The 1st few seconds of the Big Bang: What we know and what we don't

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute, host of Ask a Spaceman and Space Radio, and author of How to Die in Space. He contributed this article to Space.com's Expert Voices: Opinions and Insights.

Believe it or not, physicists are attempting to understand the universe when it was only a handful of seconds old. 

But the situation here is complex, to say the least, and while we've made significant headway, there's still a lot left to learn. From miniature black holes to exotic interactions, the infant universe was a busy place.

The universe: Big Bang to now in 10 easy steps

The known knowns

Let's start with the general framework. 13.77 billion years ago, our universe was incredibly hot (a temperature of over a quadrillion degrees) and incredibly small (about the size of a peach). Astronomers suspect that, when our cosmos was less than a second old, it went through a period of incredibly rapid expansion, known as inflation.

This inflation event was perhaps the most transformative epoch ever to occur in the history of our universe. In less than a blink, our universe became incredibly larger (enlarging by a factor of at least 10^52). When this rapid expansion phase wound down, whatever caused inflation in the first place (we're not sure what) decayed, flooding the universe with matter and radiation (we're not sure how).

A few minutes later (literally), the first elements emerged. Prior to this time, the universe was too hot and too dense for anything stable to form — it was just a giant mix of quarks (the fundamental building blocks of atomic nuclei) and gluons (the carriers of the strong nuclear force). But once the universe was a healthy dozen minutes old, it had expanded and cooled enough that the quarks could bind themselves together, forming the first protons and neutrons. Those protons and neutrons made the first hydrogen and helium (and a little bit of lithium), which went on hundreds of millions of years later to build the first stars and galaxies.

From the formation of the first elements, the universe just expanded and cooled, eventually becoming a plasma, and then a neutral gas. 

While we know that this broad-brush story is correct, we also know that we're missing a lot of details, especially in the time before the formation of the first elements. Some funky physics may have been in operation when the universe was only a few seconds old, and it's currently beyond our theoretical understanding — but that doesn't stop us from trying. 

The known unknowns

A paper recently appearing in the preprint journal arXiv, and accepted for publication in The Open Journal of Astrophysics, lays out some of the more exotic very-early-universe scenarios.

For example, there's the whole question about dark matter. We don't know what dark matter is made of, but we do know that it's responsible for over 80% of the matter in the universe. We have a well-understood story for how normal matter originated in the hot, dense soup of the early cosmos, but we have no clue when or how dark matter came on the scene. Did it appear in the first few seconds? Or much later? Did it mess up the cosmic chemistry that led to the first elements, or stay in the background?

We don't know.

Then there's inflation itself. We don't know what provided the power source for the incredible expansion event, we don't know why it lasted the length of time that it did, and we don't know what eventually stopped it. Perhaps inflation lingered for longer than we've been assuming, and made its presence known for an entire second, rather than the tiny fraction that we've been assuming. 

Here's another one: there's this massive thorn in the side of every cosmologist known as matter-antimatter asymmetry. We see from experiments that matter and antimatter are perfectly symmetrical: for every particle of matter made in reactions throughout the universe, there's also a corresponding particle of antimatter. But when we look around the cosmos, we see heaps and heaps of normal matter and not a drop of antimatter in sight. Something huge must have happened in the first few seconds of the universe's existence to throw off that balance. But as to who or what was responsible, and the exact mechanism, we're not sure.

And if dark matter and inflation and antimatter weren't enough, there's also the possibility that the early universe might have manufactured a flood of small black holes. Black holes in the present-day cosmos (i.e., the past 13 billion years) all come from the deaths of massive stars. Those are the only places where the density of matter can reach the critical thresholds necessary to trigger black hole formation. But in the exotic early universe, random patches of the cosmos may have achieved sufficient density, triggering the creation of black holes without having to go through the whole star-formation thing first. Maybe.

Images: Black holes of the universe

Digging deeper

While our theory of the Big Bang is supported by a wealth of observational data, there are plenty of mysteries to satisfy the curiosity of generations of cosmologists. Thankfully, we're not completely blind when trying to study this early epoch. 

For example, even if we can't directly see the state of the universe when it was only a few seconds old, we can try to recreate those conditions in our powerful particle colliders. It's not perfect, but it can at least teach us about the physics of those kinds of environments.

We can also look for clues left over from the first few seconds. Anything funky going on then would've left its mark on the later universe. Changing the amount of dark matter or a lingering inflation would upset the creation of hydrogen and helium, something we can measure today. 

And the universe transitioned from a plasma to a neutral gas when it was 380,000 years old. The light released then has persisted in the form of the cosmic microwave background. If the universe popped out a bunch of small black holes, they would affect this afterglow light pattern.

We might even hope to observe this epoch directly. Not with light, but with gravitational waves. That chaotic inferno must have released a torrent of ripples in the fabric of space-time, which — like the cosmic microwave background — would have survived to the present day. We don't yet have the technological capability to directly observe those gravitational waves, but every day we're inching closer.

And perhaps when we do, we'll get a glimpse of the newborn universe.

Learn more: "The First Three Seconds: A Review of Possible Expansion Histories of the Early Universe"

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