Dark Energy

Dark energy, a mysterious force that no one understands, is causing the universe to fly apart faster and faster. Only a few years ago, if you'd suggested something like that to astronomers, they would have told you to spend less time in front of the TV and more time in the "real" world.

But dark energy is real ¾ or at least, a growing number of astronomers think it is. No one, however, can truly explain understand it, but seem to know the effects of it. Kind of, got everyone one in a bit of a twist.

Dark energy entered the astronomical scene in 1998, after two groups of astronomers made a survey of exploding stars, or supernovas, in a number of distant galaxies. These researchers found that the supernovas were dimmer than they should have been, and that meant they were farther away than they should have been. The only way for that to happen, the astronomers realized, was if the expansion of the universe had sped up at some time in the past. Until then, astronomers had generally believed that the cosmic expansion was gradually slowing down, due to the gravitational tugs that individual galaxies exert on one another. But the supernova results implied that some mysterious force was acting against the pull of gravity, causing galaxies to fly away from each other at ever greater speeds.

At first, there were a lot of questions; perhaps the supernovas were dimmer because their light was being blocked by clouds of interstellar dust. Or maybe the supernovas themselves were intrinsically dimmer than scientists thought. But with careful checking, and more data, those explanations have largely been put aside, and the dark energy hypothesis has held up.

In one sense, the idea is not completely new. Einstein had included such an "anti-gravity" effect in his theory of general relativity, in his so-called cosmological constant. But Einstein himself, and later many other astronomers, came to regard this as a kind of mathematical contrivance that had little relationship to the real universe. By the 1990s no one expected that the effect would turn out to be real.

Anti-gravity may not be the right way to explain dark energy. It is not an opposite of gravity. It’s character is just as General Relativity explained it, if it had negative pressure.

If you think in terms of the universe as a very large balloon,when the balloon expands, that makes the local density of the [dark energy] smaller, and so the balloon expands some more …. because it exerts negative pressure. While it’s inside the balloon it’s trying to pull the balloon back together again, and the lower the density of it there is, the less it can pull back, and the more it expands. This is what happens in the expanding universe.

The supernova evidence suggests that the acceleration kicked in about 5 billion years ago. At that time, galaxies were far enough apart that their gravity (which weakens with distance) was overwhelmed by the relatively gentle but constant repulsive force of dark energy. Since then, dark energy's continuing push has been causing the cosmic expansion to speed up, and it seems likely now that this expansion will continue indefinitely.

It means that if you look out at the universe today, and if we wait many billions of years, everything will be flying away faster and faster, and eventually we’ll be left quite alone. As we will be so far a part from each other and the influence of another object will be nearly negligible.

Aside from such grim forecasts, dark energy is causing quite a bit of upset for astronomers who have to adjust to an unexpected and outlandish new view of the universe. Already, they have had to accept the notion of which is now thought to far outnumber ordinary matter in the universe, but which has never been detected in any laboratory. Now, the arrival of an unknown force that rules cosmic expansion has added insult to injury.

As for dark matter itself, it is quite hard to explain, Nobody’s really sure, and then dark energy makes us even more unsure. All that can be said is that the 90% - 95% of the universe is made up of two ingredients that nobody really understands. All we know is the little of physical matter. Seems like we have not moved that far, have we?

Quantum mechanics took us two generations of people to understand it comfortably. Quantum Mechanics, General Relativity and lots of other things cannot be worked out in a laboratory. The same applies to dark energy.

The only comfort dark energy provides us now, that it to some extent explains the expanding universe.