At a meeting sponsored by the American Academy, the Royal Society, and the Carnegie Institution for Science, Wendy Freedman (Crawford H. Greenewalt Chair and Director of Carnegie Observatories at the Carnegie Institution for Science) and Martin Rees (Fellow of Trinity College; Emeritus Professor of Cosmology and Astrophysics at the University of Cambridge; Astronomer Royal; and Visiting Professor at Imperial College London and at Leicester University) discussed what we know and do not know about the universe. Richard A. Meserve (President of the Carnegie Institution for Science) moderated the discussion. The meeting took place on April 29, 2014, at the Carnegie Institution for Science. An edited version of the presentations follows.
One of the defining characteristics of science is the reality that the more you know, the more you realize you don’t know. And there is perhaps no field today in which that is more evident than in astronomy. Over the last two decades, we have learned that we fundamentally do not understand the stuff that comprises 95 percent of the universe: dark energy and dark matter. In one sense, in this time of scientific achievement, our ignorance is a little embarrassing. But in another sense, this is a time of enormous excitement. There are deep mysteries to be solved, presenting a great challenge to the researchers of our time.
We first learned about dark energy about fifteen years ago. Cosmologists had long expected that the force of gravity produced by the matter in the universe would cause the universe’s expansion to slow down, and perhaps eventually to reverse course. But contrary to everyone’s expectations, observations of Type Ia supernovae by the High-Z Supernova Search Team in 1998 and by the Supernova Cosmology Project one year later suggested that the expansion of the universe is actually accelerating. Thus, we were presented with a great mystery: why is the universe’s expansion accelerating, and what could possibly be fueling it? To answer these questions, cosmologists rethought the known contents of the universe, determining that about 70 percent of its matter/energy inventory is embodied in dark energy, a substance we have not yet begun to understand.
But even before the discovery of evidence for dark energy, we had already found evidence of dark matter. In fact, Vera Rubin, a Carnegie astronomer, was responsible for the verification of the existence of dark matter. Rubin helped prove dark matter’s existence through her measurements of its influence on the movement of stars within galaxies. The trajectories she observed simply did not fit Newton’s laws of gravity; there had to be matter that we cannot observe. Once we accommodate it, we find that dark matter constitutes about 25 percent of the matter/energy inventory of the universe.
So, the stars and galaxies and the conventional matter we observe all around us really only compose 5 percent of what constitutes our universe. Our research has revealed to us deep mysteries about the remaining 95 percent, inspiring the title of tonight’s discussion, “The Universe Is Stranger Than We Thought.”
A century ago, we astronomers understood the universe to be both dominated by stars and unchanging with time. We observed the diurnal motions of stars, but they were, to us, fixed; the universe was neither expanding nor contracting. A century later, we have learned that ours is a dynamic universe: it is evolving, it is changing with time, it is filled not only with stars but with galaxies composed of stars and exotic objects like black holes, and it is overwhelmingly filled with dark energy and with matter that bears little resemblance to the matter that we know about. These findings were part of a century of far-reaching cosmological discovery. Today, I will concentrate on three discoveries in particular: the discovery of the expanding universe, the discovery of evidence supporting the presence of dark matter in the universe, and the discovery of the acceleration of the expansion of the universe.
I will begin with the discovery of the universe’s expansion, for which we are indebted to Edwin Hubble, after whom the Hubble Space Telescope is named. The history of Hubble’s discovery – and of cosmology in the twentieth century generally – is inextricably intertwined with the history of the Carnegie Institution of Science itself. Andrew Carnegie had a vision: if you hired exceptional scientists, and if you gave them resources, a laboratory, and the apparatus to do science, then interesting discoveries would follow. Likewise, George Ellery Hale, the first director of the observatories of the Carnegie Institution, had a vision of his own: if you built large telescopes with reflecting mirrors, then you would make discoveries in astronomy. Hale was fond of saying, “Make no little plans. They have no magic to stir men’s blood,” a quotation from the American architect Daniel Burnham. And Hale certainly made no little plans, arriving in Pasadena in 1903, where he identified Mount Wilson as a site for his observatory of large reflecting telescopes.
