The Expanding Universe and Cosmology 

For most of human history, people looked at the night sky and saw a Universe that did not change. While there were a few notable objects that varied in position or appearance (planets, comets and the very rare supernova), for the most part the stars appeared fixed against the icy darkness of space. Faced with this enduring observation, many philosophers and scientists assumed that the Universe was static and unchanging with neither a beginning nor an end to Time. In the first 25 years of the 20^th century that assumption was severley challenged. 

One key discovery forced scientists to rethink their ideas about the static Universe. Using the largest telescopes of his day, Edwin Hubble looked farther into space than ever before, finding that all the distant galaxies were receding from each other at fantastic speeds. From these observations Hubble concluded that the distribution of galaxies was not static and unchanging. Instead, the galaxies were all rushing away from each other. It wasn’t hard to imagine that if you ran time backwards, the galaxies - which were now all moving apart - must have started out packed together. The Universe, it seemed, must have had a beginning. 

Hubble also defined a key number now called the "Hubble Constant" (written as H₀) which expressed how fast the Universe was expanding. It was a measure of how the speed of recession of galaxies increased with their distance. According to current best estimates, at this point in cosmic history H₀ = 70 km/s / Mpc. The units are read as "kilometers per second per Mega-parsec". 

Hubble’s discovery became one of the foundations of Cosmology, the branch of astrophysics that seeks to understand the Universe as a whole. In the last 75 years Cosmology has made incredible progress, reaching the point where detailed tests of cosmological models are now possible. In this interactive you will get a chance to learn more about these models. Move to the next section to learn more or go to the "Using the Cosmology Interactive" section for instructions on using this interactive.