Sounds of Science #04

Dark Energy

So most of our universe (over 70%) is made up of something called Dark Energy. We can’t see it and we don’t really know what it is…

Hello darkness my old friend. Image: New Scientist

Matter – everything that makes up me, you, planets and stars – appears to make up only a very small fraction of the universe, about 4%. Instead, the universe seems to be filled predominantly by a very strange material known as dark energy and it is this material, with it’s anti-gravity properties, which seems to speeding up the expansion of our universe. We’ve known that the universe was expanding since Edwin Hubble made his observations in the 1920s, however it’s only in the last 20 years that we’ve realised that this expansion is actually speeding up! The problem is that we can’t directly detect dark energy and this makes it very difficult to understand what it is and whether it really does exist.

Instead we must rely on indirect observations, looking at light travelling from the far reaches of the universe to determine whether the properties of this light has changed during the time it has taken to reach us. A good way to measure the expanding universe is to make observations of distant supernovae (huge explosions which follow the death of large stars) which act as ‘standard candles‘ or ‘lighthouses’ because we know how bright these object should be. Measuring light from distant supernovae has allowed us to see that it is different to what it should be if these objects were positioned within a static universe. Instead what we see is changes in this light which indicates that these objects are being flung outwards and away from us via some sort of cosmic expansion.

A nice analogy to describe the expansion of the universe is what happens when two points are drawn on the surface of an inflating balloon. As the balloon is inflated, the two points begin to move further and further away from each other and as the material expands outwards, the distance between the two points also increases. Applying this analogy to the cosmos, we could imagine the same happening with two galaxies being pulled apart from each other as the space they exist in expands.

As dark energy is so difficult to detect, scientists have recently been looking for new ways to independently verify its presence within the universe. Whilst at the BBC I was lucky enough to interview cosmologist Dr Chris Blake from Swinburne University, Australia who has recently published two papers reconfirming dark energy via a new set of methods. Blake and his colleagues produced a galaxy map of over 200,000 galaxies and used this information to look at how these galaxies were distributed and how they grew relative to each other. Through this work Blake and his colleagues were able to reconfirm the presence of dark energy and perhaps most importantly were able to determine some of its properties.

I thought I’d use the audio from this telephone interview and spruce it for the next sounds of science episode:

It probably sounds better with headphones (or obviously decent speakers).

Useful links:

2 thoughts on “Sounds of Science #04

  1. Interesting article! I like the analogy used of an inflating balloon with two points written on it and the expansion of the universe. I have to admit I find the concept of dark energy hard to understand. I understand the Doppler Effect but how can one tell if an object moving towards or away from you is actually accelerating?

    1. Thanks for reading!!!

      Okay so I believe the key to this question lies in observing supernovae over a large range of distances, most importantly those that are nearest and those that are the furthest away.

      We can observe ‘local’ supernovae events and we know because they are relatively close to us that their light has to travel a much smaller distance and over a much smaller amount of time (and are therefore much more recent events!). So over this relatively small amount of time and distance the light has travelled to us, there has been little cosmic expansion and thus little affect on this light. Thus any redshift observed in this light should be minimal or at least much lower than objects located at much greater distances from us.

      Light coming from supernovae at the far reaches of the universe must travel phenomenally large distances and even at the speed of light, it can take billions of years to reach us. Over this time, if space is expanding it will exert an effect on the light travelling through it – stretching it and causing this redshift. If this expansion is constant then the amount of redshift would be proportional to the amount seen in light coming from relatively near by supernovae (which are relatively more recent events!) – it would only be greater because of the longer period of time and distance it had been travelling (by knowing the distance of the supernovae we can account for this).

      So we can look at distant supernovae and we can compare the degree of redshift with those from much nearer supernovae (of which light has travelled a much smaller amount of time) and by taking into account differences in the distances we should be able to see that the amount of redshift between the two objects was roughly the same.

      But what if the distant supernovae were appearing much dimmer than they should be – it would suggest that the speed of this cosmic expansion had been changing over time – so by observing the most ancient supernovae, we have been able to detect this change. Over the time and distance that this light has travelled, it has been stretched at a rate which is not in line with a linear, constant expansion and we therefore conclude that this expansion is occurring at a much greater rate.

      So by looking at the far reaches of our universe and finding that distant supernovae are far dimmer than expected we have concluded that the universe is expanding and that these objects are being flung away from us at an ever increasing rate. The light travelling from them arrives with much lower energy (greater redshift) because of the effect of this increasing expansion, stretching the light itself as the space it travels through expands!

      This is why we’ve only really known about this accelerating expansion since the 90s, because it was only really at the end of the 20th century that we had the technology to make such observations and measurements!

      Well done if you’ve read this far, I’m a Biologist in training and not a physicist so I hope that my understanding of this is correct and simple enough for you to understand! I would recommend reading this lovely and very simply written article which details how the study of supernovae have led to the development of the Dark Energy theory:

      Click to access PhysicsTodayArticle.pdf

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