Cosmological Radar Ranging in an Expanding Universe

TL;DR

This paper investigates whether radar ranging in an expanding universe proves space physically expands, concluding that matter influences cosmic motions and the expansion of space is not necessarily a physical phenomenon.

Contribution

It generalizes radar ranging to include matter effects, clarifying debates on the physical reality of space expansion in cosmology.

Findings

In empty universes, results align with special relativity.

Matter presence causes deviations from simple models.

Expansion of space is not definitively proven as physical.

Abstract

While modern cosmology, founded in the language of general relativity, is almost a century old, the meaning of the expansion of space is still being debated. In this paper, the question of radar ranging in an expanding universe is examined, focusing upon light travel times during the ranging; it has recently been claimed that this proves that space physically expands. We generalize the problem into considering the return journey of an accelerating rocketeer, showing that while this agrees with expectations of special relativity for an empty universe, distinct differences occur when the universe contains matter. We conclude that this does not require the expansion of space to be a physical phenomenon, rather that we cannot neglect the influence of matter, seen through the laws of general relativity, when considering motions on cosmic scales.