Time

December 29, 2020

What is time? Does it have a beginning? Does it have meaning or context considering our immortality? When we look at the Universe today, we know with an extraordinary amount of scientific certainty that it was not simply created as-is, but evolved to its present configuration over billions of years of cosmic history. We can use what we see today, both nearby and at great distances, to extrapolate what the Universe was like a long time ago, and to understand how it came to be the way it is now.

When we think about our cosmic origins, then, it is only human to ask the most fundamental of all possible questions: where did this all come from? It has been more than half a century since the first robust and unique predictions of the Big Bang was confirmed, leading to our modern picture of a Universe that began from a hot, dense state some 13.8 billion years ago. However, in our quest for the beginning, we know already that time could not have started with the Big Bang. In fact, it might not have had a beginning at all.

For a time, there were multiple competing ideas, which were all consistent with the observations we had.

1. An expanding Universe could have originated from a singular point — an event in space-time — where all of space and time emerged from a singularity.
2. The Universe could be expanding today because it was contracting in the past, and will contract again in the future, presenting an oscillating solution.
3. Finally, the expanding Universe could have been an eternal state, where space is expanding now and always had been and always would be where new matter is continuously created to keep the density constant.

These three examples represent the three major options: the Universe had a singular beginning, the Universe is cyclical in nature, or the Universe has always existed. In the 1960s, however, a low-level of microwave radiation was found everywhere across the sky, changing the story forever.

This radiation was not just the same magnitude everywhere, but also the same in all directions. At just a few degrees above absolute zero, it was consistent with the Universe emerging from an earlier, hot dense state, and cooling as it expanded. As improved technology and novel techniques led to better data, we learned that the spectrum of this radiation had a particular shape: that of a near-perfect blackbody. A blackbody is what you get if you have a perfect absorber of radiation heated up to a certain specific temperature. If the Universe expands and cools without changing its entropy (i.e., adiabatically), something that starts with a blackbody spectrum will remain a blackbody, even as it cools. This radiation was not only consistent with being the leftover glow from the Big Bang, but was inconsistent with alternatives like tired light or reflected starlight. According to the Big Bang, the Universe was hotter, denser, uniform and smaller in the past. It only has the properties we see today because it has been expanding, cooling, and experiencing the influence of gravitation for so long. Because the wavelength of radiation stretches as the Universe expands, a smaller Universe should have had radiation with shorter wavelengths, meaning it had higher energies and greater temperatures.

Billions of years ago, it was once so hot that even neutral atoms could not form without being blasted apart. Even earlier than that, today’s microwave radiations were so energetic that they dominated over matter as far as the Universe’s energy content was concerned. At even earlier times, atomic nuclei were instantly blasted apart, and at still earlier ones, we could not even create stable protons and neutrons.

If we extrapolate all the way back, to arbitrarily hot temperatures, small distances, and high densities, you would intuit that this would truly equate to the beginning. If you were willing to run the clock backwards as far as you could, all of the space that makes up our visible Universe today would be compressed down to a single point.

Now, it is true that if you went to these extreme conditions, compressing all the matter and energy present in today’s Universe into a tiny enough volume of space, the laws of physics would break down. You could try to calculate various properties, but you would only get nonsense for answers. This is what we describe as a singularity: a set of conditions where time and space have no meaning. At first glance, if you do the math, it appears that a singularity is inevitable, regardless of what dominates the Universe’s energy content.

Singularities are where the law of gravitation governing the Universe — Einstein’s General Relativity — yields nonsense for predictions. Relativity, remember, is the theory that describes space and time. But at singularities, both spatial and temporal dimensions cease to exist. Asking questions like “what came before this event where time began” is as nonsensical as asking “where am I” if space no longer exists.

Indeed, this is the argument that many make, including Paul Davies, when they claim that there can be no discussion of what occurred before the Big Bang. This is a tautology, of course, if you assert that the Big Bang is where time began. But as interesting as this argument is, we know that the Big Bang is not where time began anymore. Ever since we have made modern, detailed measurements of the cosmos, we have learned that this extrapolation to a singularity must be wrong.

In particular, the patterns and magnitudes of the fluctuations that we have discovered in the modern radiation left over from that early, hot, dense state teach us a number of important properties about our Universe. They teach us how much matter was present in dark matter as well as normal matter: protons, neutrons and electrons. They give us a measurement of the Universe’s spatial curvature, as well as the presence of dark energy and the effects of neutrinos.

But they also tell us something vitally important that is often overlooked: they tell us whether there was a maximum temperature for the Universe back in its earliest stages. According to the data from WMAP and Planck, the Universe never achieved a temperature greater than about 10²⁹ K. This number is enormous, but it is over 1,000 times smaller than the temperatures we would need to equate to a singularity.

The particular properties of the Universe that are imprinted upon it from the earliest stages provide a window into the physical processes that took place at those times. Not only do they tell us that we cannot extrapolate the Big Bang all the way back to a singularity, but they tell us about the state that existed prior to (and set up) the hot Big Bang: a period of cosmic inflation.

During inflation, there was a tremendous amount of energy inherent to space itself, causing the Universe to expand both rapidly and relentlessly: at an exponential rate. This period of inflation occurred prior to the hot Big Bang, set up the initial conditions that our Universe began with, and left a series of unique imprints that we searched for and discovered after the theory had already predicted them. By any metric, inflation is a tremendous success.

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.

 That is what science says, but does it really help with our day-to-day reality? The Hindu yugas tell us that time is cyclical.