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loria
2010-Apr-25, 09:02 PM
I think they key point here is that you don't understand the physics underlying the theory - as you admit and is made clear by your comments. If you don't understand that, then it is not surprising if you struggle to understand and accept the ideas behind the big bang. If your post is just a "wail of despair", a plea for help, then you might be better off asking some questions ("what is space?", "how can it expand?", "how do we know it is expanding?", etc) in the Q&A forum.
I too am new here and maybe my question would be better in Q & A.
However my question is 'What is the physics behind inflation?'
That is, what known theory drives it?
What test has been done in the laboratory to show that the theory is correct?
Thats all.

Cougar
2010-Apr-26, 07:33 PM
However my question is 'What is the physics behind inflation?'
That is, what known theory drives it?
What test has been done in the laboratory to show that the theory is correct?





Inflation is a modification of the conventional big bang theory, proposing that the expansion of the universe was propelled by a repulsive gravitational force generated by an exotic form of matter. Although Guth's initial proposal was flawed (as he pointed out in his original paper), the flaw was soon overcome by the invention of "new inflation," by Andrei Linde in the Soviet Union and independently by Andreas Albrecht and Paul Steinhardt in the US. After more than 20 years of development and scrutiny the evidence for the inflationary universe model now looks better than ever. -- Source (http://web.mit.edu/physics/people/faculty/guth_alan.html)

It is very difficult, if not impossible, to reproduce the beginning of the universe in the laboratory, so inflation hasn't been tested there. Inflation is still not a "sure thing," but several different observations are supportive of it. See Wiki (http://en.wikipedia.org/wiki/Inflation_%28cosmology%29#Observational_status) for some details.

DrRocket
2010-Apr-26, 08:06 PM
I too am new here and maybe my question would be better in Q & A.
However my question is 'What is the physics behind inflation?'
That is, what known theory drives it?
What test has been done in the laboratory to show that the theory is correct?
Thats all.

Your question and the title that you selected for the thread are not quite synchronized.

The expansion of the universe is not dependent on inflation theory. It is an observed phenomena that space is expanding. That can be explained with general relativity, and there is quite a lot of observational evidence for general relativity. It is further observed that the rate of expansion is increasing. No one really knows why that is happening, but it has been given a name -- dark energy. Dark energy can be modeled in general relativity using a positive cosmological constant, but there is currently no good understanding of or theoretical reason for dark energy.

Inflation is used to explain the extremely rapid expansion of the universe shortly after the big bang, although there are theories the extend to much larger time frames. The expansion is, in the model, driven by a scalar field, called the inflation field. No one knows what that field is or even if it really exists, although many predictions based on the model for things like the anistropy of the cosmic background radiation seem to be accurate. A fundamental understanding of the inflation field, if it really exists, will probably have to await progress in the understanding of the quantum field theories, or their successors, that are used to explain the physics of elementary particles.

For a pretty reasonable exposition of inflation, Alan Guth's book The Inflationary Universe is a good source for the non-specialist.

ShinAce
2010-Apr-26, 08:25 PM
The expansion of the universe is not dependent on inflation theory. It is an observed phenomena that space is expanding. That can be explained with general relativity, and there is quite a lot of observational evidence for general relativity.


I never saw the connection. How does GR truly explain inflation? Or is it only consistent with inflation?

Andrew D
2010-Apr-26, 08:37 PM
An excellent and straightforward explaination of the current model of inflation can be found in Leonard Susskind's video series titled "Cosmology". Here's a link to the first video; I believe there are 9.

http://www.youtube.com/watch?v=32wIKaLkvc4

EDIT: note that this is an explination of the model, not the cause. Thus, it describes the "what", but not the "why". It's also important to keep in mind that if a detailed explaination of the "why" was readily available, we wouldn't need terms like "dark energy".

Jeff Root
2010-Apr-26, 09:02 PM
ShinAce,

DrRocket will have to address your question to him in more detail, but I
can point out that he said the expansion can be explained with general
relativity, not that inflation can be so explained.

