expansion

[jimbo]

we know that the universe is expanding but has any one actually worked out a speed in which it is doing so?

[antoniseb]

A speed? No. The expansion is such that it has been expanding for about 13.7 billion years. The rate of expansion is normally expressed in Kilometers per second per megaparsec. Currently we are saying it is about 73 km/sec/Mpc. This is not a single speed per se, but it is an expansion rate. Roughly, each year [currently] the universe expands one part in 13.7 billion, or about 80 picometers per meter.

[Ola D.]

Galaxies are moving with high velocities away from each other, hence the universe is expanding.
It was concluded that the more distant galaxies are moving away from us (receding) faster than the closer ones.

Hubble's law ( v = HD ), where the v: velocity, H: Hubbles constant and D: distance, suggests that the recession velocity is directly proportional to the galaxy's distance from us.

Dividing the distance of a galaxy from us by its speed of travel away from us gives an estimate for the time taken for the expansion. But Hubble's constant is itself rather uncertian; its value is 2(+-)1 x 10^-18 s^-1.

[downunder]

Originally posted by antoniseb@Jan 4 2005, 06:51 PM
Roughly, each year [currently] the universe expands one part in 13.7 billion, or about 80 picometers per meter.

Which doesn't mean you're getting taller by this amount every year

The expansion only affects objects that aren't bound to another by any forces. Gravity being the weakest force means that galaxies don't get bigger because all the stars in it are bound by gravity. Clusters of galaxies aren't affected. Possibly even super clusters aren't affected if there's enough gravity between them to overcome the expansion.

Any object (like our bodies) held together by the far stronger EM forces can never take part in the expansion of the universe. A pity because I'd like to be taller

[Svemir]

Shall we talk about Hubble's coefficient instead of Hubble's constant?
Hubble's Law is HD for nearby galaxies,
relativistic formula is required for things further away and
"something completly different" for objects on the edge of the visible Universe.
Antoniseb wisely avoids question of "speed" and calls it "expansion".
Because someone will ask B) did the first photons get reflected from "walls" of the Universe (simply following the shortest path but at the speed of light).
What is difference between speed and expansion?
E.g. can't we say speed of "put some more space between two points" is :
25,36*^-19 m/s?