Finding Our Place in the Universe, One Discovery at a Time
Author Sean Carroll tells us how a multitude of discoveries in the last few hundred years has changed how we view our place in the universe.
The following is an excerpt from The Big Picture by Sean Carroll.
Our modern picture of our cosmos was painstakingly pieced together through data collected by astronomers, who frequently brought back results that defied conventional theoretical wisdom of the time. A century ago, in 1915, Albert Einstein put the finishing touches on his general theory of relativity, which conceives of spacetime itself as a dynamic object whose curvature gives rise to the force we know as gravity. Before that point, it’s safe to say that we knew next to nothing about what the universe was really like on large scales. Spacetime was thought to be absolute and eternal, in accordance with Newtonian mechanics, and astronomers were divided on whether the Milky Way was the only galaxy in the universe, or merely one of countless many.
Now the basics have been well established. The Milky Way we see stretching across the dark night sky is a galaxy—a collection of stars orbiting under their mutual gravitational attraction. It’s hard to count precisely how many, but there are over 100 billion stars in the Milky Way. It’s not alone; scattered throughout observable space we find at least 100 billion galaxies, typically with sizes roughly comparable to that of our own. (By coincidence, the number 100 billion is also a very rough count of the number of neurons in a human brain.) Recent studies of relatively nearby stars suggest that most of them have planets of some sort, and perhaps one in six stars has an “Earth‐like” planet orbiting around it.
Perhaps the most notable feature of the distribution of galaxies through space is that, the farther out we look, the more uniform things become. On the very largest scales, the universe is extremely smooth and featureless. There is no center, no top or bottom, no edges, no preferred location at all.
Scatter all that material throughout space, and general relativity says that it’s not just going to sit there. Galaxies are going to pull on one another, so the universe must be either expanding from a more dense state, or contracting from a less dense one. In the 1920s, Edwin Hubble discovered that our universe is indeed expanding. Given that discovery, we can use our theoretical understanding to extrapolate backward in time. According to general relativity, if we keep running the movie of the early universe backward, we come to a singularity at which the density and expansion rate approach infinity.
That scenario, developed by Belgian priest Georges Lemaître under the name “the Primeval Atom” but eventually dubbed “the Big Bang model,” predicts that the early universe was not only denser but also hotter. So hot and dense that it would have been glowing like the interior of a star, and all of that radiation should still suffuse space today, ready for detection in our telescopes. That’s just what happened in the fateful spring of 1964, when astronomers Arno Penzias and Robert Wilson at Bell Laboratories detected the cosmic microwave background radiation, leftover light from the early universe that has cooled off as space expanded. Today it is just a bit less than 3 degrees above absolute zero; it’s a cold universe out there.
When we talk about the “Big Bang model,” we have to be careful to distinguish that from “the Big Bang” itself. The former is an extraordinarily successful theory of the evolution of the observable universe; the latter is a hypothetical moment that we know almost nothing about.
The Big Bang model is simply the idea that approximately 14 billion years ago the matter in the universe was extremely hot, densely packed, and spread almost uniformly through space, which was expanding very rapidly. As space expanded, matter diluted and cooled, and stars and galaxies condensed out of the smooth plasma under the relentless pull of gravity. Unfortunately, the plasma was so hot and dense at early times that it was essentially opaque. The cosmic microwave background reveals what the universe looked like when it first became transparent, but before that, we cannot directly see.
The Big Bang itself, as predicted by general relativity, is a moment in time, not a location in space. It would not be an explosion of matter into an empty, preexisting void; it would be the beginning of the entire universe, with matter smoothly distributed all throughout space, all at once. It would be the moment prior to which there were no moments: no space, no time.
It’s also, most likely, not real. The Big Bang is a prediction of general relativity, but singularities where the density is infinitely big are exactly where we expect general relativity to break down—they are outside the theory’s domain of applicability. At the very least, quantum mechanics should become crucially important under such conditions, and general relativity is a purely classical theory.
