Cosmology and Gravitation

An exciting development in outer solar system studies is the discovery of a new class of Kuiper belt objects with orbits that lie outside that of Neptune and have semimajor axes in excess of 250 A.U. The alignment of the major axes of these objects and other orbital anomalies are the basis for the Planet Nine hypothesis that an undiscovered giant planet orbits the sun at a distance of around 500 A.U. We show that a modified gravity theory known as MOND (Modified Newtonian Dynamics) provides an alternative explanation for the observed alignment, owing to significant quadrupolar and octupolar terms in the MOND galactic field within the solar system that are absent in Newtonian gravity. Using the well-established secular approximation of solar system dynamics we predict a population of Kuiper belt objects whose major axes are aligned with the direction to the center of the galaxy and that have additional clustering in orbital parameters. These features are exhibited by known Kuiper belt objects of the newly discovered class in support of the MOND hypothesis. Thus MOND, originally developed to explain galaxy rotation without invoking dark matter, may also be observable in the outer solar system.

Inflation can be viewed as a natural "cosmological particle detector" which can probe energies as high as its Hubble scale. In this talk, I study the imprints of heavy relativistic particles during inflation on primordial correlators in situations where the scalar fluctuations have a reduced speed of sound. Breaking dS boosts allows new types of footprints of massive fields to emerge. In particular, I show that heavy particles that are lighter than Hubble divided by the speed of sound leave smoking gun imprints in the three-point function of curvature perturbations (due to the exchange of those fields) in the form of resonances in the squeezed limit which are vividly distinct from the previously explored signatures of heavy fields in de Sitter correlators. Throughout I use and extend the cosmological bootstrap techniques derived from locality, unitarity, and analyticity in order to find fully analytical formulae for the desired boost breaking correlators.

Next generation cosmic microwave background (CMB) experiments and galaxy surveys will generate a wealth of new data with unprecedented precision on small scales. Correlations between CMB anisotropies and the galaxy density carry valuable cosmological information about the largest scales, creating novel opportunities for inference. It is possible to foresee a future where reconstruction of the gravitational weak-lensing potential, velocity fields and the remote quadrupole field will provide the most precise tests of fundamental physics. The use of the second-order effects in the CMB to extract this information motivate a strong push towards low noise, high resolution frontiers of the upcoming generation CMB experiments. In this talk, I will discuss the prospects to use small-scale kinetic and polarized Sunyaev Zel’dovich effects and the moving-lens effect, in cross-correlation with ongoing galaxy surveys, to extract cosmological information. 

Zoom Link: https://pitp.zoom.us/j/91455862792?pwd=M1hFRDgyOXVKU1U4Z0pLcm1wZGdRQT09

I review a recent approach to connecting quantum gravity and the real world by deconstantizing the constants of nature, and using their conjugate as a time variable. This is nothing but a generalization of unimodular gravity. The wave functions are then packets of plane waves moving in a space that generalizes the Chern-Simons functional. For appropriate states they link up with classical cosmology in the appropriate limit. There are however deviations, namely during the matter to Lambda transition, raising the possibility that quantum gravity could be in action here and now. At the other extreme I show how this approach can be used to resolve the cosmological singularity, and perhaps more.

Zoom Link: https://pitp.zoom.us/j/94010122575?pwd=L291eHNSOG1wZmpCL1lmWHVJaEwwdz09

I will present a brief introduction to Nested Sampling, a complementary framework to Markov Chain Monte Carlo approaches that is designed to estimate marginal likelihoods (i.e. Bayesian evidences) and posterior distributions. This will include some discussion on the philosophical distinctions and motivations of Nested Sampling, a few ways of understanding why/how it works, some of its pros and cons, and more recent extensions such as Dynamic Nested Sampling. If time/interest permits, I can either (a) highlight how this can work in practice using the public Python package dynesty​ or (b) discuss the more general problem of model selection and why Bayesian evidences may (or may not) be helpful.

Zoom Link: https://pitp.zoom.us/j/95990705337?pwd=VzB4cjhzSDhoM0RCYTNwZHUzUVlzdz09

Many key cosmological observables have a built-in symmetry under rescaling of all important length scales in the problem. This scaling symmetry can be used to make partial progress toward a complete resolution of the Hubble tension. To exploit the symmetry while respecting observational constraints, we are naturally led to a “mirror world” dark sector. A successful implementation of this scaling symmetry requires a means of increasing the cosmic photon scattering rate that respects observational bounds on the primordial helium abundance. We discuss different ideas that could in principle achieve this rescaling, and discuss their advantages and drawbacks. We finally present some general observations about the fundamental nature of the “Hubble tension".

Zoom Link: https://pitp.zoom.us/j/99678466262?pwd=VTY4cWNFUERhU08vYi9lT09MZjJkdz09

Does inflation have to happen all in one go? The answer is no!

All cosmological problems can be solved by a sequence of bursts of cosmic acceleration, interrupted by short epochs of decelerated expansion.

In this rollercoaster cosmology, models that seem excluded for a single long stage become viable again, and high-scale inflation is more natural.

