Physicists just found a link between dark energy and the arrow of time

I am in the middle of writing  long piece about this. The following article is courtesy of Science Alert

For years, physicists have attempted to explain dark energy – a mysterious influence that pushes space apart faster than gravity can pull the things in it together. But physics isn’t always about figuring out what things are. A lot of it is figuring out what things cause.

And in a recent paper, a group of physicists asked this very question about dark energy, and found that in some cases, it might cause time to go forward.

When you throw a ball into the air, it starts with some initial speed-up, but then it slows as Earth’s gravity pulls it down. If you throw it fast enough (about 11 km per second, for those who want to try), it’ll never slow down enough to turn around and start falling back towards you, but it’ll still move more slowly as it moves away from you, because of Earth’s gravity.

Physicists and astronomers in the 1990s expected something similar to have occured after the big bang – an event that threw matter out in all directions. The collective gravity from all that matter should have slowed it all down, just like the Earth slows down the ball. But that’s not what they found.

Instead, everything seems to have sped up. There’s something pervading the Universe that physically spreads space apart faster than gravity can pull things together. The effect is small – it’s only noticeable when you look at far-away galaxies – but it’s there. It’s become known as dark energy – “dark”, because no one knows what it is.

Science is nothing if not the process of humans looking for things they can’t explain, so this isn’t the first time the Universe has stumped us. For centuries, one of those stumpers has been time itself: Why does time have an arrow pointing from the past to the present to the future?

It might seem like a silly question – I mean, if time didn’t go forward, then effects would precede causes, and that seems like it should be impossible – but it’s less of one than you might think.

The Universe, as far as we can tell, only operates according to laws of physics. And just about all of the laws of physics that we know are completely time-reversible, meaning that the things they cause look exactly the same whether time runs forward or backward.



 Featured image courtesy of  Paul Fleet/


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