Pseudoscalar perturbations and polarization of the cosmic microwave background

Cosmic Birefringence Triggered by Dark Matter Domination

Cosmic birefringence is predicted if an axionlike particle (ALP) moves after the recombination. We show that this naturally happens if the ALP is coupled to the dark matter density because it then acquires a large effective mass after the matter-radiation equality. Our scenario applies to a broad range of the ALP mass m_{ϕ}≲10^{-28}  eV, even smaller than the present Hubble constant. We give a simple model to realize this scenario, where dark matter is made of hidden monopoles, which give the ALP such a large effective mass through the Witten effect. The mechanism works if the ALP decay constant is of order of the grand unified theory scale without a fine-tuning of the initial misalignment angle. For smaller decay constant, the hidden monopole can be a fraction of dark matter. We also study the implications for the QCD axion, and show that the domain wall problem can be solved by the effective mass.

The stochastic axiverse

In addition to spectacular signatures such as black hole superradiance and the rotation of CMB polarization, the plenitude of axions appearing in the string axiverse may have potentially dangerous implications. An example is the cosmological overproduction of relic axions and moduli by the misalignment mechanism, more pronounced in regions where the signals mentioned above may be observable, that is for large axion decay constant. In this work, we study the minimal requirements to soften this problem and show that the fundamental requirement is a long period of low-scale inflation. However, in this case, if the inflationary Hubble scale is lower than around O(100) eV, no relic DM axion is produced in the early Universe. Cosmological production of some axions may be activated, via the misalignment mechanism, if their potential minimum changes between inflation and today. As a particular example, we study in detail how the maximal-misalignment mechanism dilutes the effect of dangerous axions and allows the production of axion DM in a controlled way. In this case, the potential of the axion that realises the mechanism shifts by a factor ∆θ = π between the inflationary epoch and today, and the axion starts to oscillate from the top of its potential. We also show that axions with masses ma ∼ O(1 − 100) H0 realising the maximal-misalignment mechanism generically behave as dark energy with a decay constant that can take values well below the Planck scale, avoiding problems associated to super-Planckian scales. Finally, we briefly study the basic phenomenological implications of the mechanism and comment on the compatibility of this type of maximally-misaligned quintessence with the swampland criteria.

Recent discoveries from the cosmic microwave background: a review of recent progress

Measurements of the anisotropies in the cosmic microwave background (CMB) radiation have provided a wealth of information about the cosmological model that describes the contents and evolution of the universe. These data have led to a standard model described by just six parameters. In this review we focus on discoveries made in the past decade from satellite and ground-based experiments, and look ahead to those anticipated in the coming decade. We provide an introduction to the key CMB observables including temperature and polarization anisotropies, and describe recent progress towards understanding the initial conditions of structure formation, and establishing the properties of the contents of the universe including neutrinos. Results are now being derived both from the primordial CMB signal that traces the behavior of the universe at 400 000 years of cosmic time, as well as from the signals imprinted at later times due to scattering from galaxy clusters, from the motion of electrons in the ionized universe, and from the gravitational lensing of the CMB photons. We describe current experimental methods to measure the CMB, particularly focusing on details relevant for ground and balloon-based instruments, and give an overview of the broad data analysis methods required to convert measurements of the microwave sky into cosmological parameters.

Tensor interpretation of BICEP2 results severely constrains axion dark matter

The recent detection of B modes by the BICEP2 experiment has nontrivial implications for axion dark matter implied by combining the tensor interpretation with isocurvature constraints from Planck observations. In this Letter the measurement is taken as fact, and its implications considered, though further experimental verification is required. In the simplest inflation models, r=0.2 implies HI=1.1×10(14)  GeV. If the axion decay constant fa<HI/2π, constraints on the dark matter (DM) abundance alone rule out the QCD axion as DM for ma≲52χ6/7  μeV (where χ>1 accounts for theoretical uncertainty). If fa>HI/2π then vacuum fluctuations of the axion field place conflicting demands on axion DM: isocurvature constraints require a DM abundance which is too small to be reached when the backreaction of fluctuations is included. High-fa QCD axions are thus ruled out. Constraints on axionlike particles, as a function of their mass and DM fraction, are also considered. For heavy axions with ma≳10(-22)  eV we find Ωa/Ωd≲10(-3), with stronger constraints on heavier axions. Lighter axions, however, are allowed and (inflationary) model-independent constraints from the CMB temperature power spectrum and large scale structure are stronger than those implied by tensor modes.

Detecting domain walls of axionlike models using terrestrial experiments

Stable topological defects of light (pseudo)scalar fields can contribute to the Universe's dark energy and dark matter. Currently, the combination of gravitational and cosmological constraints provides the best limits on such a possibility. We take an example of domain walls generated by an axionlike field with a coupling to the spins of standard-model particles and show that, if the galactic environment contains a network of such walls, terrestrial experiments aimed at the detection of wall-crossing events are realistic. In particular, a geographically separated but time-synchronized network of sensitive atomic magnetometers can detect a wall crossing and probe a range of model parameters currently unconstrained by astrophysical observations and gravitational experiments.