Questions on the Inflation Debate

[thenewmans]

The commonly accepted age of the universe is 13.7 billion years. I assume that assumes an inflationary epoch. Since inflation is in debate, what’s the age without it? And since expansion is accelerating, I assume expansion was slower 10 (or 20) billion years ago. Could it be infinitely old? (IE no bang?) Maybe Ned Wright’s calculator needs an inflation switch.

Another odd thing I notice is that discussions of inflation usually include an after size. Steinhardt’s Inflation Debate says the size after is a dime. Wikipedia says 100 million light years. Big difference. It seems odd to me to say a size since inflation requires a “flat shape”. Doesn’t that mean there’s no edge?

The Inflation Debate
http://www.physics.princeton.edu/~steinh/0411036.pdf

 

[eloheim]

I'm actually in the middle of looking for a *different* Sean Carroll blog post in response to your question, but I found the lackluster endorsement of inflation (generally) in this post a bit surprising: Do You Think Inflation Probably Happened?

What is the likelihood you would attach to the idea that some form of cosmic inflation occurred in the early universe?

My answer was 75%, which I thought was generous. It’s very hard to give a high probability to a speculative theory about what happened at energy scales to which we currently have no experimental access. But I found myself on the low end of opinions at the meeting, where the median was about 90% confidence. Of course, these are people who work on inflation professionally, and have chosen to do so. When I came home to ask the same question of my lunch crowd at Caltech, the answers were more like 25%.

 

[eloheim]

This is the post i was looking for: The Eternally Existing, Self-Reproducing, Frequently Puzzling Inflationary Universe

I know this is slightly tangential to the OP but the points Sean Carroll mentions against inflation (I also recall a similar feature article in Scientific American a few years ago) came off to me as fairly potent. The gist of it seemed to be, "Well the reasons for the whole idea (of inflation), in the first place, basically suck...but thinking of (and dealing with) other models would require a lot of work, so...you know...whatever." Here's some meat:

Note that the entire point of inflation is to make the initial conditions of our observable universe seem more “natural.” Inflation is a process, not a law of nature. If you don’t care about naturalness, and are willing to say “things just happened that way,” there is absolutely no reason to ever think about inflation. So the success or failure of inflation as a scenario depends on how natural it really is.

This raises a problem, as Roger Penrose has been arguing for years, with people like me occasionally backing him up. Although inflation does seem to create a universe like ours, it needs to start in a very particular kind of state. If the laws of physics are “unitary” (reversible, preserving information over time), then the number of states that would begin to inflate is actually much smaller than the number of states that just look like the hot Big Bang in the first place. So inflation seems to replace a fine-tuning of initial conditions with an even greater fine-tuning.

One possible response to this is to admit that inflation by itself is not the final answer, and we need a theory of why inflation started. Here, it is crucial to note that in conventional non-inflationary cosmology, our current observable universe was about a centimeter across at the Planck time. That’s a huge size by particle physics standards. In inflation, by contrast, the whole universe could have fit into a Planck volume, 10-33 centimeters across, much tinier indeed. So for some people (like me), the benefit of inflation isn’t that it’s more “natural,” it’s that it presents an easier target for a true theory of initial conditions, even if we don’t have such a theory yet.

Certainly doesn't sound reassuring...

 

[Chronos]

One thing is relatively clear, whatever happened in the very early universe must have looked a lot like inflation. Inflation flounders when you attempt to define initial conditions, but, so does virtually every other model. The Planck mission should shed some light on matters, but, will not necessarily be convincing evidence either way. LIGO, or some similar gravitational wave detector, is probably our best hope for the immediate future. The gravity wave signature of inflation would be uniquely different than just about any other option.

 

[eloheim]

LIGO, or some similar gravitational wave detector, is probably our best hope for the immediate future. The gravity wave signature of inflation would be uniquely different than just about any other option.

That's been my understanding as well.

 

[skydivephil]

One thing is relatively clear, whatever happened in the very early universe must have looked a lot like inflation. Inflation flounders when you attempt to define initial conditions, but, so does virtually every other model. The Planck mission should shed some light on matters, but, will not necessarily be convincing evidence either way. LIGO, or some similar gravitational wave detector, is probably our best hope for the immediate future. The gravity wave signature of inflation would be uniquely different than just about any other option.

My understanding is that LIGO has virtualy no chance of detecting primordial gravaitational waves and so will not make any ruling on inflation either way.
To answer the original question the age of the observable unvierse is the same with or without inflation (it was only supposed to last a tiny fraction of a second anyway).
Its also my understanding that Planck has an outiside chance of detecting the B mode polarisation but its not designed for that so we'd have to get pretty lucky for this.
What we really need is dedicated B mode polarisation such as CORE or EPIC. A space based gravitational wave observatory suchg as LISA is porbably still not going to be powerful enough to detect primordial gravitational waves. But a next gen mission such as the BIg Bang Observer hopefully will be. I think they are looking for a 1000 times more sensitivbity than LISa and don't forget LISa is currently not funded, so this is likely a long way off, read more on BBO here:
http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/37836/1/05-2157.pdf

 

[bapowell]

Yes, to further emphasize Phil's important point: 13.7 billion years is the age of the observable universe, i.e. about the age of the CMB. Since the CMB is created after inflation, one can effectively think of inflation as preceding the big bang. After inflation ends, the universe goes through nucleosynthesis, decoupling, etc just as in the standard big bang model without inflation.

 

[Lino]

Forgive me if I've got this wrong but, I thought that the CMB was 380k years (or 780k years - I can't remember which number is right ) younger than the universe? Are you just saying that as a proportion of its life this 380k years is insignificant?

Regards,

Noel.

 

[skydivephil]

380k years is the right number. That's the amount of time that's estimated to elapse from the beginning of the expansion of the universe to the release fo the CMB.
The CMb is the earliest thing we can see with light. Hence Bapowell's point that the observable universe begins with the CMB and hence inflation happened before the big bang.
However i think some clarification is helpful. People may mean different things when they say big bang.
1 the beginning of the observable universe - that's the CMB
2 the beginning of the expansion of space time, that's estimated to be 380k years before the CMb was emitted.
3 the beginning of all space and time - this is a prediction from general relativity that I think is less and less believed. Most would argue that both quantum gravity theories such AS LQC and other approaches such as eternal inflation imply there was no such ultimate beginning 13.7 bio years.
Perhap[s there is some ultimate beginning in these models but even if there is they wil be way beyond any conceivable observation we could makee.

One important point to note there are two potential ways to observe before the CMB was emitted.
1 Observe the cosmic neutrino background
2 observe the gravitational wave background.
Both of these i think its a safe bet to say are far into the future. Although if PLanck is very lucky it might see a sign in the CMB of an imprint of the gravitational wave in the form of the way the CMB is polarised.
This a good article to read from NAture magazine which also discusses the inflation debate:
http://www.nature.com/news/2009/090415/full/458820a.html

 

[Lino]

Thanks Phil (for the clarification and the article).

Regards,

Noel.