A search for ultra-light axions using precision cosmological data (1410.2896v2)

Abstract: Ultra-light axions (ULAs) with masses in the range 10^{-33} eV <m \<10^{-20} eV are motivated by string theory and might contribute to either the dark-matter or dark-energy density of the Universe. ULAs could suppress the growth of structure on small scales, or lead to an enhanced integrated Sachs-Wolfe effect on large-scale cosmic microwave-background (CMB) anisotropies. In this work, cosmological observables over the full ULA mass range are computed, and then used to search for evidence of ULAs using CMB data from the Wilkinson Microwave Anisotropy Probe (WMAP), Planck satellite, Atacama Cosmology Telescope, and South Pole Telescope, as well as galaxy clustering data from the WiggleZ galaxy-redshift survey. In the mass range 10^{-32} eV < m \<10^{-25.5} eV, the axion relic-density \Omega_{a} (relative to the total dark-matter relic density \Omega_{d}) must obey the constraints \Omega_{a}/\Omega_{d} < 0.05 and \Omega_{a}h^{2} < 0.006 at 95%-confidence. For m> 10^{-24} eV, ULAs are indistinguishable from standard cold dark matter on the length scales probed, and are thus allowed by these data. For m < 10^{-32} eV, ULAs are allowed to compose a significant fraction of the dark energy.

• The paper demonstrates how precision cosmological data constrain ultralight axion properties by limiting their relic density.
• It employs CMB and galaxy survey data to assess ULAs' impact on small-scale structure and CMB acoustic peak modifications.
• The study uses effective fluid formalism and advanced numerical methods to navigate ULA parameter degeneracies and guide future research.

Overview of the Paper on Ultralight Axions Using Precision Cosmological Data

The paper "A search for ultralight axions using precision cosmological data" by Renée Hlozek, Daniel Grin, David J.E. Marsh, and Pedro G. Ferreira presents a comprehensive investigation into the potential existence and implications of ultralight axions (ULAs) in the Universe. ULAs are hypothetical particles that could significantly contribute to the dark-matter or dark-energy content of the Universe, motivated primarily by string theory. Their masses are expected to be in the extraordinarily light range of $$10^{-33}\,\text{eV} \leq m_{a} \leq 10^{-20}\,\text{eV}$$.

Key Findings and Methods

The authors utilize cosmological observables, primarily those derived from CMB anisotropies and galaxy clustering, to search for evidence of ULAs. They employ data from various sources, including the WMAP, Planck satellite, Atacama Cosmology Telescope, South Pole Telescope, and WiggleZ galaxy-redshift survey. The paper considers the full mass range of ULAs and extends the analysis to evaluate their effects on diverse cosmological observables.

• Cosmological Impacts: ULAs are suggested to influence the small-scale structure formation by suppressing it, alter the integrated Sachs-Wolfe effect on CMB anisotropies, and modify the angular scale of CMB acoustic peaks. The suppression of small-scale structure is akin to the effects seen with neutrinos due to thermal free-streaming; however, ULAs manifest this through the de Broglie wavelength associated with their macroscopic quantum behavior.
• Constraints on Axions: The paper rigorously computes constraints on the axion relic density relative to the total dark matter density. For instance, within the mass range $$10^{-32}\,\text{eV} \leq m_{a} \leq 10^{-25.5}\,\text{eV}$$, the paper finds that the axion relic-density fraction $\Omega_{a}/\Omega_{d}$ must not exceed $0.05$, and $\Omega_{a}h^{2} \leq 0.006$ at 95%-confidence levels. This constraint implies that if ULAs exist within this mass range, their contribution to dark matter must be limited.
• Mass-dependent Degeneracy: The analysis identifies mass-dependent degeneracies between the ULA density parameters and other cosmological parameters, notably with the cold dark matter density. This degeneracy complicates parameter estimation, necessitating advanced statistical techniques such as nested sampling to explore the parameter space effectively.
• Numerical Implementation: The paper introduces the effective fluid formalism for modeling ULAs and implements a modified version of the camb code to compute the cosmological observables. This enables the tracking of ULA behavior from behaving like dark energy at early times to coherent oscillations like cold dark matter at later times.

Implications and Future Directions

The presence of ULAs would provide a novel window into fundamental physics beyond the standard model, especially in connection with string theory and potential new physics related to the dark sector. The suppression of small-scale power associated with ULAs could help reconcile observed discrepancies in small-scale structure formation, such as the missing satellites problem, small-scale halo properties, and others.

Moreover, the stringent constraints obtained for specific mass ranges set significant limits on the parameter space where ULAs could exist, guiding future observational and experimental efforts. The outlined framework and computational tools developed in this paper can be extended to analyze future high-precision data from upcoming cosmological surveys, thereby refining or further constraining the allowable ULA parameter space.

This research represents a critical step towards understanding the potential role of ultralight axions in cosmology and builds a foundation for integrating implications from theoretical physics with observational cosmology. The paper opens avenues for future inquiry into axionic dark matter, its interaction with other cosmological constituents, and its broader implications for the cosmic history and structure formation.