Beyond the local Universe: Impacts of scalar field coupled to non-relativistic neutrinos on bulk flow

This study explores the role of neutrinos in the Universe’s expansion history, tracing their transition from a relativistic phase in the early Universe to non-relativistic massive particles at later epochs. Within the framework of neutrino coupling with a scalar field, we examine cosmic evolution from radiation domination to dark energy dominance. By analyzing combined datasets (Pantheon+, Cosmic Microwave Background, Baryon Acoustic Oscillations, Cosmic Chronometers, and CMB lensing), we constrain the total neutrino mass to $\sum m_{\nu }<0.105\mathrm{eV}$ (95% CL). The transition redshifts $z_{\mathrm{nr}}$ range from 76 to 205, marking the onset of matter domination. The coupling parameter is constrained to $\alpha =5.64\pm 1.1$, consistent with growing neutrino quintessence, reinforcing the role of neutrinos despite their small mass. Late-time evolution analyses, comparing scenarios with and without neutrino coupling, reveal that non-relativistic neutrinos contribute to cosmic anisotropy. At low redshifts ($0.001<z<0.1$), the bulk flow direction aligns with the CMB dipole, while at higher redshifts, it correlates with the dark energy dipole. The evolution of neutrino density-to-redshift ratios suggests that a decreasing neutrino density weakens gravitational influence, leading to an increase in bulk velocity within $0.1<z<1$ and a decline within $1<z<1.4$. These findings highlight the role of non-relativistic neutrinos in shaping cosmic anisotropy and dark energy dynamics, offering new perspectives on the Universe’s large-scale evolution.