A couple of times when I’ve told people that I work on cosmology, they have mistaken me for a cosmetologist, which I find very funny given how poorly suited I would be for this vocation. But the mistake did motivate me to look up these words, which—if you are not listening closely—can sound strikingly similar. The Online Etymology Dictionary, which explained that both words originated in the Latinized version of the Greek kosmos, taught me that the mistake is almost justifiable. Pythagoras of Samos in the sixth century B.C. might have been the first to use kosmos to apply to the Universe. But from around A.D. 1200, the meaning of cosmos was “order, good order, or orderly arrangement.” The word didn’t gain traction, however, until the mid-nineteenth century, when the German scientist and explorer Alexander von Humboldt gave a series of lectures, which he wrote up in a treatise titled Kosmos. This treatise influenced many readers, including the writers Emerson, Thoreau, Poe, and Whitman. You might joke that Carl Sagan did the original rebooting of the popular Kosmos series.
The word cosmetic, on the other hand, traces back to the 1640s—derived from the French cosmétique, which in turn descended from the Greek kosmetikos, meaning “skilled in adornment or arrangement.” The online dictionary, presenting a dual meaning that I suspect only Los Angeles residents can fully understand, explains “thus kosmos had an important secondary sense of ‘ornaments, a woman’s dress, decoration’ as well as ‘the Universe, the world.’” In any case, the similarity—and embarrassing confusion—that I encountered was not entirely a coincidence. Both “cosmology” and “cosmetology” derive from kosmos. Like a face, the Universe has both beauty and an underlying order.
Cosmology—the science of the evolution of the Universe—has now truly come into its own. It has recently entered an era in which revolutionary advances—both experimental and theoretical—have yielded a more extensive and detailed understanding than most people would have been thought possible even thirty years ago. Improved technology combined with theories rooted in general relativity and particle physics have provided a detailed picture of the Universe’s earlier stages, and of how it evolved into the Universe we currently see. The next chapter explains how far and deep these twentieth-century advances have taken us in our understanding of the Universe’s history. But before exploring these remarkable achievements, I want to first briefly wax philosophical in order to make clear what science does and does not tell us about the answers to some of humanity’s oldest and most fundamental questions.
QUESTIONS WITHOUT ANSWERS
Cosmology is about big inquiries—nothing less than how the Universe began and subsequently developed into its current state. Prior to the scientific revolution, people tried to answer such questions with the only methods at their disposal, namely philosophy and limited observations. Some ideas they had turned out to be correct but—not surprisingly—many others were wrong.
Today too, despite our many advances, people can’t help turning to philosophy when thinking about the Universe and the questions we haven’t yet answered—in effect forcing us to confront the distinction between philosophy and science. Science concerns those ideas that at least in principle we can verify or rule out through experiments and observations. Philosophy, to a scientist at least, concerns questions we expect we will never reliably answer. Technology sometimes lags behind, but we’d like to believe that at least in principle scientific proposals will be verified or ruled out.
This leaves scientists with a dilemma. The Universe almost certainly extends beyond the domain we can observe. If indeed the speed of light is finite and if our Universe has been around for only a fixed amount of time, we can access only a finite region of space—no matter how much technology might advance. We can see only those regions that can be reached via a light ray—or something else that travels at light speed—during the lifetime of the Universe. Only from those regions can a signal possibly reach us within the time that the Universe has been around. Anything farther away—beyond what physicists call the cosmic horizon—is inaccessible to any observation we could make at this time.
This means that science in its truest form doesn’t apply beyond this domain. No one can experimentally validate or rule out conjectures that apply beyond the horizon. According to our definition of science, for those faraway regions, philosophy reigns supreme. This doesn’t mean that curious scientists never ponder the big questions about the physical principles or processes that apply there. Indeed, many do. I don’t want to dismiss such inquiries—they can often be deep and fascinating. But given the limitations, you can’t trust scientists’ answers about this domain—at least not more than anyone else’s. However, since I’m so often asked, I’ll use this chapter to offer my take on a few of the big questions people often want addressed.
One question I frequently hear is why there is something rather than nothing. Though none of us knows the true reason, I’ll give my two responses. The first, which is undeniable, is that you wouldn’t be here to ask this question and I wouldn’t be here to answer it if there were nothing. But my other answer is that I just think something is more likely. After all, nothing is very special. If you have a number line, “zero” is just one infinitesimal point among the infinity of possible numbers you can choose. “Nothing” is so special that without an underlying reason, you wouldn’t expect it to characterize the state of the Universe. But even an underlying reason is something. You at least need physical laws to explain a very nonrandom occurrence. A cause implies there must be something. Though it sounds like a joke, I really believe this. You might not always find what you are looking for, but you don’t randomly find nothing.
