# Expansion of space: Earth - Moon

Michel_vdg
Hi,

I made a (rough) calculation of how far the Moon would be moving away from Earth according to Hubble's law = 72.6 (km/s)/Mpc

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1 megaParsec = 3.08 × 1022 m ≈ 4 × 1022 m

Distance between Moon and Earth ≈ 4 x 108 m

Expansion rate ≈ 7.2 104 (m/s)/4 × 1022 m = 7.2 (m/s)/4 × 1018 m

Thus expansion for Earth - Moon ≈ 7.2 x 10-10 m/s

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Is this calculation correct? If so, than that it would be about one picometer per second, or is Earth's gravity force simply keeping the Moon in it place?

Thanks,

m.

Homework Helper
Hubble's law applies to intergalactic distances. It does not apply to stars within our own galaxy, much less distance between planets in a single solar system not distance between a planet and it moons.

Michel_vdg
ok, thanks.

Than how can we calculate the expansion of the Universe on a local scale, or is there no expansion locally because gravity keeps it all together?

Gold Member
Correct. Gravitational bound objects are essentially immune to the effects of expansion. The force of expansion is overwhelmed by gravity over 'short' distances. By 'short' distances we are talking the level of galactic clusters.

Nik_2213
FWIW, the Moon *is* receding, but due tidal friction, currently ~ 38 mm / yr.
http://en.wikipedia.org/wiki/Orbit_of_the_Moon

IIRC, this is measured by laser ranging using the retro-reflector panels positioned by Apollo astronauts or mounted on Russian rover.

Lsos
Why do I keep hearing that the expansion of space will eventually rip even atoms apart, if it's too weak to affect anything below intergalactic distances?

Gold Member
2021 Award
Why do I keep hearing that the expansion of space will eventually rip even atoms apart, if it's too weak to affect anything below intergalactic distances?

The so-called "Big Rip" scenario that you describe is hypothetical and not widely believed to be likely. There is no evidence for it, just speculation.

I SEEM to recall (sorry I can't give a link, so this is hearsay) that it has even been disproven, but that may be a stretch. Call it unlikely.

Staff Emeritus
Gold Member
The influence of the cosmological expansion on local systems
F. I. Cooperstock, V. Faraoni, D. N. Vollick
http://arxiv.org/abs/astro-ph/9803097v1

They calculate this kind of thing. The effect is extremely small. The predicted general-relativistic effect on the radius of the Earth's orbit since the time of the dinosaurs is calculated to be about as big as the diameter of an atomic nucleus.

Michel_vdg
The so-called "Big Rip" scenario that you describe is hypothetical and not widely believed to be likely. There is no evidence for it, just speculation.

I SEEM to recall (sorry I can't give a link, so this is hearsay) that it has even been disproven, but that may be a stretch. Call it unlikely.

Here's an interesting blogpost by a theoretical astrophysicist on this topic, and somewhere on it there is also a link to that paper:

http://astrokatie.blogspot.co.uk/2012/07/you-dont-have-to-blow-up-universe-to-be.html

Michel_vdg
The influence of the cosmological expansion on local systems
F. I. Cooperstock, V. Faraoni, D. N. Vollick
http://arxiv.org/abs/astro-ph/9803097v1

They calculate this kind of thing. The effect is extremely small. The predicted general-relativistic effect on the radius of the Earth's orbit since the time of the dinosaurs is calculated to be about as big as the diameter of an atomic nucleus.

Thanks, this was the kind of information I was curious about.