27,682 research outputs found
Hubble's law and faster than light expansion speeds
Naively applying Hubble's law to a sufficiently distant object gives a
receding velocity larger than the speed of light. By discussing a very similar
situation in special relativity, we argue that Hubble's law is meaningful only
for nearby objects with non-relativistic receding speeds. To support this
claim, we note that in a curved spacetime manifold it is not possible to
directly compare tangent vectors at different points, and thus there is no
natural definition of relative velocity between two spatially separated objects
in cosmology. We clarify the geometrical meaning of the Hubble's receding speed
v by showing that in a Friedmann-Robertson-Walker spacetime if the
four-velocity vector of a comoving object is parallel-transported along the
straight line in flat comoving coordinates to the position of a second comoving
object, then v/c actually becomes the rapidity of the local Lorentz
transformation, which maps the fixed four-velocity vector to the transported
one.Comment: 5 pages, 2 figures, to appear in Am. J. Phy
Background Dependent Lorentz Violation: Natural Solutions to the Theoretical Challenges of the OPERA Experiment
To explain both the OPERA experiment and all the known phenomenological
constraints/observations on Lorentz violation, the Background Dependent Lorentz
Violation (BDLV) has been proposed. We study the BDLV in a model independent
way, and conjecture that there may exist a "Dream Special Relativity Theory",
where all the Standard Model (SM) particles can be subluminal due to the
background effects. Assuming that the Lorentz violation on the Earth is much
larger than those on the interstellar scale, we automatically escape all the
astrophysical constraints on Lorentz violation. For the BDLV from the effective
field theory, we present a simple model and discuss the possible solutions to
the theoretical challenges of the OPERA experiment such as the Bremsstrahlung
effects for muon neutrinos and the pion decays. Also, we address the Lorentz
violation constraints from the LEP and KamLAMD experiments. For the BDLV from
the Type IIB string theory with D3-branes and D7-branes, we point out that the
D3-branes are flavour blind, and all the SM particles are the conventional
particles as in the traditional SM when they do not interact with the
D3-branes. Thus, we not only can naturally avoid all the known phenomenological
constraints on Lorentz violation, but also can naturally explain all the
theoretical challenges. Interestingly, the energy dependent photon velocities
may be tested at the experiments.Comment: RevTex4, 14 pages, minor corrections, references adde
Cell-free prediction of protein expression costs for growing cells
Translating heterologous proteins places significant burden on host cells, consuming expression resources leading to slower cell growth and productivity. Yet predicting the cost of protein production for any given gene is a major challenge, as multiple processes and factors combine to determine translation efficiency. To enable prediction of the cost of gene expression in bacteria, we describe here a standard cell-free lysate assay that provides a relative measure of resource consumption when a protein coding sequence is expressed. These lysate measurements can then be used with a computational model of translation to predict the in vivo burden placed on growing E. coli cells for a variety of proteins of different functions and lengths. Using this approach, we can predict the burden of expressing multigene operons of different designs and differentiate between the fraction of burden related to gene expression compared to action of a metabolic pathway
A Gauge-invariant Analysis of Magnetic Fields in General Relativistic Cosmology
We provide a fully general-relativistic treatment of cosmological
perturbations in a universe permeated by a large-scale primordial magnetic
field, using the Ellis-Bruni gauge-invariant formalism. The exact non-linear
equations for general relativistic magnetohydrodynamic evolution are derived. A
number of applications are made: the behaviour of small perturbations to
Friedmann universes are studied; a comparison is made with earlier Newtonian
treatments of cosmological perturbations and some effects of inflationary
expansion are examined.Comment: 31 pages, Latex, Submitted to Classical and Quantum Gravit
Lensing and caustic effects on cosmological distances
We consider the changes which occur in cosmological distances due to the
combined effects of some null geodesics passing through low-density regions
while others pass through lensing-induced caustics. This combination of effects
increases observed areas corresponding to a given solid angle even when
averaged over large angular scales, through the additive effect of increases on
all scales, but particularly on micro-angular scales; however angular sizes
will not be significantly effected on large angular scales (when caustics
occur, area distances and angular-diameter distances no longer coincide). We
compare our results with other works on lensing, which claim there is no such
effect, and explain why the effect will indeed occur in the (realistic)
situation where caustics due to lensing are significant. Whether or not the
effect is significant for number counts depends on the associated angular
scales and on the distribution of inhomogeneities in the universe. It could
also possibly affect the spectrum of CBR anisotropies on small angular scales,
indeed caustics can induce a non-Gaussian signature into the CMB at small
scales and lead to stronger mixing of anisotropies than occurs in weak lensing.Comment: 28 pages, 6 ps figures, eps
Geodesic Deviation Equation in Bianchi Cosmologies
We present the Geodesic Deviation Equation (GDE) for the
Friedmann-Robertson-Walker(FRW) universe and we compare it with the equation
for Bianchi type I model. We justify consider this cosmological model due to
the recent importance the Bianchi Models have as alternative models in
cosmology. The main property of these models, solutions of Einstein Field
Equations (EFE) is that they are homogeneous as the FRW model but they are not
isotropic. We can see this because they have a non-null Weyl tensor in the GDE.Comment: Submitted to Journal of Physics: Conference Series (JPCS), ERE200
General relativistic analysis of peculiar velocities
We give a careful general relativistic and (1+3)-covariant analysis of
cosmological peculiar velocities induced by matter density perturbations in the
presence of a cosmological constant. In our quasi-Newtonian approach,
constraint equations arise to maintain zero shear of the non-comoving
fundamental worldlines which define a Newtonian-like frame, and these lead to
the (1+3)-covariant dynamical equations, including a generalized Poisson-type
equation. We investigate the relation between peculiar velocity and peculiar
acceleration, finding the conditions under which they are aligned. In this case
we find (1+3)-covariant relativistic generalizations of well-known Newtonian
results.Comment: 8 pages, LaTeX2e (iopart); minor changes, matches version accepted
for publication by Classical and Quantum Gravit
Curvature blow up in Bianchi VIII and IX vacuum spacetimes
The maximal globally hyperbolic development of non-Taub-NUT Bianchi IX vacuum
initial data and of non-NUT Bianchi VIII vacuum initial data is C2
inextendible. Furthermore, a curvature invariant is unbounded in the incomplete
directions of inextendible causal geodesics.Comment: 20 pages, no figures. Submitted to Classical and Quantum Gravit
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