Spitzer seen against the infrared sky. The band of light is the glowing dust emission
from the Milky Way galaxy seen at 100 microns (as seen by the IRAS/COBE missions).

Astronomers using the Spitzer infrared space telescope have managed to refine current estimates of the Hubble constant, a measure of the rate at which the universe is expanding, down to an error of only 3%. The new value is 74.3 ± 2.1 kilometers per second per megaparsec. This means that the average speed at which objects recede from each other in the universe increases by 74.3 kilometer/second for every 3.3 million light years that they are apart.
Launched in August 2003, the Spitzer telescope wields an 85cm mirror made from Beryllium. Because it’s designed to see in infrared wavelengths, the optical assembly is chilled to only a few degrees above absolute zero, so that it wont be blinded by its own thermal radiation. Although it was only designed to last for a 2-5 years, it continues to operate and return good science data after almost 10 years in space. 
The Hubble Constant is named for Edwin Hubble, who first showed that the universe is expanding. The famous Hubble Space Telescope (HST) is named for him, and was instrumental in showing that the universe’s expansion is not simply the direct result of the big bang, but that it is in fact accelerating with time due to a still mysterious force called Dark Energy. The HST was also responsible for the previous best estimate of the Hubble Constant, although Spitzer’s new results are more accurate by a factor of 3.
Although Spitzer is much smaller than the HST, it’s infrared sensors allow it to see through dust clouds and nebulae that block our view of distant stars in visible light. This means that Spitzer was able to detect Cepheid variable stars in our own galaxy as well as the neighbouring dwarf galaxy which is visible to Southern Hemisphere astronomers as the Large Magellanic Cloud. Cepheids have a very well understood relationship between the period of their variability and their brightness, meaning that you can very accurately determine their intrinsic brightness by simply measuring how long they take to go through a cycle of dimming and brightening. Once we know the actual brightness, we can compare it to the apparent brightness to calculate how far away it is, and these distances are used to calibrate other methods used to measure the distances of larger, further objects like galaxies. It is these distances combined with speed measurements that are used to then calculate the rate of expansion of the universe.  Finally, if we know how far apart galaxies are, and how fast they’re moving, we can run a simulation backwards to see when the distances were zero, which reveals the date of the Big Bang and the age of the universe.
These findings were combined with published data from NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) to obtain an independent measurement of dark energy, one of the greatest mysteries of our cosmos. Dark energy is thought to be winning a battle against gravity, pulling the fabric of the universe apart. Research based on this acceleration garnered researchers the 2011 Nobel Prize in physics. 
“This is a huge puzzle,” said study lead author Wendy Freedman of the Observatories of the Carnegie Institution for Science in Pasadena. “It’s exciting that we were able to use Spitzer to tackle fundamental problems in cosmology: the precise rate at which the universe is expanding at the current time, as well as measuring the amount of dark energy in the universe from another angle.” Freedman led the ground-breaking Hubble Space Telescope study that earlier had measured the Hubble constant.