The Hubble Constant is the unit of measurement used to describe the expansion of the universe. The cosmos has been getting bigger since the Big Bang kick-started the growth about 13.82 billion years ago. The universe, in fact, is getting faster in its acceleration as it gets bigger.
What's interesting about the expansion is not only the rate, but also the implications, NASA explains on its website. If the expansion begins to slow down, that implies that there is something in the universe that is making the growth slow down — perhaps dark matter, which can't be sensed with conventional instruments. If the growth gets faster, though, it's possible that dark energy is pushing the expansion faster.
As of March 2013, NASA estimates the rate of expansion is about 70.4 kilometers per second per megaparsec. A megaparsec is a million parsecs, or about 3.3 million light-years, so this is almost unimaginably fast. Using data solely from NASA's Wilkinson Microwave Anisotropy Probe (WMAP), the rate is slightly faster, at about 71 km/s per megaparsec.
Discovery by Hubble
The constant was first proposed by Edwin Hubble (whose name is also used for the Hubble Space Telescope). Hubble was an American astronomer who studied galaxies, particularly those that are far away from us.
In 1929 — based on a realization from astronomer Harlow Shapley that galaxies appear to be moving away from the Milky Way — Hubble found that the farther these galaxies are from Earth, the faster they appear to be moving, according to NASA.
While scientists then understood the phenomenon to be galaxies moving away from each other, today astronomers know that what is actually being observed is the expansion of the universe. No matter where you are located in the cosmos, you would see the same phenomenon happening at the same speed.
Hubble's initial calculations have been refined over the years, as more and more sensitive telescopes have been used to make the measurements. These include the Hubble Space Telescope (which examined a kind of variable star called Cepheid variables) and WMAP, which extrapolated based on measurements of the cosmic microwave background — a constant background temperature in the universe that is sometimes called the "afterglow" of the Big Bang. [Infographic: Cosmic Microwave Background Explained]
There are many kinds of variable stars, but the one that is most useful for measuring the Hubble constant is called a Cepheid variable. These are stars that regularly change their apparent brightness on a scale that usually ranges between 2 and 100 days, according to NASA. (Polaris is among the most famous members of this group).
There also is a relation between the period of their brightness change, and their actual brightness, which means one can calculate the distance to the star. The brighter the Cepheid appears from Earth, the easier it is to measure it. Some Cepheids can even be seen from the ground, but for more accurate measurements, going into space is the best bet.
While Hubble was able to measure Cepheids about 900,000 light-years away — an astonishing distance for the time — within the universe that is still relatively close to Earth. Farther in space, the Cepheids are fainter and recede more quickly, which is where the Hubble Space Telescope was able to help in the 1990s after its launch.
Space telescope measurements
In 1999, astronomers announced a constant of 70 kilometers per second per megaparsec based on observations of nearly 800 Cepheids in 18 galaxies as far as 65 million light-years away from Earth. This pegged the universe at about 12 billion years old (which has since been refined to 13.82 billion years.) [Related: How Old is the Universe?]
"Before Hubble, astronomers could not decide if the universe was 10 billion or 20 billion years old," stated team leader Wendy Freedman of the Observatories of the Carnegie Institution of Washington in 1999 in the discovery news release. "After all these years, we are finally entering an era of precision cosmology. Now we can more reliably address the broader picture of the universe's origin, evolution and destiny."
Cepheids, however, are not perfect for this purpose. Among other difficulties, they are often located in dusty areas (which obscure some wavelengths in photographs) and the more distant ones are hard to spot because they are so faint from our perspective.
Other techniques arose to supplement Cepheid measurements, such as the Tully-Fisher relation, which describes the ratio of a spiral galaxy's rotational galaxy to its luminosity. WMAP used another technique that examines fluctuations in the cosmic microwave background to determine the constant.