New constraints on a theory that says dark matter was created just after the Big Bang – rather than at the Big Bang – have been determined by Richard Casey and Cosmin Ilie at Colgate University in the US. The duo calculated the full range of parameters in which a “Dark Big Bang” could fit into the observed history of the universe. They say that evidence of this delayed creation could be found in gravitational waves.
Dark matter is a hypothetical substance that is believed to play an important role in the structure and dynamics of the universe. It appears to account for about 27% of the mass–energy in the cosmos and is part of the Standard Model of cosmology. However, dark matter particles have never been observed directly.
The Standard Model also says that the entire contents of the universe emerged nearly 14 billion years ago in the Big Bang. Yet in 2023, Katherine Freese and Martin Winkler at the University of Texas at Austin introduced a captivating new theory, which suggests that the universe’s dark matter may have been created after the Big Bang.
Evidence comes later on
Freese and Winkler pointed out that presence of photons and normal matter (mostly protons and neutrons) can be inferred from almost immediately after the Big Bang. However, the earliest evidence for dark matter comes from later on, when it began to exert its gravitational influence on normal matter. As a result, the duo proposed that dark matter may have appeared in a second event called the Dark Big Bang.
“In Freese and Winkler’s model, dark matter particles can be produced as late as one month after the birth of our universe,” Ilie explains. “Moreover, dark matter particles produced via a Dark Big Bang do not interact with regular matter except via gravity. Thus, this model could explain why all attempts at detecting dark matter – either directly, indirectly, or via particle production – have failed.”
According to this theory, dark matter particles are generated by a certain type of scalar field. This is an energy field that has a single value at every point in space and time (a familiar example is the field describing gravitational potential energy). Initially, each point of this scalar field would have occupied a local minimum in its energy potential. However, these points could have then transitioned to lower-energy minima via quantum tunnelling. During this transition, the energy difference between the two minima would be released, producing particles of dark matter.
Consistent with observations
Building on this idea, Casey and Ilie looked at how predictions of the Dark Big Bang model could be consistent with astronomers’ observations of the early universe.
“By focusing on the tunnelling potentials that lead to the Dark Big Bang, we were able to exhaust the parameter space of possible cases while still allowing for many different types of dark matter candidates to be produced from this transition,” Casey explains. “Aside from some very generous mass limits, the only major constraint on dark matter in the Dark Big Bang model is that it interacts with everyday particles through gravity alone.” This is encouraging because this limited interaction is what physicists expect of dark matter.
For now, the duo’s results suggest that the Dark Big Bang is far less constrained by past observations than Freese and Winkler originally anticipated. As Ilie explains, their constraints could soon be put to the test.
“We examined two Dark Big Bang scenarios in this newly found parameter space that produce gravitational wave signals in the sensitivity ranges of existing and upcoming surveys,” he says. “In combination with those considered in Freese and Winkler’s paper, these cases could form a benchmark for gravitational wave researchers as they search for evidence of a Dark Big Bang in the early universe.”
Subtle imprint on space–time
If a Dark Big Bang happened, then the gravitational waves it produced would have left a subtle imprint on the fabric of space–time. With this clearer outline of the Dark Big Bang’s parameter space, several soon-to-be active observational programmes will be well equipped to search for these characteristic imprints.
Stars powered by dark matter may have been seen by the JWST
“For certain benchmark scenarios, we show that those gravitational waves could be detected by ongoing or upcoming experiments such as the International Pulsar Timing Array (IPTA) or the Square Kilometre Array Observatory (SKAO). In fact, the evidence of background gravitational waves reported in 2023 by the NANOGrav experiment – part of the IPTA – could be attributed to a Dark Big Bang realization,” Casey says.
If these studies find conclusive evidence for Freese and Winkler’s original theory, Casey and Ilie’s analysis could ultimately bring us a step closer to a breakthrough in our understanding of the ever-elusive origins of dark matter.
The research is described in Physical Review D.
