Jekyll2021-05-07T11:38:09+02:00https://curl.group/feed.xmlCURLResearch GroupChris RingevalInterferences in the Stochastic Gravitational Wave Background2021-05-06T09:00:00+02:002021-05-06T09:00:00+02:00https://curl.group/news/2021/05/06/2104.14231<p>Some cosmological sources of gravitational waves, such as cosmic
strings, are everywhere present and incessantly emitting. Their past
and present contribution can mix together to create some fine structure in
the stochastic gravitational wave background today.</p>
<p><a href="/members/dacunha.html"><strong>Disrael</strong></a> and
<a href="/members/chris.html"><strong>Christophe</strong></a> have calculated in Ref.<a class="citation" href="#dacunha:2021wyy">[1]</a> the all-length scales unequal-time strain power
spectrum \(\mathcal{P}_h\) and the energy density parameter
\(\Omega_{\mathrm{GW}}\) of gravitational waves generated by
various scaling sources. For extinct and smooth enough sources, they find that both quantities
have an oscillatory fine structure, represented as black dotted curve in the following
figures.</p>
<p><img src="/assets/images/2021.14231/singular_Ph_radinmat.png" alt="csSmat" /></p>
<p><img src="/assets/images/2021.14231/singular_OmegaGW_mat.png" alt="csEmat" /></p>
<p>On top of that, singular and never extinct sources, such as cosmic
strings, can create interferences and their spectra exhibit a
different fine structure. They are represented as red curves in the
figures. For instance, long cosmic strings in the matter era are
expected to have a rescaled strain spectrum \(k^2
\mathcal{P}_h^{\mathrm{mat}}\) oscillating around a scale invariant
plateau (blue curve above), which is quite different from the one
associated with extinct and smooth sources. It also deviates
significantly from \(\Omega_{\mathrm{GW}}^{\mathrm{mat}}\).</p>
<p>Will the fine structure of the stochastic gravitational wave
background be measured?</p>Chris RingevalSome cosmological sources of gravitational waves, such as cosmic strings, are everywhere present and incessantly emitting. Their past and present contribution can mix together to create some fine structure in the stochastic gravitational wave background today.A wibbly-wobbly timey-wimey path to salvation2020-12-10T09:42:07+01:002020-12-10T09:42:07+01:00https://curl.group/news/2020/12/10/2011.12190<p>In a recent preprint, <a href="/members/joana.html"><strong>Cristian</strong></a> and
<a href="/members/clesse.html"><strong>Sébastien</strong></a> show the robustness of
Starobinsky and Higgs inflation to inhomogeneous initial conditions of
all sizes. They have performed full numerical relativity simulations
with strong inhomogeneous initial field and velocities. After
some “wibbly-wobbly timey-wimey” evolution, the system relaxes
towards… <strong>Cosmic Inflation</strong>.</p>
<p>Their results are presented in Ref. <a class="citation" href="#joana:2020rxm">[1]</a>. Simulations show that sub-Hubble, as well as Hubble-sized, fluctuations
generically lead to inflation after an oscillatory phase between
gradient and kinetic energies. The following figure shows the
evolution of the equation of state for sub-Hubble (top), and
super-Hubble (bottom) inhomogeneous initial conditions.</p>
<p><img src="/assets/images/2011.12190/EoS.png" alt="eos" /></p>
<p>Whilst Hubble-sized fluctuations are able to form large
pre-inflationary black holes, their horizons end up being always
sub-Hubble and this allows for inflation to take place in the
neighbourhood of the black hole horizon. These results could even
suggest that the formation of large pre-inflationary black holes can
<a href="https://en.wikipedia.org/wiki/Catalysis"><strong>catalize</strong></a> the onset of
inflation. They trap gradient energies and therefore smooth the field
and metric in the surrounding regions. An equatorial slice of a black
hole forming is represented below.</p>
<p><img src="/assets/images/2011.12190/BH_slc.png" alt="bh" /></p>
<p>Paradoxically, when inhomogeneities escape the formation of black
