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).
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