An update on progress about the search for the Dark Energy by Clare Burrage.

Update on Dark Energy

By Professor Clare Burrage

Nottingham University Lecturer

There is a strange thing happening in our universe…

If you were in a vacuum and you had a ball in your hand, assuming there is no gravity in the room, when you throw the ball it would travel through the vacuum at the speed it left your hand. Without friction to stop it or gravity to pull it down, the ball would remain travelling at the same speed for as long as the vacuum and lack of gravity exists. And that simple example explains more or less what we expect for galaxies in space but that’s not what’s happening.

In 1926, using red-shifted light from distant galaxies, Edwin Hubble determined that everything is moving away from us and he thought this was happening at a constant rate. We now know that he was not quite correct – the speed of expansion is on the increase.

Since everything is moving away from us, it would be easy at this point to believe we are at the centre of the universe but that is not consider to be correct (although we cannot prove it) so if you were anywhere in the universe outside the Milky Way, it is believed you would experience the same phenomenon. Weird, intit?

There are of course exceptions because of local gravitational influences but beyond that, everything is red-shifted. To understand what this means we need to look at the visible light spectrum. The different colours of light are determined by their wavelength. Blue light has a shorter wavelength than red light. In fact, they are both at the opposite ends of the visible light spectrum. If a light source moves towards you then the light shifts towards the blue end of the spectrum. In other words, the wavelength shortens and this is described as blue-shift. If the light source is moving away from you, the wavelength gets longer and is termed as red-shifted. Specific wavelengths also known as spectral lines are looked at to detect the light shift.

Red-shift and blue shift have been used for a long time to understand the direction things are moving. However, saying that things are ‘moving’ in the universe is a little misleading. What’s really happening is that the space between objects is getting bigger hence why the term ‘expansion’ is more appropriate. Here’s how to get your head around this one – imagine you are making a cake and you have lots of currents in it. You make your cake mix and put this in the oven. At this point, the currents are all quite close to each other but as the cake cooks, it expands and the currents – which have not moved – become further apart. The cake mix between the currents has done all the expanding. This is what’s happening to the universe.

But the universe is not expanding at a constant rate. So the big question is, why is the expansion rate increasing? The answer is currently termed ‘Dark Energy’. Think of it as an as-yet-not-understood ‘thing’ that is supplementing the expansion anticipated from the effect of the Big Bang.

It is this certain something that Clare has spent the last 10 years studying at a theoretical level. Dark Energy may be something that exists outside of our current understanding of the laws of physics, it may be a particle we have not yet discovered or maybe it’s something else – if the Big Bang theory is wrong, then Dark Energy and Dark Matter, another mysterious component of our universe, might not exist.

Clare has postulated that it’s a thing called a Chameleon particle. She suggests that it is a particle whose mass changes dependent upon the situation it finds itself in. Although the idea was not received by our scientists very warmly to start with, it is now building some momentum and experiments to detect chameleon behaviour are in progress.

Clare has access to a very clever device that uses laser beams to hold clouds of atoms at the top of a tube (they are held in place using the pressure of light as a clamp!). The atoms are then dropped and the speed they fall is being measured to see if things at the quantum level might interact with gravity in an unexpected way. So far, the atoms have fallen at the expected rate but if they can build a larger device in the future, perhaps they will be able to observe new behaviours that help to support Clare’s ideas.

The talk was fascinating and the amount of questions taken during and after the talk showed how engaged everyone was. The opportunity to talk about theoretical science certainly stimulated some interesting conversations!

Our thanks to Clare for her time and we hope to see her at the observatory for another update in the future.