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Register a new userAxion-like Dark Matter Constraints from CMB Birefringence
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Axion-like particles are dark matter candidates motivated by the Peccei-Quinn mechanism and also occur in effective field theories where their masses and photon couplings are independent. We estimate the dispersion of circularly polarized photons in a background of oscillating axion-like particles (ALPs) with the standard $g_{agamma},a,F_{mu u}tilde F^{mu u}/4$ coupling to photons. This leads to birefringence or rotation of linear polarization by ALP dark matter. Cosmic microwave background (CMB) birefringence limits $Delta alpha lesssim (1.0)^circ$ enable us to constrain the axion-photon coupling $g_{agamma} lesssim 10^{-17}-10^{-12},{rm GeV}^{-1}$, for ultra-light ALP masses $m_a sim 10^{-27} - 10^{-24}$ eV. This improves upon previous axion-photon coupling limits by up to four orders of magnitude. Future CMB observations could tighten limits by another one to two orders.
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The polarization of Cosmic Microwave Background (CMB) photons is rotated as they pass through (ultralight-) axion string loops. Studying this birefringence can reveal valuable information about the axion-photon coupling and the structure of the string network. We develop an approximate analytic formalism and identify a kernel function that can be used to calculate the two-point correlation function for CMB birefringence induced by an arbitrary axion string network. Using this formalism, we evaluate the birefringence signal for some simple loop distributions (including scaling and network collapse). We find that the angular correlation function has a characteristic angular scale set by $theta_mathrm{min}$, which corresponds to the angular extent of the loops at the time of recombination. This results in a peak in the birefringence power spectrum around $ell_p sim 1/theta_mathrm{min}$. An additional scale, controlled by the axions mass, is introduced if the network collapses before today.
Updated constraints on dark matter cross section and mass are presented combining CMB power spectrum measurements from Planck, WMAP9, ACT, and SPT as well as several low-redshift datasets (BAO, HST, supernovae). For the CMB datasets, we combine WMAP9 temperature and polarization data for l <= 431 with Planck temperature data for 432 < l < 2500, ACT and SPT data for l > 2500, and Planck CMB four-point lensing measurements. We allow for redshift-dependent energy deposition from dark matter annihilation by using a `universal energy absorption curve. We also include an updated treatment of the excitation, heating, and ionization energy fractions, and provide updated deposition efficiency factors (f_eff) for 41 different dark matter models. Assuming perfect energy deposition (f_eff = 1) and a thermal cross section, dark matter masses below 26 GeV are excluded at the 2-sigma level. Assuming a more generic efficiency of f_eff = 0.2, thermal dark matter masses below 5 GeV are disfavored at the 2-sigma level. These limits are a factor of ~2 improvement over those from WMAP9 data alone. These current constraints probe, but do not exclude, dark matter as an explanation for reported anomalous indirect detection observations from AMS-02/PAMELA and the Fermi Gamma-ray Inner Galaxy data. They also probe relevant models that would explain anomalous direct detection events from CDMS, CRESST, CoGeNT, and DAMA, as originating from a generic thermal WIMP. Projected constraints from the full Planck release should improve the current limits by another factor of ~2, but will not definitely probe these signals. The proposed CMB Stage IV experiment will more decisively explore the relevant regions and improve upon the Planck constraints by another factor of ~2.
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.
If the dark matter (DM) were composed of axions, then structure formation in the Universe would be suppressed below the axion Jeans scale. Using an analytic model for the halo mass function of a mixed DM model with axions and cold dark matter, combined with the abundance-matching technique, we construct the UV-luminosity function. Axions suppress high-$z$ galaxy formation and the UV-luminosity function is truncated at a faintest limiting magnitude. From the UV-luminosity function, we predict the reionization history of the universe and find that axion DM causes reionization to occur at lower redshift. We search for evidence of axions using the Hubble Ultra Deep Field UV-luminosity function in the redshift range $z=6$-$10$, and the optical depth to reionization, $tau$, as measured from cosmic microwave background polarization. All probes we consider consistently exclude $m_alesssim 10^{-23}text{ eV}$ from contributing more than half of the DM, with our strongest constraint ruling this model out at more than $8sigma$ significance. In conservative models of reionization a dominant component of DM with $m_a=10^{-22}text{ eV}$ is in $3sigma$ tension with the measured value of $tau$, putting pressure on an axion solution to the cusp-core problem. Tension is reduced to $2sigma$ for the axion contributing only half of the DM. A future measurement of the UV-luminosity function in the range $z=10$-$13$ by JWST would provide further evidence for or against $m_a=10^{-22}text{ eV}$. Probing still higher masses of $m_a=10^{-21}text{ eV}$ will be possible using future measurements of the kinetic Sunyaev-Zeldovich effect by Advanced ACTPol to constrain the time and duration of reionization.
We apply novel, recently developed plasma ray-tracing techniques to model the propagation of radio photons produced by axion dark matter in neutron star magnetospheres and combine this with both archival and new data for the galactic centre magnetar PSR J1745-2900. The emission direction to the observer and the magnetic orientation are not constrained for this object leading to parametric uncertainty. Our analysis reveals that ray-tracing greatly reduces the signal sensitivity to this uncertainty, contrary to previous calculations where there was no emission at all in some directions. Based on a Goldreich-Julian model for the magnetosphere and a Navarro-Frank-White model for axion density in the galactic centre, we obtain the most robust limits on the axion-photon coupling, to date. These are comparable to those from the CAST solar axion experiment in the mass range $sim 4.2-60,mu{rm eV}$. If the dark matter density is larger, as might predicted by a spike model, the limits could be much stronger. The dark matter density in the region of the galactic centre is now the biggest uncertainty in these calculations.
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