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
THE BIRTH OF THE UNIVERSE
Every second, light from the beginning of time is raining down on Earth’s surface in a ceaseless torrent. Only a fraction of the light present in the Universe is visible to the naked eye, though; if we could see all of it, the sky would be ablaze with this primordial light both day and night. However, some of this hidden light is not quite a featureless glow; the long wavelength universal glow known as the Cosmic Microwave Background (CMB) in fact displays minute variations in its wavelength. The CMB carries with it an image of our universe as it was just after its birth, and this discovery has provided key evidence that the beginning really did start with the Big Bang.
It was at the Big Bang that all of spacetime came into existence. The stars and galaxies stretched away across an infinite universe and many are still to be found today. Space is stretching still; housing the old galaxies alongside numerous new star-forming regions, such as NGC 281 k.
NASA
VISIBLE LIGHT
Standing among the dunes of the Namib Desert you become aware of the sheer scale of the landscape. It is a landscape sculpted by the Sun and coloured by it at all times.
Stretching along the west coast of southern Africa is the Namib Desert. It is the oldest desert in the world; its landscape is a shifting sea of sand of over 77,700 square kilometres (30,000 square miles) which changes every minute, a consistently arid wilderness that has stubbornly avoided moisture for over fifty million years. This is a world sculpted by the Sun; its energy drives the wind that shapes the tiny grains of sand into magnificent dunes, and the colours hidden in its light paint the landscape deep orange. Yet even when the Sun has set, the desert remains awash with light and colour, but the human eye can’t see it.
Visible light is a tiny fraction of the light in the Universe. Beyond the red, the electromagnetic spectrum extends to wavelengths too long for our eyes to detect. It’s still light; still the sloshing back and forth of the electric and magnetic fields driving forwards through the void at the special universal speed, it’s just we didn’t evolve to see it. In the Namib Desert you can feel this light, though, if you hold your palms towards the sand. The dunes are warm long after sunset, and this residual heat is nothing more than long-wavelength light. A scientist would call it infrared light; the only difference between infrared and visible light is the wavelength – infrared has a longer wavelength than visible light. Travel further along the spectrum, past infrared, and we arrive at microwaves, with wavelengths unsurprisingly about the size of a microwave oven. The spectrum then seamlessly slides into the radio region, with wavelengths the size of mountains.
Throughout most of human history we have been blind to these more unfamiliar forms of light, but to detect them you don’t need expensive, hi-tech kit, just a radio. When tuning a radio you are not tuning into a sound wave, you are picking up information encoded in a wave of light. Most of the radio waves we are familiar with are artificially created and used for communication and broadcasting, but just as there is plenty of visible light in the Universe that isn’t manmade, so there are naturally occurring microwaves and radio waves too. And just like the visible photons from the most distant galaxies, the microwave and radio photons are messengers, carrying detailed information about distant places and times across the Universe and into our technologically created artificial eyes.
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Next time you are tuning a radio and can hear static, you are actually listening to a deeply profound sound – you are listening to the Big Bang.
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Next time you tune a radio, listen to the static between the stations. About 1 per cent of this is music to the ears of a physicist because it is stretched light that has travelled from the beginning of time. Deep in the static is the echo of the Big Bang. These radio waves were once visible light, but light that originated 400,000 years after the Big Bang. Prior to that, the observable universe was far smaller and hotter than it is today. At 273 million degrees Celsius, this is an order of magnitude hotter than the centre of a star, so hot that the hydrogen and helium nuclei then present in the Universe couldn’t hold onto their electrons to form atoms. The Universe was a super-heated ball of naked atomic nuclei and electrons known as a plasma. Light cannot travel far in dense plasma because it bounces off the electrically charged subatomic particles. It was only when the Universe had expanded and cooled down enough for the electrons to combine with the hydrogen and helium nuclei to form atoms that light was free to roam. This point in the evolution of the Universe, known as recombination, occurred around 400,000 years after the Big Bang, when the Universe had cooled to about 3,000 degrees Celsius and was around a thousandth of its present size. That is close to the surface temperature of red giant stars, so the whole Universe would have been glowing with visible light like a vast star. The Universe has become cooler and more diffuse since, so this ancient light has been free to fly through space, and it is some of these wandering messengers that we collect with a detuned radio today. However, as the Universe has expanded, space has stretched and so too has the light – so much so that the light is no longer in the visible part of the spectrum. It has moved beyond even the infrared, and is now visible to us only in the microwave and radio parts of the spectrum. This faint, long, wavelength universal glow is known as the Cosmic Microwave Background, or CMB, and its discovery in 1964 by Arno Penzias and Robert Wilson was key evidence in proving that the Universe began in a Big Bang
Forget state-of-the-art kit, all you need to use to detect hidden forms of light is a simple radio. As you tune, it you will pick up information encoded in a wave of light.