I would limit my own statement to saying that expansion can be described
with general relativity, rather than explained.

-- Jeff, in Minneapolis

DrRocket
2010-Apr-26, 09:57 PM
I never saw the connection. How does GR truly explain inflation? Or is it only consistent with inflation?

Expansion of the universe is a subject that is distinct from inflation.

Expansion, as it is observed now, can be handled by general relativity. To explain accelerated expansion takes more -- a positive cosmological constant, or dark energy, or something.

Inflation is primarily associated with the exponential expansion in the first fraction of a second after the big bang, and is based largely on particle physics concepts. In particular it involves some unknown scalar field, called the inflation field. Inflation is distinct from the ongoing expansion of space.

Just to confuse things a bit, there is also a notion of eternal inflation, but at this stage of the discussion I suggest that you not worry about it until the overall picture becomes a bit more clear to you.

blueshift
2010-Apr-26, 11:09 PM
Loria, welcome to the forum.

Your OP seems to be focusing on some experimental evidence for inflation. One does give a hint indirectly.
The experiment is called "inelastic scattering". If one wants to build an entire universe out of something fundamental, then the densest and most fundamental particle known, quarks and antiquarks, should be the starting place. When a high energy beam of electrons are aimed between a quark/antiquark pair that are contained in a meson, the associated gluon field will stretch and stretch until it snaps. The energy of that snapping is so great that it does not leave the pair to become isolated from one another. Instead, the energy is converted to matter, another quark/antiquark pair, each associated with the two in the beginning of the experiment. So now we have 4 quarks where before we just had two. Science cannot isolate quark/antiquark pairs. The more it tries, the more pairs it creates.

That should give you a hint. If something extremely energetic could continuously expand the gluon field of just one quark/antiquark pair, snapping it many time over, it would repeatedly multiply matter exponentially while the density of the universe remained unchanged in the beginning. But it would not have infinite energy so it would eventually dissipate that energy, slowing down inflation unless something could give that field some added kick.

Other pieces of evidence exist but this is a starter.

Ken G
2010-Apr-26, 11:28 PM
In addition to the correct answers already given, I would point out that inflation is probably more like a signpost to a theory than a theory itself, expressly because of its difficulty in testing. It stems from a series of puzzling observations, all of which can be explained at some level by inflation. In short, what inflation does is to stipulate physics that we know nothing about to bring puzzling observations into contact with physics that we know a lot about. If that sounds like a kind of "devil's bargain", I would point out that physicists have been historically pretty tight with the devil in that regard.

The puzzling observations that are brought into contact with established physics stem from some symmetries that exist that seem like they shouldn't, and other symmetries that don't exist but seem like they should. In the former group, we have a CMB that is the same temperature in all directions, even though we are looking at regions that were never in causal contact in the standard Big Bang model with no inflation. Inflation fixes that by placing them in causal contact by bringing them much closer together at early times than they otherwise would be (when we run time backward). In the latter group, we have the matter/antimatter asymmetry (more matter than antimatter), and inflation helps that by allowing our universe to come from a tiny region which might have had a statistical fluctuation in the matter/antimatter density. We also have small variations in the CMB that are also hard to explain if it was in causal contact, but inflation brings us into contact with quantum mechanical fluctuations in the very small region what we now see came from. Finally, we have another symmetry that we probably shouldn't have-- spatial hyperslices at the same universal age are very flat, much flatter than they should be-- unless flattened by inflation.

Spaceman Spiff
2010-Apr-27, 02:32 AM
Inflation is used to explain the extremely rapid expansion of the universe shortly after the big bang, although there are theories the extend to much larger time frames. The expansion is, in the model, driven by a scalar field, called the inflation field.
(my bold)

A minor nit pick: the scalar field in question is referred to as the "inflaton (http://en.wikipedia.org/wiki/Inflaton)" field. See also Andre Linde's (http://www.stanford.edu/%7Ealinde/) general description.