What is the colour of the universe?
Well one simple look into the night sky would tell you that the majority of it is black, as there are huge gaps of darkness between each of the stars that we can see. However if we were to look further into space using more powerful telescopes then in these supposed ‘empty’ spaces in the sky we would find thousands of galaxies.
Astronomers have actually performed this experiment several times using the Hubble space telescope, the first of which was an off the cuff experiment in 1995 which pointed the telescope at one tiny part of the sky for ten consecutive days. Many disagreed with this application of the Hubble telescope thinking it was a waste of time and money, but the result was staggering. In a tiny portion of the sky beside the constellation Ursa Major they discovered 3,000 previously unknown galaxies. It was one of the most important photographs ever taken, known as the Hubble Deep Field.
They have since replicated the experiment three times more, photographing the southern sky in the Hubble Deep Field South in 1998, the Hubble Ultra Deep Field in 2004, and finally the Hubble eXtreme Deep Field in 2012 (and yes the ‘x’ is supposed to be upper case!) The detail which these photographs can give us is amazing, looking back 13 billion years to some of the earliest galaxies at the very beginning of the universe.
But since there are all these objects of light in every point in the sky, then why is the sky still dark at night? Surely our night should be as bright as our day in that case.
Well the reason for the sky being dark at night is because of the expansion of our universe, as all these distant galaxies are accelerating away from our planet and as a result the light from each is being ‘red shifted’. That means that as it travels it is being stretched into parts of the electromagnetic spectrum that our eyes cannot pick up as a result of the Doppler effect, namely into ‘infra-red light’ (for more information on the Doppler effect see my previous article here , and more on the electromagnetic spectrum here)
The only reason the Hubble Space Telescope can produce these types of photographs is through detecting the infra-red light and make adjustments to turn it into something visible to our eyes.
But lets get back to the question of the colour of the universe, because every star and galaxy does have an individual colour that tells us information about it. For example, stars of a blue colour tend to be much earlier in their life cycle and extremely hot, whereas red stars tend to be nearing the end of their life cycle and cooling, but also huge in size. It’s unusual because we tend to associate colour and temperature differently, for example our taps are blue for cold and red for hot, but with stars it is the opposite way round.
So if we took every stars colour and then merged them into an average then we would get an overall average colour of the universe, and that is exactly what some researchers did in 2001. They used the 2dF Galaxy Redshift Survey which contained data from over 200,000 galaxies and broke them down into their component wavelengths, giving each a colour. Averaging these colours out gave us an overall turquoise colour of the universe.
This result captured the publics imagination, gaining huge publicity in the process. That was until other researchers went through the calculations, as happens when all scientific results are published, and they disputed the results. What had occurred was that the original researchers had used software online that was calibrated to a slightly redder, warmer light than usual, resulting in a green tint to the original results. They correctly argued that to view the correct colour of the universe you would need to be hypothetically outside the universe looking in, rather than from earth looking outwards, in an area of pure darkness.
Putting these adjustments into their data the original researchers concluded that, rather than a turquoise colour, the colour of the universe was actually more of a beige, something that they renamed ‘cosmic latte’ to give it a more sexy title. It was an embarrassing retraction but an honest mistake, and not quite the vibrant colour they were looking for.
However, the implications of this colour are more startling than meets the eye. I mentioned earlier that the colour of stars and galaxies can tell a lot about its age and temperature, and the result of this beige or ‘cosmic latte’ colour actually tells us that there are not many young blue stars left in the universe compared to older redder stars, meaning that the universe may have passed its peak point of activity and is now on the downward slope to burning out.
It is difficult to think of the universe dying out, but we maybe be at the very beginning of the long goodbye. It also tells us how fortunate we are as a human race to be living in an age where we can still look back to the very birth of the universe and find out as much as we can about our existence before very gradually the stars start to disappear over the next few billion years.
Of course the human race may be long gone by that point, which means we should not underestimate the current age that we live in, the age of space exploration. Last week NASA announced the discovery of a solar system with seven earth sized planets in orbit, three of which are in the habitable zone and great candidates for potential life, and next year will see the launch of the James Webb Telescope into a positon beyond the moon which aims to see further into space than ever before. So let us see what we can discover about this incredible universe before it finally fades into darkness.