# Inflation (The evolution of the Universe)

## Main Question or Discussion Point

Hello!

I wanted to ask about the possible inflation era when considering the Big Bang model. Can someone explain me carefully but quite briefly what this inflation was?

Thanks :)

heusdens, thanks a lot, I took a quick look at it and it seems to suit me. Thanks again ;)

heusdens, thanks a lot, I took a quick look at it and it seems to suit me. Thanks again ;)
Yes, well I could add some more, but I must confess, I do not understand inflation and only approximately know what it says.

Before dealing with inflation, be advised, you know the basics of cosmology, and are aware of for example the "flatness" problem and "horizon" problem.

Inflation is a "cure" for this otherwise inexplicable fact that the universe is so flat, since any slight deviation from flatness near the begin would have resulted in a universe already dissipated or already collapsed.
Inflation (which is a very short and rapid expansion of space) solves this, because inflation (every e-fold in which the universe becomes twice ass large) increases the flatness.

Originally the proposal for inflation was a quantum tunneling effect, as proposed by Guth. It was shown that this quantum tunneling effect (from the false vacuum state to the true vacuum state) ended too messy, and could not explain the universe as we observe it.
Also in the Soviet union there were proposals for the early universe in the form of a phase transition of matter in the early universe, amongst others proposed by Starobinsky.

Linde and others worked on new models of inflation, incorporating the same idea of exponential expansion in a vacuum state. This (the 'gracefull exit) was solved in "new inflation" in which there is a slow-roll of the potential field (a scalar field, it's quantum number is equal to a vacuum state) towards the vacuum state. The field has a self-interacting or friction term, and is somewhat similar to an oscilator. Near the minimum of the field, the field oscilates and reheats the universe, and the universe enters the normal phase of expansion, and matter at the quark level is formed, later condensining into particles and atoms (recombination).
Later progress of inflation indicated that inflation can start without requiring special initial conditions (chaotic inflation) and can be eternal (inflation goes on in other parts of the universe indefinately, outside of our universe bubble).
A number of scientists have also expressed the idea that inflation can be past time eternal, thus solving the puzzle of the initial singularity.

Inflation is nowadays part of the standard big bang theory.

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A number of scientists have also expressed the idea that inflation can be past time eternal, thus solving the puzzle of the initial singularity.

Inflation is nowadays part of the standard big bang theory.
I don 't know the details of the past eternal inflation but according to the following refrences inflation can not be past eternal
A.Borde, A.H.Guth and A.Vilenkin,Phys .Rev.lett.90, 151301(2003)[arXiv:gr-qc/0110012]
A.Borde, Phys.Rev.D50,3392(1994)

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Here is a reference to a talk that argues for an internal inflating universe.

http://streamer.perimeterinstitute.ca/mediasite/viewer/?peid=83d006a2-5ef5-4816-9045-e55e4b32b2fc [Broken]

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Interestingly, here is a more recent paper (amongst others also by Vilenkin) with the title:
"Eternal inflation, bubble collisions, and the persistence of memory"

http://arxiv.org/PS_cache/hep-th/pdf/0612/0612242.pdf [Broken]

I don't know if this is correct (I will look into the paper in your post), but as I understand the matter is that for every bubble (say, our "universe") the amount/duration of inflation is always a finite duration.
However, inflation goes on indefinately outside our universe bubble, and does not need to have started at "some" time.

Other interesting paper:
"Philosophical Implications of Inflationary Cosmology"
http://arxiv.org/PS_cache/physics/pdf/0302/0302071.pdf [Broken]

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I would like to ask the meaning of three pont correlation function if someone knows. It deals with initial perturbations in inflationary cosmology.

hellfire
I would like to ask the meaning of three pont correlation function if someone knows. It deals with initial perturbations in inflationary cosmology.
The inflationary period is assumed to be the origin of the large-scale structures that we observe today. During inflation, there exists a process that converts quantum fluctuations into real density perturbations of the energy density. This first density perturbations will be the seeds of matter structures.

To analize the spectrum of these density perturbations correlation functions are used, same than used to analize the large-scale distribution of galaxies. For example, the two-point correlation function is a measure of the probability of finding a galaxy at a distance from another. The joint probability distribution of the density fluctuations that are generated by the inflationary period is a gaussian. It turns out that for a gaussian probability distribution the two-point correlation function encodes all statistical information.

The power spectrum of density perturbations can be measured in the anisotropies of the cosmic microwave background. There, a non-vanishing three-point correlation function would indicate deviations from gaussianity that would question the inflationary paradigm at least beyond some precision. Note, however, that a non-vanishing three-point correlation function in the distribution of large-scale structure today, long after the epoch of formation of the CMB, can be traced back to other mechanisms such as non-linear clustering.

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There, a non-vanishing three-point correlation function would indicate deviations from gaussianity that would question the inflationary paradigm at least beyond some precision. Note, however, that a non-vanishing three-point correlation function in the distribution of large-scale structure today, long after the epoch of formation of the CMB, can be traced back to other mechanisms such as non-linear clustering.
Thanks a lot for nice explanation. :)

Chronos
Gold Member
Numerical simulations have nicely reproduced what is observed in the CMB spectrum and large scale structure of the universe.

Of course I don't want to hijack this thread, but whilst the attention is here I'm interested in alternative solutions to the problems inflation cures and indeed wholly alternative cosmological models.

I've read over numerous theories, and was wondering if anyone here knows of a credible website outlining the main opposition to big bang the standard model of which anyway, even if someone has a list of recognised papers I could explore. thank you.

Finite or infinite universe?

Can someone help me please, Alex Vilenkin in his book Many Worlds in One, proposed the spacetime structure where the Universe can be finite as seen from “outside” but infinite when seen from inside – from our point of view.

How is it possible that universe can be infinite and finite… I don’t want to say at the same time, but somehow from different perspective?