There is one space-related date in recent years whose importance is almost impossible to overstate. We have been fortunate in our era to have witnessed the opening of a new window on the Universe, one whose potential is truly breathtaking. As is the engineering that has facilitated it. I’m talking about the first detection of gravitational waves by an extraordinary ‘telescope’ in the United States on 14 September 2015. The news was not released until the following February, but the science press immediately grasped its significance, proclaiming the discovery with breathless enthusiasm. The discovery was made with an instrument called LIGO – the Laser Interferometer Gravitational-Wave Observatory – which is a gobbledygook way of describing a machine that senses minute changes in length.
What do I mean by minute? LIGO can detect changes in the length of a 4-kilometre-long beam of light amounting to one ten-thousandth of the diameter of a proton – the subatomic particle at the centre of a hydrogen atom. In maths-speak, that’s precision of better than 10−19 metres.
While your head is reeling from that, as mine did when I first learned about it, let me explain how LIGO’s name gives away the technology. Technically, it’s known as ‘Advanced LIGO’ to distinguish it from earlier development versions, but that needn’t worry us. The light beam I just mentioned comes from the laser, of course. What’s an interferometer? This is a device that takes a beam of light, splits it in two, and then recombines it so that the individual light waves come together in step – or very nearly so. If two waves are wildly out of step when they recombine, with the peak of one hitting a trough of the other, for example, they’ll cancel out altogether and produce darkness – a quirk of physics that I’ve always regarded as slightly magical. But two waves that come together just a fraction out of step can be compared and measured with high accuracy in an interferometer, which is why it’s such a powerful instrument.
And what’s the point of splitting the light in the first place? This is done so that the two resulting beams are at 90 degrees to each other. If one beam was sent off to the north, for example, the other would be directed west. In fact, LIGO is at a rather different compass bearing, but its two 4-kilometre-long arms are still exactly at right angles, like a gigantic ‘L’. At each tip of the ‘L’ are mirrors that send the light beams back to be recombined. So, what LIGO is sensing with that head-spinning precision is the minuscule discrepancy in length between two identical light beams that differ only in their direction in space.
And that’s where the gravitational-wave bit comes in. We’re not talking about light waves now, nor seismic waves rattling the surface of Earth, but ripples in space-time itself. Their existence is a consequence of Einstein’s General Theory of Relativity, which, you will recall, says that space is not perfectly rigid, but flexes minutely in response to matter. Things that can flex tend to be able to transmit waves, too – think of sound waves in air, ripples on a pond, heavy metal on guitar strings, and so on.
With Einstein’s general relativity withstanding every critical test that has been thrown at it over the past hundred-odd years, the prediction of gravitational waves was always taken seriously. And it had long been anticipated that when equipment could be designed and built with the required level of sensitivity, it would be able to detect them. Now, after decades of development, this has come to pass, with minute tremors in the length of LIGO’s light beams revealing the passage of a gravitational wave. Curiously, the waves detected fall within the range of frequencies to which the human ear is sensitive, so when the LIGO signals are hugely amplified, you can hear them.
I’ve rather glossed over this description of LIGO, neglecting, for example, the fact that there are two of these large interferometers – one at Livingston, Louisiana, and the other at Hanford, Washington. If you look at those two places on a map, you’ll see that they are at opposite corners of mainland United States – at least as far as local geography will allow. If you could shine a flash-lamp from one to the other, the light would traverse the distance in about 10 milliseconds, or one-hundredth of a second. That’s important because theory predicts that gravitational waves travel at the speed of light, so having two widely spaced detectors gives you a handle on what direction they’re approaching from.
SO, WHAT CAUSES GRAVITATIONAL WAVES? THEY ARE emitted when any massive object is accelerated. In the case of that first detection in 2015, the waves originated in a major gravitational disturbance some 1.3 billion years ago, when two distant black holes spiralled together and merged to form a bigger one. Such a merging takes place over time, with the black holes revolving ever more rapidly around each other during the last few seconds. The resulting gravitational waves pulse outwards with increasing intensity and frequency as the two black holes spin together – but vanish once they have merged, in what’s called the ‘ring-down’ of the resulting black hole. Played in audio, the waves sound like a short whistle whose loudness and pitch increase ever more rapidly before suddenly falling silent – a so-called ‘chirp’.
