1,093 research outputs found
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
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
The Maximal Amount of Gravitational Waves in the Curvaton Scenario
The curvaton scenario for the generation of the cosmological curvature
perturbation on large scales represents an alternative to the standard
slow-roll scenario of inflation in which the observed density perturbations are
due to fluctuations of the inflaton field itself. Its basic assumption is that
the initial curvature perturbation due to the inflaton field is negligible.
This is attained by lowering the energy scale of inflation, thereby highly
suppressing the amount of gravitational waves produced during inflation. We
compute the power-spectrum of the gravitational waves generated at second order
in perturbation theory by the curvaton (isocurvature) perturbations between the
end of inflation and the curvaton decay. An interesting property of this
contribution to the tensor perturbations is that it is directly proportional to
the amount of non-Gaussianity predicted within the curvaton scenario. We show
that the spectrum of gravitational waves may be in the range of future
gravitational wave detectors.Comment: 4 pages, laTeX; added a clarifying comment in the conclusions,
version matches publication in PRD, Rapid Communication
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
Path Integral Quantization of Cosmological Perturbations
We derive the first order canonical formulation of cosmological perturbation
theory in a Universe filled by a few scalar fields. This theory is quantized
via well-defined Hamiltonian path integral. The propagator which describes the
evolution of the initial (for instance, vacuum) state, is calculated.Comment: 16 pages, ETH-TH/94-0
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.
A Radiation Bounce from the Lee-Wick Construction?
It was recently realized that matter modeled by the scalar field sector of
the Lee-Wick Standard Model yields, in the context of a homogeneous and
isotropic cosmological background, a bouncing cosmology. However, bouncing
cosmologies induced by pressure-less matter are in general unstable to the
addition of relativistic matter (i.e. radiation). Here we study the possibility
of obtaining a bouncing cosmology if we add not only radiation, but also its
Lee-Wick partner, to the matter sector. We find that, in general, no bounce
occurs. The only way to obtain a bounce is to choose initial conditions with
very special phases of the radiation field and its Lee-Wick partner.Comment: 11 page
Non-Gaussian features of primordial magnetic fields in power-law inflation
We show that a conformal-invariance violating coupling of the inflaton to
electromagnetism produces a cross correlation between curvature fluctuations
and a spectrum of primordial magnetic fields. According to this model, in the
case of power-law inflation, a primordial magnetic field is generated with a
nearly flat power spectrum and rms amplitude ranging from nG to pG. We study
the cross correlation, a three-point function of the curvature perturbation and
two powers of the magnetic field, in real and momentum space. The
cross-correlation coefficient, a dimensionless ratio of the three-point
function with the curvature perturbation and magnetic field power spectra, can
be several orders of magnitude larger than expected as based on the amplitude
of scalar metric perturbations from inflation. In momentum space, the
cross-correlation peaks for flattened triangle configurations, and is three
orders of magnitude larger than the squeezed triangle configuration. These
results suggest likely methods for distinguishing the observational signatures
of the model.Comment: 15 pages, 2 figure
Free Quantum Fields on the Poincare' Group
A class of free quantum fields defined on the Poincare' group, is described
by means of their two-point vacuum expectation values. They are not equivalent
to fields defined on the Minkowski spacetime and they are "elementary" in the
sense that they describe particles that transform according to irreducible
unitary representations of the symmetry group, given by the product of the
Poincare' group and of the group SL(2, C) considered as an internal symmetry
group. Some of these fields describe particles with positive mass and arbitrary
spin and particles with zero mass and arbitrary helicity or with an infinite
helicity spectrum. In each case the allowed SL(2, C) internal quantum numbers
are specified. The properties of local commutativity and the limit in which one
recovers the usual field theories in Minkowski spacetime are discussed. By
means of a superposition of elementary fields, one obtains an example of a
field that present a broken symmetry with respect to the group Sp(4, R), that
survives in the short-distance limit. Finally, the interaction with an
accelerated external source is studied and and it is shown that, in some
theories, the average number of particles emitted per unit of proper time
diverges when the acceleration exceeds a finite critical value.Comment: 49 pages, plain tex with vanilla.st
New Ekpyrotic Cosmology
In this paper, we present a new scenario of the early Universe that contains
a pre big bang Ekpyrotic phase. By combining this with a ghost condensate, the
theory explicitly violates the null energy condition without developing any
ghost-like instabilities. Thus the contracting universe goes through a
non-singular bounce and evolves smoothly into the expanding post big bang
phase. The curvature perturbation acquires a scale-invariant spectrum well
before the bounce in this scenario. It is sourced by the scale-invariant
entropy perturbation engendered by two ekpyrotic scalar fields, a mechanism
recently proposed by Lehners et al. Since the background geometry is
non-singular at all times, the curvature perturbation remains nearly constant
on super horizon scales. It emerges from the bounce unscathed and imprints a
scale-invariant spectrum of density fluctuations in the matter-radiation fluid
at the onset of the hot big bang phase. The ekpyrotic potential can be chosen
so that the spectrum has a ``red'' tilt, in accordance with the recent data
from WMAP. As in the original Ekpyrotic scenario, the model predicts a
negligible gravity wave signal on all observable scales. As such ``New
Ekpyrotic Cosmology" provides a consistent and distinguishable alternative to
inflation to account for the origin of the seeds of large scale structure.Comment: 41 pages, 4 figures. v2: minor corrections, references added. v3:
small modifications in bounce section, references added. v4: version
published in PR
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