# Speed of light during inflation

1. Jun 13, 2013

### Gerinski

It is often said that although the expansion of the universe may create the illusion of objects in very far regions of the universe to recede from each other at superluminal speed, that does not violate c because locally they never move faster than c. This I can understand.

How would things look like during the inflationary period is more confusing to me though. The rate at which space expanded was huge even in a local context. Without caring to do any math, the space separation which at a certain moment was, say 1,000 km, after a few seconds would have become hundreds of thousands or billions of kilometers, as 'new kilometers got pumped in between'. Even locally, if you tried to measure the speed at which objects receded from each other (ok there were no 'objects' yet but go along with me), you would get apparent superluminal speeds, wouldn't you? (or more precisely, you would not be able to measure it because all the neighbour 'objects' would just disappear from sight with the space between you both stretching faster than the distance the emitted photon travelling at c towards you could travel). Although this would not really violate c either, the effect if it could have been measured would be that even locally things appeared to be getting apart from each other 'faster than c'. Or not?

Thanks!

2. Jun 13, 2013

### phinds

Yes, things would then, as now, have been receding from each other faster than c, it's just that as you point out it would have been happening at smaller scales. Right now things at the edge of our observable universe are receding from us at about 3c but they are about 47BILLION light years apart.

During inflation, things very close together would have been receding from each other at that speed.

Still, as you say, no speeding tickets would be issued in either case.

3. Jun 13, 2013

### Gerinski

Thanks,

So this means that (in the hypothetical case that the universe was transparent and photons could have traveled freely at c, and that there could have been any observer) any observer during the inflationary period would have just seen complete blackness around him, and effectively would have perceived that his own point was the complete existing universe, right?

4. Jun 13, 2013

### Mordred

not necessarily we do not know if the beginning of the universe was finite or infinite. The observable portion in your example would be finite. However that does not necessarily mean the entirety of the universe

5. Jun 13, 2013

### Gerinski

I mean, 'he' would not have been able to observe anything at all, so from his point of view there would be nothing else than his own point (even if with hindsight we know that there were other 'things', just that he could not observe them as they were becoming causally unrelated).

'we do not know if the beginning of the universe was finite or infinite'

What do you mean? we know that (shortly after the BB) it was spatially finite, don't we? anything that grows in size is by definition spatially finite at any given time (a different thing being that it may be infinite in time forwards, i.e. it may expand forever).

6. Jun 13, 2013

### bapowell

No, he would see things that were within his causal horizon (which includes more than just his point, but all points within a radius $\propto H^{-1}$, where H is the Hubble parameter). Photons emitted from sources within his horizon would be visible to him as long as the source remained inside his horizon. This is possible as along as the source is not "comoving" with the expansion and moving at a speed sufficient to overcome the recession velocity imparted to it by the expansion.

7. Jun 13, 2013

### Gerinski

Well, in a quick google search I could not find a precise figure for the rate of space expansion during inflation in order to calculate which sphere radius would have remained within his causal horizon but I guess that it would have been limited to a really tiny 'bubble' of space around 'him'.

The only quote I could quickly find is this (rather simplistic and possibly unprofessional one) which states that 'space itself expanded faster than the speed of light'

http://www.space.com/52-the-expanding-universe-from-the-big-bang-to-today.html

8. Jun 13, 2013

### bapowell

Yeah, that's a common and frustrating misinterpretation of inflation. It's true -- space does expand at a rate during inflation such that objects at a proper distance $\propto H^{-1}$ have superluminal recession velocities. But that's true of any expansion -- accelerated or otherwise! The key difference is that the Hubble radius expands faster than the expansion in standard, non-accelerated cosmologies, so that objects that might be receding superluminally today will not be in the future. During inflation, in contrast, the space expands more quickly than the Hubble radius, and so once objects are outside, they don't return. In the limit that the inflation is driven by a pure cosmological constant (so-called de Sitter expansion), the Hubble radius is coincident with the event horizon of the spacetime.

9. Jun 13, 2013

### Gerinski

Thanks, so, if I understand this well, it is true that during inflation space even in your close vicinity expanded at such a rate that any 'objects' in it would recede from you at superluminal velocity, right?
Is there any calculation of what would have been the maximum radius around you which remained within your causal horizon during the fastest inflation period?

