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orochi
2010-Feb-13, 03:32 PM
Hi,

I have been thinking about the size of the Universe, and whether or not inflation theory is truly correct.

It seems to be that the only way we can have a Universe that is larger than what we currently see, yet still finite, is to have inflation take place. Would it be possible to still have a finite Universe, that is larger than the observable Universe, without inflation ever having taken place?

Andrew D
2010-Feb-13, 03:49 PM
If I'm not mistaken, what seperates the observable universe from the rest of the universe is inflation. The reason we can only see to a certain distance is that at that distance the speed of recession reaches c, so all objects beyond that horizon are infinately redshifted.

There are some ATM theories floating around out there, but most have holes, and none work as well observationally or mathmatically as Hubble's law.

Cougar
2010-Feb-13, 06:00 PM
Would it be possible to still have a finite Universe, that is larger than the observable Universe, without inflation ever having taken place?

The CMB from one side of the sky has been traveling ~13 billion years to reach us. Same from the other side of the sky. There hasn't been enough time since the big bang for these different parts of the universe to interact with one another. Yet they have exactly the same temperature within a fraction of 1%. This is a problem without inflation.

Jeff Root
2010-Feb-13, 09:27 PM
The CMB from one side of the sky has been traveling ~13 billion years to
reach us. Same from the other side of the sky. There hasn't been enough
time since the big bang for these different parts of the universe to interact
with one another. Yet they have exactly the same temperature within a
fraction of 1%. This is a problem without inflation.
Are you saying that Inflation is required, or just that it is very helpful?
Would it not be possible for CMB on opposite sides of our sky to travel
so far to reach us without Inflation, given that the matter in those now
widely-separated parts of the Universe was in close contact sometime
shortly before the CMB was released?

-- Jeff, in Minneapolis

Andrew D
2010-Feb-14, 01:38 AM
The CMB from one side of the sky has been traveling ~13 billion years to reach us. Same from the other side of the sky. There hasn't been enough time since the big bang for these different parts of the universe to interact with one another. Yet they have exactly the same temperature within a fraction of 1%. This is a problem without inflation.

Do you mean that the similarities we observe in all directions show a causal connection, and inflation is the only explanation for the connection that we have now?

Kwalish Kid
2010-Feb-14, 02:50 PM
The CMB from one side of the sky has been traveling ~13 billion years to reach us. Same from the other side of the sky. There hasn't been enough time since the big bang for these different parts of the universe to interact with one another. Yet they have exactly the same temperature within a fraction of 1%. This is a problem without inflation.
This is only a problem if one believes that the universe should be significantly different at these scales. We have no means of assigning probabilities to the initial conditions of the universe.

Jeff Root
2010-Feb-14, 03:14 PM
Kwalish Kid,

You appear to be saying that Inflation theory is baseless. The whole reason
for the existence of Inflation theory is to explain how the CMB can be so similar
in widely-separated parts of the Universe. You are saying that no explanation
is needed?

-- Jeff, in Minneapolis

Jeff Root
2010-Feb-14, 03:20 PM
This is only a problem if one believes that the universe should be
significantly different at these scales.
Different from what?

-- Jeff, in Minneapolis

Cougar
2010-Feb-14, 04:11 PM
Do you mean that the similarities we observe in all directions show a causal connection...

Right. Why else would they be almost the exact same temperature?


...and inflation is the only explanation for the connection that we have now?

It may not be the only explanation, but yes, AFAIK it's the only one we currently have.

Andrew D
2010-Feb-14, 05:02 PM
Right. Why else would they be almost the exact same temperature?.

I almost answered this with "maybe we're looking at the same place", but... duh.

Quick question: how does our 'local' temperature compare with the CMB?

Cougar
2010-Feb-14, 06:07 PM
...There hasn't been enough time since the big bang for these different parts of the universe to interact with one another...

This is only a problem if one believes that the universe should be significantly different at these scales. We have no means of assigning probabilities to the initial conditions of the universe.

