Einstein's Theory of Gravity and the Problem of Missing Mass

TL;DR

This paper discusses modifications to Einstein's gravity theory as an alternative to dark matter, highlighting recent theoretical extensions and observational tests that could differentiate between these approaches.

Contribution

It reviews recent extensions of Einstein's theory that include dark fields and discusses how observational data can distinguish modified gravity from dark matter.

Findings

Modified gravity theories can explain some galactic phenomena without dark matter.

Dark fields in extended theories complicate calculations and challenge the original MOND premise.

Observational tests may help differentiate between modified gravity and dark matter hypotheses.

Abstract

The observed matter in the universe accounts for just 5 percent of the observed gravity. A possible explanation is that Newton's and Einstein's theories of gravity fail where gravity is either weak or enhanced. The modified theory of Newtonian dynamics (MOND) reproduces, without dark matter, spiral-galaxy orbital motions and the relation between luminosity and rotation in galaxies, although not in clusters. Recent extensions of Einstein's theory are theoretically more complete. They inevitably include dark fields that seed structure growth, and they may explain recent weak lensing data. However, the presence of dark fields reduces calculability and comes at the expense of the original MOND premise -- that the matter we see is the sole source of gravity. Observational tests of the relic radiation, weak lensing, and the growth of structure may distinguish modified gravity from dark matter.