arXiv subjects

Jerome Martin

Publications and source records attributed to Jerome Martin.

At least 37 records · Page 2Linked to original sources

arXiv2020

Primordial black holes from the preheating instability

After the end of inflation, the inflaton field oscillates around a local minimum of its potential and decays into ordinary matter. These oscillations trigger a resonant instability for cosmological perturbations with wavelengths that exit the Hubble radius close to the end of inflation. In this paper, we study the formation of Primordial Black Holes (PBHs) at these enhanced scales. We find that the production mechanism can be so efficient that PBHs subsequently dominate the content of the universe and reheating proceeds from their evaporation. Observational constraints on the PBH abundance also restrict the duration of the resonant instability phase, leading to tight limits on the reheating temperature that we derive. We conclude that the production of PBHs during reheating is a generic and inevitable property of the simplest inflationary models, and does not require any fine tuning of the inflationary potential.

astro-ph.CO↗

arXiv2019

Inflation after Planck: Judgment Day

Inflation is considered as the best theory of the early universe by a very large fraction of cosmologists. However, the validity of a scientific model is not decided by counting the number of its supporters and, therefore, this dominance cannot be taken as a proof of its correctness. Throughout its history, many criticisms have been put forward against inflation. The final publication of the Planck Cosmic Microwave Background data represents a benchmark time to study their relevance and to decide whether inflation really deserves its supremacy. In this paper, we categorize the criticisms against inflation, go through all of them in the light of what is now observationally known about the early universe, and try to infer and assess the scientific status of inflation. Although we find that important questions still remain open, we conclude that the inflationary paradigm is not in trouble but, on the contrary, has rather been strengthened by the Planck data.

astro-ph.CO↗

arXiv2019

Obstructions to Bell CMB Experiments

We present a general and systematic study of how a Bell experiment on the cosmic microwave background could be carried out. We introduce different classes of pseudo-spin operators and show that, if the system is placed in a two-mode squeezed state as inflation predicts, they all lead to a violation of the Bell inequality. However, we also discuss the obstacles that one faces in order to realize this program in practice and show that they are probably insurmountable. We suggest alternative methods that would reveal the quantum origin of cosmological structures without relying on Bell experiments.

astro-ph.CO↗

arXiv2019

Cosmic Inflation, Quantum Information and the Pioneering Role of John S Bell in Cosmology

According to the theory of cosmic inflation, the large scale structures observed in our Universe (galaxies, clusters of galaxies, Cosmic Background Microwave - CMB - anisotropy ...) are of quantum mechanical origin. They are nothing but vacuum fluctuations, stretched to cosmological scales by the cosmic expansion and amplified by gravitational instability. At the end of inflation, these perturbations are placed in a two-mode squeezed state with the strongest squeezing ever produced in Nature (much larger than anything that can be made in the laboratory on Earth). This article studies whether astrophysical observations could unambiguously reveal this quantum origin by borrowing ideas from quantum information theory. It is argued that some of the tools needed to carry out this task have been discussed long ago by J. Bell in a, so far, largely unrecognized contribution. A detailed study of his paper and of the criticisms that have been put forward against his work is presented. Although J. Bell could not have realized it when he wrote his letter since the quantum state of cosmological perturbations was not yet fully characterized at that time, it is also shown that Cosmology and cosmic inflation represent the most interesting frameworks to apply the concepts he investigated. This confirms that cosmic inflation is not only a successful paradigm to understand the early Universe. It is also the only situation in Physics where one crucially needs General Relativity and Quantum Mechanics to derive the predictions of a theory and, where, at the same time, we have high-accuracy data to test these predictions, making inflation a playground of utmost importance to discuss foundational issues in Quantum Mechanics

quant-ph↗

arXiv2019

Cosmic Inflation: Trick or Treat?

