Cosmology and Gravitation

More than a decade after its discovery, cosmic acceleration still 
poses a puzzle for modern cosmology and a plethora of models of dark energy 
or modified gravity, able to reproduce the observed expansion history, have 
been proposed as alternatives to the cosmological standard model. In recent 
years it has become increasingly evident that probes of the expansion his- 
tory are not sufficient to distinguish among the candidate models, and that 
it is necessary to combine those with observations that probe the dynamics 
of inhomogeneities. Future cosmological surveys will map the evolution of 
inhomogeneities to high accuracy, allowing us to test the relationships be- 
tween matter overdensities, local curvature, and the Newtonian potential on 
cosmological scales.

I will discuss theoretical issues involved in finding an optimal framework to 
study deviations from General Relativity on cosmological scales, giving an 
overview of recent progress, with a focus on model-independent, parametrized 
approaches. I will summarize where we stand and what are the next steps 
we should take.

Given a large landscape of vacua that statistically favors large values of the neutrino mass sum, $m_\nu$, I will present the probability distribution over $m_\nu$ obtained by weighting this prior by the amount of galaxies that are produced. Using Boltzmann codes to compute the smoothed density contrast on Mpc scales, we find that large dark matter halos form abundantly for $m_\nu \gtrsim 10$\,eV. However, in this regime structure forms late and is dominated by cluster scales, as in a top-down scenario. I will argue that this change of regime is catastrophic: baryonic gas will cool too slowly to form stars in an abundance comparable to our universe. Upon implementing this cooling boundary, the anthropic prediction for $m_\nu$ is consistent at better than $2\sigma$ with the entire range of values allowed by current experimental bounds, $58$\,meV $\leq m_\nu \lesssim 0.23$\,eV. A degenerate hierarchy is mildly preferred. Without a catastrophic boundary at or below $10$\,eV, the theoretical expectation would conflict strongly with the observed mass range. Thus the asserted cooling failure can be regarded as a prediction of the anthropic solution to the neutrino mass problem.

Visible matter consists mostly of hydrogen and helium, only a small fraction
of which is in stars. Until recently, the bulk of the gas in the local
universe was in fact not seen. In the largest structures, massive galaxy
clusters, the gas is seen via its x-ray emission, but in the much more
numerous groups and isolated galaxies, it has not been possible to detect
it. I will describe how, in the last year or so, the situation has changed,
with the detection of a cross-correlation between the thermal SZ effect and
lensing maps, and through the stacking of SZ images at the locations of
galaxies. These and similar techniques will tell us about the physics by
which stars and massive black holes heat and move gas in and around the
halos of galaxies and cluster, and will allow for tighter constraints on the
value of the primordial power spectrum (sigma_8).

The cosmic microwave background contains a wealth of
information about cosmology as well as high energy physics. It tells
us about the composition and geometry of the universe, the properties
of neutrinos, dark matter, and even about the conditions in our
universe long before the cosmic microwave background was emitted.
After a brief review of what we may hope to learn from studies of the
cosmic microwave background about the early universe, I will review
measurements of the angular power spectrum of temperature
perturbations from the first 15.5 months of Planck data by the Planck
collaboration and by Renee Hlozek, David Spergel and myself. I will
then discuss the implications for the early universe of the recently
released Planck full mission data as well as the joint analysis
between BICEP/KeckArray and Planck.

An exciting and largely unexplored frontier in observational and theoretical cosmology is to understand the properties of the universe between 400,000 years and one billion years after the big bang. Notably, the first galaxies formed in this time period, perhaps a few hundred million years after the big bang.  These galaxies strongly influenced the gas in their surroundings as well as the formation of subsequent generations of galaxies. The early galaxies emitted ultraviolet light and ionized "bubbles" of hydrogen gas around them.  These ionized bubbles grew, merged, and eventually filled the entire volume of the universe with ionized hydrogen in a process known as reionization. Understanding this process will constrain the properties of the first luminous sources, and fill in a significant gap in our story of structure formation, whereby the universe transitions from simple initial conditions to its present day complexity. I will briefly summarize current observational constraints and describe some new ideas for better determining when the reionization process completed using existing Lyman-alpha forest data.

It is well known that S-matrix Analyticity, Lorentz invariance and Unitarity place strong constraints on whether Effective Field Theories can be UV completed. A large class of gravitational field theories such as Massive Gravity and DGP inspired braneworld models contain as limits Galileon theories which in the past have been argued to violate the conditions necessary for a UV completion. I will present arguments that imply quite the opposite, although Galileons do not admit a standard Wilsonian UV completion, I will use the recently discovered Galileon duality to argue that these theories can exhibit a non-Wilsonian completion consistent with the `Classicalization’ proposal. These theories have quintessentially gravitational properties such as non-polynomially bounded scattering amplitudes, absence of local off-shell observables and an exponentially soft 2-2 scattering amplitude. These properties show up in a violation of the standard Wightman axioms and are consistent with many of the known properties of UV completions of gravity. I will argue that the superluminal solutions found in the low energy EFT are absent in the UV completion.

