ASTRONOMERS say all the galaxies are rushing away from us in the aftermath of the big bang. Does this mean we are at the centre of the universe and that the big bang happened in our backyard?
“Not at all,” says Steven Weinberg of the University of Texas at Austin. “The universe is in a state of universal expansion. Every bit of the universe is rushing away from every other bit. It’s true that all the galaxies are racing away from us, but if you could sit on any one of those galaxies and observe the sky from it, you would see us racing away. In fact, you would see all the galaxies racing away just as we do from our own Milky Way.”
The idea that the universe looks the same from every place is incorporated into modern theories, says Weinberg. “It’s called the Copernican principle. Copernicus discovered that there was nothing special about the position of the Earth. It was just one of a number of planets. The Copernican principle says there is nothing special about our galaxy – that the pattern of the universe’s expansion looks the same from any galaxy.”
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And this explains why it is that the further away a galaxy is from us, the faster it is moving. “It’s a direct consequence of the Copernican principle,” says Weinberg. “If the expansion of the universe is assumed to look the same wherever you are, then you can deduce that the speed of expansion has to be proportional to distance – in other words, that the speed of galaxies equals some constant times its distance.”
This constant is known as the Hubble constant and the law as Hubble’s law, after the American astronomer Edwin Hubble who discovered it in 1929. Hubble’s law states that a galaxy which is twice as far away as another is moving away from us at twice the speed. Similarly, a galaxy three times as far away is travelling three times as fast, and so on. Today, this law is known to hold true for galaxies at a distance of at least several billion light years.
“Imagine looking at a galaxy which is, say, a billion light years away,” Weinberg says. “Then imagine a fellow in that galaxy looking at a galaxy along the line of sight which is another billion light years further away. The galaxy you are looking at is travelling at 10,000 kilometres per second. But the fellow in the second galaxy must also see the third galaxy travelling at 10,000 kilometres per second. So the third galaxy, which is twice as far away from you as the second galaxy, is travelling away from you at 20,000 kilometres per second – twice as fast.”