3,022 research outputs found
Exact braneworld cosmology induced from bulk black holes
We use a new, exact approach in calculating the energy density measured by an
observer living on a brane embedded in a charged black hole spacetime. We find
that the bulk Weyl tensor gives rise to non-linear terms in the energy density
and pressure in the FRW equations for the brane. Remarkably, these take exactly
the same form as the ``unconventional'' terms found in the cosmology of branes
embedded in pure AdS, with extra matter living on the brane. Black hole driven
cosmologies have the benefit that there is no ambiguity in splitting the
braneword energy momentum into tension and additional matter. We propose a new,
enlarged relationship between the two descriptions of braneworld cosmology. We
also study the exact thermodynamics of the field theory and present a
generalised Cardy-Verlinde formula in this set up.Comment: 17 pages, no figures; v3: Minor change, References added, Version to
appear in CQ
The Gravity Field, Orientation, and Ephemeris of Mercury from MESSENGER Observations After Three Years in Orbit
We have analyzed three years of radio tracking data from the MESSENGER spacecraft in orbit around Mercury and determined the gravity field, planetary orientation, and ephemeris of the innermost planet. With improvements in spatial coverage, force modeling, and data weighting, we refined an earlier global gravity field both in quality and resolution, and we present here a spherical harmonic solution to degree and order 50. In this field, termed HgM005, uncertainties in low-degree coefficients are reduced by an order of magnitude relative to the earlier global field, and we obtained a preliminary value of the tidal Love number k(sub 2) of 0.451+/-0.014. We also estimated Mercury's pole position, and we obtained an obliquity value of 2.06 +/- 0.16 arcmin, in good agreement with analysis of Earth-based radar observations. From our updated rotation period (58.646146 +/- 0.000011 days) and Mercury ephemeris, we verified experimentally the planet's 3: 2 spin-orbit resonance to greater accuracy than previously possible. We present a detailed analysis of the HgM005 covariance matrix, and we describe some near-circular frozen orbits around Mercury that could be advantageous for future exploration
Ghosts in asymmetric brane gravity and the decoupled stealth limit
We study the spectrum of gravitational perturbations around a vacuum de
Sitter brane in a 5D asymmetric braneworld model, with induced curvature on the
brane. This generalises the stealth acceleration model proposed by Charmousis,
Gregory and Padilla (CGP) which realises the Cardassian cosmology in which
power law cosmic acceleration can be driven by ordinary matter. Whenever the
bulk has infinite volume we find that there is always a perturbative ghost
propagating on the de Sitter brane, in contrast to the Minkowski brane case
analysed by CGP. We discuss the implication of this ghost for the stealth
acceleration model, and identify a limiting case where the ghost decouples as
the de Sitter curvature vanishes.Comment: 21 page
Galileon Hairs of Dyson Spheres, Vainshtein's Coiffure and Hirsute Bubbles
We study the fields of spherically symmetric thin shell sources, a.k.a. Dyson
spheres, in a {\it fully nonlinear covariant} theory of gravity with the
simplest galileon field. We integrate exactly all the field equations once,
reducing them to first order nonlinear equations. For the simplest galileon,
static solutions come on {\it six} distinct branches. On one, a Dyson sphere
surrounds itself with a galileon hair, which far away looks like a hair of any
Brans-Dicke field. The hair changes below the Vainshtein scale, where the extra
galileon terms dominate the minimal gradients of the field. Their hair looks
more like a fuzz, because the galileon terms are suppressed by the derivative
of the volume determinant. It shuts off the `hair bunching' over the `angular'
2-sphere. Hence the fuzz remains dilute even close to the source. This is
really why the Vainshtein's suppression of the modifications of gravity works
close to the source. On the other five branches, the static solutions are all
{\it singular} far from the source, and shuttered off from asymptotic infinity.
