We come finally to the last session of Planck. The downside to a week-long conference is that I tend to suffer from burnout, so I skipped the first talk as it was another string theory inflation talk. I return for the last two, which do look a bit more interesting.
5:00 pm: Transplanckian scattering: Where do we stand?, Gabriele Veneziano
Collisions of strings with strings and strings with branes. Possibility of leading to semi-classical black holes at energies above the Planck scale. Different domains: weak gravity (IR), string gravity (UV) and strong gravity (both?).
String-string collisions at TPE: simple initial state that could lead to BH. Can we follow unitary evolution to see that state arise? Work on this started in 1987!!
String-brane collisions: brane dominates geometry, so string modifies things. Thus an easier question.
I have to say, despite the highly technical nature of this talk, the presentation is very clear. Also, the slides are a good example of not being cluttered with stuff.
Three length scales, two ratios so phase diagram is a plane. Weak gravity: High impact parameter; String gravity: string size dominant; Strong gravity: Large Schwarzschild radius.
Weak gravity: generalisation of Einstein's deflection formula for ultra-relativistic collisions, emergent metric is generated by relativistic point-particle. Large energies probe larger impact parameters for fixed impact parameter, i.e. the IR. Relation to Dvali's classicalisation? Total momentum transfer shared among large number of gravitons.
As impact parameter increases, string feels tidal forces due to finite size, string is excited. Compensate by shifting impact parameter; unitary operator eikonal.
String gravity: duality, similar behaviour in s-channel to what we saw in the t-channel above. Some relation to Hawking radiation? If kinematic distributions can be extrapolated to small energies.
5:30 pm: Dark Matter from the Multiverse: SUSY and axions, Lawrence Hall
Higgs (and nothing else) + accelerating Universe both were surprises in the last few decades, and both can be explained with the multiverse.
Our field could have turned out differently. Data tells us that nature is fine-tuned.
Here: an alternative explanation for Λ and v. Cosmological constant explained by causal patch: not by structure formation, but by number of observers. (More or less by construction, forces us to exist around time CC begins to dominate.)
As for quark masses, both up and down quark masses seem fine-tuned; points to tuning of Yukawas, rather than perhaps v. Related to boundary between hydrogen-dominated and Helium-dominated universe?
Dark matter? Silk damping of Baryonic matter makes it hard to generate structure just with DM. Is there a probability pressure pushing to low DM density? Note: Weinberg used same argument, factor of 1000 or so; DM goes like four powers, so only factor of 3 to 5. Also need pressure pushing towards small baryonic densities. Push to zero baryon density offset by need to have observers.
SUSY: might explain lack of observation due to pressures on structure, e.g. Higgsinos need to be TeV for relic density and everything else is heavier.
Higgs potential: near instability, again might be origin of critical behaviour. What is probability force pushing against this boundary? Is it axion exchange?
Question: what about e.g. second and third generations? Well, this work is only just scraping the surface of these ideas. Might reflect underlying probability distributions, i.e. no anthropic pressure.
Question: what can conclusively kill this scenario? Discover BSM physics. Falsifiability is a worry. But compare 30 years ago, what people said about inflation. The way to think is not to give up thinking about these ideas now. As we work on these ideas we might figure out new ways to prove or disprove them. But we may have to accept that we will never be as sure as we are that the W/Z/h are there.