Constraining the Tilted Dipole Cosmology with Primordial Nucleosynthesis and Baryogenesis

Abstract

The tilted dipole cosmology extends the standard model by incorporating a preferred spatial direction and separate bulk velocities for matter and radiation, offering a potential explanation for observed large-scale bulk flows and the CMB dipole. We conduct a comprehensive analysis of this anisotropic framework using Big Bang Nucleosynthesis (BBN) and Gravitational Baryogenesis (GB) to impose stringent constraints on the radiation tilt parameter ${\beta }_r$, which quantifies the magnitude of the radiation bulk flow. By deriving the modified expansion rate $H(T)$ and its impact on light element abundances, we find that the combined $2\sigma$ BBN limits from primordial helium-4 and deuterium are $|{\beta }_r|\lesssim 0.03$ for maximal shear (tightest bound) and $|{\beta }_r|\lesssim 0.2$ for minimal shear (weakest bound). These bounds are consistent with, but tighter than, those inferred from the effective neutrino species count $|\Delta N_{\mathrm{eff}}|\lesssim 0.4$. Importantly, both bounds are upper limits; ${\beta }_r=0$ is always allowed by the BBN constraints. The lithium-7 problem persists, as the ${\beta }_r$ values required to resolve it are excluded by helium-4 data. Furthermore, GB -- operating at decoupling temperatures $T_D\gtrsim 10^{12}$ GeV under the scaling ${\beta }_r\propto T$ -- yields far more severe constraints, limiting $|{\beta }_r^{\mathrm{BBN}}|\lesssim 10^{-8}$ for such high-scale baryogenesis scenarios.

20 pages, 3 figures, 1 table, to appear in CQG