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sp; Only a fraction of light is visible in the Universe. This infrared image shows the massive scale of the Universe and demonstrates how the electromagnetic spectrum extends to wavelengths that are too long for our eyes to detect. Here we can see hundreds of thousands of stars at the core of the Milky Way Galaxy, but so many are still hidden from our view.
NASA
PICTURING THE PAST
On 30 June 2001, the Wilkinson Microwave Anisotropy Probe, known as WMAP, was launched from the Kennedy Space Center in Florida. This highly specialised telescope was built with a single purpose: to capture the faint glow of the CMB and create the earliest possible photograph of the Universe. After nine years of service, WMAP has recently been retired, but its photograph is still the object of frenzied research because it contains so much rich detail about the early Universe and its expansion and evolution ever since.
This much-studied image is probably the most important picture of the sky ever taken. It may not look like much; it doesn’t have the beauty of a spiral galaxy or nebula, but to a scientist it is the most beautiful picture ever taken because it contains a vast amount of information about the history of our Universe.
The raw image from WMAP shows the glow of our Milky Way Galaxy as it creates a hot bright band across the sky, but once this detail and other observational side-effects are removed, we are left with this simplified, but equally important and informative, picture below. This photograph of the night sky documents in extraordinary detail the structure of our universe at the time of recombination. Over the nine years in which WMAP was in service, the detail of this image has been repeatedly refined, which in turn reveals more and more detailed information encoded in the primordial light.
The WMAP data is presented as a temperature map of the sky. The wavelength of the detected light at any particular point corresponds to a temperature; shorter wavelengths are higher temperatures, longer wavelengths are lower ones. The red areas are hotter than the blue, but only by around 0.0002 degrees. The average temperature of the CMB is 2.725 degrees above absolute zero. On the Kelvin temperature scale, that’s 2.725 K, or -270.425 Celsius.
Despite being incredibly tiny, these temperature differences are of overwhelming importance because they tell us that in the very first moments of our universe’s life there were regions of space that were slightly denser than others. These virtually imperceptible differences might not seem much, but without them we would not exist. That’s because these little blips in the CMB are the seeds of the galaxies. The red spots in the CMB correspond to parts of the Universe that were on average around half a per cent denser than the surrounding areas at the time of recombination. As the whole Universe expanded, these areas would have expanded slightly more slowly than their surroundings because of their higher density – effectively, their increased gravity due to their higher density would have slowed the expansion, causing their density to increase further relative to the space around them. By the time the Universe was one-fifth of its present size, just over a billion years after the Big Bang, these regions would have been twice as dense as their surroundings. By this time the matter in these regions was dense enough and cool enough to begin to collapse under its own gravity, leading to the first star formation and the emergence of the cores of the galaxies, including our own Milky Way. This is the cosmic epoch we see in the most redshifted Hubble Space Telescope data – the formation of the first galaxies – and their seeds are the minute fluctuations visible in the Cosmic Microwave Background Radiation.
This detailed picture of the Universe in its infancy was pieced together from data collected over several years by the Wilkinson Microwave Anisotropy Probe (WMAP). The different colours reveal the 13.7-billion-year-old temperature fluctuations that correspond to the seeds from which the galaxies grew.
NASA
As the Universe expanded, the denser areas within it expanded more slowly than others because of their increased gravity. By the time these areas were twice as dense as their surroundings, the matter within them was sufficiently cool and dense to collapse under their own gravity and form the first stars and cores of new galaxies.
The rest, as they say, is history. Across the cosmos, countless suns began to switch on and the fill the Universe with light. For billions of years, generations of stars lived and died until, 9 billion years after it all began, in an unremarkable piece of space known as the Orion Spur off the Perseus Arm of a galaxy called the Milky Way, a star was born that became known as the Sun. This is the story of how our solar system has its ultimate origin in those dense areas of space that appeared in the first moments of our Universe’s life. But what is the origin of those tiny fluctuations in density that we see in the CMB?
