Astrophysics

In light of the upcoming Generation 2 (G2) direct-detection experiments attempting to record dark matter scattering with nuclei in underground detectors, it is timely to inquire about their ability to single out the correct theory of dark-matter-baryon interactions, in case a signal is observed. I will present a recent study in which we perform statistical analysis of a large set of direct-detection simulations, covering a wide variety of operators that describe scattering of fermionic dark matter with nuclei. I will show that a strong signal on G2 xenon and germanium targets has enough discrimination power to reconstruct the momentum dependence of the interaction, ruling out entire classes of models. However, zeroing in on a correct UV completion will critically depend on the availability of measurements from a wide variety of nuclear targets (including iodine and fluorine) and on the availability of low energy thresholds. This study quantifies complementarity amongst different experimental designs and targets, and provides a roadmap for future data analyses. It also highlights the critical need for bringing in information from all available probes in dark matter studies.

With the completion of the Planck satellite, in order to continue collecting cosmological information it is
important to gain a precise understanding of the formation of Large Scale Structures (LSS) of the universe.
The Effective Field Theory of LSS (EFTofLSS) offers a consistent theoretical framework that aims to develop
an analytic understanding of LSS at long distances, where inhomogeneities are small. We present the recent
developments in the field covering topics of biased tracers in the EFTofLSS including the effects of baryonic
physics and primordial non-Gaussianities, finding that new bias coefficients are required. We discuss the EFT
framework for dark matter clustering in Lagrangian formalism and present its consequences on baryon acoustic
oscillations (BAO). We present analytic results and compare them with the output of N-body simulations.

We argue that theories with multiple axions generically contain a large
number of vacua that can account for the smallness of the cosmological
constant. In a theory with N axions, the dominant instantons with charges Q
determine the discrete symmetry of vacua. Subleading instantons break the
leading periodicity and lift the vacuum degeneracy. For generic integer charges
the number of distinct vacua is given by |det(Q)|~exp(N). Our construction
motivates the existence of a landscape with a vast number of vacua in
four-dimensional effective theories.

The next hope to constrain cosmological parameters observationally is in surveys of the large scale structure (LSS) of the universe. LSS has the potential to rival the CMB in cosmological constraints because the number of modes scales like the volume, but the nonlinear clustering due to gravity makes it more difficult to extract primordial parameters. In order to take full advantage of the constraining power of LSS, we must understand it in the quasi-nonlinear regime. The effective field theory (EFT) of LSS provides a consistent way to perturbatively predict the clustering of matter at large distances. In this talk, I will discuss the status of the EFT of LSS and present recent work describing the inclusion of baryons in the EFT approach, including comparisons to N-body simulations.

Cosmic neutrinos carry a wealth of information about both cosmology and particle physics, but they are notoriously difficult to observe.  Rapid advancement in measurements of the cosmic microwave background, however, have allowed us to indirectly constrain some properties of the cosmic neutrino background.  I will discuss the current status and future prospects for improving constraints on cosmic neutrinos, focusing in part of the phase shift of acoustic peaks in the cosmic microwave background which results from neutrino fluctuations.  I will also discuss how improved measurements from CMB-Stage IV will naturally constrain a wealth of beyond the standard model physics.

Magnetars are exceptional neutron stars with the highest magnetic
fields ( 10^15 gauss) in the universe, an unusual quasi steady X
radiation (10^35 ergs/sec) and also produce flares which are some of
the brightest events (10^46 ergs in one fifth of a second) to be
recorded. There is no satisfactory model of magnetars.
The talk will cover neutron stars and a new model for the origin of
the magnetic fields in which magnetars arise from a high baryon
density ( phase transition) magnetized core which forms when they are
born. The core magnetic field is initially shielded by the ambient
high conductivity plasma. With time the shielding currents dissipate
transporting the core field out, first to the crust and then breaking
through the crust to the surface of the star. Recent observations
provide support for this model which accounts for several properties
of magnetars and also enables us to identify new magnetars.

Is the graviton a truly massless spin-2 particle, or can the graviton have a small mass? If the mass of the graviton is of order the Hubble scale today, it can potentially help to explain the observed cosmic acceleration. Previous attempts to study massive gravity have been spoiled by the fact that a generic potential for the graviton leads to an instability called the Boulware-Deser ghost. Recently, a special potential has been constructed which avoids this problem while maintaining Lorentz invariance. In this talk I will present recent arguments that suggest that the requirement of avoiding the Boulware-Deser ghost (or other degrees of freedom) is so powerful that the kinetic term for a massive graviton is fixed as well. In fact it must be exactly the same as in General Relativity. This is remarkable as we derive the structure of General Relativity on the basis of stability requirements, not on a symmetry principle. 

Despite being ubiquitous throughout the Universe, the fundamental physics governing dark matter remains a mystery. While this physics plays little role in the current evolution of large-scale cosmic structures, it did have a major impact in the early epochs of the Universe on the evolution of cosmological density fluctuations on small causal length scales. Studying the astrophysical structures that resulted from the gravitational collapse of fluctuations on these small scales can thus yield important clues about the physics of dark matter. Today, most of these structures are locked in deep inside the potential wells of galaxies, making the study of their properties difficult. Fortunately, due to fortuitous alignments between high-redshift bright sources and us, some of these galaxies act as spectacular strong gravitational lenses, allowing us to probe their inner structure. In this talk, we present a unified framework to extract information about the power spectrum of gravitational potential fluctuations inside any type of lens galaxies. We argue that fully exploiting this new approach will likely require a paradigm shift in how we describe structures on sub-galactic scales. We finally discuss which properties of mass substructures are most readily constrained by lensing data.

I will talk about a novel theory of dark matter superfluidity that matches the success of LCDM model on cosmological scales while simultaneously reproducing the MOND phenomenology on galactic scales.

I will first briefly report the current status on the stability problem of global AdS space under gravitational self-interaction. I will then present evidence that the possibility of a blackhole-forming instability is strongly connected to phase-locked cascade, which is different from the usual energy cascade in turbulent flow.

Fundamental physics has reached a turning point. The most powerful experiments ever devised are revealing the structure of the universe with unprecedented clarity. On the largest scales – the whole visible universe – and the tiniest, we are discovering remarkable simplicity, which our theories do not yet explain. In between, things are complex. But here too, new technologies are allowing us to access the quantum frontier, opening up new high-precision probes of the fundamental laws of nature and revolutionary new technologies. We stand on the threshold of breakthroughs, both theoretical and experimental, which could change our picture of the world and the development of our society.