Why is most of the universe invisible?

What hot gas reveals about the invisible universe

The evening sky has always fascinated people. All early advanced civilizations studied the movement of the heavenly bodies and developed theories about the cosmic processes. For millennia, visible light was the only form of information that reached us from distant celestial bodies - and thus a very limited frequency range of electromagnetic waves. It was not until the middle of the 20th century that a completely new source of information for astronomical research could be developed: X-rays. Observations using this high-energy radiation show that much more is happening in the universe than pure light observations suggest.

The earth's atmosphere is impermeable to X-rays. This is a godsend for life on earth, but a certain challenge for X-ray astronomy. Because it means that it can only be carried out through space telescopes positioned outside the Earth's atmosphere. The astrophysicist Norbert Werner, who works at Masaryk University Brno, became enthusiastic about X-ray astronomy early in his scientific career. For his contributions in this research field, he has now been awarded the Ignaz L. Lieben Prize of the Austrian Academy of Sciences (ÖAW).

Shameful situation

The Ignaz L. Lieben Prize is the oldest and highest endowed prize of the OeAW with 36,000 US dollars (around 30,000 euros), which is awarded annually to outstanding post-scholars who have demonstrated their achievements in Bosnia-Herzegovina, Croatia and Slovakia , Slovenia, the Czech Republic, Hungary or Austria. Norbert Werner made significant contributions to the understanding of so-called hot atmospheres. "When we look at what our universe is made of, we have to find that we are in a pretty embarrassing situation," says Werner. If you look at the total energy density in the universe, around 70 percent is what is known as dark energy. So far, very little is known about it, at least it is necessary to explain why our universe is expanding - and that at an increasing pace.

Another 25 percent is accounted for by dark matter. It, too, is largely unknown territory, if not quite as mysterious as dark energy. Dark matter was introduced to explain observations of gravitational effects that cannot be traced back to the matter visible to us. Dark matter plays an essential role in explaining how galaxies are formed and what holds them together.

Extremely hot gas

That leaves five percent of the universe, which consists of the conventional matter known to us. "Of these five percent, however, only a fraction of stars, planets and galaxies have formed - that is, the part of the universe that we can see with optical telescopes," says Werner. So this is our shameful conclusion: "We know nothing about 95 percent of the universe. And of the five percent that consists of normal matter, we see less than half."

So far, so unsatisfactory. But at this point X-ray astronomy comes into play, which allows us to shift the high degree of our ignorance about the universe at least a little in our favor. Werner uses X-rays primarily to research extremely hot gas that is found between galaxies. "We can detect this hot gas around galaxy clusters," says Werner. Because in the vicinity of these massive objects, the hot gas becomes dense enough that it emits X-rays. Incidentally, the name "hot" gas seems quite appropriate, it is about temperatures of ten to one hundred million degrees Celsius.

No burning despite millions of degrees

Nevertheless, the density of the gas is comparatively very low, even around galaxy clusters. "The density is less than that of the best vacuum that we can create," says Werner. The result is a remarkable effect: imagine the extremely hypothetical case that an astronaut would get into this hot gas. "Due to the low density, the energy density would be so low that the astronaut would not burn - despite a hundred million degrees."

Using X-ray spectroscopy, Werner was able to determine the temperature of the hot gas, determine the density and the chemical composition. One of his most interesting discoveries, in his opinion, was made almost 15 years ago during his dissertation at the University of Utrecht in the Netherlands. At that time he was able to study filaments that connect galaxy clusters using X-ray spectroscopy. There is not only hot gas there, but also dark matter, as he and colleagues later demonstrated.

Black holes in the heart

In his research, the researcher also deals with black holes. Werner found out that there is a kind of feedback loop between hot gas and black holes: When the gas cools down, it is swallowed up by black holes in the center of a galaxy. But as a result, the black hole heats up the gas again. When the gas cools down again, the cycle starts all over again. Werner says: "For this reason I like to refer to black holes as the beating hearts of galaxies."

Werner is currently working on the question of how he could use nanosatellites with a size of just ten to 30 centimeters for his research. The first micro-satellite for this was launched last week on a Soyuz rocket. "As a result of technical progress, these nanosatellites are getting better and cheaper at the same time," says Werner, "I believe that every astronomy institute will be able to afford a satellite, not just the big space agencies." The astrophysicist hopes that this development will lead to a democratization and diversification of astronomical research "which will produce many brilliant ideas". (Tanja Traxler, April 3rd, 2021)