Dim view: the accelerating expantion of the Universe may leave astronomers idle.© Harvard-Smithsonian Center for Astrophysics

Astronomers working 100 billion years from now face a tedious time. Only about 1,000 galaxies will be visible from the Milky Way, compared with the many billions currently on offer. And their images will be frozen in time - not changing, just fading.

This fate will result from the Universe's ever-accelerating expansion, says cosmologist Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics. Objects will be receding from view so quickly that their new light will never reach us.

The more distant an object, the sooner it will escape our view. In 50 billion years, the Universe beyond our local galaxies will have stopped changing. The most distant object currently known, a quasar 13 billion light years away, will freeze-frame when it is six billion years old, about the same age as our Sun is now.

"There's a finite amount of information that we can gather about the Universe," says Loeb. "We'll never know how it evolves beyond a certain stage."

Ever expanding?

The Universe has been expanding since the Big Bang. Until recently, most cosmologists assumed that gravitational attraction between galaxies would slow this growth, or even reverse it, leading to a Big Crunch.

But in the past few years, observations of distant exploding stars have suggested that this expansion is accelerating, carrying everything away from everything else.

Thefurther away something is, the faster it recedes. The faster it recedes, the more slowly it changes, relative to us.

Eventually, time seems to grind to a halt. The object reaches an event horizon - like a body falling into a black hole - beyond which its light can no longer reach us.

As regions pass out of reach they also cross a threshold of how we can think about them, says astronomer Martin Rees, of the University of Cambridge. "It's a question of at what stage the unobservable parts of our Universe stop being part of science and become metaphysics," he says.

Constant problem

Finding out what is making the Universe accelerate is "probably the biggest challenge in cosmology", says Loeb. A cosmological constant may give the vacuum of space a dark energy that repels things from each other. Or something that changes with time may cause the acceleration.

Inthis second case, the acceleration might slow, and the future Universe wouldn't be so dark and lonely. But the change needs to happen relatively quickly, says Loeb, as things will be pretty much fixed when the Universe reaches five times its current age.

Any beings still existing, says Loeb, will be reliant on us to tell them about most of the cosmos. "We'd better focus our attention on the distant Universe for the next 50 billion years," he says. 

University of Cambridge

Harvard-Smithsonian Center for Astrophysics

• References

  1. Loeb, A. The long-term future of extragalactic astronomy. Physical Review D, (2001).