3,758 research outputs found
Big Bang Nucleosynthesis with Stable Be and the Primordial Lithium Problem
A change in the fundamental constants of nature or plasma effects in the
early universe could stabilize Be against decay into two He nuclei. Coc
et al. examined this effect on big bang nucleosynthesis as a function of ,
the mass difference between two He nuclei and a single Be nucleus, and
found no effects for keV. Here we examine stable Be with
larger and also allow for a variation in the rate for He + He
Be to determine the threshold for interesting effects. We
find no change to standard big bang nucleosynthesis for MeV. For MeV and a sufficiently large reaction rate, a significant fraction
of He is burned into Be, which fissions back into He when
assumes its present-day value, leaving the primordial He abundance
unchanged. However, this sequestration of He results in a decrease in the
primordial Li abundance. Primordial abundances of Li consistent with
observationally-inferred values can be obtained for reaction rates similar to
those calculated for the present-day (unbound Be) case. Even for the
largest binding energies and largest reaction rates examined here, only a small
fraction of Be is burned into heavier elements, consistent with earlier
studies. There is no change in the predicted deuterium abundance for any model
we examined.Comment: 7 pages, 2 figures, expanded discussion of 8Be binding energy, added
reference, to appear in Phys. Rev.
Mapping the Chevallier-Polarski-Linder parametrization onto Physical Dark Energy Models
We examine the Chevallier-Polarski-Linder (CPL) parametrization, in the
context of quintessence and barotropic dark energy models, to determine the
subset of such models to which it can provide a good fit. The CPL
parametrization gives the equation of state parameter for the dark energy
as a linear function of the scale factor , namely . In
the case of quintessence models, we find that over most of the ,
parameter space the CPL parametrization maps onto a fairly narrow form of
behavior for the potential , while a one-dimensional subset of
parameter space, for which , with constant,
corresponds to a wide range of functional forms for . For barotropic
models, we show that the functional dependence of the pressure on the density,
up to a multiplicative constant, depends only on and not on
and separately. Our results suggest that the CPL parametrization
may not be optimal for testing either type of model.Comment: 11 pages, 5 figures, typo corrected in Eq. (17), to appear in Phys.
Rev.
Dark energy with : Asymptotic versus pseudo-
If the dark energy density asymptotically approaches a nonzero constant,
, then its equation of state parameter
necessarily approaches . The converse is not true; dark energy with can correspond to either or
. This provides a natural division of models with into two distinct classes: asymptotic () and pseudo- (). We
delineate the boundary between these two classes of models in terms of the
behavior of , , and . We examine barotropic and
quintessence realizations of both types of models. Barotropic models with
positive squared sound speed and are always asymptotically
; they can never produce pseudo- behavior. Quintessence
models can correspond to either asymptotic or pseudo-
evolution, but the latter is impossible when the expansion is dominated by a
background barotropic fluid. We show that the distinction between asymptotic
and pseudo- models for is mathematically dual to the
distinction between pseudo-rip and big/little rip models when .Comment: 7 pages, no figures, references adde
Time variation of a fundamental dimensionless constant
We examine the time variation of a previously-uninvestigated fundamental
dimensionless constant. Constraints are placed on this time variation using
historical measurements. A model is presented for the time variation, and it is
shown to lead to an accelerated expansion for the universe. Directions for
future research are discussed.Comment: 2 pages, 2 figures, 1 tabl
Decaying dark matter mimicking time-varying dark energy
A CDM model with dark matter that decays into inert relativistic
energy on a timescale longer than the Hubble time will produce an expansion
history that can be misinterpreted as stable dark matter with time-varying dark
energy. We calculate the corresponding spurious equation of state parameter,
, as a function of redshift, and show that the evolution of
depends strongly on the assumed value of the dark matter
density, erroneously taken to scale as . Depending on the latter, one
can obtain models that mimic quintessence (), phantom
models () or models in which the equation of state
parameter crosses the phantom divide, evolving from at
high redshift to at low redshift. All of these models
generically converge toward at the present. The
degeneracy between the CDM model with decaying dark matter and the
corresponding spurious quintessence model is broken by the growth of density
perturbations.Comment: 6 pages, 2 figures. Added discussion of linear perturbation growth -
version accepted at PR
