148 research outputs found
From time to timescape - Einstein's unfinished revolution
I argue that Einstein overlooked an important aspect of the relativity of
time in never quite realizing his quest to embody Mach's principle in his
theory of gravity. As a step towards that goal, I broaden the Strong
Equivalence Principle to a new principle of physics, the Cosmological
Equivalence Principle, to account for the role of the evolving average regional
density of the universe in the synchronisation of clocks and the relative
calibration of inertial frames. In a universe dominated by voids of the size
observed in large-scale structure surveys, the density contrasts of expanding
regions are strong enough that a relative deceleration of the background
between voids and the environment of galaxies, typically of order 10^{-10}
m/s^2, must be accounted for. As a result one finds a universe whose present
age varies by billions of years according to the position of the observer: a
timescape. This model universe is observationally viable: it passes three
critical independent tests, and makes additional predictions. Dark energy is
revealed as a mis-identification of gravitational energy gradients and the
resulting variance in clock rates. Understanding the biggest mystery in
cosmology therefore involves a paradigm shift, but in an unexpected direction:
the conceptual understanding of time and energy in Einstein's own theory is
incomplete.Comment: 13 pages, 3 figures; A runner-up in the 2008 FQXi Essay Contest on
the Nature of Time; Int. J. Mod. Phys. D 18, in pres
Viable inhomogeneous model universe without dark energy from primordial inflation
A new model of the observed universe, using solutions to the full Einstein
equations, is developed from the hypothesis that our observable universe is an
underdense bubble, with an internally inhomogeneous fractal bubble distribution
of bound matter systems, in a spatially flat bulk universe. It is argued on the
basis of primordial inflation and resulting structure formation, that the
clocks of the isotropic observers in average galaxies coincide with clocks
defined by the true surfaces of matter homogeneity of the bulk universe, rather
than the comoving clocks at average spatial positions in the underdense bubble
geometry, which are in voids. This understanding requires a systematic
reanalysis of all observed quantities in cosmology. I begin such a reanalysis
by giving a model of the average geometry of the universe, which depends on two
measured parameters: the present matter density parameter, Omega_m, and the
Hubble constant, H_0. The observable universe is not accelerating. Nonetheless,
inferred luminosity distances are larger than naively expected, in accord with
the evidence of distant type Ia supernovae. The predicted age of the universe
is 15.3 +/-0.7 Gyr. The expansion age is larger than in competing models, and
may account for observed structure formation at large redshifts.Comment: 4 pages, aastex, emualteapj5.sty. v5: Complete overhaul of notation
and presentation to improve clarity. Corrected volume factor increases age of
univers
Comment on "Hubble flow variations as a test for inhomogeneous cosmology"
Saulder et al [2019, A&A, 622, A83; arXiv:1811.11976] have performed a novel
observational test of the local expansion of the Universe for the standard
cosmology as compared to an alternative model with differential cosmic
expansion. Their analysis employs mock galaxy samples from the Millennium
Simulation, a Newtonian -body simulation on a CDM background. For
the differential expansion case the simulation has been deformed in an attempt
to incorporate features of a particular inhomogeneous cosmology: the timescape
model. It is shown that key geometrical features of the timescape cosmology
have been omitted in this rescaling. Consequently, the differential expansion
model tested by Saulder et al (2019) cannot be considered to approximate the
timescape cosmology.Comment: 4 pages; v2: small changes to match published version in A&
Gravitational energy as dark energy: Average observational quantities
In the timescape scenario cosmic acceleration is understand as an apparent
effect, due to gravitational energy gradients that grow when spatial curvature
gradients become significant with the nonlinear growth of cosmic structure.
This affects the calibratation of local geometry to the solutions of the
volume-average evolution equations corrected by backreaction. In this paper I
discuss recent work on defining observational tests for average geometric
quantities which can distinguish the timescape model from a cosmological
constant or other models of dark energy.Comment: 10 pages, 7 figures; submitted to the Proceedings of the Invisible
Universe Conference, Paris, 29 June - 3 July, 2009; J.-M. Alimi (ed), AIP
Conf. Proc., to appea
Quasilocal energy and thermodynamic equilibrium conditions
Equilibrium thermodynamic laws are typically applied to horizons in general
relativity without stating the conditions that bring them into equilibrium. We
fill this gap by applying a new thermodynamic interpretation to a generalized
Raychaudhuri equation for a worldsheet orthogonal to a closed spacelike
2-surface, the "screen", which encompasses a system of arbitrary size in
nonequilibrium with its surroundings in general. In the case of spherical
symmetry this enables us to identify quasilocal thermodynamic potentials
directly related to standard quasilocal energy definitions. Quasilocal
thermodynamic equilibrium is defined by minimizing the mean extrinsic curvature
of the screen. Moreover, without any direct reference to surface gravity, we
find that the system comes into quasilocal thermodynamic equilibrium when the
screen is located at a generalized apparent horizon. Examples of the
Schwarzschild, Friedmann-Lema\^itre and Lema\^itre-Tolman geometries are
investigated and compared. Conditions for the quasilocal thermodynamic and
hydrodynamic equilibrium states to coincide are also discussed, and a
quasilocal virial relation is suggested as a potential application of this
approach.Comment: 27 pages. v2 small typos fixed, matches published versio
Brane worlds with bolts
We construct a family of (p+3)-dimensional brane worlds in which the brane
has one compact extra dimension, the bulk has two extra dimensions, and the
bulk closes regularly at codimension two submanifolds known as bolts. The
(p+1)-dimensional low energy spacetime M_{low} may be any Einstein space with
an arbitrary cosmological constant, the value of the bulk cosmological constant
is arbitrary, and the only fields are the metric and a bulk Maxwell field. The
brane can be chosen to have positive tension, and the closure of the bulk
provides a singularity-free boundary condition for solutions that contain black
holes or gravitational waves in M_{low}. The spacetimes admit a nonlinear
gravitational wave whose properties suggest that the Newtonian gravitational
potential on a flat M_{low} will behave essentially as the static potential of
a massless minimally coupled scalar field with Neumann boundary conditions.
