by Katie Mack
In any case, when we talk about the Big Bang theory, what we’re really saying is: based on our observations of the present expansion and its history, we can conclude that there was a time when the universe was, everywhere, much hotter and denser than it is today.V This is sometimes called the “Hot Big Bang,” referring to the whole span of time when the universe was hot and dense, which we now know to be the time from year 0 to somewhere around year 380,000.VI
We can even quantify what “hot and dense” means, and trace the history of the universe backward from the cool and pleasant cosmos we are enjoying now to a pressure-cooker inferno so extreme it shatters our understanding of the laws of physics.
This isn’t just a theoretical exercise, though. It’s one thing to mathematically extrapolate expansion and derive higher pressures and temperatures; it’s another to see this infernoverseVII directly.
THE COSMIC MICROWAVE BACKGROUND
The story of how we went from thinking about the Big Bang to seeing it is a classic tale of serendipitous discovery in cosmology. In 1965, a physicist named Jim Peebles at Princeton University was doing the calculations, dialing back the cosmic expansion, and coming to the startling conclusion that radiation from the Big Bang should still be streaming through the universe today. Moreover, it should be detectable. He calculated the expected frequency and intensity of that radiation and teamed up with colleagues Robert Dicke and David Wilkinson to start building an instrument to measure it. Meanwhile, unbeknownst to them, just down the road at Bell Labs, a couple of astronomers named Arno Penzias and Robert Wilson were gearing up to do some astronomy with a microwave detector that had previously been used for commercial purposes. (Microwaves are just a kind of light on the electromagnetic spectrum, higher frequency than radio but lower frequency than infrared or visible light.) When Penzias and Wilson, totally uninterested in commercial applications and keen on studying the sky, were calibrating the instrument for their research, they found a weird humming in the feed. Apparently it hadn’t interfered with the previous use of the telescope, detecting communication signals bounced off high-atmosphere balloons, so those users had ignored it. But this was for science, and it had to be fixed. The hum appeared no matter which direction they pointed the detector and was, by all accounts, extremely inconvenient.
Telescope interference is a common problem during the calibration phase of an observing run, and there are a lot of ways it can happen. There could be a loose cable somewhere, or radio interference coming from some transmitter nearby, or any number of little mechanical annoyances. (A recent big break in radio astronomy involved the discovery that tantalizing bursts of radiation seen by the Parkes radio telescope were actually due to an overeager microwave oven in the lunch room.) Penzias and Wilson examined every inch of the detector, and even considered the possibility that a small group of pigeons nesting in the antenna could be the source of the hum.VIII But no matter what they did, they couldn’t get the hum to go away, and they never found any interference that could account for it. So they had to consider the possibility that it really was coming from space, and from every direction in the sky. But what could it be? Anything coming from the planets or the Sun should only show up at certain times and directions, and even emission from our own Milky Way galaxy would not be completely uniform.
Enter the Princeton team. In a roundabout way.
To backtrack a moment, Peebles’s calculations had said that if the universe was hot everywhere early on, then we should be currently awash in the leftover radiation from it. Here’s what he was thinking. If looking farther away means looking farther into the past, and if there was a time in the distant past when the universe was basically one big all-encompassing fireball, then it should be possible to look so far away that you see a part of the universe that is still on fire. Or, thinking about it another way: if, 13.8 billion years ago, the whole possibly infinite universe was aglow with radiation, there should be parts of it so far away that the radiation from that glow is only just now reaching us, having traveled through the expanding, cooling space all this time. In any direction we look, if we look far enough away, we’ll see that distant fiery universe. We’re not looking at parts of space that are any different, but rather at a time when ALL of space was on fire.
So, this background radiation should come from everywhere. And it should come from everywhere no matter where you are, because you can always look far enough away to see the hot phase of the cosmos. The speed-of-light/time-travel connection gives you this for free. Every point in space is the center of its own sphere of ever deepening time, bounded by a shell of fire.
Peebles realized this, and, as physicists tend to do, talked to his colleagues about his extremely mind-blowing thoughts. He even passed around a preprint of a paper in which he described what he and his colleagues planned to do to detect this radiation. Then, word proceeded to travel the 60 kilometers to Bell Labs—via two unconnected physicists, an airplane, and Puerto Rico.
An attendee of Peebles’s talk, Ken Turner, went to visit the Arecibo radio telescope, and on the flight back, had a chat with fellow astronomer Bernard Burke about how cool it would be to detect this Big Bang radiation. After arriving back at the office, Burke got a phone call from Penzias about some unrelated work, and happened to mention the airplane chatter.IX At which point, I can only assume Penzias had to have a bit of a sit-down, because now he knew that he and Wilson had just become the first human beings to see the actual Big Bang. He took a couple days, talked with his colleague, and then phoned up Robert Dicke, who immediately turned around to Peebles and Wilkinson and said, “We’ve been scooped.”
