What Happened Before the Big Bang? (Script) (Patreon)

We actually have a pretty good idea of what might have happened before the Big Bang. That is, as long as we define the Big Bang as the extremely hot, dense, rapidly expanding universe that is described by Einstein’s equations. That picture of the universe is very solid down to about a trillionth of a second after the supposed beginning of time. We can make good guesses down to about 10^-30th of a second. But before that? 

The universe almost certainly did NOT explode from a singular point. We covered that misconception recently. These days, the best-accepted description of the time before is given by inflationary theory.

The idea is that the energy trapped in the so-called inflaton field caused exponential expansion of space. That was the bang in the big bang. In previous episodes we looked at why cosmologists think we need inflation, and what could possibly cause it. That last one is definitely worth a watch if you haven’t yet – because today we’re going to peer further back in time, and explore a stunning implication of inflationary theory. See, if we accept that inflation happened at all, it’s hard to escape the conclusion that it never actually stopped – and in fact our universe is but one bubble among countless others in an eternally inflating greater universe.

Cosmic inflation, if it actually happened, was driven by the inflaton field, which had the bizarre property of containing a ton of energy even in the absence of any particles. It had a non-zero vacuum energy. Last time we talked about how such a field can drive exponential expansion, but stopped short of discussing what the field actually is, and what the real implications of its existence would be. Before we make any real predictions about the behavior of an inflating universe, we probably should know more about the field that drives it.

To start with, you need a particular type of field to cause inflation – something called a scalar field. This is actually the simplest type of quantum field because it’s described by a single number – a scalar – everywhere in space. Other fields like particle fields or the electromagnetic field are described by multiple components and vectors instead of single numbers. We know that scalar fields exist – or at least one does. That’s the Higg’s field, which gives elementary particles their mass. The inflaton field would be another such scalar field. Or it might even be the Higg’s field – physicists are still arguing over that one.

I mentioned last time that quantum fields can hold energy without actually having any particles. They do this through a process called self-interaction. You can think of a field with a high field strength as being full of virtual particles. These are ephemeral vibrations in the field that are constantly tugging at the field as the field tugs at them. This self-interaction gives the field some potential energy. It’s potential energy because the field would much rather reconfigure itself into a lower energy state, in which case that stored energy would be converted into another form – for example, into real particles.

Although scalar fields are the simplest, they can exhibit complicated relationships between this potential energy and field strength. In our last inflation episode we looked at the case of old inflation, proposed by Alan Guth in 1979. Guth’s idea is that there’s a local minimum in potential energy that allows the inflaton field to get stuck in a false vacuum state. When that state decays, potential energy is released as real particles, ending inflation and reheating the universe in an expanding bubble. The random nature of the inflaton decay means that many such bubbles should form. I.e., multiple universes exist. But we saw that there were problems in this approach – old inflation predicts empty firewall bubbles that look nothing like the early phase of our universe.

A more promising idea is something called slow-roll inflation. This was proposed by

Andrei Linde, Andreas Albrecht, and Paul Steinhardt in 1982, just a few years after Guth’s proposal. The idea of slow-roll inflation is that the inflaton field isn’t stuck in a local minimum in the potential, but rather it’s on a very weakly sloping plateau leading towards a deeper valley. In that case the field strength would very slowly roll down that slope, and as it did the energy would drop very, very slowly. 

That would still give us our near-constant energy density needed to power inflation. Then, as the roll sped up towards the valley, inflation would end – but it wouldn’t end as a random process. It wouldn’t require quantum tunneling to get started. Instead, an entire region of the inflating universe would approach this minimum at the same time. Inflation would shut down smoothly and the universe would be reheated everywhere at once. That gives us the expanding, hot, dense universe that we know and love in our Big Bang model. But if slow-roll inflation stops everywhere at once, how does it last forever? And how does it give us multiple universes?

Before we get to that, I want a quick word on why the inflaton field should have one potential energy curve versus any other. The behavior of this field depends on some unverified physics, but a suitable inflaton field fits with some Grand Unified Theories – those are theories that combine the strong nuclear force with the electromagnetic and weak forces, as well as which also unify gravity, like string theory. These theories predict phase transitions in the behavior of fields as the temperature of the universe changes. As the universe cools, different vacuum states can appear, possibly trapping the inflaton field. Very flat potential energy slopes are also possible in these theories, enabling slow-roll inflation. Or a combination of both. The detailed physics requires yet more episodes. So for now, take my word for it that inflation fits some theory, even if that theory is also entirely speculative.

As speculative as inflation is, it DOES make predictions. Some are even testable. I mentioned that quantum fields fluctuate due to the intrinsic randomness of the quantum world. As the inflaton field rolls down the potential energy hill, the field strength should fluctuate slightly. That means some regions of the universe would finish inflation a little ahead of others. That would lead to very small density and temperature fluctuations in the matter produces after inflation. And we see those fluctuations in the cosmic microwave background. These same fluctuations collapsed under their own gravity to become the first galaxies.

In fact this is perhaps the best evidence we have that inflation is plausible. It can predict the pattern of temperature fluctuations in the CMB. They should, according to inflation, come in all possible sizes on the sky – and be evenly distributed in abundance, with giant fluctuations as likely to occur as tiny ones, and all physical sizes in between. And that’s exactly what we see in the CMB.

But producing all the structure in our universe is probably the least impressive thing those quantum fluctuations did. They also give eternal inflation and multiple universes. In slow-roll inflation, exponential expansion should grind to a halt over large regions as the inflaton field decays. As I mentioned, small fluctuations in the inflaton field would lead to slight differences in when inflation ends from one point to the next. But quantum fluctuations come in all sizes, and a rare, strong fluctuation would force the inflaton field back up the potential energy slope, causing inflation to last a lot longer in that spot. Such fluctuations would be extremely rare, and so you wouldn’t think they’d count for much. But remember, inflation causes exponential expansion. The further up the slope the inflaton field gets pushed, the faster that expansion. So an uphill fluctuation in a tiny patch of space would very quickly outgrow its surroundings, producing a new inflating region. That region would then continue to decay, spawning new universes, but also spawning new inflating regions.

The result is stunning. Inflation never stops, but rather forms a fractal structure of infinitely expanding space interspersed with bubble universes of all different sizes. And to get this started? You need a speck. A fraction of the Planck energy within a Planck volume - say, a millionth of a gram within a space 10^-35 m across should do it. Assuming a quantum field of the right type, that speck will start inflating, the exponential nature of the process will take over, and the speck becomes infinite universes.

OK, cool story bro. Admittedly this all raises a few questions. How plausible is this mysterious inflaton field? Can eternal inflation last infinitely into the past as well as the future? What happens when bubbles collide? There are also deep possible connections between inflation and string theory and with the Holographic principle, as described in one of Stephen Hawking’s last papers. Good material for the eternally expanding playlists of PBS Space Time.