The local Hubble flow: a manifestation of dark energy

Abstract

Our local environment at $r<10$ Mpc expands linearly and smoothly, as if ruled by a uniform matter distribution, while observations show the very clumpy local universe. This is a long standing enigma in cosmology. We argue that the recently discovered vacuum or quintessence (dark energy (DE) component with the equation of state $p_Q = w ρ_Q c^2$, $w \in [-1,0)$) from observations of the high-redshift universe may also manifest itself in the properties of the very local Hubble flow. We introduce the concept of the critical distance $r_Q$ where the repulsive force of dark energy starts to dominate over the gravity of a mass concentration. For the Local Group $r_Q$ is about 1.5 Mpc. Intriguingly, at the same distance 1.5 Mpc the linear and very "cold" Hubble flow emerges, with about the global Hubble constant. We also consider the critical epoch $t_Q$, when the DE antigravity began to dominate over the local matter gravity for a galaxy which at the present epoch is in the local DE dominated region. Our main result is that the homogeneous dark energy component, revealed by SNIa observations, resolves the old confrontation between the local Hubble flow and local highly non-uniform, fractal matter distribution. It explains why the Hubble law starts on the outskirts of the Local Group, with the same Hubble constant as globally and with a remarkably small velocity dispersion.