At Mount Wilson, Hale first built a solar telescope (he was a solar astronomer and, in fact, was the astronomer who discovered that there were magnetic fields on the Sun) and then began construction of a 60-inch mirror telescope. This 60-inch telescope is what then Carnegie astronomer Harlow Shapley used to discover that our Sun is not the center of the universe, where it had been presumed to reside ever since Copernicus had in 1543 shown that the Earth was not the center of the universe. Shapley showed that the Sun is actually located about two-thirds of the way out in a disk, a plane, of what we now know as our Milky Way galaxy. That was an extraordinary early discovery to come out of the first telescopes at Hale’s observatory. But it was the 100-inch Mount Wilson telescope, whose construction began before the 60-inch telescope was even complete, that enabled Hubble to make his discoveries about the expanding universe.
Edwin Hubble used the 100-inch telescope to study a class of objects known as “nebulae.” In the early twentieth century, nebula was the classification given to any number of diffuse objects, including interstellar clouds of dust and gas that we now know act as stellar nurseries, star clusters and galaxies beyond the Milky Way. Figure 1 features a photograph of Hubble examining a glass photographic plate, as well as an image of the nearby Andromeda nebula shown on a plate Hubble took. Glass photographic plates were the detectors in use when the 100-inch Mount Wilson telescope became operational. The black fuzzy mass centered on the glass plate is what Hubble identified as a nebula. These objects had been catalogued by astronomers for a couple hundred years. The question was, were these nebulae objects swirling around regions of gas and dust, collecting under gravity to form new stars in the Milky Way? Or were they perhaps galaxies like the Milky Way, at far greater distances? In the box in the upper right corner of the photographic plate in Figure 1, which is a negative image, you can see where Hubble marked “VAR!” “VAR” stands for variable, and the new variable Hubble had found was a class of star called a Cepheid: a star whose luminosity and pulsation period allow astronomers to measure distances to extragalactic objects.
Left: Astronomer Edwin Hubble examining plate, c. 1952
Right: Hubble’s discovery plate of a Cepheid in Andromeda
Using Cepheids, Hubble was able to show that Andromeda was well beyond the confines of our own galaxy – we now know it is about two million light years away from us. Hubble went on to make these measurements for many different galaxies and, as illustrated by Figure 2, he was able to show that when he plotted the velocity (km/s; erroneously labeled just “km” on Hubble’s graph) of the galaxy on one axis and the distance (millions of parsecs or Megaparsecs, where 1 parsec = 3.26 light years) on the other, there was a correlation between how fast the galaxy was moving and the distances he measured. That is, the farther away the galaxy is, the faster it is moving away from us.
These were two spectacular discoveries: 1) what followed is that we now know that there are about one hundred billion such galaxies in our observable universe in addition to our own, and that within galaxies like our Milky Way, there are about one hundred billion stars; and 2) that the universe is expanding and that the galaxies are participating in this overall expansion of the universe.
Hubble Diagram (1929)
We think Hubble did not actually believe that the universe was expanding, despite the evidence his empirical results provided. It was the integration of Einstein’s General Theory of Relativity that described, based on Hubble’s observational results about the linear relationship between velocity and distance, that the universe must have had a beginning. If the universe is expanding now, there must have been a time when it was compressed, hot, and dense. Einstein’s theory and Hubble’s observations led to our picture of a universe developed from the Big Bang: a furiously hot and dense explosion about 14 billion years ago. This extrapolation of Hubble’s observations has since been confirmed by more exact measurements of the Cepheid variables recently taken with the Hubble Space Telescope and its sister satellite, the Spitzer Space Telescope (which operates in the medium infrared, very long wavelengths) – which have charted out the distance scale of the universe based on many galaxies. Further, using the Hubble Space Telescope, we have estimated the age of the universe to be about 13.7 billion years, a number that has been corroborated by numerous independent findings.
