New evidence of the existence of “dark matter” in the universe may help solve one of the greatest mysteries of modern science. Research published by Prof. Rennan Barkana, an astrophysicist at Tel Aviv University, provides new information on the possible characteristics of one of the fundamental but poorly understood building blocks of the universe.
The movement of galaxies through space could not be explained by particles and forces that we can observe and understand. The theory now is that the behavior of the universe can be explained by 4.9 percent matter and energy that we know about, around 26 percent matter that we don't know about – "dark matter", and 69 percent energy we don't know about – "dark energy."
In a paper Barkana published in the prestigious journal Nature on Wednesday, he explains how a team of astronomers, led by Prof. Judd Bowman of Arizona State University, unexpectedly came across upon dark matter while using cosmic radio wave signals in an attempt to detect the earliest stars in the universe.
The radio signal, recorded using the novel EDGES radio telescope in Australia, is proof of interactions between normal matter and dark matter in the early universe, suggests Barkana, who heads the TAU astrophysics department. The discovery is the first direct proof of the existence of dark matter, and shows that it is composed of low-mass particles – and can interact with other forms of matter. The radio signal dates to 180 million years after the Big Bang that created our universe, which occurred some 13.7 billion years ago.
“We know quite a bit about the chemical elements that make up the earth, the sun and other stars,” says Barkana, “but most of the matter in the universe is invisible and known as ‘dark matter.’ The existence of dark matter is inferred from its strong gravity, but we have no idea what kind of substance it is. Hence, dark matter remains one of the greatest mysteries in physics.”
The results are very preliminary and require many more years of follow-up work, but if they do pan out, they will change the scientific understanding of dark matter and the universe, and lead to new directions in scientific research.
Scientists have been searching for clues about dark matter since the 1970s, but did not find any proof of its existence, until now. It was thought that dark matter did not interact with known particles and materials, including light. But dark matter cannot be seen directly, as can the physical world that surrounds us, including subatomic particles and atoms, as well as stars and galaxies. According to their theories and calculations, astronomers concluded additional, unseen materials must make up a large part of our universe – and this must be dark matter.
Bowman and his colleagues detected a radio wave signal at a frequency of 78 megahertz, which was largely consistent with expectations. But they also found it was much stronger than predicted, indicating that the primordial gas was colder than expected, about -263 degrees, or about 10 degrees Celsius above absolute zero.
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Barkana’s contribution was to propose was that the gas cooled through the interaction of hydrogen with cold, dark matter long before the first stars formed. He writes that this absorption can be explained by the “combination of radiation from the first stars and excess cooling of the cosmic gas induced by its interaction with dark matter.”
This is the first concrete evidence of the existence of dark matter: “Once stars formed in the early universe, their light was predicted to have penetrated the primordial hydrogen gas, altering its internal structure.” says Barkana. “This would cause the hydrogen gas to absorb photons from the cosmic microwave background, at the specific wavelength of 21 centimeters, imprinting a signature in the radio spectrum that should be observable today at radio frequencies below 200 megahertz. The observation matches this prediction except for the unexpected depth of the absorption.”
Physicists expected that any such dark matter particles would be heavy, but the discovery indicates low-mass particles. Based on the radio signal, Barkana argues that the dark-matter particle is no heavier than several proton masses. “This insight alone has the potential to reorient the search for dark matter,” he says.
In his paper in Nature, entitled “Possible Interaction Between Baryons and Dark-matter Particles Revealed by the First Stars,” Barkana predicts that the dark matter produced a very specific pattern of radio waves that can be detected with a large array of radio antennas.