When M_{low} is (p+1)-dimensional Minkowski with p\ge3 and the bulk
cosmological constant vanishes, this static scalar potential is shown to have
the long distance behaviour characteristic of p spatial dimensions.Comment: 18 pages, 1 eps figure, JHEP3 with epsfig; v2,v3 references adde
What is General Relativity?
General relativity is a set of physical and geometric principles, which lead
to a set of (Einstein) field equations that determine the gravitational field,
and to the geodesic equations that describe light propagation and the motion of
particles on the background. But open questions remain, including: What is the
scale on which matter and geometry are dynamically coupled in the Einstein
equations? Are the field equations valid on small and large scales? What is the
largest scale on which matter can be coarse grained while following a geodesic
of a solution to Einstein's equations? We address these questions. If the field
equations are causal evolution equations, whose average on cosmological scales
is not an exact solution of the Einstein equations, then some simplifying
physical principle is required to explain the statistical homogeneity of the
late epoch Universe. Such a principle may have its origin in the dynamical
coupling between matter and geometry at the quantum level in the early
Universe. This possibility is hinted at by diverse approaches to quantum
gravity which find a dynamical reduction to two effective dimensions at high
energies on one hand, and by cosmological observations which are beginning to
strongly restrict the class of viable inflationary phenomenologies on the
other. We suggest that the foundational principles of general relativity will
play a central role in reformulating the theory of spacetime structure to meet
the challenges of cosmology in the 21st century.Comment: 18 pages. Invited article for Physica Scripta Focus issue on 21st
Century Frontiers. v2: Appendix amended, references added. v3: Small
corrections, references added, matches published versio
Defining the frame of minimum nonlinear Hubble expansion variation
We characterize a cosmic rest frame in which the monopole variation of the
spherically averaged nonlinear Hubble expansion is most uniform, under
arbitrary local Lorentz boosts of the central observer. Using the COMPOSITE
sample of 4534 galaxies, we identify a degenerate set of candidate minimum
nonlinear variation frames, which includes the rest frame of the Local Group
(LG) of galaxies, but excludes the standard Cosmic Microwave Background (CMB)
frame. Candidate rest frames defined by a boost from the LG frame close to the
plane of the galaxy have a statistical likelihood similar to the LG frame. This
may result from a lack of constraining data in the Zone of Avoidance. We extend
our analysis to the Cosmicflows-2 (CF2) sample of 8162 galaxies. While the
signature of a systematic boost offset between the CMB and LG frame averages is
still detected, the spherically averaged nonlinear expansion variation in all
rest frames is significantly larger in the CF2 sample than would be reasonably
expected. We trace this to the CF2 distances being reported without a
correction for inhomogeneous distribution Malmquist bias. Systematic
differences in the inclusion of the large SFI++ subsample into the COMPOSITE
and CF2 catalogues are analysed. Our results highlight the importance of a
careful treatment of Malmquist biases for future peculiar velocities studies,
including tests of the hypothesis of Wiltshire et al [Phys. Rev. D 88 (2013)
083529; arXiv:1201.5371] that a significant fraction of the CMB temperature
dipole may be nonkinematic in origin.Comment: 25 pages, 19 figures; v4 erratum added: small corrections, no change
in conclusion
Gravitational energy and cosmic acceleration
Cosmic acceleration is explained quantitatively, as an apparent effect due to
gravitational energy differences that arise in the decoupling of bound systems
from the global expansion of the universe. "Dark energy" is a misidentification
of those aspects of gravitational energy which by virtue of the equivalence
principle cannot be localised, namely gradients in the energy due to the
expansion of space and spatial curvature variations in an inhomogeneous
universe. A new scheme for cosmological averaging is proposed which solves the
Sandage-de Vaucouleurs paradox. Concordance parameters fit supernovae
luminosity distances, the angular scale of the sound horizon in the CMB
anisotropies, and the effective comoving baryon acoustic oscillation scale seen
in galaxy clustering statistics. Key observational anomalies are potentially
resolved, and unique predictions made, including a quantifiable variance in the
Hubble flow below the scale of apparent homogeneity.Comment: 9 pages, 2 figures. An essay which received Honorable Mention in the
2007 GRF Essay Competition. To appear in a special issue of Int. J. Mod.
Phys.
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