Abstracts for Cosmology Seminars

25th Feb Emir Gumrukcuoglu (Imperial College London), “Horava gravity: suppressing the Lorentz violations in matter sector”

Abstract: Horava gravity is a carefully constructed theory which is renormalizable in the power-counting sense. As the theory is founded on an anisotropic scaling between time and space, it does not respect the local Lorentz symmetry that interchanges time and space coordinates. Instead, it has a preferred time direction allowing the high energy theory to have an improved behavior, without spoiling unitarity.

A major obstacle for any Lorentz violating theory is the restoration of Lorentz invariance at low energies in the matter sector, where constraints are very stringent. Indeed, through graviton loops, the UV Lorentz violation in the gravity sector will leak into the IR of the matter sector. There are several ways to circumvent this problem, although realizing these in concrete setups is challenging. In my talk, I will focus on a promising mechanism, proposed by Pospelov and Shang (2010), which restricts the Lorentz violations to the gravity sector at tree level and these percolate the matter sector only though graviton loops. In this set-up, an intermediate Horava scale keeps the Lorentz-violating terms in the matter sector under control, although the vector graviton loops lead to a technical naturalness problem.

The first part of the talk will be devoted to the review of Horava gravity, with an emphasis on theoretical consistency requirements and observational constraints on the Lorentz breaking scales. In the second half, I will introduce the mixed-derivative extension of the theory, designed to resolve the naturalness problems in the Pospelov-Shang mechanism of Lorentz violation suppression.

18th Feb Filippo Vernizzi (CEA Saclay) “Cosmological consistency relations”

Abstract: Cosmological consistency relations are relations between correlation functions of the cosmic density fields (e.g. CMB temperature or matter/galaxy distribution). More specifically, assuming single-field inflation and the equivalence principle, an n-point correlation function can be related to an (n+1)-point correlation function in the limit in which one of the (n+1) momenta becomes much smaller than the others. I will show how to derive these relations in the large scale structures and I will discuss some of their resummation properties. Their violation can be used to probe the presence of extra-fields during inflation or extra forces in the late-time universe.

4th Feb Alberto Sesana (Birmingham) “Supermassive black hole astrophysics with pulsar timing arrays”

Abstract: The detection of gravitational waves (GWs) may soon be a reality, opening a completely new window on the Universe. At nHz frequencies, pulsar timing arrays (PTAs) promise to detect the signal coming from the cosmological population of supermassive black hole binaries (SMBHBs) within the next few years. After reviewing the astrophysics of SMBHBs, I will describe the current status of the PTA effort, present the most recent limits, and their astrophysical implications.

28th Jan Jonathan Blazek (Ohio State University) “Challenges for accurate cosmology with galaxies: intrinsic alignments and streaming velocity bias”

Abstract: Upcoming projects such as Euclid, LSST, and DESI will measure positions and shapes for hundreds of millions of galaxies, enabling weak lensing and clustering analyses at an unprecedented level of precision to probe fundamental physics. However, in this new regime, several astrophysical effects must be understood to ensure that we draw accurate conclusions. I will discuss two effects that can impact observed correlations on cosmological scales: intrinsic alignments (the shapes and orientations of galaxies) and biasing from streaming baryon velocities. Although seemingly separate topics, similar frameworks can be used to study both. Moreover, self-consistent modeling of multiple astrophysical effects is needed as we increasingly combine multiple cosmological probes. Beyond their impact on accurate cosmology, these effects provide new windows into both fundamental physics and complex astrophysical processes.

21st Jan Marco Crisostomi (Portsmouth) “Horndeski: beyond, or not beyond?”

Abstract: The most general scalar tensor theory of gravity, giving second order field equations, takes the name from Horndeski. Recently there have been attempts to formulate theories that, although giving higher order field equations, are still free from ghost instabilities. These models are called Beyond Horndeski. In this talk I will present these new Lagrangians and show if they are really new and ghost free.

