“If we lived on a planet where nothing ever changed, there would be little to do. There would be nothing to figure out. There would be no impetus for science. And if we lived in an unpredictable world, where things changed in random or very complex ways, we would not be able to figure things out. But we live in an in-between universe, where things change, but according to patterns, rules, or as we call them, laws of nature. If I throw a stick up in the air, it always falls down. If the sun sets in the west, it always rises again the next morning in the east. And so it becomes possible to figure things out. We can do science, and with it we can improve our lives.” –Carl Sagan
As many of you know, I’m a scientist and a science educator. It’s what I’ve chosen to do with my life, and I’m happy with that choice overall. But one thing that I find amazingly difficult to deal with is trying to explain some of the biggest ideas supported by modern science — such as evolution, the big bang, and dark matter — to people who respond, “well, but your theory doesn’t explain _____________.”
I’ve got news for you. There is no Theory of Everything. Even the ones that claim to be don’t claim to explain or predict every fathomable thing.
When you have any scientific theory, it has a range of validity. Think about that phrase for a minute: range of validity. What does that mean?
Let’s start with a small idea first to illustrate this: the idea that heat rises. Sure, you put a hot thing like a burning candle in a cool room, and the flame will heat up the air around it, and the hot air will rise. Sounds simple, until you ask the question, “how will the hot air rise?”
And the answer to that one is more complex. Under some conditions (above right), the hot air will rise smoothly and the air will flow in a laminar fashion, while under other conditions (above left) the air will rise turbulently.
This doesn’t mean that there’s anything wrong with your theory that hot air rises, though. It means that your theory is incomplete, as are all scientific theories.
What, Ethan? All of them?
But, publicly, this concept is not well understood. It rears its head very frequently in discussions about evolution. (And I’m talking about Darwinian evolution here.) In a nutshell, evolution is the big idea that the inherited traits of organisms change over many successive generations. And that’s what we think produced the great diversity of life we have today.
And that part of evolution isn’t really up for debate. It happens. We see it happen, and we have observed multiple mechanisms which can drive it.
But the most frequent objection that I hear to evolution is the following: But evolution doesn’t explain the origin of life!
That’s true! Evolution doesn’t explain the origin of life, because the origin of life is outside the scope of evolution! The origin of living things from non-life is known as abiogenesis, and is an active field of scientific study.
But I digress. My point is that evolution has a range of validity, where it is useful for solving problems and explaining phenomena, and that outside of that range, either another theory or an extension of evolution is needed. But the fact that evolution doesn’t answer everything doesn’t, in any way, mean that evolution is wrong or invalid.
Now, onto the astrophysics.
The Big Bang. Another big idea that tells us that the Universe is currently expanding and cooling, and that it was hotter and denser in the past. Pretty simple, right?
There are many consequences that come out of this simple idea, including that:
- the galaxies should be expanding away from each other,
- galaxies that are farther away should be expanding away faster than ones that are closer,
- at some point in the past, it was too hot for neutral atoms to form, meaning we should observe a leftover glow from this era, and
- at some point even further back in time, it was too hot for atomic nuclei to form, allowing us to predict the primordial abundances of the light elements.
That’s it, in a nutshell. That’s all the Big Bang is. You can try to extrapolate it further back, and address questions about the very origin of the Universe, but the Big Bang doesn’t really cover that. You can try to extrapolate it well into the future and inquire about the ultimate fate of the Universe, but that’s really outside the scope of the Big Bang. You can ask about the structures that will form in your Universe, but those are highly dependent on parameters of the Universe that are independent of the points of the Big Bang Theory, above.
So when you take a look at the entirety of our cosmic history, as best as we can reconstruct it:
You see a whole bunch of other ideas mashed together with the Big Bang. You see inflation, which handles the era prior to the Big Bang. You see dark matter, which handles the imbalance between protons, neutrons, and electrons and the force of gravity. And you see dark energy, which tells you about the ultimate fate of the Universe. But these details are independent of the Big Bang framework.
