'Dark photons' may explain dark matter without heating the early universe

Full text

New research suggests that if dark matter is composed of "dark photons," it would not have heated the early cosmos like scientists previously thought. If correct, this discovery could represent a paradigm shift in the hunt for the universe's most mysterious stuff.

Dark matter remains so elusive because, despite outweighing the everyday matter that composes stars, planets, moons and our bodies by a ratio of five to one, it is effectively invisible. That is because it doesn't interact with light. And the fact that electrons, protons, and neutrons do interact with light (or, more accurately, electromagnetic radiation ) has inspired the search for particles beyond the Standard Model of particle physics, leading to lots of hypothetical candidates for dark matter.

One of these candidates is the dark photon, the dark universe's version of a photon carrying a force other than electromagnetism, which is the responsibility of standard photons.

When dark photons have been considered in the past, scientists have concluded that they would have transformed into ordinary photons while still embedded in the thick and dense soup that filled the early cosmos. This would have further heated this already blisteringly hot plasma and left detectable traces of dark photons.

This severely limits the search parameters in which dark photons could exist — so much so that many cosmological observations rule out the existence of dark photons.

Now, new computer simulations show that the dismissal of dark photons may have been a little hasty. The conversion of dark photons to photons would have shut off before significant heating could occur.

That brings previously excluded search parameters back into play.

"These exclusions were saying the strength of dark matter had to be 10^8 times weaker than it actually can be," team member Anson Hook at the University of Maryland said in a statement. "This paper opens up a lot of new possibilities to look for dark matter."

An illustration of a dark photon, a candidate for dark matter. (Image credit: Robert Lea (created with Canva)) The team behind this research first saw hints that the transformation of dark photons to photons may not be as straightforward as scientists had assumed when they realized the amount of energy involved is suspiciously large. The problem, they began to suspect, was the fact that this process had been considered to be linear; in other words, the energy released gradually and steadily converts to plasma.

"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," team member Junwu Huang of the Perimeter Institute said in the statement. "And I realized it's not possible."

Performing their first computer simulations, Huang and colleagues realized that the linear process fails to paint a complete picture.

"What we realized is that, as you are converting energy into the Standard Model plasma, the plasma actually goes crazy," Huang said. "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."

Dark matter seen at the center of a galaxy. (Image credit: Mattia Di Mauro (ESO/Fermi-Lat)) The research represents a major widening of the parameters in which dark photons could exist, expanding this metaphorical hunting ground considerably. In fact, it could also impact the hunt for hypothetical particles beyond the Standard Model of particle physics.

"By calculating the early universe plasma correctly, experiments will probe new parameter spaces and potentially actually see something," team member Mohamad Shalaby of the Perimeter Institute said in the statement.

The team's research was published on August 13 in the journal Physical Review Letters.