Beyond the Observable Universe

Our view of the universe is restricted, curtailed by its finite age and the travel time of light reaching the earth, which conceals its true, majestic form from us. What we observe to be the edge of our universe is not the actual edge of the universe, with most scientists in agreement that more space lies hidden beyond what we're able to see. Last time out we travelled to the very edge of our observable universe, but today we will be going even farther, as we wade out into the darkness of uncharted territory, dispensing with the notions of redshift and light delay, as we finally get to know the nature of the cosmos beyond our observable universe. Our last journey concluded at the cosmic light horizon, the optical boundary of our observable universe, and of every point in space in fact. Any observers view of the universe ends with a cosmic horizon, beyond which the light from distant galaxies has not had enough time to reach that observer in the 13.8 billion years since the birth of the universe. Therefore, our optical view of space starts to fade to an all-encompassing curtain of darkness, which shrouds the observable universe. But this curtain is not quite the edge of our universe. Undercutting it slightly, in the radio spectrum, we find the cosmic microwave background, the omnipresent relic radiation left behind by the big bang. As our universe's oldest and most complete detectable light signal, this map represents the holy grail of modern cosmology. There may be a couple of other, even older signals from earlier in the universe's life, such as the cosmic neutrino background, but these signals propagate at vanishing rates of energy far too ghostly for humanity to presently detect. For all intents and purposes, the cosmic microwave background marks the edge of our observable universe. And it allows us to gauge the properties of the space that may lie beyond for one key reason. On the largest cosmic scales, the universe appears strikingly smooth and broadly isotropic, its contents evenly distributed, with only local degrees of randomness. And so, according to the cosmological principle, lying beyond the cosmic horizon, we find more of the same quite simply- galaxy clusters and voids which comprise the cosmic web. In the absence of light's delay to curtail our view of space at a certain point, the all-encompassing cosmic horizon would start to fade, revealing a much larger and more chemically evolved cosmos, branching for more than 100 billion light years. The farthest, earliest galaxies like GNZ 11, HD 1 and Glass Z 13 today lie at depths in excess of 33 billion light years, having been cast to almost treble their original distance by the expansion of space. These earliest galaxies would no longer appear as torrid pockets of primordial stars and gas, and would probably instead appear similar to the galaxies in our local volume, like the Milky Way, the Andromeda Galaxy and the Silver Coin. Or perhaps they may resemble late-stage, evolved giant elliptical galaxies like Messier 87, containing several trillion stars but lacking important star-forming gases. Together, these two types of galaxies would make up the majority of what we'd expect to find beyond the cosmic horizon. In reality, the question of what lies beyond is relatively straightforward. The bigger, and harder to answer question is, how much is out there? But answering that question requires understanding the shape of the universe at large. For the longest time, dating all the way back to Aristotle, humanity believed the universe to assume the shape of a sphere, with Earth contained somewhere on the inside, sectioned off from a wider plane by a physical boundary where space stops, much like our observable universe. But mathematics is the programming language with which our cosmos was written, and even the greatest minds and machines alike have found it near-on impossible to describe what would happen at a universal boundary, or how such a boundary would even manifest in the first place. Furthermore, we don't see any indications of such a barrier. There doesn't seem to be anywhere in space, aside from black holes where photons of light run aground. And thus, we now believe that, no matter what size or shape the universe is, it almost certainly won't have a physical edge like our observable universe. Instead of its contents being enclosed within the volume of a sphere, it is more like we are mapped to the outside surface area. The surface of a sphere is both finite in extent and without boundary. Take the Earth for example. Even though it is finite, you could still fly a plane indefinitely around its expanse without meeting an edge. You would never run out of Earth to fly across, as gravity would simply bring you back round to your starting point. And it is thought that the gravity exerted by the universe induces the same effect. Therefore, the universe won't have edges, but it might have a curvature. The curvature of space is its local deviation in geometry to that of boundless, infinite, 3D Euclidean space. In other words, if the universe is not infinite, then it should curve at some large extent to close itself off, much like the surface area of Earth. And if we could detect such geometry from within the frame of our observable universe, we could use it to make reasonable estimates about the global shape, size and even eventual fate of the cosmos. This curvature is fundamentally tied to the