From ancient constellations, to strange new worlds on the edge of detection.
There’s a star almost directly over the North Pole. It is known prosaically as the Pole Star. Its more sophisticated friends call it Polaris. This position, right over the axis of the Earth’s rotation, is a very handy place for a star to be. If you can locate Polaris in the night sky, you instantly know which way is north. It’s an ancient cornerstone of navigation, and is still useful for those lost in the hills without a GPS signal.
There’s a myth that Polaris is the brightest star in the night sky. I’m not a 100 per cent certain where this hearsay comes from. I suspect that the privileged position, right over the North Pole, coupled with its usefulness for navigation, has given Polaris ideas above its station. It is moderately bright, but could never pretend to be the fulgent kingpin. Of all the stars visible from our planet, it barely scrapes into the top 50. Oh, and it’s not one star but three – though this can’t be discerned with the naked eye.
Furthermore, the exalted position of Polaris is only temporary. Over millennia, the Earth’s axis of rotation slowly shifts in a process known as precession. This means that the area of sky directly above the North Pole changes with time. Between the 4th and 2nd millennium BCE, for example, the faint star Thuban was closer to the pole. In a few centuries, Polaris will be nowhere near the axis, and we will either change or laugh at its inappropriate name. Shakespeare had Julius Caesar describe himself as ‘constant as the northern star’. We now know that there was no star to the north in Caesar’s day.
The heavens are full of myth and mythology. Stargazers in the Northern hemisphere are used to seeking out the belt of Orion or the prominent ‘W’ of Cassiopeia. Hercules is up there, and so too is Andromeda. We can also find constellations that match the 12 ‘star signs’ familiar from horoscopes. Leo, Sagittarius, Cancer, Aquarius* … they’re all there, if you know where to look.
Human brains have an unshakeable knack of finding patterns, even when none truly exist. So it is with the constellations. Since way before recorded history, our ancestors must have looked at the stars and seen lions, hunters, birds, bison and the greatest of rivers in the form of the Milky Way. It’s impossible to quantify just how important those patterns have been in shaping human cultures and traditions around the world.
The star patterns have shifted only minutely since human minds first wove them into stories. It would be easy to assume that the constellations were first defined many centuries in the past, and have stayed the same ever since. But this is not so. Of the 88 constellations in our skies, almost half were named after the time of Shakespeare.
But let’s back up a bit. What exactly do we mean by a constellation anyway? Traditionally, a constellation is a group of prominent stars that remind us of some object or person. A zig-zag of stars might resemble a serpent, while a strong V-shape could suggest the head of a bull. We can all make up our own constellations in the same way we’ve all looked for shapes in the clouds. To properly count as a constellation, though, there really needs to be some kind of consensus and agreement.
Many of the constellations we recognise today have been in use for thousands of years. Astronomers of the classical period attempted to bring it all together into a definitive description of the heavens. Men like Eudoxus of Cnidus (c.390–337BCE) and Hipparchus (c.190–120BCE) are known to have recorded the names and positions of the stars. Much of their work is lost, and so they are largely forgotten. It falls to Ptolemy (c.100-168CE), whose writings have survived, to act as our early guide to the constellations.
Ptolemy’s Almagest (c.150CE) concerned itself chiefly with the movement of the stars and planets. It was a stupendously influential book, even though its core thesis is utterly wrong. Ptolemy put the Earth as the centre of the cosmos. Everything else, including the Moon, Sun, planets and stars, rotated around the Earth in fixed spheres. This model held sway for over 1,000 years. It is often called the Ptolemaic system, even though Ptolemy was merely building on the mainstream astronomy of his time.
More importantly for present purposes, the Almagest also contains a catalogue of 1,022 stars and their respective groupings – what we would now call constellations. Here we find the 12 signs of the zodiac as well as familiar names like Orion, the Great Bear (Ursa Major) and Cassiopeia. In all, Ptolemy listed 48 constellations, many of which can still be found in modern star charts. He wasn’t the first to describe or name these patterns in the sky, but it is his work that has survived and held influence, so it is Ptolemy who is remembered.