The second discovery I want to talk about is the existence of dark matter in the universe. That story begins with the observations of Fritz Zwicky at Caltech in the 1930s, and the observations by Carnegie astronomer Horace Babcock, who in 1939 made the first measurements of the velocity of stars in the Andromeda galaxy (the same galaxy in which Hubble discovered Cepheids). Zwicky found that the velocities of galaxies in the nearby Coma cluster were so high that the galaxies could not have been bound to the cluster; they should have escaped long ago. Babcock learned that the velocity of stars and gas in the Andromeda galaxy increases and then stays constant as you move away from the center of the galaxy toward the outer regions. The expectation was that in the same manner that we observe the orbital velocities of planets in our solar system reduce proportionally to the distance from the Sun, the velocities of stars and gases in galaxies should fall off in the outer regions. For decades, these data were largely ignored because they were not expected and simply could not be explained. Then in the 1970s, Vera Rubin, of Carnegie’s Department of Terrestrial Magnetism in Washington, D.C., made her own observations. Once again, the velocities of outer stars and gases in every galaxy that Rubin and her collaborator Kent Ford measured either increased or remained flat. Other astronomers measured the velocities of hydrogen clouds within galaxies. None of these velocities decreased with distance as they did in the solar system.
Rubin’s findings signaled that there was additional matter in the outer regions of these galaxies whose gravitational influence bound these high-velocity stars to the structure. Without additional matter, there simply would not be enough mass to prevent the stars, moving at such great speeds, from escaping the galaxy. There were alternative explanations, but the evidence for what would become known as dark matter kept increasing. The measurements of velocities of other galaxies in clusters confirmed Fritz Zwicky’s measurements in the Coma cluster. Additionally, with new advancements in X-ray astronomy, astronomers were able to discover gas as hot as 100 million degrees Celsius residing in these galaxy clusters. But without additional mass to bind this gas to the cluster, it should have, at those temperatures, evaporated. Finally, Einstein’s General Relativity predicted that space would bend in the vicinity of a massive object, and light would bend around it. This phenomenon, known as gravitational lensing, reveals to us the strength of the gravitational influence of the object that is changing the light’s course. But the arcs we observe suggest that there is far more mass acting upon the light than is accounted for by the luminous matter in galaxies alone.
Ultimately, only about 4 percent of the total composition of mass and energy in the universe is ordinary visible matter. The vast majority of the matter in the universe is dark. We cannot see it and it does not emit visible light or any kind of electromagnetic radiation. So what could this dark matter be? Could it be rocks, planets, remnants of old stars that no longer shine? Could it be gas, massive compact objects, space dust, or black holes? In the 1980s, many groups embarked on searches for dark matter in such forms that we already understood, and all failed. The only option left standing was an undiscovered particle, one formed soon after the Big Bang.
The current best hypothesis is that dark matter is a relic from the early universe that interacts with ordinary matter only through gravity. That is, dark matter does not interact via electromagnetic or other known forces. Researchers are currently looking for dark matter in underground laboratories, shielded by lead from other noise sources, using detectors made of elements like germanium and silicon to look for this very faint signal from what could be these weakly interacting massive particles. The Fermi gamma-ray satellite, as well as the Large Hadron Collider – a particle accelerator between France and Switzerland that accelerates particles to very high velocities and smashes them – are also looking for evidence of dark matter candidates. Physicists and astronomers hope that these elusive particles will be discovered in the next decade – a Nobel Prize for this discovery awaits.
The third discovery I would like to discuss is the acceleration of the expansion of the universe, a discovery made in 1998 and 1999 by two independent groups studying Type Ia supernovae. Type Ia supernovae are thought to occur in a binary star system in which one of the stars is a white dwarf (a star that has completed its normal life cycle and has ceased nuclear fusion). If the white dwarf accretes enough mass from its companion star and exceeds a certain mass, that white dwarf explodes in so bright a display that you can actually see it over most of the observable universe. Another possibility is that the explosion occurs when two white dwarf stars merge. Whatever the mechanism, the supernovae themselves can be as bright as an entire galaxy. Using these supernovae, we have found that as we look back further in time (farther in space), the expansion rate has increased over time – the expansion of the universe is accelerating.
