572 research outputs found
Cosmological Neutrino Background Revisited
We solve the Boltzmann equation for cosmological neutrinos around the epoch
of the electron-positron annihilation in order to verify the freeze-out
approximation and to compute accurately the cosmological neutrino distribution
function. We find the radiation energy density to be about 0.3% higher than the
one predicted by the freeze-out approximation. As a result, the spectrum of the
Cosmic Microwave Background anisotropies changes by 0.3-05%, depending on the
angular scale, and the amplitude of the mass fluctuations on scales below about
100 h^{-1} Mpc decreases by about 0.2-0.3%.Comment: An error is corrected, figure revised; submitted to Ap
Maximum lepton asymmetry from active-sterile neutrino oscillations in the Early Universe
A large lepton asymmetry could be generated in the Early Universe by
oscillations of active to sterile neutrinos with a small mixing angle sin 2
\theta < 10^-2. The final order of magnitude of the lepton asymmetry \eta is
mainly determined by its growth in the last stage of evolution when the MSW
resonance dominates the kinetic equations. In this paper we present a simple
way of calculating the maximum possible lepton asymmetry which can be created.
Our results are in good agreement to previous calculations. Furthermore, we
find that the growth of asymmetry does not obey any particular power law. We
find that the maximum possible asymmetry at the freeze-out of the n/p ratio at
T \sim 1 MeV strongly depends on the mass-squared difference \delta m^2: the
asymmetry is negligible for \delta m^2 \ll 1 eV^2 and reaches asymptotically
large values for \delta m^2 \ge 50 eV^2.Comment: 14 pp, 4 figure
Mechanism for a Decaying Cosmological Constant
A mechanism is introduced to reduce a large cosmological constant to a
sufficiently small value consistent with observational upper limit. The basic
ingradient in this mechanism is a distinction which has been made between the
two unit systems used on cosmology and particle physics. We have used a
conformal invariant gravitational model to define a particular conformal frame
in terms of the large scale properties of the universe. It is then argued that
the contributions of mass scales in particle physics to the vacuum energy
density should be considered in a different conformal frame. In this manner a
cancellation mechanism is presented in which the conformal factor plays a key
role to relax the large effective cosmological constant.Comment: 6 pages, no figur
Avoiding BBN Constraints on Mirror Models for Sterile Neutrinos
We point out that in models that explain the LSND result for neutrino
oscillation using the mirror neutrinos, the big bang nucleosynthesis constraint
can be avoided by using the late time phase transition that only helps to mix
the active and the sterile neutrinos. We discuss the astrophysical as well as
cosmological implications of this proposal.Comment: 5 pages, latex; more discussion added; results unchange
Cosmology and New Physics
A comparison of the standard models in particle physics and in cosmology
demonstrates that they are not compatible, though both are well established.
Basics of modern cosmology are briefly reviewed. It is argued that the
measurements of the main cosmological parameters are achieved through many
independent physical phenomena and this minimizes possible interpretation
errors. It is shown that astronomy demands new physics beyond the frameworks of
the (minimal) standard model in particle physics. More revolutionary
modifications of the basic principles of the theory are also discussed.Comment: 37 pages, 5 figures; lectures presented at 9th International Moscow
School of Physics (34th ITEP Winter School
Universe Reheating after Inflation
We study the problem of scalar particle production after inflation by a
rapidly oscillating inflaton field. We use the framework of the chaotic
inflation scenario with quartic and quadratic inflaton potentials. Particular
attention is paid to parametric resonance phenomena which take place in the
presence of the quickly oscillating inflaton field. We have found that in the
region of applicability of perturbation theory the effects of parametric
resonance are crucial, and estimates based on first order Born approximation
often underestimate the particle production. In the case of the quartic
inflaton potential , the particle production
process is very efficient even for small values of coupling constants. The
reheating temperature of the universe in this case is times larger than the corresponding estimates based
on first order Born approximation. In the case of the quadratic inflaton
potential the reheating process depends crucially on the type of coupling
between the inflaton and the other scalar field and on the magnitudes of the
coupling constants. If the inflaton coupling to fermions and its linear (in
inflaton field) coupling to scalar fields are suppressed, then, as previously
discussed by Kofman, Linde and Starobinsky (see e.g. Ref. 13), the inflaton
field will eventually decouple from the rest of the matter, and the residual
inflaton oscillations may provide the (cold) dark matter of the universe. In
the case of the quadratic inflaton potential we obtain the lowest and the
highest possible bounds on the effective energy density of the inflaton field
when it freezes out.Comment: 40 pages, Preprint BROWN-HET-957 (revised version, some mistakes
corrected), uses phyzz
Why do we observe a small but non zero cosmological constant ?
