Into the Unknown, page 6
Another essential point that needs to be spelled out—scientifically, the “Big Bang” does not refer to actual creation of the universe to begin with, although the term is often conflated with this original instant of being. Rather the “Big Bang” solely refers to this period of radical inflation from whatever preexisting tiny nugget of a universe there might have been. Where that original nugget came from is another turtle down.
This is where the rubber hits the road as far as science goes: An idea as crazy as the universe expanding by a factor of 1026 in less than 10−32 seconds necessitates some compelling justification. To be sure, scientists didn’t just wake up one morning and apropos of nothing say, “Hey, I have this radical idea the universe underwent extreme inflation 14 billion years ago.” Rather, the idea of the Big Bang came about over many decades of slow burn, punctuated by a few dramatic revelations.
I do not want to go into a full and detailed history of the science that unfolded in the twentieth century that led up to the broad adoption of the Big Bang theory, which you can surely find a good account of elsewhere. However, because this book is (at least in part), about the limits of empirical inquiry, I do feel impelled to offer you a glimpse into some of these bits of empirical inquiry that convinced virtually all astronomers and physicists (and astrophysicists) to buy into this inflation hypothesis,8 which even a hardened scientist must admit sounds kind of insane at first blush.
It turns out, we didn’t even need to wait for modern astronomical observatories; I don’t know if the fact that the night sky is dark has ever bothered you, but this turns out to be a nontrivial statement. At least as far back as the sixth century, the Greek monk Cosmas Indicopleustes was wrestling with the implication of having a dark night sky—intuiting that if the universe were infinite in time and space (and uniformly populated by stars), we would burn up.9
This observation is more generally known as the “dark sky paradox” but is often attributed to Heinrich Wilhelm Olbers as Olbers’ paradox, an honor that science historians dispute his earning of.10
The dark sky paradox (which is, as usual, ultimately not an actual “paradox”) goes like this: If the universe is infinite back in time, infinite in space, and approximately uniformly populated by an infinite number of stars, every line of sight out into space ought to land on a star. If this were true, the entire night sky should be bright and virtually uniformly blanketed by stars. Clearly the night sky is dark, which is obvious to even an untrained observer (unless, perhaps, they live in Manhattan), so one or more of these assumptions about the universe has to give.
As we look farther away in the universe, individual stars appear fainter due to their distance, but there are more stars in our cone of vision as well. These two effects cancel each other out.
My students often have their intuition tricked by their innate understanding that stars that are farther away will appear fainter to us, and surely this increasing faintness matters. It does, indeed, matter. But the catch is that each shell of the universe in our cone of vision should contain exactly proportionally more stars. In other words, the apparent brightness of the stars gets fainter as the distance squared, but the number of stars in the cone of our vision should also go up by the distance squared, and the two geometric effects cancel.
There are a range of solutions to the dark sky paradox, some of which are extremely contrived. But this “paradox” completely goes away if the universe had a beginning.
An important takeaway from the dark sky paradox is that we don’t always need fancy equipment to come up with compelling hypotheses. Sometimes progress just requires asking a simple question about something we take for granted—like “Why is the night sky dark?”
Another “paradox” that burbled to the surface of physics in the 1900s was the fact that the universe had not decayed into statistical equilibrium. It was clear that applying the second law of thermodynamics to the universe resulted in a flagrant contradiction. Physicists at the time looked to the recently emerged theory of general relativity to argue that the universe was not subject to steady external conditions (i.e., it was not a closed system, as required for the second law of thermodynamics to be enforced), which provided a loophole to wiggle out of the apparent paradox.11 To my mind, this justification just kicked the can down the road, leading to the questions of why the universe might not be subject to steady external conditions and what is external to the universe anyway. Be that as it may, the clearly obvious—even to a layperson—fact that the universe had not “run down” could not be ignored. This fact was part of the slowly simmering case that might suggest the universe was not, in fact, in a steady state.
Simultaneously, two other threads of math and astronomical observation were unfolding, which resulted in the discovery that the universe itself is expanding.12 That is kind of a big deal.
I often try to imagine what it would have been like to have spent your life believing that the universe had been more or less eternally in its current state, and then be challenged to make this radical change to your worldview. Not surprisingly, there was major debate on this topic, which is a healthy thing when a potentially paradigm-shifting idea is introduced. But the data were irrefutable, and it became increasingly difficult to ignore that an expanding universe required that the universe was necessarily smaller in the past. When one followed this line of reasoning to its natural outcome, the inference was that the universe must have once been (perhaps) infinitely small. To be sure, there are other nuances that we should not be too hasty about sweeping under the rug—such as the tautology that we can only observe the region of the universe that we can observe; it could well be that there are distant regions of the universe doing radically different things. The good news is that the Big Bang hypothesis came in a nice package with very strong predictions that could be (and have been) observationally tested.
