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Dark Energy

 

We know even less about dark energy than we do about dark matter.

Dark energy is much trickier [than dark matter].  Now, we know even less about dark energy than we do about dark matter, and what’s frustrating is there’s more dark energy than there is dark matter.  The latest count is something like two thirds, or just over two thirds, of all the mass energy, , because E=mc2 means that mass and energy are interchangeable, so when we talk about the stuff that’s out there in the universe we talk about mass and energy, um, two thirds of it is, we believe, dark energy.  And then the other third is dark matter with a little bit of normal stuff, which is everything we do see.

Dark energy was first hypothesised, without him knowing it, by Albert Einstein, because when he developed his theory of general relativity he assumed the universe was static and stable.  He had no inkling that it could be expanding or collapsing and so he put into his equation a term called the cosmological constant that stabilised the universe, kept it static.  Then when it was discovered, within a decade, that the universe is actually expanding from the Big Bang, he realised there was no need for the cosmological constant.  You could do without it and the equation gets simpler.

In 1998 astronomers were studying very distant galaxies by looking at supernovae, exploding stars, because when a star runs out of its nuclear fuel, and if its big enough, it will explode as a supernova, it’s so bright it outshines the rest of its galaxy.  So it’s a very good way of learning about how far away galaxies are and how far away they’re…or how quickly they’re receding from us as the universe expands.  So they’re looking at supernovae in very distant galaxies and realised that they were much dimmer than their distance suggested.  Now, the way they measure distance is through something that Edwin Hubble discovered, which is that the further away you look, the fastest objects…galaxies are moving away from us because space itself is expanding.  So they said, right, given how fast it’s moving away from us, something that’s called the Doppler shift, so the light coming from those galaxies gets longer wavelengths towards the red end of the spectrum, it’s called a redshift, so they could measure the redshift just looking at the colour of the light.  That told them, they thought, how far away those galaxies were, but they were much dimmer than they should’ve been.  Which suggests that they were further away than they thought.

So there’s a whole sort of series of logical steps and the firm conclusion they came to is that these galaxies are actually much further away than we thought but they’re not expanding outwards, moving away from us quite as fast as they should be, given the rate of expansion of the universe.

Now, the other thing to remember is the further away you look, the further back in time you’re seeing things because of the time taken for the light to reach us from those galaxies.  So these galaxies that they’re looking at were very early on in the life of the universe, nearly 14 billion years ago.  And they said, ‘OK, so what that means is very early on in the universe, it was expanding more slowly than it is now.  It’s expansion speed now is higher.’  And that went completely against everything that we knew because, as far as astronomers and cosmologists understood, the universe started with a Big Bang, it expands very quickly and then its gravitational pull, the attraction of all the stuff in the universe, is putting the brakes on, like stretching a spring.  So the universe should expand quickly and then slow down.  

What happens afterwards, we weren’t sure.  Is it going to slow down and recollapse?  Is it just going to slow down gradually forever?  This suggested it was expanding slowly in the past and much more quickly now.  For the first half of its life, 7 billion years, ah, gravity was winning and it was slowing down the expansion of space, but then as it got beyond a certain size and all the galaxies were too far away from each other, gravity became too weak and this new dark energy took over.  And it started to win and started to push things apart ever more quickly.  And so, whatever it is, it has the nature of a force of anti-gravity, working against gravity.

We don’t know what it’s made of.  It may be something to do with the quantum vacuum but those are sort of words bandied about because people don’t really know what to call it.  A huge mystery.  But again it’s…you know, astronomers see what they see and then draw conclusions from it and then we’ve got to try and pick up the pieces and explain it.