They detect signals that would come from the first stars of the Universe

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Astronomers call it cosmic alba. It is the time when the first stars were born and the Universe left behind millions of years of darkness to fill with light for the first time. None of those stars survive today. They were huge stars that burned fast and went extinct soon in a supernova festival. But a team led by the State University of Arizona (USA) claims to have detected signals coming from those stars with a radio telescope built specially to look for them from the desert of Australia.

The signal is not what they expected. According to the results presented this week in the scientific journal Nature, the signal found indicates that the dark gas that filled the universe was colder than expected and, therefore, that something hitherto unknown was cooling it. According to the researchers, it had to be some kind of dark matter colder than gas.

If next observations confirm these results, it would be the first detection of the enigmatic dark matter that represents 85% of the mass of the Universe. “If it is true, it is an extraordinary discovery”, says Jordi Miralda-Escudé, Icrea researcher at the Institut de Ciències del Cosmos of the Universitat de Barcelona, ​​who has not participated in the research. , a door that physicists and astronomers have been searching for for decades.

The research goes back to the dark age of the Universe, when a fog of hydrogen and helium atoms filled the space without emitting light. The atoms congregated attracted by the gravity in some regions, where they reached a pressure and a sufficient temperature to enter combustion. This is how those first gigantic stars were born.

From that moment, the stars flooded the Universe of ultraviolet radiation, which affected the hydrogen of interstellar space. The hydrogen atoms began to emit then a radiation that is characteristic to them – the so-called 21-centimeter line, or hydrogen line – which is what researchers have sought with the radio telescope in Australia.

The signal they have detected contains two fundamental information. The first indicates the age of the Universe when the first stars shone. According to the results presented in Nature, they ignited 180 million years after the big bang – when the Universe was 1.3% of their current age – and went out 90 million years later forming the first supernovas and the first holes blacks

This age is derived from the frequency of the signal captured by the radio telescope, which is centered at 78 megahertz (MHz). The calculation that allows to relate the frequency of the signal with the age of the Universe is derived from the fact that the Universe is expanding. Therefore, the older the signal of a star, the farther it is from us and the farther it goes. This distance makes the signal that reaches us have a frequency lower than the original emission by the so-called Doppler effect. Knowing that the original emission of hydrogen is 1.420 MHz, the detection at 78 MHz allows us to place the birth of the first stars in about 180 million years after the big bang, a result that is in accordance with the current theoretical models on evolution of the universe.

The second fundamental information that contains the signal captured by the radio telescope indicates the temperature at which the hydrogen that filled the Universe was at that moment. This is the information that does not square with the theoretical models and that forces us to resort to dark matter to explain it.

The temperature is deducted from the intensity of the signal. If the results of the detection are correct, the hydrogen was at that time at about 3 degrees Kelvin. However, the theoretical models of the early evolution of the Universe predict that the first stars were born when the gas was still more than 7 degrees Kelvin.

“This additional cooling is only possible through the interaction (of hydrogen) with something even colder,” says astrophysicist Rennan Barkana, from Tel Aviv University (Israel), in another article published in Nature. “The only cosmic ingredient that can be colder than gas (in the period of the Universe that has been studied) is dark matter.”

The team at the University of State of Arizona agrees that “only the cooling of the gas as a result of interactions with dark matter seems able to explain” the signal detected. Even so, he claims that other radiotelescopes look for the signal to confirm their results. “If it is confirmed, it is a fundamental discovery of very profound consequences that will significantly change our understanding of the Universe”, says Jordi Miralda-Escudé, specialist in the study of this period of cosmic history. “But keep in mind that they measure a very weak signal with a large background noise from other sources of radio waves”.

It is to minimize the background noise that detection has been made from the Murchison Radioastronomy Observatory, in a desert region of western Australia. But there is still a significant background noise from radio waves coming from the Earth’s atmosphere and the Milky Way. It will also look at how solar storms affect us. “It has been a great technical challenge to make this detection, since the sources Noise can be a thousand times more intense than the signal. It’s like being in the middle of a hurricane and trying to hear the flutter of a hummingbird, “Peter Kurzynski of the US National Science Foundation, who has funded the research, said in a statement.

Although the researchers have eliminated the noise When analyzing the signal, “the question remains as to whether the signal they present is real or is an artifice,” warns Miralda-Escudé. “It is something that can only be determined with more observations”. It could also happen that the antiquity of the signal is correct and that it really is an echo of the first stars, but that its intensity is incorrect and therefore it is not an effect of dark matter.

If all the measurements are correct, “then we have learned something new and fundamental about the mysterious dark matter that represents 85% of the matter in the Universe,” Judd Bowman, the first author of the research, said in a statement. “It would be the first vision of physics beyond the Standard Model”, that is, beyond the current theory that explains the forces and particles of the Universe and that scientists know is incomplete.