The reason for this acceleration is not well understood at this time. We do not know the nature of the dark energy that is causing the acceleration of the expansion, but it makes up most of the composition of the universe. To give you some sense of what we think we know about dark energy, the density of dark energy is tiny – about 10-30 grams per cubic centimeter (for a relative comparison, the density of water is about 1 gram per cubic centimeter). It appears that there is energy in the vacuum of space, and although the density is so slight, the sheer volume of space establishes the energy’s dominance in our universe. Although astronomers have now measured the effects of dark energy and dark matter in several independent ways, we do not yet understand the fundamental nature of what is causing the universe to be stretched apart; nor do we know at this time what composes 95 percent of the universe.
To conclude, I quickly want to say a few words about what is on the horizon, because this is a very exciting time in astronomy. The successor to the Hubble Space Telescope, the James Webb Space Telescope – which features a mirror 6.5 meters (250 inches) in diameter – is due to be launched in 2018. Unlike the Hubble, which orbits the Earth about 350 miles above our heads (for comparison, the Earth-Moon distance is about 250,000 miles), the James Webb Space Telescope will reside about one million miles from the Earth, and will let us study some of the earliest moments in the universe, including the so-called Dark Ages about 400,000 years after the Big Bang, about which we know virtually nothing.
Back on the ground, I have had the pleasure of leading an international consortium planning the Giant Magellan Telescope (GMT), now poised to enter its construction phase. The GMT is a joint effort by the Carnegie Institution, the Smithsonian Institution, Harvard University, the Universities of Arizona, Chicago, Texas at Austin, and Texas A&M, as well as Australia and South Korea. The GMT is a 25-meter (1000-inch) telescope that will use seven mirrors, each over 27 feet in diameter, to capture images ten times the resolution of the Hubble Space Telescope. These mirrors are being manufactured underneath the football stadium at the University of Arizona – not what the football stadium was designed to do, but it really is a good use of the empty space in the facility! We will ship these mirrors and assemble this telescope at Carnegie’s Las Campanas Observatory in the Andes Mountains in Chile, home to our current 6.5-meter Magellan Telescopes. We hope to begin taking data with the GMT in 2021.
In summary, our universe has revealed itself to be quite extraordinary. It is stranger than we think, it is vast, it is expanding and that expansion is accelerating, it is filled with exotic objects and new kinds of matter and energy. And I would venture that it is very unlikely to be through surprising us.
Charles Darwin’s On the Origin of Species closes with these famous words: “Whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.”
But the young Earth – Darwin’s “simple beginning” – was in fact already very complicated, chemically and geologically. Astronomers aim to probe back farther than this beginning; to set our Earth in its vast cosmic context and address basic questions like: How did planets such as ours form? How did stars originate? Where did the atoms that make up planets and stars come from? Over the past few decades there has been a crescendo of progress and discovery, owed primarily to advancing instrumentation: more powerful telescopes, computers, and space technology.
Unmanned probes to other planets have beamed back pictures of varied and distinctive worlds: Venus, rendered torrid and uninhabitable by the greenhouse effect that has poisoned its atmosphere; and Mars, with its intricate geology, now being explored by the Curiosity rover. Farther afield, we have fascinating close-ups of Jupiter’s moons: icy Europa and sulphurous Io. The European probe Huygens has landed on Saturn’s giant moon Titan, revealing flowing rivers of liquid ethane at -170 degrees Celsius.
Astronomers aim not only to understand the solar system in which we live, but to trace back farther in our history – to understand how stars and planets form, and from where their constituent atoms came. We have made huge progress in delineating a process of cosmic emergence, which we can trace back to a mysterious, hot, and dense beginning 13.8 billion years ago.
But let’s start our cosmic exploration closer to home. And this leads to one of the great unknowns, which certainly would have fascinated Darwin: what creatures might be out there in space already?
Prospects for life look bleak in our solar system – even on Mars or under the ice of Saturn’s moon Enceladus. But prospects brighten if we widen our horizons to other stars – far beyond the reach of any probe we can now envisage. Indeed, a hot current topic in astronomy is the realization that many other stars – perhaps even most of them – are orbited by retinues of planets, like the Sun is.