Ptolemy was a pretty thorough guy, meticulously recording the knowledge of his times. Even so there is a huge, half-planet-sized gap in the Almagest. Ptolemy and his peers never ventured much further south than the Mediterranean. Nor did the Babylonians, Sumerians, Greeks or Romans – all were confined to a fairly narrow band of latitude. Astronomers from these ancient civilizations dwelt beneath the same firmament and enumerated the same stars. Even Chinese and Indian scholars did not stray below the equator. Had Ptolemy somehow reached Australia, he would have found the skies to be as alien as the wallabies and numbats.
Of course, many of the lands of the Southern Hemisphere were inhabited in Ptolemy’s day. The aboriginal Australians, African tribes and early settlers of South America all looked to the stars and formed their own patterns, passed on by oral tradition. Polynesian sailors were so attuned to the stars that they could navigate great distances between islands without difficulty. European explorers eventually made it to these antipodean lands, and imposed their own ideas onto the heavens. The southern constellations were defined between 1592 and 1763 by a succession of astronomers. The most prolific was Frenchman Nicolas-Louis de Lacaille (1713–62), who named 14 star patterns that are still part of the official catalogue.
These newer constellations are wider in theme than their ancient, northern counterparts. Some are inspired by exotic creatures that would have astounded Ptolemy: Tucana the toucan, or Camelopardalis the giraffe, for example. Lacaille introduced several maritime constellations, such as the Octant, the Telescope and even the Poop Deck. Mensa, adapted from Lacaille’s original coinage of Mons Mensae, is an homage to Table Mountain in South Africa – it is the only constellation to represent a geographical feature on the Earth. There’s even a constellation called Norma – Latin for ‘right angle’, and nothing to do with Marilyn Monroe. These and other names did not immediately catch on, and had to compete with rival suggestions.
In 1922, the heavens were finally united. The International Astronomical Union drew up a definitive map of the constellations. They brought together the Ptolemaic constellations of old with the recently coined star groups of the southern skies, and tidied up some of the more ambiguous corners of the star charts. The result is a cosmos with 88 official constellations. Here, ‘constellation’ has a much broader, and yet more precise, meaning. The constellation of Leo, for example, was traditionally understood to be a group of bright stars that look a bit like a lion. Since the 1922 agreement, Leo is the patch of sky that includes these stars, and anything else that happens to fall in that area no matter how faint or far away. In effect, the constellation of Leo now contains billions of stars, galaxies and dust clouds, most of which have never been viewed by human eye. Every star now belongs to one, and only one constellation.
Incidentally, some of the most famous star groupings* are not technically constellations. We’re all familiar with the Plough (known as the Big Dipper in North America, just to re-emphasize that not all star groups were named by the ancients). These seven stars are not among the 88 official constellations. Rather, they form the hindquarters of the Great Bear or Ursa Major. Such unofficial groupings are known as asterisms. Another example is the Northern Cross, which is the most prominent part of the constellation of Cygnus.
* FOOTNOTE There are actually 13 constellations in the Sun’s path. The ancient Babylonians, who first devised the zodiac system, chose to leave out Ophiuchus (‘the snake grasper’) so that the remaining 12 constellations would match up with their 12-month calendar. This is a pity. The naughty symbolism of a man grasping a snake might have substantially spiced up our modern-day horoscopes.
* FOOTNOTE To be clear, the patterns we see in the skies are only illusions. Were we somehow to travel a few light years away from Earth, the constellations would begin to shift and eventually break down. Take Orion, for example. The stars here form a pattern that resembles an hourglass or human torso. Crude drawings of Orion have been found scratched into ivory that dates back at least 32,000 years, suggesting that this group of stars has always been connected in the human mind. In reality, they’re nowhere near one another. Of the seven prominent stars in Orion, the closest, and most daintily named, is Bellatrix. It hangs some 243 light years from Earth. The farthest, by contrast, is Alnilam, 1,359 light years away. In other words, Bellatrix is closer to our sun than it is to Alnilam in its own constellation. And so it is with other star groupings.
