2,639 research outputs found
Effective field theory, large number of particle species, and holography
An effective quantum field theory (QFT) with a manifest UV/IR connection, so
as to be valid for arbitrarily large volumes, can successfully be applied to
the cosmological dark energy problem as well as the cosmological constant (CC)
problem. Motivated by recent approaches to the hierarchy problem, we develop
such a framework with a large number of particle species. When applying to
systems on the brink of experiencing a sudden collapse to a black hole, we find
that the entropy, unlike the total energy, now becomes an increasing function
of the number of field species. An internal consistency of the theory is then
used to infer the upper bound on the number of particle species, showing
consistency with the holographic Bekenstein-Hawking bound. This may thus serve
to fill in a large gap in entropy of any non-black hole configuration of matter
and the black holes. In addition, when the bound is saturated the entanglement
entropy matches the black hole entropy, thus solving the multiplicity of
species problem. In a cosmological setting, the maximum allowable number of
species becomes a function of cosmological time, reaching its minimal value in
a low-entropy post-reheating epoch.Comment: 8 pages, minor corrections, a reference added, to appear in PL
Avoiding Boltzmann Brain domination in holographic dark energy models
In a spatially infinite and eternal universe approaching ultimately a de
Sitter (or quasi-de Sitter) regime, structure can form by thermal fluctuations
as such a space is thermal. The models of Dark Energy invoking holographic
principle fit naturally into such a category, and spontaneous formation of
isolated brains in otherwise empty space seems the most perplexing, creating
the paradox of Boltzmann Brains (BB). It is thus appropriate to ask if such
models can be made free from domination by Boltzmann Brains. Here we consider
only the simplest model, but adopt both the local and the global viewpoint in
the description of the Universe. In the former case, we find that if a
parameter , which modulates the Dark Energy density, lies outside the
exponentially narrow strip around the most natural line, the theory is
rendered BB-safe. In the later case, the bound on is exponentially
stronger, and seemingly at odds with those bounds on obtained from various
observational tests.Comment: 8 pages, bounds rederived, final version to appear in PL
Entanglement in holographic dark energy models
We study a process of equilibration of holographic dark energy (HDE) with the
cosmic horizon around the dark-energy dominated epoch. This process is
characterized by a huge amount of information conveyed across the horizon,
filling thereby a large gap in entropy between the system on the brink of
experiencing a sudden collapse to a black hole and the black hole itself. At
the same time, even in the absence of interaction between dark matter and dark
energy, such a process marks a strong jump in the entanglement entropy,
measuring the quantum-mechanical correlations between the horizon and its
interior. Although the effective quantum field theory (QFT) with a peculiar
relationship between the UV and IR cutoffs, a framework underlying all HDE
models, may formally account for such a huge shift in the number of distinct
quantum states, we show that the scope of such a framework becomes tremendously
restricted, devoiding it virtually any application in other cosmological epochs
or particle-physics phenomena. The problem of negative entropies for the
non-phantom stuff is also discussed.Comment: 10 pages, version to appear in PL
Stringent constraint on the scalar-neutrino coupling constant from quintessential cosmology
An extremely light (), slowly-varying scalar
field (quintessence) with a potential energy density as large as 60% of
the critical density has been proposed as the origin of the accelerated
expansion of the Universe at present. The interaction of this smoothly
distributed component with another predominately smooth component, the cosmic
neutrino background, is studied. The slow-roll approximation for generic potentials may then be used to obtain a limit on the scalar-neutrino coupling
constant, found to be many orders of magnitude more stringent than the limits
set by observations of neutrinos from SN 1987A. In addition, if quintessential
theory allows for a violation of the equivalence principle in the sector of
neutrinos, the current solar neutrino data can probe such a violation at the
10^{-10} level.Comment: 7 pages, MPLA in press, some parts disregarded and a footnote adde
A bound on the scale of spacetime noncommutativity from the reheating phase after inflation
In an approach to noncommutative gauge theories, where the full
noncommutative behavior is delimited by the presence of the UV and IR cutoffs,
we consider the possibility of describing a system at a temperature T in a box
of size L. Employing a specific form of UV/IR relationship inherent in such an
approach of restrictive noncommutativity, we derive, for a given temperature T,
an upper bound on the parameter of spacetime noncommutativity Lambda_NC ~
|theta|^{-1/2}. Considering such epochs in the very early universe which are
expected to reflect spacetime noncommutativity to a quite degree, like the
reheating stage after inflation, or believable pre-inflation
radiation-dominated epochs, the best limits on Lambda_NC are obtained. We also
demonstrate how the nature and size of the thermal system (for instance, the
Hubble distance versus the future event horizon) can affect our bounds.Comment: 9 pages, a reference added, to appear in PL
Propagation of ultrahigh-energy neutrinos through the Earth
The dispersion relation in matter of ultrahigh-energy neutrinos above the
pole of the resonance (E_{\nu} \gsim {\rm 10}^{7} {\rm GeV} ), is
studied. We perform our calculation using the real-time formulation of Thermal
Field Theory in which the massless limit for the boson is taken. The range
of active-to-sterile neutrino oscillation parameters for which there is
significant mixing enhancement during propagation through the interior of the
Earth, and therefore significant attenuation of neutrino beams in the Earth at
high energies, is estimated. Finally, this range is considered in view of the
cosmological and astrophysical constraints.Comment: 8 pages, some comments and references added, to appear in Phys. Lett.
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