What is the difference between inflation and expansion?

[andrewkirk]

Are inflation and expansion different phenomena and if so what is different about them.
Both terms seem to refer to a process of enlargement of the spacetime manifold that increases the distance between objects in a way that is somehow different from "ordinary" relative motion and which is not limited by the speed of light.

The term inflation seems to be used only in reference to the very early universe whereas expansion is believed by many to still be occurring now, possibly even at an increasing rate.

In other threads I have seen statements that certain phenomena are caused by expansion but not by inflation, which implies that they are phenomena that differ in a way that can be empirically observed.

What is the difference between the two?

 

[marcus]

Inflation was a hypothetical very brief episode of highly accelerated expansion. We don't know if it occurred.
Such an episode is not predicted by the standard GR math that cosmology is based on, unless you assume the existence of an exotic kind of energy field that has never been observed. It has to have just the right strength and decay very quickly at just the right rate. However if there was such a thing,and if this brief abrupt highly accelerated expansion did occur, so that it increased distances by a factor of at least e⁶⁰ (socalled "60 efolds") then that would be a possible explanation for some feature of the universe.

I want to emphasize that ordinary routine expansion is a different proposition. It is a common feature of lots of solutions to the basic equations. Once it gets started somehow (various conjectures for how it might get started) it tends to persist in a well-understood way.

We observe expansion. It is not hypothetical. it doesn't require anything exotic.

Ordinary expansion can also involve a very gradual acceleration if there is a positive constant Lambda in the basic gravity equation. Again nothing exotic. It just looks like in fact there is this positive term. But the effect is very mild in any case.

 

[bcrowell]

Inflation causes growth at an exponential rate and is due to a hypothetical scalar field that has never been directly observed.

Expansion is basically just the general-relativistic version of inertia and requires no new physics.

 

[turkeyburgers]

If inflation did occur then perhaps the distance and speed of inflation was determined by the length of the Photon Epoch. If nearly all of the energy in the Universe during the early stages of the Big Bang was tied up in Photons (exerting no gravity due to not having a mass) then maybe the speed of the current expansion rate (if the vacuum/spacetime/whatever is currently increasing the rate of expansion) is determined by the amount of energy that is currently tied up in Photons.

The more that Stars create photons, i.e. convert mass with gravity into non-mass without gravity, the less gravitation there is in the universe as a whole to hold back the expansion rate.

 

[andrewkirk]

But turkeyburgers if I understand the General Relativity field equations correctly photons generate just the same gravity as the equivalent amount of mass. I don't think the stress-energy tensor distinguishes between mass and energy.

 

[Tanelorn]

Very interesting idea that mass is being converted into light reducing gravitational attraction over time. Dont know if this light has same amount of mass though, in fact I thought it had zero mass at rest. There is also the force of light pressure, but again don't know the magnitude of the effect, but likely very low.

 

[marcus]

But turkeyburgers if I understand the General Relativity field equations correctly photons generate just the same gravity as the equivalent amount of mass. I don't think the stress-energy tensor distinguishes between mass and energy.

Andrew, i think you are right. That other stuff is based on misunderstanding. You started the thread and it is about questions you asked, so I would like to focus on that.

You got some answers to your starter post:

Are inflation and expansion different phenomena and if so what is different about them.
Both terms seem to refer to a process of enlargement of the spacetime manifold that increases the distance between objects in a way that is somehow different from "ordinary" relative motion and which is not limited by the speed of light.

The term inflation seems to be used only in reference to the very early universe whereas expansion is believed by many to still be occurring now, possibly even at an increasing rate.

In other threads I have seen statements that certain phenomena are caused by expansion but not by inflation, which implies that they are phenomena that differ in a way that can be empirically observed.

What is the difference between the two?

Does the response so far suffice? Do you have followup questions?

 

[marcus]

It seems to me you might have followup questions--challenging ones in fact. But maybe the best thing is to wait and not try to guess what you might want more discussion of.

There's this however:

In other threads I have seen statements that certain phenomena are caused by expansion but not by inflation, which implies that they are phenomena that differ in a way that can be empirically observed.

I wonder what threads and/or what those "certain phenomena" could be?

I think of inflation (in cosmology) as referring to a hypothetical EPISODE. It is not an "empirically observed" process.

The word (in cosmology) is only used to refer to a scenario which is imagined to have played out right at the start of expansion. If it happened, then it happened by some exotic mechanism that we can only conjecture about.

So comparing inflation to the routine observed expansion is like comparing apples to oranges. One thing is an hypothetical scenario, not observed but imagined---the other is an observed feature of the universe that is consistent with well-tested physical law (GR) and has been observed and studied quite a lot for 80 years.

But I can't think what you mean by "certain phenomena" that are supposed to occur with routine expansion and not with the hypothetical (brief exponentially rapid) expansion! Or vice versa.

Can you recall something more specific about what was said in those other threads?

The contents of the U are supposed to be different. The percentage rate of increase is supposed to be much larger in one case than in the other. One is supposed to have self-terminated after an amazingly brief interval of time. There is no indication the other will ever stop. But at some basic level, expansion is expansion and there should be no basic difference---I think of it as more differences of degree rather than kind. I can't guess what the phenomena were that you mean. Can you remind us?

BTW, I find myself constantly urging people to get used to thinking in terms of the measured redshift that light from distant objects has undergone, and how it is interpreted in terms of distance (and travel time).

Please have a look at what you get by googling "cosmos calculator". You can put in various numbers for z (the redshift) and get distances out. Have you tried doing this? It might inspire some useful questions.
http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html

 

[andrewkirk]

Thank you very much Marcus and bcrowell for your explanations. Your answers do more than suffice, they have been very clear. I think I understand now, that inflation is an instance of a particular type of expansion that:

1. is hypothesised as an explanation for various observations but not universally agreed to have occurred
2. only occurred in the very early universe
3. was much more rapid than the expansion that is occurring now
4. cannot be predicted by existing accepted physical laws, requiring additional hypotheses

I think I'll leave inflation aside while I try and get my head around common-or-garden expansion. I've read the Scientific American article by Lineweaver and Davis and now I'm going to try to understand some of the maths that underlies it. I made a little baby mathematical model to try to understand how light from objects that are receding faster than the speed of light can still reach us (which the above article explains in a clear but non-mathematical way), and somebody very kindly solved the differential equation for me so I'll play with that. That's in https://www.physicsforums.com/showthread.php?p=3237454#post3237454".

Re the 'certain phenomena' to which I referred in the OP, the thread that refers to expansion but not inflation being a cause for something is
https://www.physicsforums.com/showthread.php?t=483578". The original question in that thread is how the universe can be only 14bn years old if we can see galaxies that were 13bn light years away 13bn years ago, which naively implies the universe must be at least 26bn years old (given it started out with everything very close to each other). Some posters (drakkith and Bill_k) explained it in terms of inflation and/or expansion but then others (nabeshin and Arbitrageur) said inflation has nothing to do with it. I think what the latter are saying is that expansion occurring since the light left the observed galaxy 13bn years ago is sufficient to resolve the apparent conundrum, with no need to assume inflation had also occurred earlier.

I expect I will have more questions, but for now I will try to digest what I've already been told.
Thanks again for your help.