Axions, hypothetical particles ubiquitous in theories of particle physics, can be produced in great profusion in the early Universe and remain relativistic to the present day, forming a relic background - a cosmic axion background (CaB). In the present paper, Jeff A. Dror, Hitoshi Murayama, and Nicolas L. Rodd explore different production mechanisms of relativistic relic axions and the potential of their discovery with extant and future experiments. They show that experiments can have sensitivities relevant to the CaB and, therefore, serve as probes of the history of the Universe.

Through the gauge-gravity duality, certain field theory quantities can provide a microscopic description of the degrees of freedom responsible for the Bekenstein-Hawking entropy of black holes. In this paper, the authors develop a powerful numerical method to study the supersymmetric index on the gauge theory side, and show how certain aspects of a class of (AdS) black holes can be extracted.

One of the serious hindrances to achieving a major goal of the next generation CMB experiments, the measurement of the sum of the neutrino masses, is the lack of sufficient knowledge of baryonic effects on the matter power spectrum. In this paper, the authors propose several mitigating strategies, which, they show, will serve to reduce these baryonic effects to a negligible level, without significantly increasing the associated statistical uncertainty.

The DAMA/LIBRA experiment’s potential dark matter detection went unconfirmed for 20 years. Now, a similar experiment offers evidence against the result.

Synopsis on:
J. Amaré et al.
Phys. Rev. D 103, 102005 (2021)

The authors present the best high-precision measurements of the longitudinal double-spin asymmetry for jet and dijet production in polarized proton collisions, improving on their previous results. The data are sensitive to the gluon helicity distribution and provide new constraints.

The authors introduce a new actor in the current drama of the Hubble tension (conflicting measurements of $H_0$), namely the age of the Universe $t_{\text{U}}$, as determined by the oldest globular clusters, and show that if local measurements are correct, the resolution of the Hubble tension implies a modification of the cosmological standard model $\mathrm{\Lambda }$CDM in the early and late Universe. Once systematic uncertainties are reduced, measuring $t_{\text{U}}$ will discriminate between the viable solutions for the $H_0$ tension, i.e. changing $\mathrm{\Lambda }$CDM or errors in the local measurements.

By employing an effective field theory approach, the authors study novel extensions of Starobinsky inflation, involving higher-order curvature corrections that imply that (fluctuation) equations are of second order in time and lead to reasonable cosmologies. Embedding these theories into anti-de Sitter space, holographic unitarity bounds are derived for the dominant corrections which allow for observational predictions despite the additional parameters, which could be tested by the upcoming generation of CMB experiments.

At the beginning of 2021, eight Physical Review journals began publishing Letters which are intended for the accelerated publication of important new results targeted to the specific readership of each journal.

APS Editor in Chief, Michael Thoennessen, discusses a new opportunity for communicating authors to include their pronouns together with their contact email in order to promote a more respectful, inclusive, and equitable environment.

From nature’s tiniest particles to waves that traverse the Universe—physicists remember resounding finds from the last half-century.

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