The Effective Theory of Quintessence: the w<-1 Side Unveiled

We study generic single-field dark energy models, by a parametrization of the most general theory of their perturbations around a given background, including higher derivative terms. In appropriate limits this approach reproduces standard quintessence, k-essence and ghost condensation. We find no general pathology associated to an equation of state w_Q < -1 or in crossing the phantom divide w_Q = -1. Stability requires that the w_Q < -1 side of dark energy behaves, on cosmological scales, as a k-essence fluid with a virtually zero speed of sound. This implies that one should set the speed of sound to zero when comparing with data models with w_Q < -1 or crossing the phantom divide. We summarize the theoretical and stability constraints on the quintessential plane (1+w_Q) vs. speed of sound squared.

Cosmology from high redshift 21 cm observations

Upcoming 21 cm experiments present the possibility of opening a new observational window into the high redshift Universe. Many challenges and opportunities face this new cosmological probe. In this talk, I'll cover some of the possibilities for measuring the neutrino mass and the tilt and running of the primordial power spectrum.

Local non-Gaussianity from inflation: beyond f_NL

fNL has become widely used to quantify the non-Gaussianity of the primordial density perturbation due to inflation, or alternative, models for the origin of structure in the very early universe. fNL is sufficient to describe the primordial bispectrum in some models, notably the curvaton or ekpyrotic models. However other models of local evolution on super-Hubble scales may produce other types of local non-Gaussianity which may have different observational signatures. One can also give more information, which may be testable observationally, including higher-order statistics (such as the trispectrum) or the full probability distribution function.

Phenomenological Connections Between Particle Physics and Dark Energy

I explore how particle physics experiments in the near future may give us a new handle on dark energy.

Slow-roll and DBI inflation with Wilson Lines

I will describe a novel model of inflation in string theory, where the inflaton field is a Wilson line in the worldvolume of a D-brane. In its simplest realisation, this construction yields a slow-roll model with a Coulomb-like potential, giving rise to a nearly flat or red tilted spectrum of (Gaussian) scalar perturbations and negligible gravitational waves. In a warped compactification, however, the inflaton acquires non-canonical kinetic terms of the DBI type, thus allowing for potentially observable non-Gaussianity. In Wilson line DBI inflation, many of the stringent constraints applicable to ordinary (position field) DBI inflation get relaxed, allowing for example a significant amount of primordial tensor perturbations.

Action approach to cosmological perturbations and inflationary trispectrum from graviton exchange

I will present the computation of the connected four-point correlation function of the primordial curvature perturbation generated during slow-roll inflation with standard kinetic terms, where the correlation is established via exchange of a graviton between two pairs of scalar fluctuations. This contribution yields a non-linearity parameter of order tau_NL ~ r, which is one order of magnitude larger than the one previously calculated on the basis of scalar perturbations interacting at a point. Finally, I will show that it satisfies a simple relation in the limit where the momentum of the graviton which is exchanged becomes much smaller than the external momenta.

From alpha (and mu) to omega

I will review the motivation for varying fundamental couplings and discuss how these measurements can be used to constrain fundamental physics scenarios that would otherwise be inaccessible to experiment. I will then focus on the relation between varying couplings and dark eenrgy, and explain how varying coupling measurements might be used to probe the nature of dark energy, with important advantages over standard methods. Assuming that the current (controversial) evidence for varying couplings is correct, a several-sigma detection of dynamical dark energy is feasible within a few years, even using current ground-based facilities. With future instruments like CODEX, a high-accuracy reconstruction of the dark energy equation of state may be possible all the way up to redshift 4.

Did you check your model against the BBN constraints?

Abstract: I'll review the present status of BBN, experimental data on light nuclei and theoretical expectations. I'll then go through several examples to illustrate how BBN can constrain physics beyond the Standard Model.

Moduli stabilization and de Sitter vacua in Supergravity/Superstrings

Supersymmetric theories have scalar and moduli fields that could account for inflation, dark energy and/or varying constants. On the other hand, our world is not supersymmetric, and in string theory and supergravity, the moduli fields that describe the size and shape of the compactied extra dimensions can be destabilized by supersymmetry breaking. I will review some recent supergravity results about the stability of de Sitter vacua, the conditions for slow roll inflation and the consistency -or otherwise- of low energy effective actions (e.g. KKLT-type models) obtained by fixing some of the compactification moduli at their critical points. The talk assumes NO prior knowledge of supergravity.

