How far could you travel in a spaceship?

HOW far could an astronaut travel in a lifetime? Billions of light years, it turns out. But they ought to be careful when to apply the brakes on the return trip.

Ever since cosmologists discovered that the universe's expansion is accelerating, many have wondered just how much this will constrain what we could see with telescopes in the future. Distant regions of the universe will eventually be expanding so fast that light from any objects there can never reach us.

Likewise, dark energy - the mysterious force behind the acceleration - places a limit on human exploration of the universe, says Juliana Kwan at the University of Sydney in New South Wales, Australia, who has now refined this limit on our travels. Even with rockets that could take us to within a whisker of light speed, expansion would still eventually leave us behind.

The furthest that light emitted from our sun today could reach, as it races in vain to outdo the accelerating expansion, currently lies around 15 billion light years away. According to previous calculations by Jeremy Heyl of the University of British Columbia in Vancouver, a super-advanced rocket could get most of this way in a human lifetime. Accelerating at around 9 metres per second per second - which would feel roughly like a comfortable 1 g - a craft could get 99 per cent of the way to the expansion "horizon". Despite the vast distance, this would take only about 50 years in the astronaut's reference frame, because time would pass slower than on Earth due to relativity (Physical Review D, DOI: 10.1103/PhysRevD.72.107302).

Now, in a paper to appear in Publications of the Astronomical Society of Australia, Kwan and her colleagues have found the trip could take even less time. Based on the latest cosmological values for dark energy and other parameters, they showed an astronaut could make the journey in only 30 years.

But their calculations also suggest that returning home presents its own challenges. Even slight uncertainties in the strength of dark energy or the total density of matter in the universe could cause a spacecraft to miss Earth by millions of light years. Beginning the deceleration just a second too late could cause you to overshoot the Milky Way, Kwan says. "You would effectively be lost in space."

Still, even if you did stop in the right place, you'd be disappointed. Some 70 billion years would have elapsed back home, so the sun would long, long since have expired, taking Earth with it, and the surrounding view would appear mostly dark.