This is perhaps the most remarkable piece of physics of all. The most popular current model for the very very early Universe is known as inflation. The idea is that around 10– 36 seconds after the Big Bang, the Universe went through an astonishingly rapid phase of expansion in which it increased in volume by a factor of around 1078! In less scientific notation, that’s a million million million million million millionths of a second after the Big Bang, and an increase in volume by a factor of million billion billion billion billion billion billion billion billion billion. This was all over by 10–32 seconds or so. Before inflation, the part of the Universe we now observe, all the hundreds of billions of galaxies in our night sky, would have been far, far smaller than a single subatomic particle. At these minute distance scales, quantum mechanics reigns supreme, and tiny quantum fluctuations before inflation would have been magnified by the rapid expansion to form the denser regions we observe in the Cosmic Microwave Background spectrum. If inflationary theory is correct, the CMB is therefore a window onto a time in the life of the Universe far earlier than 400,000 years after the Big Bang. We are seeing the imprint of events that happened in the truly mind-blowing first million million million million million millionths of a second after it all began. I find this the most astonishing idea in all of science. From a vantage point of 13.7 billion years, little beings like you and me scurrying around on the surface of a rock on the edge of one of the galaxies are able to understand the evolution of the Universe and speculate intelligently about the very beginning of time itself, just by decoding the messages carried to us across the cosmos on beams of light. The power of science is quite genuinely daunting, the richness of its stories unparalleled, the cosmos it reveals, beautiful beyond imagination.
There is one last twist to this story. Throughout our journey, light has been the messenger, carrying stories of far-flung places and the distant past to our shores. But there is evidence from one of the ancient sites on our home planet that light may have played a far more active role in our history than mere muse
The Burgess Shale is one of the most important and exciting fossil sites in the world, where a staggering amount of diverse animals are to be found, dating back over 500 million years.
FIRST SIGHT
Hidden in the high Rocky Mountains in British Columbia, Canada, is one of the most important and evocative scientific sites on Earth, and it’s where the story of light and our lives begins. Around 505 million years ago, when this whole area lay deep beneath the surface of a primordial ocean, it was hit by a huge mudflow. The mud buried everything in its path and created a snapshot of a remarkable time in the evolution of life on Earth. A whole ancient ecosystem was frozen and preserved intact in the mud; the lives of the primitive creatures documented by a chance geological event with the care and precision with which the Egyptians created their glorious tombs half a billion years later. For hundreds of millions of years, this ancient treasure trove was locked away, but in 1909 it was uncovered high on a mountainside. This is the Burgess Shale.
The Burgess Shale is one of the most important fossil sites in the world. It is not just the number and diversity of the animals found here, it’s their immense age. Before around 540 million years ago, there are no fossils of complex life forms found anywhere on the surface of Earth. We know that
there was life before this period, but the animals were very simple creatures that didn’t possess skeletons of any kind. This means that they don’t show up on the fossil record. In the geological blink of an eye in the period of time immortalised in the Burgess Shale, known as the Cambrian Era, it appears that a vast range of complex multi-cellular life emerged on the planet. Biologists call it the Evolutionary Big Bang, or the Cambrian Explosion.
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One current theory for the origin of the Evolutionary Big Bang is that the emergence of the eye in animals such as the trilobite triggered the Cambrian Explosion. Once one predatory species develops eyes, there is a powerful selection mechanism in favour of others developing and refining eyes too.
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Numerous genera of trilobites have been found in the Burgess Shale. These fossils are so detailed and well preserved that they have enabled scientists to make important observations about the structure and behaviour of these now-extinct organisms.
So what triggered the evolution of complex life? There is a clue in these fossil beds that lie high in the Canadian Rocky Mountains. The picture left shows one of the ancient animals found here; a complex organism called a trilobite. Trilobites, now long extinct, had external skeletons and jointed limbs, but most strikingly they had complex, compound eyes. These prehistoric predators could see shapes, detect movement and use their eyes very effectively to chase their prey. The ability to see made these trilobites very successful animals indeed; in fact they survived for a quarter of a billion years, only vanishing from Earth in the Permian mass extinction 250 million years ago.
Wonders of the Universe Page 7
