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Register a new userCosmic Birefringence and Electroweak Axion Dark Energy
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Taking the recently reported non-zero rotation angle of the cosmic microwave background (CMB) linear polarization $beta=0.35pm0.14{rm, deg}$ as the hint for a pseudo Nambu-Goldstone boson quintessence dark energy (DE), we study the electroweak (EW) axion quintessence DE model where the axion mass is generated by the EW instantons. We find that the observed value of $beta$ implies a non-trivial $U(1)$ electromagnetic anomaly coefficient ($c_{gamma}$), once the current constraint on the DE equation of state is also taken into account. With the aid of the hypothetical high energy structure of the model inspired by the experimentally inferred $c_{gamma}$, the model is shown to be able to make prediction for the current equation of state ($w_{rm DE,0}$) of the quintessence DE. This is expected to make our scenario distinguishable in comparison with the cosmological constant ($w=-1$) and testable in future when the error in the future measurement of $w_{rm DE,0}$ is reduced to $mathcal{O}(1)%$ level ($delta w=mathcal{O}(10^{-2})$).
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We study the axion strings with the electroweak gauge flux in the DFSZ axion model and show that these strings, called the electroweak axion strings, can exhibit superconductivity without fermionic zero modes. We construct three types of electroweak axion string solutions. Among them, the string with $W$-flux can be lightest in some parameter space, which leads to a stable superconducting cosmic string. We also show that a large electric current can flow along the string due to the Peccei-Quinn scale much higher than the electroweak scale. This large current induces a net attractive force between the axion strings with the same topological charge, which opens a novel possibility that the axion strings form Y-junctions in the early universe.
We investigate the possibility that axion-like particles (ALPs) with various potentials account for the isotropic birefringence recently reported by analyzing the Planck 2018 polarization data. For the quadratic and cosine potentials, we obtain lower bounds on the mass, coupling constant to photon $g$, abundance and equation of state of the ALP to produce the observed birefringence. Especially when the ALP is responsible for dark energy, it is possible to probe the tiny deviation of dark energy equation of state from $-1$ through the cosmic birefringence. We also explore ALPs working as early dark energy (EDE), which alleviates the Hubble tension problem. Since the other parameters are limited by the EDE requirements, we narrow down the ALP-photon coupling to $10^{-19}, {rm GeV}^{-1}lesssim glesssim 10^{-16}, {rm GeV}^{-1}$ for the decay constant $f=M_mathrm{pl}$. Therefore, the Hubble tension and the isotropic birefringence imply that $g$ is typically the order of $f^{-1}$, which is a non-trivial coincidence.
ALP domain walls without strings may be formed in the early Universe. We point out that such ALP domain walls lead to both isotropic and anisotropic birefringence of cosmic microwave background (CMB) polarization, which reflects spatial configuration of the domain walls at the recombination. The polarization plane of the CMB photon coming from each domain is either not rotated at all or rotated by a fixed angle. For domain walls following the scaling solution, the cosmic birefringence of CMB is characterized by $2^{N}$, i.e. $N$-bit, of information with $N = {cal O}(10^{3-4})$ being equal to the number of domains at the last scattering surface, and thus the name, $kilobyte~ cosmic~ birefringence$. The magnitude of the isotropic birefringence is consistent with the recently reported value, while the anisotropic one is determined by the structure of domains at the last scattering surface. The predicted cosmic birefringence is universal over a wide range of the ALP mass and coupling to photons. The detection of both signals will be a smoking-gun evidence for the ALP domain walls without strings.
Existing searches for cosmic axions relics have relied heavily on the axion being non-relativistic and making up dark matter. However, light axions can be copiously produced in the early Universe and remain relativistic today, thereby constituting a Cosmic $textit{axion}$ Background (C$a$B). As prototypical examples of axion sources, we consider thermal production, dark-matter decay, parametric resonance, and topological defect decay. Each of these has a characteristic frequency spectrum that can be searched for in axion direct detection experiments. We focus on the axion-photon coupling and study the sensitivity of current and futu
The Peccei-Quinn mechanism presents a neat solution to the strong CP problem. As a by-product, it provides an ideal dark matter candidate, the axion, albeit with a tiny mass. Axions therefore can act as dark radiation if excited with large momenta after the end of inflation. Nevertheless, the recent measurement of relativistic degrees of freedom from cosmic microwave background radiation strictly constrains the abundance of such extra relativistic species. We show that ultra-relativistic axions can be abundantly produced if the Peccei-Quinn field was initially displaced from the minimum of the potential. This in lieu places an interesting constraint on the axion dark matter window with large decay constant which is expected to be probed by future experiments. Moreover, an upper bound on the reheating temperature can be placed, which further constrains the thermal history of our Universe.
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