Hubble-Lemaître Flow in the Solar System: A Review

The Hubble-Lemaître law plays a fundamental role in cosmology. It describes the expanding Universe in terms of the velocity-distance relationship of galaxies. This review is focussed on observations, and challenges the long-held assumption that gravitationally bound bodies do not take part in cosmological expansion. Over fifty years ago it was noted that the measured rate of recession of the Moon from the Earth was close to that predicted by the Hubble-Lemaître law. That was, and continues to be, regarded by many as a coincidence. However, in the past few years, considerable further evidence has accumulated to show that many similar ‘coincidences’ occur throughout the Solar System. The conventional explanation for the orbital expansion of planetary moons is framed in terms of tidal effects, but this requires many assumptions about tidal heat flows and dissipation factors. In contrast, Hubble-Lemaître flow provides a far simpler explanation without invoking any other theoretical basis. It is concluded that while tidal effects certainly contribute to orbital evolution, they are far less important than the cosmological contribution. Furthermore, considerable indirect evidence supports the conclusion that cosmological expansion operates locally, for example the Faint Young Sun Paradox and the anomalously large orbit of Neptune.