The first serious attempt came shortly after Rubin's observations with a theory called MOND (once again, I'm not in charge of naming things). Short for Modified Newtonian Dynamics, it's exactly what it says it is—it modified Newton's fundamental laws so that the relationship of mass, acceleration, and force isn't what we're used to.6 The key is that everything appears perfectly normal on the surface of the Earth or in the solar system, but once you get to galactic scales, it breaks down and has to be, well, modified.
This works great for explaining galaxy rotation curves, because it was explicitly designed to work great for explaining galaxy rotation curves. No invisible matter here, just different physics at work. But in order to make a theory capable of coherently explaining both galactic and early-universe observations, it needed to be elevated to be fully relativistic—in other words, it had to be written in a way similar to special relativity (which, dark matter or not, nobody was arguing against). That's the game we have to play in cosmology: if you're not relativistic, you don't have enough explanatory oomph to take you everywhere you need to go.
The result of marrying MOND with relativity is called TeVeS, for Tensor-Vector-Scalar. I won't go into the messy details, but the short version is that it's largely ruled out: TeVeS predicts certain results for gravitational lensing and the cosmic microwave background that don't agree with observations.7
The real bullet that finally killed modified laws of physics was detailed observations of the appropriately named Bullet Cluster.8 That cluster of galaxies is itself a train wreck (though not to be confused with the Train Wreck Cluster, which is something else), where two massive clusters slammed into each other long ago, and we have a pretty picture of the wreckage. When we look at this system with different techniques, like a team of forensic investigators trying to understand a murder, we get a complete picture of what's going on.
First, the galaxies. Galaxies are like buzzing little bees in the giant volume of their host clusters. When clusters collide, it's like two swarms of bees headed in the same direction: for the most part, they just sail on through without interacting. So the galaxies end up on opposite sides of where they started. Fine, nothing surprising there.
Next, the hot, thin gas between the galaxies that fills up the bulk of the cluster. As thin as it is, it still can't help but get tangled up with its counterpart in the opposite cluster during the merger event. And when those two giant balls of gas slam into each other, we get all the rich and glorious physical interactions that we've come to expect with interacting balls of hot gas: cold fronts, shock waves, instabilities, the works. When we examine the Bullet Cluster with X-rays, we see all the fireworks happening in the center of the interaction, with the two sets of galaxies sitting on opposite sides, safely navigating the merger event unscathed.
Now, where's the mass? We have exquisitely good gravitational lensing maps of the Bullet Cluster, showing not only how much total material the colliding pair hosts but also where it's distributed in space. How handy is that? Those lensing maps reveal a curious pattern: the concentration of mass is not tangled up with the hot gas in the center but is more closely associated with the galaxies. Even then, though, it's not mapped directly to the galaxies themselves but, rather, smoothly distributed throughout the remnants of the clusters.
That hidden mass is much, much larger than can be provided by the galaxies alone.
There's no picture of modified gravity that can sufficiently explain what we see with the Bullet combined with every other observation we have of the universe. I know, I know. It would have been super awesome to have a handle to lever ourselves up and past Einstein's relativity. A couple of Nobel Prizes would have been tossed around, and we'd be working on the Next Big Challenge.
Oh well. Like I said, nature isn't playing fair.
So Einstein gets to stay in the game, but as I said earlier, something has to give. The universe must have some additional, previously unknown component.
Maybe it's normal stuff, just dim. Like dead or failed stars, rogue planets, or black holes. Those don't give off a lot of light (obviously), and with a bit of finagling and just the right circulation, they would have all the right properties for galaxy rotation curves, lensing, and all the rest. But the only way to make black holes is through the death of massive stars. And the only way to make botched stars is to have giant clouds of star-forming material that failed to ignite fusion. To have the dark matter really be dim but otherwise normal matter, we need a lot of…normal matter, and that's ruled out by our knowledge of the early universe, like the processes that build the first nuclei and the growth of the first galaxies.9
There are, of course, a bunch of half-baked, and sometimes quarter-baked, ideas floating around in academic circles, and it would be exhausting to give an exhaustive review. Give me five theorists, and I'll walk away with six theories on dark matter. I wanted to insert this caveat because of a profound and—dare I say—noble sense of completeness, but I'm not going to spend a lot of time on them because they're usually pretty dang awkward solutions to the dark matter problem, and not as fleshed out and agreeably comprehensive as the one I'm about to present.
Which is WIMPs. That's right: WIMPs. Astronomers, as we've seen, have a flair for the ridiculous acronym. Weakly Interacting Massive Particles. The main historical competitors to WIMPs were the MACHOs—the MAssive Compact Halo Object, the name given to the chunks of “normal” matter, like failed stars, that might have explained the observations but never quite did.
So our best solution to the dark matter problem is called a WIMP, and we're just going to have to live with that.
