The live stream can be watched
**there** while a transcripted
english translation has been published on the
**arXiv**.

At 2pm, room E349, Davide Dal Cin (SISSA), will be talking about

*The radial direction of the Peccei-Quinn field can drive cosmic
inflation, given a non-minimal coupling to gravity. This scenario has
been considered to be capable of explaining inflation, the strong
CP problem, and dark matter. We argue that Peccei-Quinn inflation
is extremely sensitive to higher-dimensional operators. Further
combining with the discussion on the axion quality required to solve
the strong CP problem, we examine the validity of this scenario. We
also show that a resonant amplification of the axion field is
unavoidable after Peccei–Quinn inflation.*

Random Apollonian Packing (RAP) is inspired by the better-known Apollonian Gasket. In mathematics, an Apollonian gasket is a fractal generated by starting with a triple of circles, each tangent to the other two, and successively filling in more circles, each tangent to another three.

In Ref. [1], Pierre examines a related mechanism in which \(d\)-dimensional spheres are randomly seeded in space, one at a time, within a finite-sized volume, and with the largest possible radius that avoids overlap.

The interest of the RAP mechanism is that it is thought to share universal properties with more general dynamic mechanisms, such as the ABK mechanism (named after Andrienko, Brilliantov and Krapivsky) in which bubbles grow linearly with time.

Pierre has built a model to explain at which rate Random Apollonian Packings grow after the insertion of one sphere after the other. The model’s prediction for the fractal properties of RAP are consistent with numerical simulations made in two, three and four dimensions.

If you want to explore a RAP in depth, feel free to zoom in that
**picture**, it
contains more than a million spheres.

At 2pm, room E349, Nilanjan Bhaumik (IIS), will be talking about

*Primordial black hole (PBH) is one of the most promising cold dark
matter candidates in recent years. The signature of PBHs in the
stochastic gravitational-wave background (SGWB) is natural as PBH
formation requires a significant amplification in inflationary scalar
curvature perturbation, which sources the tensor perturbation in
second order and amplifies the SGWB. We explore this possibility to
probe different possible reheating histories. We find that any change
in reheating histories can shift the PBH mass range and the peak
frequency of SGWB. Still, only the matter-dominated reheating phase
can lead to an additional resonant amplification in SGWB at a very
high frequency.
Isocurvature perturbation from PBH distributions
contributes to the adiabatic perturbation. It also leads to
detectable amplification in the induced GW background and can
contribute to Baryogenesis for ultra-low mass PBHs, which can
dominate the universe for a small duration. Combining these two
effects can help us probe primordial black hole scenarios and details
of reheating histories, leaving diverse implications for cosmology
and particle physics.*

Cosmic Inflation is an hypothetical early phase of accelerated expansion that has occurred before the first billionth of a second of existence of our Universe. It provides a natural mechanism to explain the observed flatness of our Universe today and naturally solves the so-called horizon problems of the Big-Bang model.

In a spectacular way, the quantum fluctuations that are inherently
sourced during the inflationary era are exactly what is needed to
explain the origin of the cosmological perturbations: the **seeds of the
galaxies** of today.

These quantum fluctuations are deeply rooted in gravity and appear as both primordial gravitational waves \(h_{ij}\) and curvature perturbations \(\zeta\), with very peculiar correlation functions. In Ref. [1], we have pushed to third order the calculation of these correlation functions. They are completely determined by the Hubble parameter during inflation \(H(N)\) and its logarithmic derivatives, \(\epsilon_i(N) \equiv \mathrm{d}\ln H / \mathrm{d} \ln N\) (the so-called Hubble flow functions). Here \(N=\ln a\) is the logarithm of the scale factor \(a\).

Slow-roll inflation **predicts** the correlation functions to be
given by these spectra:

They are expanded around an observable wavenumber
\(k_*=0.05\,\mathrm{Mpc}^{-1}\) and readily testable with the
incoming cosmological observations from the
**Euclid** and
**LiteBird** space
telescopes, but also from the ground based
**CMB-S4** telescopes and **Simons
Observatory**. Are
we going to detect a non-vanishing \(\epsilon_{3*}\)?

On display, you’ll find a black and white fractal image, extracted from Ref. [1].

*Cosmic inflation is an hypothetic phase of accelerated expansion of
the Universe, a tiny fraction of second after the Big-Bang, induced by
the existence of a scalar field in the early Universe and suspected to
be at the origin of the present structures of the Universe. During
inflation, in about \(10^{-35}\) second, the distance between any two
points in the Universe separated by one meter becomes larger than the
distance to the farthest galaxies. This makes inflation one of the
most fascinating phenomena in Science.*

*This picture shows the results of more than 4 millions numerical
simulations of inflation in a model with two scalar fields, called
hybrid inflation, for which inflation stops due a broken symmetry,
similar to the Brout-Englert-Higgs mechanism. The \(x\) and \(y\) axis
represent the initial value of each scalar field, a white dot indicate
that the simulation led to enough expansion for being compatible with
our Universe, whereas for black dots, there is no, or not enough,
inflation.*

*Before these results were obtained in Ref. [1],
at ULB and UCLouvain, it was thought that only the thin vertical white
band could lead to inflation, suggesting a fine-tuning problem. This
picture shows that, on the contrary, a large number of initial
conditions are satisfactory and form a complex fractal structure. In
analogy with anamorphosis, a phenomena consisting in the distortion of
an image through an optical instrument, one can see this fractal
structure as the image of the thin vertical band seen through the
potential associated with the scalar fields, playing the role of the
optical instrument.*

Using the recently released public domain data DR3 from the GAIA satellite (ESA), we have reconstructed a view of our night sky, as you would see it with very sensitive eyes, eyes that would allow you to actually see daylight colors but in the dark.

