One hundred years ago, it was considered fact that the Universe was completely contained within the Milky Way galaxy; our galaxy. It was not until Edwin Hubble measured the distance to 'spiral nebulae' that it was determined that other galaxies exist beyond our own. Hubble did more than that to reshape the world of cosmology as he also discovered that the Universe is expanding, based on the movement of those galaxies. Now we call the rate of that expansion Hubble's Constant and a team of researchers led by those at the Carnegie Institution have made the most accurate measurement of this constant to date.
Determining the Hubble Constant requires very precise distance measures, which is why the researchers used the Spitzer Space Telescope. This telescope operates in the infrared portion of the spectrum, which is able to pass through stellar gas, unlike visible light. Without the gas to impair the telescope, the researchers determined the brightness and distance to ninety stars that pulse in a predictable manner in our galaxy and the nearby Large Magellanic Cloud. After crunching the numbers the researchers arrived at a value of 74.3 ± 2.1 Km per second per megaparsec (so every second, one megaparsec grows an additional 74.3 Km, roughly). At just 3%, the uncertainty of this measurement is lower than any previous.
Hubble's Constant has a myriad of uses in cosmology as it is used to determine the age and size of the Universe, as well as the effects of dark energy. Dark energy is an unknown force or energy that is causing Hubble's Constant to increase and counteracting gravity's attempts to pull the Universe together.