Initial conditions for cosmological perturbations in scalar-tensor dark-energy models

We discuss the evolution and imprints of isocurvature initial conditions for the scalar field in scalar tensor extensions of Einstein gravity. We consider the simplest models of scalar tensor theories, as Induced gravity (IG, which can be recasted in form of Jordan-Brans-Dicke theory by a redefinition of the scalar field) or non-minimally coupled (NMC) scalar fields in which the acceleration of the Universe is connected to a variation of the effective Planck mass. After introducing the fundamental ideas of cosmological perturbation theory and scalar tensor theories of gravity, we give the evolution equations for matter, metric and scalar field fluctuations in synchronous gauge. We use this set of equations for both the IG and the NMC models to find a new isocurvature solution in which the scalar field fluctuations compensate for the relativistic components respectively. We also show how we can generalize the well known isocurvature modes in Einstein GR to these models. We show the different evolution of cosmological fluctuations for these isocurvature initial conditions compared to the standard adiabatic one. After that, we compute the CMB angular power spectrum for these solutions in the IG model, with the help of a modified Einstein-Boltzmann CLASS code. In particular the CMB power spectrum is computed separately for adiabatic and isocurvature initial conditions, i.e. for totally uncorrelated modes, and with arbitrary correlations leading to an interesting explanation of the lack of power in the low multipoles region of the CMB temperature power spectrum. Finally we show how a simple model of double inflation in IG could explain the generation of the new isocurvature mode

Questions on calculation of primordial power spectrum with large spikes: the resonance model case

Inflationary models predicting a scale-dependent large amplification of the density perturbations have recently attracted a lot of attention because the amplified perturbations can seed a sizable amount of primordial black holes (PBHs) and stochastic background of gravitational waves (GWs). While the power spectra in these models are computed based on the linear equation of motion, it is not obvious whether loop corrections are negligible when such a large amplification occurs during inflation. In this paper, as a first step to discuss the loop corrections in such models, we use the in-in formalism and calculate the one-loop scalar power spectrum numerically and analytically in an illustrative model where the density perturbations are resonantly amplified due to oscillatory features in the inflaton potential. Our calculation is technically new in that the amplified perturbations are numerically taken into account in the in-in formalism for the first time. In arriving at our analytical estimates, we highlight the role that the Wronskian condition of perturbations, automatically satisfied in our model, plays in obtaining the correct estimates. In addition, the analytical estimates show that the contribution originating from the quantum nature of the perturbations in the loop can be dominant. We also discuss the necessary conditions for subdominant loop corrections in this model. We find that, for the typical parameter space leading to the $\mathcal O(10^7)$ amplification of the power spectrum required for a sufficient PBH production, the one-loop power spectrum dominates over the tree-level one, indicating the breakdown of the perturbation theory.Comment: 42 pages, 15 figure

Scalar-tensor theories in light of cosmological tensions

In this PhD thesis, I study cosmologies within the simplest scalar-tensor theories of gravity consisting in a scalar field $\sigma$ non-minimally coupled to the Ricci scalar through a function $F(\sigma)$ that induces a time-variation in the Newton constant and a potential $V(\sigma)$. I explore the new physics in these cosmologies and use publicly available data to constrain them. Depending on the functional form of the non-minimal coupling and the potential, the cosmological dynamics changes significantly. For some of the models, the specific dynamics helps recover the consistency with very stringent tests of General Relativity from Solar System and laboratory experiments without the need of any screening mechanisms. When compared to publicly available data, all these models feature a value of the Hubble constant $H_0$ larger than the standard $\Lambda$CDM cosmology. This makes scalar-tensor theories one of the most interesting candidates to solve the $H_0$ tension which is becoming one of the most pressing questions in the post-Planck cosmology. In order to better characterize the phenomenology of scalar-tensor theories, I also investigate their degeneracy with parameters describing the physics of neutrinos. I show that bounds on the effective number of active neutrinos and their masses are slightly relaxed in this context, although they are only a very weakly degenerate with the modification to gravity studied in this thesis and the full inclusion of CMB and LSS data used here. Finally, I address the issue of initial conditions within these theories and present a new regular isocurvature mode connected with the variation of the Newton constant which is absent in Einstein gravity. Although the observational imprints are different, the allowed fraction of this mode, relative to the adiabatic one, is constrained by Cosmic Microwave Background data at a similar level to other known isocurvature modes

Uncovering the History of Cosmic Inflation from Anomalies in Cosmic Microwave Background Spectra

We propose an inflationary primordial feature model that can explain both the large and small-scale anomalies in the currently measured cosmic microwave background (CMB) anisotropy spectra, revealing a clip of adventurous history of the Universe during its primordial epoch. Although the model is currently statistically indistinguishable from the Standard Model, we show that future observations such as the Simons Observatory and LiteBIRD will complement each other in distinguishing the model differences due to their accurate E-mode polarization measurements, and the PICO mission, if funded, can put stringent constraints on all characteristic properties. The model predicts a signal of classical primordial standard clock, which can also be used to distinguish the inflation and alternative scenarios in a model-independent fashion.Comment: 8 pages, 4 figure

Hybrid α\alpha-attractors, primordial black holes and gravitational wave backgrounds

We investigate the two-stage inflation regime in the theory of hybrid cosmological $\alpha$-attractors. The spectrum of inflationary perturbations is compatible with the latest Planck/BICEP/Keck results, thanks to the attractor properties of the model. However, at smaller scales, it may have a very high peak of controllable width and position, leading to a copious production of primordial black holes (PBH) and generation of a stochastic background of gravitational waves (SGWB).Comment: 39 pages, 12 figure

Tracking the origin of black holes with the stochastic gravitational wave background popcorn signal

This is a pre-copyedited, author-produced PDF of an article accepted for publication in Monthly Notices of the Royal Astronomical Society following peer review. The version of record Monthly Notices of the Royal Astronomical Society 519.4 (2023): 6008-6019 is available online at: https://academic.oup.com/mnras/article-abstract/519/4/6008/6985653Unresolved sources of gravitational waves (GWs) produced by the merger of a binary of black holes at cosmological distances combine into a stochastic background. Such a background is in the continuous or popcorn regime, depending on whether the GW rate is high enough so that two or more events overlap in the same frequency band. These two regimes respectively correspond to large and small values of the so-called duty cycle. We study the detection regime of the background in models of primordial black holes (PBHs) and compare it to the one produced by black holes of stellar origin. Focusing on ground-based detectors, we show that the duty cycle of the PBH-origin background is larger than that of astrophysical black holes because of differences in their mass function and the merger rate. Our study opens up the possibility to learn about the primordial or astrophysical nature of black hole populations by examining the statistical properties of the stochastic backgroundThis work is supported by the Spanish Research Projects PGC2018-094773-B-C32 (MINECO) and PID2021-123012NB-C43 (MICINN-FEDER) and the Centro de Excelencia Severo Ochoa Program CEX2020-001007-S. MB and SK are supported by the Spanish Atracción de Talento contract no. 2019-T1/TIC-13177 granted by Comunidad de Madrid, the I+D grant PID2020-118159GA-C42 of the Spanish Ministry of Science and Innovation and the i-LINK 2021 grant LINKA20416 of CSIC. SK is partially supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant no. 20H01899 and 20H0585