Astrophysics

We discuss the general structure of finite dimensional real Lie groups, which is called the Levi decomposition theorem. According to this classical result, any Lie group (up to global topology) splits as a semi-direct product of a non-semisimple part and of a semisimple part. In traditional gauge theory / field theory folklore mostly, only semisimple groups are considered. We will argue, that quite possibly non-semisimple, in particular so-called nilpotent Lie groups could also play a role for new kind of unification mechanisms.

Upcoming CMB and large-scale structure experimental data can be cross correlated to reconstruct the large-scale matter velocity field in a process called kinetic Sunyaev–Zel'dovich (kSZ) tomography. Similar to CMB lensing reconstruction, kSZ tomography provides a large-scale probe from small scale observations. kSZ tomography is a powerful probe of cosmology, in particular of primordial non-Gaussianity, and I will discuss how the scientific returns from upcoming galaxy surveys can be enhanced with this method. I will also discuss a general bispectrum approach to kSZ estimation, which unifies several previously known methods.

I will discuss a novel and powerful way to probe dark matter particle properties using deep, high-resolution cosmic microwave background (CMB) gravitational lensing measurements. These measurements can distinguish between cold dark matter and alternative dark matter models that can explain observational puzzles of small-scale structure. I will also discuss a new experiment being developed, called CMB-HD, that can achieve this science and also open new windows on the early Universe, gas and galaxy evolution, planetary studies, and the transient sky.

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.

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.