488 research outputs found
The Well-Tempered Cosmological Constant
Self tuning is one of the few methods for dynamically cancelling a large
cosmological constant and yet giving an accelerating universe. Its drawback is
that it tends to screen all sources of energy density, including matter. We
develop a model that tempers the self tuning so the dynamical scalar field
still cancels an arbitrary cosmological constant, including the vacuum energy
through any high energy phase transitions, without affecting the matter fields.
The scalar-tensor gravitational action is simple, related to cubic Horndeski
gravity, with a nonlinear derivative interaction plus a tadpole term. Applying
shift symmetry and using the property of degeneracy of the field equations we
find families of functions that admit de Sitter solutions with expansion rates
that are independent of the magnitude of the cosmological constant and preserve
radiation and matter dominated phases. That is, the method can deliver a
standard cosmic history including current acceleration, despite the presence of
a Planck scale cosmological constant.Comment: 19 pages, 4 figure
Regularized braneworlds of arbitrary codimension
We consider a thick p-brane embedded in an n-dimensional spacetime possessing
radial symmetry in the directions orthogonal to the brane. We first consider a
static brane, and find a general fine tuning relationship between the brane and
bulk parameters required for the brane to be flat. We then consider the
cosmology of a time dependent brane in a static bulk, and find the Friedmann
equation for the brane scale factor a(t). The singularities that would
ordinarily arise when considering arbitrary codimensions are avoided by
regularizing the brane, giving it a finite profile in the transverse
dimensions. However, since we consider the brane to be a strictly local defect,
we find that the transverse dimensions must have infinite volume, and hence
gravity cannot be localized on the brane without resorting to some infra-red
cutoff.Comment: 21 page
Cluster Probes of Dark Energy Clustering
Cluster abundances are oddly insensitive to canonical early dark energy.
Early dark energy with sound speed equal to the speed of light cannot be
distinguished from a quintessence model with the equivalent expansion history
for but negligible early dark energy density, despite the different early
growth rate. However, cold early dark energy, with a sound speed much smaller
than the speed of light, can give a detectable signature. Combining cluster
abundances with cosmic microwave background power spectra can determine the
early dark energy fraction to 0.3 % and distinguish a true sound speed of 0.1
from 1 at 99 % confidence. We project constraints on early dark energy from the
Euclid cluster survey, as well as the Dark Energy Survey, using both current
and projected Planck CMB data, and assess the impact of cluster mass
systematics. We also quantify the importance of dark energy perturbations, and
the role of sound speed during a crossing of
An Expansion of Well Tempered Gravity
When faced with two nigh intractable problems in cosmology -- how to remove
the original cosmological constant problem and how to parametrize modified
gravity to explain current cosmic acceleration -- we can make progress by
counterposing them. The well tempered solution to the cosmological constant
through degenerate scalar field dynamics also relates disparate Horndeski
gravity terms, making them contrapuntal. We derive the connection between the
kinetic term and braiding term for shift symmetric theories
(including the running Planck mass ), extending previous work on monomial
or binomial dependence to polynomials of arbitrary finite degree. We also
exhibit an example for an infinite series expansion. This contrapuntal
condition greatly reduces the number of parameters needed to test modified
gravity against cosmological observations, for these "golden" theories of
gravity.Comment: 7 page
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