Shifting Heavenly Models
In the 4th Century AD, leaders of the early Christian church put forward the theory that the universe was shaped like a big tent. Looking around them, they deduced that reality had a flat bottom on which the Earth was placed, straight sides some unknown distance away, and a curved ceiling – the vault of heaven – from which the Almighty had hung the stars like so many glittering baubles. For further evidence they turned to scripture, which notes (alongside many other contrary passages describing the cosmos) that God “stretches out the heavens like a curtain, and spreads them like a tent to dwell in.” Such a shape would mean that the universe resembled the tabernacle: the canopy that sheltered the Ark of the Covenant during the exodus from Egypt. This, they thought, had a pleasing symmetry. Like the tabernacle, the tent of the cosmos was a temporary refuge; an antechamber occupied by the faithful before they could move on to the more meaningful and lasting realm of heaven. For the shape of the universe to make sense, it had to reflect a bigger plan.
Today, the shape of the universe makes no sense at all. We’re pretty sure that the cosmos is not a huge tent, but science tells us we can never know the actual dimensions of reality – or if it even has finite bounds at all – because our observations are limited by the speed of light. We can only see as far as light has been able to travel since the Big Bang took place some 13.7 billion years ago, which means that the maximum amount of the universe we can observe is a sphere centred on the planet Earth with a radius of 46.5 billion light years. What goes on beyond this limit will never be accessible to us. (The discrepancy between these two numbers – the fact that the observable universe is bigger in light years than the universe’s age in years – is due to the expansion of space that started during the Big Bang. Everything used to be much closer together.)
But, just as the cosmic tent reflected a religious belief that this world was only a divinely appointed waiting room, the limitations of today’s cosmological picture illustrate contemporary truths. In particular, that there is something fundamentally ineffable about the cosmos, an entity that is unimaginably old and unfathomably large, and which now absorbs some of the existential awe and mystery that in previous centuries was directed towards the divine. And while the external bounds of the cosmos lay beyond our grasp, we have, in recent years, been able to improve our understanding of its internal structure. We’re now pretty sure the universe – the hundreds of billions of galaxies we can see; the trillions of stars and planets they contain; the black holes, quasars, and nebulae – takes the shape of a gigantic cosmic web, with strings and clumps of galactic clusters interspersed with voids of near-empty space so large they could swallow the entire Milky Way thousands of times over.
“The cosmic web means different things to different people, but you can think about it as how the structure of the universe starts to form at a very large scale,” says Professor Xiaohui Fan, a cosmologist at the University of Arizona. “At a scale of millions of light years across, or tens of millions of light years across, the universe is not uniform in the sense that galaxies tend to group together and form larger and larger structures.” This structure is, in one sense, arbitrary, says Fan: it is simply the outcome of the starting conditions of reality, the same way that mixing together different ratios of flour, milk, and egg gives you thick or runny batter. But astronomers today also hope that by examining these final conditions we may learn more about the recipe itself. Maybe the shape of the universe will make sense after all.
Let’s put this in context first though. This conception of the universe – as something that exists on a scale beyond human comprehension – is surprisingly recent. Humans have always known that the cosmos is big, yes, but for most of history it has also been in some sense graspable. The word “cosmos” comes from the Greek kosmos, meaning orderly arrangement, and points towards an older conception of the universe as something harmonious and coherent. Ancient philosophers like Aristotle and Ptolemy thought the universe was a series of concentric spheres with Earth at the centre, surrounded by layers of celestial orbs. The stars and planets did not move as they do today, wandering through space, but were fixed into the celestial orbs which rotated around one another; a movement that some believed produced inaudible melodies heard only by the soul (the musica universalis or music of the spheres). This was a balanced cosmos: finite in extent, infinite in duration, with nothing at all outside the outermost sphere, the domain of God.
The basics of this picture persisted well into the 17th century until they were displaced by two revelations. The first was a theoretical shift towards a heliocentric model; a world in which the Earth orbited the Sun, which slowly gained acceptance thanks to the work of Nicolaus Copernicus. The second was a series of observations made by natural philosophers like Galileo Galilei using telescopes: instruments that expanded human sight beyond its natural limits for the first time in our species’ history.
The further we looked with these instruments, the more we saw, with the horizon of space always retreating to the limits of our perception. Perhaps the most striking observation from this period was that our home galaxy, the Milky Way, is itself composed of individual stars. Ancient cosmologers, whose theories blended belief and observation, had only been able to see the Milky Way as a glowing smear in the night’s sky, which they thought was located somewhere high in the Earth’s atmosphere. They explained its appearance in symbolic terms: it was the severed tail of the primordial dragon Tiamat; it was breast milk, spilled by the goddess Hera as she fed the infant Heracles. With his telescope, though, Galileo saw that the Milky Way was “nothing but a congeries of innumerable stars grouped together in clusters”. His wording is somewhat dismissive but the meaning revelatory. If these were stars, were they like our own sun? Did planets orbit around them also? And if the stars are innumerable, how small are we? How vast, the space in which we exist?
Since the 17th century, improvements in telescopes and astronomical spectroscopy (the study of star emissions, which allows us to determine their distance from Earth) have further extended the boundaries of space. In the 1920s and 30s, new evidence confirmed that spiral nebulae were not located in our galaxy but constituted their own independent “island universes” – as large and populous as our own. Then, redshift measurements confirmed that the universe was not just large but expanding: that galaxies were speeding away from one another. The logical corollary of this was the Big Bang; the idea that the universe as we see it today is but the “ashes and smoke of bright but very rapid fireworks” (in the words of Belgian astrophysicist Georges Lemaître, whose work forms the basis of the modern theory).
