Brian Greene in the New York Times:
The pinpoints of starlight we see with the naked eye are photons that have been streaming toward us for a few years or a few thousand. The light from more distant objects, captured by powerful telescopes, has been traveling toward us far longer than that, sometimes for billions of years. When we look at such ancient light, we are seeing — literally — ancient times.
During the past decade, as observations of such ancient starlight have provided deep insight into the universe’s past, they have also, surprisingly, provided deep insight into the nature of the future. And the future that the data suggest is particularly disquieting — because of something called dark energy.
This story of discovery begins a century ago with Albert Einstein, who realized that space is not an immutable stage on which events play out, as Isaac Newton had envisioned. Instead, through his general theory of relativity, Einstein found that space, and time too, can bend, twist and warp, responding much as a trampoline does to a jumping child. In fact, so malleable is space that, according to the math, the size of the universe necessarily changes over time: the fabric of space must expand or contract — it can’t stay put.