-New article published 4 days ago : Impact of the small-scale structure on the Stochastic Background of Gravitational Waves from cosmic strings !

Pierre Auclair

PhD candidate at Laboratoire Astroparticule et Cosmologie (APC), Paris

About me

  • Study cosmology and the early Universe
  • Specialized in topological defects, first order phase transitions and primordial black-holes
  • Passionate about computer sciences
  • Grew up in the Alps, loves hiking and rock climbing.
  • Member of the Virgo collaboration and LISA consortium

Publication list


Probing the gravitational wave background from cosmic strings with LISA

Cosmic string networks offer one of the best prospects for detection of cosmological gravitational waves (GWs). The combined incoherent GW emission of a large number of string loops leads to a stochastic GW background (SGWB), which encodes the properties of the string network. In this paper we analyze the ability of the Laser Interferometer Space Antenna (LISA) to measure this background, considering leading models of the string networks. We find that LISA will be able to probe cosmic strings with tensions Gμ &gtrsim 𝒪(10−17), improving by about 6 orders of magnitude current pulsar timing arrays (PTA) constraints, and potentially 3 orders of magnitude with respect to expected constraints from next generation PTA observatories. We include in our analysis possible modifications of the SGWB spectrum due to different hypotheses regarding cosmic history and the underlying physics of the string network. These include possible modifications in the SGWB spectrum due to changes in the number of relativistic degrees of freedom in the early Universe, the presence of a non-standard equation of state before the onset of radiation domination, or changes to the network dynamics due to a string inter-commutation probability less than unity. In the event of a detection, LISA's frequency band is well-positioned to probe such cosmic events. Our results constitute a thorough exploration of the cosmic string science that will be accessible to LISA.

The missing link in gravitational-wave astronomy: discoveries waiting in the decihertz range

The gravitational-wave astronomical revolution began in 2015 with LIGO’s observation of the coalescence of two stellar-mass black holes. Over the coming decades, ground-based detectors like laser interferometer gravitational-wave observatory (LIGO), Virgo and KAGRA will extend their reach, discovering thousands of stellar-mass binaries. In the 2030s, the space-based laser interferometer space antenna (LISA) will enable gravitational-wave observations of the massive black holes in galactic centres. Between ground-based observatories and LISA lies the unexplored dHz gravitational-wave frequency band. Here, we show the potential of a decihertz observatory (DO) which could cover this band, and complement discoveries made by other gravitational-wave observatories. The dHz range is uniquely suited to observation of intermediate-mass (∼102–104 M ⊙) black holes, which may form the missing link between stellar-mass and massive black holes, offering an opportunity to measure their properties. DOs will be able to detect stellar-mass binaries days to years before they merge and are observed by ground-based detectors, providing early warning of nearby binary neutron star mergers, and enabling measurements of the eccentricity of binary black holes, providing revealing insights into their formation. Observing dHz gravitational-waves also opens the possibility of testing fundamental physics in a new laboratory, permitting unique tests of general relativity (GR) and the standard model of particle physics. Overall, a DO would answer outstanding questions about how black holes form and evolve across cosmic time, open new avenues for multimessenger astronomy, and advance our understanding of gravitation, particle physics and cosmology.

Cosmic string loop production functions

Numerical simulations of Nambu-Goto cosmic strings in an expanding universe show that the loop distribution relaxes to an universal configuration, the so-called scaling regime, which is of power law shape on large scales. Precise estimations of the power law exponent are, however, still matter of debate while numerical simulations do not incorporate all the radiation and backreaction effects expected to affect the network dynamics at small scales. By using a Boltzmann approach, we show that the steepness of the loop production function with respect to loops size is associated with drastic changes in the cosmological loop distribution. For a scale factor varying as a(t)  tν, we find that sub-critical loop production functions, having a Polchinski-Rocha exponent χ < (3 ν − 1)/2, yield scaling loop distributions which are mostly insensitive to infra-red (IR) and ultra-violet (UV) assumptions about the cosmic string network. For those, cosmological predictions are expected to be relatively robust, in accordance with previous results. On the contrary, critical and super-critical loop production functions, having χ ≥ (3ν−1)/2, are shown to be IR-physics dependent and this generically prevents the loop distribution to relax towards scaling. In the latter situation, we discuss the additional regularisations needed for convergence and show that, although a scaling regime can still be reached, the shape of the cosmological loop distribution is modified compared to the naive expectation. Finally, we discuss the implications of our findings.

Particle emission and gravitational radiation from cosmic strings: observational constraints

We account for particle emission and gravitational radiation from cosmic string loops to determine their effect on the loop distribution and observational signatures of strings. The effect of particle emission is that the number density of loops no longer scales. This results in a high-frequency cutoff on the stochastic gravitational wave background, but we show that the expected cutoff is outside the range of current and planned detectors. Particle emission from string loops also produces a diffuse gamma-ray background that is sensitive to the presence of kinks and cusps on the loops. However, both for kinks and cusps, and with mild assumptions about particle physics interactions, current diffuse gamma-ray background observations do not constrain Gμ.

Impact of the small-scale structure on the Stochastic Background of Gravitational Waves from cosmic strings

Auclair, Pierre G.