Scale invariant spectrum and rapidly varying speed of sound

I want to give account of a work recently made in collaboration with Justin Khoury (arXiv:0811.3633). We show that curvature perturbations acquire a scale invariant spectrum for any constant equation of state, provided the fluid has a suitably time-dependent sound speed. In order for modes to exit the physical horizon, and in order to solve the usual problems of standard big bang cosmology, we argue that the only allowed possibilities are inflationary (albeit not necessarily slow-roll) expansion or ekpyrotic contraction. Non-Gaussianities offer many distinguish features. As usual with a small sound speed, non-Gaussianity can be relatively large, around current sensitivity levels. For DBI-like lagrangians, the amplitude is negative in the inflationary branch, and positive in the ekpyrotic branch. Unlike the power spectrum, the three-point amplitude displays strong running, peaking on smallest (largest) scales in the expanding (contracting) case. While the shape is predominantly of the equilateral type in the inflationary branch, as in DBI inflation, it is of the local form in the ekpyrotic branch. The tensor spectrum is also generically far from scale invariant. In the contracting case, for instance, tensors are strongly blue tilted, resulting in an unmeasurably small gravity wave amplitude on cosmic microwave background scales.

CMB anisotropies from acausal scaling seeds

We investigate models where structure formation is initiated by scaling seeds: We consider rapidly expanding relativistic shells of energy and show that they can fit current CMB and large scale structure data if they expand with super-luminal velocities. These acausally expanding shells provide a viable alternative to inflation for cosmological structure formation with the same minimal number of parameters to characterize the initial fluctuations. Causally expanding shells alone cannot fit present data. Hybrid models where causal shells and inflation are mixed also provide good fits.

Constraining the physics of (slow-roll) inflation

After briefly reviewing the phenomenological signatures of inflation, I will discuss the present observational status and give an outlook on what future cosmological probes may be able to reveal about the physics of inflation.

Primordial non-gaussianity from large scale structure

While simple slow-roll inflationary models predict initial conditions extremely close to being Gaussian, recently there has been renewed interest in primordial non-gaussianity, motivated mainly by Cosmic Microwave Background results. I will discuss several was to constrain primordial non gaussianity from observations of large scale structures, which are independent and highly complementary to Cosmic Microwave Background-based tests .

Strange stars: a laboratory to investigate the problem of the cosmological constant.

The well known problem of the cosmological constant can have basically two kinds of solutions: very special initial conditions of the Universe or quite peculiar properties of vacuum energy density. The two possibilities may be tested by observing very compact objects (quark stars or hybrid stars).

Current Constraints and possible pitfalls for parameter estimation in dark energy models with time-varying equation of state

I will discuss the surprising ways our knowledge of cosmological parameters is affected when generalizing dark energy to a time-varying equation of state and/or using distance priors to summarize the information in CMB or large scale structure data.

Delayed Recombination and Cosmic Parameters

Current cosmological constraints from Cosmic Microwave Background (CMB) anisotropies are typically derived assuming a standard recombination scheme, however additional resonance and ionizing radiation sources can delay recombination, altering the cosmic ionization history and the cosmological inferences drawn from CMB data. We show that for recent observations of CMB anisotropy, from the Wilkinson Microwave Anisotropy Probe satellite mission 5-year survey (WMAP5) and from the ACBAR experiment, additional resonance radiation is nearly degenerate with variations in the spectral index, n_s, and has a marked effect on uncertainties in constraints on the Hubble constant, age of the universe, curvature and the upper bound on the neutrino mass.

Lensed CMB from the Millennium Simulation

I will present all-sky CMB temperature and polarization lensed maps based on a high-resolution cosmological N-body simulation, the Millennium Simulation (MS). I have exploited the lensing potential map obtained using a map-making procedure which integrates along the line-of-sight the MS dark matter distribution by stacking and randomizing the simulation boxes up to z=127, and which semi-analytically supplies the large-scale power in the angular lensing potential that is not correctly sampled by the N-body simulation. The lensed sky has been obtained by properly modifying the LensPix code to account for the MS structures. I will also present all-sky lensed maps of the so-called E and B potentials, which are directly related to the electric and magnetic types of polarization. The angular power spectra of the simulated lensed temperature and polarization maps agree well with semi-analytic estimates up to l~2500, while on smaller scales there is a slight excess of power which we interpret as being due to non-linear clustering in the MS. I will also show how non-linear lensing power in the polarised CMB is transferred to large angular scales by suitably misaligned modes in the CMB and the lensing potential. This work is relevant in view of the future CMB probes, as a way to analyse the lensed sky and disentangle the contribution from primordial gravitational waves.

