5,861 research outputs found
The Swampland, Quintessence and the Vacuum Energy
It has recently been conjectured that string theory does not admit de Sitter
vacua, and that quintessence explains the current epoch of accelerated cosmic
expansion. A proposed, key prediction of this scenario is time-varying
couplings in the dark sector, induced by the evolving quintessence field. We
note that cosmological models with varying couplings suffer from severe
problems with quantum corrections, beyond those shared by all quintessence
models. The vacuum energy depends on all masses and couplings of the theory,
and even small variations of parameters can lead to overwhelmingly large
corrections to the effective potential. We find that quintessence models with
varying parameters can be realised in consistent quantum theories by either: 1)
enforcing exceptional levels of fine-tuning; 2) realising some unknown
mechanism that cancels all undesirable contributions to the effective potential
with unprecedented accuracy; or 3) ensuring that the quintessence field couples
exclusively to very light states, and does not backreact on heavy fields.Comment: 4
Axions and ALPs: a very short introduction
The QCD axion was originally predicted as a dynamical solution to the strong
CP problem. Axion like particles (ALPs) are also a generic prediction of many
high energy physics models including string theory. Theoretical models for
axions are reviewed, giving a generic multi-axion action with couplings to the
standard model. The couplings and masses of these axions can span many orders
of magnitude, and cosmology leads us to consider several distinct populations
of axions behaving as coherent condensates, or relativistic particles. Light,
stable axions are a mainstay dark matter candidate. Axion cosmology and
calculation of the relic density are reviewed. A very brief survey is given of
the phenomenology of axions arising from their direct couplings to the standard
model, and their distinctive gravitational interactions.Comment: This article is a longer version of material contributed to the 13th
Patras Workshop on Axions, WIMPs and WISPs, Thessaloniki, May 15 to 19, 201
Axion Cosmology
1. Introduction 2. Models: the QCD axion; the strong CP problem; PQWW, KSVZ,
DFSZ; anomalies, instantons and the potential; couplings; axions in string
theory 3. Production and I.C.'s: SSB and non-perturbative physics; the axion
field during inflation and PQ SSB; cosmological populations - decay of parent,
topological defects, thermal production, vacuum realignment 4. The Cosmological
Field: action; background evolution; misalignment for QCD axion and ALPs;
cosmological perturbation theory - i.c.'s, early time treatment, axion sound
speed and Jeans scale, transfer functions and WDM; the Schrodinger picture;
simualting axions; BEC 5. CMB and LSS: Primary anisotropies; matter power;
combined constraints; Isocurvature and inflation 6. Galaxy Formation; halo mass
function; high-z and the EOR; density profiles; the CDM small-scale crises 7.
Accelerated expansion: the c.c. problem; axion inflation (natural and
monodromy) 8. Gravitational interactions with black holes and pulsars 9.
Non-gravitational interactions: stellar astrophysics; LSW; vacuum
birefringence; axion forces; direct detection with ADMX and CASPEr; Axion
decays; dark radiation; astrophysical magnetic fields; cosmological
birefringence 10. Conclusions A Theta vacua of gauge theories B EFT for
cosmologists C Friedmann equations D Cosmological fluids E Bayes Theorem and
priors F Degeneracies and sampling G Sheth-Tormen HMFComment: v2 greatly extended: 111 pages, 38 figures. Accepted for publication
in Physics Report
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