1,935 research outputs found
Delayed Recombination
Under the standard model for recombination of the primeval plasma, and the
cold dark matter model for structure formation, recent measurements of the
first peak in the angular power spectrum of the cosmic microwave background
temperature indicate the spatial geometry of the universe is nearly flat. If
sources of Lya resonance radiation, such as stars or active galactic nuclei,
were present at z ~ 1000 they would delay recombination, shifting the first
peak to larger angular scales, and producing a positive bias in this measure of
space curvature. It can be distinguished from space curvature by its
suppression of the secondary peaks in the spectrum.Comment: submitted to ApJ
Issues for the Next Generation of Galaxy Surveys
I argue that the weight of the available evidence favours the conclusions
that galaxies are unbiased tracers of mass, the mean mass density (excluding a
cosmological constant or its equivalent) is less than the critical Einstein-de
Sitter value, and an isocurvature model for structure formation offers a viable
and arguably attractive model for the early assembly of galaxies. If valid
these conclusions complicate our work of adding structure formation to the
standard model for cosmology, but it seems sensible to pay attention to
evidence.Comment: 14 pages, 3 postscript figures, uses rspublic.st
Hyperuniformity, quasi-long-range correlations, and void-space constraints in maximally random jammed particle packings. II. Anisotropy in particle shape
We extend the results from the first part of this series of two papers by
examining hyperuniformity in heterogeneous media composed of impenetrable
anisotropic inclusions. Specifically, we consider maximally random jammed
packings of hard ellipses and superdisks and show that these systems both
possess vanishing infinite-wavelength local-volume-fraction fluctuations and
quasi-long-range pair correlations. Our results suggest a strong generalization
of a conjecture by Torquato and Stillinger [Phys. Rev. E. 68, 041113 (2003)],
namely that all strictly jammed saturated packings of hard particles, including
those with size- and shape-distributions, are hyperuniform with signature
quasi-long-range correlations. We show that our arguments concerning the
constrained distribution of the void space in MRJ packings directly extend to
hard ellipse and superdisk packings, thereby providing a direct structural
explanation for the appearance of hyperuniformity and quasi-long-range
correlations in these systems. Additionally, we examine general heterogeneous
media with anisotropic inclusions and show for the first time that one can
decorate a periodic point pattern to obtain a hard-particle system that is not
hyperuniform with respect to local-volume-fraction fluctuations. This apparent
discrepancy can also be rationalized by appealing to the irregular distribution
of the void space arising from the anisotropic shapes of the particles. Our
work suggests the intriguing possibility that the MRJ states of hard particles
share certain universal features independent of the local properties of the
packings, including the packing fraction and average contact number per
particle.Comment: 29 pages, 9 figure
Limits on the integration constant of the dark radiation term in Brane Cosmology
We consider the constraints from primordial Helium abundances on the constant
of integration of the dark radiation term of the brane-world generalized
Friedmann equation derived from the Randall-Sundrum Single brane model. We
found that -- using simple, approximate and semianalytical Method -- that the
constant of integration is limited to be between -8.9 and 2.2 which limits the
possible contribution from dark radiation term to be approximately between -27%
to 7% of the background photon energy density.Comment: 8 page
Adiabatic instability in coupled dark energy-dark matter models
We consider theories in which there exists a nontrivial coupling between the
dark matter sector and the sector responsible for the acceleration of the
universe. Such theories can possess an adiabatic regime in which the
quintessence field always sits at the minimum of its effective potential, which
is set by the local dark matter density. We show that if the coupling strength
is much larger than gravitational, then the adiabatic regime is always subject
to an instability. The instability, which can also be thought of as a type of
Jeans instability, is characterized by a negative sound speed squared of an
effective coupled dark matter/dark energy fluid, and results in the exponential
growth of small scale modes. We discuss the role of the instability in specific
coupled CDM and Mass Varying Neutrino (MaVaN) models of dark energy, and
clarify for these theories the regimes in which the instability can be evaded
due to non-adiabaticity or weak coupling.Comment: 20 pages, 2 figures; final published versio
Particle linear theory on a self-gravitating perturbed cubic Bravais lattice
Discreteness effects are a source of uncontrolled systematic errors of N-body
simulations, which are used to compute the evolution of a self-gravitating
fluid. We have already developed the so-called "Particle Linear Theory" (PLT),
which describes the evolution of the position of self-gravitating particles
located on a perturbed simple cubic lattice. It is the discrete analogue of the
well-known (Lagrangian) linear theory of a self-gravitating fluid. Comparing
both theories permits to quantify precisely discreteness effects in the linear
regime. It is useful to develop the PLT also for other perturbed lattices
because they represent different discretizations of the same continuous system.
In this paper we detail how to implement the PLT for perturbed cubic Bravais
lattices (simple, body and face-centered) in a cubic simulation box. As an
application, we will study the discreteness effects -- in the linear regime --
of N-body simulations for which initial conditions have been set-up using these
different lattices.Comment: 9 pages, 4 figures and 4 tables. Minor corrections to match published
versio
The Measure of Cosmological Parameters
New, large, ground and space telescopes are contributing to an exciting and
rapid period of growth in observational cosmology. The subject is now far from
its earlier days of being data-starved and unconstrained, and new data are
fueling a healthy interplay between observations and experiment and theory. I
briefly review here the status of measurements of a number of quantities of
interest in cosmology: the Hubble constant, the total mass-energy density, the
matter density, the cosmological constant or dark energy component, and the
total optical background light.Comment: 12 pages, 4 figures, to be published in "2001: A Spacetime Odyssey:
Proceedings of the Inaugural Conference of the Michigan Center for
Theoretical Physics", Michael J. Duff & James T. Liu, eds., (World
Scientific, Singapore), in pres
Direct Detection Rates of Dark Matter Coupled to Dark Energy
We investigate the effect of a coupling between dark matter and dark energy
on the rates for the direct detection of dark matter. The magnitude of the
effect depends on the strength of this new interaction relative to
gravity. The resulting isothermal velocity distribution for dark matter in
galaxy halos is still Maxwell-Boltzmann (M-B), but the characteristic velocity
and the escape velocity are increased by . We adopt a
phenomenological approach and consider values of near unity. For such
values we find that: (i) The (time averaged) event rate increases for light
WIMPs, while it is somewhat reduced for WIMP masses larger than 100 GeV. (ii)
The time dependence of the rate arising from the modulation amplitude is
decreased compared to the standard M-B velocity distribution. (iii) The average
and maximum WIMP energy increase proportionally to , which, for
sufficiently massive WIMPs, allows the possibility of designing experiments
measuring rays following nuclear de-excitation.Comment: 16 pages, 7 figure
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
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