Is Inflation the Key to Understanding a Flat Universe?

[HowardTheDuck]

Hi, I've been reading about inflation, and I'm not totally convinced about how it is proposed to result in a flat universe.

If I understand it correctly, inflation expanded the universe so rapidly that the actual size of the universe now is 23 orders of magnitude larger than the observable universe (according to the second section of this Wikipedia article: http://en.wikipedia.org/wiki/Observable_universe)

So the initial universe got stretched to a a hugely larger size, and hence our observable universe appears flat (like a bowling ball stretched to the size of the universe - it would appear flat locally).

So the part of the universe we can see is a tiny fraction of the total universe.

But here's the bit which doesn't make sense to me. When we use the Hubble telescope to look deep into the observable universe, we see galaxies which are in early stages of construction. The galaxies look noticeably different to the galaxies closer to us.

("The Hubble Deep Field galaxies contained a considerably larger proportion of disturbed and irregular galaxies than the local universe" - quoted from here).

This would appear to indicate that we are actually seeing a large proportion of the total universe. If we could only see a fraction of the universe then we would expect the distant galaxies to look very much the same as the galaxies nearer us.

So this says to me like inflation to solve the flatness problem is wrong. And maybe inflation on the whole is wrong.

Anyone? Thanks.

 

[mfb]

Galaxies in a large distance can be seen how they looked billions of years ago - the difference to galaxies nearby is just the time evolution, not a spatial variance.

So this says to me like inflation to solve the flatness problem is wrong. And maybe inflation on the whole is wrong.

If you don't understand something as established as inflation, your first, second, third and at least forth idea should be that the problem is at you.

 

[HowardTheDuck]

Thanks, yes.

But like I say, according to inflation the vast bulk of the universe is outside of the observable universe - we shouldn't be able to see these things at all.

Here's that quote from Wikipedia:

"According to the theory of cosmic inflation and its founder, Alan Guth, if it is assumed that inflation began about 10^−37 seconds after the Big Bang, then with the plausible assumption that the size of the universe at this time was approximately equal to the speed of light times its age, that would suggest that at present the entire universe's size is at least 10^23 times larger than the size of the observable universe."

So we should be able to see only a fraction of the total universe. No? This is why the local part - the part we can see - appears flat. Because we can only see a fraction of the total universe.

"If you don't understand something as established as inflation, your first, second, third and at least forth idea should be that the problem is at you." - Cheers, mate.

 

[mfb]

So we should be able to see only a fraction of the total universe. No?

There is no hint that we see any significant fraction of the whole universe. The universe could even be infinite in size.

 

[HowardTheDuck]

Well, my point it that those Hubble images seem to indicate that the observable universe is a considerable fraction of the entire universe. We seem to be able to see a significant fraction of the lifespan of those galaxies. I appreciate we are looking back in time when we look into space, but those galaxies are moving at a certain speed so that time translates to a distance as well - we're not just looking back in time.

I just can't believe from that that the total universe is 10^23 times larger than the observable universe - as inflation suggests.

 

[mfb]

Well, my point it that those Hubble images seem to indicate that the observable universe is a considerable fraction of the entire universe.

No, they do not.

We seem to be able to see a significant fraction of the lifespan of those galaxies.

We are not. For the most distant galaxies, we can only observe how they looked like billions of years ago. We cannot observe how they look like today.
In addition, that has nothing to do with distance, it is just time. If you would have been able to capture an image of our galactic neighborhood 10 billion years ago, it would have looked similar.

I appreciate we are looking back in time when we look into space, but those galaxies are moving at a certain speed so that time translates to a distance as well - we're not just looking back in time.

Sure, but today those distant galaxies look like the ones we have nearby.

I just can't believe from that that the total universe is 10^23 times larger than the observable universe - as inflation suggests.

That is not a problem of the theory.

 

[HowardTheDuck]

Thanks for your help. I just think the distant galaxies look different. They look less formed, more basic. Like I said in that quote from Wikipedia "a considerably larger proportion of disturbed and irregular galaxies than the local universe".

