63 research outputs found
An introduction to inflation after Planck: from theory to observations
These lecture notes have been written for a short introductory course on the
status of inflation after Planck and BICEP2, given at the Xth Modave School of
Mathematical Physics. The first objective is to give an overview of the theory
of inflation: motivations, homogeneous scalar field dynamics, slow-roll
approximation, linear theory of cosmological perturbations, classification of
single field potentials and their observable predictions. This includes a
pedagogical derivation of the primordial scalar and tensor power spectra for
any effective single field potential. The second goal is to present the most
recent results of Planck and BICEP2 and to discuss their implications for
inflation. Bayesian statistical methods are introduced as a tool for model
analysis and comparison. Based on the recent work of J. Martin et al., the best
inflationary models after Planck and BICEP2 are presented. Finally a series of
open questions and issues related to inflation are mentioned and briefly
discussed.Comment: 39 pages, 9 figures, to be published in the proceedings of the Xth
Modave School in Mathematical Physics. Important parts of those lecture notes
draw from chapters 1 and 2 of the author's PhD thesis, arXiv:1109.557
Anamorphosis in hybrid inflation: How to avoid fine-tuning of initial conditions?
In order to generate more than 60 e-folds of accelerated expansion in
original hybrid inflation, 2-fields trajectories are usually required to be
initially fine-tuned in a very narrow band along the inflationary valley or in
some isolated points outside it. From a more precise investigation of the
dynamics, these points which can cover a non-negligible proportion of the space
of sub-planckian initial field values, depending on the potential parameters,
are shown to be organised in connected domains with fractal boundaries. They
correspond to trajectories first falling towards the bottom of the potential,
then climbing and slow-rolling back along the inflationary valley. The full
parameter space, including initial velocities and all the potential parameters,
is then explored by using Monte-Carlo-Markov-Chains (MCMC) methods. Results
indicate that successful initial conditions (IC) outside the valley are not
localized in the parameter space and are the dominant way to realise inflation,
independently of initial field velocities. Natural bounds on parameters are
deduced. The genericity of our results is confirmed in 5 other hybrid models
from various framework.Comment: AIP Proceedings of the "Invisible Universe" conference, Palais de
l'Unesco, Paris, 29 June - 4 July 200
Updated Constraints on Large Field Hybrid Inflation
We revisit the status of hybrid inflation in the light of Planck and recent
BICEP2 results, taking care of possible transient violations of the slow-roll
conditions as the field passes from the large field to the vacuum dominated
phase. The usual regime where observable scales exit the Hubble radius in the
vacuum dominated phase predicts a blue scalar spectrum, which is ruled out. But
whereas assuming slow-roll one expects this regime to be generic, by solving
the exact dynamics we identify the parameter space for which the small field
phase is naturally avoided due to slow-roll violations at the end of the large
field phase. When the number of e-folds generated at small field is negligible,
the model predictions are degenerated with those of a quadratic potential.
There exists also a transitory case for which the small field phase is
sufficiently long to affect importantly the observable predictions.
Interestingly, in this case the spectral index and the tensor to scalar ratio
agree respectively with the best fit of Planck and BICEP2. This results in a
\Delta \chi^2 \simeq 5.0 in favor of hybrid inflation for Planck+BICEP2 (\Delta
\chi^2 \simeq 0.9 for Planck only). The last considered regime is when the
critical point at which inflation ends is located in the large field phase. It
is constrained to be lower than about ten times the reduced Planck mass. The
analysis has been conducted with the use of Markov-Chain-Monte-Carlo bayesian
method, in a reheating consistent way, and we present the posterior probability
distributions for all the model parameters.Comment: 13 pages, 9 figures, comments welcom
Detecting the gravitational wave background from primordial black hole dark matter
The black hole merging rates inferred after the gravitational-wave detection
by Advanced LIGO/VIRGO and the relatively high mass of the progenitors are
consistent with models of dark matter made of massive primordial black holes
(PBH). PBH binaries emit gravitational waves in a broad range of frequencies
that will be probed by future space interferometers (LISA) and pulsar timing
arrays (PTA). The amplitude of the stochastic gravitational-wave background
expected for PBH dark matter is calculated taking into account various effects
such as initial eccentricity of binaries, PBH velocities, mass distribution and
clustering. It allows a detection by the LISA space interferometer, and
possibly by the PTA of the SKA radio-telescope. Interestingly, one can
distinguish this background from the one of non-primordial massive binaries
through a specific frequency dependence, resulting from the maximal impact
parameter of binaries formed by PBH capture, depending on the PBH velocity
distribution and their clustering properties. Moreover, we find that the
gravitational wave spectrum is boosted by the width of PBH mass distribution,
compared with that of the monochromatic spectrum. The current PTA constraints
already rule out broad-mass PBH models covering more than three decades of
masses, but evading the microlensing and CMB constraints due to clustering.Comment: 12 pages, 4 figure
Fractal initial conditions and natural parameter values in hybrid inflation
We show that the initial field values required to produce inflation in the
two fields original hybrid model, and its supergravity F-term extension, do not
