Cosmology and Gravitation

Inflation is proposed as a means of explaining why the Universe is currently so homogeneous on larger scales, solving both the horizon and flatness problems in early universe cosmology. However, if inflation itself requires homogeneous conditions to get started, then inflation is not a solution to the horizon problem. Most work up until now has focussed on a dynamical systems approach to classifying the stability of inflationary models, but recently Numerical Relativity (NR) has been used to simulate the actual evolution of the inflaton field, leading to new insights. I will describe a recent work (https://arxiv.org/abs/1608.04408) in which we used NR to consider the robustness of generic small and large field inflationary models to initial inhomogeneities in the inflaton field and the extrinsic curvature of the metric.

I will discuss the potential for the LISA space-based interferometer to detect the stochastic Gravitational Wave (GW) background produced from different mechanisms during inflation. In particular, I will present the GW contributions from particle production during inflation, inflationary spectator fields with varying speed of sound, effective field theories of inflation with specific patterns of symmetry breaking and models leading to the formation of primordial black holes. I will show that LISA is able to probe these inflationary scenarios beyond the irreducible vacuum tensor modes expected from any inflationary background.

I discuss the phenomenology of models of inflation with periodic particle production. Particle production occurs as the mass of heavy particles goes through non-adiabatic modulations as the inflaton field rolls. I show that this process can lead to significant emission of scalar and gravitational waves during inflation, with distinct observational signatures.

We argue that moduli stabilization severely constrains the evolution following transitions between weakly coupled de Sitter vacua and can induce a strong selection bias towards inflationary cosmologies. We carefully discuss gravitational vacuum decays and resolve a naive sign ambiguity in the exponential of the decay rate. Equipped with this clear understanding of vacuum decay we then turn towards constraints on the cosmological evolution after transitions in weakly coupled flux compactifications. The energy density of domain walls between vacua typically destabilizes Kahler moduli and triggers a runaway towards large volume. This decompactification phase can collapse the new de Sitter region unless inflation lasts for more than roughly 60 efolds. High scale inflation is vastly favored. Our results illustrate the necessity to understand inflationary initial conditions at least at a basic level, before making predictions in the landscape.

If heavy fields are present during inflation, they can leave an imprint in late-time cosmological observables. The main signature of these fields is a small amount of distinctly shaped non-Gaussianity, which if detected, would provide a wealth of information about the particle spectrum of the inflationary Universe. Here we investigate to what extent these signatures can be detected or constrained using futuristic 21-cm surveys. This part of my talk is based on 1610.06559. In the second part of my talk I will discuss how non-adiabatic production of heavy particles, as recently studied in 1606.00513, can generate an interesting and so far unconstrained class of non-Gaussianity in the CMB.   

The precision of current and future cosmological observations at Megaparsec scales demands a detailed understanding of the effects of baryonic processes on the clustering of matter at these scales. In this talk, I will explore how to use measurements of cosmic shear to constrain the impact of these processes on the total matter power spectrum. I will present forecasts demonstrating that shear measurements from Stage III surveys (such as DES and HSC) and beyond will be able to strongly constrain (or even rule out) current simulation-based implementations of baryonic physics (such as AGN feedback). These forecasts make use of a model-independent parametrization of the impact of baryons on the matter power spectrum, and marginalize over several key observational and theoretical systematics. The results indicate that cosmic shear can likely be used as a robust probe of the physics of galaxy formation, and provide an important observational input for future simulations or modeling efforts.

It has long been wondered to what extent the observable properties of an inhomogeneous universe will be measurably different from a corresponding FLRW model. Here, we use tools from numerical relativity to study the properties of photons traversing an inhomogeneous universe. We evolve the full, unconstrained Einstein field equations for a spacetime containing dust, with a spectrum of long-wavelength density perturbations similar to the observed one. We then integrate the optical scalar equations along paths through this numerical spacetime, with all paths terminating at an observer situated similarly to ourselves, and construct the resulting Hubble diagrams.

