Jekyll2019-04-09T15:04:14+02:00https://curl.group/CURLResearch GroupChris RingevalThe shape of cosmic string loop production functions2019-04-07T15:00:00+02:002019-04-07T15:00:00+02:00https://curl.group/news/2019/04/07/1903.06685<p>We have recently made public a rather technical paper in which we
explicitly and unambiguously calculate the cosmic string loop number
density distribution in our universe coming from various motivated
loop production functions. Such a density distribution crucially
determines the spectrum of their emitted gravitational waves, a
smooking gun for their potential discovery.</p>
<p>Together with <a href="https://www.apc.univ-paris7.fr/APC_CS/fr/auclair-pierre"><strong>Pierre Auclair</strong></a>,
<a href="https://www.kcl.ac.uk/nms/depts/physics/people/academicstaff/sakellariadou"><strong>Mairi Sakellariadou</strong></a>
and <a href="http://www.apc.univ-paris7.fr/APC_CS/fr/users/steer"><strong>Danièle Steer</strong></a>, we have
carefuly explored in Ref.<a href="#auclair:2019zoz">[1]</a> the consequences of
changing the slope of the Polchinski-Rocha loop production function to the actual
observable cosmic string loop distribution. This is the parameter
<script type="math/tex">\chi</script> in the next plot:</p>
<p><img src="/assets/images/1903.06685/loopprod.png" alt="lpf" /></p>
<p>For the cases we have referred to as “sub-critical”, <script type="math/tex">% <![CDATA[
\chi < (3 \nu -
1)/2 %]]></script>, corresponding to steep slopes in the previous figure, we
recover the results of Ref. <a href="#lorenz:2010sm">[2]</a>. The parameter
<script type="math/tex">\nu</script> encodes the growing rate of the scale factor <script type="math/tex">a(t)</script>, namely,
we assume <script type="math/tex">a(t) \propto t^\nu</script>.</p>
<p>For shallower slopes, the so-called “critical” and “super-critical”
cases, <script type="math/tex">\chi \ge (3\nu-1)/2</script>, we find that either the loop
distribution incessantly grows, or, with some regularisation, reaches a
stationnary distribution whose shape depends on what happens on the
larger length scales. This is best illustrated by the following plot:</p>
<p><img src="/assets/images/1903.06685/diraccomp.png" alt="diraccomp" /></p>
<p>It compares the loop number density produced by assuming an infinitely
sharp loop production function peaked at <script type="math/tex">\gamma = 0.1</script> (green
curve) with a (regularised) super-critical Polchinski-Rocha
distribution having <script type="math/tex">\chi > 0.25</script> (purple curve), in the radiation
era (<script type="math/tex">\nu=1/2</script>). As this plot shows, the gravitational backreaction
scale, <script type="math/tex">\gamma_\mathrm{c}</script>, at which the loop production function is
cut <strong>always matters</strong> and is responsible for the plateau on the
purple curve at small <script type="math/tex">\gamma</script> values. The existence of a second
plateau around <script type="math/tex">\gamma \simeq 10^{-6}</script> comes from the Infra-Red
sensitivity of all super-critical loop production functions.</p>Chris RingevalWe have recently made public a rather technical paper in which we explicitly and unambiguously calculate the cosmic string loop number density distribution in our universe coming from various motivated loop production functions. Such a density distribution crucially determines the spectrum of their emitted gravitational waves, a smooking gun for their potential discovery.A large mass hierarchy from a small non-minimal coupling2019-04-02T15:00:00+02:002019-04-02T15:00:00+02:00https://curl.group/news/2019/04/02/1903.03544<p>In a recent preprint, we discuss the possibility that both the Planck
mass and the dark energy scale are dynamically generated by stochastic
effects during inflation.</p>
<p>Two of the energy scales involved in gravitation, the Planck mass
<script type="math/tex">M_\mathrm{Pl} \simeq 10^{18}\,\mathrm{GeV}</script> and the cosmological
constant energy scale <script type="math/tex">\rho_{\Lambda}^{1/4} \simeq 10^{-3}\,
\mathrm{eV}</script> differ by many orders of magnitude.</p>
<p>Together with <a href="https://curl.group/members/suyama.html"><strong>Teruaki