holes, the dynamics becomes the richest. On sub-Hubble scales, the
non-linear dynamics of the deeply inhomogeneous scalar field sources
metric backreactions in the form of shear and tensor modes. These
modes are found to decay slower than the energy density of the field
and can potentially dominate the energy content of the Universe for a
while. However, when this occurs, the beginning of inflation is simply
delayed but never prevented. The next figure represents a snapshot of
the gravitational modes “energy” produced in the sub-Hubble dynamics.</p>
<p><img src="/assets/images/2011.12190/ECM_slc.png" alt="preinf" /></p>
<p>Our analysis further supports the robustness of inflation to any size
of inhomogeneity, in the field, velocity or equation-of-state. At
large scalar field values, the pre-inflation dynamics only marginally
depends on the field potential and it is expected that such behaviour
is universal and it applies to any inflation potential of
plateau-type, as the ones favoured by the CMB observations after
Planck.</p>Chris RingevalIn a recent preprint, Cristian and Sébastien show the robustness of Starobinsky and Higgs inflation to inhomogeneous initial conditions of all sizes. They have performed full numerical relativity simulations with strong inhomogeneous initial field and velocities. After some “wibbly-wobbly timey-wimey” evolution, the system relaxes towards… Cosmic Inflation.~~Post-doctoral opening in Virgo/LIGO data analysis for the SGWB~~2020-11-25T08:00:00+01:002020-11-25T08:00:00+01:00https://curl.group/jobs/2020/11/25/ARC<p>The Virgo gravitational wave group at the <a href="https://uclouvain.be/en/research-institutes/irmp"><strong>Institute of Mathematics
and Physics</strong></a>, is
searching for an senior post-doctoral researcher to take a leading
role in the data analysis carried out within the LIGO and Virgo
searches for a Stochastic Gravitational Wave Background.</p>
<p>Deadline is the 10th of January 2021, more information and online
applications here:</p>
<p><a href="https://cp3.irmp.ucl.ac.be/jobs/65"><strong>https://cp3.irmp.ucl.ac.be/jobs/65</strong></a></p>
<hr />Chris RingevalThe Virgo gravitational wave group at the Institute of Mathematics and Physics, is searching for an senior post-doctoral researcher to take a leading role in the data analysis carried out within the LIGO and Virgo searches for a Stochastic Gravitational Wave Background.Irreducible relic abundance of vortons2020-10-12T09:00:00+02:002020-10-12T09:00:00+02:00https://curl.group/news/2020/10/12/2010.04620<p>Vortons are centrifugally supported cosmic string loops that can be
continuously created at all times during the cosmological
expansion. Could they be all of the dark matter?</p>
<p>Together with <a href="https://www.apc.univ-paris7.fr/APC_CS/fr/auclair-pierre"><strong>Pierre
Auclair</strong></a>,
<a href="http://www2.iap.fr/users/peter/"><strong>Patrick Peter</strong></a> and <a href="http://www.apc.univ-paris7.fr/APC_CS/fr/users/steer"><strong>Danièle
Steer</strong></a>, we have
calculated in Ref.<a class="citation" href="#auclair:2020wse">[1]</a> their inclusive
cosmological distribution, taking into account the incessant
production of vortons from a scaling string network all along the
history of the Universe. They are a viable dark matter candidate!</p>
<p><img src="/assets/images/2010.04620/dmkvortons_vv_tot.png" alt="omegatot" /></p>
<p>This figure shows the density parameter of vortons today as a function
of the two fundamental parameters of the theory. They are the string
energy per unit length, \(G \mu\), and the ratio of this energy scale
to the one at which a current condenses on the strings, \(\mathcal{R}\). The green line represents values of the relic density
\(\Omega_\mathrm{tot} \in [0.2,0.4]\), around the current abundance of
dark matter. For all possible values of \(G \mu\), one can find a
current-carrier energy scale allowing for vortons to be all of the