In other words, if you want to take down the Big Bang, you need an alternative that explains all of my earlier points without having an expanding, cooling Universe that was much hotter and denser in the past! And for the record, there are no alternatives that we’ve hit upon that can explain the leftover glow (that was discovered in 1964).
Every scientific theory has its limits. It never means that the theory is entirely wrong, but pushing up against those limits often gives you clues towards either extending your theory (such as adding inflation to the Big Bang) or overthrowing your theory (such as replacing Newton’s gravity with Einstein’s General Relativity). In either case, the old theory is still a good theory over its range of validity, and if you overthrow your old theory, your new theory must explain all of the observations of the old theory.
Why do I bring all of this up now? Because Dark Matter — an extraordinarily important part of modern cosmology (which you can learn about here, here, here, here, and here) — is under attack by people who don’t understand this concept.
In a nutshell, the “dark matter problem” is that on large scales (galaxies, clusters of galaxies, and even larger structures), the amount of normal matter that’s present — stuff from stars, gas, planets, and dust — is insufficient to explain the gravity caused by these structures. The speeds of the galaxies in the image above? They’re too fast, unless there’s some extra unseen mass. The bent arcs of light, caused by gravitational lensing? Also require more unseen mass, but coincidentally, the same amount that the speeds require.
What about the large-scale structure that forms in the Universe?
It also requires dark matter, or gravity wouldn’t work correctly. But there’s something extra, that constitutes insurmountable evidence for the existence of dark matter. We’ve caught galaxy clusters in the act of colliding! According to the theory of dark matter, the gas should “splat” in the middle of the collision, but the dark matter from each cluster should pass right through, unaffected by the collision. When we map out the effects of gravity, if dark matter is correct, it shouldn’t line up with where the gas is. Let’s take a look at these colliding clusters, known as the Bullet Cluster. And let’s put the hot X-ray emitting gas (from the “splat”) in pink, and the map of gravitational force (as measured from lensing) in blue.
The fact that these don’t line up is a smoking gun for dark matter. So if you want an alternative for dark matter, that alternative needs to explain all of these observations. And none of the theories — not MOND, MOG, TeVeS, or any others — can do so.
But there are limits to the theory of dark matter. Dark matter is great for explaining, gravitationally, what happens on large scales. But when you start looking at small cosmic scales, you start pushing up against the limits of what saying “the Universe is full of dark matter” can tell you. Some dark matter mysteries?
- What goes on with dark matter close to the center of a galaxy or cluster?
- What’s going on with the rotation of low-mass (or low-surface brightness) galaxies?
- What happens with very small satellite galaxies in the Universe?
In other words, when you come down to small scales, and to details about small-scale structure, we need to know more about dark matter before we can make definitive predictions. This is where the present limitation of the theory of dark matter lives.
Well, there’s a new paper out pushing these limits, which is good. The authors state some specific examples of what difficulties exist for dark matter on these small scales. (And you can trust me; I know something about Dark Matter on the Smallest Scales.) But the authors assert that these difficulties mean that dark matter may be wrong! And assertions like that have a way of making it into the press. Over here:
In the news over here:
and, I’m sure, in many other places.
Look. Dark matter doesn’t do everything. Its predictions on small scales are uncertain, primarily because we don’t know what makes it up, and we don’t know what its properties are! If we knew these things, we could make predictions. But saying that, “since the naive predictions we can make are inadequate, the entire idea of dark matter needs to be thrown out” is absurd.
This isn’t a house of cards, where if you find one defect, the whole thing will collapse. It’s a stepladder that’s missing a rung or two. It still works. In fact, it totally works! Its flaw is that it doesn’t serve your needs for every single possible class of applications. (At least, not yet.)
So yes, my theory doesn’t do everything. And that’s okay. It means it’s just like every other scientific theory that’s out there. Which is to say, it makes it possible to figure things out.