Suyama</strong></a> and <a href="http://www.th.phys.titech.ac.jp/cosmo/"><strong>Masahide
Yamaguchi</strong></a> from
<a href="http://www.th.phys.titech.ac.jp"><strong>TiTech</strong></a> (Tokyo), we explore in
Ref.<a href="#ringeval:2019bob">[1]</a> the possibility that both have been
dynamically generated from a new and unique ultra light scalar field
that would have experienced super-Hubble quantum fluctuations during
primordial inflation.</p>
<p>The scenario is currently passing all cosmological and astrophysical
constraints, and could be proven at work with future gravitational
wave measurements of the non-minimal coupling.</p>Chris RingevalIn a recent preprint, we discuss the possibility that both the Planck mass and the dark energy scale are dynamically generated by stochastic effects during inflation.Informal meeting on Primordial Black Holes2019-04-01T09:00:00+02:002019-04-01T09:00:00+02:00https://curl.group/news/2019/04/01/MiniPBH<p>We hold an informal meeting, organized one day before the Solvay
Workshop on <a href="http://www.solvayinstitutes.be/event/workshop/dark_2019/dark_2019.html">“The dark side of Black
Holes”</a>
in Brussels (but not officially related to it), with a few experts on
PBH to discuss recent developments in the field. This takes place
Tuesday 2nd of April, seminar room E349 from 10 am.</p>
<p>Registration <a href="https://agenda.irmp.ucl.ac.be/event/3341/">here</a>.</p>
<hr />
<p>The program starts at 10:00 with Ilia Musco (ICC U. Barcelona)</p>
<h3 id="threshold-and-abundance-of-primordial-black-holes-dependence-on-the-profile-of-the-cosmological-perturbations">Threshold and abundance of primordial black holes: dependence on the profile of the cosmological perturbations</h3>
<p><em>Primordial black holes can form in the early Universe from the
collapse of cosmological perturbations after the cosmological horizon
crossing. They are possible candidates for the dark matter as well as
for the seeds of supermassive black holes observed today in the
centre of galaxies. In calculations of spherically symmetric
collapse, using a large curvature perturbation in the super horizon
regime, the initial conditions are specified using the gradient
expansion approximation in the long wave length limit. The non linear
evolution is then simulated using a Lagrangian relativistic
hydrodynamical code. If the perturbation is larger than a threshold
depending on the equation of state and on the specific shape of the
perturbation, a black hole is formed. In this talk I will discuss the
dependence of PBH formation from the initial shape of the curvature
profile showing the relation between the threshold amplitude and the
steepness of the perturbation which is linked to the amplitude of the
pressure gradients that are developing during the collapse. I will
show how to derive the initial curvature profile form the shape of
the inflationary power spectrum, which affects also the abundance of
PBHs. Depending on the model, a proper calculation, using the shape
of the power spectrum, shows that the abundance of PBHs is
significantly increased by several order of magnitudes compared to
previous estimations.</em></p>
<hr />
<p>At 10:50, Teruaki Suyama (TiTech, Tokyo) will speak about</p>
<h3 id="hidden-universality-in-the-merger-rate-distribution-in-the-primordial-black-hole-scenario">Hidden universality in the merger rate distribution in the primordial black hole scenario</h3>
<p><em>It has been proposed that primordial black holes (PBHs) form binaries
in the radiation dominated era. Once formed, some fraction of them
may merge within the age of the Universe by gravitational radiation
reaction. We investigate the merger rate of the PBH binaries when the
PBH mass function is not monochromatic, which is a generalization of
the previous studies where the PBHs are assumed to have the same
mass. After deriving a formula for the merger time probability
distribution in the PBH mass plane, we evaluate it under two
different approximations. We identify a quantity constructed from the
mass-distribution of the merger rate density whose value is close to
unity for all binary masses independently of the PBH mass
function. This result suggests that the measurement of this quantity
