2,418 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
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
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
Classical Propagation of Strings across a Big Crunch/Big Bang Singularity
One of the simplest time-dependent solutions of M theory consists of
nine-dimensional Euclidean space times 1+1-dimensional compactified Milne
space-time. With a further modding out by Z_2, the space-time represents two
orbifold planes which collide and re-emerge, a process proposed as an
explanation of the hot big bang. When the two planes are near, the light states
of the theory consist of winding M2-branes, describing fundamental strings in a
particular ten-dimensional background. They suffer no blue-shift as the M
theory dimension collapses, and their equations of motion are regular across
the transition from big crunch to big bang. In this paper, we study the
classical evolution of fundamental strings across the singularity in some
detail. We also develop a simple semi-classical approximation to the quantum
evolution which allows one to compute the quantum production of excitations on
the string and implement it in a simplified example.Comment: 38 pages, 19 figure
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
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
The Transition Amplitude for 2T Physics
We present the transition amplitude for a particle moving in a space with two
times and D space dimensions having a Sp(2,R) local symmetry and an SO(D,2)
rigid symmetry. It was obtained from the BRST-BFV quantization with a unique
gauge choice. We show that by constraining the initial and final points of this
amplitude to lie on some hypersurface of the D+2 space the resulting amplitude
reproduces well known systems in lower dimensions. This provides an alternative
physical interpretation for two times physics which is derived in a single
framework.Comment: 4 pages, typos corrected, references adde
Neutrino oscillations in a Robertson-Walker Universe with space time foam
In Phys. Rev. D77 (2008) 105001, we have studied decoherence models for
flavour oscillations in four-dimensional stochastically fluctuating space times
and discussed briefly the sensitivity of current terrestrial and astrophysical
neutrino experiments to such models. In this addendum we extend these results
to incorporate the effects due to the expansion of the Universe, so that our
analysis can be useful in studies of extragalactic high-energy neutrinos, such
as those coming from Gamma Ray Bursts at cosmological distances. Unfortunately
for some microscopic models of foam, constructed in the string theory
framework, we arrive at pessimistic conclusions about the detectability of the
decoherence effects via flavour oscillation meaurements.Comment: Addendum to Phys. Rev. D77 (2008) 10500
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