In the late 1990s, two separate teams of astrophysicists examined very rare and bright stars called supernovas and then made the same startling announcement. Each team of scientists reported that the universe was expanding at an accelerating rate. Since then other scientists have gathered additional observational evidence to support the idea of cosmic acceleration. Pressed to explain what was going on and what could possibly be causing the universe to expand at a faster rate, cosmologist are now cautiously revisiting Einstein's cosmological constant and embracing a refashioned version of a gravitationally repulsive term—one that has its origins in quantum vacuum energy. Modern astronomical observations are also having profound implications for particle physics and could help scientists understand the fundamental forces of nature, especially the relation between gravity and quantum mechanics.
Today the main attraction of a cosmological constant term is that it significantly improves the agreement between theory and observation. The most interesting example of this is the recent effort to measure how much the expansion of the universe has changed in the last few billion years. Within the framework of contemporary big bang cosmology, scientists previously postulated that the gravitational pull exerted by matter in the universe slows the expansion. Now, if the universe is indeed expanding at an accelerating rate, they need to figure out what is causing this to occur. Cosmologists must now seriously consider that the universe may contain some bizarre form of matter or energy that is, in effect, gravitationally repulsive. The concept of a cosmological constant based on quantum vacuum energy is one candidate. But much work lies ahead before big bang cosmologists can be comfortable with suggesting the ultimate destiny of the expanding universe.
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