What’s up with that?

In April’s What Hubble Taught Us (p51), it said that astronomers are questioning why the Universe is still expanding, when it should be slowing down or at a plateau. “We still don’t really understand,” quotes Jennifer Wiseman.

Surely, unless I’m missing something, the Universe simply hasn’t reached that point. By what calculation has science deemed that we’ve reached the plateau?

Murray Cornwell, Florida

It’s not surprising that the Universe is still expanding, as you said. We would expect that the Universe exploded outwards at a high speed and gradually came to a stop. The confusing part is that it isn’t slowing down, but speeding up!

It’s like if you threw a ball up in the air, and instead of coming back down, it shot out of the atmosphere into space. That would be

surprising – and hint that there was something strange going on.

Sara Rigby, online assistant, BBC Science Focus