Cosmology and Gravitation

I will discuss phenomena associated with particle production and field excitation during inflation. In the first part of the talk I will present several
signatures that can originate from the coupling of an axion inflaton to gauge fields. In the second part I will test the robustness of the standard
implications associated with the detection of a gravity wave (GW) signal at CMB scales, by discussing what conditions can allow a visible
sourced GW background that exceeds the vacuum one (without simultaneously overproduce scalar perturbations), and by providing a concrete
example. In the final part I will review the phenomenology of trapped inflation, which is an interesting and minimal mechanism where particle production
provides the main friction for the motion of the inflaton.

Alternative theories of gravity are popular alternatives to the LCDM model because they can self-accelerate without a cosmological constant. On smaller scales, consistency with solar system tests of gravity is achieved by utilising screening mechanisms, which act to hide fifth-forces locally. This makes them difficult to distinguish from general relativity. In this talk I will describe recent work using astrophysical objects---stars, galaxies, and clusters---as new and novel probes of alternative gravity theories.

The kinetic Sunyaev-Zel'dovich effect is a direct probe of the distribution and velocity of electrons on cosmological scales.  Recent progress in Cosmic Microwave Background observations allow statistical detections of this subtle effect originating from a number of different tracers populations.  In my talk, I will review the observational status, highlight the consequences for astrophysics and cosmology and discuss future directions.

The global redshifted 21-cm radiation background is expected to be a powerful probe of the re-heating and re-ionization of the intergalactic medium. However, its measurement is technically challenging: one must extract the small, frequency-dependent signal from under much brighter and spectrally smooth foregrounds. Traditional approaches to study the global signal have used single-antenna systems, where one must calibrate out frequency-dependent structure in the overall system gain, as well as remove the noise bias from auto-correlating a single amplifier output. I will review these approaches, and critically examine several recent proposals to measure the global background using interferometric setups. In particular, using very general principles, I will show that the latters' sensitivity is directly related to two characteristics: the cross-talk between the readout channels (i.e. the signal picked up at one antenna when the other one is driven) and the correlated noise due to thermal fluctuations of lossy elements (e.g. absorbers or the ground) radiating into both channels. I will also briefly discuss the implications and future prospects for interferometric methods.

The Atacama Cosmology Telescope (ACT) has been pushing our measurements of the Cosmic Microwave Background on small scales to high resolution and deeper sensitivity since 2008. While ACT stopped taking temperature-only measurements in 2010, ACTPol is now operating with polarisation-sensitive detectors. I will present some of the current ACTPol results in terms of the power spectrum constraints. In addition, I'll highlight some of our recent results using ACTPol data, including detections of both the thermal and the kinetic Sunyaev- Zel'dovich effects through cross-correlation with other probes. I'll discuss the lensing of the small-scale microwave sky as measured by ACT and discus how cross-correlating CMB lensing with other probes gives us a handle on galaxy properties like bias. Finally, I'll discuss the next stage in small-scale CMB cosmology: Advanced ACTPol, which will improve our understanding of the reionisation of the universe - arguably our least constrained epoch to date.

One possibility for studying reheating is to link the duration and final temperature after reheating, and its equation of state, to inflationary observables. By restricting the equation of state to lie within a broad physically allowed range, one can bracket an allowed range of $n_s$ and $r$ for models of inflation. The results are similar to, but do a little better, than requiring the length of inflation lie between 50 and 60 efolds. The added constraints can help break degeneracies between inflation models that otherwise overlap in their predictions.

According to the standard model of cosmology 96% of the matter and energy in the universe is invisible. The dark matter particles comprising the invisible material have so far not been detected in laboratory and astrophysical experiments.  The dark energy responsible for the acceleration of the universe is still a controversial issue. A modified gravity theory is presented that can potentially fit current cosmological and astrophysical data. The black holes and their shadows predicted by MOG can differ from the predictions of Einstein gravity.