You probably won’t be surprised to read that the first gravitational-wave detection on 14 September 2015 is designated GW150914. But you might be more impressed to know that the analysis of its signal yielded not only the distance of the event (1.3 billion light years) but also the masses of the two merging black holes (35.6 and 30.6 times the mass of the Sun) plus the final black hole mass of 63.1 solar masses. Note that those mass figures don’t add up, and what the discrepancy tells you is that the merger also produced the energy equivalent of 3.1 solar masses in gravitational waves. (You can work out how much energy that is by using a rather famous equation that links it with mass and c, the speed of light, squared.) The colossal energy release is why those ripples in space were still detectable after more than a billion years of travelling through the Universe.
As might be expected, that first detection yielded a handsome return in international awards, with the 2017 Nobel Prize for Physics going to Rainer Weiss, Kip Thorne and Barry Barish for their role in it. And since then, significant progress has been made in gravitational-wave astronomy. Advanced LIGO has been joined by a European instrument known as Advanced Virgo, located near Galileo’s old stamping ground in Pisa. Others are in development. The benefit of adding to the world’s suite of gravitational-wave observatories is that more well-spaced detectors enhance not only our sensitivity, but also our ability to pinpoint the direction from which a given signal has come.
At the time of writing, GW150914 has been joined by ten further confirmed event detections with many more in the pipeline (one of which, reported in May 2019, may be the first example of a black hole devouring a neutron star). Of the ten confirmed, just one – GW170817 – did not come from merging black holes, but from merging neutron stars at a distance of about 130 million light years. The gravitational-wave signal came from the final 100 seconds of this event, and once again, the chirp revealed the masses of the progenitor objects – 1.5 and 1.3 times the mass of the Sun. But while a merger of stellar-mass black holes is not expected to produce an electromagnetic pulse, a merger of neutron stars is. And, in a triumph of international collaboration, GW170817 was detected by 70 observatories worldwide (and in orbit) across the entire electromagnetic spectrum. The detections ranged from gamma rays, which arrived 1.7 seconds after the gravitational-wave signal, to radio waves detected 16 days later. When the physics of the various emission mechanisms was taken into account, these observations provided spectacular confirmation that gravitational waves do, indeed, travel at the speed of light.
I WANT TO TURN, NOW, TO A PICTURE THAT IS EVEN bigger than mergers of exotic objects in deep space. The generally accepted theory of the beginning of the Universe is something called the Big Bang, which postulates an origin of everything (space, time and matter) some 13.8 billion years ago. It’s not the only theory espoused by the science of cosmology – which studies the origin and evolution of the Universe as a whole – but it’s the one with the most solid evidence. It’s based on a mixture of observation and general relativity, which forms its theoretical foundation. While ‘Big Bang’ is an evocative description, cosmologists often refer to a more precise mathematical formulation known as the ‘Lambda CDM model’, which I’ll explain in a few minutes. Other theories, such as a regenerative Universe, or even multiple ones, are more speculative, even though they’re great to talk about at parties. Well, the kind of parties I go to, anyway.
So what is the Big Bang theory? It owes its origin to the work of a Russian mathematician named Alexander Friedmann, and a Belgian priest called Georges Lemaître, who independently formulated some of the properties of an expanding Universe in the 1920s – before Edwin Hubble discovered the actual expansion in 1929. Lemaître, in particular, focused on the idea of a ‘primaeval atom’ from which the Universe and its contents have evolved. Over the next few decades, the theory was refined to include the idea of an extremely hot and dense beginning, which, today is regarded as a singularity – but with infinite temperature as well as infinite density. Present-day physics is not equipped to probe the innards of this singularity, but provides a surprisingly good understanding of its immediate aftermath.
