1,721,012 research outputs found
Challenging the Cosmological Constant
No full textKaloper, Nemanja. (2012). Challenging the Cosmological Constant. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/129785
Anisotropies in nonthermal distortions of cosmic light from photon-axion conversion
No full textUltralight axions which couple sufficiently strongly to photons can leave imprints on the sky at diverse frequencies by mixing with cosmic light in the presence of background magnetic fields. We explore such direction dependent grey-body distortions of the cosmic microwave background spectrum, enhanced by resonant conditions in the intergalactic medium plasma. We also find that if such axions are produced in the early Universe and represent a subdominant dark radiation component today, they could convert into x-rays in supervoids and brighten them at x-ray frequencies
Quantum Field Theory of Interacting Dark Matter/Dark Energy: Dark Monodromies
No full textWe discuss how to formulate a quantum field theory of dark energy interacting with dark matter. We show that the proposals based on the assumption that dark matter is made up of heavy particles with masses which are very sensitive to the value of dark energy are strongly constrained. Quintessence-generated long range forces and radiative stability of the quintessence potential require that such dark matter and dark energy are completely decoupled. However, if dark energy and a fraction of dark matter are very light axions, they can have significant mixings which are radiatively stable and perfectly consistent with quantum field theory. Such models can naturally occur in multi-axion realizations of monodromies. The mixings yield interesting signatures which are observable and are within current cosmological limits but could be constrained further by future observations.We discuss how to formulate a quantum field theory of dark energy interacting with dark matter. We show that the proposals based on the assumption that dark matter is made up of heavy particles with masses which are very sensitive to the value of dark energy are strongly constrained. Quintessence-generated long-range forces and radiative stability of the quintessence potential require that such dark matter and dark energy are completely decoupled. However, if dark energy and a fraction of dark matter are very light axions, they can have significant mixings which are radiatively stable and perfectly consistent with quantum field theory. Such models can naturally occur in multi-axion realizations of monodromies. The mixings yield interesting signatures which are observable and are within current cosmological limits but could be constrained further by future observations.We discuss how to formulate a quantum field theory of dark energy interacting with dark matter. We show that the proposals based on the assumption that dark matter is made up of heavy particles with masses which are very sensitive to the value of dark energy are strongly constrained. Quintessence-generated long range forces and radiative stability of the quintessence potential require that such dark matter and dark energy are completely decoupled. However, if dark energy and a fraction of dark matter are very light axions, they can have significant mixings which are radiatively stable and perfectly consistent with quantum field theory. Such models can naturally occur in multi-axion realizations of monodromies. The mixings yield interesting signatures which are observable and are within current cosmological limits but could be constrained further by future observations
Power-law Inflation Satisfies Penrose's Weyl Curvature Hypothesis
Full text linkBased on entropy considerations and the arrow of time Penrose argued that the
universe must have started in a special initial singularity with vanishing Weyl
curvature. This is often interpreted to be at odds with inflation. Here we
argue just the opposite, that Penrose's persuasions are in fact consistent with
inflation. Using the example of power law inflation, we show that inflation
begins with a past null singularity, where Weyl tensor vanishes when the metric
is initially exactly conformally flat. This initial state precisely obeys
Penrose's conditions. The initial null singularity breaks -reversal
spontaneously and picks the arrow of time. It can be regulated and interpreted
as a creation of a universe from nothing, initially fitting in a bubble of
Planckian size when it materializes. Penrose's initial conditions are favored
by the initial symmetry of the bubble, selected by extremality of the
regulated Euclidean action. The predicted observables are marginally in tension
with the data, but they can fit if small corrections to power law inflation
kick in during the last 60 efolds.Comment: 19 pages LaTeX, 5 figures png format; v2: added comments, discussion
and references; published versio
Monodromy inflation at strong coupling: in the sky
No full textWe present a simple effective field theory formulation of a general family of single-field flux monodromy models for which strong coupling effects at large field values can flatten the potential and activate higher-derivative operators. Both of these effects can suppress the tensor amplitude. These models are radiatively and nonperturbatively stable and can sustain ≳60 e folds of inflation. The dynamics combines features of both large-field chaotic inflation and k inflation. Reducing the tensor-scalar ratio below the observational bound r≲0.1 while keeping the scalar spectral index ns within experimental bounds either yields equilateral non-Gaussianity fNLeq≃O(1), close to the current observational bounds, or gives very small r.We present a simple effective field theory formulation of a general family of single field flux monodromy models for which strong coupling effects at large field values can flatten the potential and activate operators with higher powers of derivatives. These models are radiatively and non-perturbatively stable and can easily sustain \ga 60 efolds of inflation. The dynamics combines features of both large field chaotic inflation and -inflation, both of which can suppress the tensor amplitude. Reducing the tensor-scalar ratio below the observational bound while keeping the scalar spectral index within experimental bounds either yields equilateral nongaussianity , close to the current observational bounds, or ultimately gives very small
Super-GZK photons from photon–axion mixing
No full textWe show that photons with energies above the GZK cutoff can reach us from very distant sources if they mix with light axions in extragalactic magnetic fields. The effect which enables this is the conversion of photons into axions, which are sufficiently weakly coupled to travel large distances unimpeded. These axions then convert back into high energy photons close to the Earth. We show that photon-axion mixing facilitates the survival of super-GZK photons most efficiently with a photon-axion coupling scale M ≳ 1011 GeV, which is in the same range as the scale for the photon-axion mixing explanation for the dimming of supernovae without cosmic acceleration. We discuss possible observational consequences of this effect
Very Hairy Inflation
Full text linkWe revisit the rollercoaster cosmology based on multiple stages of monodromy
inflation. Working within the framework of effective flux monodromy field
theory, we include the full range of strong coupling corrections to the
inflaton sector. We find that flattened potentials
with , limited to efolds in the first
stage of inflation, continue to fit the CMB. They yield , and produce relic gravity waves with , in full agreement with the most recent bounds from BICEP/{\it Keck}.