dark matter.</p>Chris RingevalVortons are centrifugally supported cosmic string loops that can be continuously created at all times during the cosmological expansion. Could they be all of the dark matter?~~Post-doctoral opening in Gravitational Wave Cosmology~~2020-10-01T09:00:00+02:002020-10-01T09:00:00+02:00https://curl.group/jobs/2020/10/01/ARC<p>We are pleased to open a <strong>post-doctoral position</strong> in Gravitational
Wave Cosmology at CURL (Cosmology, Universe and Relativity at
Louvain), Louvain University (Belgium), starting fall 2021.</p>
<p>Applicants interested in stochastic gravitational wave background,
gravitational wave signatures of early universe physics, such as
cosmic strings, primordial black holes, cosmic inflation, as well as
data analysis for current and future gravitational wave
interferometers, such as Pulsar Timing Arrays, LIGO/Virgo and LISA,
are particularly encouraged to apply.</p>
<p>Appointment is available for two years.</p>
<p>The position is supported by a joint ARC research program in gravitational
wave physics between the <a href="https://cp3.phys.ucl.ac.be">CP3 center</a> and
the <a href="https://curl.irmp.ucl.ac.be">CURL group</a> at UCLouvain and the <a href="https://www.star.uliege.be">STAR
institute</a> at Université de
Liège. Computing resources and support for travelling are excellent.</p>
<hr />
<p>Letters of application (including a curriculum vitae, a list of
publications, a brief statement of research interests) and at least
two recommendation letters from senior scientists should be submitted
on-line, by the 10th of January 2021, at:</p>
<p><a href="https://cp3.irmp.ucl.ac.be/jobs/63"><strong>https://cp3.irmp.ucl.ac.be/jobs/63</strong></a></p>
<p>For more information, please contact:</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>christophe.ringeval@uclouvain.be
Cosmology, Universe and Relativity at Louvain
Institute of Mathematics and Physics
Louvain University
2, Chemin du Cyclotron
1348 Louvain-la-Neuve
Belgium
</code></pre></div></div>
<p>A summary of our current interests can be consulted on the CURL
website: <a href="https://curl.group"><strong>curl.group</strong></a></p>
<hr />Chris RingevalWe are pleased to open a post-doctoral position in Gravitational Wave Cosmology at CURL (Cosmology, Universe and Relativity at Louvain), Louvain University (Belgium), starting fall 2021.Detection of a very very massive black holes merger2020-09-03T09:00:00+02:002020-09-03T09:00:00+02:00https://curl.group/news/2020/09/03/GW190521<p>The gravitational waves emitted during the merging of two black holes
of 85 and 66 times the sun mass have been detected on May 25, 2019 by
the LIGO detectors in the United States and the Virgo detector in
Italy.</p>
<p><img src="/assets/images/outreach/ligovirgo_merger.jpg" alt="GW190521" /></p>
<p>The existence of so massive black holes is quite unexpected because
theoretical models disfavour the formation of black holes between
65 \(M_{\odot}\) and 120 \(M_{\odot}\) when the most massive stars explode in
supernovae. They may come from previous mergers, but, then, they should
live in a dense black hole environment, like a black hole or star
cluster, in order to have a chance to form a binary. An exotic
alternative: these black holes could be primordial and have formed
less than a milli-second after the Big-Bang, from pre-existing
inhomogeneities in the primeval plasma. This scenario could explain
some intriguing properties of black hole mergers detected so far. An
idea proposed by Stephen Hawking in 1971 that comes back at the
forefront of hot topics in Science.</p>
<p>CURL members Sébastien Clesse and Christophe Ringeval are members of
the Virgo collaboration, working on the gravitational waves from
cosmic strings, and from primordial black holes. Upcoming LIGO/Virgo
observations will help to distinguish the different models of black
hole formation<sup id="fnref:1" role="doc-noteref"><a href="#fn:1" class="footnote" rel="footnote">1</a></sup>. See Ref. <a class="citation" href="#clesse:2020ghq">[1]</a> and <a href="/news/2020/07/13/2007.06481.html">this
post</a>.</p>
