Our own universe may be but one bubble of space-time in a much bigger multiverse.

Credit: Greg Smye-Rumsby

This article is an edited extract of the book From Here to Infinity: The Royal Observatories Greenwich Guide to Astronomy, published by the University of Western Australia Press.

Credit: University of Western Australia Press

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COSMIC MICROWAVE BACKGROUND RADIATION

The explanation for the existence of the CBR is that the universe began in a hot fireball, much hotter than a star. As it expanded, the fireball cooled, in much the same way that gas expanding out of an aerosol spray can cools.

At first, radiation (photons) bounced around between charged particles in the same way that photons bounce around in the heart of the Sun. But when the whole universe cooled to the point where charged particles got locked up in electrically neutral atoms, the radiation could range freely through space, just as it escapes from the surface of a star.

Inevitably, this happened when the temperature of the entire universe was the same as the temperature at the surface of a star today, a few thousand degrees.

When the electromagnetic radiation escaped, a few hundred thousand years after the beginning of the expansion, it was very much like sunlight. But since then, it has been redshifted by the expansion of the universe and stretched to longer wavelengths, turning it into microwaves.

The entire universe is now filled with this radiation, like a very cold microwave oven. When we use radio telescopes to 'look' at the cosmic background radiation, we are seeing a highly redshifted but direct view of the Big Bang itself, using 'light' that was radiated more than 13.5 billion years ago.

Of course, this understanding of the CBR did not emerge overnight. Following the discovery of the background radiation in the 1960s, it was studied in detail, first using radio telescopes on the ground and then using satellites designed specifically for the job.

It is only in the past ten years or so that the more detailed studies of the background radiation made using satellites have shown that it is not quite perfectly uniform, and have used it to tell us more about the nature of the universe we live in, and where structures like galaxies come from.

Just before the universe cooled to the point where radiation stopped interacting with matter, the radiation and the matter were coupled together very strongly, like milk spread evenly through a cup of tea.

Where the matter was a little less dense than the average, the radiation could cool off slightly, but where the matter was a bit more compressed than average it would be at a slightly hotter temperature. After the radiation and the matter decoupled, these differences in temperature remained imprinted in the radiation, while the matter got clumped together under the influence of gravity, collapsing into sheets and filaments within which clusters of galaxies formed.

Stretching the analogy slightly, the picture is a bit like the way milk that is slightly 'off' forms little lumps when added to tea, instead of being spread out evenly. If the universe had been perfectly smooth when the matter and radiation decoupled, then the CBR would be perfectly smooth today. But there would be nobody around to notice, since if this had been the case matter could never have clumped together to form galaxies and we would not be here.

The fluctuations required to do the job are so tiny, though, that for a long time it seemed unlikely that they would ever be measured. When satellites became sophisticated enough to do the job, in the first decade of the twenty-first century, they found that the average temperature of the CBR is 2.725 K, and that the fluctuations range from 2.7251 K to 2.7249 K.

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