IN THE MIDDLE YEARS OF THE 20TH CENTURY, THE BIG Bang theory was pitted against a competing model that envisaged matter as being continuously created within an infinitely old Universe. That was the ‘steady-state’ theory, espoused by British astronomer Fred Hoyle and others. But two discoveries knocked the steady-state theory on its head. The first came from a new generation of sensitive radio telescopes that were introduced in the mid-1960s. One of them revealed a mysterious background hiss in the microwave spectrum that seemed to cover the entire sky. It took a while before scientists realised that what they were picking up was something that had been predicted nearly two decades earlier, in 1948. It was effectively the afterglow of the Big Bang – the brilliant light that had filled the infant Universe, stretched in wavelength into microwaves by its subsequent expansion. We give this afterglow a technical name – it’s called the Cosmic Microwave Background Radiation, or CMBR.
Why can we still perceive this ancient fossil radiation? Once again, it arises because whenever we look into space, we are always looking back in time. In fact, once you get beyond the confines of our own Milky Way Galaxy, the so-called ‘look-back time’ becomes a more relevant concept than the actual distance. So, while the eight-minute look-back time to the Sun, or the 4.3-year look-back time to the nearest bright star (Alpha Centauri), or even the 2.5-million-year look-back time to the nearest big galaxy (Andromeda) aren’t particularly momentous in evolutionary terms, once you get to more distant galaxies, you’re looking back to a significantly earlier epoch. Which, incidentally, was the second blow that took out the steady-state theory – because astronomers could see that at look-back times of several billion years, galaxies were considerably different from today’s galaxies, implying that they have undergone evolutionary changes. That would not be the case in a steady-state Universe.
But back to the CMBR. To understand its origin, you have to appreciate that for the first few hundred thousand years after the Universe came into being, it was filled with a fog of brilliant radiation. It was essentially a fireball. As with a fog of water droplets here on Earth, there was no way of seeing through it. Water droplets scatter light, and, in a sense, so did the radiation permeating the cosmos. But then, some 380 000 years after the Big Bang, the fog cleared fairly rapidly throughout the whole Universe, rendering it transparent, as it is today. So, as we look further and further back in time through our transparent Universe, we eventually come to the instant when the fog cleared, and can see it as a wall of radiation covering the whole sky. We call it the ‘last scattering surface’, and it’s the fact that the Universe has expanded by around 1300 times since it became transparent that has stretched the radiation into microwaves. Were it not for that, the sky would be an encompassing sphere of brilliant light, and there would be no such thing as night.
A moment’s thought will show that this sphere of microwave radiation (the last scattering surface) is receding from us at the speed of light – because the moment when the fog cleared is retreating second by second into our past. The best way to get your head around this is to imagine yourself in the Universe at the moment the dazzling fog cleared. Knowing that it cleared everywhere exactly simultaneously (in our thought experiment, at least), do you immediately see darkness? The answer is no, because one second after it clears, you’ll be seeing a wall of illumination 300 000 kilometres away whose light has only just reached you. That brilliant wall will be 600 000 kilometres away after two seconds…and so on. Right from the start, the flash of the Big Bang is receding into your past – and it still is.
Effectively, the CMBR is a gigantic optical illusion, because it’s not a physical barrier. Enclosed within it, however, is everything we can see in the Universe. And it has another attribute, too, which is of immense importance in studies of the Universe’s evolution. As the waveband of the CMBR has been changed by the expansion of the Universe from visible light to microwaves, so has its temperature, falling from several thousand degrees when it was emitted to 2.7 degrees above absolute zero today. Effectively, that’s the temperature of space. And it’s almost uniform over the whole sky. But not quite – the CMBR has ripples of temperature in it, at the minute level of about one part in 100 000.
Remarkably, those ripples originated in sound waves reverberating through the primordial fireball – the sound of the Big Bang, if you like. Despite the vanishingly small range of temperatures they cover, the ripples have been mapped in detail by a succession of space-borne radio telescopes over the past two decades, revealing much about conditions in the hot early Universe. They also provide a baseline for our investigations of the way the Universe has evolved, because the slightly cooler spots are regions of higher density in the fireball. They are thought to have been the seeds of the large-scale structure we see in the Universe today, as revealed by the way galaxies are distributed in space. Comparing today’s Universe with the CMBR tells us about the details of the expansion, including, for example, the contribution of dark matter.