Numerical simulations and analytical models suggest that infinite cosmic strings produce cosmic string loops of all sizes with a given power-law. Precise estimations of the power-law exponent are still matter of debate while numerical simulations do not incorporate all the radiation and back-reaction effects expected to affect the network at small scales. Previously it has been shown, using a Boltzmann approach, that depending on the steepness of the loop production function and the gravitational back-reaction scale, a so-called Extra Population of Small Loops (EPSL) can be generated in the loop number density. We propose a framework to study the influence of this extra population of small loops on the Stochastic Background of Gravitational Waves (SBGW). We show that this extra population can have a significant signature at frequencies higher than $H_0(\Gamma G\mu)^{-1}$ where $\Gamma$ is of order $50$ and $H_0$ is the Hubble constant. We propose a complete classification of the gravitational wave power spectra expected from cosmic strings into four classes, including the model of Blanco-Pillado, Olum and Shlaer and the model of Lorenz, Ringeval and Sakellariadou. Finally we show that given the uncertainties on the Polchinski-Rocha exponents, two hybrid classes of gravitational wave power spectrum can be considered giving very different predictions for the SBGW.

Irreducible cosmic production of relic vortons

The existence of a scaling network of current-carrying cosmic strings in our Universe is expected to continuously create loops endowed with a conserved current during the cosmological expansion. These loops radiate gravitational waves and may stabilise into centrifugally supported configurations. We show that this process generates an irreducible population of vortons which has not been considered so far. In particular, we expect vortons to be massively present today even if no loops are created at the time of string formation. We determine their cosmological distribution, and estimate their relic abundance today as a function of both the string tension and the current energy scale. This allows us to rule out new domains of this parameter space. At the same time, given some conditions on the string current, vortons are shown to provide a viable and original dark matter candidate, possibly for all values of the string tension. Their mass, spin and charge spectrum being broad, vortons would have an unusual phenomenology in dark matter searches.

Primordial black holes from metric preheating: mass fraction in the excursion-set approach

We calculate the mass distribution of Primordial Black Holes (PBHs) produced during metric preheating. After inflation, the oscillations of the inflaton at the bottom of its potential source a parametric resonant instability for small-scale scalar perturbations, that may collapse into black holes. After reviewing in a pedagogical way different techniques that have been developed in the literature to compute mass distributions of PBHs, we focus on the excursion-set approach. We derive a Volterra integral equation that is free of a singularity sometimes encountered, and apply it to the case of metric preheating. We find that if the energy density at which the instability stops, $\rho_\Gamma$, is sufficiently smaller than the one at which inflation ends, $\rho_\mathrm{end}$, namely if $\rho_\Gamma^{1/4}/\rho_\mathrm{end}^{1/4}< 10^{-5}(\rho_\mathrm{end}^{1/4}/10^{16}\mathrm{GeV})^{3/2}$, then PBHs dominate the universe content at the end of the oscillatory phase. This confirms the previous analysis of arXiv:1907.04236 . By properly accounting for the "cloud-in-cloud" mechanism, we find that the mass distribution is more suppressed at low masses than previously thought, and peaks several orders of magnitude above the Hubble mass at the end of inflation. The peak mass ranges from $10$ g to stellar masses, giving rise to different possible cosmological effects that we discuss.

Seminars and conferences


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Department of Theoretical Physics, UPV/EHU, Bilbao
Online seminar on the subject :
Primordial black holes from metric preheating: mass fraction in the excursion-set approach

Instituto de Física Teórica, Universidad Autónoma de Madrid
Online seminar on the subject :
Primordial black holes from metric preheating: mass fraction in the excursion-set approach

Institut de FIsica Corpuscular
Online seminar on the subject :
Constraints on cosmic strings from gravitational waves, diffuse gamma-ray background and dark matter

Theoretical Particle Physics & Cosmology group at King's College London
Online seminar on the subject :
Constraints on cosmic strings from gravitational waves, diffuse gamma-ray background and dark matter

Institute of Cosmology & Gravitation, University of Portsmouth
Online seminar on the subject :
Constraints on cosmic strings from gravitational waves, diffuse gamma-ray background and dark matter

University of Oxford
Online presentation on the subject :
TBA

Zooming in on Strings and Vortons, CERN
Online presentation on the subject :
Irreducible cosmic production of relic vortons

DESY, University of Hamburg
Online seminar on the subject :
Constraints on cosmic strings from gravitational waves, diffuse gamma-ray background and dark matter

Troisième AG du groupe de recherche Ondes Gravitationnelles
Online presentation on the subject :
Cordes cosmiques, ondes gravitationnelles et abondance des vortons

Theoretical subatomic physics and cosmology, University of Stavanger
Online seminar on the subject :
Constraints on cosmic strings from gravitational waves, diffuse gamma-ray background and dark matter

Particle Cosmology and Quantum Gravity groups, University of Nottingham
Online seminar on the subject :
Constraints on cosmic strings from gravitational waves, diffuse gamma-ray background and dark matter

Institut d'Astrophysique de Paris, Séminaire du GReCO
Seminar on the subject :
Probing cosmic string networks with gravitational waves

Laboratoire APC, groupe théorie
Seminar on the subject :
Probing cosmic string networks with gravitational waves

3rd Paris Primordial Cosmology Meetings
Talk on the subject :
Particle emission and gravitational radiation from cosmic strings: observational constraints

Deuxième Assemblée Générale du GdR Ondes Gravitationnelles
Talk on the subject :
Latest observational signatures of cosmic strings: gravitational waves and particle emission

Workshop: Gravitational Waves from the Early Universe, Nordita
Talk on the subject :
Latest observational signatures of cosmic strings: gravitational waves and particle emission

University of Helsinki
Seminar on the subject :
Cosmic strings and gravitational waves

PhD Student Conference organized by the STEP'UP doctoral school
Poster on the subject :
Gravitational waves from cosmic strings : relics of the early Universe

6th LISA Cosmology Working Group Workshop, Institute of Theoretical Physics (IFT) in Madrid
Talk on the subject :
Results from the LISA cosmic string working group: Constraints on the string tension from stochastic GW background

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