“Even though we can trace our cosmic history all the way back to the earliest stages of the hot Big Bang, that isn’t enough to answer the question of how (or if) time began. Going even earlier, to the end-stages of cosmic inflation, we can learn how the Big Bang was set up and began, but we have no observable information about what occurred prior to that. The final fraction-of-a-second of inflation is where our knowledge ends.
Thousands of years after we laid out the three major possibilities for how time began — as having always existed, as having begun a finite duration ago in the past, or as being a cyclical entity — we are no closer to a definitive answer. Whether time is finite, infinite, or cyclical is not a question that we have enough information within our observable Universe to answer. Unless we figure out a new way to gain information about this deep, existential question, the answer may forever be beyond the limits of what is knowable.”
If you didn’t know anything about the Universe, you might intuit three possibilities for how time originated. Either it had a beginning a finite duration ago, or it existed for an eternity into the past, or it is cyclical in nature, with no beginning, end, or true delineation between past and future. But we have lots of physical evidence today. We know about the Big Bang and what its limits are. We know about cosmic inflation, which preceded and set up the Big Bang. And we know about dark energy, which determines the fate of our Universe.
Ask Ethan: Are The Smallest Particles Of All Truly Fundamental?
“Is there theoretical or experimental evidence which unambiguously establishes the existence of fundamental particles?”
When we talk about the Standard Model, including the quarks, leptons, their antiparticles, and the bosons that make up the Universe, we implicitly assume that these are fundamental particles. When we say fundamental, we often imply that these are the smallest possible, indivisible structural components of all that exists. Yet there’s a limit as far as how well we actually know this goes. Our experimental reach is limited in terms of energy; better deep inelastic scattering experiments might yet reveal a composite structure to the particles that we presently think are fundamental. There might be a more fundamental structure that makes up these particles, and those structures may not be particles. Dark matter and dark energy may not be particles at all, and space and time might be continuous or discrete, quantum or not, and either fundamental or emergent.
“Every time you see a diagram, an article, or a story talking about the “big bang singularity” or any sort of big bang/singularity existing before inflation, know that you’re dealing with an outdated method of thinking. The idea of a Big Bang singularity went out the window as soon as we realized we had a different state — that of cosmic inflation — preceding and setting up the early, hot-and-dense state of the Big Bang. There may have been a singularity at the very beginning of space and time, with inflation arising after that, but there’s no guarantee. In science, there are the things we can test, measure, predict, and confirm or refute, like an inflationary state giving rise to a hot Big Bang. Everything else? It’s nothing more than speculation.”
The Universe, as we observe it today, is expanding and cooling, with the overall density dropping as the volume of space increases. If we ran the clock backwards, however, instead of forwards, things would appear to contract, become denser, and grow hotter. If you go back farther and farther in time, you’d come to an epoch before there were stars and galaxies; before neutral atoms could stably form; before atomic nuclei could remain; etc. You’d go all the way back to hotter and denser states, eventually compressing all the matter and energy in the Universe into a single point: a singularity. This was the ultimate beginning of everything according to the original Big Bang: the birth of time and space.
Are Space And Time Quantized? Maybe Not, Says Science
“Incredibly, there may actually be a way to test whether there is a smallest length scale or not. Three years before he died, physicist Jacob Bekenstein put forth a brilliant idea for an experiment where a single photon would pass through a crystal, causing it to move by a slight amount. Because photons can be tuned in energy (continuously) and crystals can be very massive compared to a photon’s momentum, it ought to be possible to detect whether the “steps” that the crystal moves in are discrete or continuous. With a low-enough energy photon, if space is quantized, the crystal would either move a single quantum step or not at all.”
When it comes to the Universe, everything that’s in it appears to be quantum. All the particles, radiation, and interactions we know of are quantized, and can be expressed in terms of discrete packets of energy. Not everything, however, goes in steps. Photons can take on any energy at all, not just a set of discrete values. Put an electron in a conducting band, and its position can take on a set of continuous (not discrete) values. And so then there’s the big question: what about space and time? Are they quantized? Are they discrete? Or might they be continuous, even if there’s a fundamental quantum theory of gravity.
“The Universe began not with a whimper, but with a bang! At least, that’s what you’re commonly told: the Universe and everything in it came into existence at the moment of the Big Bang. Space, time, and all the matter and energy within began from a singular point, and then expanded and cooled, giving rise over billions of years to the atoms, stars, galaxies, and clusters of galaxies spread out across the billions of light years that make up our observable Universe. It’s a compelling, beautiful picture that explains so much of what we see, from the present large-scale structure of the Universe’s two trillion galaxies to the leftover glow of radiation permeating all of existence. Unfortunately, it’s also wrong, and scientists have known this for almost 40 years.”
