670 research outputs found
The multi-stream flows and the dynamics of the cosmic web
A new numerical technique to identify the cosmic web is proposed. It is based
on locating multi-stream flows, i.e. the places where the velocity field is
multi-valued. The method is local in Eulerian space, simple and computaionally
efficient. This technique uses the velocities of particles and thus takes into
account the dynamical information. This is in contrast with the majority of
standard methods that use the coordinates of particles only. Two quantities are
computed in every mesh cell: the mean and variance of the velocity field. In
the cells where the velocity is single-valued the variance must be equal to
zero exactly, therefore the cells with non-zero variance are identified as
multi-stream flows. The technique has been tested in a N-body simulation of the
\L CDM model. The preliminary analysis has shown that numerical noise does not
pose a significant problem. The web identified by the new method has been
compared with the web identified by the standard technique using only the
particle coordinates. The comparison has shown overall similarity of two webs
as expected, however they by no means are identical. For example, the
isocontours of the corresponding fields have significantly different shapes and
some density peaks of similar heights exhibit significant differences in the
velocity variance and vice versa. This suggest that the density and velocity
variance have a significant degree of independence. The shape of the
two-dimensional pdf of density and velocity variance confirms this proposition.
Thus, we conclude that the dynamical information probed by this technique
introduces an additional dimension into analysis of the web.Comment: 19 pages, 10 figure
The Value of the Cosmological Constant
We make the cosmological constant, {\Lambda}, into a field and restrict the
variations of the action with respect to it by causality. This creates an
additional Einstein constraint equation. It restricts the solutions of the
standard Einstein equations and is the requirement that the cosmological wave
function possess a classical limit. When applied to the Friedmann metric it
requires that the cosmological constant measured today, t_{U}, be {\Lambda} ~
t_{U}^(-2) ~ 10^(-122), as observed. This is the classical value of {\Lambda}
that dominates the wave function of the universe. Our new field equation
determines {\Lambda} in terms of other astronomically measurable quantities.
Specifically, it predicts that the spatial curvature parameter of the universe
is {\Omega}_{k0} \equiv -k/a_(0)^(2)H^2= -0.0055, which will be tested by
Planck Satellite data. Our theory also creates a new picture of self-consistent
quantum cosmological history.Comment: 6 pages. This article received Third Prize in the 2011 Gravity
Research Foundation Awards for Essays on Gravitatio
Non-minimal neutral Higgs bosons at LEP2
We study the phenomenology of the neutral Higgs sector of a non-SUSY
non-minimal Standard Model. Models with more than one Higgs doublet are
possible, and may contain neutral Higgs scalars with branching ratios
significantly different to those of the Minimal Standard Model Higgs boson. We
show how these differences may be exploited at LEP2 in order to distinguish the
non-minimal Standard Model from the minimal version.Comment: 12 pages inc 4 figures, Latex, to appear in Physics Letters
Fluctuations in the Cosmic Microwave Background I: Form Factors and their Calculation in Synchronous Gauge
It is shown that the fluctuation in the temperature of the cosmic microwave
background in any direction may be evaluated as an integral involving scalar
and dipole form factors, which incorporate all relevant information about
acoustic oscillations before the time of last scattering. A companion paper
gives asymptotic expressions for the multipole coefficient in terms of
these form factors. Explicit expressions are given here for the form factors in
a simplified hydrodynamic model for the evolution of perturbations.Comment: 35 pages, no figures. Improved treatment of damping, including both
Landau and Silk damping; inclusion of late-time effects; several references
added; minor changes and corrections made. Accepted for publication in Phys.
Rev. D1
Lattice Sigma Models with Exact Supersymmetry
We show how to construct lattice sigma models in one, two and four dimensions
which exhibit an exact fermionic symmetry. These models are discretized and
{\it twisted} versions of conventional supersymmetric sigma models with N=2
supersymmetry. The fermionic symmetry corresponds to a scalar BRST charge built
from the original supercharges. The lattice theories possess local actions and
in many cases admit a Wilson term to suppress doubles. In the two and four
dimensional theorie s we show that these lattice theories are invariant under
additional discrete symmetries. We argue that the presence of these exact
symmetries ensures that no fine tuning is required to achieve N=2 supersymmetry
in the continuum limit. As a concrete example we show preliminary numerical
results from a simulation of the O(3) supersymmetric sigma model in two
dimensions.Comment: 23 pages, 3 figures, formalism generalized to allow for explicit
Wilson mass terms. New numerical results added. Version to be published in
JHE
Fermiophobic Higgs bosons at the Tevatron
Higgs bosons with negligible couplings to fermions can arise in various
non--minimal Higgs sectors. We show that such a particle could be discovered
during the current run at the Tevatron, and would be evidence against a minimal
supersymmetric Higgs sector.Comment: 10 pages, Latex, 5 figures, to appear in Phys. Lett. B, figures
include
Scalaron the mighty: producing dark matter and baryon asymmetry at reheating
In R^2-inflation scalaron slow roll is responsible for the inflationary
stage, while its oscillations reheat the Universe. We find that the same
scalaron decays induced by gravity can also provide the dark matter production
and leptogenesis. With R^2-term and three Majorana fermions added to the
Standard Model, we arrive at the phenomenologically complete theory capable of
simultaneously explaining neutrino oscillations, inflation, reheating, dark
matter and baryon asymmetry of the Universe. Besides the seesaw mechanism in
neutrino sector, we use only gravity, which solves all the problems by
exploiting scalaron.Comment: 13 pages; v2: minor corrections; v3: 14 pages, journal versio
Constraining holographic inflation with WMAP
In a class of recently proposed models, the early universe is strongly
coupled and described holographically by a three-dimensional, weakly coupled,
super-renormalizable quantum field theory. This scenario leads to a power
spectrum of scalar perturbations that differs from the usual empirical LCDM
form and the predictions of generic models of single field, slow roll
inflation. This spectrum is characterized by two parameters: an amplitude, and
a parameter g related to the coupling constant of the dual theory. We estimate
these parameters, using WMAP and other astrophysical data. We compute Bayesian
evidence for both the holographic model and standard LCDM and find that their
difference is not significant, although LCDM provides a somewhat better fit to
the data. However, it appears that Planck will permit a definitive test of this
holographic scenario.Comment: 24 pages, 9 figs, published versio
Cosmological model with interactions in the dark sector
A cosmological model is proposed for the current Universe consisted of
non-interacting baryonic matter and interacting dark components. The dark
energy and dark matter are coupled through their effective barotropic indexes,
which are considered as functions of the ratio between their energy densities.
It is investigated two cases where the ratio is asymptotically stable and their
parameters are adjusted by considering best fits to Hubble function data. It is
shown that the deceleration parameter, the densities parameters, and the
luminosity distance have the correct behavior which is expected for a viable
present scenario of the Universe.Comment: 6 pages, 8 figure
Relating the Cosmological Constant and Supersymmetry Breaking in Warped Compactifications of IIB String Theory
It has been suggested that the observed value of the cosmological constant is
related to the supersymmetry breaking scale M_{susy} through the formula Lambda
\sim M_p^4 (M_{susy}/M_p)^8. We point out that a similar relation naturally
arises in the codimension two solutions of warped space-time varying
compactifications of string theory in which non-isotropic stringy moduli induce
a small but positive cosmological constant.Comment: 7 pages, LaTeX, references added and minor changes made, (v3) map
between deSitter and global cosmic brane solutions clarified, supersymmetry
breaking discussion improved and references adde
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