Discovered almost forty years ago, inflation has become the leading paradigm for the early universe. Originally invented to avoid the fine-tuning puzzles of the standard model of cosmology, the so-called hot Big Bang phase, inflation has always been the subject of intense debates. In this article, after a brief review of the theoretical and observational status of inflation, we discuss the criticisms that have been expressed against it and attempt to assess whether inflation can really be viewed as a successful solution to the above mentioned issues.

astro-ph.CO↗

arXiv2018

The Theory of Inflation

This article contains a concise review of the theory of inflation. We discuss its main theoretical aspects as well as its observational predictions. We also explain how the most recent astrophysical observations constrain the inflationary scenario.

astro-ph.CO↗

arXiv2018

Non Gaussianities from Quantum Decoherence during Inflation

Inflationary cosmological perturbations of quantum-mechanical origin generically interact with all degrees of freedom present in the early Universe. Therefore, they must be viewed as an open quantum system in interaction with an environment. This implies that, under some conditions, decoherence can take place. The presence of the environment can also induce modifications in the power spectrum, thus offering an observational probe of cosmic decoherence. Here, we demonstrate that this also leads to non Gaussianities that we calculate using the Lindblad equation formalism. We show that, while the bispectrum remains zero, the four-point correlation functions become non-vanishing. Using the Cosmic Microwave Background measurements of the trispectrum by the Planck satellite, we derive constraints on the strength of the interaction between the perturbations and the environment and show that, in some regimes, they are more stringent than those arising from the power spectrum.

astro-ph.CO↗

arXiv2017

Shortcomings of New Parametrizations of Inflation

In the hope of avoiding model dependence of the cosmological observables, phenomenological parametrizations of Cosmic Inflation have recently been proposed. Typically, they are expressed in terms of two parameters associated with an expansion of the inflationary quantities matching the belief that inflation is characterized by two numbers only, the tensor-to-scalar ratio and the scalar spectral index. We give different arguments and examples showing that these new approaches are either not generic or insufficient to make predictions at the accuracy level needed by the cosmological data. We conclude that disconnecting inflation from high energy physics and gravity might not be the most promising way to learn about the physics of the early Universe.

astro-ph.CO↗

arXiv2016

Information Gain on Reheating: the One Bit Milestone

We show that the Planck 2015 and BICEP2/KECK measurements of the Cosmic Microwave Background (CMB) anisotropies provide together an information gain of 0.82 +- 0.13 bits on the reheating history over all slow-roll single-field models of inflation. This corresponds to a 40% improvement compared to the Planck 2013 constraints on the reheating. Our method relies on an exhaustive CMB data analysis performed over nearly 200 models of inflation to derive the Kullback-Leibler entropy between the prior and the fully marginalized posterior of the reheating parameter. This number is a weighted average by the Bayesian evidence of each model to explain the data thereby ensuring its fairness and robustness.

astro-ph.CO↗

arXiv2016

Bell's Inequalities for Continuous-Variable Systems in Generic Squeezed States

Bell's inequality for continuous-variable bipartite systems is studied. The inequality is expressed in terms of pseudo-spin operators and quantum expectation values are calculated for generic two-mode squeezed states characterized by a squeezing parameter $r$ and a squeezing angle $φ$. Allowing for generic values of the squeezing angle is especially relevant when $φ$ is not under experimental control, such as in cosmic inflation, where small quantum fluctuations in the early Universe are responsible for structures formation. Compared to previous studies restricted to $φ=0$ and to a fixed orientation of the pseudo-spin operators, allowing for $φ\neq 0$ and optimizing the angular configuration leads to a completely new and rich phenomenology. Two dual schemes of approximation are designed that allow for comprehensive exploration of the squeezing parameters space. In particular, it is found that Bell's inequality can be violated when the squeezing parameter $r$ is large enough, $r\gtrsim 1.12$, and the squeezing angle $φ$ is small enough, $φ\lesssim 0.34\,e^{-r}$.

quant-ph↗

arXiv2015

Observing the Inflationary Reheating

Reheating is the the epoch which connects inflation to the subsequent hot Big-Bang phase. Conceptually very important, this era is however observationally poorly known. We show that the current Planck satellite measurements of the Cosmic Microwave Background (CMB) anisotropies constrain the kinematic properties of the reheating era for most of the inflationary models. This result is obtained by deriving the marginalized posterior distributions of the reheating parameter for about 200 models taken in Encyclopaedia Inflationaris. Weighted by the statistical evidence of each model to explain the data, we show that the Planck 2013 measurements induce an average reduction of the posterior-to-prior volume by 40%. Making some additional assumptions on reheating, such as specifying a mean equation of state parameter, or focusing the analysis on peculiar scenarios, can enhance or reduce this constraint. Our study also indicates that the Bayesian evidence of a model can substantially be affected by the reheating properties. The precision of the current CMB data is therefore such that estimating the observational performance of a model now requires to incorporate information about its reheating history.

