Talks

The Gaia mission is in the process of mapping nearly 1% of the Milky Way’s stars. This data set is unprecedented and provides a unique view into the formation history of our Galaxy and its associated dark matter halo. My talk will focus primarily on recent work using deep learning methods to classify Gaia stars that were born inside the Milky Way, versus those that were accreted from satellite mergers. Using these techniques, we discovered a vast stellar stream, called Nyx (after the Goddess of the Night), in the Solar vicinity that co-rotates with the Galactic disk. If Nyx is the remnant of a disrupted dwarf galaxy, it may provide the first evidence for an accreted stellar disk and a dark matter disk.

The Einstein action has made us very accustomed to black holes and their “no drama” event horizons. But the Einstein action will eventually be subsumed into a UV complete theory of gravity, and in such a theory there can be a new class of solutions that are not quite black holes. Within a Planck length of the would-be horizon, strong gravity and high curvatures quickly turn on. These solutions are analogous to the hadrons and/or the quark matter states of QCD. They are very close to being completely black, but not quite. An ideal probe to test for not quite black holes are the low frequency gravitational waves that are excited in and around them when they are newly formed, as in the merger events observed by LIGO. There are some key features of waves that escape the interior of not quite black holes, and from this we describe our own search and search results using LIGO data.

I will discuss the problem of an observer's S-matrix in de Sitter space, i.e. the mapping between fields on the initial and final horizons of a de Sitter static patch. I will show how the S-matrix of free massless fields can be packaged in a spinor-helicity language. This involves “cheating” the static patch’s painfully low symmetry, by relating each horizon separately to global, de Sitter-invariant data.

I will then discuss the important special case of higher-spin gravity in de Sitter, and argue that higher-spin symmetry prevents any corrections to the free S-matrix, except in the “soft" sector of zero frequencies.

A new quantization prescription is able to endow quantum field theory with a new type of “particle”, the fakeon (fake particle), which mediates interactions, but cannot be observed. A massive fakeon of spin 2 (together with a scalar field) allows us to build a theory of quantum gravity that is both renormalizable and unitary, and basically unique. After presenting the general properties of this theory, I discuss its classical limit, which carries important remnants of the fakeon quantization prescription. I also discuss the implications for cosmology and the possibility that the Higgs boson might be a fakeon.

A "quasi-classical" picture of the transition from an evaporating black hole to a white hole is described, which is based on a resolution of the Schwarzschild singularity suggested by loop quantum gravity. All quantum information trapped by the black hole is eventually released from the white hole, without any Cauchy horizons, consistent with unitarity. The effective stress-energy tensor suggests that inflow of negative energy associated with Hawking "partners" in the interior of the black hole becomes, at least initially, an outflow of negative energy from the white hole. Alternative scenarios for the further evolution of the white hole and their implications will be discussed.

Quantizing the black hole can be done without String Theory, fuzz balls, AdS/CFT and such. We just assume matter to keep the form of point particles until they come close to the horizon. The gravitational back reaction of these particles generates a novel relation between particles going in and particles going out, enabling us to transform in-going particles into out-going ones. This transformation removes "firewalls" along the future and past horizons, but it strongly affects space-time inside a black hole. It subsequently allows us, and indeed forces us, to identify antipodal points on the horizon. We argue that this is the only way to restore unitarity for the quantum evolution operator, and to identify the black hole microstates. Some mysteries, however, remain unresolved.