But there is also a scientific, rather than a philosophical question, that comes up when physicists consider the matter we are made of—the stuff we are supposed to understand. Why in our Universe do we have as much of the matter we are made of—protons, neutrons, electrons—as we do? Though we understand a great deal about ordinary matter, we don’t fully understand why so much of it is still here. The amount of energy in ordinary matter is an unsolved problem. We don’t yet know why it has survived as abundantly as it has to today.
The problem boils down to the question of why there wasn’t always an equal amount of matter and antimatter. Antimatter is the stuff with the same mass but opposite charges to ordinary matter. Physical theories tell us that for every matter particle, an antimatter particle must exist. For example, knowing that an electron has charge –1 tells us there must also be an antiparticle—it’s called a positron—with the same mass but opposite charge, +1. To avoid any confusion, let me state explicitly that antimatter is not dark matter. Antimatter carries the same types of charges as ordinary matter and therefore interacts with light. The only difference is that the antimatter charges are the opposite to those of the associated matter.
Because antimatter carries opposite charges to our usual matter, the net charge of matter and antimatter is zero. Since matter and antimatter together carry no charge, charge conservation and Einstein’s famous formula E = mc2 tell us that matter can meet up with antimatter to disappear into pure energy—which also has no charge.
We would have expected that as the Universe cooled, essentially all known matter would have annihilated with antimatter, meaning matter and antimatter would have combined to turn into pure energy, and thereby disappeared. But as we are here to discuss the question, clearly this was not the case. We are left with matter—that five percent of the Universe’s energy that you see in Figure 3—so the amount of matter in the Universe must be greater than the amount of antimatter. A critical feature for our Universe—and ourselves—is that in contrast to standard thermal expectations, ordinary matter sticks around and survives in sufficient quantities to create animals and cities and stars. This is possible only because matter dominates over antimatter—there is a matter-antimatter asymmetry. If the amounts had always been equal, matter and antimatter would have found each other, annihilated, and disappeared.
In order to have matter stick around until today, an asymmetry between matter and antimatter had to have been established at some time in the early Universe. Physicists have suggested many workable scenarios for what could have created this imbalance, but we don’t yet know which if any of these ideas are correct. The origin of the asymmetry remains one of the important unsolved problems in cosmology. This means that not only do we not understand the dark components, but we don’t even fully understand ordinary matter—the small piece of the cosmic pie that represents known matter. Something special had to have occurred early in the evolution of the Universe to explain why this piece of pie remains.
A second currently unanswerable question is what exactly happened during the Big Bang. Scientists and the popular press frequently refer to the Big Bang explosion that happened back when the Universe was less than 10−43 seconds old and the Universe was 10−33 cm big, and even “illustrate” the explosion with gorgeous multicolored images. But the term “Big Bang” is misleading, as I will further discuss in the chapter that follows. The astronomer Fred Hoyle, who preferred a static Universe, invented the term in 1949 as a pejorative for use on his BBC radio show to refer to this theory that he didn’t believe.
Regardless of your attitude toward Big Bang cosmology, which very successfully describes the Universe’s evolution only a fraction of a second after the Universe we know began, no one knows what happened at that earliest moment. A reliable characterization of the Big Bang—and possibly what happened before—requires a theory of quantum gravity. On the tiny distance scales that are relevant to this earliest time, both quantum mechanics and gravity are important, and no one has yet found a solvable theory that applies to this infinitesimal distance regime. We will gain insight into the very beginning of the Universe only when we know more about physical processes on this tiny distance scale. And even then observations to validate the conclusions will very likely be impossible.
An even more impossible to answer question I often hear is, “What came before the Big Bang?” Answering this presumably requires even more knowledge than understanding the Big Bang itself. We don’t know what happened at the time of the Big Bang and neither I nor anyone else knows what came before. But before you become too disappointed by this omission, let me reassure you that you would probably find any answer to this question unsatisfactory. Either the Universe was around an infinite length of time or it started at some particular time. Both answers can seem disturbing, but those are the options.
Taking this a step further, if the Universe existed forever and the Big Bang was part of it, either our Universe was all there was or other universes also emerged from their own Big Bangs. The multiverse is the name associated with a cosmos in which, in addition to our own universe, there are many others. In this scenario, there would be many different expanding regions—each constituting its own universe.