The words ‘astronomer’ and ‘telescope’ are as intimately linked as ‘surgeon’ and ‘scalpel’, or ‘firefighter’ and ‘hose’. Do an image search for astronomer and see how many of the top results feature a man – and occasionally a woman – peering through a tube at the heavens. Is astronomy really like that?
In a word, no. The traditional telescope is next to useless for professional astronomy. Even the largest and best optical telescopes are only as good as the human retina squinting through the eyepiece. Modern astronomy demands the use of sophisticated sensors and recording equipment so that data can be captured and shared.
Astronomers do still use telescopes. It’s just that they tend to look very different from the popular notion of what a telescope should be. Today’s instruments often capture light that the human eye cannot see, such as infrared and radio waves. The latter are best collected through a large dish – a radio telescope – which bears little resemblance to the traditional optical telescope. The most powerful astronomy of all is performed with space-based telescopes. Up there, observations can be made in the UV and X-ray end of the spectrum, radiation that does not penetrate the atmosphere.
A recent breakthrough in astronomy is the detection of gravitational waves, first announced in 2016, exactly 100 years after Einstein had predicted their existence. This achievement is already seen as a major landmark in astronomy. It promises fresh insights into the Universe that we can’t get from conventional telescopes. The proof-of-concept was made at a facility in the USA known as the Laser Interferometer Gravitational-Wave Observatory (LIGO). This couldn’t be more dissimilar from a traditional telescope if it coated itself in hummus and started tap dancing.
It doesn’t even look like an observatory, never mind a telescope. For starters, LIGO operates simultaneously from two locations in Washington state and Louisiana, so that any local background vibration can be ruled out by checking against the twin. Each LIGO site consists of a slender concrete ‘L’ large enough to enclose a small town. The vacuum tubes within these concrete arms serve as the detectors. They do not focus radiation, as per a traditional telescope, but ‘feel’ the effects of any gravitational wobbles. Those effects are tiny. LIGO can pick up on a distortion less than one-thousandth the diameter of a proton. It would be technically inept to suggest that LIGO is a telescope – but it is an example of the kind of cutting-edge tool used by astronomers today.
Professional astronomers rarely turn to traditional telescopes, but they do still make use of visible light. The Hubble Space Telescope, for example, captures images at these frequencies. But even then, astronomers don’t get to linger over the hardware. Hubble is about six times oversubscribed – despite operating 24 hours a day. Clever software works out the best schedules to bunch together different observations like a cosmic version of the travelling salesman problem. Even so, astronomers spend much more time writing proposals or analyzing previously collected data than they do with the telescope itself.
Now to contradict everything I just said. The traditional telescope does still hold a very important place among the ranks of amateur astronomers. It is a popular hobby. Estimates range from 200,000 to 500,000 enthusiasts in the USA alone, depending on how you define ‘amateur astronomer’. They vastly outnumber professional astronomers. Most of these enthusiasts peer through their telescopes for the simple pleasure of studying the heavens. Some, though, have made important discoveries – supernovae, comets and asteroids – that have eluded the professional community. Which leads us nicely on to our next topic …
In August 2007 a new class of object was discovered in the constellation of Leo Minor. This swirling green blob seemed to be associated with a nearby spiral galaxy, as though a plume of material had been thrown out into deep space. It glows strongly, yet contains few stars. Astronomers think that the glow is an ‘echo’ from a superbright light source, called a quasar, in the nearby galaxy. This has since dimmed, but its effects live on in the radiation from the dust. It is a remarkable object. Still more remarkable is that its discoverer was a Dutch schoolteacher without a telescope.
