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 inflation event, which at very early times after the Big Bang sent our universe into a period of extremely rapid expansion. When inflation ended, the exotic quantum fields that powered this event decayed, transforming into the stream of particles and radiation that remains today.
When our universe was less than 20 minutes old, these particles began to assemble into the first protons and neutrons during what we call Big Bang Nucleosynthesis. Big Bang Nucleosynthesis is a pillar of modern cosmology, as the calculations behind it accurately predict the amount of hydrogen and helium in the cosmos.
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But despite the success of our picture of the early universe, we still don’t understand dark matter, which is the mysterious and invisible form of matter that makes up the vast majority of mass in the cosmos. The standard assumption in Big Bang models is that whatever process generated particles and radiation created dark matter. And after that, the dark matter just hung around, ignoring everyone else.
But a team of researchers has proposed a new idea. They argue that our inflation and Big Bang Nucleosynthesis eras were not alone. Dark matter may have evolved along an entirely separate path. In this scenario, when inflation ended, it still flooded the universe with particles and radiation. But not dark matter. Instead, some quantum field remained that did not decay. As the universe expanded and cooled, the extra quantum field eventually transformed itself, triggering the formation of dark matter.
The advantage of this approach is that it decouples the evolution of dark matter from normal matter, allowing Big Bang nucleosynthesis to continue as we currently understand it, while the dark matter evolves along a separate track.
This approach also opens up opportunities to explore a rich variety of theoretical models of dark matter, because now that it has a separate evolutionary track, it’s easier to keep track of the calculations to see how it compares to observations. For example, the team behind the paper could determine that if there was a so-called Dark Big Bang, it had to happen when our universe was less than a month old.
The research also found that the appearance of a dark Big Bang released a very unique signature of strong gravitational waves that would continue into the present universe. Ongoing experiments such as pulsar-timing arrays should be able to detect these gravitational waves, if they exist.
We don’t yet know if a Dark Big Bang happened, but this work provides a clear way to test the idea.