Maps of the CMBR are, like gravitational waves, Nobel Prize material, and form a vital basis for contemporary cosmology. Because they conventionally depict the CMBR in a range of colours, and because this radiation is behind everything else we can see in the Universe, I sometimes refer to it as the ‘cosmic wallpaper’. But, vital though it is, the cosmic wallpaper has a downside. It forms a horizon beyond which we can never see with radio telescopes, visible-light telescopes, or any other kind that depends on electromagnetic radiation. It is impenetrable. But why should we want to see beyond it? The answer to that lies in the fact that ‘beyond’ in this context means ‘earlier’. That is, if there was some means of penetrating the cosmic microwave background, we could detect events that took place before the Universe became transparent. And this would allow us to probe details of the Big Bang that, at present, are only in the realm of theory.
WHAT ARE THE KINDS OF THINGS WE’D LIKE TO KNOW about? One concerns the origin of something I’ve hardly dared touch on in this book. It’s another ‘dark’ mystery, next to which the mystery of dark matter pales into insignificance. While our ongoing quest into the nature of dark matter has, at least, some possibility of success, this one stubbornly defies the efforts of theoretical physicists. And it’s to do with the expansion of space itself.
In the 1970s and 80s, most cosmologists assumed that there would be enough matter in the Universe to gradually slow down its expansion by the mutual gravitational attraction of everything in it. Perhaps even to the extent that at some time in the distant future, the expansion might stop and turn into a contraction, with the ultimate fate of the Universe being a ‘big crunch’ as it collapsed back into a singularity. Nobel laureate Brian Schmidt (today the Vice-Chancellor of the Australian National University) famously referred to this reversal of the Big Bang at the end of the Universe as the ‘Gnab Gib’.
During the 1990s, Schmidt was leading one of two groups of scientists that were independently using observations of distant supernovae to chart this expected deceleration of the Universe. But what they found was the reverse. To the astonishment of everyone, both groups discovered that for the past six billion years or so, the expansion of the Universe has been accelerating. Announced in 1998, that was the discovery that earned Schmidt the 2011 Nobel Prize in Physics, along with his colleague Adam Riess, and Saul Perlmutter, leader of the other group.
Of course, the immediate question was ‘Why?’ The supernova work, together with comparisons of the Universe’s present large-scale structure with that in the CMBR, suggest that space itself is endowed with a pressure that’s causing the acceleration. For want of a better term, we call it dark energy, and it’s a property of the Universe as a whole rather than a local effect. While accelerating expansion is definitely the situation today, it may not always have been. We believe that during the first six or seven billion years of the Universe’s history, the matter in it was sufficiently closely packed that the braking effect of its mutual gravity was enough to decelerate it. The acceleration kicked in only when galaxies were far enough apart for dark energy to begin to overcome gravity.
The most recent work suggests that the dark energy of a portion of space is related to its volume, so, as space expands, it gets more energetic and further accelerates the expansion. In that respect, it resembles a mathematical entity that Einstein introduced into his relativity equations in 1917, which he called the ‘cosmological constant’. He denoted this constant by the Greek symbol lambda, which is why the name ‘Lambda CDM model’ is used. Lambda represents dark energy, and CDM stands for ‘cold dark matter’. Those two entities, together with normal matter (which, for the record, we call baryonic matter in the trade, and is dominated by hydrogen) make up the mass-energy budget of the Universe. The best observational determinations have them in the ratio 68:27:5 (dark energy to dark matter to normal matter). Once again, the Universe has put us in our place by the fact that everything we can see only amounts to a measly 5 per cent of its contents. And it puts astronomers firmly in their place with the admission that we don’t understand what makes up the other 95 per cent.
