There may have been a second Big Bang, new research suggests
The Big Bang may have been accompanied by a shadow, the “Dark” Big Bang that flooded our cosmos with mysterious dark matter, cosmologists have proposed in a new study. And we may be able to see evidence for that event by studying ripples in the fabric of spacetime.
After the Big Bang, most cosmologists think, the universe underwent a period of rapid and tremendous expansion in its earliest moments, known as inflation. No one knows what caused inflation, but it is necessary to explain a number of observations, such as the extreme geometric flatness of the universe on a large scale.
Inflation was apparently driven by an exotic quantum field, which is a fundamental entity that permeates all of space. At the end of inflation, that field broke down in a shower of particles and radiation, causing the “Hot Big Bang” that physicists commonly associate with the beginning of the universe. These particles would continue to coalesce into the first atoms when the cosmos was about 12 minutes old and—hundreds of millions of years later—begin to aggregate into stars and galaxies.
But there is another ingredient in the cosmological mix: dark matter. Once again, cosmologists are not sure what dark matter is, but they see evidence for its existence through its gravitational influence on normal matter.
In the simplest models, the end of inflation and the subsequent Hot Big Bang also flooded the universe with dark matter, which evolved along an independent path. But this assumption is made simply for the sake of simplicity, two cosmologists proposed in a paper that appeared in February in the preprint database arXiv (opens in new tab). Scientists see no evidence for the existence of dark matter until much later in the evolution of the universe, since the elusive substance had enough time to exert gravitational influence, so there is no need for it to have filled the universe in the Hot Big Bang alongside normal. matter. Plus, because dark matter doesn’t interact with normal matter, it may have had its own “Dark” Big Bang, the researchers claim.
The Dark Big Bang
In their paper, the researchers explored what a dark Big Bang would look like. First, they hypothesized the existence of a new quantum field—a so-called “dark field”—which is necessary to allow dark matter to form completely independently.
In this new scenario, the Dark Big Bang begins only after inflation fades and the universe expands and cools enough to force the dark field into its phase transition, where it turns into dark matter particles.
Researchers found that the Dark Big Bang had to obey several constraints; if it is too early, there would be a lot of dark matter today, and if too late, there would be too little. But if the Dark Big Bang happened when the universe was less than a month old, it could agree with all known observations.
Introducing a dark Big Bang has several advantages. First, it is consistent with what scientists know about dark matter: if it does not interact with normal matter, then there is no reason for them to have a common origin. Second, it allows researchers to create models of dark matter without having to worry about how it will affect the behavior of normal matter at very early times, which gives scientists much more flexibility in creating models.
But more importantly, the researchers found that a dark Big Bang produces a distinct signature in gravitational waves, which are ripples in space-time that still swirl around the universe today. This means that the theory may one day be testable.
The researchers admit that current gravitational wave experiments lack the sensitivity to find signs of the Big Dark Bang. But another probe of gravitational waves using distances to distant pulsars, known as pulsar time groups like the NANOGrav experiment (opens in new tab), may be able to do the trick.