Figure 3: A cartoon map of the observable universe. At different distances from our vantage point on Earth, we can see different epochs in the past. The lookback time (the number of years before today) is labeled for each sphere around us in this diagram. The farthest we can see, even in principle, corresponds to a distance from Earth from which light leaving that point at the very beginning of the universe would be reaching us right now. This defines a sphere around us known as the observable universe.
And indeed they had. Penzias and Wilson went on to win the Nobel Prize in 1978 for the first observation of what became known as the cosmic microwave background.X
The cosmic microwave background, or CMB, went on to become one of the most important tools we have for studying the history of the universe. It’s hard to overstate its significance, both as an astronomical data set and as a technological achievement. We can now collect, analyze, and map the glow of the hot early cosmos. The first thing it tells us is: the hypothesis that the early universe was one big inferno, glowing with heat, is completely confirmed.
But how do we know for sure that the background light we detect is actually from the primordial fireball, and not from, say, some collection of weird faraway stars or something? It turns out the light’s spectrum—the way it’s brighter or dimmer when measured at different frequencies—contains a dead giveaway.
Let’s say you have a fireplace, and you stick a poker in the fire until it starts to glow red. That red glow isn’t a property of the metal itself, it’s a phenomenon that happens to anything that gets heated up (without bursting into flames). That glow is called “thermal radiation,” and the color of it depends only on the temperature. Something glowing blue is hotter than something glowing red. In fact, if you could see infrared light, you’d see thermal radiation coming from people and warm food and sun-soaked sidewalks all the time. Human thermal radiation comes out at the low frequency of infrared light because we’re much cooler than open flames, unless things are going very badly for us.
The color you see isn’t the entirety of the light produced, though. Aside from lasers, anything that produces light makes it at a spread of different frequencies (or colors), and the color it appears to the eye is just the color where the light is most intense. (This is why incandescent light bulbs are hot to the touch: even though much of the light they produce is visible, a lot of extra lig
ht is produced in the infrared part of the spectrum, which makes them feel hot.) For any thermal radiation, including that emitted by pokers and people and the little blue flames on gas stovetops, the intensity of the light changes with frequency in exactly the same way. The light is brightest at some peak color depending on the temperature, and it dims quickly for colors on either side. Plotting out on a graph how the intensity rises and falls with frequency gives you a shape we call the blackbody curve—a curve reproduced by anything that glows because it is hot.XI And it turns out that if you measure the intensity of the cosmic microwave background at different frequencies, you get the most precise, most perfect blackbody curve ever measured in nature. The only way to explain this is that the universe itself was once, everywhere, extremely hot.
Legend has it that the first time this result was presented as a graph in a conference talk, the audience actually cheered. Part of their enthusiasm was certainly because the measurement was extremely impressive and precise, and a perfect fit to the theory (which is always nice to see). But I’m fairly sure that another part was the fact that people realized they were SEEING THE BIG BANG. Actually seeing it. I, for one, am still not quite over this.
Figure 4: The blackbody spectrum of the cosmic microwave background. The height of the curve indicates the intensity of the radiation at a given frequency or wavelength. The data points are shown with error bars indicating the uncertainties in the measurement, but with the size of the uncertainties magnified 400 times so they’re not all completely hidden behind the width of the line, which is the spectrum you’d expect for something glowing at a temperature of 2.725 Kelvins (-270˚C).
Aside from the mind-blowing-ness of it, the CMB gives us an invaluable window into the universe’s first moments, and into how it has grown and evolved over time. It also gives us some hints about where it is all going, as we’ll see in later chapters.
That said, when you make a map of the CMB that shows the color variation of the light across the sky, it actually looks fairly boring: it is very nearly exactly the same color everywhere. The minute deviations you can detect, though, as tiny as they are, tell us a lot. When they crank up the contrast enough to get some color variation, astronomers can see that the CMB looks ever so slightly blotchy, as if someone did an abstract pointillism painting on the sky with a brush as big around as the full Moon viewed from Earth. These blotches collect in clumps of one color in some places and mix in others, with some of the spots a little tiny bit redder, some a little tiny bit bluer.XII The color variations reveal places where the roiling primordial cosmic plasma was just slightly cooler or hotter, due to very, very slight changes in density—each point has a density that deviates from the average by no more than about one part in 100,000. (For some perspective on what one part in 100,000 looks like, empty a soda can into a backyard swimming pool.)
We can, with careful calculations, work out how those tiny variations in density are destined to grow over time, starting from minuscule blips and, over the millennia, growing up into entire clusters of galaxies. Gravitational collapse is a powerful thing. If you have a little bit of matter that’s denser than the matter around it, it will pull in more from those less dense places, which increases the contrast, which pulls in more, and so on. The rich get richer and the poor get poorer.
Figure 5: The cosmic microwave background. This is a microwave-frequency map of the whole sky projected onto an oval in a Mollweide projection (with the emission from our own galaxy removed). The darker regions indicate slightly cooler (lower frequency, or redder) microwave emission and the light regions are slightly hotter (higher frequency, or bluer) emission. These indicate, respectively, parts of the early universe that were slightly denser or slightly less dense than their surroundings, at a level of one part in 100,000.
