# Cosmic inflation

1. Jun 29, 2015

### DiracPool

1. Inflation--I've been doing a little study into cosmic inflation, and a question that many people (including myself) seem to have a hard time wrapping their head around is why was there a need for inflation rather than just assuming that the universe just started off flat, homogeneous, and even "temperatured." I can imagine a tiny sphere right around the time of the big bang whose geometry is (almost) perfectly flat and whose temperature is (almost) perfectly the same everywhere. I can then imagine this sphere (the universe) expanding at a steady rate to the point where at the size it is today it would have remained almost perfectly flat with an even temperature across the sky. However, when you present this scenario to a cosmologist they always seem to say "oh no, that could never happen." But then when they go into explaining why, I don't get the explanation.

The explanation always seems to go along the lines that 1) light wouldn't have time to communicate among the varied regions in order to keep the temperature even, and 2) that the inflation was needed to "flatten out" all the bumpiness in the pre-inflation situation. But my quandary is that, didn't we already take care of this in the description in the first paragraph? Is this just some kind of anomaly that just falls out of the mathematics that can't really be communicated visually? Or is it something like the size of the quantum fluctuations pre-inflation created a bumpiness in the primordial sphere I alluded to above such that it couldn't have started off that uniform, and that's the problem and that's why need the inflation concept? Or is it something else? I'm hoping someone can explain this fairly simply so that it makes sense, or can direct me to some literature/videos that do so.

2. Magnetic monopoles--Susskind has said that another problem inflation seems to solve is the monopole problem, saying that inflation diluted these monopoles to a concentration that may not be detectable (if my memory serves correct), and that is why no one has detected them. However, he also says that there was pretty much nothing pre the reheating epoch that existing other than the inflaton field. But now we also seem to have these monopoles existing before the reheating epoch as well as some "bumpiness" in the early inflaton field that needed smoothing or flatting out by the inflation process itself.

So my question is what, if anything, did the inflaton field consist of other than "inflaton potential energy" pre the inflationary phase and during the inflationary phase before reheating?

2. Jun 29, 2015

### Berenices

Last edited: Jun 29, 2015
3. Jun 29, 2015

### Orodruin

Staff Emeritus
You might have taken care of it with the assumption that you started out with an even temperature in the entire Universe. The point is that this is not necessarily your initial condition. With inflation, you can allow for initial conditions that are not homogeneous and still end up with a homogeneous universe.

You (essentially) do not have anything else than the inflaton field precisely because of inflation. This is why inflation solves the magnetic monopole problem. The point is that the inflationary phase dilutes away anything which is not the inflaton field and thus right before reheating the inflaton field is the only thing that is around.

4. Jun 29, 2015

### bapowell

You raise a good question. The inflaton field existed along with all the other matter and gauge fields in the early universe prior to inflation. There may or may not have been monopoles prior to inflation (they might be created after depending on the particle physics model). In order for inflation to get underway, there needed to be a sufficiently smooth region of the universe dominated by the inflaton field energy. (Notice that there is a little bit of circularity in inflationary theory -- that one needs a suitably smooth and uniform region of the universe for inflation to render a larger smooth and uniform region of the universe.) If there were monopoles in existence prior to inflation, they must still have been sufficiently dilute in some region of the universe so as not to spoil the uniformity needed for inflation to begin.

The other possibility is that the operative particle physics model does not produce monopoles prior to inflation, but after. Recall that monopoles are formed in GUTs as topological defects -- places in the universe where various scalar fields take on different vacuum expectation values (VEV). Inflation results in a smooth universe, with all such VEVs correlated and so at most we expect to see one monopole per horizon volume.

5. Jun 29, 2015

### DiracPool

Ok, I think that may be what I was looking for. If everything actually were completely smooth an homogenous at the very beginning, maybe we wouldn't need inflation at all, but that wasn't very likely due to, perhaps, quantum fluctuations very early on pre the inflationary epoch? So what inflation gives us is the opportunity to take a perhaps somewhat bumpy primordial sphere and smooth it out? In other words, it just allows for a wider variety of heterogeneity in the pre-inflationary universe?

6. Jun 30, 2015

### bapowell

Inflation takes an initially small homogeneous patch and inflates to a size greater than or equal to the observable universe.

Last edited: Jun 30, 2015
7. Jun 30, 2015

### phinds

As stated, that seems to imply that the entire size of the currently observable universe was caused by inflation, with no further added size due to the expansion, and accelerated expansion, of the universe. I'm sure that's not what you mean, but my point is that it's not clear WHAT you mean, exactly.

8. Jun 30, 2015

### bapowell

The universe is homogeneous across distances at least as great as the cosmological horizon. One way of understanding that is to suppose that during inflation length scales grew to be at least as large as 1/H.

9. Jun 30, 2015

### phinds

I have no idea what that means nor how it addresses what I said. I don't doubt that it DOES, I just don't understand it. Are you saying that a meter wasn't a meter at the beginning of inflation?

10. Jun 30, 2015

### bapowell

Sorry for the confusion. What I'm saying is that if $L_0$ is the proper size of the pre-inflationary patch, then $e^N L_0 \geq 1/H$, where $N$ is the number of e-foldings of inflation.

11. Jun 30, 2015

### phinds

I wonder, could you possibly address my post #7 in English, not math?

12. Jun 30, 2015

### bapowell

I don't understand what this means. Expansion, inflation or otherwise, has nothing to do with the size of the observable universe. Perhaps maybe you can clarify what you mean by this. All I am saying is that the initial, pre-inflationary patch must have been stretched by inflation to a size at least as large as the present observable universe. Of course there is additional, non-inflationary expansion beyond this. But we need *at least* enough inflationary expansion to explain the homogeneity observed on horizon scales.

13. Jun 30, 2015

### phinds

This seems to be saying that the volume that is currently encompassed by the observable universe was always the size it is now, because expansion and inflation had nothing to do with making it the size it is (and I don't know what else could have).
I agree w/ that but it seems to contradict the sentence directly above.
Again, I agree.

14. Jun 30, 2015

### bapowell

OK, good. In the sentence above I realize I'm not being very clear. What I'm trying to say is that the post-inflation particle horizon knows nothing about the amount of expansion that occurs during inflation.

15. Jun 30, 2015

### phinds

Well, again we agree. Seems we have, at least to some extent, been talking past each other in all this.

16. Jun 30, 2015

### bapowell

Well, I'm happy to have helped you past 9000 posts at least ;)

17. Jun 30, 2015

### phinds

Ha. I hadn't even noticed. Thanks

18. Jul 2, 2015

### Khashishi

Do people actually believe in magnetic monopoles? The non-observance of magnetic monopoles is only evidence for inflation if magnetic monopoles can be shown to exist.

The initial conditions of the universe could have been expected to be close to uniform in temperature by a simple probability argument. If you assume every particle in the universe was given some random position and velocity (you have to make some assumptions), then there are vastly more configurations with uniform energy distribution than non-uniform. This quickly becomes a uniform temperature distribution.

19. Jul 2, 2015

### bapowell

They are a generic prediction of most grand unified models. The lack of magnetic monopoles is not considered as evidence for inflation, rather, it is one of the puzzles that motivated cosmologists to propose inflation in the first place.
By this line of reasoning it is vastly more likely that instead of a uniform universe you'd end up with just a single, fully functioning brain: see https://en.wikipedia.org/wiki/Boltzmann_brain

20. Jul 2, 2015

### Khashishi

I don't accept the anthropic principle, so there is no reason to believe a Boltzmann brain is more likely than a uniform thermal bath. In fact, it is far less likely.