Inverse Gertsenshtein effect as a probe of high-frequency gravitational waves

Abstract

We apply the inverse Gertsenshtein effect, i.e., the graviton-photon conversion in the presence of a magnetic field, to constrain high-frequency gravitational waves (HFGWs).Using existing astrophysical measurements, we compute upper limits on the GW energy densities ΩGW at 16 different frequency bands.Given the observed magnetisation of galaxy clusters with field strength B ∼ μG correlated on 𝒪(10) kpc scales, we estimate HFGW constraints in the 𝒪(102) GHz regime to be ΩGW ≲ 1016 with the temperature measurements of the Atacama Cosmology Telescope (ACT).Similarly, we conservatively obtain ΩGW ≲ 1013 (1011) in the 𝒪(102) MHz (𝒪(10) GHz) regime by assuming uniform magnetic field with strength B ∼ 0.1 nG and saturating the excess signal over the Cosmic Microwave Background (CMB) reported by radio telescopes such as the Experiment to Detect the Global EoR Signature (EDGES), LOw Frequency ARray (LOFAR), and Murchison Widefield Array (MWA), and the balloon-borne second generation Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE2)with graviton-induced photons. The upcoming Square Kilometer Array (SKA) can tighten these constraints by roughly 10 orders of magnitude, which will be a step closer to reaching the critical value of ΩGW = 1 or the Big Bang Nucleosynthesis (BBN) bound of ΩGW ≃ 1.2 × 10-6.We point to future improvement of the SKA forecast and estimate that proposed CMB measurement at the level of 𝒪(100-2) nK, such as Primordial Inflation Explorer (PIXIE) and Voyage 2050, are needed to viably detect stochastic backgrounds of HFGWs.