Cosmology and Gravitation

Thermodynamics is a closed field of research. The laws of thermodynamics, established in the nineteenth century, are still standing unchallenged.  However, they do not include gravity. Inclusion of gravity into the thermodynamical system can significantly modify the expected behavior of the system. We will demonstrate that gravity dynamically induces a maximal temperature that can be reached in a gas of particles.  We will also show how gravity can significantly change the Poincare recurrence theorem, and sometimes even prevent the recurrence from happening.

Many of the multi-planet systems discovered around other stars are maximally packed. This implies that simulations with masses or orbital parameters too far from the actual values will destabilize on short timescales; thus, long-term dynamics allows one to constrain the orbital architectures of many closely packed multi-planet systems. I will present a recent such application in the TRAPPIST-1 system, with 7 Earth-sized planets in the longest resonant chain discovered to date. In this case the complicated resonant phase space structure allows for strong constraints. A central challenge in such studies is the large computational cost of N-body simulations, which preclude a full survey of the high-dimensional parameter space of orbital architectures allowed by observations. I will discuss our recent successes in training machine learning models capable of predicting orbital stability a million times faster than N-body simulations, and the discovery space that this opens up for exoplanet characterization and planet formation studies.

The study of super-Eddington accretion is essential to our understanding of the growth of super-massive black holes in the early universe, the accretion of tidally disrupted stars, and the nature of ultraluminous X-ray sources.  Unfortunately, this mode of accretion is particularly difficult to model because of the multidimensionality of the flow, the importance magnetohydrodynamic turbulence, and the dominant dynamical role played by radiation forces.  However, recent increases in computing power and advances in algorithms are facilitating major improvements in our ability to model radiation in numerical simulations of astrophysical plasmas.  I will briefly describe our new radiation transfer modules and discuss our efforts to model super-Eddington accretion flows around stellar mass and supermassive black holes.  I will focus on applications to ultraluminous X-ray sources, which must be radiating well above their Eddington luminosity unless they harbour intermediate mass black holes. I will argue that most of these sources can be (and likely are) "normal" ~10 solar mass black holes accreting and radiating with luminosities well above their Eddington "limit”.

The 21cm transition of atomic hydrogen is rapidly becoming one of our most powerful tools for probing the evolution of the universe.  The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is a planned 1,024-element array to be built in South Africa that will study the (possible) evolution of dark energy from z=0.8 to 2.5. Its design also makes it an excellent pulsar and fast radio burst (FRB) search machine.  While the FRB rates in the HIRAX band are uncertain, we expect it will discover dozen(s) per day.  HIRAX will use inexpensive 6m satellite television-class dishes.  The same dishes can be used to build relatively inexpensive outrigger stations in southern Africa to localize a large fraction of the FRBs discovered by HIRAX to ~0.1 arcseconds, potentially transforming them into probes of the universe.

Universality classes of inflation as phases of condensed matter: slow-roll, solids, gaugids etc.
Abstract: Cosmology and condensed matter systems share the same basic symmetries: both are characterised by a (spontaneous) breaking of Lorentz boosts and by approximate translation and rotation invariances. It is therefore not completely surprising that the basic mechanisms for cosmic acceleration be in one to one correspondence with different condensed matter realisations. This point of view has recently unveiled inflationary models well beyond single-field inflation. The different inequivalent ways in which translation and rotation invariance can be recovered at low energy lead to different "universality classes” for cosmic acceleration. Among those, solid inflation and the recently proposed gaugid inflation have substantially different properties than those of single-field slow-roll scenarios.

