For the past year astronomers of the world have been in a state of high excitement. Observations of supernovas - exploding stars - billions of light-years away, suggest a surprising result: The universe, known to be expanding since the 1920's, actually is "accelerating." Distant galaxies are not only receding from us, they are flying away faster and faster.

Since these events are taking place at distances so great as to be almost unimaginable, the natural reaction to the new observations might well be, so what? How can things so remote have any possible relevance to human affairs, here on Earth?

To answer that question, we need to explain why it is so surprising for far-off galaxies to be moving away increasingly fast. The place to start is with the "standard model" of the universe, the mental picture of the cosmos that scientists have been developing and testing for the past seventy years.

According to that model, our universe began somewhere between twelve and twenty billion years ago in a "Big Bang" that sent all parts of that original tightly compressed universe rushing away from each other. We have to point out that it is not that other parts of the universe are receding "from us," which would imply we are in some special position. All parts are running away from each other. And Edwin Hubble, after whom the Hubble Space Telescope is named, provided the first evidence of this expansion in 1929. All observations since then confirm his result.

Will the expansion continue forever, or it will it stop at some future time? That question proved difficult to answer. The force of gravity operates on every galaxy, no matter how far away, and it acts to pull them all closer together. Given enough material in the universe, the expansion might one day slow down and even reverse, with everything falling back together to end in a "Big Crunch." Or, with less density of material, the expansion might go on forever, with the force of gravity gradually slowing the expansion rate. But in either case, gravity can only serve to pull things together. It can't push; and a push is what you need in order to explain how the expansion of the universe can possibly be accelerating.

Where could such a push - a repulsive force between the galaxies - possibly come from? The only possible source, according to today's science, arises from space itself. There must be a "vacuum energy," present even in empty space, and providing an expansion force powerful enough to overcome the attraction of gravity.

Einstein introduced the idea of such a source of force over eighty years ago, as a so-called "cosmological constant." Einstein used this constant to explain why the universe did "not" expand (this was before Hubble's observations showing that it did) and Einstein called his failure to imagine an expanding universe the biggest blunder of his life. Until recently, most cosmologists preferred to assume that the value of the cosmological constant was zero, which meant there was no repulsive force associated with space itself.

The new observations, of an accelerating universe, imply that this is no longer an option. The cosmological constant can't be equal to zero if space itself is to be the origin of a repulsive force more than strong enough to balance gravitational attraction.

And now for the so what? Can such esoteric ideas, originating so far away, have any relevance to everyday life?

I can't really answer that. But I will point out that the proposed vacuum energy is present here on Earth, as well as in remote locations. And notions equally abstract, published by Einstein in 1905 and concerning the nature of space and time, led very directly to atomic energy and the atomic bomb. That development took less than forty years.

If history is any guide, many of us might live to see practical consequences of a non-zero cosmological constant.

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