<p>The official announcement: <a href="https://www.ligo.org/detections/GW190521/files/pr-english.pdf"><strong>Press Release</strong></a></p>
<div class="footnotes" role="doc-endnotes">
<ol>
<li id="fn:1" role="doc-endnote">
<p>Artist illustration: <a href="https://commons.wikimedia.org/wiki/File:Illu_LIGO-Virgo_20200902_2.jpg">Ingrid Bourgault</a> <a href="#fnref:1" class="reversefootnote" role="doc-backlink">↩</a></p>
</li>
</ol>
</div>Chris RingevalThe gravitational waves emitted during the merging of two black holes of 85 and 66 times the sun mass have been detected on May 25, 2019 by the LIGO detectors in the United States and the Virgo detector in Italy.Two candidate mergers of primordial black holes from the QCD epoch2020-07-13T10:42:07+02:002020-07-13T10:42:07+02:00https://curl.group/news/2020/07/13/2007.06481<p>The two recent gravitational-wave events GW190425 and GW190814 from
the third observing run of LIGO/Virgo have both a companion which is
unexpected if originated from a neutron star or a stellar black
hole. <a href="/members/clesse.html"><strong>Sébastien</strong></a> and <a href="https://uam.academia.edu/JuanGarciaBellido"><strong>Juan
García-Bellido</strong></a>
investigate the possibility that these objects are Primordial Black
Holes (PBHs) in Ref. <a class="citation" href="#clesse:2020ghq">[1]</a>.</p>
<p>The known thermal history of the Universe predicts that the PBH
formation is boosted at the time of the QCD transition, inducing a
peak in their distribution at this particular mass scale, and a bump
around 30−50 \(M_\odot\). We find that the merging rates inferred from GW190425
and GW190814 are consistent with PBH binaries formed by capture in
dense halos. At the same time, the rate of black hole mergers around
30 \(M_\odot\) and of sub-solar PBH mergers do not exceed the LIGO/Virgo
limits. Such PBHs could explain a significant fraction, or even the
totality of the Dark Matter, but they must be sufficiently strongly
clustered in order to be consistent with current astrophysical limits.</p>
<p><img src="/assets/images/2007.06481/rates.png" alt="frates" /></p>
<p>This figure shows the expected merging rate distribution for
primordial black holes, as a function of the two component masses and
for tidal capture in dense haloes. The green and orange lines
indicate the rates inferred from the GW190425 and GW190814 events
(dashed: best fit, solid: \(90\%\) confidence limits), in agreement with
the primordial black hole predictions.</p>Chris RingevalThe two recent gravitational-wave events GW190425 and GW190814 from the third observing run of LIGO/Virgo have both a companion which is unexpected if originated from a neutron star or a stellar black hole. Sébastien and Juan García-Bellido investigate the possibility that these objects are Primordial Black Holes (PBHs) in Ref. [1].Cosmology Seminar2020-02-07T08:00:00+01:002020-02-07T08:00:00+01:00https://curl.group/news/2020/02/07/seminar<p>Our next cosmology seminar takes place on <strong>Thursday 13th</strong> of
February, seminar <strong>room E349</strong> at <strong>4pm</strong>. The speaker is <strong>Dida
Markovic</strong> from the Jet Propulsion Laboratory, NASA (Caltech, Pasadena
U.S.).</p>
<hr />
<p>At 16:00, room E349, Dida Markovic (JPL, NASA), will be talking about</p>
<h3 id="cosmology-from-the-anisotropic-power-spectrum-with-euclid">Cosmology from the anisotropic power spectrum with Euclid</h3>
<p><em>Euclid will be the first experiment to observe redshift space
distortions from space. This way it will be able to survey the sky,
not limited by distortions in the Earth’s atmosphere. The design of
the survey and the instrument will enable us to reach 1% precision in
the power spectrum. This will give us the power to conduct
unprecedented tests of Einstein’s gravity and the expansion history of
the universe. However this will also mean that we must reach an
unprecedented understanding of our systematic effects, whether coming
from our instrument or the foreground sky. It will also mean we will
need to ensure our theories are adequate to be tested against such