In models of inflation driven by an axion-like pseudoscalar field, the inflaton, a, may couple to the standard model hypercharge gauge field via a Chern-Simons-type interaction, L ⊃ a F F̃. This coupling results in the explosive production of hypermagnetic fields during inflation, which has two interesting consequences: (1) The primordial hypermagnetic field is maximally helical. It is therefore capable of sourcing the generation of nonzero baryon number around the electroweak phase transition (via the chiral anomaly in the standard model). (2) The gauge field production during inflation feeds back into the stochastic background of gravitational waves (GWs). In this talk, I am going to discuss the correlation between these two phenomena. To this end, I will (a) present an updated study of baryogenesis via hypermagnetic fields after pseudoscalar inflation and (b) describe the corresponding implications for GWs. As it turns out, successful baryogenesis is feasible---provided the axion coupling to the gauge fields is suppressed by a decay constant \Lambda ~ 3 x 10^17 GeV. Moreover, in the case of successful baryogenesis, one expects a characteristic peak in the GW spectrum at frequencies in the MHz range

The unrenormalised energy momentum tensor is both huge and fluctuating from point to point. Taking this seriously we (Qingdi Wang, Zhen Zhu, and myself) argue that the slow exponential expansion of the universe (on time scales of 10^10 years) comes from a very weak parametric resonance induced by the fluctuating energy mementum tensor on the rapidly fluctuating scale factor (on time scales much shorter than the Planck scale). We see only the slow exponential growth because we avarage over the scale factor squared.

In 1977, Blandford and Znajek  discovered a process by which a spinning 

black hole can transfer rotational energy to a force-free plasma, offering a possible mechanism for energy and jet emissions from quasars and other astrophysical sources.  This Blandford-Znajek (BZ) mechanism is a Penrose process, which exploits the presence of an ergosphere supporting negative energy states, and it involves currents of electrical charge sourcing the toroidal  magnetic field component of the emitted Poynting flux.  

In this talk, I will discuss a version of the BZ process requiring only vacuum electromagnetic fields outside the black hole.  The setting is somewhat artificial, since it assumes the black hole is cylindrical rather than spheroidal, or that the black hole lives in 2 spatial dimensions, but it is nevertheless of theoretical interest.  The radiation power and horizon regularity relations are identical to those of the BZ mechanism with plasma, and the solution can be given in simple, closed form for a wide class of metrics, so it helps to illuminate the nature of the original mechanism.  For asymptotically Anti-de Sitter black holes it presumably has an interesting dual CFT description, but we haven't quite yet figured out  what that is.

We present an analytic, perturbative solution that describes dynamical black holes in slow-roll inflation with a general potential. It is shown that the spacetime evolves quasi-statically through a sequence of quasi-Schwarzchild-deSitter metrics with time dependent cosmological constant and mass parameters, such that the cosmological constant is instanteously equal to the value of the scalar potential.  The areas of the black hole and cosmological horizons each increase in time as the effective cosmological constant decreases, and the the factional area increase is proportional to the fractional change of the cosmological constant, times a geometrical factor. For black holes ranging in size from much smaller than to comparable to the cosmological horizon, the pre-factor varies from very small to order one. The change in the horizon areas happens in such a way that the first law of thermodynamics is obeyed between successive time steps.

One of the basic puzzles of black hole thermodynamics is the simplicity and universality of the  Bekenstein-Hawking entropy. The idea that this entropy might be governed by a symmetry at the horizon is an old one, but until now efforts have focused on conformal symmetries, either at infinity or on a "stretched horizon."  I argue that a better approach uses a BMS-like symmetry of the horizon itself.  This avoids the limitations of previous attempts (including my own), and explains the entropy in terms of a generalization of the Cardy formula for the density of states.

The Lambda Cold Dark Matter framework successfully accounts for observational constraints on large (> 1 Mpc) scales, from the clustering of galaxies to the angular dependence of the Cosmic Microwave Background to the structure and matter content of galaxy clusters. On the scale of individual galaxies and, in particular, of dwarf systems much fainter than the Milky Way, a number of apparent conflicts with LCDM expectations have been reported. These have prompted the consideration of a number of radical modifications to LCDM, such as the possibility that dark matter might be "self-interacting", or that it might not be “cold”. I will review the status of these alleged problems and will report on recent work that reevaluates the observational evidence and reexamines the role of systematic uncertainties in the comparison between observation and model predictions. In particular, I will propose a possible resolution to the “cusp vs core” problem that requires no cores; an explanation for the mass discrepancy-acceleration relation that requires no changes to LCDM halos; and a plausible tidal origin for the enigmatic population of galaxies inhabiting “extremely cold” dark matter halos, such as the recently discovered Crater 2 satellite.