The surprising answer is: nobody knows. Physicists, despite trying for almost a century to rule it out, have so far failed. The known laws of physics do not forbid time travel. In fact, they appear to make it easy - at least in principle.
The discovery of the fifth dimension changes everything we thought to know about our universe and opens a completely new window for our views on our existence, including the two questions "how" and "why".
28.000 light years away from Earth, towards the center of our Milky Way, an unexpectedly mighty hurricane blows around a tiny black hole. It is the strongest wind ever observed by astronomers and a great surprise because these pushed off particles exceed the speed of 32 million k.p.h
The Nobel Prize in Physics was awarded on Tuesday to Saul Perlmutter, Brian P. Schmidt and Adam G. Riess for their leadership of the teams that discovered the apparent "accelerating expansion of the Universe through observations of distant supernovae" in 1998.
We don't yet know what dark energy is, or where it comes from. We can't tax it, we can't use it to bolster economic growth and it is not going to solve rising fuel prices or alleviate climate change. But sometimes knowledge is its own reward. Even if it is nutty-sounding.