during the 1980s and 1990s, observations of cosmic background radiation and Type Ia supernovas in very distant galaxies (which also means the galaxies were formed relatively shortly after the Big Bang) showed cosmologists that not only was the universe expanding, but that the expansion was accelerating! This ran counter to the widely accepted belief that the expansion was slowing down. Some kind of energy source was required to account for this increasingly rapid expansion. As a result, it made sense to reincorporate the cosmologi- cal constant into the relativistic equations of gravity. If these observations are correct, then the biggest blunder of Einstein's life may actually have been that he ended up refuting his own cosmological constant. If Einstein had declared that the cosmological constant was absolutely necessary, he might be even more renowned as a genius. The Mystery of Dark Energy So what's causing the universe to expand faster and faster? It could be that Einstein's idea of the cosmological constant is correct, meaning that some constant repulsive force is accel- erating the expansion of the universe. One explanation for this is that the very vacuum of empty space has some energy that drives the acceleration. Astronomers call this unknown energy dark energy. That energy may well be the cosmological constant (denoted with the Greek letter lambda, L). By observing distant supernovas, astronomers have been able to look back in time and discover that the universe is acting as predicted in the models in Figure 5-7. In each model, the gravity exerted by galaxies slows the universe's expansion early in its life. This happens because the galaxies are close together, so the gravitational pull they exert on each other