How far does time go back? It’s a simple enough question that a child could have the temerity to ask. Any attempt at a meaningful answer, however, leads us headfirst into the limits of understanding what time even is. At the most solipsistic level, it only goes as far back as our earliest memories. Before that, the starting point of our sentience appears to be shrouded in oblivion, or perhaps a fragmented haze of impressions. At the most objective level, this may not be such an inaccurate metaphor, at least according to our current understanding of what might appear to be the “beginning” of the universe, the point at which the very notion of a continuum spacetime breaks down.
I must have been around 5 years old when I made the mistake of asking my dad about history. What was the world like when he was born? What about 20, or 50 years before that? By then, my old man had developed the habit of messing with me for his own amusement. So when I got around to asking him about what the world was like 100 years ago—a number that, to my sapling mind, was about as close to infinity as I could conceive—he playfully dismissed me by telling me that there was no history back then. Time only went back 100 years, he declared. I don’t remember the exact nature of my reaction to this fiat, but kicks and screams were involved, very likely aggravated by his giggling. I still wonder to this day whether my dad was just being a jerk, or whether this was his way of planting the seeds that eventually provoked me into becoming a theoretical physicist.
There may be certain questions that will remain forever beyond our reach from our vantage point.
The true nature of the beginning of time is a question that many ancient traditions, including that of the culture I was born into, Hinduism, seem at first glance to evade—by putting time on a circle. The writings of the historian and philosopher John Gray first alerted me to the idea that Abrahamic thought might have revolutionized the historiography of time, by putting it on a line. In doing so, one is immediately forced to confront questions and imperatives that may not occur, or be dismissed by those that posited a cyclicity to time, questions such as: When and how did it begin? If time has a beginning, does it have an end? Although the object of Gray’s critique was the implicitly millenarian subtext to post-Enlightenment notions of progress,1 the broader implications of his assertion were not easy for the likes of me to digest.
In spite of the remarkable though often overlooked, and sometimes co-opted, contributions to knowledge multiple intellectual traditions outside the Abrahamic fold have made, it was in those that we find sustained, if flawed, inquiries into a peculiar possibility: that time had its origin in a moment of creation. As a detail in the annals of science, it might therefore come as a bit too on the nose that the first mind to have mathematically derived a beginning to the universe, as a consequence of Einstein’s theory of General Relativity, was an ordained Catholic priest.
Georges Lemaître, who also happened to be a professor of physics, independently arrived at a solution to Einstein’s theory (first written down by the Russian mathematician Alexander Friedmann several years before). The solution described a universe filled with matter or radiation, with no fixed center nor preferred direction, that had its origin in a “primeval atom.”2 In doing so, Lemaître accounted for Edwin Hubble’s startling discovery, a few years earlier, that distant galaxies in the night sky were receding from us in an ordered manner, the only implication of which could be that the universe was expanding.
The Big Bang, as Friedman and Lemaître’s solution came to be known, is the cornerstone of modern cosmology, most notable for the fact that it is an incomplete account. It raises as many questions as it answers, the most obvious one being: How did the big bang itself come about in the first place as the dense, thermalized, rapidly expanding, yet almost uniform fireball that was Lemaître’s primeval atom? Moreover, the primordial plasma came seeded with microscopic fluctuations, which went on to collapse under their own gravity once the universe cooled enough, and in doing so, condensed into the weblike scaffolding around which all the galaxies we observe eventually formed. Where did these seed perturbations come from?
It is widely accepted today that these seeds are none other than microscopic quantum fluctuations—ripples set up by some dynamical process that preceded the big bang. Perhaps this process was a phase of primordial inflation, in which the big bang was the terminal, explosive release of energy stored in some as yet unknown field which set up the initial conditions for the big bang. Perhaps the universe periodically expanded and contracted in a series of cycles, with a bang at the culmination of each cycle. Perhaps it was neither. We may, given our intrinsically limited observations, be merely accessing a partial projection of the underlying reality, one we interpret through the theories currently at our disposal, perhaps not unlike that of the ancient astronomer Ptolemy, whose geocentric interpretations of the movements of planets and stars were once prevalent in Europe.
