3,345 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
On Necessary and Sufficient Conditions for Preserving Convergence Rates to Equilibrium in Deterministically and Stochastically Perturbed Differential Equations with Regularly Varying Nonlinearity
This paper develops necessary and sufficient conditions for the preservation
of asymptotic convergence rates of deterministically and stochastically
perturbed ordinary differential equations with regularly varying nonlinearity
close to their equilibrium. Sharp conditions are also established which
preserve the asymptotic behaviour of the derivative of the underlying
unperturbed equation. Finally, necessary and sufficient conditions are
established which enable finite difference approximations to the derivative in
the stochastic equation to preserve the asymptotic behaviour of the derivative
of the unperturbed equation, even though the solution of the stochastic
equation is nowhere differentiable, almost surely
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
The Well-Tempered Cosmological Constant: The Horndeski Variations
Well tempering is one of the few classical field theory methods for solving
the original cosmological constant problem, dynamically canceling a large
(possibly Planck scale) vacuum energy and leaving the matter component intact,
while providing a viable cosmology with late time cosmic acceleration and an
end de Sitter state. We present the general constraints that variations of
Horndeski gravity models with different combinations of terms must satisfy to
admit an exact de Sitter spacetime that does not respond to an arbitrarily
large cosmological constant. We explicitly derive several specific
scalar-tensor models that well temper and can deliver a standard cosmic history
including current cosmic acceleration. Stability criteria, attractor behavior
of the de Sitter state, and the response of the models to pressureless matter
are considered. The well tempered conditions can be used to focus on particular
models of modified gravity that have special interest -- not only removing the
original cosmological constant problem but providing relations between the free
Horndeski functions and reducing them to a couple of parameters, suitable for
testing gravity and cosmological data analysis.Comment: 25 pages, 3 figure
The Well-Tempered Cosmological Constant: Fugue in B
Zero point fluctuations of quantum fields should generate a large
cosmological constant energy density in any spacetime. How then can we have
anything other than de Sitter space without fine tuning? Well tempering --
dynamical cancellation of the cosmological constant using degeneracy within the
field equations -- can replace a large cosmological constant with a much lower
energy state. Here we give an explicit mechanism to obtain a Minkowski
solution, replacing the cosmological constant with zero, and testing its
attractor nature and persistence through a vacuum phase transition. We derive
the general conditions that Horndeski scalar-tensor gravity must possess, and
evolve in a fugue of functions, to deliver nothing and make the universe be
flat.Comment: 15 pages, 3 figure
Relaxing a large cosmological constant in the astrophysical domain
We study the problem of relaxing a large cosmological constant in the
astrophysical domain through a dynamical mechanism based on a modified action
of gravity previously considered by us at the cosmological level. We solve the
model in the Schwarzschild-de Sitter metric for large and small astrophysical
scales, and address its physical interpretation by separately studying the
Jordan's frame and Einstein's frame formulations of it. In particular, we
determine the extremely weak strength of fifth forces in our model and show
that they are virtually unobservable. Finally, we estimate the influence that
the relaxation mechanism may have on pulling apart the values of the two
gravitational potentials Psi and Phi of the metric, as this implies a departure
of the model from General Relativity and could eventually provide an
observational test of the new framework at large astrophysical scales, e.g.
through gravitational lensing.Comment: 14 pages, 3 figures, accepted in Mod. Phys. Lett. A, extended
discussion, references adde
- …