It is a crystal-clear night far away from the bright lights of a big city. A luminous full Moon is pulling itself free of the treetops. Against the velvet-black sky stars are winking like diamonds.
But the night sky is not all it seems …
The visible light our eyes see makes up only a vanishingly small portion of all the light that is streaming through the Universe. Raining down on the Earth from space is a ceaseless torrent of invisible ‘light’.
For most of human history we have been entirely blind to this light. But in recent years astronomers have opened up our eyes. New telescopes have been built which can see X-rays, infrared, radio waves and every other kind of invisible light. Now, for the first time, we can behold the greater glories of the Universe.
Imagine that you can see what the astronomers see simply by putting on a pair of ‘magic’ glasses. To ‘tune’ them to different types of light you need only twiddle a knob on the frame. No longer are you almost blind. Now you can have infrared eyes, radio eyes, eyes that see ultraviolet light, gamma rays or X-rays.1
What can you see with these impressively enhanced lenses?
At first, nothing appears to be changing. Then you realise that the Moon is fading. So, too, are most of the stars. Soon the Moon is hardly visible and the stars have begun to wink out one by one. But as the stars disappear new ones pop into view in places where no stars were visible before. Some of the new stars are shrouded in clouds of misty white.
This is the ultraviolet sky. Your glasses are registering the kind of invisible light that causes sunburn when you lie too long on a beach. Only the very hottest stars shine brightly with ultraviolet light.
Twiddle on.
The stars change again. Now there are no familiar signposts in the heavens. The intensely bright pinpricks that dot the sky mark places where stars are cannibalising other stars and where blisteringly hot gas is plunging headlong into black holes. Wherever matter is heated to hundreds of thousands of degrees it shines brightly with X-rays.
Keep twiddling.
Everything is fading now. We have come to gamma rays, the most energetic light in the Universe, created by the most violent events imaginable. Now the sky looks utterly black.
But there is a tiny, brilliant flash of light. You turn your head to stare. But there is nothing to see. The black sky is utterly empty. But if you were very patient indeed and watched the gamma-ray sky for a few days at a stretch you would see another brilliant flash from an entirely different part of the sky. And after a few more days you would see another. Astronomers call these ‘gamma-ray bursters’. They are the most powerful explosions in the Universe and we are seeing them at the very edge of the Universe. No one is completely sure what they are, but they may be the birth cries of black holes.
There is nothing more to see by tuning any further – except darkness and yet more darkness. Turn the knob back the other way, through the X-ray and ultraviolet skies to the familiar visible sky with its full Moon and familiar stars. But don’t stop. Keep going. Keep tuning.
You are now seeing infrared light. Instead of the Universe’s hot bodies, you are seeing relatively cool ones. Even human beings give out infrared. It’s the same kind of light earthquake-rescue teams use to detect people trapped beneath rubble.
The Moon has reappeared in the sky. But instead of shining brightly from reflected sunlight, it is glowing dully from its own meagre internal heat. The sky is full of unfamiliar stars. Cold stellar embers. There are bloated red giants in their death throes and stars so new that they are still swathed in the shimmering gas out of which they were formed.
But now you have left even the infrared sky behind. You are seeing microwaves, the same type of light used for radar and for heating food in the ubiquitous ovens. Now if our glasses are working, something very odd will begin to happen: the sky will light up. Not just a part of it – all of it.
The whole sky, from horizon to horizon, is glowing a uniform pearly white. You tune further into the microwave region but the sky simply gets brighter. The whole of space seems to be glowing. It is as though you are inside a giant light bulb. And what you are seeing is quite real. It is the relic of the Big Bang, the titanic fireball in which the Universe was born. Incredibly, it still permeates every pore of space 13.7 billion years after the event.
There is more energy tied up in this universal ‘cosmic microwave background’ than there is in the visible light of all the stars put together. In fact, the Big Bang radiation accounts for 99.9 per cent of all the particles of light streaming through the Universe at this moment.
Yet although the technology to detect microwaves was developed for radar during the Second World War, remarkably it was not until 1965 that anyone noticed this ‘afterglow of creation’. And even then it was noticed only by accident. The two astronomers who stumbled on it carried off the Nobel Prize for Physics despite not believing in the cosmic origin of what they had found for at least a year after their discovery, and despite initially mistaking it for the microwave glow of pigeon droppings.
The extraordinary story of the discovery of the relic radiation from the Big Bang forms the backbone of this book. With its tortuous twists and turns, accidents and missed opportunities, it provides a wonderful example of the way in which science is really carried out.
