4,684 research outputs found
K-Chameleon and the Coincidence Problem
In this paper we present a hybrid model of k-essence and chameleon, named as
k-chameleon. In this model, due to the chameleon mechanism, the directly strong
coupling between the k-chameleon field and matters (cold dark matters and
baryons) is allowed. In the radiation dominated epoch, the interaction between
the k-chameleon field and background matters can be neglected, the behavior of
the k-chameleon therefore is the same as that of the ordinary k-essence. After
the onset of matter domination, the strong coupling between the k-chameleon and
matters dramatically changes the result of the ordinary k-essence. We find that
during the matter-dominated epoch, only two kinds of attractors may exist: one
is the familiar {\bf K} attractor and the other is a completely {\em new},
dubbed {\bf C} attractor. Once the universe is attracted into the {\bf C}
attractor, the fraction energy densities of the k-chameleon and
dust matter are fixed and comparable, and the universe will undergo
a power-law accelerated expansion. One can adjust the model so that the {\bf K}
attractor do not appear. Thus, the k-chameleon model provides a natural
solution to the cosmological coincidence problem.Comment: Revtex, 17 pages; v2: 18 pages, two figures, more comments and
references added, to appear in PRD, v3: published versio
Constraining f(R) Gravity as a Scalar Tensor Theory
We search for viable f(R) theories of gravity, making use of the equivalence
between such theories and scalar-tensor gravity. We find that models can be
made consistent with solar system constraints either by giving the scalar a
high mass or by exploiting the so-called chameleon effect. However, in both
cases, it appears likely that any late-time cosmic acceleration will be
observationally indistinguishable from acceleration caused by a cosmological
constant. We also explore further observational constraints from, e.g., big
bang nucleosynthesis and inflation.Comment: 15 pages, 5 figure
A new test of conservation laws and Lorentz invariance in relativistic gravity
General relativity predicts that energy and momentum conservation laws hold
and that preferred frames do not exist. The parametrised post-Newtonian
formalism (PPN) phenomenologically quantifies possible deviations from general
relativity. The PPN parameter alpha_3 (which identically vanishes in general
relativity) plays a dual role in that it is associated both with a violation of
the momentum conservation law, and with the existence of a preferred frame. By
considering the effects of alpha_3 neq 0 in certain binary pulsar systems, it
is shown that alpha_3 < 2.2 x 10^-20 (90% CL). This limit improves on previous
results by several orders of magnitude, and shows that pulsar tests of alpha_3
rank (together with Hughes-Drever-type tests of local Lorentz invariance) among
the most precise null experiments of physics.Comment: Submitted to Classical Quantum Gravity, LaTeX, requires ioplppt.sty,
no figure
Testing General Relativity with Current Cosmological Data
Deviations from general relativity, such as could be responsible for the
cosmic acceleration, would influence the growth of large scale structure and
the deflection of light by that structure. We clarify the relations between
several different model independent approaches to deviations from general
relativity appearing in the literature, devising a translation table. We
examine current constraints on such deviations, using weak gravitational
lensing data of the CFHTLS and COSMOS surveys, cosmic microwave background
radiation data of WMAP5, and supernova distance data of Union2. Markov Chain
Monte Carlo likelihood analysis of the parameters over various redshift ranges
yields consistency with general relativity at the 95% confidence level.Comment: 11 pages; 7 figures; typographical errors corrected; this is the
published versio
The Gravitomagnetic Influence on Gyroscopes and on the Lunar Orbit
Gravitomagnetism--a motional coupling of matter analogous to the Lorentz
force in electromagnetism--has observable consequences for any scenario
involving differing mass currents. Examples include gyroscopes located near a
rotating massive body, and the interaction of two orbiting bodies. In the
former case, the resulting precession of the gyroscope is often called ``frame
dragging,'' and is the principal measurement sought by the Gravity Probe-B
experiment. The latter case is realized in the earth-moon system, and the
effect has in fact been confirmed via lunar laser ranging (LLR) to
approximately 0.1% accuracy--better than the anticipated accuracy of the
Gravity-Probe-B result. This paper shows the connnection between these
seemingly disparate phenomena by employing the same gravitomagnetic term in the
