Photon-to-graviton conversion in blazar jets dominates a new graviton background that sets a floor for high-frequency GW detectors, analogous to the neutrino floor.
Stringent Limit on Primordial Magnetic Fields from the Cosmic Microwave Background Radiation
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abstract
Primordial magnetic fields (PMFs), being present before the epoch of cosmic recombination, induce small-scale baryonic density fluctuations. These inhomogeneities lead to an inhomogeneous recombination process that alters the peaks and heights of the large-scale anisotropies of the cosmic microwave backround (CMB) radiation. Utilizing numerical compressible MHD calculations and a Monte Carlo Markov chain analysis, which compares calculated CMB anisotropies with those observed by the \textit{WMAP} and \textit{Planck} satellites, we derive limits on the magnitude of putative PMFs. We find that the {\it total} remaining present day field, integrated over all scales, cannot exceed 47 pG for scale-invariant PMFs and 8.9 pG for PMFs with a violet Batchelor spectrum at 95\% confidence level. These limits are more than one order of magnitude more stringent than any prior stated limits on PMFs from the CMB which have not accounted for this effect.
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Maximally helical primordial U(1)_Y magnetic fields can generate both intergalactic magnetic fields and baryon asymmetry; non-helical fields may work if Higgs dynamics compensate helicity loss to ≲10^{-9-10} precision during electroweak crossover.
Primordial magnetic fields enhance low-mass halo abundance at Cosmic Dawn, advancing Lyman-alpha coupling, X-ray heating and reionization and imprinting detectable signatures on global and fluctuating 21-cm signals observable by HERA and SKA.
ALP-assisted first-order phase transitions can explain observed intergalactic magnetic fields and produce detectable gravitational waves, linking cosmology with particle physics searches.
A review summarizing the Hubble tension as a persistent crisis and discussing resolutions via interacting dark energy models that combine early-time and late-time modifications.
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Graviton Floor
Photon-to-graviton conversion in blazar jets dominates a new graviton background that sets a floor for high-frequency GW detectors, analogous to the neutrino floor.