530 research outputs found
Scalar Non-Luminous Matter in Galaxies
As a candidate for dark matter in galaxies, we study an SU(3) triplet of
complex scalar fields which are non-minimally coupled to gravity. In the
spherically symmetric static spacetime where the flat rotational velocity
curves of stars in galaxies can be explained, we find simple solutions of
scalar fields with SU(3) global symmetry broken to U(1) X U(1), in an
exponential scalar potential, which will be useful in a quintessence model of
the late-time acceleration of the Universe.Comment: 6 pages, no figure, LaTex. Submitted to IJMP
Accelerating Universes with Scaling Dark Matter
Friedmann-Robertson-Walker universes with a presently large fraction of the
energy density stored in an -component with , are considered. We
find all the critical points of the system for constant equations of state in
that range. We consider further several background quantities that can
distinguish the models with different values. Using a simple toy model
with a varying equation of state, we show that even a large variation of
at small redshifts is very difficult to observe with measurements up
to . Therefore, it will require accurate measurements in the range
and independent accurate knowledge of (and/or
) in order to resolve a variable from a constant .Comment: submitted to IJMPD (uses Latex, 12 pages, 6 Figures) Minor
corrections, Figures 4, 6 revised. Conclusions unchange
Is Cosmology Solved?
We have fossil evidence from the thermal background radiation that our
universe expanded from a considerably hotter denser state. We have a well
defined and testable description of the expansion, the relativistic
Friedmann-Lemaitre model. Its observational successes are impressive but I
think hardly enough for a convincing scientific case. The lists of
observational constraints and free hypotheses within the model have similar
lengths. The scorecard on the search for concordant measures of the mass
density parameter and the cosmological constant shows that the high density
Einstein-de Sitter model is challenged, but that we cannot choose between low
density models with and without a cosmological constant. That is, the
relativistic model is not strongly overconstrained, the usual test of a mature
theory. Work in progress will greatly improve the situation and may at last
yield a compelling test. If so, and the relativistic model survives, it will
close one line of research in cosmology: we will know the outlines of what
happened as our universe expanded and cooled from high density. It will not end
research: some of us will occupy ourselves with the details of how galaxies and
other large-scale structures came to be the way they are, others with the issue
of what our universe was doing before it was expanding. The former is being
driven by rapid observational advances. The latter is being driven mainly by
theory, but there are hints of observational guidance.Comment: 13 pages, 3 figures. To be published in PASP as part of the
proceedings of the Smithsonian debate, Is Cosmology Solved
Dynamics of a Generalized Cosmological Scalar-Tensor Theory
A generalized scalar-tensor theory is investigated whose cosmological term
depends on both a scalar field and its time derivative. A correspondence with
solutions of five-dimensional Space-Time-Matter theory is noted. Analytic
solutions are found for the scale factor, scalar field and cosmological term.
Models with free parameters of order unity are consistent with recent
observational data and could be relevant to both the dark-matter and
cosmological-"constant" problems.Comment: 13 page
Variational dynamics in open spacetimes
We study the effect of non-vanishing surface terms at spatial infinity on the
dynamics of a scalar field in an open FLRW spacetime. Starting from the
path-integral formulation of quantum field theory we argue that classical
physics is described by field configurations which extremize the action
functional in the space of field configurations for which the variation of the
action is well defined. Since these field configurations are not required to
vanish outside a bounded domain, there is generally a non-vanishing
contribution of a surface term to the variation of the action. We then
investigate whether this surface term has an effect on the dynamics of the
action-extremizing field configurations. This question appears to be
surprisingly nontrivial in the case of the open FLRW geometry, since surface
terms tend to grow as fast as volume terms in the infinite volume limit. We
find that surface terms can be important for the dynamics of the field at a
classical and at a quantum level, when there are supercurvature perturbations.Comment: 21 pages, Latex, no figure
Diffusion of ions through some Indian timbers
A simple diffusion cell (which can be easily constructed from perspex sheeting) for studying the passage of molecules, ions, gases, vapours and liquids through wood and other membranes is described. The diffusion of ions through some species of Indian timbers under variety of conditions has been studied and the results reported. The results show that the diffusion of ions through wood obeys Fick's law as long as the ion does not react with any constituents of the wood. In other cases there is deviation from this law. The diffusion constant is largest in the axial direction and smallest in the tangential direction. As against other claims, for the species tested, ionic diffusion is higher through sapwood than through heartwood. Temperature increases the rate of diffusion of ions. Diffusion of copper sulphate through cellophane is in conformity with Fick's law and is of the same order as for some timbers
A conjecture on the origin of dark energy
The physical origin of holographic dark energy (HDE) is investigated. The
main existing explanations, namely the UV/IR connection argument of Cohen et
al, Thomas' bulk holography argument, and Ng's spacetime foam argument, are
shown to be not satisfactory. A new explanation of the HDE model is then
proposed based on the ideas of Thomas and Ng. It is suggested that the dark
energy might originate from the quantum fluctuations of spacetime limited by
the event horizon of the universe. Several potential problems of the
explanation are also discussed.Comment: 11 pages, no figure
Big Bang Nucleosynthesis Constraints on Primordial Magnetic Fields
We reanalyze the effect of magnetic fields in BBN, incorporating several
features which were omitted in previous analyses. We find that the effects of
coherent magnetic fields on the weak interaction rates and the electron
thermodynamic functions (\rhoe, \Pe, and \drhoedt ) are unimportant in
comparison to the contribution of the magnetic field energy density in BBN. In
consequence the effect of including magnetic fields in BBN is well approximated
numerically by treating the additional energy density as effective neutrino
number. A conservative upper bound on the primordial magnetic field,
parameterized as , is (). This bound can be stronger than the conventional bound coming from
the Faraday rotation measures of distant quasars if the cosmological magnetic
field is generated by a causal mechanism.Comment: Latex, 20 pages, 3 uuencoded figures appende
Magnetohydrodynamics of the Early Universe and the Evolution of Primordial Magnetic Fields
We show that the decaying magnetohydrodynamic turbulence leads to a more
rapid growth of the correlation length of a primordial magnetic field than that
caused by the expansion of the Universe. As an example, we consider the
magnetic fields created during the electroweak phase transition. The expansion
of the universe alone would yield a correlation length at the present epoch of
1 AU, whereas we find that the correlation length is likely of order 100 AU,
and cannot possibly be longer than AU for non-helical fields. If the
primordial field is strongly helical, the correlation length can be much
larger, but we show that even in this case it cannot exceed 100 pc. All these
estimates make it hard to believe that the observed galactic magnetic fields
can result from the amplification of seed fields generated at the electroweak
phase transition by the standard galactic dynamo.Comment: 15 pages, REVTeX. Added results of numerical simulation, enlarged and
revise
CMB anisotropies due to cosmological magnetosonic waves
We study scalar mode perturbations (magnetosonic waves) induced by a helical
stochastic cosmological magnetic field and derive analytically the
corresponding cosmic microwave background (CMB) temperature and polarization
anisotropy angular power spectra. We show that the presence of a stochastic
magnetic field, or an homogeneous magnetic field, influences the acoustic
oscillation pattern of the CMB anisotropy power spectrum, effectively acting as
a reduction of the baryon fraction. We find that the scalar magnetic energy
density perturbation contribution to the CMB temperature anisotropy is small
compared to the contribution to the CMB -polarization anisotropy.Comment: 17 pages, references added, version accepted for publication in Phys.
Rev.
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