2,042 research outputs found

### Quantum Corrections in Quintessence Models

We investigate the impact of quantum fluctuations on a light rolling
quintessence field from three different sources, namely, from a coupling to the
standard model and dark matter, from its self-couplings and from its coupling
to gravity. We derive bounds for time-varying masses from the change of vacuum
energy, finding \Delta m_e/m_e << 10^{-11} for the electron and \Delta m_p/m_p
<< 10^{-15} for the proton since redshift z~2, whereas the neutrino masses
could change of order one. Mass-varying dark matter is also constrained. Next,
the self-interactions are investigated. For inverse power law potentials, the
effective potential does not become infinitely large at small field values, but
saturates at a finite maximal value. We discuss implications for cosmology.
Finally, we show that one-loop corrections induce non-minimal gravitational
couplings involving arbitrarily high powers of the curvature scalar R,
indicating that quintessence entails modified gravity effects.Comment: 10 pages + appendix, added reference

### Perturbative quantization of two-dimensional space-time noncommutative QED

Using the method of perturbative quantization in the first order
approximation, we quantize a non-local QED-like theory including fermions and
bosons whose interactions are described by terms containing higher order
space-time derivatives. As an example, the two-dimensional space-time
noncommutative QED (NC-QED) is quantized perturbatively up to O(e^2,\theta^3),
where e is the NC-QED coupling constant and \theta is the noncommutativity
parameter. The resulting modified Lagrangian density is shown to include terms
consisting of first order time-derivative and higher order space-derivatives of
the modified field variables that satisfy the ordinary equal-time commutation
relations up to O(e^2,\theta^3. Using these commutation relations, the
canonical current algebra of the modified theory is also derived.Comment: 22 pages, no figure

### Comments on branon dressing and the Standard Model

This technical note shows how Electrodynamics and a Yukawa model are dressed
after integrating out perturbative brane fluctuations, and it is found that
first order corrections in the inverse of the brane tension occur for the
fermion and scalar wave functions, the couplings and the masses. Nevertheless,
field redefinitions actually lead to effective actions where only masses are
dressed to this first order. We compare our results with the literature and
find discrepancies at the next order, which, however, might not be measurable
in the valid regime of low-energy brane fluctuations.Comment: 12 page

### A Note on Tachyon Moduli and Closed Strings

The collective behavior of the SL(2,R) covariant brane states of non-critical
c=1 string theory found in a previous work, is studied in the Fermi liquid
approximation. It is found that such states mimick the coset WZW model, whereas
only by further restrictions one recovers the double-scaling limit which was
purported to be equivalent to closed string models. Another limit is proposed,
inspired by the tachyon condensation ideas, where the spectrum is the same of
two-dimensional string theory. We close by noting some strange connections
between vacuum states of the theory in their different interpretations.Comment: PDFLaTeX, 17 pages, 2 figures; Section 2 rewritten, several fixes
throughout the text to improve clarit

### Quantum fields near phantom-energy `sudden' singularities

This paper is committed to calculations near a type of future singularity
driven by phantom energy. At the singularities considered, the scale factor
remains finite but its derivative diverges. The general behavior of barotropic
phantom energy producing this singularity is calculated under the assumption
that near the singularity such fluid is the dominant contributor. We use the
semiclassical formula for renormalized stress tensors of conformally invariant
fields in conformally flat spacetimes and analyze the softening/enhancing of
the singularity due to quantum vacuum contributions. This dynamical analysis is
then compared to results from thermodynamical considerations. In both cases,
the vacuum states of quantized scalar and spinor fields strengthen the
accelerating expansion near the singularity whereas the vacuum states of vector
fields weaken it.Comment: 6 pages RevTe

### Vacuum fluctuations in a supersymmetric model in FRW spacetime

We study a noninteracting supersymmetric model in an expanding FRW spacetime.
A soft supersymmetry breaking induces a nonzero contribution to the vacuum
energy density. A short distance cutoff of the order of Planck length provides
a scale for the vacuum energy density comparable with the observed cosmological
constant. Assuming the presence of a dark energy substance in addition to the
vacuum fluctuations of the field an effective equation of state is derived in a
selfconsistent approach. The effective equation of state is sensitive to the
choice of the cut-off but no fine tuning is needed.Comment: 19 pages, accepted for publication in Phys. Rev.

### First-order quantum correction to the Larmor radiation from a moving charge in a spatially homogeneous time-dependent electric field

First-order quantum correction to the Larmor radiation is investigated on the
basis of the scalar QED on a homogeneous background of time-dependent electric
field, which is a generalization of a recent work by Higuchi and Walker so as
to be extended for an accelerated charged particle in a relativistic motion. We
obtain a simple approximate formula for the quantum correction in the limit of
the relativistic motion when the direction of the particle motion is parallel
to that of the electric field.Comment: 12 pages, 2 figures, accepted for publication in Physical Review

### Dark Matter from R^2-gravity

The modification of Einstein gravity at high energies is mandatory from a
quantum approach. In this work, we point out that this modification will
necessarily introduce new degrees of freedom. We analyze the possibility that
these new gravitational states can provide the main contribution to the
non-baryonic dark matter of the Universe. Unfortunately, the right ultraviolet
completion of gravity is still unresolved. For this reason, we will illustrate
this idea with the simplest high energy modification of the Einstein-Hilbert
action: R^2-gravity.Comment: 5 pages, 2 figure

### Quantum Dynamics for de Sitter Radiation

We revisit the Hamiltonian formalism for a massive scalar field and study the
particle production in a de Sitter space. In the invariant-operator picture the
time-dependent annihilation and creation operators are constructed in terms of
a complex solution to the classical equation of motion for the field and the
Gaussian wave function for each Fourier mode is found which is an exact
solution to the Schr\"odinger equation. The in-out formalism is reformulated by
the annihilation and creation operators and the Gaussian wave functions. The de
Sitter radiation from the in-out formalism differs from the Gibbons-Hawking
radiation in the planar coordinates, and we discuss the discrepancy of the
particle production by the two methodComment: LaTex 12 pages, no figure; CosPA2011, Peking Univ., Oct. 28-31, 2011;
references added; to be published in International Journal of Modern Physics:
Conference Serie

### Cosmology and the S-matrix

We study conditions for the existence of asymptotic observables in cosmology.
With the exception of de Sitter space, the thermal properties of accelerating
universes permit arbitrarily long observations, and guarantee the production of
accessible states of arbitrarily large entropy. This suggests that some
asymptotic observables may exist, despite the presence of an event horizon.
Comparison with decelerating universes shows surprising similarities: Neither
type suffers from the limitations encountered in de Sitter space, such as
thermalization and boundedness of entropy. However, we argue that no realistic
cosmology permits the global observations associated with an S-matrix.Comment: 16 pages, 5 figures; v2: minor editin

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