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.