290 research outputs found
Observables in theories with a varying fine structure constant
We show how two seemingly different theories with a scalar multiplicative
coupling to electrodynamics are actually two equivalent parametrisations of the
same theory: despite some differences in the interpretation of some
phenemenological aspects of the parametrisations, they lead to the same
physical observables. This is illustrated on the interpretation of observations
of the Cosmic Microwave Background.Comment: 14 pages, matched published versio
Breaking of the equivalence principle in the electromagnetic sector and its cosmological signatures
This paper proposes a systematic study of cosmological signatures of
modifications of gravity via the presence of a scalar field with a
multiplicative coupling to the electromagnetic Lagrangian. We show that, in
this framework, variations of the fine structure constant, violations of the
distance duality relation, evolution of the cosmic microwave background (CMB)
temperature and CMB distortions are intimately and unequivocally linked. This
enables one to put very stringent constraints on possible violations of the
distance duality relation, on the evolution of the CMB temperature and on
admissible CMB distortions using current constraints on the fine structure
constant. Alternatively, this offers interesting possibilities to test a wide
range of theories of gravity by analysing several datasets concurrently. We
discuss results obtained using current data as well as some forecasts for
future data sets such as those coming from EUCLID or the SKA.Comment: 14 pages, 4 figures, matched published version. Note: title changed
upon suggestion of PRD editor
Classification Of Nanopolymers
Nanopolymers with different structures, shapes, and functional forms have recently been prepared using several techniques. Nanopolymers are the most promising basic building blocks for mounting complex and simple hierarchical nanosystems. The applications of nanopolymers are extremely broad and polymer-based nanotechnologies are fast emerging. We propose a nanopolymer classification scheme based on self-assembled structures, non self-assembled structures, and on the number of dimensions in the nanometer range (nD). © 2008 IOP Publishing Ltd.1001Ikkala, O., Brinke Ten, G., (2004) Chem. Commu., p. 213
Is the local Hubble flow consistent with concordance cosmology?
Yes. In a perturbed Friedmann model, the difference of the Hubble constants
measured in two rest-frames is independent of the source peculiar velocity and
depends only on the relative velocity of the observers, to lowest order in
velocity. Therefore this difference should be zero when averaging over
sufficient sources, which are at large enough distances to suppress local
nonlinear inhomogeneity. We use a linear perturbative analysis to predict the
Doppler effects on redshifts and distances. Since the observed redshifts encode
the effect of local bulk flow due to nonlinear structure, our linear analysis
is able to capture aspects of the nonlinear behaviour. Using the largest
available distance compilation from CosmicFlows-3, we find that the data is
consistent with simulations based on the concordance model, for sources at
Mpc.Comment: 10 pages, 3 figures. Version accepted by JCA
Averaging inhomogeneities in scalar-tensor cosmology
The backreaction of inhomogeneities on the cosmic dynamics is studied in the
context of scalar-tensor gravity. Due to terms of indefinite sign in the
non-canonical effective energy tensor of the Brans-Dicke-like scalar field,
extra contributions to the cosmic acceleration can arise. Brans-Dicke and
metric f(R) gravity are presented as specific examples. Certain representation
problems of the formalism peculiar to these theories are pointed out.Comment: Comments and references added. 14 page
Probing beyond-Horndeski gravity on ultra-large scales
The beyond-Horndeski gravity has recently been reformulated in the dark energy paradigm—which has been dubbed, Unified Dark Energy (UDE). The evolution equations for the given UDE appear to correspond to a non-conservative dark energy scenario, in which the total energy-momentum tensor is not conserved. We investigate both the background cosmology and, the large-scale imprint of the UDE by probing the angular power spectrum of galaxy number counts, on ultra-large scales; taking care to include the full relativistic corrections in the observed overdensity. The background evolution shows that only an effective mass smaller than the Planck mass is needed in the early universe in order for predictions in the given theory to match current observational constraints. We found that the effective mass-evolution-rate parameter, which drives the evolution of the UDE, acts to enhance the observed power spectrum and, hence, relativistic effects (on ultra-large scales) by enlarging the UDE sound horizon. Conversely, both the (beyond) Horndeski parameter and the kineticity act to diminish the observed power spectrum, by decreasing the UDE sound horizon. Our results show that, in a universe with UDE, a multi-tracer analysis will be needed to detect the relativistic effects in the large-scale structure. In the light of a multi-tracer analysis, the various relativistic effects hold the potential to distinguish different gravity models. Moreover, while the Doppler effect will remain significant at all epochs and, thus can not be ignored, the integrated Sachs-Wolfe, the time-delay and the potential (difference) effects, respectively, will only become significant at epochs near z = 3 and beyond, and may be neglected at late epochs. In the same vein, the Doppler effect alone can serve as an effective cosmological probe for the large-scale structure or gravity models, in the angular power spectrum—at all z
Integrability of anisotropic and homogeneous Universes in scalar-tensor theory of gravitation
In this paper, we develop a method based on the analysis of the Kovalewski
exponents to study the integrability of anisotropic and homogeneous Universes.
The formalism is developed in scalar-tensor gravity, the general relativistic
case appearing as a special case of this larger framework. Then, depending on
the rationality of the Kovalewski exponents, the different models, both in the
vacuum and in presence of a barotropic matter fluid, are classified, and their
integrability is discussed.Comment: 16 pages, no figure, accepted in CQ
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