Wonders of the Universe

REDSHIFT

Although first discovered in the early twentieth century, redshifts were really put into their cosmological context through the work of Edwin Hubble. He discovered that there is a very simple relationship between the distance and the redshift of a galaxy – the further away a galaxy is, the greater its redshift. This is because the further light has had to travel, the more the travelling light is stretched, and this occurs when the Universe is expanding.

Nathalie Lees © HarperCollins

There is a vast amount of information contained within Hubble’s simple graph. Redshift can be expressed as the amount of stretching you would see if something were flying away from you at a particular speed. The ratio of the redshift expressed in this way to the distance to the galaxy – which is the gradient of the line on Hubble’s graph – is called the Hubble constant. Its value as measured today is 68 kilometres (42 miles) per second, per megaparsec. A megaparsec is a measure of distance commonly used by astronomers – 1 megaparsec is 3.3 million light years. So, another way to think of Hubble’s law is that a galaxy that is 3.3 million light years away will be receding from us at a velocity of about 70 kilometres (45 miles) per second. That’s pretty slow! A galaxy that is 6.6 million light years away will be receding at about 140 kilometres (90 miles) per second, and so on. And further, if you simply invert the Hubble constant, then you get a number with the units of time. For a Hubble constant of 70 kilometres (45 miles) per second per megaparsec, this corresponds to 14.3 billion years, which can be interpreted as the age of the Universe! (For the more mathematically inclined, you can calculate this number easily by converting megaparsecs to kilometres.) As an aside, the attentive reader might have noticed that our current best measurement for the age of the Universe is slightly lower than this, at 13.75 billion years; this is because precision measurements over the last few decades have shown us that the expansion of the Universe is not in strict accord with Hubble’s simple law. The best data we have today tells us that the Universe is accelerating in its expansion due to the presence of something called dark energy.

Hubble’s discovery of the cosmological redshift brought about another important discovery: we are living in an expanding universe.

This might seem complicated, but the conclusion is simple and profound. The reddening of the distant galaxies tells us that the Universe is expanding. This means that the galaxies we see in the sky today must have been closer together in the past. If, in your mind’s eye, you keep winding back time and you watch the galaxies getting closer and closer together, then, at a time given by the inverse of the Hubble constant, you will find that they must have all been on top of each other. In other words, the Universe we see today must have been incredibly tiny. This all happened around fourteen billion years ago, and that event is what we call the Big Bang. So Hubble’s remarkable observation is direct evidence that the Universe began with a big bang around fourteen billion years ago. All this was deduced in the 1920s simply by capturing the light from Cepheid variable stars and distant galaxies.

The Big Bang is difficult to visualise; it is easy to think of it as a vast explosion that flung matter out into a pre-existing void – a giant empty box, if you like – but this is completely wrong. The currently accepted picture is that all of space came into existence at the Big Bang. In fact, in the spirit of Einstein we should more correctly say that all of spacetime came into existence at the Big Bang. This means that the Big Bang didn’t just happen out there somewhere in the Universe, it happened everywhere at once. So the Big Bang happened in the bit of space between you and this book; it happened inside your head, across the road, at every point in the Solar System and inside the most distant galaxies. In other words, it happened at every point in the Universe. All of space was there at the Big Bang, and all it has done is stretch ever since. This has the rather mind-bending consequence that if the Universe is infinite today, it was born infinite. Everywhere that is here now was there then, but just squashed a lot! Nobody said cosmology was easy. So when we look at the distant galaxies and we see them all flying away from us, this is not because they were flung out in some massive explosion at the beginning of time; it is because space itself is stretching, and it’s been stretching since the Big Bang.

The Hubble expansion is one piece of evidence for the Big Bang, but there is another, perhaps more remarkable, fingerprint of the Universe’s violent beginning, delivered to us by the most ancient light in the cosmos