Informal discussion on applying the Halo Model to Large Scale Structure Measurements of the Luminous Red Galaxies

We present a method to reconstruct the halo density field traced by LRGs. We use a large set of N-body simulations to calibrate our model P(k) up to k = 0.2 h/Mpc. We use the halo model as a framework to understand the effects of our uncertainties on the galaxy-dark matter mapping on the recovered cosmological parameters. We present preliminary constraints from the SDSS DR7 LRG P(k) and contrast our method with previous approaches

Gravitino Dark Matter

We will review the scenario of gravitino DM and discuss in detail the two cases of a colored NLSP with R-parity conservation and of a small R-parity violation, which can both help in relaxing some of the Nucleosynthesis constraints.

Strong and Weak Gravitational Lensing as Probes of Dark Matter and Dark Energy

Among the observational tests of the halo density profiles predicted by Cold Dark Matter, strong gravitational lensing in cluster cores is one of the most promising. Recent work on the use of gravitational lensing in cluster cores to constrain the density profile of dark matter will be reviewed and discussed. Possibilities to constrain dark energy models with weak gravitational lensing will also be discussed.

Massive neutrinos and the nonlinear power spectrum

I will present a brief review of the effects of neutrino masses on cosmological observables such as the cosmic microwave background anisotropies and the large-scale structure distribution, as well as the current status of and future prospects for cosmological neutrino mass measurements. I will discuss some recent efforts towards modelling nonlinear effects in the large-scale matter power spectrum in the presence of massive neutrinos.

BAO: photometry vs spectroscopy

We discuss the requirements for a photometric survey using Luminous Red Galaxies (LRG) to competitively measure the radial baryonic acoustic oscillations (BAO) signal. Spectral variability of the LRG substantially worsens the achievable photometric redshift errors. We estimate the etendue times exposure necessary to obtain a high enough density of galaxies to measure radial BAO with sufficient low shot noise up to a given redsift.

The CMBPol Mission Concept Study

I will present some of the conclusions of the CMBPol workshops which took place last yearto study the feasibility and the science benefits of a future satellite mission dedicated to the measurement of the polarization of the CMB. I will review the scientific motivations for such a mission, the systematic effects and show some possible instrumental setups developed for the Experimental Probe for Inflationary Cosmology (EPIC) proposal, a candidate for the future satellite.

Looking for non-Gaussianity: going beyond fNL

I will review the recent evidence for non-Gaussianity in the CMB maps and the techniques by which these measurements are made. Then I will briefly discuss some alternative models for sourcing non-Gaussianity from the early universe.

Clustering in interacting dark energy cosmologies

I will illustrate the case of coupled dark Energy, that is to say cosmologies in which dark energy interacts with other components in the universe as cold dark matter or neutrinos. After briefly presenting the topic, I will in particular focus on the case of 'growing neutrinos': in these models, neutrinos with a mass increasing with time might be driven to cluster at very large scales, due to a new interaction stronger than gravity and mediated by the dark energy scalar field.

Super-inflation in Loop Quantum Cosmology

We will be looking at the cosmological effects of small scale corrections that arise in Loop Quantum Cosmology (LQC). In particular, we will see how the Friedmann equation is modified under inverse volume and quadratic corrections. We will see that a positively curved universe can undergo a period of several expanding and contracting phases during which a scalar field is displaced in its potential and consequently establishing the initial conditions for standard slow-roll inflation. We will also compute the power spectrum of the perturbed scalar field generated during a super-inflationary phase of the universe and show that it can be scale invariant and that the horizon problem is solved with only a few e-folds of super-inflationary expansion. Finally we will see that the tensor spectrum is strongly blue, which means that the abundance of gravitational waves, in these models, is strongly suppressd on the large scales.

Higher inverse derivative gravity

We consider gravitational alternatives to dark energy defined by actions of apparently nonlocal forms with (negative) powers of the d'Alembertian acting on curvature invariants. In particular the effects to cosmological expansion and structure formation will be discussed.

Measuring the galaxy-mass and galaxy-dust correlations through magnification and reddening

We thought the following paper might be interesting arXiv:0902.4240 [ps, pdf, other]

Title: Measuring the galaxy-mass and galaxy-dust correlations through magnification and reddening

Authors: Brice Ménard, Ryan Scranton, Masataka Fukugita, Gordon Richards