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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After the discovery of the redshift–distance relationship, Einstein and other mathematicians realised that this was exactly what his equations described – space itself stretching and carrying clusters of galaxies along with it. This was the beginning of modern cosmology.

The cosmological redshift is not a Doppler effect. It is not caused by galaxies moving through space, but by the space between the galaxies stretching during the time it takes light to get from one galaxy to another, and stretching the light to longer wavelengths.

Galaxies do move through space, producing Doppler effects in their spectra, but these are simply added to or subtracted from the cosmological redshift – which is why, for example, Andromeda shows a blueshift. It is moving towards us through space faster than the space between us and Andromeda is expanding. But for all except the nearest galaxies, the cosmological redshift dominates.

The usual way to get a picture of what is going on is to imagine a perfectly round rubber ball with spots of paint spattered on it. If the ball is pumped up so that it expands, all the spots will move apart from one another, not because they are moving through the rubber skin of the ball, but because the skin is stretching.

And, crucially, there will be no centre to the expansion – from whichever spot you choose to measure, you will see all the other paint spots receding, exactly in line with Hubble's Law. Once again, we see that there is nothing special about the Milky Way. The view of the universal expansion would be the same from any galaxy in the universe.

THE BIG BANG

The startling implication of this discovery was that if the universe is getting bigger now, it must have been smaller in the past. Go back far enough, and you would come to a time when all the galaxies were piled up on top of each other.

If you imagine winding the expansion backwards even further, you would come to a time when the whole universe was concentrated at a point. Using Hubble's Law and the latest calibration of the redshift–distance relation, we can calculate that this corresponds to a time about 14 billion years ago.

This realisation is what led to the idea that the universe emerged from a hot fireball, the Big Bang, and has been expanding and cooling ever since. But the Big Bang model of the universe really only began to be taken seriously in the 1960s, when radio astronomers Arno Penzias and Robert Wilson discovered a weak hiss of radio noise that they identified as the fading echo of the Big Bang itself.

This background radiation is in the microwave part of the spectrum, and comes from all directions in space, so it is known as the cosmic microwave background radiation (CMB), or just as the cosmic background radiation (CBR).

Different parts of the electromagnetic spectrum correspond to radiation with different temperatures. If the radiation peaks in the part of the spectrum corresponding to the yellow-white of the Sun, this is equal to a few thousand degrees on the Kelvin scale; a red-hot lump of iron is cooler, the invisible infrared radiation you feel if you hold your hand near a radiator cooler still, and so on.

Similarly, objects that emit mainly ultraviolet radiation or X-rays are much hotter than the Sun. The temperature of the cosmic background radiation, however, is just 2.7 K, or -270.5 ºC. But it fills the entire universe.

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