These planets are not detected directly but inferred by precise measurement of their parent star. One technique is very simple. From our vantage point, a star dims slightly when a planet is “in transit” in front of it. An Earth-like planet transiting a Sun-like star causes a fractional dimming, recurring once per orbit, of one part in ten thousand.
NASA’s Kepler spacecraft spent three years monitoring the brightness of over one hundred and fifty thousand stars, at least twice every hour, with this precision. It has determined the orbits of more than two thousand planets, and allowed us to infer their sizes from the depth of the dip during transit. We are especially interested in possible “twins” of our Earth: planets the same size as ours, on orbits with temperatures such that water neither boils nor stays frozen. The best such candidate so far is one of five planets orbiting a star half the mass of the Sun (and much fainter). The outermost planet has 1.2 times the Earth’s radius, and it orbits at a distance from the parent star such that liquid water might just exist. There may be better candidates still to be retrieved from the Kepler data. Moreover, Kepler has only looked at a thousandth of the area of the sky; so we would expect, after scanning it all, to find a candidate planet that is ten times closer and one hundred times less faint than this one.
The real goal, of course, is to see Earth-like planets directly – not just their shadows. But that is hard. To realize just how hard, suppose an alien astronomer with a powerful telescope was viewing the Earth from thirty light years away – the distance of a nearby star. Our planet would seem to be, in Carl Sagan’s phrase, a “pale blue dot,” very close to a star (our Sun) that outshines it: like a firefly next to a searchlight. But if the aliens could detect this dot, there is a lot they could infer. The shade of blue would be slightly different, depending on whether the Pacific Ocean or the Eurasian land mass was facing them (of course, also depending on the global pattern of cloud cover). So the alien astronomers could infer the length of our “day,” the length of our seasons, the gross topography, and the climate. By analyzing the faint light, they could infer that the Earth had a biosphere. In the 2020s, telescopes like the Giant Magellan Telescope and its European counterpart, the Extremely Large Telescope (with a mirror 39 meters across), will be drawing such inferences about planets the size of our Earth that orbit other Sun-like stars.
Could there be life on these planets? Here we are still in the realm of speculation. Even if simple life is common, it is a separate question whether it is likely to evolve into anything we might recognize as intelligent or complex – whether Darwin’s writ runs through the wider cosmos. Perhaps the cosmos teems with life; on the other hand, our Earth could be unique among the billions of planets that surely exist.
What has surprised people about these planetary systems is their great variety: Jupiter-mass planets very near their stars; planets on extremely eccentric orbits; and planets orbiting double-star systems, a relationship that produces two “suns” in the planet’s sky. But the existence of these planets was not surprising given what we have learned about how stars form via the contraction of clouds of dusty gas. If a proto-stellar cloud has any angular momentum, it will spin faster as it contracts and spin off a dusty disc around the protostar, in which gas condenses and dust agglomerates into rocks and planets. We believe this to be a generic process in all protostars.
Flashback to Newton, who famously explained why planets move in ellipses, but did not understand why they were orbiting on roughly the same plane: the ecliptic. Newton believed it was providence, but we now understand it as a natural outcome of formation from a dusty proto-stellar disc. We have pushed back the causal chain farther than Newton could. Indeed, as Wendy Freedman has adumbrated, we have pushed it right back to the cosmos’s hot, dense beginning. We can trace cosmic history back to one second after the Big Bang, when the temperature was 1 MeV and helium and deuterium formed via nuclear fusion. Indeed we can probably be confident back to a nanosecond after the Big Bang, when each particle had about 50 GeV of energy – as much as can be achieved in the Large Hadron Collider accelerator in Geneva.