suffer from any fine-tuning problem, even when the fields are restricted to be
sub-planckian and for almost all potential parameter values. This is due to the
existence of an initial slow-roll violating evolution which has been overlooked
so far. Due to the attractor nature of the inflationary valley, these
trajectories end up producing enough accelerated expansion of the universe. By
numerically solving the full non-linear dynamics, we show that the set of such
successful initial field values is connected, of dimension two and possesses a
fractal boundary of infinite length exploring the whole field space. We then
perform a Monte-Carlo-Markov-Chain analysis of the whole parameter space
consisting of the initial field values, field velocities and potential
parameters. We give the marginalised posterior probability distributions for
each of these quantities such that the universe inflates long enough to solve
the usual cosmological problems. Inflation in the original hybrid model and its
supergravity version appears to be generic and more probable by starting
outside of the inflationary valley. Finally, the implication of our findings in
the context of the eternal inflationary scenario are discussed.Comment: 16 pages, 16 figures, uses RevTeX. Lyapunov exponents and references
added, matches published versio
Probing Modified Gravity with Atom-Interferometry: a Numerical Approach
Refined constraints on chameleon theories are calculated for
atom-interferometry experiments, using a numerical approach consisting in
solving for a four-region model the static and spherically symmetric
Klein-Gordon equation for the chameleon field. By modeling not only the test
mass and the vacuum chamber but also its walls and the exterior environment,
the method allows to probe new effects on the scalar field profile and the
induced acceleration of atoms. In the case of a weakly perturbing test mass,
the effect of the wall is to enhance the field profile and to lower the
acceleration inside the chamber by up to one order of magnitude. In the
thin-shell regime, results are found to be in good agreement with the
analytical estimations, when measurements are realized in the immediate
vicinity of the test mass. Close to the vacuum chamber wall, the acceleration
becomes negative and potentially measurable. This prediction could be used to
discriminate between fifth-force effects and systematic experimental
uncertainties, by doing the experiment at several key positions inside the
vacuum chamber. For the chameleon potential and a coupling function , one finds , independently of the power-law index.
For , one finds . A sensitivity of would probe
the model up to the Planck scale. Finally, a proposal for a second experimental
set-up, in a vacuum room, is presented. In this case, Planckian values of
could be probed provided that , a limit
reachable by future experiments. Our method can easily be extended to constrain
other models with a screening mechanism, such as symmetron, dilaton and f(R)
theories.Comment: 13 pages, 12 figures, version accepted by PR
LIGO Lo(g)Normal MACHO: Primordial Black Holes survive SN lensing constraints
It has been claimed in Ref.[arXiv:1712.02240] that massive primordial black
holes (PBH) cannot constitute all of the dark matter (DM), because their
gravitational-lensing imprint on the Hubble diagram of type Ia supernovae (SN)
would be incompatible with present observations. In this paper, we critically
review those constraints and find several caveats on the analysis. First of
all, the constraints on the fraction of PBH in matter seem to be
driven by a very restrictive choice of priors on the cosmological parameters.
In particular, the degeneracy between and is ignored
and thus, by fixing , transferred the constraining power of SN
magnitudes to . Furthermore, by considering more realistic physical
sizes for the type-Ia supernovae, we find an effect on the SN lensing
magnification distribution that leads to significantly looser constraints.
Moreover, considering a wide mass spectrum of PBH, such as a lognormal
distribution, further softens the constraints from SN lensing. Finally, we find
that the fraction of PBH that could constitute DM today is bounded by , for JLA (Union 2.1) catalogs, and thus it is perfectly
compatible with an all-PBH dark matter scenario in the LIGO band.Comment: 7 pages, 7 figure
Primordial black holes from the QCD epoch: Linking dark matter, baryogenesis and anthropic selection
If primordial black holes (PBHs) formed at the quark-hadron epoch, their mass
must be close to the Chandrasekhar limit, this also being the characteristic
mass of stars. If they provide the dark matter (DM), the collapse fraction must
be of order the cosmological baryon-to-photon ratio , which
suggests a scenario in which a baryon asymmetry is produced efficiently in the
outgoing shock around each PBH and then propagates to the rest of the Universe.
We suggest that the temperature increase in the shock provides the ingredients
for hot spot electroweak baryogenesis. This also explains why baryons and DM
have comparable densities, the precise ratio depending on the size of the PBH
relative to the cosmological horizon at formation. The observed value of the
collapse fraction and baryon asymmetry depends on the amplitude of the
curvature fluctuations which generate the PBHs and may be explained by an
anthropic selection effect associated with the existence of galaxies. We
propose a scenario in which the quantum fluctuations of a light stochastic
spectator field during inflation generate large curvature fluctuations in some
regions, with the stochasticity of this field providing the basis for the
required selection. Finally, we identify several observational predictions of
our scenario that should be testable within the next few years. In particular,
the PBH mass function could extend to sufficiently high masses to explain the
black hole coalescences observed by LIGO/Virgo.Comment: 37 pages, 3 figures, published in MNRA
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