At the same time, we expect interesting predictions at several different length scales, such as gravitational wave signals potentially detectable by LISA.

I will describe the general framework, and focus on a realization with two stages of monodromy inflation.

Zoom Link: https://pitp.zoom.us/j/99206384855?pwd=dUpDdWVxV1lXc0g1bU9uZ2lrMzVXUT09

The disk of the Milky Way comprises some 100 billion stars on nearly circular orbits about the Galactic centre. Over the next few years, the Gaia Space Telescope will measure positions and velocities for over 1% of these stars. By combining equilibrium models of the Galaxy with these observations we can construct the Galactic rotation curve, which allows us to infer the large-scale structure of the dark matter halo. We can also construct a model for the mass distribution in the Solar Neighbourhood, which allows us to infer the local density of dark matter.  However, even a cursory study of the Milky Way reveals structures that signal a departure from equilibrium. The most prominent of these are the Galactic bar, spiral arms, and warping of the outer disk.  I will describe recent observations of some more subtle departures from equilibrium and discuss ways in which these observations can lead to refined models of the Galaxy and a more complete picture of the Galaxy's dynamics.

Zoom Link: https://pitp.zoom.us/j/98802402146?pwd=K3RPYlNMR2hMcXFMUm5SclU3djdiZz09

Intensity mapping of redshifted 21cm emission from neutral hydrogen holds great promise for learning about cosmology, as it provides an efficient way to map large volumes of the universe without the need to characterize individual luminous sources. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) is a cylinder telescope located in Western Canada that was custom-built for this purpose, with additional science targets including fast radio bursts, pulsars, and Galactic radio emission. In this talk, I will provide an overview of the design and operational status of the telescope (in the context of the cosmology science case), and then present its first 21cm science results: detection of a cross-correlation between CHIME sky maps and galaxy/quasar catalogs from the extended Baryon Oscillation Spectroscopic Survey (eBOSS). In particular, I will discuss our data processing pipeline and how we model the measured signal, as well as the physical implications and prospects for more precise future measurements.

21-cm emission from neutral hydrogen atoms provides a unique window into galaxy formation and cosmology in the first billion years of cosmic history. As the first galaxies form after the Big Bang, they generate intense ultraviolet and X-ray radiation fields, which ionize and heat their otherwise neutral surroundings. The resulting modulations in the brightness of 21-cm emission relative to the background can be detected in principle by a single, well-calibrated dipole receiver, as features in the sky-averaged radio spectrum below ~200 MHz. Spatial fluctuations in the 21-cm background are expected also, and can in principle be detected statistically with the current generation of interferometers. In just the last few years, enormous progress has been made on both fronts. The sky-averaged 78 MHz feature reported by the EDGES collaboration in 2018 caused a flurry of activity, largely aimed at explaining its anomalous amplitude. Since then, the MWA, LOFAR, and HERA have all reported upper limits on the 21-cm power spectrum. In this talk, I will focus in particular on the first limits from HERA -- the most stringent limits reported to date -- and describe their implications for galaxy formation and cosmology. I will also discuss the ongoing EDGES controversy, and how JWST and SPHEREx can provide independent tests of astrophysical scenarios that produce EDGES-like 21-cm absorption troughs at frequencies below ~100 MHz.

In the fuzzy dark matter model, dark matter consists of “axion-like” ultra-light scalar particles of mass around 10⁻²² eV. This candidate behaves similarly to cold dark matter on large scales, but exhibits different properties on smaller (galactic) scales due to macroscopic wave effects arising from the extremely light particles’ large de Broglie wavelengths. It has both particle physics motivations and a rich astrophysical phenomenology, giving rise to notable differences in the structures on highly non-linear scales due to the manifestation of wave effects, which can impact a number of contentious small-scale tensions in the standard cosmological model, ΛCDM. Some of the unique features include transient wave interference patterns and granules, the presence flat-density cores (solitons) at the centers of dark matter halos, and the formation of quantized vortices. I will present large numerical simulations of cosmic structure formation with this dark matter model – including the full non-linear wave dynamics – using a pseudo-spectral method to numerically solve the Schrödinger–Poisson equations, and the significant computational challenges associated with these equations. I will discuss several observables, such as the evolution of the matter power spectrum, the fuzzy dark matter halo mass function, dark matter halo density profiles, and the question of a fuzzy dark matter core–halo mass relation, using results obtained from these simulations, and contrast them with corresponding results for the cold dark matter model.

Most of the efforts in searching for dark energy (DE) have focused on its gravitational signatures, and in particular on constraining its equation of state. However, there is a lot to be learned about DE by getting off the beaten track. I will first focus on non-gravitational interactions of DE with visible matter, leading to the possibility of "direct detection of dark energy" (analogous to direct detection of dark matter): I will argue that such interactions can and potentially may already have been detected in experiments such as XENON1T, while discussing complementary cosmological and astrophysical signatures. I will then discuss early- and late-time consistency tests of LCDM, and how these may shed light on (early and late) DE in relation to the Hubble tension. I will present two such tests based on the early ISW effect and the ages of the oldest astrophysical objects in the Universe.