Frankly, I'm just aping what I've read from a number of sources, the latest being Gribbin's The Origins of the Future [2006] at p.55. Yes, as we detect it, CMB radiation has been traveling ~13 billion years from its original emission on opposite sides of the sky. But of course it hasn't been traveling through a static universe, but rather an expanding one, which complicates the logical deduction that can be drawn therefrom. I've given this picture a modicum of thought but have not personally been able to arrive at any necessary conclusion. Likely just a personal shortcoming.

Andrew D
2010-Feb-14, 06:52 PM
Yes, as we detect it, CMB radiation has been traveling ~13 billion years from its original emission on opposite sides of the sky. But of course it hasn't been traveling through a static universe, but rather an expanding one, which complicates the logical deduction that can be drawn therefrom. I've given this picture a modicum of thought but have not personally been able to arrive at any necessary conclusion. Likely just a personal shortcoming.

Heres a great diagram. It represents visible light, but I believe the CMB can be thought of in the same terms in this case:

http://upload.wikimedia.org/wikipedia/commons/3/32/Embedded_LambdaCDM_geometry.png

"An isometric embedding of part of the visible universe over most of its history, showing how a light ray (red line) can travel an effective distance of 28 billion light years (orange line) in just 13 billion years of cosmological time. Click the images to zoom.
"(Mathematical details): Euclidean embedding of a part of the Lambda-CDM spacetime geometry, showing the Milky Way (brown), a quasar at redshift z = 6.4 (yellow), light from the quasar reaching the Earth after approximately 12 billion years (red), and the present-era metric distance to the quasar of approximately 28 billion light years (orange). Lines of latitude (purple) are lines of constant cosmological time, spaced by 1 billion years; lines of longitude (cyan) are world lines of objects moving with the Hubble flow, spaced by 1 billion light years in the present era (less in the past and more in the future)."

Jeff Root
2010-Feb-15, 12:14 AM
OOOooo! I really like that diagram, because I figured out everything going
on in it without any explanation! I noticed that the red line crosses the grid
at a 45-degree angle, so I knew it was the path of a pulse of light. At first
I didn't see that the red and orange lines were different colors, and thought,
"That can't be right! It has to be a different color, because they're different
things!" Just my eyes playing tricks on me.

-- Jeff, in Minneapolis

Kwalish Kid
2010-Feb-15, 01:29 AM
Kwalish Kid,

You appear to be saying that Inflation theory is baseless. The whole reason
for the existence of Inflation theory is to explain how the CMB can be so similar
in widely-separated parts of the Universe. You are saying that no explanation
is needed?

-- Jeff, in Minneapolis
What I am saying is that one cannot use the existence of differences in different areas of the universe as strong evidence for an inflation theory. There is no way to judge the probability that different areas of the universe should have a noticeably different density and content from other areas in the early eras of the universe. It could be that the initial conditions of the universe are very, very uniform.

There are other ways to produce evidence for an inflation theory. Different forms of inflation can lead to different dynamics that one can see the trace of.

Andrew D
2010-Feb-15, 02:00 AM
OOOooo! I really like that diagram, because I figured out everything going
on in it without any explanation! I noticed that the red line crosses the grid
at a 45-degree angle, so I knew it was the path of a pulse of light. At first
I didn't see that the red and orange lines were different colors, and thought,
"That can't be right! It has to be a different color, because they're different
things!" Just my eyes playing tricks on me.

-- Jeff, in Minneapolis

I had that same problem. The first time I saw it I read the description about 8 times and convinced myself that both lines were the same color and I just didn't understand it. But yes, good diagram. If you follow the sourcecode back to the article, there's another view of it, from straight on, so the bell shape forms a circle.

Jeff Root
2010-Feb-15, 02:02 AM
So you're discarding the reason Inflation theory was invented, and depending
on observation of some other, lesser effects to keep the theory alive.

-- Jeff, in Minneapolis