The discovery came via a web interface called Galaxy Zoo. This ground-breaking project was set up in July 2007 to help astronomers with a most frustrating problem. Modern telescopes are able to record images of millions of distant galaxies. So many, in fact, that the challenge is now one of interpretation. How do you make sense of all the data?
The obvious thing to do would be to throw computer power at the problem. Computers are ideally suited to scanning and categorizing information. They break into a digital sweat, though, when asked to pull meaning out of images. Show a computer a photo of a distant galaxy, and it would struggle to decide whether that galaxy is spiral, elliptical or somewhere in-between. At least, it would have done back in 2007, when image-recognition software was not as sophisticated as today.
The solution was to marshal the curiosity of the ‘general public’. Anyone can log onto the Galaxy Zoo website and try their hand – after a brief tutorial – at classifying galaxies. The results were astonishing. In the first year alone, some 50 million classifications had been received. Accuracy was higher than an algorithm could achieve. Many of the galaxies were classified by dozens of people, adding to the reliability.
Hanny van Arkel was one of the first volunteers at Galaxy Zoo. Like most people on the site, she had no formal training in astronomy, just an innate curiosity about the Universe. Even so, she noticed a peculiar blob on one of her images that didn’t look like anything she’d seen before. She pointed out the mysterious smear in Galaxy Zoo’s discussion forum and soon had the attention of professional astronomers. It turned out to be a cloud of glowing dust, as described at the start of this section. ‘Hanny’s object’ has been translated into the far more mellifluous Dutch equivalent and is now officially known as Hanny’s Voorwerp. Other voorwerps have since been spotted, but Hanny holds the distinction of finding the first, with nothing more exotic than a public website and the power of the human brain.
Galaxy Zoo then moved on to other tasks. Users were asked to describe the size and shape of the brightest galaxies in more detail – for example, to count the number of spiral arms or measure the central bulge. A third project pushed deeper out into space, and therefore further back in time, to see if galaxies in the remote past differ much from our neighbouring galaxies. And so the work goes on, with ever-more data sets covering fresh areas of sky, and at better resolution. It is a hugely successful project that has spawned dozens of scientific papers and expanded our knowledge of galactic appearance. Almost all of the key work was done by amateurs, with no specialist equipment and little training.
Galaxy Zoo is one of the largest and best known examples of ‘citizen science’, and has itself inspired many further projects through the spin-off Zooniverse website. You can now hunt for planets around other stars, map our own galaxy, sift for gravitational waves or make sense of the surface of Mars – not to mention projects in other scientific disciplines.
Galaxy Zoo follows a long tradition in astronomy, which has always welcomed the contributions of amateurs. This makes sense. The Universe is so terrifically gigantic that even modern computers and telescopes cannot appraise it all. The right person, looking at the right piece of sky at just the right moment can make a telling difference.
The tradition goes all the way back to William Herschel (1738–1822) who discovered the planet Uranus from his back garden in Bath, England, in 1781. He was a music director, pursuing astronomy only as a hobby. It would later become his main profession. The famous Hale–Bopp comet, which caused much excitement in the mid-1990s, was independently discovered by two amateur astronomers: Alan Hale (b.1958) and Thomas Bopp (b.1949). Bopp spotted the comet through a borrowed, home-made telescope. Far from a professional astronomer, he was working in a factory for construction materials at the time.
One of the most celebrated skywatchers is the Australian astronomer and Christian minister Robert Evans (b.1937). In a long life of stargazing, Evans has memorized the skies like no one else alive. He has the rare ability to look at a galaxy (through a telescope) and immediately notice if something has changed. He uses this skill to spot exploding stars – supernovae that burn bright, but briefly. Evans has bagged 42 supernovae, far more than anybody else – although automated telescopes are now at a comparable level.
Notwithstanding the lyrics to Don McLean’s ‘Vincent’ – in which the starry, starry night is painted blue and grey – most of us would agree that space is black. It looks black from Earth; it looks black from space; it looks black from the Moon. It is, in most meaningful ways, black.
