Related papers: Axion Perturbations: A General Analytical Treatmen…
Possible existence of the primordial magnetic fields has affected the structure formation of the universe. In this paper it is shown that the initial conditions for density perturbations with magnetic fields derived in previous works are…
We consider the most general primordial cosmological perturbation in a universe filled with photons, baryons, neutrinos, and a hypothetical cold dark matter (CDM) component within the framework of linearized perturbation theory. We give a…
The observed values of the cosmological constant {\it and} the abundance of Dark Matter (DM) can be successfully understood, using certain measures, by imposing the anthropic requirement that density perturbations go non-linear and…
We review current cosmological constraints on the axion. We describe the basic mechanisms by which axions are created in the early universe, focussing on the standard thermal scenario where the dominant process is through axion radiation by…
Cosmological moduli generically come to dominate the energy density of the early universe, and thereby trigger an early matter dominated era. Such non-standard cosmological histories are expected to have profound effects on the evolution…
We review the study of inhomogeneous perturbations about a homogeneous and isotropic background cosmology. We adopt a coordinate based approach, but give geometrical interpretations of metric perturbations in terms of the expansion, shear…
For the minimal QCD axion model it is generally believed that overproduction of dark matter constrains the axion mass to be above a certain threshold, or at least that the initial misalignment angle must be tuned if the mass is below that…
The two dark sectors of the universe - dark matter and dark energy - may interact with each other. Background and linear density perturbation evolution equations are developed for a generic coupling. We then establish the general conditions…
Relativistic axions are good candidates for the dark radiation for which there are mounting observational hints. The primordial decays of heavy fields produce axions which are ultra-energetic compared to thermalised matter and inelastic…
The axion arises in well-motivated extensions of the Standard Model of particle physics and is regarded as an alternative to the weakly interacting massive particle paradigm to explain the nature of dark matter. In this contribution, we…
Axions arise in many theoretical extensions of the Standard Model of particle physics, in particular the "string axiverse". If the axion masses, $m_a$, and (effective) decay constants, $f_a$, lie in specific ranges, then axions contribute…
Laboratory searches for ultralight axion dark matter (DM) have traditionally assumed the terrestrial density of axions is equal to the average density of DM in the solar system. However, quadratic couplings to matter introduce a non-trivial…
The axion has emerged in recent years as a leading particle candidate to provide the mysterious dark matter in the cosmos, as we review here for a general scientific audience. We describe first the historical roots of the axion in the…
The Cosmic Background (CB) is defined as the isotropic diffuse radiation field with extragalactic origin found across the electromagnetic spectrum. Different astrophysical sources dominate the CB emission at different energies, such as…
The default assumption of early universe cosmology is that the postinflationary universe was radiation dominated until it was about 47000 years old. Direct evidence for the radiation dominated epoch extends back until nucleosynthesis, which…
Axions are hypothetical pseudoscalar particles introduced initially as a solution to the Strong CP problem in Quantum Chromodynamics (QCD), and they also arise naturally in a broad class of low-energy compactifications of string theory.…
We revisit the isocurvature density perturbations induced by quantum fluctuations of the axion field by extending a recently developed analytic method and approximations to a time-dependent scalar potential, which enables us to follow the…
The evolution of cosmic perturbations in a two-component ultralight axionic universe is investigated. We present the first spectral computation of perturbations in multi-component universes. A particular case composed of light extreme…
We study the cosmology of multi-field Dark Energy, using a well-motivated axio-dilaton model that contains the minimal number of fields to have the 2-derivative sigma-model interactions that power-counting arguments show naturally compete…
Recently, two of the present authors showed that even when the axion momentum is much smaller than its mass, the axion can still behave like radiation if its energy density greatly exceeds the maximum potential energy set by the cosine-type…