energy density of space. And therefore the universe's density parameter, represented by the Greek character Omega, can tell us about the curvature of space on large scales. Einstein showed us that mass-gravity curves space-time. And so, a universe dominated by the self-gravitation of its cosmic web should curve all the way round, so to speak, completely closing off its finite volume. Like the sphere example we mentioned earlier, such a universe would have a positive curvature with an omega value greater than one, representing a high density for gravity inducing matter and dark matter. In a positively curved universe, two parallel photons of light would eventually converge at some extremely distant point, not least because they would eventually be brought back within range of each other by a universe destined to collapse. A universe dominated by the gravity of its cosmic web is capable of both halting its expansion and reversing the process within a finite amount of time, as gravity eventually triumphs over the expansionary force. This is theorised to trigger a period of runaway contraction, eventually crushing the cosmic web as it is cramped back into a comparably minuscule area, returning it to a hot and dense high-energy state reminiscent of its earliest moments. It's an interesting possibility, and one that scientists believed for the longest time. However, in 1997, it became apparent thanks to the Hubble telescope that the universe is not on a path to a big crunch any time soon, as its expansion appears to have accelerated with time. But a universe dominated by gravity cannot speed up its expansion. That would be like throwing a ball into the air on earth and watching it fly off to escape the atmosphere at an ever-increasing rate. Instead, cosmic acceleration implies that the universe is not dominated by gravity on the largest of scales, and must instead be powered by an anti-gravity-like pressure, which we call dark energy. Dark energy is attributed to the repulsive vacuum energy of space, which exerts negative pressure tension that smooths out and inverts space-time, where gravity would otherwise curve. If the lion's share of the universe's contents owes to this dark energy, then it would mould space into a bizarre, open-ended, unclosing hyperbolic curvature. In a universe that is negatively curved, with a density parameter less than one, the gravity exerted by the cosmic web would be too insubstantial to halt the universe's growth, leading to a period of runaway expansion that proceeds for an eternity. Such a universe would have an irregular, peculiar saddle or funnel shape, or perhaps a horn shape, all of which are difficult to describe and enumerate. Such a universe could be infinite or finite in an ever-expanding future, and thus two parallel photons of light would soon diverge as they barrel off to infinitum, with the universe's expansion giving rise to new space forevermore. And then finally balanced on a cosmic knife-edge between these two eventualities, we have a universe at the critical density, where omega is exactly equal to one, about 5.7 hydrogen atoms per cubic metre of space. At this cosmic density, the positively curving influence of gravity is perfectly poised and balanced by the negative, inverting tension released by dark energy, resulting in no significant global curvature and a flat geometry. A so-called flat universe without curvature is not capable of closing itself up to curtail its volume, and is therefore the geometry we'd expect to find in a universe that is truly infinite. Euclidean space would apply on all length scales, as two parallel photons of light would remain parallel for an eternity, encountering new space forevermore like a boundless, unending sheet of cosmic paper. With gravity and dark energy balanced at equilibrium, a universe at the critical density is smooth, and is not capable of reversing its expansion. Rather, a flat universe sits on the boundary for re-collapse, a strange theoretical state where the universe's self-gravity would eventually be sufficient to bring its expansion to a halt, without causing it to collapse, but only after an infinite amount of time. In other words, like the negatively curved universe, a flat universe would be destined to expand forever. With these three equally tantalising eventualities on offer, scientists were keen to ascertain the geometry of space. But how exactly does one measure the curvature of the universe? Well, we have two main methods. The first is to sum up the total energy densities in the universe, by observing its various large-scale properties. The discovery of the nearly uniform microwave sky in the 60s sowed suspicions that the universe must be comparable with the critical density, and the 1997 discovery of cosmic acceleration demonstrated that space must either be negatively curved, or very close to flat indeed. The second, and more rigorous method for deducing universal curvature is to probe the edge of our observable universe, the CMB. This left-over radiation from the closing moments of the Big Bang is like a screenshot of the entire observable universe, from when it was only 379,000 years old. Therefore, if curvature exists to be found anywhere in space, it'll probably be found here. As mentioned, the CMB is strikingly smooth, appearing similar from all directions, with only minor, localised fluctuations in its near-uniform average temperature. These fluctuations have a number of