378 research outputs found
Testing Two-Field Inflation
We derive semi-analytic formulae for the power spectra of two-field inflation
assuming an arbitrary potential and non-canonical kinetic terms, and we use
them both to build phenomenological intuition and to constrain classes of
two-field models using WMAP data. Using covariant formalism, we first develop a
framework for understanding the background field kinematics and introduce a
"slow-turn" approximation. Next, we find covariant expressions for the
evolution of the adiabatic/curvature and entropy/isocurvature modes, and we
discuss how the mode evolution can be inferred directly from the background
kinematics and the geometry of the field manifold. From these expressions, we
derive semi-analytic formulae for the curvature, isocurvature, and cross
spectra, and the spectral observables, all to second-order in the slow-roll and
slow-turn approximations. In tandem, we show how our covariant formalism
provides useful intuition into how the characteristics of the inflationary
Lagrangian translate into distinct features in the power spectra. In
particular, we find that key features of the power spectra can be directly read
off of the nature of the roll path, the curve the field vector rolls along with
respect to the field manifold. For example, models whose roll path makes a
sharp turn 60 e-folds before inflation ends tend to be ruled out because they
produce strong departures from scale invariance. Finally, we apply our
formalism to confront four classes of two-field models with WMAP data,
including doubly quadratic and quartic potentials and non-standard kinetic
terms, showing how whether a model is ruled out depends not only on certain
features of the inflationary Lagrangian, but also on the initial conditions.
Ultimately, models must possess the right balance of kinematical and dynamical
behaviors, which we capture in a set of functions that can be reconstructed
from spectral observables.Comment: Revised to match accepted PRD version: Improved discussion of
background kinematics and multi-field effects, added tables summarizing key
quantities and their links to observables, more detailed figures, fixed typos
in former equations (103) and (117). 49 PRD pages, 11 figure
Testing Multi-Field Inflation: A Geometric Approach
We develop an approach for linking the power spectra, bispectrum, and
trispectrum to the geometric and kinematical features of multifield
inflationary Lagrangians. Our geometric approach can also be useful in
determining when a complicated multifield model can be well approximated by a
model with one, two, or a handful of fields. To arrive at these results, we
focus on the mode interactions in the kinematical basis, starting with the case
of no sourcing and showing that there is a series of mode conservation laws
analogous to the conservation law for the adiabatic mode in single-field
inflation. We then treat the special case of a quadratic potential with
canonical kinetic terms, showing that it produces a series of mode sourcing
relations identical in form to that for the adiabatic mode. We build on this
result to show that the mode sourcing relations for general multifield
inflation are extension of this special case but contain higher-order covariant
derivatives of the potential and corrections from the field metric. In
parallel, we show how these interactions depend on the geometry of the
inflationary Lagrangian and on the kinematics of the associated field
trajectory. Finally, we consider how the mode interactions and effective number
of fields active during inflation are reflected in the spectra and introduce a
multifield consistency relation, as well as a multifield observable that can
potentially distinguish two-field scenarios from scenarios involving three or
more effective fields.Comment: 21 pages, 4 figures + tables. Revised to clarify several points and
reorganized Section III for pedagogical reasons. Error in one equation and
typos were corrected, as well as additional references adde
Enhancing the tensor-to-scalar ratio in simple inflation
We show that in theories with a nontrivial kinetic term the contribution of
the gravitational waves to the CMB fluctuations can be substantially larger
than that is naively expected in simple inflationary models. This increase of
the tensor-to-scalar perturbation ratio leads to a larger B-component of the
CMB polarization, thus making the prospects for future detection much more
promising. The other important consequence of the considered model is a higher
energy scale of inflation and hence higher reheating temperature compared to a
simple inflation.Comment: 9 pages, 1 figure and references are added, discussion is slightly
extended, published versio
Back-reaction of Cosmological Fluctuations during Power-Law Inflation
We study the renormalized energy-momentum tensor of cosmological scalar
fluctuations during the slow-rollover regime for power-law inflation and find
that it is characterized by a negative energy density at the leading order,
with the same time behaviour as the background energy. The average expansion
rate appears decreased by the back-reaction of the effective energy of
cosmological fluctuations, but this value is comparable with the energy of
background only if inflation starts at a Planckian energy. We also find that,
for this particular model, the first and second order inflaton fluctuations are
decoupled and satisfy the same equation of motion. To conclude, the fourth
order adiabatic expansion for the inflaton scalar field is evaluated for a
general potential V(\phi).Comment: 9 pages, no figures, revtex. Some changes made, comments and
references added, conclusions unchanged, version accepted for pubblication in
Phys. Rev.