is useful for testing the PBH scenario.</em></p>
<hr />
<p>After lunch, we will have Florian Kuhnel (Oscar Klein Centre,
Stockholm U.) with</p>
<h3 id="primordial-black-holes---formation-constraints-uncertainties">Primordial Black Holes - Formation, Constraints, Uncertainties</h3>
<p><em>Primordial black holes are black holes that may have formed in the
early Universe. Their masses potentially span a range from as low as
the Planck mass up to many orders of magnitude above the solar
mass. This, in particular, includes black holes of the order of 10
solar masses, like those recently discovered by LIGO. These may be of
primordial origin. In order to quantitatively asses this and related
scenarios, a profound understanding of the holes’ formation mechanism
necessary. After a general introduction on primordial black holes, I
will discuss the most consequential aspects of their formation, and
elaborate on the observable imprints these may leave. I will give an
overview about recent abundance limits, discuss the uncertainty of
these constraints.</em></p>
<hr />
<p>And finally, Michael Hawkins (Edimburg O.) will close the day talking about</p>
<h3 id="the-signature-of-primordial-black-holes-in-galaxy-dark-matter-haloes">The signature of primordial black holes in galaxy dark matter haloes</h3>
<p><em>There are several well-known gravitationally lensed quasar systems
where the quasar images appear to lie well clear of the stars in the
lensing galaxy. I shall show that in some such cases the stellar
population cannot account for the observed microlensing. In these
cases the most plausible bodies responsible for thr lensing are
primordial black holes.</em></p>Chris RingevalWe hold an informal meeting, organized one day before the Solvay Workshop on “The dark side of Black Holes” in Brussels (but not officially related to it), with a few experts on PBH to discuss recent developments in the field. This takes place Tuesday 2nd of April, seminar room E349 from 10 am.Inflation after Planck: Judgment Day2019-03-01T09:42:07+01:002019-03-01T09:42:07+01:00https://curl.group/news/2019/03/01/1902.03951<p>The final release of the Planck data 2018, see <a href="/news/2018/07/17/Planck2018.html">this
post</a>, invited
us to finalize a long project in which we discuss the <em>actual</em> status of
<a href="https://en.wikipedia.org/wiki/Inflation_(cosmology)"><strong>Cosmic Inflation</strong></a> with
respect to what we have learned about the early universe.</p>
<p>In a quite long preprint <a href="#chowdhury:2019otk">[1]</a>, we are trying to
exhaustively address all the criticisms raised against the inflationary
paradigm and confront them to what the Planck data told
us. The conclusion is short and summarized in the abstract:</p>
<blockquote>
<p>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.</p>
</blockquote>
<p>You’re skeptic? Read the paper…</p>Chris RingevalThe final release of the Planck data 2018, see this post, invited us to finalize a long project in which we discuss the actual status of Cosmic Inflation with respect to what we have learned about the early universe.Post-doctoral openings in Gravitational Waves2019-02-28T08:00:00+01:002019-02-28T08:00:00+01:00https://curl.group/jobs/2019/02/28/IISN<p>We are happy to announce the opening of a few <strong>post-doctoral
positions</strong> in Gravitational Waves research at the <a href="https://uclouvain.be/en/research-institutes/irmp"><strong>Institute of
Mathematics and
Physics</strong></a>. The
positions are shared with our <a href="https://cp3.irmp.ucl.ac.be/"><strong>CP3</strong></a>
colleagues and the
<a href="https://www.star.uliege.be/cms/c_3300779/en/star-portail"><strong>STAR</strong></a>
institute. We search for candidates over a wide range of expertises,
from VIRGO related hardware design to GW data analysis for
Cosmology. Don’t hesitate to apply, the number of openings is not
rigidly fixed yet.</p>
<p>All infos on the <a href="https://inspirehep.net/record/1716842"><strong>Inspire
Job</strong></a> post.</p>
<hr />