*This image represents the integrated light flux coming from all faint
sources of light, of magnitude greater than \(10\), in human-visible
colors (sRGB). These sources are mostly stars from our own Galaxy and
have been measured by the Gaia
satellite
(ESA/Gaia/DPAC). There are more than 1.5 billion of stars accounted
for in this image. The image making code is available here:
gaialaxy.*

Extracted from Ref. [2], a small piece of a computer generated all sky map of the Cosmic Microwave Background generated by Cosmic Strings.

*These objects are line-like defects in the fabric of space-time and
could have been formed in the earliest times of the Universe history.
This picture is a small part of a full sky image computed using
\(12000\) processors of one of the largest world computer at that
time: the Cray XE6
Hopper
at the NERSC. This image has been used
to search for Cosmic Strings in the Planck
satellite data
(ESA).*

In Ref. [1], we have run new simulations of Nambu-Goto cosmic strings evolving during the radiation, the transition, and the matter eras to compute the unequal time correlators of the anisotropic stress tensor associated with the long strings. The following figure shows a snapshot of one of these simulation, the long strings have been represented in white whereas all the other objects are loops, see Ref. [2] and this post.

This correlator sources the gravitational waves and it allows us to solve for their creation, and propagation, all along the Universe history. By using the Green’s function method we can then predict the strain, \(k^2 \mathcal{P}_h\), and the energy density parameter \(\Omega_{\mathrm{GW}}^{\mathrm{mat}}\) of the gravitational waves that can be measured today. Their power spectra are represented below as a function of the wavenumber \(k\).

In these figures, the exact numerical result is represented in black while the blue and red curves show some semi-analytical approximations that we had proposed in a previous paper, calibrated using the amplitude found with the simulations, see Ref. [3]. The only significant deviations show up around \(k/\mathcal{H}_0 \simeq 100\), which corresponds to the transition between the radiation and matter era. An interesting point to notice is that most of strain signal is actually generated by the long cosmic strings in the matter era, i.e., close to us.

As we discuss in Ref. [1], this signal is quite
small, but reachable by the
**LISA**
satellites. These ones will be sensitive to long strings that are
undetectable today in the Cosmic Microwave Background.

LISA can potentially detect GW signals from first order phase transitions occurring in the energy range \(100\,\mathrm{GeV}\) – \(1 \, \mathrm{TeV}\). There are several processes possibly leading to sizeable emission of GWs:

- Bubble percolation, with the consequent breaking of spherical symmetry, is the most direct one
- Bubble collision
- Sound waves produced in the fluid by expanding bubbles
- Decaying turbulence in the fluid

In Ref. [1], we propose a new model to the decay and decorrelation of a purely vortical velocity field and calculate the subsequent GW signal. To confirm the accuracy and relevance of the model, we have carried out a campaign of massively parallel direct numerical simulations of decaying turbulence. The figure below shows a slice across one of these simulations.

The following figure shows the initial conditions for the velocity field in real space

After some evolution, the velocity field decays and small scale
structures develop. The next figure shows the velocity field
after \(20\)
**eddy**
turnover times

Based on this modelling, we provide an approximate analytical form for the GW signal which could be readily used within the LISA collaboration to search for it. In the figure below, coloured lines show the results of the simulations, the black line the semi-analytical model and the grey line the analytical form usable for LISA.

Other new findings are presented in Ref. [1]. For instance, including an initial growth phase for the turbulent flow is shown to heavily influences the spectral shape of the SGWB. This highlights the importance of a complete understanding of the turbulence generation mechanism.

]]>At 11:00, room E349, Hugo Rousille (APC), will be talking about

*General relativity can be tested at many scales using various
physical systems. A particularly interesting probe is the study of
the ringdown phase of a binary black hole merger, during which the
gravitational waves emitted by the newly-formed black hole are well
described by perturbation theory over a single black hole
background. The features of these waves depend heavily on the theory
of gravity underlying the solution and the solution
itself. Therefore, they can be used both to test GR and rule out some
black hole solutions. We focus in this talk on the study of new
behaviours for these waves in scalar-tensor theories in the
Degenerate Higher-Order Scalar-Tensor (DHOST) class. In such
theories, the usual dichotomy between axial (odd parity) and polar
(even parity) perturbations no longer enables one to decouple all
degrees of freedom since the even parity sector contains one
additional scalar degree of freedom. Axial perturbations can still be
completely decoupled: we show that they propagate in an effective
metric that differs from the background metric, and is not always a
black hole metric. In the case of polar perturbations, we present a
systematic approach that extracts the asymptotic behaviour of
perturbations at spatial infinity and near the horizon directly from
the perturbed Einstein’s equations. This asymptotic behavior gives
physical insight about the modes, such as their direction and speed
of propagation, and allows us to rule out some solutions that
explicit bad behavior. Furthermore, we can use the obtained
asymptotic behavior to numerically solve the propagation equations
and compute quasinormal modes in systems where this was not possible
before.*

At 11:00, room E349, Fabien Lacasa (IAS), will be talking about

*The large scale structure of the Universe, the cosmic web, is a key
probe for late-time cosmology, and is indeed targeted by several of
the major observational efforts in the coming decades. However, its
analysis is hampered by the complex non-linear dynamics it undergoes
as structure grow. A lot of efforts has focused on the impact of
non-linearity on the observables, in particular the power
spectrum. However, the non-linearity also affects error bars, and thus
the information content, by generating non-Gaussian tails that induce
new covariance term. After sufficient introduction, I will present a
first exhaustive derivation of the covariance of the galaxy angular
power spectrum. The impact will then be presented for a baseline
cosmological analysis up to mildly non-linear scale, showing
decreasing returns. By contrast, I will finally show that there is a
large motivation to jump more boldly and push into the strongly
non-linear regime at high resolution.*