All publications

Classification of inflationary models and constraints on fundamental physics

Date of publication: 

2016-11-11 11:00:00

Author: 

Mauro Pieroni

Abstract
This work is focused on the study of early time cosmology and in particular on the study of inflation. After an introduction on the standard Big Bang theory, we discuss the physics of CMB and we explain how its observations can be used to set constraints on cosmological models. We introduce inflation and we carry out its simplest realization by presenting the observables and the experimental constraints that can be set on inflationary models. The possibility of observing primordial gravitational waves (GWs) produced during inflation is discussed. We present the reasons to define a classification of inflationary models and introduce the \beta-function formalism for inflation by explaining why in this framework we can naturally define a set of universality classes for inflationary models. Theoretical motivations to support the formulation of inflation in terms of this formalism are presented. Some generalized models of inflation are introduced and the extension of the \beta-function formalism for inflation to these models is discussed. Finally we focus on the study of models where the (pseudo-scalar) inflaton is coupled to some Abelian gauge fields that can be present during inflation. The analysis of the problem is carried out by using a characterization of inflationary models in terms of their asymptotic behavior. A wide set of theoretical aspects and of observational consequences is discussed.

Consistency of the Planck CMB data and ΛCDM cosmology

Date of publication: 

2016-10-24 12:00:00

Author: 

Arman Shafieloo, Dhiraj Kumar Hazra

We test the consistency between Planck temperature and polarization power spectra and the concordance model of Λ Cold Dark Matter cosmology (ΛCDM) within the framework of Crossing statistics. We find that Planck TT best fit ΛCDM power spectrum is completely consistent with EE power spectrum data while EE best fit ΛCDM power spectrum is not consistent with TT data. However, this does not point to any systematic or model-data discrepancy since in the Planck EE data, uncertainties are much larger compared to the TT data. We also investigate the possibility of any deviation from ΛCDM model analyzing the Planck 2015 data. Results from both TT and EE data analysis indicate that no deviation is required beyond the flexibility of the concordance ΛCDM model. Our analysis thus rules out any strong evidence for beyond the concordance model in the Planck spectra data. We also report a mild amplitude difference comparing temperature and polarization data, where temperature data seems to have slightly lower amplitude than expected (consistently at all multiples), as we assume both temperature and polarization data are realizations of the same underlying cosmology.

Metastable Dark Energy with Radioactive-like Decay

Date of publication: 

2016-10-17 11:00:00

Author: 

Arman Shafieloo, Dhiraj Kumar Hazra, Varun Sahni, Alexei A. Starobinsky

We propose a new class of metastable dark energy (DE) models in which the DE decay rate does not depend on external parameters such as the scale factor or the curvature of the Universe. Instead, the DE decay rate is a function only of the intrinsic properties of DE and, in this sense, resembles the radioactive decay of particles and nuclei. As a consequence, the DE energy density becomes a function of the proper time elapsed since its formation, presumably in the very early Universe. Such a natural type of DE decay can profoundly affect the expansion history of the Universe and its age. Metastable DE can decay in three distinct ways: (i) exponentially, (ii) into dark matter, (iii) into dark radiation. Testing metastable DE models with observational data we find that the decay half-life must be larger than the age of the Universe. Models in which dark energy decays into dark matter lead to lower values of the Hubble parameter at large redshifts relative to ΛCDM. Consequently these models provide a better fit to cosmological BAO data (especially data from from high redshift quasars) than concordance (ΛCDM) cosmology.

General Relativity and Cosmology: Unsolved Questions and Future Directions

Date of publication: 

2016-09-28 14:00:00

Author: 

Ivan Debono, George F. Smoot

For the last 100 years, General Relativity (GR) has taken over the gravitational theory mantle held by Newtonian Gravity for the previous 200 years. This article reviews the status of GR in terms of its self-consistency, completeness, and the evidence provided by observations, which have allowed GR to remain the champion of gravitational theories against several other classes of competing theories. We pay particular attention to the role of GR and gravity in cosmology, one of the areas in which one gravity dominates and new phenomena and effects challenge the orthodoxy. We also review other areas where there are likely conflicts pointing to the need to replace or revise GR to represent correctly observations and consistent theoretical framework. Observations have long been key both to the theoretical liveliness and viability of GR. We conclude with a discussion of the likely developments over the next 100 years.

A Brief History of Gravitational Waves

Date of publication: 

2016-09-26 14:00:00

Author: 

Jorge L. Cervantes-Cota, Salvador Galindo-Uribarri, George F. Smoot

This review describes the discovery of gravitational waves. We recount the journey of predicting and finding those waves, since its beginning in the early twentieth century, their prediction by Einstein in 1916, theoretical and experimental blunders, efforts towards their detection, and finally the subsequent successful discovery.