Did the Universe begin with the Big Bang? When we discovered the cosmic microwave background, and its properties matched exactly the prediction of the Big Bang theory, it was a watershed moment for cosmology. For the first time, we had uncovered the origins to the entire Universe, having learned where all of this came from at long last. Emerging from a hot, dense, expanding, and cooling state, the matter-and-radiation-filled early Universe gave rise to everything we see today. Except there were a few pesky problems that the Big Bang couldn’t explain. If the Universe truly emerged from an arbitrarily hot, dense state, and if space and time themselves were born at that exact moment, the Universe would have signatures that we simply don’t see. Instead, theorists came up with an alternative beginning: cosmic inflation. Inflation made a bold prediction about the scale and magnitude of the fluctuations that should arise from this early state, and when our technology finally caught up to our imaginations, we measured them.
Ask Ethan: Does Light Always Move At The Same Speed?
“Does light always move at the same speed? If it is slowed down by something, will is stay slower after it is no longer being slowed down? Will [it] speed back up to the speed of light?”
Throughout the entire Universe, there’s a fundamental law that governs the motions of all particles: Einstein’s relativity. It states that all particles with mass can never attain the speed of light, no matter how much energy you put into it. Additionally, all massless particles only move at the speed of light, no matter what you do to either them or to the device/person observing them. No matter what reference frame you’re in, the speed of light in a vacuum is a constant. But light isn’t always in a vacuum! From air to quartz to acrylic to glass to many other media, light can pass through transparent material, and when it does, it slows down. Not only that, but light of different energy slows down by different amounts. In what ways is the speed of light always the same, and in what ways can it change?
“You might then ask, at what distance does the expansion start to take over? That happens when you average over a volume so large that the density of matter inside the volume has a gravitational self-attraction weaker than the expansion’s pull. From atomic nuclei up, the larger the volume you average over, the smaller the average density. But it is only somewhere beyond the scales of galaxy clusters that expansion takes over. On very short distances, when the nuclear and electromagnetic forces aren’t neutralized, these also act against the pull of gravity. This safely prevents atoms and molecules from being torn apart by the universe’s expansion.”
The Universe is expanding. The farther away a galaxy is, the faster it appears to be receding from us. The standard story tells us that space itself is expanding, and that’s the cause, but it’s only natural to wonder if perhaps space is static, and everything else within it isn’t shrinking instead? Many laypersons choose to go this route, and question the entire field of cosmology as a result. But is this fair? Or is this a road to not only ruin, but to physical inconsistencies? Could we flip the story on its head, and do some sort of test to see if atoms, the planet, or some other ‘local’ entity is shrinking, instead? Or, using the principle of relativity, could we declare that all frames are equally valid, and choose a frame where space isn’t expanding, after all?
Ask Ethan: What does the edge of the Universe look like?
“If the universe is finite in volume, then is there a boundary? Is it approachable? And what might the view in that direction be?”
When we look at the nearby Universe, it looks a lot like we, ourselves, appear. Nearby galaxies are similar in structure to our own; the stars inside them have the same properties, masses, ages and distributions as our own. But as we look to greater and greater cosmic distances, we find that more distant galaxies appear younger, bluer, smaller, and less evolved. If we go back beyond a certain point, there are no more galaxies at all. What’s going on? Does this mean that, if we go back far enough, there’s an edge to the Universe? And if so, what does that mean? What lies beyond it? And what would someone living on that edge see and perceive? It turns out that there is a boundary to our Universe, but not in space.
“You can witness the evolution and destruction of humanity; the end of the Earth and Sun; the dissociation of our galaxy; the heat death of the Universe itself. So long as you have enough power in your space ship, you can travel as far into the future as you like.”
Have you ever wondered about time travel? Perhaps you have your destination in the far future, and want to see how it all turns out? Maybe you want to return to the past, and alter the future or present by your actions there? Or maybe you want to freeze time altogether? If you want to know whether it’s possible, the physics of relativity holds the answer. Special relativity allows us to control our motion through time by manipulating our motion through space. The more we move through space, the less we move through time, allowing us to travel as far as we want into the future, limited only by our energy available for space travel. But going to the past requires some specific solutions to general relativity, which may (or may not) describe our physical Universe.