Background Essay: Gravity and the Expanding Universe

Theoretical physicists use mathematics to describe and make predictions about the physical world. Since technological limitations make them unable to observe the behavior of the universe through experimentation, they must operate in the abstract. Working at the frontier of scientific understanding means grappling with the unknown -- and possibly the unknowable. Proposing theories is thus particularly risky for a theoretical physicist, as proof for bold new ideas may be years in coming.

In the early 1900s, scientists thought that the universe was static -- neither contracting nor expanding. Albert Einstein explored the problem of why, with all its combined matter, the universe did not succumb to gravity and collapse in on itself. As an explanation, he introduced the idea of a universal force that opposed gravity. He called this anti-gravity force the cosmological constant. Although he could not measure it, Einstein was convinced that this force had to exist. How else could anyone explain the fact that the universe was static?

Scientific understanding is always subject to change. Not long after Einstein proposed the cosmological constant, scientists observed that the universe was much bigger than previously thought; the astronomer Edwin Hubble concluded it was, in fact, expanding. Faced with this knowledge, Einstein renounced his own idea, calling it his "biggest blunder." Though Hubble's new findings were widely accepted, scientists now faced new questions: If the universe was so big and continuously expanding, what was driving expansion? And shouldn't the force of gravity at least be slowing it down?

Today, theoretical physicists studying exploding stars, called supernovae, are learning more about the rate of expansion of the universe and how it has changed over time. Instead of finding that the expansion rate is slowing, they've learned that it's accelerating. To explain this, some of the scientists have proposed the existence of something called dark energy, a form of energy that is pushing space outward. Dark energy, while still very speculative, might relate to Einstein's earlier idea of the cosmological constant.