1,480 research outputs found

### Inflation over the hill

We calculate the power spectrum of curvature perturbations when the inflaton
field is rolling over the top of a local maximum of a potential. We show that
the evolution of the field can be decomposed into a late-time attractor, which
is identified as the slow roll solution, plus a rapidly decaying non-slow roll
solution, corresponding to the field rolling ``up the hill'' to the maximum of
the potential. The exponentially decaying transient solution can map to an
observationally relevant range of scales because the universe is also expanding
exponentially. We consider the two branches separately and we find that they
are related through a simple transformation of the slow roll parameter $\eta$
and they predict identical power spectra. We generalize this approach to the
case where the inflaton field is described by both branches simultaneously and
find that the mode equation can be solved exactly at all times. Even though the
slow roll parameter $\eta$ is evolving rapidly during the transition from the
transient solution to the late-time attractor solution, the resultant power
spectrum is an exact power-law spectrum. Such solutions may be useful for
model-building on the string landscape.Comment: 11 pages, 1 figure (V3: Version accepted by PRD, title changed by
journal

### On Adiabatic Renormalization of Inflationary Perturbations

We discuss the impact of adiabatic renormalization on the power spectrum of
scalar and tensor perturbations from inflation. We show that adiabatic
regularization is ambiguous as it leads to very different results, for
different adiabatic subtraction schemes, both in the range v\equiv k/(aH)
\gsim 0.1 and in the infrared regime. All these schemes agree in the far
ultraviolet, $v\gg 1$. Therefore, we argue that in the far infrared regime,
$v\ll 1$, the adiabatic expansion is no longer valid, and the unrenormalized
spectra are the physical, measurable quantities. These findings cast some doubt
on the validity of the adiabatic subtraction at horizon exit, $v=1$, to
determine the perturbation spectra from inflation which has recently advocated
in the literature.Comment: 7 pages, 3 figures, revtex. New version with more results and
modified plot

### Nucleosynthesis Without a Computer

I derive completely analytically the time evolution and final abundances of
the light elements (up to Be-7) formed in the big-bang nucleosynthesis.This
highlights an interesting physics taking place during the formation of light
elements in the early universe.Comment: 3 figures; uses tcilate

### Cosmological Inflation and the Quantum Measurement Problem

According to cosmological inflation, the inhomogeneities in our universe are
of quantum mechanical origin. This scenario is phenomenologically very
appealing as it solves the puzzles of the standard hot big bang model and
naturally explains why the spectrum of cosmological perturbations is almost
scale invariant. It is also an ideal playground to discuss deep questions among
which is the quantum measurement problem in a cosmological context. Although
the large squeezing of the quantum state of the perturbations and the
phenomenon of decoherence explain many aspects of the quantum to classical
transition, it remains to understand how a specific outcome can be produced in
the early universe, in the absence of any observer. The Continuous Spontaneous
Localization (CSL) approach to quantum mechanics attempts to solve the quantum
measurement question in a general context. In this framework, the wavefunction
collapse is caused by adding new non linear and stochastic terms to the
Schroedinger equation. In this paper, we apply this theory to inflation, which
amounts to solving the CSL parametric oscillator case. We choose the
wavefunction collapse to occur on an eigenstate of the Mukhanov-Sasaki variable
and discuss the corresponding modified Schroedinger equation. Then, we compute
the power spectrum of the perturbations and show that it acquires a universal
shape with two branches, one which remains scale invariant and one with nS=4, a
spectral index in obvious contradiction with the Cosmic Microwave Background
(CMB) anisotropy observations. The requirement that the non-scale invariant
part be outside the observational window puts stringent constraints on the
parameter controlling the deviations from ordinary quantum mechanics...
(Abridged).Comment: References added, minor corrections, conclusions unchange

### A note on the equivalence of a barotropic perfect fluid with a K-essence scalar field

In this short note, we obtain the necessary and sufficient condition for a
class of non-canonical single scalar field models to be exactly equivalent to
barotropic perfect fluids, under the assumption of an irrotational fluid flow.
An immediate consequence of this result is that the non-adiabatic pressure
perturbation in this class of scalar field systems vanishes exactly at all
orders in perturbation theory and on all scales. The Lagrangian for this
general class of scalar field models depends on both the kinetic term and the
value of the field. However, after a field redefinition, it can be effectively
cast in the form of a purely kinetic K-essence model.Comment: 4 pages, 1 figure; v2: References and footnotes 3 and 4 added.
Replaced to match published versio

### Possible Explanation to Low CMB Quadrupole

The universe might experience many cycles with different vacua. The slow-roll
inflation may be preceded by kinetic-dominated contraction occurring in
"adjacent" vacua during some cycles. In this report we briefly show this
phenomenon may lead to a cutoff of primordial power spectrum. Thus in some
sense the CMB at large angular scale might encode the information of other
vacua.Comment: 10 pages, 3 eps figures, accepted for publication in PRD, v2 revised
with published versio

### Quantum modes in DBI inflation: exact solutions and constraints from vacuum selection

We study a two-parameter family of exactly solvable inflation models with
variable sound speed, and derive a corresponding exact expression for the
spectrum of curvature perturbations. We generalize this expression to the slow
roll case, and derive an approximate expression for the scalar spectral index
valid to second order in slow roll. We apply the result to the case of DBI
inflation, and show that for certain choices of slow roll parameters, the
Bunch-Davies limit (a) does not exist, or (b) is sensitive to stringy physics
in the bulk, which in principle can have observable signatures in the
primordial power spectrum.Comment: 10 pages, LaTeX; V2: version submitted to PRD. References added,
minor error in text correcte

### Non-Gaussianity of scalar perturbations generated by conformal mechanisms

We consider theories which explain the flatness of the power spectrum of
scalar perturbations in the Universe by conformal invariance, such as conformal
rolling model and Galilean Genesis. We show that to the leading {\it
non-linear} order, perturbations in all models from this class behave in one
and the same way, at least if the energy density of the relevant fields is
small compared to the total energy density (spectator approximation). We then
turn to the intrinsic non-Gaussianities in these models (as opposed to
non-Gaussianities that may be generated during subsequent evolution). The
intrinsic bispectrum vanishes, so we perform the complete calculation of the
trispectrum and compare it with the trispecta of local forms in various limits.
The most peculiar feature of our trispectrum is a (fairly mild) singularity in
the limit where two momenta are equal in absolute value and opposite in
direction (folded limit). Generically, the intrinsic non-Gaussianity can be of
detectable size.Comment: 28 pages, 5 figures. Journal version. A comment on the size of the
non-Gaussianities inserted. Misprints corrected. A reference adde

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