equation of motion to obtain both gyroscopic precession and modification of the
lunar orbit. Since lunar ranging currently provides a part in a thousand fit to
the gravitomagnetic contributions to the lunar orbit, this feature of
post-Newtonian gravity is not adjustable to fit any anomalous result beyond the
0.1% level from Gravity Probe-B without disturbing the existing fit of theory
to the 36 years of LLR data.Comment: 4 pages; accepted for publication in Physical Review Letter
Detecting a Lorentz-Violating Field in Cosmology
We consider cosmology in the Einstein-aether theory (the generally covariant
theory of gravitation coupled to a dynamical timelike Lorentz-violating vector
field) with a linear aether-Lagrangian. The 3+1 spacetime splitting approach is
used to derive covariant and gauge invariant perturbation equations which are
valid for a general class of Lagrangians. Restricting attention to the
parameter space of these theories which is consistent with local gravity
experiments, we show that there are tracking behaviors for the aether field,
both in the background cosmology and at linear perturbation level. The
primordial power-spectrum of scalar perturbations in this model is shown to be
the same that predicted by standard general relativity. However, the
power-spectrum of tensor perturbation is different from that in general
relativity, but has a smaller amplitude and so cannot be detected at present.
We also study the implications for late-time cosmology and find that the
evolution of photon and neutrino anisotropic stresses can source the aether
field perturbation during the radiation and matter dominated epochs, and as a
result the CMB and matter power spectra are modified. However these effects are
degenerate with respect to other cosmological parameters, such as neutrino
masses and the bias parameter in the observed galaxy spectrum.Comment: 13 pages, 3 figures; modified version to appear in Physical Review
Testing Gravity in the Outer Solar System: Results from Trans-Neptunian Objects
The inverse square law of gravity is poorly probed by experimental tests at
distances of ~ 10 AUs. Recent analysis of the trajectory of the Pioneer 10 and
11 spacecraft have shown an unmodeled acceleration directed toward the Sun
which was not explained by any obvious spacecraft systematics, and occurred
when at distances greater than 20 AUs from the Sun. If this acceleration
represents a departure from Newtonian gravity or is indicative of an additional
mass distribution in the outer solar system, it should be detectable in the
orbits of Trans-Neptunian Objects (TNOs). To place limits on deviations from
Newtonian gravity, we have selected a well observed sample of TNOs found
orbiting between 20 and 100 AU from the Sun. By examining their orbits with
modified orbital fitting software, we place tight limits on the perturbations
of gravity that could exist in this region of the solar system.Comment: 20 pages, 4 figures, 2 tables, uses AASTex v5.x macro
Translational Invariance and the Anisotropy of the Cosmic Microwave Background
Primordial quantum fluctuations produced by inflation are conventionally
assumed to be statistically homogeneous, a consequence of translational
invariance. In this paper we quantify the potentially observable effects of a
small violation of translational invariance during inflation, as characterized
by the presence of a preferred point, line, or plane. We explore the imprint
such a violation would leave on the cosmic microwave background anisotropy, and
provide explicit formulas for the expected amplitudes of
the spherical-harmonic coefficients.Comment: Notation improve
Extended coherence time on the clock transition of optically trapped Rubidium
Optically trapped ensembles are of crucial importance for frequency
measurements and quantum memories, but generally suffer from strong dephasing
due to inhomogeneous density and light shifts. We demonstrate a drastic
increase of the coherence time to 21 s on the magnetic field insensitive clock
transition of Rb-87 by applying the recently discovered spin self-rephasing.
This result confirms the general nature of this new mechanism and thus shows
its applicability in atom clocks and quantum memories. A systematic
investigation of all relevant frequency shifts and noise contributions yields a
stability of 2.4E-11 x tau^(-1/2), where tau is the integration time in
seconds. Based on a set of technical improvements, the presented frequency
standard is predicted to rival the stability of microwave fountain clocks in a
potentially much more compact setup.Comment: 5 pages, 4 figure
Don\u27t Send The Little Ones Crying To Bed
https://digitalcommons.library.umaine.edu/mmb-vp/1328/thumbnail.jp
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