precise data. In this talk I will describe Euclid - our instrument,
our survey, and our science. I will describe the challenges in
understanding it’s systematics and I will present some results from my
resent work on testing two competing models for non-linear large scale
structure.</em></p>Chris RingevalOur next cosmology seminar takes place on Thursday 13th of February, seminar room E349 at 4pm. The speaker is Dida Markovic from the Jet Propulsion Laboratory, NASA (Caltech, Pasadena U.S.).Cosmology Seminar2020-01-13T08:00:00+01:002020-01-13T08:00:00+01:00https://curl.group/news/2020/01/13/seminar<p>Our next cosmology seminar takes place on <strong>Friday 24th</strong> of January,
seminar <strong>room E349</strong> at <strong>2pm</strong>. The speaker is <strong>Disrael Camargo Neves da Cunha</strong> who have recently joined us at CURL.</p>
<hr />
<p>At 14:00, room E349, Disrael Camargo Neves da Cunha (CURL, Louvain U.), will be talking about</p>
<h3 id="signatures-of-cosmic-strings-on-the-dark-matter-distribution">Signatures of Cosmic Strings on the Dark Matter Distribution</h3>
<p><em>In the first part of the talk, I will present a brief review of
cosmic strings and some of its observational consequences. One of the
observable object produced by a cosmic string is the wake, which is a
high-density sheet-like structure in the matter distribution. The
second part will be a description of the work
<a href="https://arxiv.org/abs/1508.02317"><strong>arXiv:1508.02317</strong></a> in which the
stability of the wake is analyzed. The principal conclusion is that
this object could be globally undisrupted until today. Finally, I will
mention the consequences of this work for wake presence and possible
detection in the large scale structures, using the results from N-body
simulation of dark matter distribution with a wake:
<a href="https://arxiv.org/abs/1804.00083"><strong>arXiv:1804.00083</strong></a>,
<a href="https://arxiv.org/abs/1810.07737"><strong>arXiv:1810.07737</strong></a></em></p>Chris RingevalOur next cosmology seminar takes place on Friday 24th of January, seminar room E349 at 2pm. The speaker is Disrael Camargo Neves da Cunha who have recently joined us at CURL.Cosmology Seminar2019-11-22T08:00:00+01:002019-11-22T08:00:00+01:00https://curl.group/news/2019/11/22/seminar<p>Our next cosmology seminar takes place on <strong>Friday 29th</strong> of November,
seminar <strong>room E349</strong> at <strong>2pm</strong>. The speaker is <strong>Atsuhisa Ota</strong>
from the DAMPT (Cambridge, U.K.).</p>
<hr />
<p>At 14:00, room E349, Atsuhisa Ota (DAMPT, Cambridge), will be talking about</p>
<h3 id="induced-superhorizon-tensor-perturbations-from-anisotropic-non-gaussianity">Induced superhorizon tensor perturbations from anisotropic non-Gaussianity</h3>
<p><em>While we have not detected large scale tensor perturbations, they are
essential for cosmology because we consider their origin is
microscopic quantum fluctuations during inflation. However, we should
note that the classical secondary effects of scalar perturbations also
source them after inflation. We usually consider that these secondary
effects are not significant on the CMB anisotropy scale, but we will
revisit this issue with non-Gaussian initial conditions for the scalar
perturbations. In this talk, we will show that induced tensor modes
can be generated on superhorizon scales through the intrinsic
long-short mode coupling due to the non-Gaussianity. The induced
tensor power spectrum has similar scale dependence with the primordial
contribution. Hence we can discuss the detectability by the CMB
anisotropy measurements. Interestingly, the induced tensor
perturbations can be significantly enhanced, depending on the
reheating scenario. Also, we will present the observational
consequences of various non-Gaussian initial conditions based on the
present framework.</em></p>Chris RingevalOur next cosmology seminar takes place on Friday 29th of November, seminar room E349 at 2pm. The speaker is Atsuhisa Ota from the DAMPT (Cambridge, U.K.).