4,782 research outputs found
Cosmographic constraints on a class of Palatini f(R) gravity
Modified gravity, known as gravity, has presently been applied to
Cosmology as a realistic alternative to dark energy. For this kind of gravity
the expansion of the Universe may accelerate while containing only baryonic and
cold dark matter. The aim of the present investigation is to place cosmographic
constraints on the class of theories of the form within
the Palatini approach. Although extensively discussed in recent literature and
confronted with several observational data sets, cosmological tests are indeed
inconclusive about the true signal of in this class of theories. This is
particularly important to define which kind of corrections (infra-red or
high-energy) to general relativity this class of theory indeed represent. We
shed some light on this question by examining the evolution of the deceleration
parameter for these theories. We find that for a large range of
, models based on gravity in the Palatini
approach can only have positive values for , placing thus a broad
restriction on this class of gravity.Comment: 4 pages, 2 figures, Latex, Submitte
Type II Seesaw and a Gauge Model for the Bimaximal Mixing Explanation of Neutrino Puzzles
We present a gauge model for the bimaximal mixing pattern among the neutrinos
that explains both the atmospheric and solar neutrino data via large angle
vacuum oscillation among the three known neutrinos. The model does not include
righthanded neutrinos but additional Higgs triplets which acquire naturally
small vev's due to the type II seesaw mechanism. A combination of global
and symmetries constrain the mass matrix for both
charged leptons and neutrinos in such a way that the bimaximal pattern emerges
naturally at the tree level and needed splittings among neutrinos at the one
loop level. This model predicts observable branching ratios for , which could be used to test it.Comment: Latex file, 8 pages, five figures include
Charged and Neutral Currents in a 3-3-1 Model with Right-Handed Neutrinos
The charged and the neutral currents are obtained by using a formal
algebraical approach (developed and applied by the author) within the exact
solution of a 3-3-1 gauge model with right-handed neutrinos. The entire
Standard Model phenomenology is recovered without imposing any supplemental
condition, but only by choosing an adecquate set of parameters from the very
beginning of the calculus. A new and rich phenomenology regarding the particles
and their currents occurs as well. The appealing feature of our results resides
in the exact expressions of the currents which need not the adjustment usually
due to the small mixing angle between neutral bosons and
(like in the most of the papers in the literature treating the
same issue). The required mixing was considered and aleready performed as an
intermediate step by the solving method itself, since the physical eigenstates
of those bosons were determined and then identified in the neutral currents.Comment: 14 pages, 1 Table, no figure
Neutrino Decay and Neutrinoless Double Beta Decay in a 3-3-1 Model
In this work we show that the implementation of spontaneous breaking of the
lepton number in the 3-3-1 model with right-handed neutrinos gives rise to fast
neutrino decay with majoron emission and generates a bunch of new contributions
to the neutrinoless double beta decay.Comment: Version accepted for publication in the Phys. Rev.
The Left-Right SU(3)(L)xSU(3)(R)xU(1)(X) Model with Light, keV and Heavy Neutrinos
We construct a full left-right model for the electroweak interactions based
on the gauge symmetry. The fermion
content of the model is such that anomaly cancellation restricts the number of
families to be a multiple of three. One of the most important features of the
model is the joint presence of three light active neutrinos, three additional
neutrinos at keV mass scale, and six heavy ones with masses
around\textbf{} GeV. They form a well-motivated part of the spectrum
in the sense they address challenging problems related to neutrino oscillation,
warm dark matter, and baryogenesis through leptogenesis.Comment: 11 pages. Small corrections and typos fixed. Accepted for publication
in PR
Lookback time bounds from energy conditions
In general relativity, the energy conditions are invoked to restrict general
energy-momentum tensors on physical grounds. We show that in the standard
Friedmann-Lemaitre-Robertson-Walker (FLRW) approach to cosmological modeling,
where the energy and matter components of the cosmic fluid are unknown, the
energy conditions provide model-independent bounds on the behavior of the
lookback time of cosmic sources as a function of the redshift for any value of
the spatial curvature. We also confront such bounds with a lookback time sample
which is built from the age estimates of 32 galaxies lying in the interval
and by assuming the total expanding age of the
Universe to be Gyr, as obtained from current cosmic microwave
background experiments. In agreement with previous results, we show that all
energy conditions seem to have been violated at some point of the recent past
of cosmic evolution.Comment: 7 pages, 3 figures. v2: Minor changes, published in Phys.Rev.D in the
present for
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