Inflation theory; dead or alive?

[Peter Watkins]

Not so long ago, any article, book or treatise regarding the history of the universe seemed to take the "inflation" theory as more or less established fact, whilst also admitting that the finer details were not quite figured out. Is this spectacular theory still regarded as being part of our early history, or has it died, or is it dying, a death?

 

[nicksauce]

Well (in my opinion) it is certainly not dead or dying. It solves a number of problems in the standard cosmology that can't be solved in any other way. It is true though that there are still more details to be worked out; there are a number of competing models of inlfation.

A crucial test of inflation that will be done in the next few years is measuring the polarization of the CMB to look for the influence of gravitational waves due to inflation.

 

[Chronos]

Inflation theory remains popular, as Nick noted. No other explanation is satisfactory for now. That may change as observation and theory advance. For now, however, it is well entrenched.

 

[Wallace]

Every new piece of evidence that has emerged over the last decade or more has provided more, not less, evidence for inflation. For instance every new data release from WMAP has shown more and more evidence for inflation. The number types of inflation models allowed have been reduced, for instance WMAP and other data have shown very strong evidence that the spectral index of the inflationary power spectrum is not the most simple value of 1, but is more like 0.97.

In more plain English, a value of 1 for this quantity says that fluctuations on any scale are equally likely (when expressed in a suitable volume normalised way). Any value away from this indicates some scale dependence to the fluctuations. This is important because inflationary models make firm predictions about this, so this has meant some popular models have been ruled out. Others that do predict scale dependence remain. Basically we not only have strong evidence for inflation, but we also are starting to learn more information about the underlying physics that must have driven it.

 

[matt.o]

Remind me, Wallace, does a power spectrum with index 0.97 place more power at larger or smaller scales? The reason I ask is because of the recent paper by Jee et al. on the galaxy cluster XMMU J2235.3-2557 at z=1.4, which is apparently very massive -- perhaps too massive for vanilla LCDM at that redshift? Does a lower spectral index allow more leeway for these massive clusters to form earlier?

 

[Wallace]

The primordial power spectrum is written as

[tex] P(k) = k^n [/tex]

where n is the spectral index we are talking about. So less than unity indicates less fluctuations at high k, which corresponds to small scales. So the 0.97 tells you the tilt is against small scale power in favour of larger scales.

But, there is a big caveat. This is is primordial power spectrum, which we don't access directly but have to infer from what we observe in the CMB, which involves taking into account all the details of the plasma physics that goes on before re-combination. The spectrum that really matters for structure formation is the post re-combination one, which you can tweak in a variety of ways since you observe it via the angular power spectrum of the CMB and need to convert to a physical one via the cosmologically dependant angular diameter distance.

Shifting the primordial index alone would put you away from WMAP constraints, although there is probably a degeneracy plane or two that might allow you to move it a reasonable way as long as you change other parameters and not be too far from the WMAP allowed region.

Assuming the LCDM angular distances are not too bad, the CMB constrains the fluctuations on scales relevant to cluster formation pretty well independant of the value of n, so I don't think tweaking this gets you out of any glaring model/observation mismatch.

Cluster mass measurements are pretty tricky though right? I had a quick look at the paper and it's certainly an interesting result, but I could think of plenty of ways to wriggle out of it. A z=1.4 is not going to be a nice, relaxed, spherical blob without sub-structure, so getting any kind of mass would be a nighmare, so I'm not sure if I'd trust their ~20% error on that. Still, it gives hope for dark energy, since there's a bazillion smalls tweaks that you can make to amplify the number of high z clusters in DE models compared to LCDM.

 

[matt.o]

The primordial power spectrum is written as

[tex] P(k) = k^n [/tex]

where n is the spectral index we are talking about. So less than unity indicates less fluctuations at high k, which corresponds to small scales. So the 0.97 tells you the tilt is against small scale power in favour of larger scales.

But, there is a big caveat. This is is primordial power spectrum, which we don't access directly but have to infer from what we observe in the CMB, which involves taking into account all the details of the plasma physics that goes on before re-combination. The spectrum that really matters for structure formation is the post re-combination one, which you can tweak in a variety of ways since you observe it via the angular power spectrum of the CMB and need to convert to a physical one via the cosmologically dependant angular diameter distance.

Shifting the primordial index alone would put you away from WMAP constraints, although there is probably a degeneracy plane or two that might allow you to move it a reasonable way as long as you change other parameters and not be too far from the WMAP allowed region.

Assuming the LCDM angular distances are not too bad, the CMB constrains the fluctuations on scales relevant to cluster formation pretty well independant of the value of n, so I don't think tweaking this gets you out of any glaring model/observation mismatch.

I need to brush up on this stuff. Might be time to dust off Peebles or Peacock!

Cluster mass measurements are pretty tricky though right? I had a quick look at the paper and it's certainly an interesting result, but I could think of plenty of ways to wriggle out of it. A z=1.4 is not going to be a nice, relaxed, spherical blob without sub-structure, so getting any kind of mass would be a nighmare, so I'm not sure if I'd trust their ~20% error on that. Still, it gives hope for dark energy, since there's a bazillion smalls tweaks that you can make to amplify the number of high z clusters in DE models compared to LCDM.