Our complex cosmos today manifests a huge range of temperature and density – from blazingly hot stars to the dark night sky. People sometimes worry about how this intricate complexity emerged from an amorphous fireball. It might seem to violate a hallowed physical principle – the second law of thermodynamics – which describes an inexorable tendency for patterns and structure to decay or disperse. The answer to this seeming paradox lies in the force of gravity. Gravitating structures have a negative specific heat. As they lose energy, they get hotter. If the nuclear reactions that generate its power were switched off, the Sun would gradually contract, but in the process its center would get hotter: higher pressure would be needed to balance gravity as the Sun shrunk.
In the expanding universe, gravity enhances, density contrasts. Any patch of the universe that starts off slightly denser than average would decelerate more because it feels extra gravity; its expansion lags farther and farther behind, until it eventually stops expanding and separates out. Computer simulations of part of a “virtual universe” clearly show incipient structures unfolding and evolving. Within the resulting galaxy-scale clumps, gravity enhances the contrasts still further: gas is pulled in and compressed into stars. Simulations of this kind, displayed as movies, portray how galaxies emerged sixteen powers of ten times faster than it actually happened! Each galaxy is an arena within which stars, planets, and perhaps life can emerge.
And there is one important point: the initial irregularities fed into the computer models are not arbitrary; they are inferred from the observed fluctuations in the temperature of the cosmic microwave background. The amplitude is only one part in one hundred thousand, but computing forward, the fluctuations are amplified by gravity into the conspicuous structures – galaxies, galaxy clusters – in the present universe. This vindicates the claim that structure emerges by clustering of the gravitationally dominant dark matter during cosmic expansion.
As I said, we can trace cosmic history back to a nanosecond after the Big Bang, when the entire visible universe was squeezed to the size of our solar system. But questions like “where did the fluctuations come from?” and “why did the early universe contain the actual mix we observe of protons, photons, and dark matter?” take us back to an even younger universe, where matter was hugely more compressed still.
The physics at that era are of course still conjectural. But an astonishingly bold theory called “inflation” suggests that the fluctuations could have been generated by microscopic quantum fluctuations that are stretched by the subsequent expansion right up to the scales of galaxies, and beyond. The generic idea of inflation has achieved success in predicting two features of the fluctuations: that they are Gaussian, and that their amplitude depends on scale in a distinctive way. As well as generating the density fluctuations that evolve into galaxies, quantum effects could generate a second kind of fluctuation: gravitational waves that generate transverse motions, without changing the density.
Recent claims to have detected the latter would, if confirmed, offer further support for “inflation”; their strength is an important discriminant among different models.
Now for another basic question: How much space is there altogether? How large is physical reality? We can only see a finite volume, a finite number of galaxies. That is essentially because there is a horizon, a shell around us delineating the distance light could have travelled since the Big Bang. But that shell has no more physical significance than the circle that delineates your horizon if you are in the middle of the ocean. There is no perceptible gradient across the visible universe, which suggests that, if finite and bounded, it stretches thousands of times farther. But that is just a minimum. If it stretched far enough, then all combinatorial possibilities would be repeated. Far beyond the horizon, we could all have avatars. Even conservative astronomers are confident that the volume of space-time within range of our telescopes – what astronomers have traditionally called “the universe” – is only a tiny fraction of the aftermath of our Big Bang.
But that is not all. Plausible models for 1016 GeV physics lead to so-called eternal inflation. “Our” Big Bang could be just one island of space-time in a vast cosmic archipelago. This is speculative physics – it is perplexing today, just as the shape of the Solar System was to Newton and the “Big Bang” was until fifty years ago. But it is physics, not metaphysics; we can hope to push the casual chain back farther still.
So a challenge for twenty-first-century physics is to address two fundamental questions. First, are there many big bangs rather than just one? Second, if there are many, are they all governed by the same physics or not? Many string theorists do not think so. They think there could be a huge number of different vacuum states – arenas for different microphysics. If they are right, what we call “laws of nature” may in this grander perspective be local bylaws governing our cosmic patch. Many patches could be still-born or sterile: the laws prevailing in them might not allow any kind of complexity. We therefore would not expect to find ourselves in a typical universe; rather, we would be a typical member of the subset where an observer could evolve. This is sometimes called anthropic selection.
