Value of Hubble Constant, tricky/contradictory information?

KennyB is 100% correct.

> "I know it can expand faster than light, but once you get to that distance isn't it impossible for photons to reach us because the Universe is expanding too fast?"

This is true. The "edge" of the observable universe is the point at which no photons will ever reach us because they are too far and cannot ever reach us, even if they are travelling at the speed of light. This point is known as the Particle Horizon.

http://en.wikipedia.org/wiki/Observable_universe#P...

> "They say that the Hubble constant is 70.4 (km/s)/Mpc."

This is pretty much correct. I believe the most authoritative source on the value of the Hubble Constant is the WMAP satellite. They have made it a goal over the past 7 years to nail down an accurate value of the Hubble Constant (amongst other values) and they suggest it is equal to 69.32 +/- 0.80 km/s/Mpc. You will generally find sources quoting values in the range of 68 to 76 km/s/Mpc. Anything outside this range, I think, is outdated and incorrect.

> "By doing some algebra and using the speed of light you should see that the edge of the Observable Universe is expanding at the speed of light, right?"

Actually no. If you do out the math - and the math is actually kind of complicated - you find that the edge of the observable universe is expanding away at three times the speed of light.

> "c /(Hubb_cnst) = Size of Universe

c/H = 14,000 megaparsecs

Solve for H and you get that it is 21.4 km/s/Mpc BUT on the same page it says H =70Km/s/Mpc"

This actually doesn't work, for a variety of reasons. First off, c / H does not give you the size of the universe. Second, the edge of the universe is traveling faster than c, as I mentioned above. Third, H is not constant. I know that may sound wrong, but the term "Hubble's Constant" is somewhat of a misnomer. The constant is actually a function of time. And because looking farther into space is equivalent to looking back in time, effectively Hubble's Constant is a function of distance from us as well. That means you need a function that tells you how Hubble's Constant changes over time and space. It is really only constant for the local region of our universe. You cannot apply it any farther than about the Virgo cluster.

If you really want to check up on the math, get a cosmology book and study the Friedmann Equation.

The 14 Gpc number is the "comoving proper distance" from here to the edge of the observable universe. This is the distance you would observe yourself to cover if you maintained an acceleration that kept you slightly ahead of the Hubble flow, and always measured the distance traveled relative to local objects in small time intervals. Because of relativistic length contraction, this number will be greater that what an observer at your starting point would measure.

There isn't much I can add to permeative pedagogy's explanation. I think the main confusion was that the hubble sphere isn't a horizon, but the particle horizon is.

The latest value of the Hubble constant I know of is 74.3km/s/Mpc, which is from the Spitzer telescope.

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Hubble's constant is not pinned down yet to better than about 3.5%. You're going to get differing answers if you're using one method value for this and another value method for that. Mixing and matching makes for huge differences. Blessed be the error bars.

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