Lunar Recession Acceleration

The Moon recedes from Earth at a rate of approximately 3.82 cm/yr, a well-measured quantity from lunar laser ranging (LLR) that has been tracked since the Apollo retroreflectors were placed in 1969. This recession is primarily driven by tidal dissipation — the Moon raises tidal bulges on Earth, those bulges lead the Earth-Moon axis due to Earth's rotation, and the resulting gravitational torque transfers angular momentum from Earth's spin to the Moon's orbit. The expected rate from tidal models calibrated to Earth's measured tidal dissipation factor Q is roughly consistent with LLR, but when deep geological records (tidal rhythmites) are used to infer the Earth-Moon distance in the distant past, the implied ancient recession rate is systematically lower than the current rate — a discrepancy sometimes called the "tidal time paradox.

This discrepancy has traditionally been treated as a purely geophysical problem involving changes in Earth's ocean configuration and tidal dissipation over time. However, some researchers have raised the question of whether any component of the observed recession could involve a cosmological or gravitational contribution — for example, a time-varying gravitational constant G, or a coupling to the expanding cosmic background. ΛCDM predicts absolutely no local influence of cosmic expansion on solar system dynamics (the expansion is negligible at scales far below galaxy clusters), so any confirmed cosmological contribution to lunar recession would be a direct falsification of the clean separation between local physics and cosmological expansion that ΛCDM assumes. Current data do not require such a contribution, but the tidal paradox remains incompletely resolved and keeps the question formally open.