Friday, 1 April 2016

#4: The last scattering surface

Did you know that we can look back in time to see the universe as it was 30,000 years after The Big Bang? This is one of the fascinating things I have been learning from Lawrence Krauss’s book A Universe from Nothing.

I’ll try to explain. Seventy-five percent of the universe is now hydrogen (measured by weight). But for 30,000 years after The Big Bang this was not the case. Then, the universe was so hot (3000° [Kelvin] above absolute zero) that there were no hydrogen atoms – instead, there were separate protons and electrons, or as Krauss puts it, ‘a dense “plasma” of charged particles interacting with radiation’. This ‘plasma’ is not transparent (unlike hydrogen) – it’s opaque and we cannot see past it. 

Why does this matter? Because since we cannot see past it, it stops us viewing the origin of the universe. When we look out into space we are also looking back in time, since we are looking at light which may have been emitted aeons ago because it has taken so long to reach us. So we can thus see events which happened millions or even billions of years ago. 

Could we look right back to The Big Bang? In principle yes, says Krauss, but in practice no, because of this ‘plasma’, which we cannot see past, which he calls ‘the last scattering surface’. This forms a kind of wall between us and The Big Bang.

But radiation coming from that wall of ‘plasma’ or ‘last scattering surface’ can be photographed. This is cosmic microwave background radiation (CMBR). A striking image in the book shows a piece of sky over Antarctica with the ‘hot spots’ and ‘cold spots’ of this radiation imaged, our blue sky now turned into a mass of squiggly shapes. 

Measurements of these spots can give us strong indications of the overall shape of the universe – closed, flat or open. For further explanation, I recommend Krauss’s book (published in 2012). But it strikes me how useful shortcut words like ‘plasma’ and ‘scattering surface’ are in giving us lay people a quick route to some kind of appreciation of these concepts. And also what a shame it is we cannot view The Big Bang – what a show that would be. I’m reminded of Douglas Adams’s Restaurant at the End of the Universe, in which diners can time travel to take their evening meal while watching the universe die outside the window. A breakfast bar at the beginning of the universe would be wonderful.

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