389 research outputs found
A terrestrial search for dark contents of the vacuum, such as dark energy, using atom interferometry
We describe the theory and first experimental work on our concept for
searching on earth for the presence of dark content of the vacuum (DCV) using
atom interferometry. Specifically, we have in mind any DCV that has not yet
been detected on a laboratory scale, but might manifest itself as dark energy
on the cosmological scale. The experimental method uses two atom
interferometers to cancel the effect of earth's gravity and diverse noise
sources. It depends upon two assumptions: first, that the DCV possesses some
space inhomogeneity in density, and second that it exerts a sufficiently strong
non-gravitational force on matter. The motion of the apparatus through the DCV
should then lead to an irregular variation in the detected matter-wave phase
shift. We discuss the nature of this signal and note the problem of
distinguishing it from instrumental noise. We also discuss the relation of our
experiment to what might be learned by studying the noise in gravitational wave
detectors such as LIGO.The paper concludes with a projection that a future
search of this nature might be carried out using an atom interferometer in an
orbiting satellite. The apparatus is now being constructed
Constraints On The Topology Of The Universe From The WMAP First-Year Sky Maps
We compute the covariance expected between the spherical harmonic
coefficients of the cosmic microwave temperature anisotropy if the
universe had a compact topology. For fundamental cell size smaller than the
distance to the decoupling surface, off-diagonal components carry more
information than the diagonal components (the power spectrum). We use a maximum
likelihood analysis to compare the Wilkinson Microwave Anisotropy Probe
first-year data to models with a cubic topology. The data are compatible with
finite flat topologies with fundamental domain times the distance to
the decoupling surface at 95% confidence. The WMAP data show reduced power at
the quadrupole and octopole, but do not show the correlations expected for a
compact topology and are indistinguishable from infinite models.Comment: 16 pages, 5 figure
Time-Varying Fine-Structure Constant Requires Cosmological Constant
Webb et al. presented preliminary evidence for a time-varying fine-structure
constant. We show Teller's formula for this variation to be ruled out within
the Einstein-de Sitter universe, however, it is compatible with cosmologies
which require a large cosmological constant.Comment: 3 pages, no figures, revtex, to be published in Mod. Phys. Lett.
Correspondence between Electro-Magnetic Field and other Dark Energies in Non-linear Electrodynamics
In this work, we have considered the flat FRW model of the universe filled
with electro-magnetic field. First, the Maxwell's electro-magnetic field in
linear form has been discussed and after that the modified Lagrangian in
non-linear form for accelerated universe has been considered. The corresponding
energy density and pressure for non-linear electro-magnetic field have been
calculated. We have found the condition such that the electro-magnetic field
generates dark energy. The correspondence between the electro-magnetic field
and the other dark energy candidates namely tachyonic field, DBI-essence,
Chaplygin gas, hessence dark energy, k-essenece and dilaton dark energy have
been investigated. We have also reconstructed the potential functions and the
scalar fields in this scenario.Comment: 11 pages, 7 figure
De-Sitter-spacetime instability from a nonstandard vector field
It is found that de-Sitter spacetime, the constant-curvature matter-free
solution of the Einstein equations with a positive cosmological constant,
becomes classically unstable due to the dynamic effects of a certain type of
vector field (fundamentally different from a gauge field). The perturbed
de-Sitter universe evolves towards a final exotic singularity. The relevant
vector-field configurations violate the strong and dominant energy conditions.Comment: 10 pages, v7: published versio
A Testable Solution of the Cosmological Constant and Coincidence Problems
We present a new solution to the cosmological constant (CC) and coincidence
problems in which the observed value of the CC, , is linked to other
observable properties of the universe. This is achieved by promoting the CC
from a parameter which must to specified, to a field which can take many
possible values. The observed value of Lambda ~ 1/(9.3 Gyrs)^2\Lambda$-values
and does not rely on anthropic selection effects. Our model includes no
unnatural small parameters and does not require the introduction of new
dynamical scalar fields or modifications to general relativity, and it can be
tested by astronomical observations in the near future.Comment: 31 pages, 4 figures; v2: version accepted by Phys. Rev.
Agreement of MSmetrix with established methods for measuring cross-sectional and longitudinal brain atrophy
Introduction
Despite the recognized importance of atrophy in multiple sclerosis (MS), methods for its quantification have been mostly restricted to the research domain. Recently, a CE labelled and FDA approved MS-specific atrophy quantification method, MSmetrix, has become commercially available. Here we perform a validation of MSmetrix against established methods in simulated and in vivo MRI data.