Direct search for features in the primordial bispectrum

Date of publication: 

2016-07-04 11:00:00

Author: 

Stephen Applebya , Jinn-Ouk Gongb, Dhiraj Kumar Hazrab, Arman Shafielooe, Spyros Sypsasb

 Abstract

We study features in the bispectrum of the primordial curvature perturbation correlated with the reconstructed primordial power spectrum from the observed cosmic microwave background temperature data. We first show how the bispectrum can be completely specified in terms of the power spectrum and its first two derivatives, valid for any configuration of interest. Then using a model-independent reconstruction of the primordial power spectrum from the Planck angular power spectrum of temperature anisotropies, we compute the bispectrum in different triangular configurations. We find that in the squeezed limit at k∼0.06 Mpc−1 and k∼0.014 Mpc−1 there are marginal 2σ deviations from the standard featureless bispectrum, which meanwhile is consistent with the reconstructed bispectrum in the equilateral configuration.

A simple model for the evolution of a non-Abelian cosmic string network

Date of publication: 

2016-06-16 14:00:00

Author: 

G. Cella (INFN, Pisa) , M. Pieroni (APC, Paris & Paris Cent. Cosmol. Phys.)

Abstract (arXiv)

In this paper we present the results of numerical simulations intended to study the behavior of non-Abelian cosmic strings networks. In particular we are interested in discussing the variations in the asymptotic behavior of the system as we variate the number of generators for the topological defects. A simple model which should generate cosmic strings is presented and its lattice discretization is discussed. The evolution of the generated cosmic string networks is then studied for different values for the number of generators for the topological defects. Scaling solution appears to be approached in most cases and we present an argument to justify the lack of scaling for the residual cases.

Sub-Femto-g Free Fall for Space-Based Gravitational Wave Observatories: LISA Pathfinder Results

Date of publication: 

2016-06-07 11:00:00

Author: 

P. Binetruy and 117 more

We report the first results of the LISA Pathfinder in-flight experiment. The results demonstrate that two free-falling reference test masses, such as those needed for a space-based gravitational wave observatory like LISA, can be put in free fall with a relative acceleration noise .....

Primordial features and Planck polarization

Date of publication: 

2016-05-06 10:00:00

Author: 

Dhiraj Kumar Hazra, Arman Shafieloo, George F. Smoot, Alexei A. Starobinsky

With the Planck 2015 Cosmic Microwave Background (CMB) temperature and polarization data, we examine possibility of having features in the primordial power spectrum (PPS). We revisit the Wiggly Whipped Inflation (WWI) framework and demonstrate how generation of some particular primordial features can improve the fit to Planck data. WWI potential allows the scalar field to transit from a steeper potential to a nearly flat potential through a discontinuity either in potential or in its derivatives. Using Planck 2015 data, we constrain the primordial features in the context of Wiggly Whipped Inflation and present the features that are supported both by temperature and polarization. WWI model provides upto ∼12−14 improvement in χ2 fit to the data with respect to the best fit power law model considering combined temperature and polarization data from Planck and B-mode polarization data from BICEP and Planck dust map. We use 2-4 extra parameters in the WWI model compared to the featureless strict slow roll inflaton potential. WWI offers a wide variety of features in the primordial power spectra incorporating a number of models discussed in the literature that can address localized and non-local features in the temperature and polarization angular power spectrum. We find that the difference between the temperature and polarization data in constraining background cosmological parameters such as baryon density, cold dark matter density are reduced to a good extent if we use primordial power spectra from WWI. We also discuss the extent of bispectra obtained from the best potentials in arbitrary triangular configurations using the BI-spectra and Non-Gaussianity Operator (BINGO).

Primordial gravitational waves for universality classes of pseudoscalar inflation

Date of publication: 

2016-03-03 14:00:00

Author: 

Valerie Domcke, Mauro Pieroni, Pierre Binétruy

Abstract

Current bounds from the polarization of the CMB predict the scale-invariant gravitational wave (GW) background of inflation to be out of reach for upcoming GW interferometers. This prospect dramatically changes if the inflaton is a pseudoscalar, in which case its generic coupling to any abelian gauge field provides a new source of GWs, directly related to the dynamics of inflation. This opens up new ways of probing the scalar potential responsible for cosmic inflation. Dividing inflation models into universality classes, we analyze the possible observational signatures. One of the most promising scenarios is Starobinsky inflation, which may lead to observational signatures both in direct GW detection as well as in upcoming CMB detectors. In this case, the complementarity between the CMB and direct GW detection, as well as the possibility of a multi-frequency analysis with upcoming ground and space based GW interferometers, may provide a first clue to the microphysics of inflation.

Pages

Connect with us

We're on Social Networks. Follow us & get in touch.