Observing dark matter at cosmic dawn

After almost a century of speculation, proposals and searches for dark matter, physicists now know that it currently comprises about 27% of the universe’s mass-energy, with an abundance over five times that of ordinary matter like you, oceans and exoplanets.

Most of the matter in the universe is dark. On large scales, it is cold and doesn’t collide with anything we recognize, and so, it is called “cold dark matter.” Many candidates have been proposed that could explain the large scale structure of the universe, but none has been established by experiments.

But on smaller scales, dark matter may be different and may leave different signatures, especially on the early universe. Of course, those are harder to observe.

Baryons such as protons and neutrons were in the early universe as well, and their effects have to be distinguished from any dark matter that was present; both would affect the formation of smaller structures.

A host of discrepancies exist at galactic and sub-galactic distances, and it is unknown if all these discrepancies can be explained by baryon physics while keeping the cold dark matter scenario. On length scales of one megaparsec or less and mass scales smaller than 100 billion solar masses, this has proved not easy to do.

A group led by Jo Verwohlt of the University of Copenhagen in Denmark has now shown that there is a way that could unveil dark matter by using a deeply redshifted line in the hydrogen spectrum, from the first stars and galaxies now at the far edge of the universe. Their work appears in the journal Physical Review D.

Some ideas about dark matter propose that it interacts with dark radiation, also known as dark electromagnetism or dark photons. As photons are exchanged in electromagnetic forces, dark radiation would mediate the interactions between dark matter particles.

Just as does dark matter, dark radiation would not interact with the other forces of the Standard Model, the weak force and the strong force. It’s unknown if dark radiation exists; one candidate is a sterile neutrino, if it exists.

Dark radiation could have heated the dense early universe, as hot dark radiation interacted with dark matter, raising its temperature. The warming may have been enough for large dark matter concentrations to form “dark matter halos,” hypothetical regions in which the dark matter is gravitationally bound and has decoupled from the expansion of the universe, bound together locally and expanding as a whole instead, much like galaxies and clusters today.

These halos would temporarily and repeatedly resist collapsing gravitationally, cycles that are called “dark acoustic oscillations”—acoustic because they are fluctuations in density, just as sound waves are fluctuations in the density of air or some other fluid.

These dark matter cycles would have quickly died out, but first would have affected the beginning of “cosmic dawn” when the first ordinary matter galaxies formed from primordial gas that was drawn into the halos.

Verwohlt and her team explored “how well we could measure properties of dark matter using the 21-cm power spectrum at z > 10.” (“z” is a redshift parameter astronomers use to denote how fast another object or region is moving away from us due to cosmic expansion, the Doppler effect that includes relativistic velocities. The region where z=10 is expanding at 99.8% of the speed of light away from Earth.)

Conditions around cosmic dawn would affect the 21-cm light. (21-cm light is emitted when a neutral hydrogen atom, with one proton and one electron, transitions from a state with both particles having their spins in the same direction to a state where the electron spin is opposite to the unchanged proton spin, a so-called hyperfine spin-flip transition.)

Early on there would be a net absorption (or emission) of the 21-cm photons from the cosmic microwave background by the 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,” they wrote.

They used an “effective theory of structure formation,” which enables the formation of cosmological structure to be determined in almost any microphysical model of dark matter, and models of other physical processes to link the 21 cm signal to the star formation rate density.

Their end result found that the radio telescope HERA in South Africa would need almost a year and a half of observing the redshifted 21-centimeter line to determine whether dark acoustic oscillations exist and to distinguish among several different dark model models.

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