In this seminar, I will present two promising ways in which the cosmic microwave background (CMB) sheds light on critical uncertain physics and systematics of the large-scale structure.
       Shear calibration with CMB lensing (arXiv:1607.01761):
Realizing the full potential of upcoming weak lensing surveys requires an exquisite understanding of the errors in galaxy shape estimation. In particular, such errors lead to a multiplicative bias in the shear, degenerate with the matter density parameter and the amplitude of fluctuations. Its redshift-evolution can hide the true evolution of the growth of structure, which probes dark energy and possible modifications to general relativity. I will show that CMB lensing from a stage 4 experiment (CMB S4) can self-calibrate the shear for an LSST-like optical lensing experiment. This holds in the presence of photo-z errors and intrinsic alignment.
       Evidence for the kinematic Sunyaev-Zel'dovich (kSZ) effect (arXiv:1510.06442); cluster energetics:

Through the kSZ effect, the baryon momentum field is imprinted on the CMB. I will report significant evidence for the kSZ effect from ACTPol and peculiar velocities reconstructed from BOSS. I will present the prospects for constraining cluster gas profiles and energetics from the kSZ effect with SPT-3G, AdvACT and CMB S4. This will provide constraints for galaxy formation and feedback models.

Despite tremendous recent progress, gaps remain in our knowledge of our cosmic history. For example, we have yet to make observations of Cosmic Dawn or the subsequent Epoch of Reionization. Together, these represent the important period when the first stars and galaxies were formed, dramatically altering their surroundings in the process. Radio telescopes targeting the 21cm line will open up these crucial epochs to direct observations in the next few years, filling in a missing chapter in our cosmic story. I will review our recent results from the Precision Array to Probe the Epoch of Reionization (PAPER) experiment. These results have begun to shed light on heating processes during reionization. I will also motivate unconventional ideas in experiment design that have been proposed and implemented to deal with the unique technical challenges of 21cm cosmology. Cognizant of "lessons learned" from the current generation of instruments, I will describe our recently commenced Hydrogen Epoch of Reionization Array (HERA), including its forecasted promise to provide exquisite constraints on reionization astrophysics as well as on fundamental parameters such as the neutrino mass.

If we want a mechanism for the current cosmic expansion that is alternative to (and possibly more “natural” than) the cosmological constant, there exist intriguing proposals within the dark energy and modified gravity realm.

First, I will briefly review the status of one of the most promising ideas, massive gravity: cosmological solutions, some formal aspects and recent developments. Then, I will present recent work aimed at constraining such models with LSS probes. 

In the coming decade, ground based CMB telescopes could face a substantial upgrade, to so-called CMB-S4. There are two main science drivers behind this initiative: B-modes, and neutrino mass, and I will focus on the latter. Thought of more generally, constraints on neutrinos can be thought of as generic tests of dark matter. I will discuss the prospects for CMB-S4 in the dark sector, with emphasis on searches for axions and neutrinos. It will be possible to detect percent level departures from standard cold dark matter at many sigma over a wide range of scales: a vast improvement over Planck (+ existing ground based). A large part of the improvement over Planck comes from precision lensing measurements at high multipole. I will briefly discuss some technical challenges in this measurement, based on modelling of dark matter clustering in the non-linear regime.

In this talk, I will show how entropy perturbations created during a contracting phase and converted into adiabatic/curvature perturbations after a bounce form the dominant contribution to the observed temperature fluctuations in the CMB.
In [arXiv:1607.05663] we have studied two-field bouncing cosmologies in which primordial perturbations are created in either an ekpyrotic or a matter-dominated contraction phase. We then used a non-singular ghost condensate bounce model to follow the perturbations through the bounce into the expanding phase of the universe. In contrast to the adiabatic perturbations, which on large scales are conserved across the bounce, entropy perturbations can grow significantly during the bounce phase. If they are converted into adiabatic/curvature perturbations after the bounce, they typically form the dominant contribution to the observed temperature fluctuations in the microwave background, which can have several beneficial implications. For ekpyrotic models, this mechanism loosens the constraints on the amplitude of the ekpyrotic potential while naturally suppressing the intrinsic amount of non-Gaussianity. For matter bounce models, the mechanism amplifies the scalar perturbations compared to the associated primordial gravitational waves with the consequence that the tensor-to-scalar ratio is typically reduced by an order of magnitude or so, bringing it close to current observational bounds.