Universe Today
More than fifty years after it was proposed, the search for Dark Matter continues, whether through cosmic surveys or theoretical studies. This mysterious, invisible matter is theorized to account for 85% of the Universe's mass. While it remains unconfirmed, substantial indirect evidence supports its existence, ranging from the rotational curves of galaxies and Dark Matter Haloes to gravitational lenses. In addition, several candidate particles have been proposed and tested against observational data.
These include WIMPs (Weakly Interacting Massive Particles), primordial black holes (PBHs), axions, and "dark photons." In theory, this particle was responsible for heating the early Universe and acts as a bridge between the "visible" and dark sectors of the cosmos. According to a new paper, dark photons would not have heated the early Universe as previously thought. Their findings could alter the hunt for Dark Matter by suggesting that dark photons could be hiding in more places than previously thought.
The study was part of a cross-disciplinary research effort led by Professor Anson Hook of the Maryland Center for Fundamental Physics (MCFP). He was joined by Senior Postdoctoral Researcher Junwu Huang and Horizon AstroPhysics Initiative (HAPI) Fellow Mohamad Shalaby of the Perimeter Institute for Theoretical Physics (PITP) at the University of Waterloo, Ontario. Their results are published in Physical Review Letters.
The largest structure in existence — the cosmic web of gas and dark matter stretching across billions of light years, connecting galaxies (Credit: NASA)
For some time, physicists have predicted that dark photons would have converted into ordinary light in the hot clouds of neutral hydrogen that permeated the early Universe. If true, this heating process would have left detectable traces of dark photons, and a large portion of the matter-energy density of the Universe (parameter space) has been excluded from cosmological models. In essence, this means these excluded ranges could hold the key to dark photons.
“These exclusions were saying the strength of dark matter had to be 108 [times] weaker than it actually can be,” said Hook in a PITP press release. “This paper opens up a lot of new possibilities to look for dark matter.” In general, researchers assumed the conversion process was linear, and the energy would slowly convert into plasma. However, the energy requirement would be enormous, which presents problems for cosmological models. This led to the collaborative effort between Huang, Hook, and Shalaby, who specializes in plasma physics.
Together, they ran simulations that proved that the standard linear conversion theory was incomplete. Instead, the simulations showed that once dark photon energy enters the plasma, the system becomes violently nonlinear, and the conversion process would have shut down before significant heating could occur. Said Huang in a PITP press release:
The treatment for the last 15 years is a linear treatment. If you use that approximation, you can compute the amount of energy transfer, and it’s very large. And I realized it’s not possible. What we realized is that, as you are converting energy into the standard model plasma, the plasma actually goes crazy. There are a lot of nonlinearities in the system, and these nonlinearities basically shut off the energy conversion after a tiny amount of energy is converted.
This is a test case in cosmology. A lot of astrophysical systems have also been used to look for similar effects, and we need to rethink all of them. Linear approximation, which is easy to compute, might have nothing to do with how a neutron star magnetosphere [or] a white dwarf magnetosphere actually behaves.
According to the team's analysis, the conventional cosmological constraint on dark photons is invalid across roughly ten orders of magnitude, ranging from frequencies of about 10⁻¹⁵ electron volts (eV) to 10⁻⁶ eV, which correspond roughly to the kilohertz and gigahertz parts of the radio spectrum. This represents a significant range previously excluded from parameter space and could open new avenues in cosmological studies. According to Shalaby, it could be expanded to help in the search for other elusive particles.
"By calculating the early universe plasma correctly, experiments will probe new parameter spaces and potentially actually see something. It's truly interdisciplinary. It's the interaction between plasma physics and particle physics,” he said. “And this will directly impact people who do experiments.”
Further Reading: Perimeter Institute, Physical Review Letters