Such conjectures motivate us to explore what range of parameters would allow complexity to emerge. Those who are allergic to multiverses can regard this just as an exercise in counterfactual history (rather as historians speculate on what might have happened to America if the British had fought more competently in 1776, and biologists conjecture how our biosphere might have evolved if the dinosaurs had not been wiped out).
Anthropic arguments are irrelevant if the constants are unique. Otherwise, they are the best explanation we will ever have. It is reminiscent of planetary science four hundred years ago, even before Newton. At that time, Kepler thought that the Earth was unique, its orbit related to the other planets by beautiful mathematical ratios. We now realize that even within our own galaxy there are billions of stars, each with planetary systems. Earth’s orbit is special only insofar as it is in the range of radii and eccentricities compatible with life. Maybe we are due for an analogous conceptual shift on a far grander scale. Our Big Bang may not be unique any more than planetary systems are. Its parameters may be “environmental accidents,” like the details of the Earth’s orbit. The hope for neat explanations in cosmology may be as vain as Kepler’s numerological quest.
Mention of a multiverse often triggers the response that unobservable domains are not part of science. I want to contest this by way of aversion therapy, the psychological process of increased exposure whereby you are, for example, at first presented with a spider a long way away, but end up at ease even with tarantulas crawling over you. I mentioned that there are galaxies beyond our horizon: in a decelerating universe, their existence is untroublesome, since as the universe’s expansion slows, they will eventually be observable. However, as Wendy Freedman explained, we realize now that these galaxies are accelerating away from us, which means that they will never in principle be observable. But does that make them any less “real”? They are the aftermath of “our” Big Bang. But since they will never be observable, why is their reality more acceptable than that of galaxies in the aftermaths of other big bangs (if there are other big bangs, which we, of course, do not know)? We will only take other big bangs seriously if they are a prediction of a unified theory that gains credibility by being “battle tested” in other ways.
If there is a multiverse, it will take our Copernican demotion one stage further: our Big Bang may be one among billions. It may disappoint some physicists if some of the key numbers they are trying to explain turn out to be mere environmental contingencies. But in compensation, we would realize space and time were richly textured, but on a scale so vast that astronomers are not directly aware of it – not any more than a plankton whose “universe” was a spoonful of water would be aware of the world’s topography and biosphere.
The bedrock nature of space and time and the unification of cosmos and quantum are surely among science’s great “open frontiers.” But calling this the quest for a “theory of everything” is hubristic and misleading. It is irrelevant to 99 percent of scientists. Problems in biology and in environmental and human sciences remain unsolved because it is hard to elucidate the complexities of Darwin’s “forms most wonderful,” not because we do not understand subatomic physics well enough.
Now let’s focus back on the Earth. I have lived my life among astronomers, and I can assure you that their awareness of vast expanses of space and time does not make them more serene in everyday life. But there is one special perspective that astronomers can offer: an awareness of a vast future. The stupendous time spans of the evolutionary past are now part of common culture. But most people still somehow think that humans are the culmination of the evolutionary tree. That hardly seems credible to astronomers.
Our Sun formed 4.5 billion years ago and has 6 billion more years before its fuel runs out. It will then flare up, engulfing the inner planets. The expanding universe will continue – perhaps forever – destined to become ever colder, ever emptier. Any creatures witnessing the Sun’s demise 6 billion years hence won’t be human – they will be as different from us as we are from a bug. Posthuman evolution – here on Earth and far beyond – could be as prolonged as the Darwinian evolution that has led to us, and could be even more wonderful. And, of course, the evolution is even faster now: machines may take over.
However, even in this concertinaed timeline – extending billions of years into the future, as well as into the past – this century may be a defining moment, for good or for ill. It is the first century when complex entities – technologically empowered humans – have mapped the cosmos and have begun to understand how they emerged. But it is also the first century where one species – ours – holds the Earth’s future in its hands, and could jeopardize life’s immense potential here and far beyond.
This pale blue dot in the cosmos is a special place. It may be a unique place. And we are its stewards at a crucial era. That is a message for us all, whether we are interested in astronomy or not.