Cosmology

In the second lecture, I will extend the previous discussion to gravity, and show that the conformal trace anomaly must play a special role in the effective field theory of low energy gravity. Classical General Relativity receives an infrared relevant modification from the conformal trace anomaly of the energy-momentum tensor of massless, or nearly massless, quantum fields, which implies the existence of a new long range massless scalar degree of freedom, not present in the classical Einstein theory, that contributes to gravitational scattering processes and has long range gravitational effects. Similar to the axial anomaly, the local form of the effective action associated with the conformal anomaly is expressible in terms of a dynamical scalar field that couples to the conformal factor of the spacetime metric, allowing it to propagate over macroscopic distances. Among the significant implications of this effective field theory of gravity are the prediction of scalar gravitational wave solutions—a spin-0 breather mode— in addition to the transversely polarized tensor waves of the classical Einstein theory. Astrophysical sources for scalar gravitational waves are considered, with the excited gluonic condensates in the interiors of neutron stars in merger events with other compact objects likely to provide the strongest burst signals. The conformal anomaly also implies generically large quantum back reaction effects and conformal correlators in the vicinity of black hole horizons which are relevant to the formation of a non-singular interior, as well as an additional scalar degree of freedom in cosmology, providing a theoretical foundation for dynamical dark energy.

A key prediction of the Lambda CDM framework of structure formation is that a host halo containing a Milky Way sized disk galaxy should contain hundreds of thousands of sub-dwarf galaxy mass dark matter subhalos. Devoid of stars, these substructures remain undetected. Detecting them will not only corroborate the existence of dark matter but also give crucial information on the particle nature of dark matter and how they cluster at small scale. Cold stellar streams originate when globular clusters are tidally disrupted in the Milky Way potential. Gravitational encounters with dark matter subhalos will perturb these streams' density. I will present a novel method of probing the fundamental nature of dark matter by statistically analyzing the power spectrum of the stream density. I will discuss the uncertainties that the known baryonic structures such as the Galactic bar, the spiral arms, the Galactic population of giant molecular clouds and globular clusters can impart. Understanding their effects will be crucial for deciding whether a particular stream can be used to probe dark matter substructures.  

Line Intensity Mapping has emerged as a powerful tool to probe the large-scale structure across redshift, with the potential to shed light on dark energy at low redshift and the cosmic dawn and reionization process at high redshift.  Multiple spectral lines, including the redshifted 21cm, CO, [CII], H-alpha, and Lyman-alpha emissions, are promising tracers in the intensity mapping regime, with several experiments on-going or in the planning.  I will discuss results from current pilot programs, prospects for the upcoming TIME experiment, and the outlook of future space missions such as SPHEREx.  I will illustrate how the use of cross-correlation between multiple line intensity maps will enable unique and insightful measurements, revealing for example the tomography of reionization and the physics of multiple phases of the interstellar medium across redshift.

Dr. Avery Broderick will provide a highly accessible and interesting lecture on the Event Horizon Telescope (EHT) and international efforts to interpret horizon-resolving images of numerous supermassive black holes. Black holes are among the most powerful and mysterious phenomena in the universe. Almost every galaxy has at its core a supermassive black hole, millions or even billions of times more massive than our sun. Despite composing a small fraction of the galactic mass budgets, they set the stage for astrophysical dramas that dictate the fates of their hosts. Though black holes are in theory the ultimate manifestation of strong gravity’s impact on the visible universe, placing these exotic phenomena on concrete empirical footing has been impossible - until now.

Stars orbiting in the halo of our galaxy, the Milky Way, are a window into the distribution of dark matter. In particular, tidally disrupted star clusters, which produce thin stellar streams, are optimal tracers of matter. Based on a Fisher-information calculation, we expect that the current data on the known Milky Way streams should constrain the radial profile and the shape of the inner halo to a precision of a few percent. In addition, stellar streams retain a detailed record of the matter field on small scales. Typically, streams are very thin -- approximately 100 times longer than they are wide, so even small gravitational perturbations introduce observable variations in the density of stars along the stream. Recently, significant gaps were detected in the GD-1 stellar stream, as well as stream stars displaced from the main stream. The observed structure is naturally reproduced in simulations where a stream like GD-1 encounters a massive perturber. None of the known Milky Way satellites has recently crossed paths with GD-1. If confirmed, the encounter scenario indicates the presence of an unknown, massive (~10^7 Msun) and compact (~<10 pc) object in the Milky Way -- the first evidence of dark substructure in a galactic halo.

In galaxy redshift surveys, the line-of-sight velocity information is encoded in the observed redshift as a Doppler component that radially distorts the galaxy positions. The linear component of such `Redshift-Space Distortions' (RSD) is directly proportional to the growth rate of structure, f(z), and motivates the interest in RSD as a powerful way to constrain gravity. However, the non-linear evolution of the density and velocity fields requires the use of sophisticated theoretical models to extract reliable cosmological information from quasi-linear scales. Alternatively, one can use tracers of the LSS less sensitive to the non-linear motions at small scales. After an overview of VIPERS I will present a novel method that uses the clustering of luminous blue galaxies to obtain an accurate estimate of the growth rate while keeping the theoretical model relatively simple.