This raises an interesting question. If the Universe is infinite then why don’t we see a firmament that is awash with starlight? Why is there so much blackness when every piece of sky, no matter how minute, must eventually point out to a star or galaxy?
This is called Olbers’ paradox, after Wilhelm Olbers (1758–1840), the 19th-century astronomer who popularized this conundrum. It wasn’t resolved until midway through the 20th century. Part of the reason lies in Olbers’ assumptions. He thought that the Universe was ageless and infinite. We now know it had an origin in the Big Bang 13.8 billion years ago. It is also of finite size, meaning that the number of light sources is also finite – not enough to fill the sky. And then stars and galaxies have lifespans. They do not stick around forever but come and go over the aeons.
In another sense, Olbers’ instinct was correct. Billions of years ago, about 400,000 years after the Big Bang, the Universe really was full of light in every direction. This was a time before the cosmos had cooled sufficiently to form atoms and molecules. Everywhere was bathed in the light of a hot, orange plasma – not that any eyeball could have existed to see it. As the Universe expanded further, this plasma became more tenuous and cool, allowing the formation of regular matter and the temperate existence we know today. Black was the new orange.
That ‘orange everywhere’ universe is long in the past, but we can still get a glimpse of it Remember, the deeper you look into space, the further back in time you see. Look really, really deep and you will get back to the days of orange plasma. It doesn’t look orange to us now. The light has become distorted, and stretched out by a phenomenon known as ‘red shift’. It has passed beyond the range of visible light but it can be picked up as microwave radiation.
This is known as the cosmic microwave background or, more romantically, the ‘afterglow of creation’. We cannot see the Big Bang itself, but we can observe and even map its aftermath. It is one of the greatest discoveries of all time but remains relatively unknown, since it requires at least four challenging paragraphs to explain.
If we look through other filters – infrared, ultraviolet, gamma rays, radio waves – we see still other sources of radiation. The light we pick up with our eyes – visible light – is but a small piece of the electromagnetic spectrum. The night sky is only black, then, to our limited vision. With reconfigured eyeballs, the night sky would be radiant.
There’s yet another sense in which space is not black. In 2002, a team of US astronomers performed a curious experiment. What would happen, they asked, if you took all of the light from nearby galaxies and mixed it all together, like an infant let loose with a paintbox? Using data from a survey of 200,000 galaxies, they did just that. The computer spat out an answer that captured the imagination of news editors everywhere: when you take the mean gleam of everything, the Universe is beige.
The precise shade, given in the RGB colour model, is (255, 248, 231). Various names have been put forward for this empyrean hue, with Skyvory, Univeige and Big Bang Buff among the leading contenders. The winner, in as much as it has its own Wikipedia page, is Cosmic Latte.
In space, nobody can hear you scream. The Alien movies have to be applauded for reversing one of the commonest film bloopers and turning it into a tagline. Indeed, how many films have you seen where spaceships zoom loudly past the camera, or fire their laser cannons with a pszew, pszew, pszew? The truth is that space is a quiet place. Sound can only propagate if it has some kind of substance – water, air, metal – to pass through. Sound is simply our perception of vibrating molecules. When molecules are all but absent – as in space – you couldn’t hear a 21-gun salute, never mind a scream.
I’ll admit it. This is a pretty robust truth that’s nearly impossible to argue with. You could hover close to a nuclear explosion in space and not hear a peep – although you’d probably have bigger things running through your mind, like various lethal types of radiation. That said, with a little stretching, we can find ways to perceive sounds in space.
Stretching is the right word. While we don’t encounter the more familiar type of sound wave in space, we can ‘hear’ something analogous from the stretching of space itself. As discussed elsewhere in the book, 2016 saw the first reports of gravitational waves – ripples in spacetime that had been predicted by Einstein a century before. Gravitational waves are caused when stupendous masses accelerate – an example would be two black holes orbiting one another, then spiralling inward to coalesce. The gravitational ripples sent out by such events are analogous (and I can’t stress that word enough) to sound waves. They have amplitude and frequency like any other wave. It is relatively simple to convert them into sound waves. And just what is the sound of two supermassive black holes colliding? Very much like a plop of water into a bucket – the ultimate example of bathos.