<p>Mind the close deadline: the 30th of March 2019.</p>Chris RingevalWe are happy to announce the opening of a few post-doctoral positions in Gravitational Waves research at the Institute of Mathematics and Physics. The positions are shared with our CP3 colleagues and the STAR institute. We search for candidates over a wide range of expertises, from VIRGO related hardware design to GW data analysis for Cosmology. Don’t hesitate to apply, the number of openings is not rigidly fixed yet.Interview for ScienceToday2019-01-31T08:00:00+01:002019-01-31T08:00:00+01:00https://curl.group/outreach/2019/01/31/ScienceToday<p>The ScienceToday UCL website features the researches of Sébastien Clesse
on primordial black holes.</p>
<p><a href="https://uclouvain.be/en/sciencetoday/news/seven-hints-point-to-primordial-black-holes.html"><strong>Seven Hints point for Primordial Black Holes</strong></a></p>
<p><img src="/assets/images/7hints/BH_LMC.png" alt="pbh" /></p>
<p>More details can be found in the scientific paper Ref. <a href="#clesse:2017bsw">[1]</a>. The photo is a simulated view of a black hole in
front of the Large Magellanic Cloud<sup id="fnref:1"><a href="#fn:1" class="footnote">1</a></sup>.</p>
<div class="footnotes">
<ol>
<li id="fn:1">
<p>Credit: A. Riazuelo <a href="#fnref:1" class="reversefootnote">↩</a></p>
</li>
</ol>
</div>Chris RingevalThe ScienceToday UCL website features the researches of Sébastien Clesse on primordial black holes.~~Post-doctoral opening in Cosmology~~2018-12-19T08:00:00+01:002018-12-19T08:00:00+01:00https://curl.group/jobs/2018/12/19/PDR<p>We are happy to announce the opening of a <strong>post-doctoral position</strong> in
Cosmology at CURL (Cosmology, Universe and Relativity at Louvain),
Louvain University (Belgium), starting fall 2019.</p>
<p>Applicants interested in early universe models, cosmic defects,
primordial black holes, cosmic inflation, as well as data analysis for
the future observational missions (CMB, GW, galaxy surveys, 21cm,
Euclid, LISA…) are encouraged to apply. Please notice that
applicants should have obtained their PhD no longer than 5 years
before the starting date of the fellowship, plus one year of allowance
for parenthood.</p>
<p>Appointment is available for up to three years, depending on results
and performance, and is supported by the Belgian National Fund for
Scientific Research (FNRS).</p>
<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 />
<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 sent by
the 15th of February 2019 to <strong>christophe.ringeval@uclouvain.be</strong> with
the subject containing the wordings <strong>[JOB19]</strong>.</p>
<p>Postal mail applications can be addressed to:</p>
<div class="highlighter-rouge"><div class="highlight"><pre class="highlight"><code>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>Chris RingevalWe are happy to announce the opening of a post-doctoral position in Cosmology at CURL (Cosmology, Universe and Relativity at Louvain), Louvain University (Belgium), starting fall 2019.IRMP colloquium on crowd-sourcing optical follow-up searches2018-11-14T08:00:00+01:002018-11-14T08:00:00+01:00https://curl.group/news/2018/11/14/IRMP<p>Some echos from Ref. <a href="#boucher:2018xvk">[1]</a> and <a href="/news/2018/01/01/1801.04124.html">this
post</a>. Thursday,
2pm, room CYCL 01, C. Ringeval gives a talk for the <a href="https://agenda.irmp.ucl.ac.be/event/3162/"><strong>IRMP
colloquium</strong></a>.</p>
<hr />
<h3 id="crowd-sourcing-galaxy-clusters-confirmation-with-non-professional-telescopes">Crowd-sourcing galaxy clusters confirmation with non-professional telescopes</h3>
<p><em>The new all sky and multi-wavelength astrophysical and cosmological
surveys are, and will be, providing an unprecedented amount of
information on a plethora of cosmic objects. Many of them are
foregrounds for the original science goals and cannot received the
same attention as the mission main objectives. Within the Planck
satellite maps of our universe, a space telescope dedicated to the
observation of the Cosmic Microwave Background, a few thousand of so
far undetected galaxy clusters are letting very peculiar imprints
through the so-called Sunyaev Zel’dovich effect. These cluster
candidates have never been observed with optical telescopes and
confirmation of their true nature requires a significant amount of