Being a cosmologist is not unlike being an archaeologist. You’re trying to reconstruct what might have happened in the distant past using the laws of physics and whatever evidence history has been kind enough to leave behind, obscured by layers of galactic dust and various other detritus through which we’re trying to peer. The evidence is in the form of surveys of distant galaxies and quasars, themselves a tracer of an underlying web of clustered dark matter, and maps of the relic microwave radiation left over from the big bang. In the near future cosmologists aim to also observe the relic bath of cosmic neutrinos, the faint radiation emitted from clouds of neutral hydrogen that made up most of the universe before stars had formed, and, of course, the gravitational waves that could have traveled unimpeded to us from the big bang.
There may come a time in the not too distant future where we can peer directly into the big bang.
It is a tantalizing prospect that young people alive today could live to see the day when most if not all gravitationally bound structures within our cosmic horizon, whether they shine light at us or not, be mapped in totality. Like the surface of the Earth, our cosmic horizon is an ever expanding but finite volume, after all. A remarkably accurate picture has emerged over decades of observations that indicate the consistency of the predictions of a hot big bang seeded with microscopic, almost scale-free fluctuations, and a universe made up mostly of dark matter and an as yet undetermined yet energetically significant component that imbues spacetime with an “anti-elasticity,” causing its expansion rate to accelerate, referred to as dark energy.
As cosmic archaeologists, we are limited by the historical contingencies that place us exactly where we are, as we are. If, for instance, the earliest stars shone much more brightly, evidence for the initial seed fluctuations in the big bang that imprinted themselves as tiny temperature variations in the cosmic microwave background would have been erased, and our task of inferring the initial conditions of the big bang would have been made much more difficult. Our coming into existence as sentient observers occurred at a very particular moment in cosmic history. What an observer might conclude from their observations in another epoch might be very different depending on when they are.
This is dramatically illustrated by a simple thought experiment. Let’s assume for the sake of argument that our current cosmological model is in fact correct, that our inference is spot on, and that we have not been deceived by the assumptions that go into them—a prospect that I’ve never been able to completely shake. In this very universe that humans managed to glimpse around 13.8 billion years after the big bang, in the far future, say a trillion years from now, what would a sentient observer located in our galaxy conclude?
For starters, our nearest galactic neighbor, the Andromeda galaxy, and the Milky Way will have merged into a single galaxy—Milkomeda. All other galaxies beyond our immediate galactic neighbourhood would have accelerated beyond our cosmic horizon. The relic radiation from the big bang would have a wavelength larger than our horizon, and so impossible to detect. An observer a trillion years from now, looking up from the galactic plane of Milkomeda, would see an empty universe in which they were the center, and probably declare that to be that. Their universe always was, always will be, unchanging. This observer could only infer a time without boundary in the infinite past and the infinite future, and be justified in their narcissism in concluding that they are at the center of it all.
Fortunately, we are not this future observer, and are in a relatively privileged position among all the cosmic archaeologists that might ever exist. Although we ourselves have our own limits in what we can infer given our location in history. Frustratingly, there may be certain questions that will remain forever beyond our reach from our vantage point. It could be, for instance, that the dust and detritus within our galaxy—foregrounds as they’re technically known—might fatally obscure our attempts to map the structure of the universe before stars even had a chance to form. Or, it could be that whatever preceded the big bang itself had an epoch preceding it, as did that epoch, and so on. Even if this process doesn’t regress infinitely, any information from the initial state of the universe will have been recycled through and possibly overwritten by multiple cycles of cosmic cataclysm.
Any observer at that scale would be adrift in a froth of creation and destruction.
As a species, however, we are nothing if not stubbornly determined and enterprising in our right to continue asking questions, and there are possibilities that may yet offer a glimpse further back than any probe available to us at present. Gravitational waves for instance, unlike electromagnetic waves, are barely obscured by intervening structures and clumps of matter, and so may yet offer us a metaphorical X-ray into the very first moments. There may come a time in the not too distant future where we can peer directly into the big bang using futuristic gravitational wave detectors that are sensitive to a wide range of frequencies, possibly even probing for what might have preceded it.
It’s natural to wonder whether we’ll ever be able to answer the questions for which we seek answers to the degree of certitude proffered by our more commonly venerated creation myths. Whether the chain of explanation of scientific inquiry ever reaches an end, or whether it’s turtles all the way down. Fortunately, or unfortunately depending on your predisposition, there is an ultimate veil of ignorance beyond which cosmologists are absolved of any further responsibility, where the inquiry into the origins of space and time disintegrates into meaninglessness. That is to say, when the very framework within which we’re even able to frame such questions breaks down.