By converting other types of wave into sound, we can listen in on all kinds of objects throughout the Universe. Radio telescopes, for example, pick up naturally generated radio waves from stars and planets. The signal can be converted into sound waves, just as in your car stereo. We can then ‘hear’ the sounds of outer space (and, again, this is not true sound, but a close analogy). The Sun gives out a hiss. Jupiter sounds like someone walking over bags of cornflakes. Pull the same trick with another part of the electromagnetic spectrum and we can get the ‘sound’ of the cosmic microwave background, which we met in the previous section (see here). The ‘oldest sound you’ll ever hear’ is nondescript, rather like the background murmur in a shopping centre. It is perhaps appropriate for a Cosmic Latte universe.
There’s plenty of stuff in space: planets, stars, comets, moons, spacecraft, discarded bags of poop (see here). Between them is a void, an emptiness, an expanse of nothingness. In short, a vacuum.
It’s true that outer space lacks atmosphere. But it’s not a complete vacuum. No vacuum is ever truly perfect. Look close enough at any area of ‘empty space’ round the Earth and you’ll find energetic particles from the Sun, a small scattering of hydrogen (about five atoms per cu. m/35⅓ cu. ft), helium that has escaped Earth’s atmosphere, and the occasional particle of dust. Get away from the Solar System, and the vacuum may be tidier, but even here we find atomic and subatomic litter.
Light (in all its flavours) propagates through space. It is usually thought of as a wave, but can also be interpreted as a stream of particles known as photons. Likewise, other tiny entities such as neutrinos and cosmic rays permeate the cosmos. The so-called vacuum is also teeming with ‘virtual particles’, fleeting objects that pop in and out of existence at the smallest, quantum scales. A particle and its antiparticle spontaneously appear, then immediately annihilate one another. It happens all the time, everywhere, and adds further clutter to our increasingly busy vacuum.
Where should we go to find the best vacuum? It would make sense to travel as far from any star or other source of radiation and particles as possible. And yet the emptiest space in our solar system is right here on Earth. The beam vacuum at the Large Hadron Collider holds that record. By some accounts, it reaches conditions about 100 times more rarified than the space above the Moon.
By the same token, space is often considered to be an inhospitably cold place – as chilly as it gets. In truth, the temperature is highly variable. In Earth orbit, conditions swing from around -100ºC (-148ºF) in the shade to 260ºC (500ºF) in direct sunlight. Heat is more of a problem than cold for astronauts. Spacesuits and stations are typically padded in white material to deflect the Sun’s rays and limit heating. Further out from the Sun, the temperature plummets. We’re down to a mean temperature of about -229ºC (-380ºF) at Pluto, where the sunlight is puny. Yet even if we travel as far from any star as it is possible to be, there will still be some residual temperature. The cosmic microwave background, which we met earlier in this chapter, keeps space at a few degrees above absolute zero (-273ºC/-460ºF) – the theoretical temperature at which particles are at complete rest.
The coldest place in the known universe is not some random spot between galaxies. Again, you’ll find it here on Earth. The chilliest location in all of existence can be found in the USA on the banks of the Charles River in Boston. In 2015, researchers at Massachusetts Institute of Technology cooled a sodium gas down to just a half-billionth of a degree above absolute zero. Such experiments use a complex set-up of magnetic fields and lasers that are unlike anything found in nature. Unless aliens have a freezer box that’s still more biting, Earth is the coolest place in the Universe.
The first true telescopes were raised to the heavens in the 17th century by the likes of Hans Lippershey, Christiaan Huygens and Sir Isaac Newton. We’ve since had 400 years to improve on our knowledge of the Universe. We’ve fared pretty well. Astronomers have tracked down millions of galaxies; thousands of planets are now known around other stars. We’ve even taken images of the cosmic microwave background, the radiation left over from the Big Bang. Is there much left to discover?
