2,832 research outputs found
On Semi-classical Degravitation and the Cosmological Constant Problems
In this report, we discuss a candidate mechanism through which one might
address the various cosmological constant problems. We first observe that the
renormalization of gravitational couplings (induced by integrating out various
matter fields) manifests non-local modifications to Einstein's equations as
quantum corrected equations of motion. That is, at the loop level, matter
sources curvature through a gravitational coupling that is a non-local function
of the covariant d'Alembertian. If the functional form of the resulting
Newton's `constant' is such that it annihilates very long wavelength sources,
but reduces to ( being the 4d Planck mass) for all sources
with cosmologically observable wavelengths, we would have a complimentary
realization of the degravitation paradigm-- a realization through which its
non-linear completion and the corresponding modified Bianchi identities are
readily understood. We proceed to consider various theories whose coupling to
gravity may a priori induce non-trivial renormalizations of Newton's constant
in the IR, and arrive at a class of non-local effective actions which yield a
suitably degravitating filter function for Newton's constant upon subsequently
being integrated out. We motivate this class of non-local theories through
several considerations, discuss open issues, future directions, the inevitable
question of scheme dependence in semi-classical gravitational calculations and
comment on connections with other meditations in the literature on relaxing of
the cosmological constant semi-classically.Comment: 15 pages, 2 appendices. References added
Market Concentration, Firm Size and Innovative Activity: A Firm-level Economic Analysis of Selected Indian Industries under Economic Liberalization
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Relaxing the Electroweak Scale: the Role of Broken dS Symmetry
Recently, a novel mechanism to address the hierarchy problem has been
proposed \cite{Graham:2015cka}, where the hierarchy between weak scale physics
and any putative `cutoff' is translated into a parametrically large field
excursion for the so-called relaxion field, driving the Higgs mass to values
much less than through cosmological dynamics. In its simplest incarnation,
the relaxion mechanism requires nothing beyond the standard model other than an
axion (the relaxion field) and an inflaton. In this note, we critically
re-examine the requirements for successfully realizing the relaxion mechanism
and point out that parametrically larger field excursions can be obtained for a
given number of e-folds by simply requiring that the background break exact de
Sitter invariance. We discuss several corollaries of this observation,
including the interplay between the upper bound on the scale and the order
parameter associated with the breaking of dS symmetry, and entertain
the possibility that the relaxion could play the role of a curvaton. We find
that a successful realization of the mechanism is possible with as few as
e-foldings, albeit with a reduced cutoff GeV
for a dark QCD axion and outline a minimal scenario that can be made consistent
with CMB observations.Comment: 16 pages, 3 figures. Version to appear in JHE
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