In 1998, two teams of astronomers independently reported amazing and bizarre news: the Universal expansion known for decades was not slowing down as expected, but was speeding up. Something was accelerating the Universe.
Since then, the existence of this something was fiercely debated, but time after time it fought with and overcame objections. Almost all professional astronomers now accept it's real, but we still don't know what the heck is causing it. So scientists keep going back to the telescopes and try to figure it out.
[Click to galactinate, or grab the cosmic 3500 x 4000 pixel browser bruiser.]
This gorgeous image is of the nearby spiral galaxy NGC 5584, where of course "nearby" to an astronomer means 72 million light years. This galaxy is loaded with a specific type of variable star -- called Cepheids -- which are very important: the way they change their brightness depends on how luminous they are. Measure the change, and you measure the luminosity, and if you measure how bright they appear in the sky you get their distance. It's a bit like judging how far away a car is by gauging how bright its headlights are. Except in this case astronomers use Hubble instead of their eyes. It's a tad more accurate.
It so happens that in 2007, NGC 5584 was the host of a Type Ia supernova, the Golden Standard of distance indicators. These are so bright they can be seen clear across the Universe! By knowing the distance to the one in NGC 5584, we can then use that to get the distances to supernovae much, much farther away.
It's a bootstrappy way of measuring the cosmic distance scale.
But it appears to work. By measuring the Cepheids in eight galaxies that also hosted Type 1a supernovae, astronomers (led by my old pal Adam Riess; we were grad students together when he cracked the Type 1a code that led to the discovery of the universal acceleration -- which will win him the Nobel one day, I'd wager) were able to hone the distance bootstrap even better.
And now, by measuring first the Cepheids and then using them to determine the distance to far-flung supernovae, they have nailed down just how fast the Universe is expanding: 73.8 (plus or minus 2.4) km/sec/megaparsec.
OK, so what does that mean?
We see galaxies rushing away from us. Moreover, the farther away they are, the faster they appear to be moving. The rate of that expansion is what was measured. If you find a galaxy 1 megaparsec away (about 3.26 million light years), the expansion of space would carry it along at 73.8 km/sec (fast enough to cross the United States in about one minute!). A galaxy 2 megaparsecs away would be traveling away at 147.6 km/sec, and so on*.
The last time this was measured accurately, the speed was seen to be 74.2 +/- 3.6 km/sec/mpc. Note the uncertainty; both the old and new measurements overlap to within their uncertainty (which is good!), but the new one has a smaller uncertainty. In other words it's most likely more accurate.
By knowing this number so well, it allows better understanding of how the Universe is behaving. It also means astronomers can study just how much the Universe deviates from this constant rate at large distances due to the acceleration. And that in turn allows us to throw out some ideas for what dark energy is, and entertain notions of what it might be. For example, one idea was that instead of dark energy accelerating the Universe, it was an illusion due to our region in the local Universe being unusually underdense -- that would make it look like distant galaxies are receding faster than expected. However, the precision of the new measurement rules this out; the value needed by the void model would have to be much lower. So right away we know that this explanation doesn't work, and the idea of dark energy survives another battle.
So that number of 73.8 +/- 2.4 km/sec/mpc may sound arcane and weird to you, but it is in fact the key to understanding the Universe itself. It's amazing that so much can ride on one number... but that's the Universe for you.
* In reality it's a little more complicated than that. Due to gravity of galaxies and clusters, the rate of expansion doesn't really kick in until you get out to a distance of a handful of megaparsecs. But the concept is the same.