To explain what the heck a WIMP is and why you should care, I need to mention one other property that dark matter, whatever it is, has to have: it has to be cold. That means that at the time it comes on the scene in a big way, it has to have speeds much lower than that of light. The reason is structure. If the invisible part of our universe is too “hot,” then it has a much easier time ignoring the effects of gravity, and this “washes out” smaller structures like galaxies. Since galaxies very much exist, the dark matter has to help rather than hurt the formation of structure billions of years ago, which means it has to be amenable to the flirtatious whisperings of gravity.10
Thus particles like the neutrino, which absolutely inundate the universe and (this turned out to be a surprise) have a little bit of mass, aren't a good candidate to be the dark matter—they're too hot, and if you added too many of them into the recipe of the universe, enough to point to it and say, “That's the dark matter—we've known it all along!,” the resulting pastry will be too bland and smooth, lacking the delicious, flaky layers of our modern cosmos.
I've been dancing around this issue up until now, so I'll get right to it: as best as we can tell, the explanation for the dark matter problem in our universe is a new kind of subatomic particle previously unknown to science. Something that doesn't interact with light, probably doesn't even interact with itself, and is cold.
And our best theoretical candidates for that particle is the WIMP, a (P)article that is (M)assive and (I)nteracts only via the (W)eak nuclear force. Why this one? Well, cosmologists aren't the only people on the planet hunting for never-before-seen particles. Those crafty high-energy theorists are constantly churning out idea after idea, coming up with ways to break the chains of the standard model of particle physics and extend our knowledge of the subatomic realm. The initial forays into the uncharted territory of new physics predict the existence of a host of new particles, just as Dirac's musings led us on the path to antimatter.
These predicted particles just so happen, by sheer happy coincidence, to have the right properties to explain the dark matter as we see it in the universe, and especially to have the right properties in the early universe to form the seeds of the kinds of structures that we observe at large scales today.11
Of course there are many untested routes beyond the standard model, and these routes differ in their predictions of what precisely the
dark matter might be—if there is even a single particle responsible for all that mass, and not a family. But the good news of WIMPs is that these hypotheses make testable predictions, so we can at least try to rule them out.
If dark matter is truly a WIMP, then each and every galaxy is flooded with them. The same way you are constantly surrounded and bombarded by radiation, which you notice, and neutrinos, which you don't, trillions upon countless trillions of WIMP particles are zipping through your room right now, through the hands holding this book and through the brain thinking these thoughts. You're drowning in dark matter.
You don't notice because—as per the definition—it only interacts via the weak force, which is very rare. We don't notice the effects of its gravity on small scales like the solar system because it's relatively smooth. Gravity only cares about differences in density, so you have to get out to supergalactic scales to notice any substantial variations and the interesting gravitational properties that come into play.
Right now, as I type and probably as you read, there are detectors all around the world dedicated to hunting for a dark matter particle, hoping to catch enough rare chance interactions to confirm a detection of the dark matter particle. So far, they've turned up empty, which has helped rule out some models and tighten the noose on the exact properties of the WIMP.
While they're still the most appealing candidate for the dark matter, the attitude toward WIMPs has taken a more skeptical bent as of late, especially after the Large Hadron Collider smashed some particles together to look for paths leading away from the standard model. Unfortunately, those searches have—so far—turned up empty, and the simplest and easiest paths, some of them containing a WIMP candidate, appear to be dead ends.
That said, there are a few other candidate ideas floating around the journals and blackboards of academia that predict new particles without them being strictly WIMPy, so there are still a lot of uncovered stones.
Again bowing to that noble sense of completeness, I need to mention that dark matter as an explanation for our cosmic observation does have some weaknesses, dealing with the detailed structures of galaxies and the numbers of satellites a galaxy might have. Those are, of course, topics of active research and vigorous debate.12
Disclaimers out of the way, this is how we do science. You pick the explanation that has the fewest assumptions and is able to explain the most observations. It's a very simple strategy that has uncovered and unlocked mystery after mystery in the cosmos. And despite our best efforts to develop a viable alternative, it appears that we live in a universe dominated by an exotic, cold, invisible form of matter.
This particle, if it exists, lives outside the standard model. Nothing we know of—not the neutrino, not any of the quarks or mesons or whatever-ons—can explain the cosmic observations. Something funny is going on in the universe at large scales, and that funny something is deeply related to our unquenchable thirst to understand physics at its deepest level.
Whatever that particle is, whatever its true nature and intent, it is by a wide margin the dominant player of the matter game in our universe, beating our familiar light-loving baryons at least five to one. So when you enjoy a clear dark night dotted with countless sparkling stars, you're looking at the mere representatives of cosmic structures. All the particles you know and love, the ones that constitute the physics of the familiar, of the warmth of sunlight on a summer's day, of the solid rocks underneath your feet, of the exchange of ions in your blood vessels, are a minority.
Stars and even galaxies are lighthouses on a distant, hidden shore. A beacon of light, signaling the presence of larger masses, tracing their outlines without revealing more. The journey started by Zwicky and Rubin, and continuing to the present day, leads us to an inescapable and uncomfortable conclusion: we do indeed live in a dark, cold universe.
The cosmic web is insultingly big. To state the bare fact that it's the single largest pattern found in nature does supreme disservice to the word largest. If there were a superlative to express a quantity greater than the greatest, that adjective might come close to addressing the magnitude of the cosmic web.
