Relativistic approach to the kinematics of large-scale peculiar motions. (arXiv:1906.05164v3 [astro-ph.CO] UPDATED)

Relativistic approach to the kinematics of large-scale peculiar motions. (arXiv:1906.05164v3 [astro-ph.CO] UPDATED)
<a href="http://arxiv.org/find/astro-ph/1/au:+Tsaprazi_E/0/1/0/all/0/1">Eleni Tsaprazi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tsagas_C/0/1/0/all/0/1">Christos G. Tsagas</a>

We consider the linear kinematics of large-scale peculiar motions in a
perturbed Friedmann universe. In so doing, we take the viewpoint of the “real”
observers that move along with the peculiar flow, relative to the smooth Hubble
expansion. Using relativistic cosmological perturbation theory, we study the
linear evolution of the peculiar velocity field, as well as the
expansion/contraction, the shear and the rotation of the bulk motion. Our
solutions show growth rates considerably stronger than those of the earlier
treatments, which were mostly Newtonian. On scales near and beyond the Hubble
radius, namely at the long-wavelength limit, peculiar velocities are found to
grow as $a^2$, in terms of the scale factor, instead of the Newtonian
$a^{1/2}$-law. We attribute this to the fact that, in general relativity, the
energy flux, triggered here by the peculiar motion of the matter, also
contributes to the local gravitational field. In a sense, the bulk flow
gravitates, an effect that has been bypassed in related relativistic studies.
These stronger growth-rates imply faster peculiar velocities at horizon
crossing and higher residual values for the peculiar-velocity field.
Alternatively, one could say that our study favours bulk peculiar flows larger
and faster than anticipated.

We consider the linear kinematics of large-scale peculiar motions in a
perturbed Friedmann universe. In so doing, we take the viewpoint of the “real”
observers that move along with the peculiar flow, relative to the smooth Hubble
expansion. Using relativistic cosmological perturbation theory, we study the
linear evolution of the peculiar velocity field, as well as the
expansion/contraction, the shear and the rotation of the bulk motion. Our
solutions show growth rates considerably stronger than those of the earlier
treatments, which were mostly Newtonian. On scales near and beyond the Hubble
radius, namely at the long-wavelength limit, peculiar velocities are found to
grow as $a^2$, in terms of the scale factor, instead of the Newtonian
$a^{1/2}$-law. We attribute this to the fact that, in general relativity, the
energy flux, triggered here by the peculiar motion of the matter, also
contributes to the local gravitational field. In a sense, the bulk flow
gravitates, an effect that has been bypassed in related relativistic studies.
These stronger growth-rates imply faster peculiar velocities at horizon
crossing and higher residual values for the peculiar-velocity field.
Alternatively, one could say that our study favours bulk peculiar flows larger
and faster than anticipated.

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