100 research outputs found
Consistency check of {\Lambda}CDM phenomenology
The standard model of cosmology LCDM assumes general relativity, flat space,
and the presence of a positive cosmological constant. We relax these
assumptions allowing spatial curvature, time-dependent effective dark energy
equation of state, as well as modifications of the Poisson equation for the
lensing potential, and modifications of the growth of linear matter density
perturbations in alternate combinations. Using six parameters characterizing
these relations, we check LCDM for consistency utilizing cosmic microwave
background anisotropies, cross correlations thereof with high-redshift galaxies
through the integrated Sachs-Wolfe effect, the Hubble constant, supernovae and
baryon acoustic oscillation distances, as well as the relation between weak
gravitational lensing and galaxy flows. In all scenarios, we find consistency
of the concordance model at the 95% confidence level. However, we emphasize
that constraining supplementary background parameters and parametrizations of
the growth of large-scale structure separately may lead to a priori exclusion
of viable departures from the concordance model.Comment: 15 pages, 14 figures, 4 tables; revision with minor change
Challenges to Self-Acceleration in Modified Gravity from Gravitational Waves and Large-Scale Structure
With the advent of gravitational-wave astronomy marked by the aLIGO GW150914
and GW151226 observations, a measurement of the cosmological speed of gravity
will likely soon be realized. We show that a confirmation of equality to the
speed of light as indicated by indirect Galactic observations will have
important consequences for a very large class of alternative explanations of
the late-time accelerated expansion of our Universe. It will break the dark
degeneracy of self-accelerated Horndeski scalar-tensor theories in the
large-scale structure that currently limits a rigorous discrimination between
acceleration from modified gravity and from a cosmological constant or dark
energy. Signatures of a self-acceleration must then manifest in the linear,
unscreened cosmological structure. We describe the minimal modification
required for self-acceleration with standard gravitational-wave speed and show
that its maximum likelihood yields a 3-sigma poorer fit to cosmological
observations compared to a cosmological constant. Hence, equality between the
speeds challenges the concept of cosmic acceleration from a genuine
scalar-tensor modification of gravity.Comment: 5 pages; v2 updated with newer data; v3 extended titl
Dark Energy vs. Modified Gravity
Understanding the reason for the observed accelerated expansion of the
Universe represents one of the fundamental open questions in physics. In
cosmology, a classification has emerged among physical models for the
acceleration, distinguishing between Dark Energy and Modified Gravity. In this
review, we give a brief overview of models in both categories as well as their
phenomenology and characteristic observable signatures in cosmology. We also
introduce a rigorous distinction between Dark Energy and Modified Gravity based
on the strong and weak equivalence principles.Comment: 29 pages, 4 figures; invited review submitted to Annual Reviews of
Nuclear and Particle Science; v2: some pertinent references added; v3: table
with constraints added, reflects published version; v4 [trivial]: fixed
missing references in arxiv versio
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