Search Talks

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.

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.

CHIME is a new interferometric telescope at radio frequencies 400-800 MHz. The mapping speed (or total statistical power) of CHIME is among the largest of any radio telescope in the world, and the technology powering CHIME could be used to build telescopes which are orders of magnitude more powerful. Recently during precommissioning, CHIME started finding new fast radio bursts (FRB's) at an unprecedented rate, including a new repeating FRB.Understanding the origin of fast radio bursts is a central unsolved problem in astrophysics, and we anticipate that CHIME's statistical power will play an important role in solving it. In this talk, I'll give a status update on CHIME, with emphasis on FRB's.

FRBs are the only known sources of extragalactic coherent radiation, that show interference phenomena after traveling over cosmological distances. The interferometric probe allows equivalent strain measurements of $h\sim 10^{-26}$, opening new windows for gravitational wave detection, dark matter properties, and emission physics. I describe new directions, theoretical and observational tools, and current and future experiments.

I will show how to derive new positivity bounds for scattering amplitudes in theories with a massless graviton in the spectrum in four spacetime dimensions. The bounds imply that extremal black holes are self-repulsive, M/|Q|<1 once higher dimensional operators are taken into account, and that they are unstable to decay to smaller extremal black holes, hence providing an S-matrix proof of the weak gravity conjecture.

The discovery of gravitational waves from a binary black hole merger in 2015 opened up a new window to study the Universe, including the origin of black holes, the nature of dark matter, and the expansion history of the Universe. However, gravitational waves emitted from binary mergers propagate through the inhomogeneous Universe, which can have a considerable impact on observations of gravitational waves, in good or bad ways. I will highlight some examples of the effects of the inhomogeneity on gravitational wave observations, including their possible applications and implications.