The cosmic microwave background is the oldest ‘fossil’ in creation. It has come to us directly from the Big Bang and has been travelling across space for 13.7 billion years. The cosmic microwave background was given out by matter cooling in the fireball, so it carries with it an imprint of the Universe as it was soon after the Big Bang. When you look at the microwave sky, you are seeing a snapshot of the Universe 13.7 billion years ago.
The early Universe must have been an extremely boring place, you think. After all, there is not a single feature anywhere in the microwave sky. However, the beauty of this featureless, uniform Universe is that it is a lot easier for scientists to understand than a complicated one. The smoothness of the cosmic microwave background is telling us that matter in the early Universe must have also been spread amazingly smoothly throughout space. And herein lies a great puzzle. Today’s Universe is anything but smooth. In fact, the Universe is full of stars, and the stars are grouped together into galaxies, and these galaxies are in turn linked into great chains and clusters that snake their way across space. And between these groupings of galaxies are great voids of utterly empty space. Far from being smooth, the luminous material in today’s Universe has the appearance of Swiss cheese.
So how did such an uneven and complicated universe arise from such a smooth and simple beginning?
Clearly, at some point the stuff of the Universe must have begun to clump together, like milk curdling. So, although the cosmic microwave background looks remarkably smooth, it cannot be dead smooth. If we look closely at it, we ought to be able to see signs of the first structures in the Universe beginning to clump together under gravity soon after the Big Bang.
For more than 25 years after the discovery of the cosmic microwave background astronomers peered at it closely. But, try as they might, they were unable to find any variation in the brightness of the microwave background.2 There were no signs of the lumps of matter which would later form galaxies like our own Milky Way. The evidence of the cosmic microwave background seemed to be contradicting one of our most cherished ideas: that we and our world exist!
In 1989, NASA launched an obscure satellite called COBE (pronounced ‘co-bee’) into an orbit just above the Earth to study the fireball radiation. Previously, this had been difficult because the Earth’s atmosphere glows brightly with microwaves.3 COBE’s sensitive instruments listened carefully for the faint whisper of the cosmic explosion which started the Universe’s expansion 13.7 billion years ago. For more than two years the satellite found nothing. There were jittery mutterings among scientists.
But, in April 1992, COBE hit the jackpot. It found ‘ripples in the cosmic background radiation’. In some parts of the sky the cosmic microwave background was ever so slightly brighter than in others. It was a tiny effect. The ‘hot spots’ in the sky were only a few parts in 100,000 hotter than the ‘cold spots’, but the outpouring of relief among scientists was unprecedented. ‘It’s like seeing the face of God,’ declared one of the scientists on the COBE team. ‘It’s the discovery of the century, if not of all time,’ declared the physicist Stephen Hawking.
Many thought these remarks a little extravagant, but the fact remained that COBE had found the ‘seeds’ of galaxies in the early Universe. Those regions that were slightly denser than others would grow and grow as the Universe expanded in the aftermath of the Big Bang, getting bigger as their gravity pulled in more and more matter. They would eventually become the clusters and superclusters of galaxies we see around us today. COBE had not quite seen the face of God but it had seen the largest and oldest structures in the Universe.
At the time of the discovery the world’s media went wild. The story was splashed across TV screens and the front pages of newspapers all over the planet. It is probably true that no other scientific story has received such blanket coverage in the media.
Why so many people lost their heads over such an obscure and esoteric story is a bizarre tale in itself, and one that I tell later in this book. But before you can understand what all the fuss was about, you need to know a little background to the cosmic background. In particular, you need to know about the Big Bang.
The story begins in the first decades of the twentieth century, when a new generation of giant telescopes allowed astronomers to probe the remote depths of space and discover for the first time just what kind of Universe we were living in …
1. Strictly speaking, you would have to go into space to use your magic glasses because most invisible light is absorbed by the atmosphere. But don’t let that worry you. This is only a story.
2. This is not strictly true. In the late 1970s, astronomers discovered that the microwave background is slightly hotter in the direction the Earth is moving in space and slightly colder behind us. But this is due to our motion through the microwave background and is not inherent in the background radiation itself.
3. In fact, the ground glows with microwaves, as do buildings, trees, people and even clouds of hydrogen gas floating in space. These competing sources of microwaves make the uniform glow of the fireball radiation a little more difficult to spot than I have led you to believe. They explain why detecting the fireball radiation is a challenge and why it was not discovered until 1965.