astro-ph.CO↗

arXiv2015

The Observational Status of Cosmic Inflation after Planck

The observational status of inflation after the Planck 2013 and 2015 results and the BICEP2/Keck Array and Planck joint analysis is discussed. These pedagogical lecture notes are intended to serve as a technical guide filling the gap between the theoretical articles on inflation and the experimental works on astrophysical and cosmological data. After a short discussion of the central tenets at the basis of inflation (negative self-gravitating pressure) and its experimental verifications, it reviews how the most recent Cosmic Microwave Background (CMB) anisotropy measurements constrain cosmic inflation. The fact that vanilla inflationary models are, so far, preferred by the observations is discussed and the reason why plateau-like potential versions of inflation are favored within this subclass of scenarios is explained. Finally, how well the future measurements, in particular of $B$-Mode CMB polarization or primordial gravity waves, will help to improve our knowledge about inflation is also investigated.

astro-ph.CO↗

arXiv2014

How Well Can Future CMB Missions Constrain Cosmic Inflation?

We study how the next generation of Cosmic Microwave Background (CMB) measurement missions (such as EPIC, LiteBIRD, PRISM and COrE) will be able to constrain the inflationary landscape in the hardest to disambiguate situation in which inflation is simply described by single-field slow-roll scenarios. Considering the proposed PRISM and LiteBIRD satellite designs, we simulate mock data corresponding to five different fiducial models having values of the tensor-to-scalar ratio ranging from $10^{-1}$ down to $10^{-7}$. We then compute the Bayesian evidences and complexities of all Encyclopaedia Inflationaris models in order to assess the constraining power of PRISM alone and LiteBIRD complemented with the Planck 2013 data. Within slow-roll inflation, both designs have comparable constraining power and can rule out about three quarters of the inflationary scenarios, compared to one third for Planck 2013 data alone. However, we also show that PRISM can constrain the scalar running and has the capability to detect a violation of slow roll at second order. Finally, our results suggest that describing an inflationary model by its potential shape only, without specifying a reheating temperature, will no longer be possible given the accuracy level reached by the future CMB missions.

astro-ph.CO↗

arXiv2014

Compatibility of Planck and BICEP2 in the Light of Inflation

We investigate the implications for inflation of the detection of B-modes polarization in the Cosmic Microwave Background (CMB) by BICEP2. We show that the hypothesis of primordial origin of the measurement is only favored by the first four bandpowers, while the others would prefer unreasonably large values of the tensor-to-scalar ratio. Using only those four bandpowers, we carry out a complete analysis in the cosmological and inflationary slow-roll parameter space using the BICEP2 polarization measurements alone and extract the Bayesian evidences and complexities for all the Encyclopaedia Inflationaris models. This allows us to determine the most probable and simplest BICEP2 inflationary scenarios. Although this list contains the simplest monomial potentials, it also includes many other scenarios, suggesting that focusing model building efforts on large field models only is unjustified at this stage. We demonstrate that the sets of inflationary models preferred by Planck alone and BICEP2 alone are almost disjoint, indicating a clear tension between the two data sets. We address this tension with a Bayesian measure of compatibility between BICEP2 and Planck. We find that for models favored by Planck the two data sets tend to be incompatible, whereas there is a moderate evidence of compatibility for the BICEP2 preferred models. As a result, it would be premature to draw any conclusion on the best Planck models, such as Starobinsky and/or Kahler moduli inflation. For the subset of scenarios not exhibiting data sets incompatibility, we update the evidences and complexities using both data sets together.

astro-ph.CO↗

arXiv2014

Sharp inflaton potentials and bi-spectra: Effects of smoothening the discontinuity