Yes, my initial reaction was to question the mass measurement and the assumptions inherent in those measurements. But the different methods (lensing, X-ray temperature and velocity dispersion) all seem to add up to give fairly consistent results. I would believe that value to a factor of 2. The surprising thing is that this appears fairly relaxed at X-ray wavelengths with a cool core, although there is some elongation, and it even has the canonical one-third solar metal abundance. It appears to be in a fairly evolved state with a bright central massive galaxy and old, passive galaxies in the central regions with evidence for more recent star formation activity in galaxies at larger radii.

That said, I've found it does take a lot of work to find evidence for even major mergers in clusters, so there could be more to the story as the data piles up!

 

[Wallace]

Yeah, I think it's certainly a case of wait and see, but an interesting find none the less. These kind of '1%' results are tricky, it can only take a relatively small shift in some qauntity for the stats to suddenly look a lot less convincing. If we found ten objects like this, with pretty good mass measurments, that would start to be a lot more interesting.

 

[matt.o]

Yeah, I think it's certainly a case of wait and see, but an interesting find none the less. These kind of '1%' results are tricky, it can only take a relatively small shift in some qauntity for the stats to suddenly look a lot less convincing. If we found ten objects like this, with pretty good mass measurments, that would start to be a lot more interesting.

I totally agree. More telescope time for clusters!

 

[Skolon]

About inflation theory I found not so long ago an interesting new theory of http://gravity.psu.edu/people/Ashtekar/" (Director, Institute for Gravitational Physics and Geometry The Pennsylvania State University) and others called Big Bounce Theory.
This theory is based on LQC (Loop Quantum Cosmology - an extension of Loop Quantum Mechanics ideas to cosmology) and its basic idea is that the Big Bang does not start with a singularity, the (our) Universe is just an expanding part of a cyclic Universe, before Big Bang an contracting Universe exists (a final stage of an Universe like ours).
The contracting stage stopped when a critical density occurred (so-called Planck density) and a expanding stage begin. The inflation is just the result of this big repulsion created by Planck density limit.

For a brief describe of this theory you can look here.
It looks like this theory is a good one because has a strong mathematical description and fit with all observations.

Did you hear about it? It is so good as it seems?

 

[twofish-quant]

It looks like this theory is a good one because has a strong mathematical description and fit with all observations.

The theory is interesting since it may make some predictions that you can observe. Theories that fit with all observations are useless, you want theories that *could* not fit with observations, but happen to do so.

One good/bad thing about inflation is that it tends to wipe out the conditions that happen before the inflation happened. It's sort of the cosmic equivalent of destroying evidence by sticking it into a hydrogen bomb. It's good since it means that the predictions of the theory don't depend much on the details. It's bad because it means that you can't look at observations and piece together exactly what happened before inflation.

Now the big crunch seems to predict that there is superinflation before inflation, but it's going to be tough to come up with a "smoking gun". Even if you do see evidence of superinflation, it's going to be tough to say that it's because of loop quantum gravity.

Did you hear about it? It is so good as it seems?

It's "useful". They are trying to come up with experimentally testable predictions.

 

[justwondering]

Not so long ago, any article, book or treatise regarding the history of the universe seemed to take the "inflation" theory as more or less established fact, whilst also admitting that the finer details were not quite figured out. Is this spectacular theory still regarded as being part of our early history, or has it died, or is it dying, a death?

Inflation theory seems to be more of a fudge-factor useful Supernatural event than a Natural, and plausible, one. i.e. If the Model has a 'gap' or two, fudge it.

 

[twofish-quant]

Inflation theory seems to be more of a fudge-factor useful Supernatural event than a Natural, and plausible, one. i.e. If the Model has a 'gap' or two, fudge it.

It is a fudge factor, but the important thing is that it is *one* fudge factor that explains about four or five things that people have no idea how to explain otherwise. Better *one* big fudge factor than ten little ones.

One of my teachers had what was called the "tooth fairy rule" which is that with any theoretical paper, you can invoke the tooth fairy once.

If you have any better ideas, then let's hear them...

 

[Wallace]

Once upon a time, the prevailing wisdom in Western philosophy was that all truths must be obtainable purely via deductive reasoning as the senses could be decided. From this belief the foundations of geometry and logic theory were developed. But it proved to be a flawed process when trying to do physics, you simply couldn't work things out logically, you had to observe the world and the attempts to work out physics deductively lead to some very strange misconeptions that lasted for more than a thousand years.

Eventually, a new breed of natural philosophers came along and decided that empiricism and inductive reasoning was fine, and in fact preferable to using logic alone. There was a great deal of mistrust of the new wave and some prominent figures were persecuted by the establishment for their views. In the end though, the utility of empiricism won the day, it simply worked.

Why am I banging on about philosophy? Because empiricism is nothing but adding in successive 'fudge factors' as you go, based on what you observe. There is no requirement to understand these things you introduce, it is a matter of smply explaining what you observe. The most famous fudge factor is the theory of gravity, which explained why Keplers laws hold. We are no closer to knowing why there should be a law of gravity than Newton was, we just have a much better idea of what fudge is needed to get our theory to match what we observe.

I'm constantly amazed at people who think the modification of a theory due to new data is some kind of dodgy illegitimate practice. I guess to them, the Renaissance is just something that happened to other people?