A Hubble Space Telescope captures the blue glow of young stars within a galaxy: Scientists say the universe is expanding at an accelerating rate, and that they've pinned down the exact figure.

NASA

cientists discovered in 1998 that not only is the universe expanding, but it's expanding at a rate that continues to accelerate. Now, using the brightness of a specific type of star and supernova as a guide, they've been able to work out exactly how much faster that expansion happens as the universe grows. Hint — it's very, very fast indeed. Here, a quick guide:

So, how fast is the universe expanding?
The universe is expanding at 73.8 kilometers per second per 3.26 million light years, give or take 2.4 km, according to a study published in the Astrophysical Journal.

Uh, what?
The expansion of space means that galaxies are speeding away from us. The farther away they get, the faster they move. So, according to this equation, a galaxy 3.26 million light years away — or one megaparsec — is moving away from us at around 73.8 kilometers per second. A galaxy two megaparsecs away would be travelling twice as quickly, and so on. 

What does this tell us?
It's just more proof that the universe is actually expanding, says Phil Plait at Discover. Some theorized that evidence of expansion was merely an illusion caused by our galaxy's location in a giant void. But the precision of this measurement rules out the so-called "void theory." Now we can go back to studying exactly what is making the universe expand.

What do scientists think is expanding the universe?
Dark energy. That's the name scientists have given to the mysterious force that is causing the universe to expand, rather than give in to the gravitational attraction within it. Understanding this "unknown and undetectable repulsive force" is something of a holy grail for astrophysicists, says Alasdair Wilkins at I09.

Sources: Discover, I09, Astrophysical Journal, New Scientist