This is in the regime where notions of continuum geometry—the idea that space and time are infinitely divisible—stops making sense. The arena in which things happen starts to instead resemble a violent magma, possibly fractal in nature—a quantum foam, first anticipated by John Wheeler in the 1950s. That is, instead of an infinitely divisible continuum, spacetime begins to resemble something more like a continuously bubbling and re-forming froth. That this is the true nature of spacetime at the most fundamental scale, and that the continuum geometry that we’re familiar with is emergent at larger scales, is something that multiple independent and complementary approaches to quantum gravity have arrived at. These approaches go by various different names, such as string theory, loop quantum gravity, causal dynamical triangulations, among others, but can also be understood on rather general grounds.
That there is very likely a smallest-possible scale below which space and time themselves break down is one of the more jarring consequences of attempts to make quantum mechanics and gravity sit together within a self-consistent framework. Below the Planck length, a resolution of about 10-35 meters—or between intervals shorter than the amount of time it would take light to traverse this distance, or about 10-43 seconds (known as the Planck time)—there is no meaningfully definable geometry or topology to spacetime. These are the scales at which the physics responsible for the ultimate origin of the universe operates at. To understand this, one only needs to invoke Heisenberg’s uncertainty principle in the presence of gravity.
According to the uncertainty principle, were we to localize a point in spacetime with finer and finer precision, we become more and more uncertain of the momentum or energy within it. Any attempt to probe spacetime beyond the Planck scale will result in quantum fluctuations with so much energy localized within that region, that it can spontaneously collapse into a black hole, and will break off from geometry as a spacetime singularity (geometry itself, let alone the notion of a point, loses all meaning at the Planck scale). Only to evaporate again in the smallest resolvable instant. Any observer at that scale would be adrift in a froth of creation and destruction, where moments and islands of space appear and disappear at every instant.
The usual continuum nature of spacetime emerges at macroscopic distances from this chaotic, non-geometric, microscopic reality as an averaged effect, much like the illusion of solidity of everyday objects. These are in fact constituted entirely of fundamental particles of no resolvable spatial extent, stacked in highly energetic configurations that manifest as nucleons and atoms at larger length scales. From this, we can appreciate the category error inherent in even asking whether time had a beginning or not, as it turns out to be just as meaningless as the question of where the edge of the earth is.
It’s entirely possible that we may never get to confront this ultimate veil of ignorance given our place in time. If, as is widely accepted, an epoch of primordial inflation was the origin of what looked like a big bang in our past, it’s entirely possible that the universe at its largest scales consists of multiple, self-reproducing regions,3 each with their own cosmic horizons, unaware of the other. By peering back at what might look like the big bang to them, any observer in this multiverse would merely be looking back at their nucleation—not unlike a bubble nucleating in a pot of boiling water—in their branch of a much bigger universe whose ultimate origin might never fully be revealed to them. It’s also possible that the universe undergoes periodic cycles of creation and destruction, rendering the very question of its origin moot.
The remarkable thing is that, in my view, there’s a chance that we may find out either way within a human lifetime. I happen to be in a minority among cosmologists in being uneasy with how much our current understanding seems to flatter our partial theoretical understanding of physics at very high energies, close to the initial moment, where quantum gravitational effects can become important. I’m unsure whether we’re justified in extrapolating our spectacular success in decoding the initial conditions of the thermal big bang with our limited observations to necessarily taking for granted that a phase of inflation preceded it. I’m constantly mindful of the paradigm of inflationary cosmology being the butt of a cautionary anecdote by future historians and sociologists of science. The remarkable and invigorating thing is that none of my doubts matter. Cosmologists are nowhere near being done yet with banging their heads against hard questions, and I’m thoroughly energized by the prospect of having these doubts dispelled in my lifetime.
As to whether I’m any closer to losing my wariness of being mocked, as my old man once did to me, for the naivete of my questions, I’m still not so sure. A form of occupational imposter syndrome, perhaps, that lingers, yet anchors me.
Subodh Patil is an assistant professor at the Lorentz Institute for Theoretical Physics at Leiden University. He tweets on occasion at @_subodhpatil.
Lead image: Art Furnace / Shutterstock
1. Gray, J. The Silence of Animals Penguin, New York, NY (2014).
2. LeMaître, C.G. The Primeval Atom D. Van Nostrand Company, Inc., New York, NY (1950).3. Kinney, W. An Infinity of Worlds MIT Press, Cambridge, MA (2022).
3. Kinney, W. An Infinity of Worlds MIT Press, Cambridge, MA (2022).