I’m at Current Themes in High Energy Physics and Cosmology this week, the yearly conference of the Niels Bohr International Academy. (I talked about their trademark eclectic mix of topics last year.)
This year, the “current theme” was broadly gravitational (though with plenty of exceptions!).
For example, almost getting kicked out of the Botanical Garden
There were talks on phenomena we observe gravitationally, like dark matter. There were talks on calculating amplitudes in gravity theories, both classical and quantum. There were talks about black holes, and the overall shape of the universe. Subir Sarkar talked about his suspicion that the expansion of the universe isn’t actually accelerating, and while I still think the news coverage of it was overblown I sympathize a bit more with his point. He’s got a fairly specific worry, that we’re in a region that’s moving unusually with respect to the surrounding universe, that hasn’t really been investigated in much detail before. I don’t think he’s found anything definitive yet, but it will be interesting as more data accumulates to see what happens.
Of course, current themes can’t stick to just one theme, so there were non-gravitational talks as well. Nima Arkani-Hamed’s talk covered some results he’s talked about in the past, a geometric picture for constraining various theories, but with an interesting new development: while most of the constraints he found restrict things to be positive, one type of constraint he investigated allowed for a very small negative region, around thirty orders of magnitude smaller than the positive part. The extremely small size of the negative region was the most surprising part of the story, as it’s quite hard to get that kind of extremely small scale out of the math we typically invoke in physics (a similar sense of surprise motivates the idea of “naturalness” in particle physics).
There were other interesting talks, which I might talk about later. They should have slides up online soon in case any of you want to have a look.
4 gravitons 
The current theme might be broadly gravitational, but does anybody there know how gravity works? I imagine not.
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Thanks for the pointer to Sarkar et al’s arXiv:1808.04597.
Curiously, there are several independent groups that are and have been arriving at the same kind of conclusions, namely that careful analysis reveals that it is at least open whether the actual inhomogeneity/un-isotropy of the observable universe may cause at least some fraction of the effect that appers as apparent dark energy/cosmological constant when seen through the lens of the standard FRW model.
I have been collecting pointers to this here.
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Urs, there’s an “edit” option on that page. It looked like I could change it.
A gravitational field is a place where space is neither homogeneous nor isotropic. See http://iopscience.iop.org/article/10.1088/0256-307X/25/5/014/meta, and note that the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy. Inhomogeneous space is associated with dark matter as opposed to dark energy.
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Your last sentence doesn’t follow from any of the rest of the paragraph. Regardless, the point Sarkar is making is in some sense quite straightforward: if we measure that the universe is accelerating around us, it could be that the whole universe is accelerating…or just that our portion of the universe is. At that point, it’s a question of how good the data is, and how well data from different scales match up.
If you think that you have justification for the statement “Inhomogeneous space is associated with dark matter as opposed to dark energy” then I encourage you to either:
a) Read up on the FRW metric, and the assumptions that go into it, if you have the relevant background,
b) If you don’t have the relevant background, phrase statements about things you’re not confident you understand in terms of questions rather than certainties, or
c) If you’re basing your reasoning on a non-mainstream (read crackpot) understanding of gravity, then don’t try to hide that. Trying to sneak in crackpot content when it’s not relevant, or mislead people as to the content, is a good way to get your posts classified as spam.
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I’m satisfied that the universe is accelerating, we’ve had evidence of that for over a hundred years. See Cormac O’Raifeartaigh’s paper on the contribution of VM Slipher to the discovery of the expanding universe. As for justifying “inhomogeneous space is associated with dark matter as opposed to dark energy”, those are Einstein quotes above. It was Einstein who described a gravitational field as space that was neither homogeneous nor isotropic. It was Einstein who said the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy. That’s a non-uniform spatial energy causing gravity, and it isn’t made of WIMPs. If you think this sort of thing is crackpot, I’m afraid you’ve got a bit of a shock coming about the GR you’ve been taught. Sorry.
I know all about the FRW metric thanks. Note that Alexander Friedmann’s 1922 paper was on the curvature of space. And that space isn’t curved where a gravitational field is. Instead it’s inhomogeneous in a non-linear fashion. Google on inhomogeneous vacuum.
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