Effect of the Cosmological Constant on Halo Size

TL;DR

This paper explores how the cosmological constant influences galactic halo sizes using a relativistic model, deriving a scale that matches observed halo sizes and extends to larger cosmic structures.

Contribution

It introduces a simple geometric scale for halo sizes based on the Schwarzschild-de Sitter metric, aligning well with observed galactic and larger structure sizes.

Findings

Derived a maximum orbit size scale $r_\Lambda$ matching galactic halos.

Showed the scale applies to clusters and superclusters.

Compared with detailed dynamical models, finding agreement within a factor of one.

Abstract

In this work, we consider the effect of the cosmological constant on galactic halo size. As a model, we study the general relativistic derivation of orbits in the Schwarzschild-de Sitter metric. We find that there exists a length scale $r_\Lambda$ corresponding to a maximum size of a circular orbit of a test mass in a gravitationally bound system, which is the geometric mean of the cosmological horizon size squared, and the Schwarzschild radius. This agrees well with the size of a galactic halo when the effects of dark matter are included. The size of larger structures such as galactic clusters and superclusters are also well-approximated by this scale. This model provides a simplified approach to computing the size of such structures without the usual detailed dynamical models. Some of the more detailed approaches that appear in the literature are reviewed, and we find the length scales agree to within a factor of order one. Finally, we note the length scale associated with the effects of MOND or Verlinde's emergent gravity, which offer explanations of the flattening of galaxy rotation curves without invoking dark matter, may be expressed as the geometric mean of the cosmological horizon size and the Schwarzschild radius, which is typically 100 times smaller than $r_\Lambda$.