Ready to travel? (Image: Jacobs Stock Photography/Getty)

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 …