Using computer simulations that show billions of years passing in the span of seconds, we can watch as a patch of matter that is only a tiny fraction denser than the one next to it pulls in enough of its surrounding gas to form the first star in the universe. These stars form within the first galaxies, which gather into clusters of galaxies, which en masse turn the splotchy patchwork of the CMB into what we now see as a cosmic web: a veiny arrangement of nodes and filaments and voids traced out by the galaxies shining along it, like dew drops on a spider’s web. If you compare the results of one of these simulations with an actual map of the cosmos, where each galaxy is one point in a giant 3D map, they are in such incredible agreement that you won’t be able to tell the difference.
So. The Big Bang happened. We’ve seen it, we’ve calculated it, the physics adds up. Now, let’s all gather around in the glow of the cosmic blackbody and tell the origin story of the cosmos.
IN THE BEGINNING
Not all of cosmic history is as directly visible as the cosmic microwave background. The few hundred thousand years before the end of the fireball stage, and the half million or so years right after, are extremely hard to observe. In the former case, it’s because of too much light (imagine trying to look through a wall of fire) and in the latter, because of too little (imagine trying to look at some specks of dust in the air between you and a wall of fire). But the CMB, right in the middle, gives us a solid anchor to extrapolate from in each direction, and now we have a compelling narrative for how the universe evolved over time, starting from the first billionth of a billionth of a billionth of a second and arriving at today, 13.8 billion years later.
Shall we?
In the beginning, there was the singularity.
Well, maybe. A singularity is what most people think of when they think of the Big Bang: an infinitely dense point from which everything in the universe exploded outward. Only, a singularity doesn’t have to be a point—it could just be an infinitely dense state of an infinitely large universe. And, as discussed above, there’s no explosion per se, since an explosion implies an expansion into something, rather than an expansion of everything. The idea that everything started with a singularity comes from observing the current expansion of the universe, applying Einstein’s equations of gravity, and extrapolating backward. But that singularity might never have happened. What most physicists do think happened, a fraction of a second after whatever was the true “beginning,” was a dramatic super-expansion that effectively erased all trace of whatever went on before it. So the singularity is one hypothesis for what might have started everything off, but we can’t really be sure.
There’s also the question of what was “before” the singularity. That question might be, depending on who you ask, incoherent nonsense (because the singularity was the beginning of time as well as space, so it had no “before”), or one of the most crucial questions in cosmology (because the singularity might have been the end point of a previous phase of a cyclic universe: one that goes from Big Bang to Big Crunch and back again forever). We’ll talk about the latter possibility in Chapter 7, but in the meantime, there’s not much to say about the singularity other than that it might have happened. Even if we did trust ourselves to dial back expansion all the way to that point, a singularity represents a state of matter and energy so extreme that nothing we currently know about physics can describe it.
To a physicist, a singularity is pathological. It’s a place in the equations where some quantity that is normally well behaved (like the density of matter) goes to infinity, at which point there is no longer any way to calculate things that makes any sense. Most of the time, when you encounter a singularity, it is telling you that something has gone wrong in your calculations and you need to go back to the drawing board. Finding a singularity in your theory is like having your satellite navigation direct you to the edge of a lake, then instruct you to disassemble your car, reassemble it as a boat, and paddle your new car-boat to the other side. Maybe this really is the only way to get where you’re trying to go, but more likely, you’ve made a wrong turn somewhere a few miles back.
In practice, though, it doesn’t even take something as clearly dysfunctional as a true singularit
y to lay waste to physics as we know it. Any time you have a lot of energy in a very small space, you have to deal with both quantum mechanics (the theory governing particle physics) and general relativity (the theory of gravity) at the same time. Under normal circumstances, you’re only dealing with one or the other, because when gravity is important, it’s usually because you have a massive thing, so you can ignore the individual particles, and when quantum mechanics is important, at the particle scale, you’re dealing with so little mass that gravity is a totally negligible part of the interaction. But at extreme densities you have to contend with both, and they don’t work well together, at all. Extreme gravity involves well-defined massive objects that warp space and alter the flow of time; quantum mechanics allows particles to pass through solid walls or exist only as fuzzy probability clouds. The fundamental incompatibility of our theories of the very massive and the very small is one of the things that hints at the direction we should go in creating new, more complete theories. But it is also rather inconvenient when we’re trying to explain the very early universe.
Without a full theory of quantum gravity (something that reconciles particle physics with gravity), there’s a limit to how far back we can extrapolate the universe in a way that makes sense. We inevitably reach a moment before which all bets are off. During that time, the densities are high enough that we expect extreme gravitational effects to be competing with the inherent fuzziness of quantum mechanics, and we just don’t know what to do in that scenario. Do microscopic black holes form (because of the strong gravity) but then pop in and out of existence at random (because of the quantum uncertainty)? Does time have any meaning when the shape of space is no more predictable than a roll of dice? If you zoom in to a small enough scale, do space and time act like discrete particles, or perhaps waves that interfere with each other? Are there wormholes? Are there dragons??? We have no idea.