"You know, dark matter matters."
-Neil deGrasse Tyson, American Physicist (1958-), 2017
"Dark matter is interesting. Basically, the universe is heavier than it should be. There's whole swathes of stuff we can't account for." - Talulah Riley, English Actress (1985-)
We know from another of my blogs on the Universe ("How vast is the Universe? Let’s look at it on a human scale"), that the entire cosmos we inhabit is unbelievably big. But it’s getting bigger as we speak. This must be the case, as the entire Universe now is a lot larger than it was when it was less than the size of a pinhead. Cosmologists call the Universe at this point (the very moment it came into existence) the gravitational or spacetime singularity.
Everything happened from this mysterious trigger event. We know it as the Big Bang—the point at which the Universe exploded into being.
It was an incredibly long time ago: almost 14 billion years, according to our best estimates. The Universe has been getting bigger and bigger all this time. Right now it’s expanding at the rate of 45 miles (72 kilometers) per second.
So far, so logical. A while back, astronomers assumed that the Universe couldn’t keep expanding indefinitely, because as it grew, gravity would operate on it, slow it down, and pull it back from runaway enlargement. As a consequence, all the galaxies would have to stop relentlessly pulling away from each other.
Sounds right? Wrong.
In theory, the Universe should slow. Gravity is the strange force you think you know (because it keeps you on the planet and makes soccer balls return to earth) that pulls things together. Ok, so it keeps the planets revolving round the sun, and us rooted to the Earth. And it is an inhibitory force on galaxies moving apart.
Despite gravity’s pulling properties, the Universe is not just continuing its expansionary ways. It's accelerating. Every galaxy in the Universe is moving away from every other galaxy. Faster and faster.
No one, not even the smartest cosmological theorists, has any definitive idea what is acting, contrary to gravity's tendency to pull things together, to push the galaxies apart.
But this hasn’t stopped them giving this force a name, and trying to figure it out. Dark energy. It might be called dark, but that’s just a metaphor. We don’t even know if it’s energy or, if it is, what sort. But dark energy brilliantly expresses all the mystery of this potent and elusive force.
It turns out that the “normal” Universe—the one that we can see with a telescope, and the bits of it we experience here on Earth in all the matter that we see and interact with—makes up only about 4.9% of what we can observe. That accounts for all the mass of the stars, planets, and galactical gasses, for example: all the physical stuff in the Universe. The rest, physicists estimate, around 68.3% of the Universe, is made up of dark energy, this pulling-apart force.
And then there’s the remaining 26.8% of the Universe. This is even more mysterious (if anything can be more enigmatic than some sort of energy that must exist, but we can't see or understand, and we can only theorize about): dark matter.
That all adds up to 100.0%. But remember, less than 5.0% of the Universe is composed of atoms that we know about, like the ones that comprise us. The search is on for the remaining 95.1% of the Universe that’s missing, presumed lost.
To explain how we know this story to date, and the limits of our knowledge, we need to brush up a little more on our physics. That will help us understand dark energy a bit better.
Space is not nothing. Space has characteristics, properties, features. One essential property of space, is that more of it can come into existence. Another is that space can embody—possess, if you like—forms of energy. So if you add space to the boundaries of the Universe as it expands, and this additional space takes on more energy, you get more expansion. This pushes the Universe out, in conflict with gravity pulling it in. That’s the dark energy.
(There’s another possible explanation where we don’t need the dark energy explanation. Maybe gravity doesn’t behave as Einstein’s theory suggests, and it’s somehow weaker than we estimate it to be. We’d have to re-write the textbooks, and we’d need a deeper explanation of gravity than we have now, but we can’t exclude anything for a problem this taxing. But very few cosmologists believe this. Either way, it’s still a mystery.)
Dark energy, whatever its composition, is either everywhere, spread out, or it’s clumpy. The “everywhere” explanation suggests that dark energy permeates the entire Universe, filling all of space equally. A kind of insidious, homogeneous infusion, imbuing everything.
The alternative, “clumpy” suggestion, also known as the quintessence theory, says that the energy density differs in different parts of the Universe, and can change dynamically. There is no real evidence for the clumpy theory, and so most physicists think that the “dark energy everywhere” theory is the most plausible.
A visualization of how dark matter ‘clumps’ in the cosmos – if it does
The Universe’s acceleration, as distinct from merely expanding, is thought to have begun about five billion years ago, i.e., eight billion years after it all got started. And it’s speedng up. Eventually, and incredibly, we’ll no longer be able to see any galaxies beyond our own—their velocity will move them too far away from us.
Once they are over our “event horizon,” even if they emit a signal, i.e., a star shines light in our direction, an observer will not be able to see it. Not even if the future is infinite, and therefore there is unlimited time for it to reach that observer.
Startlingly, some estimates say that this means that in the case of anything that is more than 16 billion light years away from us now, no signal in the distant future would ever reach us. In short, galaxies all around us will disappear from sight. They’d vanish. Literally. An observer would never know that any other galaxy ever existed in the Universe—they’d have shifted away from our vantage point.
