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Did Cosmic Inflation really happen?

Post of the Month: January 2010


Subject:    | The Big-Bang Theory is flawed.
Date:       | 21 Jan 2010
Message-ID: | hjajoj$1l6n$

jillery asks:
> If you don't mind, I would like to ask you a question. I have an interested
> layman's understanding of these things, so bear with me if I don't use the
> right words the right way. My understanding is that Alan Guth introduced the
> concept of Inflattion, as a separate stage of expansion, because the original
> description of the Big Bang didn't allow for the Universe to be as uniform as
> it is.

Steve Carlip replies:
Well, roughly.

One of the basic issues is that without inflation, or something like it, points more than a few degrees apart from each other in the sky would not yet have had time to "communicate" with each other since the big bang. But we observe that the cosmic microwave background radiation ("CMBR" or "CMB") is uniform to a part in 100,000. There's no obvious reason why the CMBR should look the same in patches of the sky that have never been in contact with each other.

We *could* just hypothesize that the Universe started from a very uniform early state -- since we don't know the relevant physics yet, this is certainly possible. The issue isn't that the big bang model is inconsistent without inflation, just that it requires an assumption that many people considered "unnatural." There were a bunch of attempts to explain this uniformity -- for instance, a chaotic early stage that more thoroughly "mixed" the Universe -- but they mostly didn't work in detail.

Inflation gives a different explanation: basically, that the whole visible Universe grew from some very tiny initial region, in which an assumption of uniformity might be more natural. It's not yet completely clear whether this is a sensible explanation -- you still need a tiny uniform patch of space for inflation to start, and a major topic of research these days is how common such patches should be.

The major success of inflation was something that wasn't foreseen at the start. As I said, the CMBR is uniform to a part in 100,000. But at smaller levels, it has a very specific pattern of fluctuations. Inflation explains these: they are ordinary quantum mechanical fluctuations, due to the uncertainty principle, in the tiny initial patch, blown up by inflation to the size of the Universe. Note that this is not just a "story" -- it's a set of very detailed quantitative predictions, which have been tested to very high accuracy. Again, this is not a "proof" of inflation, but most of the alternatives that have been suggested so far seem very contrived.

> What I don't understand is what mechanism(s) caused inflation to start and stop
> when it did. Can you offer an explanation that I might understand?

Part of the problem is that the answer isn't unique -- there are a number of different mechanisms that can cause inflation to start and stop. I'll try to describe the simplest (and perhaps the most likely).

The closest everyday analogy is bubbles of steam "inflating" in a pot of boiling water. The basic feature is a phase transition, like the transition from liquid to gas. When a quantum field undergoes a phase transition, its pressure and energy change, again like water. But according to general relativity, pressure and energy determine the gravitational field and the structure of spacetime, and a phase transition can drastically change the expansion of the Universe.

In particular, inflation will take place if a quantum field has a constant, or nearly constant, nonzero potential energy. You should think of this as the energy of the field's interaction with itself, roughly analogous to the binding energy of molecules of water. If a field jumps from one value of potential energy to another in some small region -- either by quantum tunneling of through ordinary thermal fluctuations -- this can nucleate an inflating "bubble" of space. Inflation will continue until the potential energy drops down to zero.

(By conservation of energy, that potential energy has to go somewhere. It goes into "reheating" the Universe, creating a hot stew of particles and antiparticles. That's where the standard hot big bang model starts from.)

What I've described so far is a model, or a set of models, rather than a detailed theory. To go farther, we would have to know exactly what quantum field is responsible for this behavior, and we'd have to understand its interactions, which determine its potential energy. That's an open question. The hope is that more precise measurements of the CMBR can help pin this down -- the *very* fine details of the fluctuations depend on the details of inflation -- and that, conceivably, a relevant particle could show up in accelerator experiments. But I suspect this will take a while.

Steve Carlip

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