The last years have seen a renewed interest in theories of massive gravity. They represent an infra-red modification of gravity where the gravitational force weakens at very large scales. Heuristically, they provide the playground to understand a possible modification of GR which could potentially provide a dynamical solution to the cosmological constant problem. In this talk I will discuss a number of theoretical aspects of massive gravity theories, focusing on the relevance of the so-called Vainstein mechanism, both at the classical and the quantum level. I will also discuss how we can use cosmology from the early and late universe to constrain these theories. For example, what does the Cosmic Microwave Background Radiation know about the graviton mass?

This talk will cover a number of fun topics related to studying and understanding the Cosmic Microwave Background, including where it came from, how it is like an inside-out star, CMB numerology, how much information it contains and how it is evolving in time.

Models such as Natural Inflation that use Pseudo-Nambu-Goldstone bosons (PNGB's) as the inflaton are attractive for many reasons. However, they typically require trans-Planckian field excursions $\Delta \Phi>M_{\rm Pl}$, due to the need for an axion decay constant $f>M_{\rm Pl}$ to have both a sufficient number of e-folds {\em and} values of $n_s,\ r$ consistent with data.  Such excursions would in general require the addition of all other higher dimension operators consistent with symmetries, thus disrupting the required flatness of the potential and rendering the theory non-predictive. We show that in the case of Natural Inflation, the existence of spinodal instabilities (modes with tachyonic masses) can modify the inflaton equations of motion to the point that versions of the model with $f<M_{\rm Pl}$ can still inflate for the required number of e-folds. The instabilities naturally give rise to two separate phases of inflation with different values of the Hubble parameter $H$ one driven by the zero mode, the other by the unstable fluctuation modes. The values of $n_s$ and $r$ typically depend on the initial conditions for the zero mode, and, at least for those examined here, the values of $r$ tend to be unobservably small.

We explore the brightness frontier in time domain radio astronomy and its possible usefulness for cosmology.  It is argued that the brightest known source of emission, Crab nanoshots, are caused by Schwinger pair production.  The same mechanism may be the source of Fast Radio Bursts (FRBs) if this emission is form coalescing neutron stars.  It is then shown how using FRBs as triggers can extend the reach of gravitational radiation and neutrino telescopes.  Finally we discuss how combining FRB monitoring, large neutrino telescopes, combined with preexisting galaxy catalogs could provide an accurate cosmological distance estimator.

In this talk, I will review the main ideas underlying stochastic inflation, by introducing the formalism in two independent ways. First I will start from the intuitive picture stemming from the equations of motion of the system. I will then introduce a more rigorous approach based on the in-in formalism, and show how the usual set of Langevin equations can emerge from a path integral formulation. With this understanding, I will then formulate a new, recursive method which allows to solve consistently both in slow-roll parameters and in quantum corrections. I will then discuss examples of how this method can be applied to derive corrected predictions for cosmological observables in the case of hybrid inflation, multi-field inflation, and inflation on modulated potentials.

We propose a new way to search for (hidden) cool molecular hydrogen H2 in the Galaxy through diffractive and refractive effects: Stars twinkle because their light crosses the atmosphere. The same phenomenon is expected on a longer time scale when the light of a remote star crosses an interstellar turbulent molecular cloud, but it has never been observed at optical wavelengths. Our simulations and test observations show that in favorable cases, the light of a background star can be subject to stochastic fluctuations on the order of a few percent at a characteristic time scale of a few minutes.

We  searched for scintillation caused by molecular gas within visible dark nebulae as well as by hypothetical halo clumpuscules of cool molecular hydrogen (H2-He) with the ESO-NTT telescope. Within a few thousands of densely sampled light-curves, we found one candidate that shows variabilities compatible with a strong scintillation effect through a turbulent structure of the B68 nebula. Furthermore, since no candidate has been found toward the SMC, we were also able to establish upper limits on the contribution of gas clumpuscules to the Galactic halo mass.

I will discuss the perspectives of synchronized observations with two large distant telescopes, to observe the time decorrelation between the light curves, an undisputable signature of the scintillation process. I will then show that a few nights of observation using the so-called « movie-mode » of LSST should allow one to significantly constrain the last unknown baryonic contribution to the Galactic mass.