What is inflation and why do we need it?

[neutralino]

The thread by antonio in the universe expansion forum reminded me that Pat asked a while ago about inflation in my expansion of the universe thread and until now I had forgotten about it. So, here's a brief overview of what inflation is and why the standard model currently needs it (answered in the reverse order!)

Why does the standard model currently need inflation?
The big bang theory on its own has three major problems. These are

• The Horizon Problem
• The Flatness Problem
• The Monopole Problem

A brief discussion, suitable for the lay-reader, can be found in [1]. Inflation was invented in the 1980's by Alan Guth as a solution to some of the problems of the big bang. His form of inflation was not entirely correct, and so it has been modified a little, but the main idea was his, and he is known as the "father" of inflation. So...

What is inflation?
The simplest answer to this question is that inflation is a period when the universe is accelerating in its expansion. If any of you have read Ned Wright's tutorial that I linked to a while ago, we see him introduce [2] the scale factor a(t). This is basically a measure of distance in the universe, and so the definition above is equivalent to saying that a period of inflation occurs when [math]\frac{d^2a}{dt^2}=\ddot{a}>0[/math]; that is, the second time derivative of the scale factor is positive. We can draw the analogy with everyday life, since we know that if given a particle at a displacement, x, then the second time derivative of x is equal to the acceleration of the particle.

The actual ideas behind inflation are quite simple. The theory says that in the very early universe (and we're talking much less than 1 second here, since this had to happen before nucleosynthesis happened) the universe was dominated by exotic matter which exhibited negative pressure: that is, was of the form of a cosmological constant. It turns out, that by inserting this, the horizon and flatness problems are solved. It's obvious that such a mechanism can solve the horizon problem (which, in essence, questions why we see the cosmic microwave background radiation the same temperature from all directions when, in the inflationless big bang model, these areas were not causally connected) since the rapid acceleration in the early universe means that areas were now causally connected. It's a little harder to explain to the layperson why inflation solves the flatness problem, but it does. See [3] for an explanation.

So, as I've said earlier, inflation was invented to solve some problems of the bb model, but it turns out to give us more than that; it can also explain large scale structure formation in the universe.

Finally, to close my brief summary, the keener eyed amongst you may have noticed that in fact we are, today, going through a period of acceleration of expansion of the universe; i.e. inflation. This is a different period of inflation, and thus driven by a different cosmological constant to the one I discuss above (the cosmological constant driving acceleration at present time is often referred to as dark energy). These two types of inflation are mathematically the same ([imath]\ddot{a}>0[/imath]) but physically not. We note that the energy of the cosmological constant that drove early time inflation is much larger than the energy of the current cosmological constant. There have been theories that suggest one particle (recall, a cosmological constant is equivalent to an exotic particle with negative pressure) that was responsible for both, but to the best of my knowledge, these theories have some problems. Apologies to all, but I do not have any links to this last comment.

So, that's my summary of inflation. If anyone has any questions, or thoughts, please post and ask them. Finally (I mean it this time!) reference [4] is a very good site giving information on inflationary cosmology, but it is quite technical.

[1] http://csep10.phys.utk.edu/astr162/l...bproblems.html
[2] http://www.astro.ucla.edu/~wright/cosmo_02.htm
[3] http://csep10.phys.utk.edu/astr162/l...inflation.html
[4] http://nedwww.ipac.caltech.edu/level..._contents.html

(I'll try and find some more links giving more understandable explanations and post them here as and when I find them).

[Profpat]

Well I asked and you delivered now it's time for me to start reading.

Best,

Pat

[neutralino]

I hope you find it useful, Pat. I should add a caveat here. I said that inflation also solves the problem of large scale structure, however it does not necessarily need to be the same particle (or scalar field), that is the inflaton, that is responsible for large scale structure. There has been postulated another scalar field (or particle) called the curvaton that does this. However, the important point is that the scalar field was invented to solve the BB problems, and the same technology has been used to solve the large scale structure. Whether the two are the same is important for the specific model, but not for the general idea.

[Profpat]

I always find good information useful, Neutralino.

[swoarg]

thanks Neutralino i find the subject very interesting i will get reading
what are your thoughts about the void or beyond space having a force but not enough to hold back the expansion of space as space expands it utilisers this force and converts it into the vital ellements to enable more expansion

thanks swoarg

[neutralino]

thanks Neutralino i find the subject very interesting i will get reading
what are your thoughts about the void or beyond space having a force but not enough to hold back the expansion of space as space expands it utilisers this force and converts it into the vital ellements to enable more expansion

thanks swoarg

What do you mean by "the void"? Are you talking about something "beyond space"? If so, it doesn't exist; space has no boundary.

[swoarg]

Neutralino sorry i have maybe put this in the wrong thread but that is what i mean the void,i just find it hard to understand that space has no boundrys no doubt i have to read more
thanks Swoarg

[Profpat]

Neutralino sorry i have maybe put this in the wrong thread but that is what i mean the void,i just find it hard to understand that space has no boundrys no doubt i have to read more
thanks Swoarg

Don't feel bad swoarg I think I'm probably more confused than you. Even Stephen Hawkings has a little confusion when it comes to no boundries. Quote:



The newest development is that Hawking has recently realized that the no boundary proposal does not require the universe to be closed and finite, as he thought before. In a lecture Inflation; An Open and Shut Case, Hawking included the following statement.

• ...I thought the no boundary proposal, implied that the universe had to be spatially closed, and finite in size. But a few months ago, Neil Turok and I, were talking about his ideas on open inflation. We realized that they could be fitted in with the no boundary proposal. The universe would still be closed and finite, in one way of looking at it. But in another, it would appear open and infinite.
  
  You like I are probably more confused now.
  
  Best to you, don't let confusion get you down.
  
  Pat

[force5]

What do you mean by "the void"? Are you talking about something "beyond space"? If so, it doesn't exist; space has no boundary.

A simple question.

Can a volume of space exist without containing energy in some form?

Thanks, John

[swoarg]

thanks Pat and yes i am more confused but thanks for the reasurance that does seem to answer my question or it does seem to indicate that the question is still being asked ( i think)
must be heaven to be able to understand and work such theory,s out
thanks Swoarg