A question about dark energy and the expansion/acceleration of the Universe.

Pattonias

I was curious about whether or not the Universe was accelerating or decelerating and how we could know one way or another.

I read an article I found online http://curious.astro.cornell.edu/question.php?number=63 and it said that, among other things, that the universe is accelerating partly due to dark energy and its "repulsive force of empty space that counteracts gravity on large scales." I had never heard of anything actually "repulsive" to gravity and was wondering if this was right or wrong.

nicksauce

It is correct that the expansion of the universe is accelerating. The way this has been typically measured is to look at things, typically Type IA supernovae, which we know how bright they should be. If the universe is accelerating then they'll look less bright as a function of redshift than they would if the universe wasn't accelerating.

It can indeed be shown that a term with negative pressure, like dark energy, causes the expansion of the universe to accelerate.