The current observations seem to suggest that the universe has a positive
cosmological constant of the order of while the most natural value for
the cosmological constant will be where
is the Planck length. This reduction of the cosmological constant from
to may be interpreted as due to the ability of
quantum micro structure of spacetime to readjust itself and absorb bulk vacuum
energy densities. Being a quantum mechanical process, such a cancellation
cannot be exact and the residual quantum fluctuations appear as the ``small''
cosmological constant. I describe the features of a toy model for the spacetime
micro structure which could allow for the bulk vacuum energy densities to be
canceled leaving behind a small residual value of the the correct magnitude.
Some other models (like the ones based on canonical ensemble for the four
volume or quantum fluctuations of the horizon size) lead to an insignificantly
small value of with showing that obtaining the
correct order of magnitude for the residual fluctuations in the cosmological
constant is a nontrivial task, becaue of the existence of the small
dimensionless number .Comment: couple of references added; matches with published versio
Inhomogeneous Neutrino Degeneracy and Big Bang Nucleosynthesis
We examine Big Bang nucleosynthesis (BBN) in the case of inhomogenous
neutrino degeneracy, in the limit where the fluctuations are sufficiently small
on large length scales that the present-day element abundances are homogeneous.
We consider two representive cases: degeneracy of the electron neutrino alone,
and equal chemical potentials for all three neutrinos. We use a linear
programming method to constrain an arbitrary distribution of the chemical
potentials. For the current set of (highly-restrictive) limits on the
primordial element abundances, homogeneous neutrino degeneracy barely changes
the allowed range of the baryon-to-photon ratio. Inhomogeneous degeneracy
allows for little change in the lower bound on the baryon-to-photon ratio, but
the upper bound in this case can be as large as 1.1 \times 10^{-8} (only
electron neutrino degeneracy) or 1.0 \times 10^{-9} (equal degeneracies for all
three neutrinos). For the case of inhomogeneous neutrino degeneracy, we show
that there is no BBN upper bound on the neutrino energy density, which is
bounded in this case only by limits from structure formation and the cosmic
microwave background.Comment: 6 pages, no figure
Neutrino oscillations: Quantum mechanics vs. quantum field theory
A consistent description of neutrino oscillations requires either the
quantum-mechanical (QM) wave packet approach or a quantum field theoretic (QFT)
treatment. We compare these two approaches to neutrino oscillations and discuss
the correspondence between them. In particular, we derive expressions for the
QM neutrino wave packets from QFT and relate the free parameters of the QM
framework, in particular the effective momentum uncertainty of the neutrino
state, to the more fundamental parameters of the QFT approach. We include in
our discussion the possibilities that some of the neutrino's interaction
partners are not detected, that the neutrino is produced in the decay of an
unstable parent particle, and that the overlap of the wave packets of the
particles involved in the neutrino production (or detection) process is not
maximal. Finally, we demonstrate how the properly normalized oscillation
probabilities can be obtained in the QFT framework without an ad hoc
normalization procedure employed in the QM approach.Comment: LaTeX, 42 pages, 1 figure; v2: minor clarifications, matches
published version; v3: Corrected the discussion of the conditions under which
an oscillation probability can be sensibly defined in the QFT approach (sec.
5.2.4
Attractor Universe in the Scalar-Tensor Theory of Gravitation
In the scalar-tensor theory of gravitation it seems nontrivial to establish
if solutions of the cosmological equations in the presence of a cosmological
constant behave as attractors independently of the initial values. We develop a
general formulation in terms of two-dimensional phase space. We show that there
are two kinds of fixed points, one of which is an attractor depending on the
coupling constant and equation of state. In the case with a power-law potential
in the Jordan frame, we also find new type of inflation caused by the coupling
to the matter fluid
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