So the Big Bang doesn’t actually mark the beginning of our universe; it marks the end of our theoretical understanding. We have a very good idea, on the basis of observational data, what happened soon after the Bang. The microwave background radiation tells us to a very high degree of precision what things were like a few hundred thousand years afterward, and the abundance of light elements tells us what the universe was doing when it was a nuclear fusion reactor, just a few minutes afterward. But the Bang itself is a mystery. We shouldn’t think of it as “the singularity at the beginning of time”; it’s a label for a moment in time that we currently don’t understand.
Ever since the expansion of the universe was discovered, the question of the future fate of the universe has preoccupied the minds of cosmologists. Would it keep expanding forever, or eventually reverse course, contracting down to an ultimate “Big Crunch”?
A major clue was uncovered just as the twentieth century was ending, when in 1998 two teams of astronomers announced that the universe wasn’t only expanding; it was accelerating. If you focused on a particular faraway galaxy and measured its velocity, then came back a few million or billion years later and measured it again, you would find that it’s now moving away from you even faster. (That’s not what the astronomers did, of course; they compared the velocities of galaxies at different distances.) If this behavior continues forever—which seems quite plausible—the universe will continue to expand and dilute in perpetuity.
Normally we’d expect the expansion of the universe to slow down as the gravitational forces between the galaxies worked to pull them together. The observed acceleration must be due to something other than matter as we know it. There is a very obvious, robust candidate for what the culprit might be: vacuum energy, which Einstein invented and called the cosmological constant. Vacuum energy is a kind of energy that is inherent in space itself, remaining at a constant density (amount of energy per cubic centimeter) even as space expands. Due to the interplay of energy and spacetime in general relativity, vacuum energy never runs out or fades away; it can keep pushing forever.
We don’t know for sure whether it will keep pushing forever, of course; we can only extrapolate our theoretical understanding into the future. But it’s possible, and in some sense would be simplest, for the accelerated expansion to simply continue without end.
That leads to a somewhat lonely future for our universe. Right now the night sky is alive with brightly shining stars and galaxies. That can’t last forever; stars use up their fuel, and will eventually fade to black. Astronomers estimate that the last dim star will wink out around 1 quadrillion (1015) years from now. By then other galaxies will have moved far away, and our local group of galaxies will be populated by planets, dead stars, and black holes. One by one, those planets and stars will fall into the black holes, which in turn will join into one supermassive black hole. Ultimately, as Stephen Hawking taught us, even those black holes will evaporate. After about 1 googol (10100) years, all of the black holes in our observable universe will have evaporated into a thin mist of particles, which will grow more and more dilute as space continues to expand. The end result of this, our most likely scenario for the future of our universe, is nothing but cold, empty space, which will last literally forever.
We are small, and the universe is large. It’s hard, upon contemplating the scale of the cosmos, to think that our existence here on Earth plays an important role in the purpose or destiny of it all.
That’s just what we see, of course. For all we know, the universe could be infinitely big; or it could be just a bit larger than what we observe. The uniformity that characterizes our observable region of space could extend on indefinitely, or other regions could be extremely different from our own. We should be modest when making pronouncements about the universe beyond what we can measure.
One of the most striking features of the universe is the contrast between its uniformity in space and its dramatic evolution over time. We seem to live in a universe with a pronounced temporal imbalance: about 14 billion years between the Big Bang and now, and perhaps an infinite number of years between now and the eventual future. To the best of our knowledge, there’s a legitimate sense in which we find ourselves living in a young and vibrant period in the universe’s history—a history that will mostly be cold, dark, and empty.
Why is that? Maybe there’s a deeper explanation, or maybe that’s just how it is. The best a modern cosmologist can do is to take these observed features of the universe as clues to its ultimate nature, and keep trying to put it all into a more comprehensive picture. A crucial question along the way is, why did the matter in the universe evolve over billions of years in such a way as to create us?
Excerpt from The Big Picture by Sean Carroll. Reprinted by arrangement with DUTTON, a member of Penguin Group (USA) LLC, A Penguin Random House Company. Copyright © 2016 by Sean Carroll