Methods
Whole-brain and gray matter (GM) volume were measured with the cross-sectional pipeline of MSmetrix and compared to the outcomes of FreeSurfer (cross-sectional pipeline), SIENAX and SPM. For this comparison we investigated 20 simulated brain images, as well as in vivo data from 100 MS patients and 20 matched healthy controls. In fifty of the MS patients a second time point was available. In this subgroup, we additionally analyzed the whole-brain and GM volume change using the longitudinal pipeline of MSmetrix and compared the results with those of FreeSurfer (longitudinal pipeline) and SIENA.
Results
In the simulated data, SIENAX displayed the smallest average deviation compared with the reference whole-brain volume (+ 19.56 ± 10.34 mL), followed by MSmetrix (− 38.15 ± 17.77 mL), SPM (− 42.99 ± 17.12 mL) and FreeSurfer (− 78.51 ± 12.68 mL). A similar pattern was seen in vivo. Among the cross-sectional methods, Deming regression analyses revealed proportional errors particularly in MSmetrix and SPM. The mean difference percentage brain volume change (PBVC) was lowest between longitudinal MSmetrix and SIENA (+ 0.16 ± 0.91%). A strong proportional error was present between longitudinal percentage gray matter volume change (PGVC) measures of MSmetrix and FreeSurfer (slope = 2.48). All longitudinal methods were sensitive to the MRI hardware upgrade that occurred during the time of the study.
Conclusion
MSmetrix, FreeSurfer, FSL and SPM show differences in atrophy measurements, even at the whole-brain level, that are large compared to typical atrophy rates observed in MS. Especially striking are the proportional errors between methods. Cross-sectional MSmetrix behaved similarly to SPM, both in terms of mean volume difference as well as proportional error. Longitudinal MSmetrix behaved most similar to SIENA. Our results indicate that brain volume measurement and normalization from T1-weighted images remains an unsolved problem that requires much more attention
Deforming the Maxwell-Sim Algebra
The Maxwell alegbra is a non-central extension of the Poincar\'e algebra, in
which the momentum generators no longer commute, but satisfy
. The charges commute with the momenta,
and transform tensorially under the action of the angular momentum generators.
If one constructs an action for a massive particle, invariant under these
symmetries, one finds that it satisfies the equations of motion of a charged
particle interacting with a constant electromagnetic field via the Lorentz
force. In this paper, we explore the analogous constructions where one starts
instead with the ISim subalgebra of Poincar\'e, this being the symmetry algebra
of Very Special Relativity. It admits an analogous non-central extension, and
we find that a particle action invariant under this Maxwell-Sim algebra again
describes a particle subject to the ordinary Lorentz force. One can also deform
the ISim algebra to DISim, where is a non-trivial dimensionless
parameter. We find that the motion described by an action invariant under the
corresponding Maxwell-DISim algebra is that of a particle interacting via a
Finslerian modification of the Lorentz force.Comment: Appendix on Lifshitz and Schrodinger algebras adde
Proper Size of the Visible Universe in FRW Metrics with Constant Spacetime Curvature
In this paper, we continue to examine the fundamental basis for the
Friedmann-Robertson-Walker (FRW) metric and its application to cosmology,
specifically addressing the question: What is the proper size of the visible
universe? There are several ways of answering the question of size, though
often with an incomplete understanding of how far light has actually traveled
in reaching us today from the most remote sources. The difficulty usually
arises from an inconsistent use of the coordinates, or an over-interpretation
of the physical meaning of quantities such as the so-called proper distance
R(t)=a(t)r, written in terms of the (unchanging) co-moving radius r and the
universal expansion factor a(t). In this paper, we use the five non-trivial FRW
metrics with constant spacetime curvature (i.e., the static FRW metrics, but
excluding Minkowski) to prove that in static FRW spacetimes in which expansion
began from an initial signularity, the visible universe today has a proper size
equal to R_h(t_0/2), i.e., the gravitational horizon at half its current age.
The exceptions are de Sitter and Lanczos, whose contents had pre-existing
positions away from the origin. In so doing, we confirm earlier results showing
the same phenomenon in a broad range of cosmologies, including LCDM, based on
the numerical integration of null geodesic equations through an FRW metric.Comment: Accepted for publication in Classical and Quantum Gravit
- …