You said: "If you would have been able to capture an image of our galactic neighborhood 10 billion years ago, it would have looked similar." Yes, our local galaxies would have been 10 billion years younger, in a less-formed state - like those distant galaxies appear to my eyes.

We'll have to agree to disagree! Thanks for your help.

 

[mfb]

They look less formed, more basic.

Sure. As we currently see them, they had 10-13 billion years less time than galaxies we see nearby.

You said: "If you would have been able to capture an image of our galactic neighborhood 10 billion years ago, it would have looked similar." Yes, but clearly in a less-formed state - like those distant galaxies.

Right, our Milky Way started like those galaxies as well. Where is the problem?

We'll have to agree to disagree!

Usually, that is a sign that one side is wrong. One side is supported by basically all cosmologists. Guess which one is right.

 

[HowardTheDuck]

Yes, I think I have a much better understanding now.

We might only be seeing a fraction of the total universe. The Hubble telescope is just looking at the only galaxies whose light has had time to reach us. The universe can expand much faster than the speed of light, so there can indeed be a huge number of galaxies outside our observable universe.

I'm happy now! Thanks a lot. Take a chill pill, though.

 

[Chronos]

Galactic evolution is not highly 'evolved' as theories go. There are plenty of enigmas out there that resist explanation - like how did supermassive black holes form in the early universe, and how did highly metallized, 'old' galaxies [giant ellipticals] form? They obviously had to evolve from something less well structured, but, plenty of details remain to be fleshed out. Many cosmologists believe the observable universe is just a small part of a much larger 'megaverse', but, is uproven thus far. If we assume the inflation model is correct, as suggested by the latest Planck data, the observable universe appears uncomfortably small at a mere 10^26 meters in diameter.

 

[bapowell]

Yes, I think I have a much better understanding now.

We might only be seeing a fraction of the total universe. The Hubble telescope is just looking at the only galaxies whose light has had time to reach us. The universe can expand much faster than the speed of light, so there can indeed be a huge number of galaxies outside our observable universe.

I think you are suffering from some conceptions regarding the big bang. Have you had a look at the balloon analogy sticky? There could be an infinite number of galaxies infinitely far away from us, and this has nothing to do with "time since the big bang" or any such thing. The universe did not evolve out of a single point, as some of the popular (and incorrect) conceptions of the big bang might suggest. Imagine an infinite rubber sheet that starts expanding -- this is one way to imagine the big bang (and the rubber sheet need not be infinite, it just helps with the illustration.) The expansion is happening everywhere on the sheet. Galaxies are forming everywhere on the sheet. We happen to be sitting somewhere on the sheet, and as we look out, we see an ever expanding circle of galaxies come into view. The farther out we look, the younger these galaxies look. But there's no shortage of galaxies! I hope this helps.

 

[Chalnoth]

Hi, I've been reading about inflation, and I'm not totally convinced about how it is proposed to result in a flat universe.

Mathematically it's very simple. During inflation, the energy density of the inflaton behaved rather like a cosmological constant. This means that the density didn't change with the expansion.

While the spatial curvature is not an energy density, it sits alongside the other energy densities in a similar manner in the Friedmann equations:

H(a)^2 = {8\pi G \over 3} \rho - {kc^2 \over a^2}

Here the energy density \rho from inflation was roughly constant (not exactly: inflation has to end, after all, but it has to be pretty close to constant to inflate at all). So the bit on the left was remaining the same, while the bit on the right, the curvature term, was shrinking as 1/a^2. So if the universe expanded by a factor of 10^{30}, then the effect of curvature on the expansion became roughly 10^{30} times smaller (because the curvature scales as 1/a^2, but the left hand side has the expansion rate squared...there's also another factor of 1/2 in there, but that doesn't change the overall point).

Because of this, there might have been quite a bit of curvature when inflation started. But it diluted away due to the rapid accelerated expansion.

 

[Naty1]

Hi Howard...
with the above under your belt, you might enjoy this if you have not yet seen it:

THE BALLOON ANALOGY

http://www.phinds.com/balloonanalogy/


It's from these forums, is an introduction to some basic concepts, and explains what can be inferred from the balloon analogy of the universe and what should not be.