professional telescope time. In this talk, I will present a
proof-of-concept of crowd-sourcing galaxy clusters optical
confirmation with amateur astronomers. Using a non-professional
telescope of 62cm provided by the Astroqueyras association in
Saint-Véran (France), we have been able to confirm one of the Planck
cluster candidate and to extract various of its properties, such as
distance, redshift and mass. Although the exposure time required to
image one cluster candidate is an order of magnitude larger than with
a professional telescope, there are orders of magnitude more amateur
astronomers than professionals thereby rendering the concept useful
to many other surveys. They are the South Pole Telescope, the Atacama
Cosmology Telescope, future X-ray spatial missions such as eRosita,
and all of the searches for gravitational wave optical counterparts.</em></p>Chris RingevalSome echos from Ref. [1] and this post. Thursday, 2pm, room CYCL 01, C. Ringeval gives a talk for the IRMP colloquium.Georges Lemaître Chair 20182018-11-06T08:00:00+01:002018-11-06T08:00:00+01:00https://curl.group/news/2018/11/06/GLChair<p>The “Chaire Georges Lemaître 2018” is taking place at
<a href="https://uclouvain.be/en/research-institutes/irmp"><strong>IRMP</strong></a> the week
of the 26th of November. <strong>Prof. Roberto Trotta</strong> will be giving a
serie of lectures from Monday to Friday on “Astrostatistics in
Action”.</p>
<p><img src="/assets/images/glchair/gl2018.png" alt="glchair" /></p>
<hr />
<p>The program consists in an Inaugural Lecture followed by a reception
(<a href="https://agenda.irmp.ucl.ac.be/event/3164/"><strong>Registration here</strong></a>),
Monday at 16:15, CYCL01.</p>
<h3 id="if-you-dont-need-astrostatistics-you-have-done-the-wrong-experiment">If You Don’t need (Astro)Statistics, You Have Done the Wrong Experiment</h3>
<p><em>At the beginning of last century, the physicist and Nobel Prize
Winner Ernest Rutherford reportedly believed that “If your experiment
needs statistics, you ought to have done a better experiment”. If he
were alive today, he probably would not recognize the way cosmology
(the study of the Universe on its largest scale) has developed:
essentially all of the exciting discoveries in the last two decades
have relied on sophisticated statistical analyses of very large and
complex datasets. Today, advanced astrostatistical methods belong to
the toolbox of almost every cosmologist. In this talk I will give an
overview of how cosmologists have established a “cosmological
concordance model” that explains extraordinarily well very accurate
observations ranging from the relic radiation from the Big Bang to the
distribution of galaxies in the sky in the modern Universe. The
emerging picture of a cosmos remains puzzling: 95% of the Universe is
constituted of unknown components, dark matter and dark energy. Our
understanding of the Universe is – already today – limited by our
statistical and computational methods. I will discuss how
astrostatistics will meet the challenges posed by upcoming extremely
large data sets and thereby be instrumental in answering some of the
most fundamental questions about the physical reality of the cosmos.</em></p>
<hr />
<p>The 2 hours lectures start on Tuesday 10:00 am, room E-349, till
Friday. If you intend to follow one, or more, lectures, you <strong>MUST</strong>
register below.</p>
<p><a href="https://agenda.irmp.ucl.ac.be/event/3165/"><strong>REGISTRATION</strong></a></p>
<h3 id="lecture-1-how-to-learn-from-experience-principle-of-bayesian-inference">Lecture 1: How to Learn from Experience: Principle of Bayesian Inference</h3>
<p><em>The problem of inference from noisy and/or partial data is ubiquitous
in science. It is particularly acute in observational disciplines,
like astrophysics and cosmology, where we don’t have the luxury of
being able to control our experiments. I will introduce the
fundamental principles of Bayesian inference as a complete theory for
how to learn from experience, and contrast this approach with the more
traditional frequentist view of probability. I will also discuss
practical Bayesian methods and technology to determine posterior