different causes, but they mostly owe to density gradients arising in the early universe's plasma. These gradients manifest on remarkably consistent scales, and so by measuring the angular sizes and distances between the various hotspots, we can search for signs of lensing causing apparent distortions in the radiation. Scientists can then plot lines through these areas to form a triangle, and determine the universe's curvature by measuring its interior angles. In a spherical, positively curved universe, these angles would add up to more than 180 degrees, like they would on the surface of the Earth. Whereas, in a saddle-shaped, negatively curved universe, they would sum to less than 180 degrees. And in an infinite, flat universe with no significant curvature, these angles would sum to exactly 180 degrees. Using this technique, the Boomerang probe was the first to map the CMB's anisotropies in enough detail to reveal its geometry, and it concluded Omega to be exactly 1, with only a 10% margin of error, further fuelling long-held suspicions that the universe may actually be flat. Shortly after, NASA's flagship CMB mapping mission, the Wilkinson Microwave Anisotropy Probe, started to return its first estimates of the universe's density and curvature. And by 2006, it had reaffirmed to an even greater degree of precision that the universe is largely flat and critically dense. In 2010, this probe was decommissioned to coincide with the launch of the European Space Agency's Planck satellite, which gave us our most extensive and detailed dataset of the early universe to date. It also drew the same conclusion, that the universe must be very close to the critical density for flatness. And in the years since, a number of follow-up experiments have independently verified this assertion. As far as we can tell, the universe is flat, with a density matching the critical density. And with that, the new crisis in cosmology was well and truly established. The question remains, why is the universe so perfectly balanced in this knife-edge state? Even if it is not bang on the critical density, and does have a slight curvature, either positive or negative, the fact cosmologists still cannot determine which, after almost 14 billion years, suggests it must have been impeccably close to critical at the moment of the big bang, as any slight disparity would have been magnified over time. Why the universe was born with such perfect properties, and has managed to maintain said properties for so long, is known as the flatness problem. And understanding its underlying cause is a frontier in 21st century cosmology. What we do know is that there are three possible solutions to the flatness problem. Either the universe is actually flat, and therefore likely extends infinitely. Or perhaps it is curved, but on a scale too large for us to detect. Or it may have a topology which hides its true shape as a continuous object. The first of these three possibilities is the most profound. That the universe is truly flat. Euclidean in all directions, and likely infinitely extending. The idea of an infinite cosmos carries its own terrifying existential possibilities, but one does have a hard time imagining how an expanding universe could have reached infinitude in a finite time, if the universe really did start out small. The big bang theory lends itself to a cosmos of finite volume. And our universe has other properties you'd expect to find only in a finite universe as well. Namely, limiting factors. On scales of the sky greater than 60 degrees, the Planck satellite confirmed that the level of variability in the CMB's power spectrum starts to flatten out significantly, assuming a stubborn uniformity and homogeneity. Which implies a cut-off in wave-strength on scales larger than our observable universe. But we would not expect to find such a limiting scale in a universe that is not limited or scaled. And it is statistically improbable, though not impossible, that such wave-strength limits would arise naturally in an infinite cosmos. This is one of the major arguments in favour of a finite universe, whatever size it may be. And so, the universe's apparent flatness is more likely to have an alternative underlying cause. For example, the topology at large may comprise a shape that hides its true dimensions for inside observers, with a complex, multi-connected topology. So far, most of the possible forms we've visualised for the universe have been simply connected. Meaning a straight line could be drawn to connect any two points and be contracted down to a single point, without needing to tunnel through the overarching topology. As is the case for the surface of a sphere, but not for a 2D circle. A multi-connected topology, on the other hand, is one which describes a sophisticated, non-trivial shape for the universe, with interlinking holes, gaps and coplanar extensions that render it impossible to bridge certain disparate point pairings. The simplest, best and most popular example of a multiply-connected space is the donut-shaped universe, known more scientifically as the toroidal universe. Torii are a fundamental shape of geometry, arising in many natural settings, most notably in magnetic fields. And in 1984, a pair of Soviet physicists, Yakov Zel'dovich and his student, Alexey Starobinsky, proposed a mainfold model of the global universe, describing its shape as a three-torus. That is, one where all of its faces are connected. And it