Gravitational lensing has long served as a unique probe of the growth of structure, which is sensitive on large scales to the properties of dark energy and gravity.  In particular, lensing is instrumental in forming the statistic E_G, which is constructed to measure the growth rate of structure in a way independent of galaxy clustering bias.  Measurements of E_G using both CMB lensing and galaxy lensing have proliferated, but new challenges await as the data from upcoming surveys become more precise.  One such challenge is the bias on E_G due to the inhomogeneous magnification of galaxies.  In this talk, we will discuss how to identify this bias and calibrate it to remove from survey data.  We also consider a new tracer of gravitational lensing, namely upcoming 3D line intensity maps of emission lines such as the 21 cm line from neutral hydrogen.  Previous considerations of this new probe have assumed the flat-sky approximation, valid over small areas, where all light rays are assumed to come from the same direction.  We will present our extension of this treatment to the full-sky case which will be needed for large-area, 21 cm surveys such as HIRAX.  We will then discuss how this enhancement could improve intensity mapping probes of gravity.

 One of the most important discoveries of the 20th century has been the finding of neutrino oscillations. That phenomena implies that neutrinos are massive and shows the existence of physics beyond the standard model. Fundamental questions associated to this discovery are: what are the absolute neutrino masses? and what is their hierarchy? In this talk I will discuss how to use cosmological observables to answer these questions. I will first show one of the predictions of the Big Bang theory: the existence of a cosmic neutrino background. I will then present the effects that it introduces on several cosmological observables such as the matter power spectrum, halo/galaxy properties, cluster counts, halo/galaxy clustering and cosmic voids. I will show how to use these effects to weigh neutrinos. I will emphasize the importance of having accurate theory predictions, and to search for new observables, in order to achieve a 5-sigma detection of the minimum sum of the neutrino masses.

I will first outline an effective field theory for cosmology (EFTC) that is based on the Standard Model coupled to General Relativity and improved with Weyl symmetry. Any version of quantum gravity (QG), including string theory, must include the same improvement, otherwise QG will not be geodesically complete.  Based on arguments of an underlying QG theory, this effective field theory banishes higher curvature terms in the action,  thus making this EFTC  mathematically sufficiently well behaved to make unique predictions for the classical solutions and the quantum wavefunction at the singularity, as well as past the singularity, for a complete cosmology that includes the past, future and detailed quantitative solution at the big bang.
Using this EFTC, I will show its predictions of surprising behavior of the universe at the singularity that cosmologists usually "conveniently" ignore, but they should not, because this predicts the initial conditions. I will illustrate this behavior with detailed formulas and plots of the classical and quantum solutions. The solutions are given in the geodesically complete mini-superspace that has the flavor of the extended spacetime of a black hole or extended Rindler spacetime. The analytic continuation of the quantum wavefunction across the horizons describes the passage through the singularities. The mathematical solution of this analytic continuation solves a long-standing problem of the singular 1/r^2 potential in quantum mechanics that dates back to Von-Neumann. The analytic properties of the wavefunction  also reveals an infinite stack of universes sewn together at the horizons of the geodesically complete space. Remaining open questions and problems will be outlined.

Burst phenomena are ubiquitous in astrophysics. Understanding the origin of bright and rapid bursts, like FRBs, is an important goal of contemporary astrophysics.  We apply Dicke's superradiance, a coherent quantum mechanical radiation mechanism, to explain these burst phenomena. We show that bursts lasting from a few milliseconds (FRBs) to a few years (e.g. OH masers) can be produced by very large groups of entangled atoms/molecules. This is in contrast with the common assumption that, in the interstellar medium, the atoms/molecules in a radiating gas act independently from each other. Superradiance, a well-known and intensely studied phenomenon in the physics community, was first discussed by R. H. Dicke in 1954. I will present our superradiance models developed to explain some maser flares and FRBs, and discuss our results for the 6.7-GHz methanol, 1612-MHz OH, and 22-GHz water spectral lines. Our analyses suggest that the aforementioned groups of entangled atoms/molecules, developing over distances of up to a few kilometers for maser flares and 1000 AU for FRBs, can reproduce the observed light curves

Axions are attractive candidates for theories of large-field inflation that are capable of generating observable primordial gravitational wave backgrounds. These fields enjoy shift-symmetries that protect their role as inflatons from being spoiled by coupling to unknown UV physics. This symmetry also restricts the couplings of these axion fields to other matter fields. At lowest order, the only allowed interactions are derivative couplings to gauge fields and fermions. These derivative couplings lead to the biased production of fermion and gauge-boson helicity states during and after inflation. I will discuss preheating in axion-inflation models that are derivatively coupled to Abelian gauge-fields and fermion axial-currents.

For an axion coupled to U(1) gauge fields preheating is efficient for a wide range of parameters. In certain cases the inflaton is seen to transfer all its energy to the gauge fields within a few oscillations. Identifying the gauge field as the hypercharge sector of the Standard Model can lead to the generation of cosmologically relevant magnetic fields.

Coupling the inflaton-axion to Majorana fermions leads to the biased production of fermion helicity-states which can have interesting phenomenological implications for leptogenesis.