distributions, including Markov Chain Monte Carlo and related sampling
schemes.</em></p>
<h3 id="lecture-2-shaving-theories-with-occams-razor-bayesian-model-comparison">Lecture 2: Shaving Theories with Occam’s Razor: Bayesian Model Comparison</h3>
<p><em>Many of the questions in astroparticle physics, cosmology and
particle physics aim at establishing which theoretical model is the
best explanation for the available data. Classically, theories can
only be falsified by ruling them out (the Popperian view). I will
demonstrate how the Bayesian model comparison approach is more
general, enabling physicists to correctly reward models that make
precise predictions that are then verified by experiment or
observation. I will contrast this with the hypothesis testing
paradigm, and discuss consequences for our understanding of what a
“significant result” means. I will illustrate analytical and numerical
methods for the implementation of Bayesian model comparison with
examples from cosmology and particle physics.</em></p>
<h3 id="lecture-3-bayesian-hierarchical-models-and-applications-to-supernova-type-ia-cosmology">Lecture 3: Bayesian Hierarchical Models and Applications to Supernova Type Ia Cosmology</h3>
<p><em>Thanks to large and accurate measurements obtained in the last 2 decades, and to
sophisticated statistical analyses, cosmologists have established a “cosmological
concordance model” that reproduces well observations ranging from the relic radiation
from the Big Bang to the distribution of galaxies in the sky in the modern Universe. I will
review the observational and theoretical underpinnings of this so-called “Lambda-CDM”
concordance model of cosmology, which strongly points to the existence of both dark
matter and dark energy.
I will then focus on recent advances in supernova type Ia cosmology. Supernovae type Ia are
a type of stellar explosion that can be used as standard candles to measure extragalactic
distances, and have been instrumental in determining the accelerated expansion of the
Universe – a smoking gun observation for the existence of dark energy. I will present recent
results and a novel, powerful (Bayesian) statistical framework for interpreting the data,
including cosmological parameter inference, selection effects and classification in the
absence of spectroscopic data. I will discuss upcoming challenges for the field as the
quantity and quality of upcoming data sets increases.</em></p>
<h3 id="lecture-4-applications-of-astrostatistics-to-dark-matter-phenomenology-and-beyond-the-standard-model-theories">Lecture 4: Applications of Astrostatistics to Dark Matter Phenomenology and Beyond the Standard Model Theories</h3>
<p><em>The study of dark matter phenomenology and of underlying theoretical
models has advanced dramatically in recent years thanks to the
development of “global fits” an approach to combine all available
data in a statistically convergent fit of Beyond the Standard Model
theoretical parameters. This approach now includes sophisticated
methods to simultaneously use direct detection, indirect detection and
collider data to constrain the parameter space for dark matter, and
map this onto the underlying theory parameters, e.g. in
Supersymmetry. I will describe the “global fits” approach and its
ability to deliver quantitative inference both from a Bayesian and a
frequentist (profile likelihood) point of view. I will review current
results in the field and associated challenges.</em></p>Chris RingevalThe “Chaire Georges Lemaître 2018” is taking place at IRMP the week of the 26th of November. Prof. Roberto Trotta will be giving a serie of lectures from Monday to Friday on “Astrostatistics in Action”.Public conference series on Black Holes, Big-Bang and Universe2018-10-10T09:00:00+02:002018-10-10T09:00:00+02:00https://curl.group/outreach/2018/10/10/UDA<p>S. Clesse is giving a series of public lectures for the Université des
Aînés, the 10th, 17th and 24th of October.</p>
<p>More information there: <a href="https://universitedesaines.be/fiche-formation.php?categorie=82"><strong>UDA</strong></a></p>Chris RingevalS. Clesse is giving a series of public lectures for the Université des Aînés, the 10th, 17th and 24th of October.