14th Jan Cora Dvorkin (Harvard) “Traces of the Early Universe in the CMB and the Large-Scale Structure”

Abstract: Cosmological observations have provided us with answers to age-old questions, involving the age, geometry, and composition of the universe. However, there are profound questions that still remain unanswered. In this talk, I will describe ongoing efforts to shed light on some of these questions. The origin of the small anisotropies that later grew into the stars and galaxies that we see today is still unknown. However, the nature of the anisotropies in the Cosmic Microwave Background (CMB) provides strong evidence that they were generated long before the CMB radiation had its last interaction with ordinary matter. In the first part of this talk, I will explain how we can use measurements of the CMB, which was last scattered when the universe was 380,000 years old, to reconstruct the detailed physics of much earlier epochs, when the universe was only a tiny fraction of a second old. In the last part of the talk, I will discuss how we can use observations of the CMB and the large-scale structure of the universe to improve our understanding of another open question in fundamental physics. Cosmological observations and galaxy dynamics seem to imply that 84% of all matter in the universe is composed of dark matter, which is not accounted for by the Standard Model of particles. The particle nature of dark matter is one of the most intriguing puzzles of our time. I will identify cosmological processes in which the particle interactions of dark matter are of relevance and show how we can use current and future cosmological data to probe these interactions both at large and small scales.

19 November Garrett Goon “Black Holes, Superluminality and EFT”

Abstract: We expect that any reasonable effective field theory (EFT) should have group velocities smaller than one. Any superluminal theory violating this condition should, at the very least, be heavily scrutinized, if not outright rejected. Yet, EFTs which display superluminality are of current research interest, ranging from the speculative (massive gravity) to the foundational (QED). In this talk I will discuss the diagnosis of superluminalities within the EFT framework and demonstrate how certain EFTs have built-in protection mechanisms against superluminal pathologies, an exploration which will involve fascinating black hole physics.

13 November Shaun Hotchkiss “Simulating relativistic effects in cosmology”

Abstract: General Relativity is believed to be the correct theory of gravitation. Despite this, almost all simulations of the large scale structure of the universe use Newton’s theory of gravitation, with small changes to account for the expansion of the universe. For simple cold dark matter this *may* be acceptable; however it is unlikely to remain true when other relativistic degrees of freedom are considered (e.g. neutrinos or more exotic fields). Moreover, modifications to General Relativity are a popular field of research and many of these modified theories do not have natural “Newtonian” limits. I will discuss a framework for including relativistic effects in the simulation of large scale structure. I will also show how this framework can be applied to spherically symmetric initial conditions where exact relativistic solutions exist. This makes it possible to prove that this framework is accurate in a regime where Newtonian simulations would not be applicable. I will also speculate as to where this framework will be most useful in the future.

5 November Stefano Camera (Manchester): “Innovative Techniques To Test the Fundaments of Cosmology”

Abstract: In this talk, I shall review the innovative techniques that will allow next-generation experiments to tackle some of the most fundamental questions of contemporary cosmology: the nature of gravity, inflation and dark matter. On the one hand, oncoming experiments such as the ESA Euclid satellite or the Square Kilometre Array will, for the first time, open a window onto the largest cosmic scales. Such scales, near or above the horizon, are uncontaminated by the non-linear growth of structure and by baryonic/astrophysics feedback, which is difficult to model. They can teach us about inflation and the physics of the early Universe, and allow us to further confirm or rule out Einstein’s theory of gravity on cosmological distances, as there are relativistic effects which only become detectable on those extremely large scales. On the other hand, novel multi-wavelength synergies will enable us to lift degeneracies in the dark sector. For instance, by cross-correlating the extragalactic gamma-ray background with weak gravitational lensing, we shall be able to disentangle, in the diffuse gamma-ray emission, the tiny contribution due to annihilations or decays of dark matter particles (if they exist) to the overwhelming background of unresolved astrophysical sources.