is the way in which these faces are connected that holds the key to understanding how a toroidal universe can appear flat and smooth, while also being finite and positively curved. Geometrically, a torus is what we get when we close a simply connected surface, like a sheet of paper or open-ended cylinder, so that it may assume a finite volume. In the context of 3D space, we can achieve this by rolling up our sheet of paper to connect its opposite edges, which gives us an open-ended cylinder that we can then deform further by bending it back on itself to connect the faces. This gives us our closed, rounded 3D donut shape, with a finite volume and positive curvature. But this shape is simply a means to an end, a consequence which the torus inherits from its place in its wider environment. In 3D space, there is no way for us to connect the faces or edges of a flat space without curving it. But in the context of our universe, which is mapped to the outside surface of the torus, the wider plane of 3D space doesn't exist. And from all other vantage points bar the exterior, the space in a toroidal universe would appear no different from the flat, simply connected sheet it started off as. The only difference now being that the sheet is closed, in a way that going far enough in one direction causes you to pop out of the other side, like a game of pacman. And so, instead of experiencing the apparent positive curvature associated with the torus at large, the CMB photons would simply appear as having streamed through flat, Euclidean space, barreling indefinitely through a dark, endless cosmic hallway, despite travelling in closed loops around the torus' geometry. This would explain why the CMB looks the same everywhere, as the seemingly unnatural scale of its smoothness would likely be the result of tiling projections of a smaller, more diffused volume of space, repeating around the torus. But if that were the case, we would expect to see repeating patterns of photons reflected in the background radiation. Alas, no such sequences have ever been identified in one of the CMB datasets. But in truth, this expectation would rely on the global torus being smaller than the perceived extent of our observable universe. In the arguably more likely event that the torus is larger, but not incomparably larger than our observable universe, say around 300 billion light-years by its largest extension, then it would fit reasonably well with a number of observed properties seen in our apparently flat, simply connected universe. But the story of the donut universe doesn't end here either, because we can also factor in the flow of time to give us the hypertorus. In this case, each opening in the torus' topology represents the distant past and future horizon, with space-time flowing unidirectionally from one side to the other. This explains the apparent expansion of the universe thus far as simply space-time following the torus' geometry. Emerging from the past horizon hole associated with the big bang, space expands rapidly early on, before decelerating until it reaches a midpoint, where it starts to contract en route to the future horizon. Remember that big crunch we talked about earlier? Well in this model, as space-time begins to flow into the other opening of the torus, its contents become much smaller and more condensed, compressed into a state reminiscent of the one it started off in. But in the hypertorus, this isn't the end of the line. Rather, the space-time is blended and recycled as it is forced back through into its original half of the torus, where it re-emerges as a new big bang, born again in a fresh repetition of a cyclical universe. If our universe really is shaped like a donut, then perhaps our entire existence is just one go-round in an endless oscillating cycle of universes being born, expanding, and then collapsing into big crunches, with the process repeating at infinitum. The final solution to the flatness problem is that our universe is curved, either positively or negatively, but on a scale too large for us to gauge from within our observable field, just as Earth is indistinguishable from flat on all ground-level vantage points. If the universe is positively curved and spherical to some extent, then it would need to be extraordinarily large in order to hide such a curvature. Estimates vary on how much larger, but we're talking hundreds of times the radius of the observable universe, and millions of times the volume, correlating to a whopping diameter in excess of 20 trillion light-years. Alternatively, if the universe is negatively curved, then it may be even larger, owing to its extreme runaway expansion. But in both cases, how has the universe managed to get this insanely large, in only 13.8 billion years? One has a difficult time making all this fit with the classical description of the Big Bang Theory, which postulates a physically microscopic singular origin for the universe, with maximal heat and density that suddenly began to expand. But if the story were that simple, then our universe would not be nearly as perfect as it appears today. A cosmos which began its journey at Planck-scale temperatures would be expected to bear magnetic monopole particles, among other by-products and blemishes from such an extreme primordial state. We would also expect to see contrasting temperatures on the largest scales, as disparate, causally disconnected regions of space would not have had time to