Fast radio bursts (FRBs) are bright, broadband, non-repeating, millisecond flashes of unknown astronomical origin. The dispersion of these bursts by intervening plasma suggests that the sources of the 16 bursts reported to date= are at 0.2<z<1. I will discuss the possibility of using dispersion, instead of redshift, to study the large-scale structure of the Universe in three dimensions. Like redshift, which is distorted by peculiar velocities, dispersion is an imperfect proxy for distance as it is distorted by inhomogeneities in the electron density. These dispersion-space distortions are calculable and actually greatly enhance the signal. The clustering signal in dispersion space could be detected in a survey of 10 000 FRBs, as is expected to be produced by the CHIME telescope over three years.

The greatest uncertainty in whether this technique will be successful is the unknown nature of FRB sources. A new observation tells us more about the environment of a source than ever before through the polarization and scattering properties of a burst. More observations of this type, along with observations that identify host galaxies, will soon tell whether FRBs will provide a new probe of structure.

In this talk I derive the evolution equations for two scalar fields with non-canonical field space metric up to third order in perturbation theory, employing the covariant formalism. These equations can be used to calculate the local bi- and trispectra of the non-minimal ekpyrotic model. Remarkably, the nearly scale-invariant entropy perturbations have vanishing bi- and trispectra during the ekpyrotic phase. However, an efficient conversion process to curvature perturbations induces local non-Gaussianity parameters f_NL and g_NL at levels that should be detectable by near-future observations.
In the second part of the talk I construct a new kind of cosmological model – conflation. The universe undergoes accelerated expansion, but with crucial differences compared to ordinary inflation. In particular, the potential energy is negative, which is of interest for supergravity and string theory where both negative potentials and the required scalar-tensor couplings are rather natural. A distinguishing feature of the model is that it does not amplify adiabatic scalar and tensor fluctuations, and in particular does not lead to eternal inflation and the associated infinities.

In light of the upcoming Generation 2 (G2) direct-detection experiments attempting to record dark matter scattering with nuclei in underground detectors, it is timely to inquire about their ability to single out the correct theory of dark-matter-baryon interactions, in case a signal is observed. I will present a recent study in which we perform statistical analysis of a large set of direct-detection simulations, covering a wide variety of operators that describe scattering of fermionic dark matter with nuclei. I will show that a strong signal on G2 xenon and germanium targets has enough discrimination power to reconstruct the momentum dependence of the interaction, ruling out entire classes of models. However, zeroing in on a correct UV completion will critically depend on the availability of measurements from a wide variety of nuclear targets (including iodine and fluorine) and on the availability of low energy thresholds. This study quantifies complementarity amongst different experimental designs and targets, and provides a roadmap for future data analyses. It also highlights the critical need for bringing in information from all available probes in dark matter studies.

With the completion of the Planck satellite, in order to continue collecting cosmological information it is
important to gain a precise understanding of the formation of Large Scale Structures (LSS) of the universe.
The Effective Field Theory of LSS (EFTofLSS) offers a consistent theoretical framework that aims to develop
an analytic understanding of LSS at long distances, where inhomogeneities are small. We present the recent
developments in the field covering topics of biased tracers in the EFTofLSS including the effects of baryonic
physics and primordial non-Gaussianities, finding that new bias coefficients are required. We discuss the EFT
framework for dark matter clustering in Lagrangian formalism and present its consequences on baryon acoustic
oscillations (BAO). We present analytic results and compare them with the output of N-body simulations.

We argue that theories with multiple axions generically contain a large
number of vacua that can account for the smallness of the cosmological
constant. In a theory with N axions, the dominant instantons with charges Q
determine the discrete symmetry of vacua. Subleading instantons break the
leading periodicity and lift the vacuum degeneracy. For generic integer charges
the number of distinct vacua is given by |det(Q)|~exp(N). Our construction
motivates the existence of a landscape with a vast number of vacua in
four-dimensional effective theories.