233 research outputs found
A First-Quantized Formalism for Cosmological Particle Production
We show that the amount of particle production in an arbitrary cosmological
background can be determined using only the late-time positive-frequency modes.
We don't refer to modes at early times, so there is no need for a Bogolubov
transformation. We also show that particle production can be extracted from the
Feynman propagator in an auxiliary spacetime. This provides a first-quantized
formalism for computing particle production which, unlike conventional
Bogolubov transformations, may be amenable to a string-theoretic
generalization.Comment: 18 pages, LaTeX; v2: significantly revised for clarity; conclusions
unchange
The Volume of the Past Light-Cone and the Paneitz Operator
We study a conjecture involving the invariant volume of the past light-cone
from an arbitrary observation point back to a fixed initial value surface. The
conjecture is that a 4th order differential operator which occurs in the theory
of conformal anomalies gives when acted upon the invariant volume of the
past light-cone. We show that an extended version of the conjecture is valid
for an arbitrary homogeneous and isotropic geometry. First order perturbation
theory about flat spacetime reveals a violation of the conjecture which,
however, vanishes for any vacuum solution of the Einstein equation. These
results may be significant for constructing quantum gravitational observables,
for quantifying the back-reaction on spacetime expansion and for alternate
gravity models which feature a timelike vector field.Comment: 22 pages, no figures, 5 tables. Version 2 substantially extended to
cover nonzero spatial curvature, and with simplified derivation
The stress-energy tensor for trans-Planckian cosmology
This article presents the derivation of the stress-energy tensor of a free
scalar field with a general non-linear dispersion relation in curved spacetime.
This dispersion relation is used as a phenomelogical description of the short
distance structure of spacetime following the conventional approach of
trans-Planckian modes in black hole physics and in cosmology. This
stress-energy tensor is then used to discuss both the equation of state of
trans-Planckian modes in cosmology and the magnitude of their backreaction
during inflation. It is shown that gravitational waves of trans-Planckian
momenta but subhorizon frequencies cannot account for the form of cosmic vacuum
energy density observed at present, contrary to a recent claim. The
backreaction effects during inflation are confirmed to be important and generic
for those dispersion relations that are liable to induce changes in the power
spectrum of metric fluctuations. Finally, it is shown that in pure de Sitter
inflation there is no modification of the power spectrum except for a possible
magnification of its overall amplitude independently of the dispersion
relation.Comment: 18 pages, 2 figures. Version to appear in PRD (minor modifications
Minimal modifications of the primordial power spectrum from an adiabatic short distance cutoff
As a simple model for unknown Planck scale physics, we assume that the
quantum modes responsible for producing primordial curvature perturbations
during inflation are placed in their instantaneous adiabatic vacuum when their
proper momentum reaches a fixed high energy scale M. The resulting power
spectrum is derived and presented in a form that exhibits the amplitude and
frequency of the superimposed oscillations in terms of H/M and the slow roll
parameter epsilon. The amplitude of the oscillations is proportional to the
third power of H/M. We argue that these small oscillations give the lower bound
of the modifications of the power spectrum if the notion of free mode
propagation ceases to exist above the critical energy scale M.Comment: 10 pages; matches version accepted by PR
Ten Proofs of the Generalized Second Law
Ten attempts to prove the Generalized Second Law of Thermodyanmics (GSL) are
described and critiqued. Each proof provides valuable insights which should be
useful for constructing future, more complete proofs. Rather than merely
summarizing previous research, this review offers new perspectives, and
strategies for overcoming limitations of the existing proofs. A long
introductory section addresses some choices that must be made in any
formulation the GSL: Should one use the Gibbs or the Boltzmann entropy? Should
one use the global or the apparent horizon? Is it necessary to assume any
entropy bounds? If the area has quantum fluctuations, should the GSL apply to
the average area? The definition and implications of the classical,
hydrodynamic, semiclassical and full quantum gravity regimes are also
discussed. A lack of agreement regarding how to define the "quasi-stationary"
regime is addressed by distinguishing it from the "quasi-steady" regime.Comment: 60 pages, 2 figures, 1 table. v2: corrected typos and added a
footnote to match the published versio
Constraining Cut-off Physics in the Cosmic Microwave Background
We investigate the ability to constrain oscillatory features in the
primordial power spectrum using current and future cosmic microwave background
observations. In particular, we study the observability of an oscillation
arising from imprints of physics at the cut-off energy scale. We perform a
likelihood analysis on the WMAP data set, and find that the current data set
constrains the amplitude of the oscillations to be less than 0.77 at 2-sigma,
consistent with a power spectrum without oscillations. In addition, we
investigate the fundamental limitations in the measurement of oscillation
parameters by studying the constraints from a cosmic variance limited
experiment. We find that such an experiment is capable of constraining the
amplitude of such oscillations to be below 0.005, implying that reasonable
models with cut-off energy scales Lambda>200 H_infl are unobservable through
the microwave background.Comment: 16 pages, 7 figures; PRD accepted versio
Wnt signalling and cancer stem cells
[Abstract] Intracellular signalling mediated by secreted Wnt proteins is essential for the establishment of cell fates and proper tissue patterning during embryo development and for the regulation of tissue homeostasis and stem cell function in adult tissues. Aberrant activation of Wnt signalling pathways has been directly linked to the genesis of different tumours. Here, the components and molecular mechanisms implicated in the transduction of Wnt signal, along with important results supporting a central role for this signalling pathway in stem cell function regulation and carcinogenesis will be briefly reviewed.Ministerio de Ciencia e InnovaciĂłn; SAF2008-0060
Theory of Cosmological Perturbations and Applications to Superstring Cosmology
The theory of cosmological perturbations is the main tool which connects
theories of the early universe (based on new fundamental physics such as string
theory) with cosmological observations. In these lectures, I will provide an
introduction to this theory, beginning with an overview of the Newtonian theory
of fluctuations, moving on to the analysis of fluctuations in the realm of
classical general relativity, and culminating with a discussion of the quantum
theory of cosmological perturbations. I will illustrate the formalism with
applications to inflationary cosmology. I will review the basics of
inflationary cosmology and discuss why - through the evolution of fluctuations
- inflation may provide a way of observationally testing Planck-scale physics.Comment: Writeup of lectures delivered at the 2004 Cargese Summer School on
String Theory, 49 pages, 3 figure
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