© NASA Goddard Space Flight Center.
This edge-on image of the Milky Way galaxy was taken by the COBE satellite that made the first full-sky map of the cosmic microwave background. Our solar system is located near the edge of the visible portion of the Milky Way. When Einstein was developing his theory of general relativity, there was no evidence of a universe outside the Milky Way. Unaware of the existence of other galaxies and their relative motions, Einstein added a—cosmological constant—term in his equations to ensure that the universe remained the same size throughout time. The cosmological constant prevented the matter in the universe from collapsing in on itself under the influence of gravity. Later on, after it was understood that our universe contains many galaxies that are all moving apart, Einstein removed the cosmological constant from general relativity, calling it his greatest blunder. (Unit: 11)