One strong and specific prediction from the Big Bang model is the precise relative abundances of elements in the universe. For us humans living on this rock with lots of heavy elements, you can be forgiven for thinking that things like oxygen and carbon are abundant in the universe. But once again, we have such a very limited view of reality. If you’ve never thought about where the various elements on the periodic table that haunted you in chemistry class came from, now is your chance. This is one of those things that exist that are easy to take for granted, like the darkness of the night sky. But when you dig a little deeper and start asking questions, suddenly new problems emerge. In this case, with all these elements on the periodic table, it seems terribly unfair that hydrogen gets roughly three-quarters of all the atomic mass in the universe today (and helium gets the remaining one-quarter). The other elements really got stiffed.
Back in the day, by which I mean within like the first 10−6 seconds after the Big Bang, we didn’t even have any elements—rather just an exceptionally hot soup of elementary particles like quarks and gluons. Shortly thereafter (or “immediately” by modern time keeping), this soup cooled enough that protons, electrons, and neutrons could emerge. At that point, the universe only had about twenty minutes to create whatever elements it could before it cooled off too much to slam nuclei together with enough speed. Instead of taking a twenty-minute power nap, the universe squeezed in some element production, but all it had time to do in these twenty minutes was make some helium nuclei (by slamming hydrogen nuclei—in other words, protons—together) and an itty-bitty trace amount of lithium. At that point, the universe ran out of steam (but not literally, because we didn’t have oxygen yet, so we didn’t have H2O), and those were the elements we had—for a very long time—until stars formed, which took a couple hundred million years. All the other elements were forged in the lives and deaths of stars. As the saying goes, you are literally made of stardust (although I would argue it is more like star guts, but that doesn’t sound nearly as poetic).
I know that the physics of all of this might sound complex, but it is actually straightforward thermodynamics and particle physics. The point of which is that we can make very precise predictions about the exact abundances of hydrogen, helium, lithium, and even more importantly—their isotopes.13 Chief among these isotopes is deuterium, or “heavy hydrogen” (because the nucleus has both a proton and a neutron). Unlike the other elements that are created in stars, deuterium is destroyed in stars, so as the universe gets older and stars burn through their fuel, the amount of deuterium is getting ever lower. Moreover, there are no known viable paths to create more than trace amounts of deuterium other than Big Bang nucleosynthesis. The ratio of deuterium to standard hydrogen (also known as “protium”) provides exquisite constraints on the conditions and timescales of the first twenty minutes of the universe. And guess what, the data exactly support the Big Bang theory.
Another bit of evidence is admittedly circumstantial, but a challenge to explain without the Big Bang. If we look out toward the left 10 billion light-years toward the horizon,14 and then we look to the right 10 billion light-years toward the other side of the horizon, the two regions (which were something like 20 billion light-years away from each other when their light headed our way)15 appear to be statistically indistinguishable. In fact, every direction we look out to the horizon, the universe appears to be homogeneous.
If the universe had been sitting around in some kind of steady state, these regions cannot have been in causal contact and sharing information for 20 billion years, in which case the horizon has no business being homogeneous. Once again, cosmic inflation comes to the rescue. With a cosmic inflation scenario, the entire observable universe would have been in causal contact—at the end of inflation, today’s observable universe would have been about a meter in diameter, or about 3x10−9 light seconds. In other words, there was loads of time for causal contact between different places in the universe when it had the size of a large dog. Just like that, the horizon problem goes away.
The Big Bang theory made one more key prediction that would be hard to reconcile with any other physical model: roughly 400,000 years after the Big Bang, the universe should have cooled enough to allow electrons to combine with protons and make bona fide atoms. For reasons I am totally glossing over (let’s just call these reasons “physics”), when the protons and electrons are able to combine, light is able to stream freely out into the universe, and travel in whatever direction it was heading until it runs into something. This background source of light, which should be coming from every direction in the sky, has been dubbed the cosmic microwave background (or CMB). This CMB light should have a very specific observational signature that tells us exactly the temperature of the universe when it was emitted. Couple that with our knowledge of exactly what that temperature should be when the protons and electrons combined into atoms, and we have a very specific and testable prediction. The physics here is exceedingly well understood and easily done by hapless undergraduate physics majors as homework problems. What was less easy was testing this precise prediction.
Lucky for us, some of this light that escaped when electrons and protons combined into atoms proceeded to travel across the universe for more than 13 billion years and run into Earth. There have been several major experiments to measure the properties of the cosmic background, with steadily increasing precision. I think it will not surprise you at this point to learn that the observations exactly agree with predictions from the Big Bang theory. Again.
The bottom line is that there is now a mountain of evidence, using radically different predictions and experiments, that supports the Big Bang theory. The only other hypothesis to date that is 100 percent consistent with the experimental data is that a higher power contrived this universe as a set-up job to fake out us mere mortals into thinking there was a Big Bang. This hypothesis, however, notably lacks testability or predictive power.
Epochs of Confidence
While we have an extraordinary amount of evidence supporting that the Big Bang happened, that is not at all the same thing as understanding how or why it happened. As we consider eras in the cosmos before the present, as a general rule, our level of understanding diminishes. That being said, it may surprise you to hear that in many respects, our understanding of the physics in the universe—between the time when it was about twenty minutes old and 400,000 years old—is more complete than our understanding of the physical processes that have unfolded over the last billion or so years.
