Corals and Cosmology
Many questions have come in lately. Today Kea's curiosity about coral reefs will be answered. Mahndisa's questions will be next, with both graphics and equations. Since we can't afford a SNAP spacecraft, we must be creative. The coral reefs of Queensland and Hawaii are an indescribable adventure. Snorkelling in the reef is a visit to another world. Among the reef's ancient wisdom are clues to cosmology.
Among the equipment left on the Moon by Apollo astronauts was the Lunar Laser Ranging Experiment (LLRE). This simple passive device uses corner reflectors like those on a bicycle. By bouncing laser beams from Earth, astronomers can measure distances to the Moon with great accuracy. Data from the LLRE has told us that the Moon still has a liquid core, that Newton's G is indeed constant, and provides one more test of General Relativity.
Most important for this narrative, the LLRE tells us that the Moon is slowly drifting away from the Earth. Most of this apparent drift is due to tidal forces. As the Moon creates tides, the tidal bulge outraces the Moon due to Earth's 24-hour rotation. This bulge pulls the Moon forward by a tiny amount, increasing the Moon's orbital velocity. In this way angular momentum is tranferred from Earth to Moon across 384,000 kilometres of Space. This small acceleration is causing the Moon to slowly drift away. After 35 years of lunar ranging, this drift is measured to be 3.84 cm per year.
Geologists and paleontologists can tell more precisely how the Moon's orbit has changed. Coral gains layers on both daily and yearly cycles, dependent upon tides caused by the Moon. By studying fossilised coral, paleontologists can tell the length of Earth's day, therefore how much angular momentum Earth has lost. Growth rings in coral tell the height of lunar tides, indicating how close the Moon was in the past. Earth's record tells us that the Moon's average recession over the last 650 million years is only 2.17 cm per year.
Small discrepancies in orbits can be very significant. Copernican theory finally triumphed over Ptolemy because it could predict planetary orbits more precisely than epicycles. Mercury's orbital ellipse precesses at 5,600 arc seconds per century, yet a change of only 43 arc seconds per century was enough to verify General Relativity. With the additional momentum, the Moon's recession today is no more than about 2.9 cm per year. If the Moon appears to recede 1/3 faster than geology says, it is a serious anomaly.
If the speed of light slows, that would increase the time for light to return from the Moon, making the Moon appear to recede faster. From GM=tc^3, predicted c change per year is 1 in 41 billion. Multiplied by the Moon's distance of 384,402 km, that distance will appear to increase by 0.94 cm per year. Change in c may precisely account for the discrepancy in the Moon's drift. Like Mercury's orbit, the Moon may give us clues about cosmology and the Universe. As c change makes expansion appear to accelerate, it also makes the Moon appear to recede faster.
Above picture is from Australia, below is Hanauma Bay, Oahu