This reasoning leaves us with three choices. Either our universe started with the Big Bang, the Universe has been around forever but eventually went over to the expansion that the Big Bang theory predicts, or we are one of many universes that grew out of a universe/multiverse that has always existed. This covers all the possibilities. The last one seems most likely to me in that it doesn’t assume that our world or even our particular universe is special, which is reasoning that has been invoked since the time of Copernicus. This choice also implies that just as the spatial extent of the universe—at least to my way of thinking—is more likely to be infinite than finite in size, the evolving universe is unlikely to have a beginning or end in time—even though our particular universe might. The existence of multiple emerging and eventually disappearing universes is probably the least unsatisfying of three not completely graspable possibilities.
This brings me to the last philosophical inquiry—prompted by the one above—which is whether such a multiverse exists. Existing physical theories suggest that multiverses are rather likely, especially given the many possible solutions in quantum gravity theories as currently formulated. Whether or not those calculations stand up to scrutiny, I would wager that other inaccessible universes should be present. Why shouldn’t they? Given that we know the limitations of physical laws and current technology, it is both figuratively and literally shortsighted to decide they aren’t there. Nothing about our world is inconsistent with the existence of a multiverse.
But that doesn’t mean we will ever know. If nothing travels faster than the speed of light, any region that is too far away—beyond the cosmic horizon—is off-limits to observations. Yet these other regions could in principle contain other universes that are completely separate from our own. Some signals of other universes could potentially be found in the cases where over time separate universes come into contact. But this is highly unlikely, and in general other universes are inaccessible.
For my faithful, returning readers, I’ll now make an aside to be clear that in discussing multiverses, I am not referring to the multidimensional scenarios that I described in Warped Passages. Universes might also exist that are closer than the horizon but that are separated from us across another dimension of space—a dimension beyond the three that we observe: left-right, up-down, and forward-backward. Although no one has yet seen such a dimension, it might exist and in principle a universe separated from us along this dimension might as well. This type of universe is known as a braneworld. As those who read my first book know, the braneworlds that interest me most could potentially have observable consequences because they are not necessarily so far away. However, braneworlds are generally not what people mean when discussing the more general multiverse scenario involving many separate universes that won’t interact even via gravity. Multiverses are so far away that even something traveling at the speed of light from one of these other universes wouldn’t have time to reach us in the lifetime of our Universe.
Nonetheless, there is a lot of interest in the multiverse idea in the popular imagination. I was recently talking to a friend who was very excited about the idea of a multiverse and didn’t understand why I didn’t necessarily find it as interesting as he did. For me the first reason is the one stated above: in all likelihood, we will never know with certainty whether or not we live in a multiverse. Even if other universes exist, they are likely to remain undetectable. My friend found this only mildly disappointing and his interest persisted. I suspect that he—along with many others—likes the idea because he thinks a copy of himself is living in one of these distant realms. Just for the record, I don’t hold this view. If other universes exist, they are most likely nothing like our own. They probably don’t even contain the same forms of matter or forces as we do. If there were life there, we most likely wouldn’t even recognize it and probably couldn’t detect it in the first place—even if it weren’t so far away. The infinite number of confluences that create any single human being would be even less likely. After I explained how—even with many other universes—there can be an even larger universe of possibilities, my friend began to see my point.
In fact, even if the multiverse scenario holds, most of the other universes will be untenable and will either collapse or explode, in which case they will dilute to nothing almost instantaneously. Only a few, like our own, might last long enough to develop structure and perhaps even life. Despite Copernicus’s insightful perspective, our particular Universe does seem to have a number of peculiar properties—ones that permit galaxies, the Solar System, and life. Some people try to explain the special properties of our Universe by assuming the existence of multiple universes, at least one of which has the special properties we require for our existence. Many who think this way attempt anthropic reasoning, which tries to justify particular properties of our Universe on the grounds that they are essential to life—or at least galaxies that can support life. The problem here is that we don’t know which properties are anthropically determined and which are based on fundamental physical laws, or which properties are essential to life and which are simply essential to the life that we see. Anthropic reasoning might be correct in some cases, but we have the usual problem that we don’t know how to test the ideas. In all likelihood, we will only rule out such ideas if a better, more predictive idea takes its place.
Ideas such as those discussed above are speculations. They are intriguing, but we won’t have answers—at least not anytime soon. In my research, I prefer to think about the “multiverse” of communities of matter that’s right here and which we can hope to understand. I’m using the term metaphorically, but it’s not so far from the truth. A universe of dark matter is right under our noses. Yet we don’t generally interact with it and we don’t yet know what it is. But theoretical and experimental physicists are currently advancing our knowledge about what this “dark universe” might be. Someday soon we might know the answer, and such a discovery would be worth the wait.