First CMB Constraints on the Inflationary Reheating Temperature
We present the first Bayesian constraints on the single field inflationary
reheating era obtained from Cosmic Microwave Background (CMB) data. After
demonstrating that this epoch can be fully characterized by the so-called
reheating parameter, we show that it is constrained by the seven years
Wilkinson Microwave Anisotropies Probe (WMAP7) data for all large and small
field models. An interesting feature of our approach is that it yields lower
bounds on the reheating temperature which can be combined with the upper bounds
associated with gravitinos production. For large field models, we find the
energy scale of reheating to be higher than those probed at the Large Hadron
Collider, Ereh > 17.3 TeV at 95% of confidence. For small field models, we
obtain the two-sigma lower limits Ereh > 890 TeV for a mean equation of state
during reheating = -0.3 and Ereh > 390 GeV for = -0.2. The
physical origin of these constraints is pedagogically explained by means of the
slow-roll approximation. Finally, when marginalizing over all possible
reheating history, the WMAP7 data push massive inflation under pressure (p <
2.2 at 95% of confidence where p is the power index of the large field
potentials) while they slightly favor super-Planckian field expectation values
in the small field models.Comment: 18 pages, 15 figures, uses RevTeX. References added, matches
published versio
Hunting Down the Best Model of Inflation with Bayesian Evidence
We present the first calculation of the Bayesian evidence for different
prototypical single field inflationary scenarios, including representative
classes of small field and large field models. This approach allows us to
compare inflationary models in a well-defined statistical way and to determine
the current "best model of inflation". The calculation is performed numerically
by interfacing the inflationary code FieldInf with MultiNest. We find that
small field models are currently preferred, while large field models having a
self-interacting potential of power p>4 are strongly disfavoured. The class of
small field models as a whole has posterior odds of approximately 3:1 when
compared with the large field class. The methodology and results presented in
this article are an additional step toward the construction of a full numerical
pipeline to constrain the physics of the early Universe with astrophysical
observations. More accurate data (such as the Planck data) and the techniques
introduced here should allow us to identify conclusively the best inflationary
model.Comment: 12 pages, 2 figures, uses RevTeX. Misprint corrected, references
added. Matches published versio
Inflation without Slow Roll
We draw attention to the possibility that inflation (i.e. accelerated
expansion) might continue after the end of slow roll, during a period of fast
oscillations of the inflaton field \phi . This phenomenon takes place when a
mild non-convexity inequality is satisfied by the potential V(\phi). The
presence of such a period of \phi-oscillation-driven inflation can
substantially modify reheating scenarios.
In some models the effect of these fast oscillations might be imprinted on
the primordial perturbation spectrum at cosmological scales.Comment: 9 pages, Revtex, psfig, 1 figure, minor modifications, references
adde
Second Order Gauge-Invariant Perturbations during Inflation
The evolution of gauge invariant second-order scalar perturbations in a
general single field inflationary scenario are presented. Different second
order gauge invariant expressions for the curvature are considered. We evaluate
perturbatively one of these second order curvature fluctuations and a second
order gauge invariant scalar field fluctuation during the slow-roll stage of a
massive chaotic inflationary scenario, taking into account the deviation from a
pure de Sitter evolution and considering only the contribution of super-Hubble
perturbations in mode-mode coupling. The spectra resulting from their
contribution to the second order quantum correlation function are nearly
scale-invariant, with additional logarithmic corrections to the first order
spectrum. For all scales of interest the amplitude of these spectra depend on
the total number of e-folds. We find, on comparing first and second order
perturbation results, an upper limit to the total number of e-folds beyond
which the two orders are comparable.Comment: 17 pages, 6 figures. Final version to appear in Phys. Rev.
Sub-eV scalar dark matter through the super-renormalizable Higgs portal
The Higgs portal of the Standard Model provides the opportunity for coupling
to a very light scalar field via the super-renormalizable operator
. This allows for the existence of a very light scalar dark
matter that has coherent interaction with the Standard Model particles and yet
has its mass protected against radiative corrections. We analyze ensuing
constraints from the fifth-force measurements, along with the cosmological
requirements. We find that the detectable level of the fifth-force can be
achieved in models with low inflationary scales, and certain amount of
fine-tuning in the initial deviation of from its minimum.Comment: 6 pages, 3 figures. References added in the revised version
Observational tests of inflation with a field derivative coupling to gravity
A field kinetic coupling with the Einstein tensor leads to a gravitationally
enhanced friction during inflation, by which even steep potentials with
theoretically natural model parameters can drive cosmic acceleration. In the
presence of this non-minimal derivative coupling we place observational
constraints on a number of representative inflationary models such as chaotic
inflation, inflation with exponential potentials, natural inflation, and hybrid
inflation. We show that most of the models can be made compatible with the
current observational data mainly due to the suppressed tensor-to-scalar ratio.Comment: 11 pages, 5 figure
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