Do Direct Detection Experiments Constrain Axionlike Particles Coupled to Electrons?

Several laboratory experiments have published limits on axionlike particles (ALPs) with feeble couplings to electrons and masses in the kilo-electron-volt to mega-electron-volt range, under the assumption that such ALPs comprise the dark matter. We note that ALPs decay radiatively into photons, and show that for a large subset of the parameter space ostensibly probed by these experiments, the lifetime of the ALPs is shorter than the age of the Universe. Such ALPs cannot consistently make up the dark matter, which significantly affects the interpretation of published limits from GERDA, Edelweiss-III, SuperCDMS, and Majorana. Moreover, constraints from x-ray and γ-ray astronomy exclude a wide range of the ALP-electron coupling, and supersede all current laboratory limits on dark matter ALPs in the 6 keV to 1 MeV mass range. These conclusions are rather model independent, and can only be avoided at the expense of significant fine-tuning in theories where the ALP has additional couplings to other particles.

Exacerbating the Cosmological Constant Problem with Interacting Dark Energy Models.

Future cosmological surveys will probe the expansion history of the Universe and constrain phenomenological models of dark energy. Such models do not address the fine-tuning problem of the vacuum energy, i.e., the cosmological constant problem (CCP), but can make it spectacularly worse. We show that this is the case for "interacting dark energy" models in which the masses of the dark matter states depend on the dark energy sector. If realized in nature, these models have far-reaching implications for proposed solutions to the CCP that require the number of vacua to exceed the fine-tuning of the vacuum energy density. We show that current estimates of the number of flux vacua in string theory, N_{vac}∼O(10^{272 000}), are far too small to realize certain simple models of interacting dark energy and solve the cosmological constant problem anthropically. These models admit distinctive observational signatures that can be targeted by future gamma-ray observatories, hence making it possible to observationally rule out the anthropic solution to the cosmological constant problem in theories with a finite number of vacua.

Simple Emergent Power Spectra from Complex Inflationary Physics.

We construct ensembles of random scalar potentials for N_{f}-interacting scalar fields using nonequilibrium random matrix theory, and use these to study the generation of observables during small-field inflation. For N_{f}=O(few), these heavily featured scalar potentials give rise to power spectra that are highly nonlinear, at odds with observations. For N_{f}≫1, the superhorizon evolution of the perturbations is generically substantial, yet the power spectra simplify considerably and become more predictive, with most realizations being well approximated by a linear power spectrum. This provides proof of principle that complex inflationary physics can give rise to simple emergent power spectra. We explain how these results can be understood in terms of large N_{f} universality of random matrix theory.

Excess astrophysical photons from a 0.1-1 keV cosmic axion background.

Primordial decays of string theory moduli at z~10(12) naturally generate a dark radiation cosmic axion background with 0.1-1 keV energies. This cosmic axion background can be detected through axion-photon conversion in astrophysical magnetic fields to give quasithermal excesses in the extreme ultraviolet and soft x-ray bands. Substantial and observable luminosities may be generated even for axion-photon couplings <<10(-11) GeV(-1). We propose that axion-photon conversion may explain the observed excess emission of soft x rays from galaxy clusters, and may also contribute to the diffuse unresolved cosmic x-ray background. We list a number of correlated predictions of the scenario.

Studies of the rhizoplast from Naegleria gruberi.

A procedure utilizing homogenization and centrifugation in a low ionic strength buffer containing Triton X-100, has been used to facilitate the isolation of the rhizoplast from flagellates of Naegleria gruberi. This has enabled a study to be made of the physical and biochemical properties of this organelle. The rhizoplast is shown to be a proteinaceous structure with chemical properties similar to those of the molluscan gill ciliary rootlet. Polyacrylamide gel electrophoresis gives a possible subunit molecular weight of approximately 240 000 Daltons. Studies with antisera raised against the rhizoplast fraction demonstrated the absence of rhizoplast antigens in amoeboid forms of Naegleria gruberi and is taken as evidence that the organelle is synthesized de novo during transformation of the amoeba to the flagellate form. Results of optical diffraction studies on isolated rhizoplasts are also presented.