It gets worse. According to some physicists, dark energy will eventually dominate gravity to such an extent that it tears apart everything—galaxies, solar systems, the underlying forces in the Universe, and particles. Every atom in the Universe will be ripped to shreds. Ouch.
Meanwhile, what about dark matter? It’s got intriguing properties: it’s concealed, it’s cold, there’s lots of it, it’s stable as it hasn’t dissapated, and therefore we can assume it doesn’t readily decay. Like dark energy, dark matter has never been seen, but it is thought to be made up of some sort of particles. A widely held theory is that it is composed of weakly interacting massive particles (or WIMPS, as physicists humorously like to call them) that are somehow an interactive product of gravity and another force found in nature called the weak force.
Stay with me please. It gets curiouser and curiouser.
Another even wackier theory hypothesizes (because we can’t find any other evidence to confirm dark matter’s existence) that there is one or more parallel Universes (which have little to do with our own) where dark matter resides—quietly lurking, I guess, and doing its thing—whatever that is. Patiently waiting, hanging around, I guess. Physicists (who clearly like their little jokes) refer to this as the “hidden valley” theory.
Whichever theory is the case, what almost no one questions is that the Universe contains far more matter than can be observed as ordinary mass, that is, made up of the atoms comprising that skimpy 4.9%. Writer Bill Bryson’s bemused definition of dark matter in his A Short History of Nearly Everything sums up the continuing mystery brilliantly: “For the moment we might very well call them DUNNOS (for Dark Unknown Nonreflective Nondetectable Objects Somewhere).”
All this might be a purely academic argument to many, and simply not worth the paper it’s printed on (or pixels you are reading from if you are scanning a digital version of this), for all the effect it has on our lives. But consider this suggestion from Lisa Randall, a Harvard physicist who has thought about this issue more than most. Her popular science book Dark Matter and the Dinosaurs suggests that the extinction of every kid’s favorite Mesozoic creatures, 66 million years ago, came about through dark matter. I'm guessing you want a little explanation for this claim.
We all know the story of the dinosaurs: track back 66 million years, when they were dominating the Earth and being hugely successful, and were then wiped out by a nine-mile long body, probably a comet. This comet, which struck Earth with huge force at a crater impact site in the Yucatán Peninsula in Mexico called Chicxulub, created a kind of nuclear winter for up to decade.
The beneficiaries were tiny little shrew-like animals, early mammals, who eventually filled the evolutionary gap left by the dinosaurs. Over the millennia these little creatures proliferated in various evolved forms, eventually became bipedal, bigger-brained primates, and ultimately evolved into the species which invented baseball, take-out pizza, the Game of Thrones franchise, Marxism and capitalism.
How does dark matter figure in this particular narrative, you ask? In Lisa Randall’s account not only does dark matter cluster in big bubbles on the outskirts of galaxies, it also bunches together in regions within galaxies. Our solar system revolves around the center of our own galaxy, the Milky Way, but oscillates—shifting a little from a completely circular orbit. This oscillating orbit means we pass through the plane of our galaxy every 32 million years or so.
In Randall’s reckoning, that’s about the timeframe for mass extinctions to occur on Earth—every 32 million years, give or take a bit. So she postulates that the comet that led to the demise of the dinosaurs passed through the mass of dark matter lying around as a kind of wrinkle in the cosmos, and this nudged that fateful comet in our direction, on a collision course. And it’s happened before, in other extinctions on Earth going back into the deep past.
Understanding dark matter (and for that matter, dark energy) isn’t just the equivalent of bored physicists playing Minecraft on rainy days, then. It can have a profound effect on properties of the Universe and how it behaves, and how those attributes and behaviors can affect us. Randall concludes her book, thus:
“In some global sense, we are all descendants of Chicxulub. It’s a part of our history that we should want to understand. If true, the additional wrinkle presented in this book would mean that not only was dark matter responsible for irrevocably changing our world, but also that some of it played a crucial role in allowing our existence.”
So, did dark matter play a direct role in wiping out the dinosaurs and paving the way, eventually, through 66 million years of evolution, for us? It’s an intriguing proposition.
In the end, this is all pretty bizarre. We have dark energy, a repulsive force overcoming gravity, pushing everything in the Universe apart. We have dark matter, additional stuff that we can’t begin to describe or explain, but it is the title we give for all the missing mass that the Universe consists of.
And we can’t see, feel, taste, touch or smell either of them. But without dark matter and dark energy, our knowlege of the Universe isn’t complete, and nothing in it, including us, makes sense. Did I say that fact is stranger than fiction?
Bryson, Bill (2003). A Short History of Nearly Everything. New York, NY: Broadway Books
Carroll, Sean (2015). Why is there dark matter? Preposterous Universe Blog. July 7. http://www.preposterousuniverse.com/blog/2015/07/07/why-is-there-dark-matter/
Randall, Lisa (2015). Dark Matter and the Dinosaurs: The Astounding Interconnectedness of the Universe. New York, NY: Ecco
Randall, Lisa, Reece, Matthew (2014). Dark matter as a trigger for periodic comet impacts. Physical Review Letters 112: 161301