The first talk of the day was a cosmology/dark matter review. I arrived late, halfway through the talk, and it was explicitly a review so I left this talk alone. We follow this with the first plenary talk of the day.
10:00 am: Cliff Burgess, "Acceleration, then and now"
Subtitle: Inflation and Dark Energy after Planck; apparently "pleading two theoretical cases".
A conversation going on in the arxiv about inflation and cyclic models post-Planck. Speaker seems to support the standard inflationary paradigm...?
Inflation: Occam vs Wilson. That is, simplicity (Occam's razor) vs Wilson (low energy EFT; what is stable and generic?) Many times, to address the objections/problems with inflation, you must understand the UV theory. Sensitivity to initial conditions is sensitivity to UV behaviour; quantum fluctuations mean inflation is on the edge of understanding gravity; very often, inflation involves large fields pushing us into the UV domain.
Note that string models of inflation were predicted before Planck satellite to lie in the regions Planck found. In particular, we are talking about very small tensor ratio r and reasonable ns. In other words, small r is generic in string inflation is what we have found.
Additionally, despite their complexity, string inflation models often (though certainly not exclusively) give small non-Gaussianity, again as observed.
In both cases, people tried to make string inflation models give large r or large non-Gaussianity, and found it difficult to do so. What about sufficiently many e-folds?
Second half of the talk: dark energy. Even ignoring cut-off effects, the contribution of a particle of mass m to the vacuum energy is proportional to m4; already far too large for an electron.
Solution: break the connection between vacuum energy and curvature using extra dimensions, e.g. cosmic strings.
There seem to be solutions involving 4-branes in higher dimensions that give no curvature in the brane; no details given. Unfortunately, these solutions are generically not stable to loop corrections; unless the extra dimensions are supersymmetric.
Wow. A slide containing a big list of objections... and the response is just "I think solutions exist, next slide"! I mean, sure, time and general audience but that's still not the way to convince people.
That's my problem with this talk. Bold claims, but I don't see enough detail to support them. Of course, I'm not a string theorist.