When Albert Einstein applied his theory of general relativity to the universe the paradigm was that the universe was static. Since matter and energy gravitate, they drive the universe to collapse on itself. This was physically unacceptable, so Einstein introduced a cosmological constant term in his equations to balance the attractive force of gravity. It was latter discovered by Edwin Hubble that other galaxies appear to be moving away from us, that the universe was actually expanding. It was these observations that caused Einstein to claim that the inclusion of the cosmological constant was his biggest blunder, and was subsequently dropped from cosmological theories.

There are reasons to believe that the cosmological constant may still be a viable part of cosmology. Today's paradigm is that the universe went through a period of rapid expansion, called inflation, early in its history. This inflation would act to smooth out the universe and make it very geometrically flat. Mathematically, it causes to be very near one today. This causes problems that the cosmological constant may remedy. When astronomers measure the amount of matter and energy in the universe today they only come up with about 30% of what is needed to make the universe flat. The cosmological constant can pick up the slack and make the universe flat. Even if inflation is wrong and there is no reason to believe the universe is spatially flat, there is still an apparent problem with the age of the universe. The age derived for an open universe with matter at the level we observe and with no cosmological constant is younger that the age of the oldest stars, this can not be. However, a flat universe with both matter (at the level observed) and a cosmological constant is a much older universe, and is indeed as old as the oldest stars.

Another reason to expect a cosmological constant is the existence of quantum mechanical vacuum energy. Quantum mechanical theory predicts that the vacuum is not really empty but has some amount of energy associated with it. Since general relativity states that all forms of matter and energy should gravitate, we might associate the cosmological constant with the energy of the vacuum. The coupling of vacuum energy and gravity is not unprecedented, inflationary theory relies on the gravitational influence of a vacuum energy.

It is for these reasons that interest in the cosmological constant is still alive today, and why it is worth bothering to think about the cosmological constant in present day cosmology.

How does the cosmological constant fit into general relativity?

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intro | why | general relativity | evolution | observations | plausibility | blunder | links & references

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These pages were created by Eli Michael (michaele@colorado.edu), with the inspiration of Andrew Hamilton.
We are in the Department of Astrophysical and Planetary Sciences at the University of Colorado, Boulder.

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