The universe may have started with a dark Big Bang

The Big Bang may not have been the only one. The appearance of all particles and radiation in the universe may have been joined by another Big Bang which flooded our universe with dark matter particles. And we may be able to detect it.

In the standard cosmological picture, the early universe was a very exotic place. Perhaps the most significant thing to happen in our cosmos was the event of inflation, which, very soon after the Big Bang, sent our universe into a period of extremely rapid expansion. When the inflation ended, the exotic quantum fields that caused this event decayed, turning into a stream of particles and radiation that remain today.

When our universe was less than 20 minutes old, these particles began to come together to form the first protons and neutrons in what we call Big Bang nucleosynthesis. Big Bang nucleosynthesis is a mainstay of modern cosmology, as the calculations behind it accurately predict the amount of hydrogen and helium in the cosmos.

However, despite the success of our picture of the early universe, we still don’t understand dark matter, which is the mysterious, invisible form of matter that occupies the vast majority of mass in the cosmos. The standard assumption in Big Bang models is that whatever process generated particles and radiation also created dark matter. And after that, the dark matter just sat there, ignoring everyone.

But a team of researchers has come up with a new idea. They argue that our eras of Big Bang inflation and nucleosynthesis were not alone. Dark matter may have evolved along a completely separate trajectory. In this scenario, when inflation ended, it further flooded the universe with particles and radiation. But no dark matter. Instead, there remained a quantum field that did not decay. As the universe expanded and cooled, this extra quantum field eventually transformed, triggering the formation of dark matter.

The advantage of this approach is that it decouples the evolution of dark matter from normal matter, so that Big Bang nucleosynthesis can proceed as we currently understand it while dark matter evolves along a separate path.

This approach also opens up avenues for exploring a rich variety of theoretical models of dark matter, because now that it has a distinct evolutionary pathway it is easier to follow in calculations to see how it might compare to observations. For example, the team behind the paper was able to determine that if there was a so-called Dark Big Bang, it must have happened when our universe was less than a month old.

The research also revealed that the onset of a Dark Big Bang released a very unique signature of strong gravitational waves that would persist in the current universe. Ongoing experiments like pulsar synchronization networks should be able to detect these gravitational waves, if they exist.

We still don’t know if a Dark Big Bang happened, but this work opens a clear path to test the idea.