A hypothetical particle that could help explain dark matter may have been dismissed too quickly, according to a new study.
Scientists have taken another look at “dark photons,” proposed particles that are similar in some ways to ordinary particles of light, or photons.
Dark photons are considered a possible form of dark matter, the mysterious material thought to make up most of the matter in the universe.
A study published in Physical Review Letters suggests that previous calculations may have placed overly strict limits on where scientists should look for these particles. The reason involves the strange behavior of plasma in the early universe.
Shortly after the Big Bang, the universe was filled with extremely hot plasma—a gas of electrically charged particles. For years, physicists assumed that if dark photons existed at that time, some of their energy would have been converted into ordinary electromagnetic energy inside this plasma.
That process should have heated the early universe. Because astronomers can measure the effects of conditions in the young cosmos, the apparent absence of this extra heating has been used to rule out a huge range of possible dark photon properties.
But researchers from Perimeter Institute and the University of Maryland now argue that this reasoning overlooked an important part of plasma physics.
Previous calculations treated the transfer of energy from dark photons into ordinary plasma as a relatively simple, gradual process. In physics, this is known as a linear approximation. Using that assumption, researchers calculated that dark photons could transfer enormous amounts of energy into the plasma.
Junwu Huang of Perimeter Institute began questioning whether such a large transfer of energy was physically realistic. Working with particle physicist Anson Hook and plasma physicist Mohamad Shalaby, the team investigated what would actually happen as energy entered the early universe’s plasma.
Computer simulations produced a very different picture.
As dark photon energy begins entering the plasma, the plasma does not simply absorb more and more energy in a smooth way. Instead, its behavior quickly becomes highly complicated and nonlinear. These effects effectively shut down the conversion process after only a tiny amount of energy has been transferred.
That means dark photons would not necessarily have heated the early universe enough to leave the signals scientists previously expected.
The implications could be substantial. According to the researchers, conventional cosmological limits on dark photons may not apply across about 10 orders of magnitude in possible particle mass, ranging from approximately 10⁻¹⁵ to 10⁻⁶ electron volts.
This reopens a huge area that scientists had largely considered unavailable for dark photon searches. Future experiments may now be able to explore this territory and potentially discover signals that previous theories suggested should not exist.
The finding could also reach beyond dark matter.
Scientists frequently use simplified linear models to predict how unusual particles and electromagnetic fields behave in extreme environments, including around neutron stars and white dwarfs. If nonlinear plasma effects become important in these environments as well, some existing astrophysical limits on new particles may need to be reconsidered.
The research highlights how combining different areas of physics can challenge long-standing assumptions. By bringing together expertise in particle physics, cosmology and plasma physics, the team found that the early universe may have behaved very differently from the simplified picture scientists had been using.
And that difference could give the hunt for dark matter a surprisingly large new place to look.
