What is the cosmological constant?

David N. asks, "Dr. Volt, what is the cosmological constant, and is it really constant? And does it really matter?"

Excellent question, David! You have hit upon one of the great unanswered questions of the universe. That is, is the universe expanding, contracting, or is it stationary? The cosmological constant was first introduced by Einstein into his equations for General Relativity in order to allow for the possibility of a static universe. While Einstein later considered this one of his "biggest blunders," the evidence later seen in the 1990s that suggested an expanding universe (i.e. the redshift in light from distant galaxies) led astronomers to consider the cosmological constant once again. While certainly believed to be a constant, there remains great debate as to the value of this constant, which will in turn tell us if we live in an open universe (forever expanding) or a closed universe that will eventually contract and collapse. Essentially the cosmological constant is an energy density, and if the value of this density in the universe is small enough it means the universe will continue to expand forever, but if it is great enough it means the universe will eventually collapse back in on itself leading to what is often termed a "big crunch." (Some astronomers argue that the universe could then expand again, leading to a cyclical universe of forever repeating "big bangs" and "big crunches," although one has to wonder how this could be compatible with the Second Law of Thermodynamics.) Pick a value right in the middle and the universe is "flat," neither expanding nor contracting.

So does it matter? As a purely practical point if the universe will someday collapse then anyone around at that point may certainly feel that it matters, but as such a time is billions and billions of years away we need not worry about the end of the universe as we know it quite yet. In the meantime, establishing once and for all the value of this constant will aid in our understanding of the structure of the universe and its ultimate fate. Many of us in the mad scientist community are eagerly awaiting such a result in order to perfect our own diabolical contraptions for creating localized regions of collapsing space (very useful in holding cities for ransom, quite necessary when mad scientists usually find themselves ineligible for most research grants).

Thanks for your question, David. I hope you find this information useful as you devise your own schemes for world domination!

The Sombrero Galaxy, courtesy of the Hubble Telescope