HERA radio telescope working at a wavelength of 21 cm let scientists not only look into the first hundreds of millions of years of existence of our Universe, but also find out about the way dark matter behaved in it. It is believed that it played a significant role in the formation of galaxies.
Dark matter in the early Universe
After nearly a century of speculation, suggestions and searches for dark matter, physicists now know that it makes up about 27% of the mass-energy of the Universe, and is five times more than the amount of ordinary matter such as you, oceans and exoplanets.
Most of the matter in the Universe is dark matter. On a large scale, it is cold and does not come in contact with anything we know, which is why it is called cold dark matter. Many candidates have been proposed that could explain the large-scale structure of the Universe, but none have been confirmed experimentally.
But on smaller scales, dark matter can be different and leave different traces, especially in the early Universe. Of course, they are harder to observe.
Baryons such as protons and neutrons also existed in the early Universe, and their influence should be distinguished from any dark matter that was present; both influenced the formation of smaller structures.
There are many divergences at galactic and sub-galactic distances, and it is unknown whether all of these divergences can be explained by baryon physics keeping the cold dark matter scenario intact. On length scales of one megaparsec or less and masses smaller than 100 billion solar masses, this proved difficult to accomplish.
How to detect dark matter?
A team led by Jo Verwohlt at the University of Copenhagen in Denmark has shown that there is a way to detect dark matter using the deeply shifted red line in the spectrum of hydrogen, starting with the first stars and galaxies now at the far edges of the Universe. Their work is published in the journal Physical Review D.
Some ideas about dark matter suggest that it interacts with dark radiation, also known as dark electromagnetism or dark photons. Since photons exchange in electromagnetic forces, dark radiation could mediate interactions between dark matter particles.
As with dark matter, dark radiation will not interact with the other forces of the Standard Model, weak and strong. It is unknown whether dark radiation exists; one candidate is the sterile neutrino, if it exists.
Dark radiation may have heated the dense early Universe because the hot dark radiation interacted with dark matter, raising its temperature. The warming may have been sufficient for large concentrations of dark matter to form “dark matter halos” — hypothetical regions where dark matter was gravitationally bound and separated from the expansion of the Universe, bound locally and expanding as a unit, similar to today’s galaxies and clusters.
These halos would temporarily and repeatedly resist gravitational collapse — cycles called dark acoustic oscillations. Acoustic because they are density fluctuations, just as sound waves are density fluctuations of air or other liquid.
These dark matter cycles would have quickly extinguished, but would have first influenced the beginning of the “cosmic dawn” when the first galaxies of ordinary matter formed from the original gas that was pulled into the halo.
Dark matter and spectrum measurements
Verwohlt and her team investigated how well the properties of dark matter can be measured using a 21-centimeter power spectrum at z > 10. (“z” is the redshift parameter that astronomers use to indicate how fast another object or region moves away from us due to cosmic expansion, the Doppler effect, which includes relativistic velocities. The region where z=10 is expanding at 99.8% of the speed of light, moving away from the Earth).
At the beginning, there would be a net absorption (or emission) of 21-centimeter photons from the cosmic microwave background by neutral hydrogen atoms in the medium between galaxies.
“Thus, the evolution of the 21-cm signal (both global and fluctuations) can be used to infer the presence of dark matter damping at small scales,” the researchers wrote.
They used an efficient structure formation theory, which can determine cosmological structure formation in practically any microphysical model of dark matter, as well as models of other physical processes, to relate the 21-cm signal to the density of the star formation rate.
Their final result showed that the HERA radio telescope in South Africa would need nearly a year and a half of 21-centimeter redshift observations to determine whether dark acoustic oscillations exist and to distinguish between several different dark matter models.
Provided by phys.org
