Expansion of the universe

[azafran]

I'm trying to get my head around the rate of expansion and limitations of the speed of light?

 

If the expansion of the universe is approx. 71 km/sec / megaparsec, and the distance to the observable universe horizon is 14 gigaparsecs, then the rate of expansion at the furthest observable point works out at about 3 x the speed of light.

 

Isn't this impossible so what am I missing?

[swansont]

The rule about things traveling below c refers to massive objects, and that velocity is a local measurement. Space is not a massive object, and the measurement is not local.

[Strange]

The other point is that the speed of light limit comes from relativity theory and so does the description of expanding space. As it is a single consistent theory, there is obviously no contradiction. (The apparent contradiction is, as swansont says, resolved by the difference between relative motion through (local) space and the change in the metric of pace over cosmological distances.)

[azafran]

I'm still confused. Space contains stuff and this stuff at the horizon point of the observable universe appears to be moving away from us at 3 (ish) x the speed of light. Therefore we shouldn't be able to observe it, should we?

 

In fact our observations would indicate the universe is shrinking because stuff that is expanding away from us approaches then eventually exceeds the speed of light and therefore becomes lost from our point of view.

Edited January 21, 2014 by azafran

[Spyman]

While the expansion of space is constant over distance it is not constant over time, in current models of big bang theory the Hubble constant has been changing over time. Shortly after the big bang event the expansion was very rapid but then gravity slowed it down until the universe became to large and dark energy started to speed up the expansion again.

 

When the expansion is accelerating the radius to the sphere where space is expanding with the speed of light is decreasing, blocking out more and more of the universe outside but when the expansion is slowing down, as it did for billions of years after the BB event, the sphere was growing and light that was close outside, but unable to pass, got engulfed by its increasing radius. On the inside space is expanding slower than the speed of light so light that gets swallowed by a growing Hubble sphere can start to make progress and eventually reach us.

 

There is currently a lot of very old photons inside our huge sphere moving towards us and event though the expansion of the universe is currently accelerating, they will keep coming for billions of years until the view from outside of Milky Way will go dark.

 

"Since the Hubble "constant" is a constant only in space, not in time, the radius of the Hubble sphere may increase or decrease over various time intervals. The subscript '0' indicates the value of the Hubble constant today. Current evidence suggests the expansion of the universe is accelerating (see Accelerating universe), meaning that for any given galaxy, the recession velocity dD/dt is increasing over time as the galaxy moves to greater and greater distances; however, the Hubble parameter is actually thought to be decreasing with time, meaning that if we were to look at some fixed distance D and watch a series of different galaxies pass that distance, later galaxies would pass that distance at a smaller velocity than earlier ones."

http://en.wikipedia.org/wiki/Hubble's_law