Sharp shapes in the inflaton potentials often lead to short departures from slow roll which, in turn, result in deviations from scale invariance in the scalar power spectrum. Typically, in such situations, the scalar power spectrum exhibits a burst of features associated with modes that leave the Hubble radius either immediately before or during the epoch of fast roll. Moreover, one also finds that the power spectrum turns scale invariant at smaller scales corresponding to modes that leave the Hubble radius at later stages, when slow roll has been restored. In other words, the imprints of brief departures from slow roll, arising out of sharp shapes in the inflaton potential, are usually of a finite width in the scalar power spectrum. Intuitively, one may imagine that the scalar bi-spectrum too may exhibit a similar behavior, i.e. a restoration of scale invariance at small scales, when slow roll has been reestablished. However, in the case of the Starobinsky model (viz. the model described by a linear inflaton potential with a sudden change in its slope) involving the canonical scalar field, it has been found that, a rather sharp, though short, departure from slow roll can leave a lasting and significant imprint on the bi-spectrum. The bi-spectrum in this case is found to grow linearly with the wavenumber at small scales, a behavior which is clearly unphysical. In this work, we study the effects of smoothening the discontinuity in the Starobinsky model on the scalar bi-spectrum. Focusing on the equilateral limit, we analytically show that, for smoother potentials, the bi-spectrum indeed turns scale invariant at suitably large wavenumbers. We also confirm the analytical results numerically using our newly developed code BINGO. We conclude with a few comments on certain related points.

astro-ph.CO↗

arXiv2014

The Best Inflationary Models After Planck

We compute the Bayesian evidence and complexity of 193 slow-roll single-field models of inflation using the Planck 2013 Cosmic Microwave Background data, with the aim of establishing which models are favoured from a Bayesian perspective. Our calculations employ a new numerical pipeline interfacing an inflationary effective likelihood with the slow-roll library ASPIC and the nested sampling algorithm MULTINEST. The models considered represent a complete and systematic scan of the entire landscape of inflationary scenarios proposed so far. Our analysis singles out the most probable models (from an Occam's razor point of view) that are compatible with Planck data, while ruling out with very strong evidence 34% of the models considered. We identify 26% of the models that are favoured by the Bayesian evidence, corresponding to 15 different potential shapes. If the Bayesian complexity is included in the analysis, only 9% of the models are preferred, corresponding to only 9 different potential shapes. These shapes are all of the plateau type.

astro-ph.CO↗

arXiv2013

Superimposed Oscillations in Brane Inflation

In canonical scalar field inflation, the Starobinsky model (with a linear potential but discontinuous slope) is remarkable in that though slow-roll is violated, both the power-spectrum and bi-spectrum can be calculated exactly analytically. The two-point function is characterised by different power on large and small scales, and a burst of small amplitude superimposed oscillations in between. Motivated by string-theory models with stuck branes, we extend this analysis to Dirac Born Infeld (DBI) inflation, for which generalised slow-roll is violated at the discontinuity and a rapid variation in the speed of sound c_s occurs. In an attempt to characterise the effect of non-linear kinetic terms on the oscillatory features of the primordial power-spectrum, we show that the resulting power spectrum has a shape and features which differ significantly from those of the standard Starobinsky model. In particular, the power-spectrum now takes very similar scale invariant values on large and small scales, while on intermediate scales it is characterised by much larger amplitude and higher frequency superimposed oscillations. We also show that calculating non-Gaussianities in this model is a complicated but interesting task since all terms in the cubic action now contribute. Investigating whether the superimposed oscillations could fit to the Planck Cosmic Microwave Background (CMB) data (for instance by explaining the large scale Planck anomalies) with, at the same time, small non-Gaussianities remains an intriguing and open possibility.

hep-th↗

arXiv2013

Inflation after Planck: and the winners are

We review the constraints that the recently released Cosmic Microwave Background (CMB) Planck data put on inflation and we argue that single field slow-roll inflationary scenarios (with minimal kinetic term) are favored. Then, within this class of models, by means of Bayesian inference, we show how one can rank the scenarios according to their performances, leading to the identification of ``the best models of inflation''.

astro-ph.CO↗