interact and diffuse their contents. And most of all, we would expect to see some kind of curvature, or topological defect in the universe following a classical Big Bang, as an expanding Planck-scale singularity would have no way to remain balanced at the critical density for so long. Alas, the CMB bears none of these scars. It is so stunningly smooth and free from defects, that cosmologists have often wondered whether there's a physical reason behind it. And thus, in recent decades, we've tweaked our descriptions of the Big Bang Theory, challenging, among other things, the notion that the universe grew from a singularity. A singular origin for the universe is simply what scientists arrive at when they extrapolate its expansion back to time zero, using the framework of general relativity. And while it is plausible that space and time were born in such an environment, the invariability and pristineness of the universe today tells us that, between then and the events we associate with the Big Bang, the universe must have been scaled up in size very vigorously and exponentially, before the events we know as the birth of the universe had had time to occur. This scaling event is known as Cosmic Inflation, and it is cosmologists contemporary answer to the flatness problem, explaining our universe's size, smoothness and apparent lack of curvature, without the need for any complex, non-trivial topology. Cosmic Inflation was a theory developed in the late 70s and early 80s, by MIT physicist Alan Guth, as a solution to the magnetic monopole problem plaguing the conventional Big Bang model. He and a colleague, Henry Tai, theorised that a period of super-cooled expansion may have driven the universe to a size so enormous that its curvature was rendered indistinguishable from flat, within the frame of our observable universe. They proposed this runaway expansion to be the result of a form of vacuum decay, catalyzed when the strong nuclear binding force separated from the other fundamental forces, just 10 to the power of minus 36 seconds after the birth of the universe. For some reason, this enabled a rogue, high-energy quantum field to hijack the universe's scale factor, enlarging the small primordial cosmos at a rate equivalent to billions of times the speed of light, scaling it up by at least a factor of 26. This event resized the minuscule, pre-inflationary contents of the universe, which was capable of diffusing, so quickly that its homogeneity would have been preserved for some time after. The only imbalances in this inflationary field would have been caused by quantum ripples, which also became caught up in the scaling of space. This seeded the universe with propagations of a variety of length scales, many of which would have been larger than the relative travel distance of light. At the end of cosmic inflation, just 10 to the power of minus 31 seconds after time zero, this field is believed to have decayed into the first subatomic matter, releasing its enormous potential energy in the process and reheating the universe to the levels associated with the big bang. The resulting immeasurable universe was then flooded with a hot plasma, which followed the density gradients sown by inflationary ripples, many of which now propagated across distances far exceeding their respective light horizons. And burned into the cosmic microwave background to this day are abundances of these super-horizon fluctuations. These fluctuations are one of the great pieces of observational evidence for cosmic inflation, proving, if nothing else, that larger-than-light propagations were indeed a reality in the early universe. This event may have left other evidential imprints on the microwave background as well, including an aftershock signal even older than the cosmic neutrino background. The same process through which inflation seeded the universe with density propagations should also have released a series of primordial gravitational waves, which may still be vibrating the universe, albeit at a vanishing rate of energy, to this day. For now, however, and like the CNB, this gravitational wave background lies well beyond the capacity of our current wave-detecting technology. But even without this smoking gun of a signal, cosmic inflation has nonetheless established itself in the Standard Model and is now an accepted reality among most cosmologists, being the simplest all-encompassing solution to the flatness problem, addressing all the universal properties we've covered today without the need for any fancy traits or unlikely shapes. And though the hunt for more definitive proof goes on, it would seem that a new variant of the process is on the rise in our present-day universe. In fact, it was Alan Guth himself, along with Alexey Starobinsky, who first described an anti-gravity field exerting negative pressure, to explain cosmic inflation. The same type of field we today use to describe dark energy driving the universe's accelerating expansion. A lot more research needs to be conducted to establish what, if anything, links the fields driving these periods of expansion. But it would seem that the tendency to go rogue and accelerate its growth, against all the odds, is a peculiar natural instinct our universe was born with. To quote an email from one of the discoverers of dark energy, Dr. Adam Rees, maybe the universe does this from time to time. And with that, I wish you all a very Merry Christmas, and I'll see you in 2023.