String Axiverse (0905.4720v2)

Abstract: String theory suggests the simultaneous presence of many ultralight axions possibly populating each decade of mass down to the Hubble scale 10^-33eV. Conversely the presence of such a plenitude of axions (an "axiverse") would be evidence for string theory, since it arises due to the topological complexity of the extra-dimensional manifold and is ad hoc in a theory with just the four familiar dimensions. We investigate how upcoming astrophysical experiments will explore the existence of such axions over a vast mass range from 10^-33eV to 10^-10eV. Axions with masses between 10^-33eV to 10^-28eV cause a rotation of the CMB polarization that is constant throughout the sky. The predicted rotation angle is of order \alpha~1/137. Axions in the mass range 10^-28eV to 10^-18eV give rise to multiple steps in the matter power spectrum, that will be probed by upcoming galaxy surveys. Axions in the mass range 10^-22eV to 10^-10eV affect the dynamics and gravitational wave emission of rapidly rotating astrophysical black holes through the Penrose superradiance process. When the axion Compton wavelength is of order of the black hole size, the axions develop "superradiant" atomic bound states around the black hole "nucleus". Their occupation number grows exponentially by extracting rotational energy from the ergosphere, culminating in a rotating Bose-Einstein axion condensate emitting gravitational waves. This mechanism creates mass gaps in the spectrum of rapidly rotating black holes that diagnose the presence of axions. The rapidly rotating black hole in the X-ray binary LMC X-1 implies an upper limit on the decay constant of the QCD axion f_a<2*10^17GeV, much below the Planck mass. This reach can be improved down to the grand unification scale f_a<2*10^16GeV, by observing smaller stellar mass black holes.

• The paper demonstrates that ultralight axions spanning 10⁻³³ to 10⁻¹⁰ eV can induce observable signatures in CMB polarization, matter power spectrum, and black hole superradiance.
• It employs string theory compactifications and extra-dimensional models to predict distinct mass-dependent phenomena such as polarization rotation and step-like features in galaxy surveys.
• It underscores the potential for using advanced experiments like Planck, BOSS, and black hole spin analyses to validate the theoretical predictions of the string axiverse.

Exploring the String Axiverse

The concept of the "String Axiverse," closely examined in this paper, emerges from the intricacies of string theory, which suggests the existence of a multitude of ultralight axions. These hypothetical particles span an expansive mass range, potentially providing an observational window into the underpinnings of string theory itself. The axiverse is characterized by axions whose masses extend from as low as $10^{-33}$ eV to $10^{-10}$ eV, distributing across several orders of magnitude on a logarithmic scale.

String theory, through its complex extra-dimensional topologies, naturally predicts a diverse axionic landscape. The primary investigation in this paper concerns how these axions may manifest in various astrophysical observations and experiments. Crucial to this exploration are various mass ranges of axions and the respective phenomena they could affect, such as the rotation of the cosmic microwave background (CMB) polarization, the structure of matter power spectrum, and the dynamics of astrophysical black holes.

Key Observational Windows

1. CMB Polarization Rotation: Axions with masses between $10^{-33}$ eV and $4 \times 10^{-28}$ eV could induce a rotation in the polarization of the CMB due to their coupling with electromagnetic fields. This effect is described as an axion-induced polarization rotation, which is detectable to the order of $\Delta\beta \sim 10^{-3}$, well within the anticipated sensitivity of experiments like the Planck satellite and forthcoming CMB observational missions.
2. Steps in the Matter Power Spectrum: Axions in the mass range of $10^{-28}$ eV to $10^{-18}$ eV have implications for the small scale fluctuations in the matter power spectrum. Axions contribute a mass-dependent characteristic suppression scale and exhibit "step-like" features proportionate to their densities relative to cold dark matter. Approaching these scales with high-precision galaxy surveys like BOSS and 21 cm line tomography offers a promising prospect for detecting these effects in power spectrum variations.
3. Black Hole Dynamics via Superradiance: Axions can influence rotating black holes through the Penrose superradiance process. Black holes in the mass range of $10^7$ solar masses, coupled with axion masses from $10^{-22}$ eV to $10^{-10}$ eV, are particularly vulnerable to this. The interaction sets up a superradiant instability where axions extract energy from the black hole, potentially influencing black hole spin dynamics and leading to observable consequences in black hole mass spectra.

Implications and Future Directions

The paper provides substantial insights into the plausible ties between low-energy phenomenological predictions and high-energy string theory. Observing light axions could yield pivotal evidence for string theory compactifications, further constraining models of the early universe and inflation. The identification of multiple axions through diversified cosmic signals would support the theoretical premise that the real world mirrors the string axiverse's plenitude.

Significantly, axions with properties aligned with QCD axion constraints represent a connection back to established particle physics, situating string axions as both cosmological and particle physics probes. This interconnectivity extends the reach of experimental and observational platforms, signaling a future where theoretical predictions are continually refined by empirical discoveries.

Overall, advancements in this direction would require precise and collaborative effort across cosmology, astrophysics, and experimental particle physics. The potential for a unified modeling paradigm that ties gravitational wave observations, CMB anomalies, galaxy clustering, and ground-based axion searches exemplifies a forward trajectory for the exploration of the string axiverse.