Big Bang nucleosynthesis with a stiff fluid
Models that lead to a cosmological stiff fluid component, with a density
that scales as , where is the scale factor, have been
proposed recently in a variety of contexts. We calculate numerically the effect
of such a stiff fluid on the primordial element abundances. Because the stiff
fluid energy density decreases with the scale factor more rapidly than
radiation, it produces a relatively larger change in the primordial helium-4
abundance than in the other element abundances, relative to the changes
produced by an additional radiation component. We show that the helium-4
abundance varies linearly with the density of the stiff fluid at a fixed
fiducial temperature. Taking and to be the stiff
fluid energy density and the standard density in relativistic particles,
respectively, at MeV, we find that the change in the primordial helium
abundance is well-fit by . The
changes in the helium-4 abundance produced by additional radiation or by a
stiff fluid are identical when these two components have equal density at a
"pivot temperature", , where we find MeV. Current estimates
of the primordial He abundance give the constraint on a stiff fluid energy
density of .Comment: 6 pages, 2 figures. Clarification added: element abundances derived
using a full numerical calculation. Version accepted at PR
Oscillating and Static Universes from a Single Barotropic Fluid
We consider cosmological solutions to general relativity with a single
barotropic fluid, where the pressure is a general function of the density, . We derive conditions for static and oscillating solutions and provide
examples, extending earlier work to these simpler and more general single-fluid
cosmologies. Generically we expect such solutions to suffer from instabilities,
through effects such as quantum fluctuations or tunneling to zero size. We also
find a classical instability ("no-go" theorem) for oscillating solutions of a
single barotropic perfect fluid due to a necessarily negative squared sound
speed.Comment: 5 pages; v2: additional references, minor clarification in Sec. IIC,
matches version published in JCA
Classifying the behavior of noncanonical quintessence
We derive general conditions for the existence of stable scaling solutions
for the evolution of noncanonical quintessence, with a Lagrangian of the form
, for power-law and exponential
potentials when the expansion is dominated by a background barotropic fluid.
Our results suggest that in most cases, noncanonical quintessence with such
potentials does not yield interesting models for the observed dark energy. When
the scaling solution is not an attractor, there is a wide range of model
parameters for which the evolution asymptotically resembles a zero-potential
solution with equation of state parameter , and oscillatory
solutions are also possible for positive power-law potentials; we derive the
conditions on the model parameters which produce both types of behavior. We
investigate thawing noncanonical models with a nearly-flat potential and derive
approximate expressions for the evolution of . These forms for
differ in a characteristic way from the corresponding expressions for canonical
quintessence.Comment: 6 pages, 1 figure, minor clarifications and correction
Hilltop Quintessence
We examine hilltop quintessence models, in which the scalar field is rolling
near a local maximum in the potential, and w is close to -1. We first derive a
general equation for the evolution of the scalar field in the limit where w is
close to -1. We solve this equation for the case of hilltop quintessence to
derive w as a function of the scale factor; these solutions depend on the
curvature of the potential near its maximum. Our general result is in excellent
agreement (delta w < 0.5%) with all of the particular cases examined. It works
particularly well (delta w < 0.1%) for the pseudo-Nambu-Goldstone Boson
potential. Our expression for w(a) reduces to the previously-derived slow-roll
result of Sen and Scherrer in the limit where the curvature goes to zero.
Except for this limiting case, w(a) is poorly fit by linear evolution in a.Comment: 7 pages, 9 figures, label on Fig. 4 correcte
A new generic evolution for -essence dark energy with
We reexamine -essence dark energy models with a scalar field and a
factorized Lagrangian, , with A value of the equation of state parameter,
, near requires either or . Previous
work showed that thawing models with evolve along a set of unique
trajectories for , while those with can result in a
variety of different forms for . We show that if is small and
is roughly constant, then the latter models also converge
toward a single unique set of behaviors for , different from those with
. We derive the functional form for in this case, determine
the conditions on for which it applies, and present observational
constraints on this new class of models. We note that -essence models with
correspond to a dark energy sound speed .Comment: 7 pages, 2 figures, discussion and references adde
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