PHYSICISTS ARE CURRENTLY WORKING HARD TO TIE DOWN dark energy. More observations of the Universe’s rate of expansion at different epochs in the past might give us more insights, and that is being accomplished using fibre optics technology. It could be assisted by gravitational-wave observations of distant neutron star mergers. They could be used to better calibrate our standard light sources, for example, improving the cosmic distance scale. But wouldn’t it be wonderful if we could probe beyond the CMBR to see what was going on in the works while the Universe was still glowing brilliantly? That might seem a bit fanciful, but there is at least one more pressing reason why we’d love to achieve that.
It’s one of the fervent hopes for the bright new future offered by the detection of gravitational waves. Remember, they’re not just emitted when black holes or neutron stars collide, but when any massive object is accelerated. And we believe the granddaddy of all massive accelerations occurred a tiny fraction of a second after the Big Bang itself, when the whole Universe underwent a fleetingly brief episode of violent expansion. There are good reasons to believe that the infant Universe expanded by at least 1026 times (that’s a 1 followed by 26 zeroes) when it was about 10–36 of a second old (and that’s a 1 preceded by 35 zeroes and a decimal point). Yes, I know these numbers seem ridiculous. They mean that instantaneously after the Big Bang, the Universe went from being the diameter of a hair to the diameter of a galaxy. And, with consummate understatement, we call this the period of inflation. It was followed immediately afterwards by the much gentler expansion that is still taking place today.
The inflation theory was developed in the late 1970s in order to overcome some of the problems of the Big Bang model as it was then understood. The almost perfect smoothness of the CMBR’s temperature was one of them, because physics wouldn’t allow the fireball to achieve such a level of uniformity before expansion had carried different segments of it too far apart to interact with each other. This is known as the ‘horizon problem’. There were other problems, too, but the new inflationary model dealt with them all rather well. Which is why it’s now a part of the standard Big Bang model that is generally accepted – despite it having no direct observations to support it, other than the smoothness of the CMBR.
Inflation was an acceleration of space itself, rather than of objects moving through space. So its gravitational-wave signature is not as straightforward as that from conventional accelerating masses – if black holes and neutron stars can ever be described as conventional. In fact, the gravitational waves expected to have been produced during the inflationary period are of such a low frequency that they would show no change during normal human timescales, appearing simply as a frozen pattern imprinted on the CMBR. This pattern is known in the trade as the B-mode polarisation, and may eventually be detected in microwave observations of the CMBR rather than by direct gravitational-wave signals. The bottom line, though, is that the cosmic wallpaper is no barrier to the gravitational signal of inflation. Nor to any of the other physical processes taking place in the Universe’s earliest phase.
WHILE THE TWO LIGO DETECTORS THAT MADE THE recent ground-breaking discoveries are amazing in their sensitivity, they are nowhere near sensitive enough to pick up cosmic inflation. Nor are they tuned to the low-frequency gravitational waveband that is necessary to detect events in the immediate aftermath of the Big Bang. However, their descendants almost certainly will be. Today’s gravitational-wave technology is still in its infancy, in both design and implementation. More improvements are planned to the LIGO detectors, and there will be more of them. Eventually, it is expected that there will be a network of LIGO-like detectors all around the globe, combining their results to give us a high-frequency gravitational-wave detector the size of Earth.
And beyond that is a space-based detector of even more exquisite sensitivity, which the European Space Agency (ESA) proposes to launch in around 2034. The Laser Interferometer Space Antenna – LISA – will bounce laser beams backwards and forwards over millions of kilometres rather than the 4-kilometre beams of LIGO. It will be sensitive to exactly the low-frequency signals we have just been discussing, with the added possibility of detecting distant supermassive black-hole mergers and the detailed mechanics of galaxy formation. In December 2015, ESA launched a technology demonstrator spacecraft called LISA Pathfinder, whose 16-month mission exceeded all expectations. As a proof-of-concept, LISA Pathfinder has succeeded with flying colours, inspiring confidence in the prospects for LISA itself.
With such improvements in technology, there is real hope that we may be able to use gravitational waves to probe the secrets of the early Universe. It might not be too much to hope that one day, we will not only know the physical details of dark energy and cosmic inflation, but the mechanism of the Big Bang itself. And what an astonishing discovery that would be.