Our levels of confidence in what happened fall precipitously along with our ability to empirically test our theories. For example, the earliest time from which we can ever receive light is when the universe was ~400,000 years old—specifically the time when the universe cooled down enough for electrons and protons to settle down and make atoms. To be sure, there was plenty of light before that time, but it was trapped in a kind of particle soup. As a result, this is the earliest time period we can directly observe with light. Ever.
Just because we can’t actually observe the universe at times before about 400,000 years doesn’t mean we are totally in the dark. For example, our ability to detect gravitational waves is rapidly advancing, which will give us a whole new window on the universe. At the moment, our main tools to explore earlier times are fantastical experiments in physics labs. With modern equipment, it is commonplace to observe energetic conditions that would have been present when the universe was only a few minutes old, so the physics that unfolds under these conditions is well understood.
We are now astoundingly able to probe energy levels present when the universe was less than 10−32 seconds old, which is—at least to me even with my desensitized astrophysics brain—utterly mind-blowing. These insane energy levels can be achieved with the Large Hadron Collider in Europe. Please take a moment and let that sink in—modern experiments can explore the conditions of a 10−32-second-old universe.
That being said, even with the Large Hadron Collider, we don’t have unfettered access to these early times—the experiments are brief and limited. By analogy, our progress with the Large Hadron Collider is like trying to map the room around you by looking through a drinking straw and having no control over where the straw is pointed. Actually, never mind, it is a lot harder than that; the straw also needs to be out of operation frequently for repairs, and the room is dark most of the time.
At the moment, we have no window—observationally or experimentally—into the universe at times earlier than the Large Hadron Collider can recreate the energetics of. This includes the period of cosmic inflation. We have a working hypothesis for what might have caused inflation, which involves states called “false vacuums” and something called an “inflation field,” which colluded to generate negative pressure, which in turn triggered the incredible cosmic inflation. But, as of the time I’m writing this, we haven’t been able to go back 13.7 billion years to test this hypothesis, and we also haven’t (yet) recreated these conditions in the lab.
The cause of the inflation is still an unsolved mystery in its own right. But—and this is an important “but”—I think this is one of those mysteries that might actually be solvable soonish—maybe even with the next generation of supercolliders.
When we try to think about even earlier times, like, say, the Planck time of ~10−43 seconds, we are now in a real pickle because even the laws of physics as we currently understand them break down; a consequence of Heisenberg’s uncertainty principle is that any measure of time smaller than the Planck time is indeterminant. Similar to the situation we will encounter at the heart of black holes, we confront an apparent singularity at the creation of the universe, and the only way out seems to be a theory of quantum gravity.
That being said, thinking about what generated cosmic inflation—or even what was going on at the Planck time—isn’t what keeps many people up at night thinking about what caused the Big Bang. Rather, the deeper conundrum arises when we go a turtle down and consider why there was even something—however compact or bizarre—to be inflated to begin with. Or in other words, why is there something instead of nothing?
Something from Nothing
I don’t know how you feel about the idea that the universe came from “nothing,” but this plagues me. This is the root of our bewilderment—whether there can be a “nothing” from which “something” is generated. This question truly keeps me awake many nights, but I draw some solace from knowing that thinkers have wrestled with this topic for perhaps as long as there have been thinkers to wrestle with it. Creatio ex nihilo (creation out of nothing) has a venerable history; Thomas Aquinas, for example, considered this question to inherently offer proof of a god as a “first cause.”16
If you have felt stymied by this “something from nothing” question, at least you can know that you are not alone.
When I was a graduate student, I had the impression that the solution to this philosophical knot was obvious to others, and I just didn’t get the right memo. The party line in physics seemed to be a version of “The birth of the universe came about from a quantum fluctuation,” and that was that. To be sure, this explanation is still very much on the table, and we will come back to it in more detail. However, what always bothered me about the cursory response to how the universe came into being was an implicit avoidance of what was (at least to me) the deeper question—fine, let’s say a quantum fluctuation caused the universe to come into being, but what enabled a framework for quantum mechanics to even exist and for the laws of physics to have an arena in which to operate? Or put another way, what is the realm in which these laws that govern the behavior of everything live? I kept these questions to myself for a very long time, but they didn’t go away.
Many physicists will argue (and have argued with me) that there is no point in trying to go another turtle down past the cause of inflation. This apparently philosophical question of why there is something instead of nothing frequently exasperates many folks as “outside the realm of science.” I will concede that they may have a point, and it may truly be outside the realm of science. If this question is ultimately unanswerable, perhaps dwelling on it has no practical value beyond making astrophysicists like me even more sleep deprived. However, I would also argue this avoidance position becomes self-fulfilling; if scientists just give up and stop studying something because there might not be a scientific explanation, then we have very little chance indeed of coming up with a scientific explanation. Somehow knowing the possible futility is not sufficient to deter my mind from going down this rabbit hole, and I don’t think I’m alone.