The nonlinear derivative corrections generated by strong dynamics in EFT also
lead to equilateral non-Gaussianity , close to the current observational bounds. Finally, in multi-stage
rollercoaster, an inflaton-hidden sector coupling can produce a
tachyonic chiral vector background, which converts rapidly into tensors during
the short interruption by matter domination. The produced stochastic gravity
waves are chiral, and so they may be clearly identifiable by gravity wave
instruments like LISA, Big Bang Observatory, Einstein Telescope, NANOgrav or
SKA, depending on the precise model realization. We also point out that the
current attempts to resolve the tension using early dark energy
generically raise . This may significantly alter the impact of BICEP/{\it
Keck} data on models of inflation.Comment: 25+1 pages LaTeX, 8 figures. v2: Minor changes, references added,
published in PR
Recommended from our members
Axions in Cosmology
Full text linkThis thesis is based on two main projects completed during my graduate studies. The first project consists of showing that hybrid inflationary theories are still viable meaning that there is a parameter space that give us cosmological parameters in agreement with the Planck data.The second project explore the mixing between gravity wave, electromagnetic waves and axion waves in a curved space time as a way to potentially detect dark matter.
We revisit two-field hybrid inflation as an effective field theory for low-scale inflation with sub- Planckian scalar field ranges. We focus on a prototype model by Stewart because it allows for a red spectral tilt, which still fits the current data. We describe the constraints on this model imposed by current CMB measurements. We then explore the stability of this model to quantum corrections. We find that for relevant, marginal, and at least a finite set of irrelevant operators, some additional mechanism is required to render the model stable to corrections from both quantum field theory and quantum gravity. We outline a possible mechanism by realizing the scalars as compact axions dual to massive 4-form field strengths, and outline how natural hybrid inflation may be supported by strong dynamics in the dual theory.
We describe bosonic (scalar, electromagnetic and gravitational) wave mixing in curved space- time. Curved spacetime adds a new length scale, the Schwarzschild radius, which significantly alters the oscillation probabilities in comparison to the standard flat spacetime computations. The alterations are analogous to the Mikheyev-Smirnov-Wolfenstein (MSW) effect for neutrinos and are “frozen- in” as the outgoing gravitational and/or electromagnetic wave propagates away from a compact object. Although we consider the axion and axion-like particles, our computations
iii
are largely model independent and applicable for generic spin-zero dark matter. We describe the probabilities for axions and generic bosonic dark matter oscillations. We describe some of the ob- servational consequences of the mixing including the energy and polarization of the waves exiting the compact object
An Etude on Global Vacuum Energy Sequester
Full text linkRecently two of the authors proposed a mechanism of vacuum energy sequester as a means of protecting the observable cosmological constant from quantum radiative corrections. The original proposal was based on using global Lagrange multipliers, but later a local formulation was provided. Subsequently other interesting claims of a different non-local approach to the cosmological constant problem were made, based again on global Lagrange multipliers. We examine some of these proposals and find their mutual relationship. We explain that the proposals which do not treat the cosmological constant counterterm as a dynamical variable require fine tunings to have acceptable solutions. Furthermore, the counterterm often needs to be retuned at every order in the loop expansion to cancel the radiative corrections to the cosmological constant, just like in standard GR. These observations are an important reminder of just how the proposal of vacuum energy sequester avoids such problems
de Sitter Space Decay and Cosmological Constant Relaxation in Unimodular Gravity with Charged Membranes
No full textGeneral covariant unimodular gravity frameworks, based on the
Henneaux-Teitelboim formulation, are, in disguise, precisely -form field
theories corrected with higher dimension operators. In the presence of charged
tensional membranes, any de Sitter space in all such theories is unstable and
decays. If the fluxes sourced by membranes are mutually incommensurate, de
Sitter geometries comprise a very refined discretuum of states. Whenever the
-form sector is dominated by terms linear in flux the almost-Minkowski space
is the unique long-time attractor. As a result, a tiny cosmological constant is
natural in all such frameworks, without appealing to anthropic reasoning.Comment: 28 pages, 5 figures; v2: Final version as published in PRD: title
changed in journal, added discussion, added labels in a figure, extra
reference
- …