10. Jun 13, 2013

### Mordred

during the inflationary epoch the universe was too hot to have matter, so your left with energy/radiation. Matter formed later so you wouldn't have objects as per se. Unfortunately one could calculate the rate of inflation, however their is no agreement on which inflationary paradigm is correct. Each inflationary model has differences in the number of e-foldings some may coincide but thats more coincidence than design. Some models say 60 e-folds other models say higher or lower.

One of the reasons we do not know which is correct is that we cannot observe this period of time, we simply cannot see that far back due to the lack of transparency of the plasma state. So any values we have are based on calculations of what we do know. Related calculations is the number of e-folds to solve the flatness problem and the horizon problem.

Last edited: Jun 13, 2013
11. Jun 13, 2013

### bapowell

No, only objects outside the event horizon, approximately $d \sim H^{-1}$ away, must move superluminally. You can figure out exactly what this distance is if you know the energy scale of inflation, which has an upper limit around $H^2 = 10^{16}$ GeV or so and a lower limit set by about the scale of nucleosynthesis.

12. Jun 13, 2013

### bapowell

This is the amount of inflation -- not the rate. The rate is set by the energy scale, which is also not known. But, it's possible to get a sense of the size of the horizon by calculating what it should be for reasonable energy scales.

13. Jun 13, 2013

### Mordred

good point not sure why I stated rate wasn't thinking clearly.

14. Jun 13, 2013

### phinds

My understanding is that the plasma was still dense enough during inflation that any hypothetical observer would have had his hypothetical *** fried off. It wasn't until 400,000 year AFTER inflation that the plasma cooled enough to allow light to disperse instead of constantly banging into things. Google "surface of last scattering"

15. Jun 13, 2013

### bapowell

Inflation is essentially a supercooled phase transition -- the temperature is that of the vacuum during inflation.

Last edited: Jun 13, 2013
16. Jun 13, 2013

### phinds

Oh. Boy, did I have THAT wrong. Thanks.

17. Jun 13, 2013

### Gerinski

Well, that's why I said 'hypothetical', meaning 'as a thought exercise even if historically not applicable'. This does not invalidate the question as a matter of principle. The question refers to the expansion of spacetime during the inflationary period, irrelevant of what was occupying it at that particular moment.

18. Jun 13, 2013

### marcus

I glanced at a paper of Liddle where he was estimating the energy scale of inflation and saw and estimate of H ≈ 10-6 mplanck

The basic unit of H is frequency (i.e. reciprocal time) so it makes sense to state H in either energy terms or mass terms. I think Liddle's figure was not an upper limit, but rather was what he judged sufficient to get 60 efolds.

Since Planck mass is about 1019 GeV, that would make Liddle's estimate around
H ≈ 1013 GeV

It would be nice to have a source for that upper limit figure that you give, if there's an arxiv link and it's not too hard to understand. As I recall you've published some about early universe/inflation yourself, but it would save fumbling around to have a link. The Liddle paper I looked at is over ten years old.

19. Jun 13, 2013

### Mordred

this paper provides one estimate

http://arxiv.org/pdf/1008.5258v3.pdf "Observational constraints on the energy scale of inflation"

In this paper we have placed observational constrains
on the potential energy scale, the first and second deriva-
tive of the potential by using the 7-year WMAP data,
combined with the latest distance measurements from
the baryon acoustic oscillations in the distribution of
galaxies and measurement of the present-day Hubble con-
stant from supernova data. A previous upper limit from
the first WMAP data release, combined with large scale
structure data from the 2dF galaxy redshift survey found
V01/4<∼2.7 × 1016 GeV at 90% C.L. [7, 8]. Our new up-
per limit on the energy scale of inflation is only slightly
stronger V0 1/4<∼2.3 × 1016 GeV at 95% C.L., and shows
a degeneracy with the upper limit on the first derivative of the inflaton potential,
V 131<∼2.7 × 1015 GeV at 95%

this article covers this as well but is not easy to find the info nor understand partly due to its extreme length.
http://arxiv.org/abs/1303.3787 "Encyclopædia Inflationaris" it utilizes the slow roll approximation to compare alternate inflationary models and either contraints them or invalidates (in rare cases).
The article states that over 64 models are still viable.

Last edited: Jun 13, 2013
20. Jun 13, 2013

### Mordred

Found this paper covering constraints on inflation according to the latest Planck results

http://arxiv.org/abs/1303.5082

in the conclusions it places the upper energy-scale at 1.9*1016 Gev Planck+WMAP

Last edited: Jun 13, 2013