Cosmology & the Expanding Universe
Einstein asked the question, what curved spacetime describes our Universe on very large scales? Making the approximation that matter is spread out uniformly, he found that our Universe cannot be static. It must be either expanding, or contracting.
This seemed horribly wrong to Einstein. He found a way to fix it by adding a new element to his theory - a cosmological constant - a background energy density with negative pressure. This allowed a static (but unstable) Universe.
In 1929, the astronomer Edwin Hubble discovered that, in fact, the universe is expanding. Einstein famously described the cosmological constant as his “greatest blunder”.
COSMOS to HUMANITY, Spring 2008
Lecture 2 , slide 10