The first stars of the Universe located 180 million years after the Big Bang

The first stars of the Universe located 180 million years after the Big Bang

The first stars of the Universe located 180 million years after the Big Bang

About 380,000 years after the Big Bang, the temperature of the observable universe has dropped because of its expansion, and sufficiently so that the first neutral hydrogen and helium atoms are formed from a plasma of light nuclei and electron. The primordial nucleosynthesis must have produced a few nuclei of oxygen, carbon and nitrogen but in proportion a million billion times less, so in negligible quantity. It is the stars that will eventually produce heavy nuclei.

The phenomenon was accompanied by the emission of the famous fossil radiation that reaches us today from regions located more than 45 billion light-years away from us, because of the expansion of the observable cosmos. Recall that it is about 13.7 billion years old. This is known as the "recombination period", which has not been instantaneous. This curious term is a historical vestige going back at least to the 1960s, at the time of the discovery of this radiation. Robert Dicke and James Peebles, among other pioneers of cosmology at the time, considered that the initial hydrogen and helium atoms may have come from an earlier phase of contraction of the observable universe. (which would be cyclic), and dissociated nuclei and electrons at high temperatures, before a new phase of expansion begins, the one we observe.

The dark ages and the first stars

Still, for at least 100 million years, the observable universe was not illuminated by stars and even less by galaxies, whereas the temperature of the fossil radiation had fallen enough to no longer visible At the moment of recombination, the cosmos had to be as bright as the surface of the Sun since it was filled with a plasma almost at the same temperature. This dark period in the history of the universe is the so-called dark ages.

The stars finally began to form in large numbers. They must have been very massive, probably 100 to 1,000 solar masses, and their intense ultraviolet radiation began to re-ionize the atoms, perhaps together with that of the first quasars. The beginning of this period of reionization of the ordinary matter of the universe has sometimes been called the "cosmic renaissance" and this period itself is logically called reionization.

Cosmologists would like to specify the dates of these phenomena and of course study them. But how do you get information on what happened during the dark ages when there were no stars to shine and how to observe those very first stars called people III stars?

Wouthuysen-Field effect and fossil radiation

There are strategies that researchers have been exploring for more than a decade. One is based on a curious phenomenon called the Wouthuysen-Field effect, named after the Dutch physicist Siegfried Adolf Wouthuysen and the American astrophysicist George B. Field. He uses the famous 21 cm line of hydrogen that has made it possible to map the Milky Way. Neutral hydrogen can in fact absorb or emit photons at this wavelength because of particular energy levels due to the interactions between the proton and the electron of a hydrogen atom driving the spins of these particles to be either parallel or antiparallel. This line can therefore be used to detect and map the distribution of neutral hydrogen masses during the dark ages.

However, at the moment when the first stars ignited, quantum mechanics predicts that their ultraviolet photons will allow the Wouthuysen-Field effect to manifest in such a way that the still neutral hydrogen atoms begin to absorb the photons of the fossil radiation lying precisely at the wavelength of 21 cm. The spectrum of this radiation must therefore be notched with a small hollow corresponding to this depopulation effect.

However, because of the expansion of the universe, this hollow in the spectrum of fossil radiation is shifted to lower wavelengths, in the field of radio waves. The sooner the reionization occurred, the more the expansion of the observable cosmos would have had time to stretch the wavelength of these radio photons. By measuring it precisely, we detect the ignition of the first stars and, above all, it is possible to date this event.

A team of researchers led by astronomer Judd Bowman of the Arizona State University School of Earth and Space Exploration began the adventure more than 12 years ago by building a detector suitable for highlighting the signal researched as part of the Edges (Experiment to Detect the Global Signature EoR). She has just presented in the journal Nature the observation of the birth of the first stars. According to this result, this event would have occurred about 180 million years after the Big Bang.

The first stars of the Universe located 180 million years after the Big Bang

The Wouthuysen-Field effect and evidence of dark matter

This measurement is a tour de force because the desired signal is embedded in the radio background, including that of the Milky Way. As explained in the video above, this is like listening to the humming of a humming bird's wings in the middle of a hurricane. It was also necessary for the researchers to isolate themselves as much as possible from radio sources of human origin and that is why Edges took place far from everything in Australia, at the MRO (Murchison Radio-astronomy Observatory), not far from one of the famous Square Kilometer Array radio telescope.

There may be a cherry on the cake, as explained in an article filed on arXiv by astrophysicist Rennan Barkana of Tel Aviv University. The intensity of photon absorption from the Wouthuysen-Field effect is more than double what was expected. The anomaly (3.8 σ) is however not at the level of the 5 sigma required for the result to be considered a discovery. Caution is therefore required. But according to the researcher, this would indicate that the atoms of the hydrogen gas at that time were colder than we thought. The reason ? Before reionization, the universe was denser and collisions with dark matter particles (which are a cooler gas in the standard cosmological model) were more frequent, which would have cooled the normal matter gas.

But this hypothesis only works under two conditions. On the one hand, dark matter particles must interact a little with normal matter through forces other than gravity. On the other hand, the particles constituting it should not be too heavy, less than five times the mass of the proton approximately, which would lead to models of Wimps lighter than expected and perhaps to models of warm dark matter and not cold.

Is Barkana right? It is too early to say but if that is the case, it would be the first direct evidence of the existence of dark matter particles. A spectacular result to credit to particle astrophysics.