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Register a new userThe Spectrum of the Axion Dark Sector, Cosmological Observable and Black Hole Superradiance Constraints
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Matthew J. Stott
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Consistent frameworks of quantum gravity often predict the existence of large numbers of ultralight pseudoscalar degrees of freedom, forming the phenomenological landscape of the String Axiverse. The complexity of the extra-dimensional compactification manifolds and vacua determine that these fields could possess parameters with cosmologically significant scales, which span many decades. Astrophysical observations of stellar binary and supermassive black hole systems can be used to exclude the existence of certain ultralight massive bosons, via the superradiance phenomenon. In this work it is shown how these measurements can be used to constrain properties of statistical distributions for the masses of multiple bosonic field theories, inspired by axion field alignment models and an explicit realisation of the string axiverse in M-theory. Such a methodology can exclude $N_{rm ax} geq 30$ axion-like fields with a range of mass distribution widths and central values spanning many orders of magnitude, covering axion phenomenologies important to the dark sector and grand unified theories. This is demonstrated for several examples of axions in string theory and M-theory, where the mass distributions in certain cases take universal forms found in random matrix theory.
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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 to the cosmological dark matter and dark energy densities. We compute the background cosmological (quasi-)observables for models with a large number of axion fields, $n_{rm ax}sim mathcal{O}(10-100)$, with the masses and decay constants drawn from statistical distributions. This reduces the number of parameters from $2n_{rm ax}$ to a small number of hyperparameters. We consider a number of distributions, from those motivated purely by statistical considerations, to those where the structure is specified according to a class of M-theory models. Using Bayesian methods we are able to constrain the hyperparameters of the distributions. In some cases the hyperparameters can be related to string theory, e.g. constraining the number ratio of axions to moduli, or the typical decay constant scale needed to provide the correct relic densities. Our methodology incorporates the use of both random matrix theory and Bayesian networks.
Astrophysical observations of spinning BHs, which span $ 5M_odotlesssim M_{rm BH}lesssim 5times 10^8 M_odot$, can be used to exclude the existence of certain massive bosons via the superradiance phenomenon. In this work, we explore for the first time how these measurements can be used to constrain properties of statistical distributions for the masses of multiple bosonic fields. Quite generally, our methodology excludes $N_{rm ax}gtrsim 30$ scalar fields with a range of mass distribution widths and central values spanning many orders of magnitude. We demonstrate this for the specific example of axions in string theory and M-theory, where the mass distributions in certain cases take universal forms. We place upper bounds on $N_{rm ax}$ for certain scenarios of interest realised approximately as mass distributions in M-theory, including the QCD axion, grand unified theories, and fuzzy dark matter.
We consider an interacting field theory model that describes the interaction between dark energy - dark matter interaction. Only for a specific interaction term, this interacting field theory description has an equivalent interacting fluid description. For inverse power law potentials and linear interaction function, we show that the interacting dark sector model is consistent with $textit{four cosmological data sets}$ -- Hubble parameter measurements (Hz), Baryonic Acoustic Oscillation data (BAO), Supernova Type Ia data (SN), and High redshift HII galaxy measurements (HIIG). More specifically, these data sets prefer a negative value of interaction strength in the dark sector and lead to the best-fit value of Hubble constant $H_0 = 69.9^{0.46}_{1.02}$ km s$^{-1}$ Mpc$^{-1}$. Thus, the interacting field theory model $textit{alleviates the Hubble tension}$ between Planck and these four cosmological probes. Having established that this interacting field theory model is consistent with cosmological observations, we obtain quantifying tools to distinguish between the interacting and non-interacting dark sector scenarios. We focus on the variation of the scalar metric perturbed quantities as a function of redshift related to structure formation, weak gravitational lensing, and the integrated Sachs-Wolfe effect. We show that the difference in the evolution becomes significant for $z < 20$, for all length scales, and the difference peaks at smaller redshift values $z < 5$. We then discuss the implications of our results for the upcoming missions.
We explore the model-independent constraints from cosmology on a dark-matter particle with no prominent standard model interactions that interacts and thermalizes with other particles in a hidden sector. Without specifying detailed hidden-sector particle physics, we characterize the relevant physics by the annihilation cross section, mass, and temperature ratio of the hidden to visible sectors. While encompassing the standard cold WIMP scenario, we do not require the freeze-out process to be nonrelativistic. Rather, freeze-out may also occur when dark matter particles are semirelativistic or relativistic. We solve the Boltzmann equation to find the conditions that hidden-sector dark matter accounts for the observed dark-matter density, satisfies the Tremaine-Gunn bound on dark-matter phase space density, and has a free-streaming length consistent with cosmological constraints on the matter power spectrum. We show that for masses <1.5 keV no region of parameter space satisfies all these constraints. This is a gravitationally-mediated lower bound on the dark-matter mass for any model in which the primary component of dark matter once had efficient interactions -- even if it has never been in equilibrium with the standard model.
The process of superradiance can extract angular momentum and energy from astrophysical black holes (BHs) to populate gravitationally-bound states with an exponentially large number of light bosons. We analytically calculate superradiant growth rates for vectors around rotating BHs in the regime where the vector Compton wavelength is much larger than the BH size. Spin-1 bound states have superradiance times as short as a second around stellar BHs, growing up to a thou- sand times faster than their spin-0 counterparts. The fast rates allow us to use measurements of rapidly spinning BHs in X-ray binaries to exclude a wide range of masses for weakly-coupled spin-1 particles, $5 times 10^{-14} - 2 times 10^{-11}$ eV; lighter masses in the range $6 times 10^{-20} - 2 times 10^{-17}$ eV start to be constrained by supermassive BH spin measurements at a lower level of confidence. We also explore routes to detection of new vector particles possible with the advent of gravitational wave (GW) astronomy. The LIGO-Virgo collaboration could discover hints of a new light vector particle in statistical analyses of masses and spins of merging BHs. Vector annihilations source continuous monochromatic gravitational radiation which could be observed by current GW observatories. At design sensitivity, Advanced LIGO may measure up to thousands of annihilation signals from within the Milky Way, while hundreds of BHs born in binary mergers across the observable universe may superradiate vector bound states and become new beacons of monochromatic gravitational waves.
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