One of them, however, is really the self-accelerating branch, and the
singularity is removed by turning on time dependence. We give examples of
regulated solutions, where the Dyson sphere explodes outward, and its
self-accelerating side is nonsingular. These constructions may open channels
for nonperturbative transitions between branches, which need to be addressed
further to determine phenomenological viability of multi-branch gravities.Comment: 29+1 pages, LaTeX, 2 .pdf figure
Solar System Expansion and Strong Equivalence Principle as Seen by the NASA MESSENGER Mission
The NASA MESSENGER mission explored the innermost planet of the solar system and obtained a rich dataset of range measurements for the determination of Mercury's ephemeris. Here we use these precise data collected over seven years to estimate parameters related to General Relativity and the evolution of the Sun. These results confirm the validity of the Strong Equivalence Principle with a significantly refined uncertainty of the Nordtvedt parameter eta=(-6.6 plus or minus 7.2)x10(exp -5) By assuming a metric theory of gravitation, we retrieved the Post-Newtonian parameter beta = 1 + (-1.6 plus or minus 1.8)x10(exp -5) and the Sun's gravitational oblateness, J(sub 2 solar)=(2.246 plus or minus 0.022)x10(exp -7). Finally, we obtain an estimate of the time variation of the Sun gravitational parameter, G (raised dot)solar mass/G solar mass =(-6.13 plus or minus 1.47)x10(exp -14), which is consistent with the expected solar mass loss due to the solar wind and interior processes. This measurement allows us to constrain |G(raised dot)|/G to be less than 4 x 10(exp -14) yr(exp -1)
A short review of "DGP Specteroscopy"
In this paper we provide a short review of the main results developed in
hep-th/0604086. We focus on linearised vacuum perturbations about the
self-accelerating branch of solutions in the DGP model. These are shown to
contain a ghost in the spectrum for any value of the brane tension. We also
comment on hep-th/0607099, where some counter arguments have been presented.Comment: Minor typos correcte
Stealth Acceleration and Modified Gravity
We show how to construct consistent braneworld models which exhibit late time
acceleration. Unlike self-acceleration, which has a de Sitter vacuum state, our
models have the standard Minkowski vacuum and accelerate only in the presence
of matter, which we dub ``stealth-acceleration''. We use an effective action
for the brane which includes an induced gravity term, and allow for an
asymmetric set-up. We study the linear stability of flat brane vacua and find
the regions of parameter space where the set-up is stable. The 4-dimensional
graviton is only quasi-localised in this set-up and as a result gravity is
modified at late times. One of the two regions is strongly coupled and the
scalar mode is eaten up by an extra symmetry that arises in this limit. Having
filtered the well-defined theories we then focus on their cosmology. When the
graviton is quasi-localised we find two main examples of acceleration. In each
case, we provide an illustrative model and compare it to LambdaCDM.Comment: 32 pages, 5 figure
Simulated recovery of Europa's global shape and tidal Love numbers from altimetry and radio tracking during a dedicated flyby tour
The fundamental scientific objectives for future spacecraft exploration of Jupiter's moon Europa include confirmation of the existence of subsurface ocean beneath the surface ice shell and constraints on the physical properties of the ocean. Here we conduct a comprehensive simulation of a multiple-flyby mission. We demonstrate that radio tracking data can provide an estimate of the gravitational tidal Love number k2 with sufficient precision to confirm the presence of a liquid layer. We further show that a capable long-range laser altimeter can improve determination of the spacecraft position, improve the k2 determination (2 (3-4% error), which is directly related to the amplitude of the surface tidal deformation. These measurements, in addition to the global shape accurately constrained by the long altimetric profiles, can yield further constraints on the interior structure of Europa. Key Points A multiple-flyby mission to Europa can recover key geophysical parameters Laser altimetry can uniquely and accurately recover the global shape of Europa Laser altimetry enables the recovery of h2 to constrain the ice shell thicknes
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Diabetes reversal by inhibition of the low-molecular-weight tyrosine phosphatase.
Obesity-associated insulin resistance plays a central role in type 2 diabetes. As such, tyrosine phosphatases that dephosphorylate the insulin receptor (IR) are potential therapeutic targets. The low-molecular-weight protein tyrosine phosphatase (LMPTP) is a proposed IR phosphatase, yet its role in insulin signaling in vivo has not been defined. Here we show that global and liver-specific LMPTP deletion protects mice from high-fat diet-induced diabetes without affecting body weight. To examine the role of the catalytic activity of LMPTP, we developed a small-molecule inhibitor with a novel uncompetitive mechanism, a unique binding site at the opening of the catalytic pocket, and an exquisite selectivity over other phosphatases. This inhibitor is orally bioavailable, and it increases liver IR phosphorylation in vivo and reverses high-fat diet-induced diabetes. Our findings suggest that LMPTP is a key promoter of insulin resistance and that LMPTP inhibitors would be beneficial for treating type 2 diabetes
Braneworld holography in Gauss-Bonnet gravity
We investigate holography on an (n-1)-dimensional brane embedded in a
background of AdS black holes, in n-dimensional Gauss-Bonnet gravity. We
demonstrate that for a critical brane near the AdS boundary, the Friedmann
equation corresponds to that of the standard cosmology driven by a CFT dual to
the AdS bulk. We show that there is no holographic description for non-critical
branes, or when the brane is further away from the AdS boundary. We then derive
a Cardy-Verlinde formula for the dual CFT on the critical brane near the
boundary. This gives us insight into the remarkable correspondence between
Cardy-Verlinde formulae and Friedmann equations in Einstein gravity.Comment: 24 pages, no figures; references added, minor changes, version to
appear in CQ
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