The perceived irrationality of this theory irritated many scientists but was met with triumph in religious circles. Pope Pius XII announced in 1952 that the theory affirmed key Catholic teachings like the doctrine of creation ex nihilo while leaving certain other questions – like what came before the Big Bang and what instigated it – unanswered. Wrote Pius: “will the path, undertaken by the spirit of man, which so far has been to his undisputed honour, later be open indefinitely to him and followed incessantly until the last mystery the universe has in store is solved? Or, on the contrary, is the mystery of nature so vast and hidden that the human spirit, for its intrinsic limitations and disproportions will never fathom it entirely?” This harmony between religious and scientific thought is a reminder that when it comes to cosmology, there is only so far direct observation can take us.
We are, of course, no closer to understanding the ultimate origins of the universe than our ancestors who believed the Milky Way was divine breast milk, but our observations have certainly become more expansive and grounded, forming the fairly recent concept of the cosmic web. Dr Joseph Hennawi, a cosmologist and associate professor at the University of California, says the cosmic web is both a map and a model of the universe; something that helps locate us but also explains how galaxies and stars have developed as they have. “Every civilization was map making. Ancient astronomers used stars as a form of navigation while also trying to map the cosmos, and cosmologists are making maps on the largest scale” he says. “What the cosmic web also does is it gives you the physical framework to interpret that map.” More specifically, the shape of the cosmic web is determined by the energy contents of the universe – by the amount of matter, dark matter, and dark energy that together constitute all reality – and by studying one we learn about the other.
First, though, a quick primer on these three types of stuff. Regular matter – atoms and protons and neutrons – is self-explanatory, but dark matter and dark energy less so. The first thing to remember is that although they sound closely related, these two concepts play very different roles in the cosmological picture. In brief, dark energy is a force that seems to be responsible for pushing galaxies away from one another, while dark matter is stuff that seems to be pulling them together. And while dark energy is thought to be distributed evenly throughout the universe and inherent to the fabric of reality, dark matter clumps together much like ordinary matter does, often forming a sort of halo around galaxies. Both are “dark” in that they’re invisible to scientific instruments: they don’t reflect, emit, or absorb visible light, and nor do they interact with any of the other forms of electromagnetic radiation we use to “look” at things. But we know that they exist because when we model the universe as we see it today, we know that the way gravity has pulled and pushed things together can’t be explained simply by the matter we can see. It’s like you’re at a gig, looking down at a crowd of people from a balcony when space suddenly opens up around someone. You think to yourself: hmm, maybe that guy smells. You’re too far away to know for sure, but why else would people move away in an otherwise crowded room? This is how we know dark energy and dark matter exist, because when we look at the shape of the cosmic web, its distribution of galaxies, clusters, and voids, we know that there has to be something in addition to regular matter pushing and pulling things about. The cosmic web is both map and model.
In recent years, our observations of the web have become much more detailed, in part thanks to data collected by the James Webb telescope, which was launched into orbit in 2021, and capable of retrieving data and images of regions of space more distant than any before. Both Fan and Hennawi are members of a team that used Webb’s data to identify some of the oldest filaments of the cosmic web itself; identifying in June last year a thread of 10 galaxies that formed just 830 million years after the Big Bang took place. The group used the telescope to hunt for quasars, supermassive black holes at the heart of galaxies that are incredibly luminous, using them as beacons to identify clusters of galactic matter in the web to study further. “The theory says these are the most massive black holes, which should be in the most massive galaxies, which are in the most massive dark matter structures, and these are supposed to be the nodes or connections in the cosmic web. That was what we wanted to study,” says Fan.
The team’s findings suggest that the web developed quickly in the history of the universe and was taking shape during the Epoch of Reionization — an era that began around 380,000 years after the Big Bang. Prior to this period, the universe was simply a vast fog of hot, dense gas through which light could not penetrate. Over hundreds of thousands of years, tiny variations in the density of this gas slowly coalesced, eventually collapsing into fiery crucibles of matter: the first stars and galaxies. As these stars shone in what had previously been a lightless void, the radiation they emitted stripped electrons from the surrounding clouds of hydrogen, ionizing them in the process and allowing light to travel freely through the cosmos for the first time. It’s this light that we see today, and which now reveals the shape of the cosmic web itself, in a form we can capture with telescopes like the James Webb. Already, Fan and his colleagues are working on a new survey 10 times larger than the one that revealed these ancient filaments, with observations starting later this year. It’ll look at a portion of the night’s sky roughly equivalent to the size of the Moon as seen from Earth but in even more detail. It’ll hopefully reveal new information about both the structure of the universe and its origins.
Thinking about the universe – about reality – on these grand scales might seem daunting, even maddening; the type of work likely to trigger an existential breakdown in even the hardiest of souls. But Fan is phlegmatic about the enterprise. He says he doesn’t get distracted by questions about meaning and reality while he’s working on mapping the earliest formations of the universe. After all, our understanding has changed massively over the millennia: from heavenly tents to celestial spheres, and now the cosmic web. Fan says we can only look around us, observe what we see, and draw conclusions from that data. “I think the stars have been there longer than the theory, and the theory keeps on changing,” he says. “The theory we have now is almost certainly wrong, but I can try and make sure the observations are right.”
This article is taken from Port Issue 35. To continue reading, buy the issue or subscribe here
