Scientists Think Dark Matter May Answer 'Hydrogen Forest'

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

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

Using PRIYA, the team studied spectrograms, which are pictures of hydrogen emission lines in the universe. The spectrograms capture the Lyman-Alpha forest, a dense mass of absorption lines in spectra from quasars, the brightest light sources in the universe.

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

Dark matter is the name that holds about 27% of the content of the universe. It is so named because it has never been observed directly, but its presence is known through its gravitational effects. Instruments like the Euclid Space Telescope collect data that can reveal makeup in dark space.

At the same time, ground-based instruments, such as the DM Radio project, are gradually reducing the potential range of particles that could cause dark matter. Some popular candidates for dark matter are Weakly Interacting Massive Particles (WIMPs), axions, and hidden (or dark) photons, among others.

Mapping the global distribution of dark matter can also reveal how well theoretical models of the universe fit the observational data. In their latest book, Lyman-Alpha Forest reveals the darkest places in the universe.

“Dark matter pulls harder so it has gravity,” said Bird. “Hydrogen gas falls into it, and you use it as a tracer for dark matter.”

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

Bird presented two leading theories as to why the two might not be the same. One possibility is that supermassive black holes at the core of galaxies are confounding the team's calculations about the composition of the universe, while another is a new, yet-to-be-discovered physics.

“If this holds up in recent data sets, it could be new particles or some kind of physics, rather than black holes messing up our calculations,” Bird said.

In other words, more data will be needed to solve one of the great mysteries of the universe. Fortunately we have many observatories available and designed to collect that data.