Yes. Most of it. In fact, the parts of the Universe we don’t know about are so overwhelmingly large that, to a close approximation, we haven’t even started looking.
Let’s begin our catalogue of ignorance with the familiar nuts and bolts of the Universe, the galaxies and stars. Nobody’s sure exactly how many galaxies are out there. We’ve seen or photographed only a tiny fraction, yet we know there must be billions and billions. How?
A landmark moment in astronomy occurred in 1995. The Hubble Space Telescope was ordered to turn away from the stars and nebulae, and instead stare at a portion of seemingly empty space. This was a very small piece of empty space – about a 24-millionth of the entire sky. The telescope collected light from this region for over 100 hours. With such exposures, dim and distant objects were teased into the open. In all, scientists counted some 3,000 objects, almost all of them galaxies. All this, in an incomprehensibly minute portion of the sky. Extrapolating from the Hubble Deep Field and later observations, astronomers now suppose there to be some two trillion galaxies in the observable universe. Everybody on Earth could lay claim to 270 galaxies – and the 100 billion stars in each one. There’s plenty of scope for exploration.
This is just the stuff we have a chance of seeing with modern instruments. Most normal matter in the Universe is effectively invisible to us. It might be too far away to see, very small, hidden behind something else, or non-luminescent. Black holes, by definition, are impossible to see – though we may detect their effects on nearby objects and matter.
Even if we could observe all of the normal matter in the Universe, we still wouldn’t be close to knowing the whole of existence. There’s another kind of ‘stuff’ out there called dark matter. It has never been observed. Nobody knows quite what it is. Yet it must be there to account for the distribution and motion of visible matter. The current thinking is that normal matter (stars, galaxies, dust …) accounts for just 5 per cent of the Universe. Dark matter, by comparison, makes up about 27 per cent. It is more than five times more common than everyday matter, yet remains elusive.
That still leaves about 68 per cent of the Universe hiding down the back of the sofa somewhere. Theoretical physicists point the finger at something called dark energy*. It is again unobserved, but must be everywhere. We’ve noted elsewhere that the Universe seems to be expanding at an increasing rate. Dark energy is the force behind that acceleration. Otherwise, its nature remains a total mystery.
Finally, there are parts of the Universe that will forever remain beyond our knowledge, unless we find a way to bend the laws of physics. The limiting factor here is the speed of light. Objects that are really, really far away can never be seen because the Universe hasn’t existed long enough for their light to reach us yet. You could build a telescope with a detector the size of the Solar System, but it could never observe anything beyond this barrier.
How far are we talking? It’s 13.8 billion years since the Big Bang, give or take, so light has had this long to travel outwards. It’s tempting – almost logical – to assume that the maximum distance we might possibly see is 13.8 billion light years. Yet scientists reckon that the ‘observable universe’ is more like 47 billion light years in any given direction. That’s because the Universe itself has expanded significantly since that initial light set out. The observable universe will continue to grow with time, as light from still longer ago finally reaches us, but there are some parts about which we will never have knowledge. What lies beyond in the ‘unobservable universe’ is – by definition – entirely unknown. All we can say is that it must be many, many times larger than our observable universe. And that’s without going into the possibility of additional and alternative universes separate to our own.
It seems that no matter what wondrous technology we develop, we will only ever be able to study an infinitesimally small fraction of our surroundings. We are like a mole who knows his tunnel well but has no chance of learning about the mountains, valleys, cities and seas that lie just beyond his realm. It is humbling, and just a little bit frightening to contemplate. Time to move on to lighter things.
* FOOTNOTE According to the laws of physics – specifically Einstein’s famous formula that E = mc2 – mass and energy are two sides of the same coin. One can be converted to the other. Astrophysicists therefore talk of the mass-energy of the Universe, and hence why ‘dark energy’ is considered part of the makeup alongside more tangible forms of matter.