Think of the largest thing you possibly can. A planet? The Earth is so large that even though it's round, it appears flat in your backyard. A solar system? NASA's New Horizons probe traveled at thirty-six thousand miles per hour and took nine and half years just to make the hop to Pluto. A galaxy? A bustling stellar megalopolis, home to hundreds of billions of stars and a hundred billion suns’ worth of gas.
The cosmic web is made of galaxies, the same way that your body is made of cells. But even that metaphor breaks down—as do all metaphors—when describing the cosmic web. The cosmic web is made of galaxies, the same way your body is made of cells…if your cells were a million times smaller than they are.
But the galaxies themselves are only representatives of the true bones of the web: dark matter. The matter in our universe appears (or doesn't—ha, sorry, cosmology joke) to be made of some nonluminous particle or family of particles, one that doesn't interact with light or really anything. It's the dark matter that began pooling together billions of years ago; the “normal” matter simply fell into the already-existing gravitational valleys. When we see a galaxy, we must imagine a “halo” of dark matter surrounding it; likewise for a gigantic cluster. When we see the web, the light-emitting galaxies are tracers of the true structure underneath. Thus while everything I'll talk about below concerns stars and galaxies, keep in mind that those are only the metaphorical tips of the cosmic icebergs.
Let's start with some baby steps and work our way up. Voyager 1, launched in the late 1970s on a grand tour of the outer planets of the solar system, finally penetrated the bubble of our sun's influence, as defined by the boundary where the stream of charged particles racing outward from the sun's surface begins to mix with the general galactic milieu, in 2012. In three hundred years, that little spacecraft—which is no bigger than a small car, mind you—will reach the inner boundary of the Oort cloud, a thin, diffuse shell of frozen debris left over from the formation of the solar system.
Voyager 1 now enjoys the privilege of being the only humanmade object in interstellar space, the long gulfs of emptiness between the stars that make up the Milky Way. It will eventually pass by another star, coming within 1.6 light-years of Gliese 445, an unremarkable red dwarf currently situated about 18 light-years from the sun.
In forty thousand years.
While it's difficult to predict exactly, astronomers are pretty sure that's the closest Voyager 1 will come to another star. Ever.
In 230 million years, traveling at a steady thirty-eight thousand miles per hour, it will complete a single circumnavigation of the Milky Way without meeting anything larger than a stray bit of dust.
Here's another perspective. The sun's nearest neighbor is Proxima Centauri, another unremarkable red dwarf (they're rather common) about four light-years from our home. If you were to build a scale model of our galactic neighborhood, and you were to put the Earth a scant three feet away from the sun, Proxima Centauri would be two hundred miles away.
And we're just getting warmed up.
The Milky Way galaxy itself is around one hundred thousand light-years across. Simple math reveals that you could fit twenty-five thousand sun-Proxima distances across its breadth. In our scale model, with the sun three feet from the Earth and Proxima two hundred miles from that, the Milky Way in its entirety would stretch five million miles, which would put the edge about twenty times farther than the moon.
That's a big model.
Let's revisit our exploration of the deep universe using the handy new phrasing we learned when we first encountered Hubble and his fantastic result: the parsec, the quintessential astronomical jargon word. Like this: the Milky Way is about thirty thousand parsecs, or thirty kiloparsecs, across. It's still unimaginably gigantic, but at least I don't have to type a bunch of “illions” anymore.
The Andromeda Galaxy, the nearest major neighbor to ou
r own galaxy, is about a megaparsec, or million parsecs, away from us. That simple statement hides an intriguing fact. Although galaxies are tens of thousands of times bigger than their constituent solar systems, the distances between galaxies are only a few times larger than galaxies themselves, making them sort of close together. Relatively speaking.
But the most interesting thing about the large-scale structure of the universe on scales bigger than even galaxies, and the reason it has a name like cosmic web, is that galaxies aren't just arranged randomly throughout the cosmos, a fact that quite surprised the early cosmographers—the mappers of the universe.
But in order to reveal the structure of the universe, you have to go much larger than our local patch. The work of Hubble and others to establish the expansion of the universe relied on a comparatively nearby sample of galaxies—not nearly enough to reveal anything other than a scattered smattering of so-called spiral nebulae. Sure, there were dense beehives of galaxies like the nearby Coma Cluster, which flirtatiously suggested the existence of greater structures, but for decades cosmologists weren't sure if that was a significant feature or just a random collection.
But beginning in the 1970s, astronomers began to perfect the technology necessary to systematically survey the locations of galaxies far from the familiar, using a potent combination of improved telescopes and computerized search algorithms. It was like the classic duo of scope + camera that transformed our understanding (or lack thereof) of the nineteenth-century universe, but on steroids. The new surveys pushed both wider and deeper, creating the first-ever maps of our universe on a truly universal scale. Galaxy by galaxy, insignificant point of light by insignificant point of light, each dot representing a hundred billion warm nuclear hearths set against the deep, vast coldness of our cosmos, structures began to appear.1
Your Place in the Universe Page 18