks and slopes of mountains with warm air masses from the relatively high layers of the dwarf planet’s atmosphere.

Previously, as noted by Bertrand and his colleagues, scientists did not suspect that this was possible. The fact is that they did not take into account that due to the deposition of even a small amount of methane snow and ice, the reflectivity of the peaks and slopes of mountains in the Cthulhu region increases. As a result, their surface temperature drops sharply, and snow forms even faster.

Scientists suggest that another mysterious feature of Pluto’s relief could have arisen in a similar way – the so-called Tartarus Ridges, located east of the Sputnik plain. A distinctive feature of this mountainous region is strange peaks that are shaped like skyscrapers or blades. Bertrand and his colleagues suggest that these peaks are also methane ice deposits that grow “from top to bottom.”

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