After years of research, scientists can finally account for all the normal matter of the universe

Astronomers using a powerful quasar to study a huge invisible swirl full of overheated gas say that they have finally discovered the "missing" detectable matter of the universe.

The results, published in the journal Nature, solve a decades-old mystery and could help scientists probe further the structure and evolution of the cosmos.

All atoms in stars, galaxies and planets make up about 5% of the mass density of cosmic energy. The overwhelming majority, about 70%, is made up of dark energy – a mysterious and repulsive force that makes the universe grow faster and faster. The remaining quarter is composed of invisible substance, invisible, untouchable whose presence can be felt only by its gravitational influence on the galactic scales. Dark matter connects clusters of galaxies to massive tendrils, forming a cosmic network that serves as an invisible skeleton to the universe.

Scientists have estimated these actions largely using two different methods, said study co-author J. Michael Shull, an astrophysicist at the University of Colorado, Boulder. Several years ago, researchers roughly calculated how much material would have formed as a result of the "big bang" that gave birth to the universe. Astronomers have also studied the cosmic microwave background – the oldest light in the universe, which pervades the entire sky – and found roughly the same proportions of normal matter, dark matter, and dark energy.

This small slice of normal matter that we can directly detect, which scientists call baryonic matter, is the best known of the three: it emits light (like the sun) or reflects it (like the moon), making it visible to us or detectable by telescopes. And yet, it also presents its own mystery because for decades scientists have not been able to find everything.

"More than 20 years ago, people have noted that if you add all the starlight and all the mass in the galaxies that go with that light, you only get about 10% of that 5% of ordinary matter." "So there was a problem of" missing material "going back more than 20 years: where are the gases, where are the baryons, which are not collapsed in stars and galaxies?"

"That's why we worried," he added. "It really goes to the heart of the key predictions in cosmology on the big bang."

Researchers have slowly reduced this gap by adding to the census all the hot and diffuse gas in the huge halos of galaxies and even in the largest clusters of galaxies. But they wondered if even more of the missing matter could be suspended in the huge filaments of dark matter that make up the cosmic network.

Here is the problem of finding this missing material: It would be mainly made of hydrogen, the simplest element and by far the most abundant in the universe. When the hydrogen atoms are ionized, they can become invisible at optical wavelengths, making them very difficult to detect.

Fortunately, if an ionized hydrogen cloud sits between the Earth and a source of ultraviolet light, this hydrogen will absorb certain wavelengths, leaving a distinct chemical footprint that astronomers can detect once. they reach their telescopes. Shull and his colleagues added to the census by finding this ionized gas.

The problem is that when the gas gets hotter and hotter, for example, above one million degrees Kelvin, the ionized hydrogen stops leaving a clear signal in the ultraviolet. So, for this article, the researchers also targeted much rarer oxygen ions, and searched for their fingerprint in X – rays, which are much more energy – consuming light wavelengths.

Scientists used the European Space Agency's XMM-Newton X-ray space telescope to study the quasar BL Lacertae 1ES 1553 + 113, an active, supermassive black hole in the center of a galaxy. The quasars engulf the material and shine in many wavelengths of light, X-ray radio waves. These celestial headlights can essentially back-light the material that crosses the path of the beam, just like a beam of Flashlight illuminates grains of dust unnoticed in the air.

By studying the chemical fingerprint of oxygen in the x-rays of the quasar light, scientists found a large amount of extremely hot intergalactic gas – to such an extent that they calculated that this gas could represent up to 40% of the baryonic matter in the cosmos, which could be enough to explain the missing problem.

The researchers believe that these ions may have started in the hearts of stars that went supernova, and were thrown out of their galaxies at home by these explosive stellar deaths. They may have been overheated by shocks. Atoms need to interact with each other to radiate energy, and because individual atoms in this sparse gas were so far apart, unable to touch each other, they remained extremely hot.

Taotao Fang of the Jiujiang Research Institute in China, who did not participate in the study, suggested some possible explanations, namely that the ionized gas signal could come from a galaxy rather than from a galaxy. an intergalactic gas embedded in a dark matter filament.

Yet, Fang wrote in a comment, the results "offer a tantalizing glimpse of where the elusive missing baryons have been hidden."

The next step, says Shull, is to repeat these observations using other quasars, to see if the part of the baryonic matter that they have found is held in other parts of the world. sky.

Explore further:
The researchers find the last ordinary matter missing from the universe

More information:
F. Nicastro et al, Observations of missing baryons in the hot-hot intergalactic environment, Nature (2018). DOI: 10.1038 / s41586-018-0204-1