Physicists Think Dark Matter May Be Responsible for ‘Hydrogen Forest’

Using a supercomputer, a team of physicists has confirmed a discrepancy between observations of the universe and theoretical predictions about its structure.

The team used PRIYA, a suite of simulations that takes optical light data from two surveys to refine cosmological parameters, to determine constraints on measurements of the universe and its evolution. The team’s research was published earlier this month in the Journal of Cosmology and Astroparticle Physics.

Using PRIYA, the team studied spectrograms, which are images of hydrogen emission lines in the universe. The spectrograms captured the Lyman-Alpha forest, a dense crowd of absorption lines in spectra from quasars, extremely bright light sources in the universe.

In the team’s spectrograms, spikes of missing frequencies indicated the “atoms and molecules that the light has encountered along the way,” said Simeon Bird, a physicist at UC Riverside and co-author of the research, in a university release. “Since each type of atom has a specific way of absorbing light, leaving a sort of signature in the spectrogram, it is possible to trace their presence, especially that of hydrogen, the most abundant element in the universe,” he added.

Dark matter is the catch-all name for about 27% of the universe’s content. It is so named because it’s never been directly observed, but its presence is evident in its gravitational effects. Instruments like the Euclid Space Telescope are collecting data that could reveal the makeup on the dark universe.

Simultaneously, instruments on the ground, like the DM Radio project, are slowly reducing the potential mass range of particles that may be responsible for dark matter. Some of the popular candidates for dark matter are Weakly Interacting Massive Particles (WIMPs), axions, and hidden (or dark) photons, among others.

Mapping dark matter’s distribution across the universe can also reveal how well theoretical models of the universe line up with observational data. In their recent work, the Lyman-Alpha Forest revealed the locations of dark matter in the universe.

“Dark matter gravitates so it has a gravitational potential,” Bird said. “The hydrogen gas falls into it, and you use it as a tracer of the dark matter.”

The team uses its model to monitor dark matter concentrations in the universe, but to also investigate discrepancies between observations of the cosmos and theoretical predictions of its structure.

Bird presented two leading ideas as to why the two may not match up. One possibility is that supermassive black holes at the cores of galaxies are confounding the team’s calculations about the structure of the universe, while the other is that new, yet-to-be-discovered physics are at play.

“If this holds up in later data sets, then it is much more likely to be a new particle or some new type of physics, rather than the black holes messing up our calculations,” Bird said.

In other words, more data will be needed to figure out one of the biggest outstanding mysteries about the universe. Thankfully we have plenty of observatories both presently available and planned to collect that data.