The Fate of the Universe: Expansion, Matter Decay, and the Ultimate End

[Chalnoth]

This is not terribly convincing without even rough numbers. If you presume that matter and anti-matter repel each other, then you have several hundred thousand years for the matter and anti-matter to separate, and you can make the matter/anti-matter annihilation end up as low as you want. That gets rid of the non-thermal spectrum.

I did a quick calculation of gamma ray flux and to make the numbers work, you have to assume a suppression factor of 10^-2 or 10^-3. That's not a crazy number if matter and anti-matter repel.

At that point you'd have much less plasma at the domain walls, but the temperature would have time to thermalize at which point that you'd have a thermal spectrum and no temperature anisotropy.

This doesn't help, because it would still be much dimmer at the domain walls, which I would be willing to bet would be glaringly obvious in the CMB spectrum. Specifically, it would be glaringly obvious in the spectrum of the anisotropies (because instead of differences in temperature causing the anisotropies, differences in density would cause some of them, which would lead to different spectral effects).

Which means that if there is something funny happening at low multipoles, you aren't going to see it.

Right, but domain walls would affect multipoles on many scales, because they are linear features.

Right, but at high multipoles everything goes thermal so Dirac-Milne gives you the same basic spectrum.

Why? The optical thickness of the CMB washes out features at high multipoles overall, but the effect of the domain walls should be visible at all scales relative to the CMB anisotropies (which are also washed out at high multipoles due to this effect).

But the topological defect mechanism as far as I can tell could work for the inflaton. Why do we think the inflaton is a massive particle? It's because we need inflation to happen at a specific time and having a massive particle makes the phase transition happen at the right time. Well, what if you have collections of small particles?

The inflaton is typically modeled as a field, with the quanta of that field being inflatons. I'm not sure a field of solitons makes sense.

Saying that something is unlikely presumes a meta-theory. One problem with meta-theories is that whether something is contrived or not is a matter of taste. One reason Dirac-Milne is interesting is that it seems less contrived than the standard model, but this is a matter of taste, and the problem with aesthetic arguments is that if someone says it "looks contrived" and you disagree, there's no way of easily resolving the argument.

I would be willing to bet that Dirac-Milne simply cannot work on purely empirical grounds, just given our current observations of the CMB, regardless of any arguments regarding simplicity.

As for simplicity, however, there are reasonably good measures of simplicity, such as the number of parameters required to describe the model. If a model requires more parameters to describe it, it sure as heck had better explain a lot more experimental evidence than the competing model, or else it's most likely wrong. Even though it's not possible to prove that this is a good way of doing things, and even though there are sometimes arguments about just how simple or complex various theories are, it seems to be a pretty good heuristic that has worked rather well in the past. And there are some rather rough probabilistic justifications for it that at least seem reasonable.

So we need more data, but then we have to ask what data do we need. It's not a matter of "wait and see" and "wait and see what?"

Yes, it is a matter of wait and see, because it takes an overwhelmingly-compelling theory to push people to base new experiments about it.

 

[twofish-quant]

Why? The optical thickness of the CMB washes out features at high multipoles overall, but the effect of the domain walls should be visible at all scales relative to the CMB anisotropies (which are also washed out at high multipoles due to this effect).

A lot depends on the geometry of the domain walls, and on the processing that people do to get the multipoles. If the thickness of the domain walls are large compares to the features that people care about, then the only thing in the higher order multipoles are going to be harmonics and it's not hard for those to get lost.

One thing is that if some says "yes I've actually put in domain walls" here is what they look like, that would convince me, but I think that the Dirac-Milne have put enough of a case that I don't think that it's valid to dismiss their challenges without some numbers.

The inflaton is typically modeled as a field, with the quanta of that field being inflatons. I'm not sure a field of solitons makes sense.

What if the inflaton is a soliton? The reason you need a high mass particle is so that you get the phase transition at the right time. You can have the inflationary particle be relatively low mass but the phase transition happen because of a soliton.

Also, this is a different argument than Dirac-Milne.

I would be willing to bet that Dirac-Milne simply cannot work on purely empirical grounds, just given our current observations of the CMB, regardless of any arguments regarding simplicity.

It's not that one is willing to bet but how much. I'd be willing to bet US $25K-$50K that Dirac-Milne is wrong. I wouldn't bet my life on it. As far as primordial baryongenesis. I'd be willing to bet several hundred dollars that primordial baryon number is irrelevant, but I wouldn't bet any more than that.

Yes, it is a matter of wait and see, because it takes an overwhelmingly-compelling theory to push people to base new experiments about it.

Or one weird observation. All that has to have happen to have people take Dirac-Milne seriously is to drop some anti-protons and watch them go up.

 

[Chalnoth]

One thing is that if some says "yes I've actually put in domain walls" here is what they look like, that would convince me, but I think that the Dirac-Milne have put enough of a case that I don't think that it's valid to dismiss their challenges without some numbers.

It's up to them to put forward their case, not the rest of us to disprove it. And yes, I really think that the domain walls would produce brightness anisotropies that are much, much larger than the temperature anisotropies we see.

Or one weird observation. All that has to have happen to have people take Dirac-Milne seriously is to drop some anti-protons and watch them go up.

Well, right, and there are groups that are trying. The problem is that it's really, really hard given that gravity is some 40 orders of magnitude weaker than electromagnetism, so that the electric charges of the anti-protons tend to react far more strongly than do their masses.