74 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
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
On the Predictiveness of Single-Field Inflationary Models
We re-examine the predictiveness of single-field inflationary models and
discuss how an unknown UV completion can complicate determining inflationary
model parameters from observations, even from precision measurements. Besides
the usual naturalness issues associated with having a shallow inflationary
potential, we describe another issue for inflation, namely, unknown UV physics
modifies the running of Standard Model (SM) parameters and thereby introduces
uncertainty into the potential inflationary predictions. We illustrate this
point using the minimal Higgs Inflationary scenario, which is arguably the most
predictive single-field model on the market, because its predictions for ,
and are made using only one new free parameter beyond those measured
in particle physics experiments, and run up to the inflationary regime. We find
that this issue can already have observable effects. At the same time, this
UV-parameter dependence in the Renormalization Group allows Higgs Inflation to
occur (in principle) for a slightly larger range of Higgs masses. We comment on
the origin of the various UV scales that arise at large field values for the SM
Higgs, clarifying cut off scale arguments by further developing the formalism
of a non-linear realization of in curved space. We
discuss the interesting fact that, outside of Higgs Inflation, the effect of a
non-minimal coupling to gravity, even in the SM, results in a non-linear EFT
for the Higgs sector. Finally, we briefly comment on post BICEP2 attempts to
modify the Higgs Inflation scenario.Comment: 31 pp, 4 figures v4: Minor correction to section 3.1. Main arguments
and conclusions unchange
Features and New Physical Scales in Primordial Observables: Theory and Observation
All cosmological observations to date are consistent with adiabatic, Gaussian
and nearly scale invariant initial conditions. These findings provide strong
evidence for a particular symmetry breaking pattern in the very early universe
(with a close to vanishing order parameter, ), widely accepted as
conforming to the predictions of the simplest realizations of the inflationary
paradigm. However, given that our observations are only privy to perturbations,
in inferring something about the background that gave rise to them, it should
be clear that many different underlying constructions project onto the same set
of cosmological observables. Features in the primordial correlation functions,
if present, would offer a unique and discriminating window onto the parent
theory in which the mechanism that generated the initial conditions is
embedded. In certain contexts, simple linear response theory allows us to infer
new characteristic scales from the presence of features that can break the
aforementioned degeneracies among different background models, and in some
cases can even offer a limited spectroscopy of the heavier degrees of freedom
that couple to the inflaton. In this review, we offer a pedagogical survey of
the diverse, theoretically well grounded mechanisms which can imprint features
into primordial correlation functions in addition to reviewing the techniques
one can employ to probe observations. These observations include cosmic
microwave background anisotropies and spectral distortions as well as the
matter two and three point functions as inferred from large-scale structure and
potentially, 21 cm surveys.Comment: Invited review to IJMPD, 101 pages + 2 appendices, 29 figures,
references added, matches journal versio
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