Welcome to my holiday column, wherein I am inclined to wander even further afield than normal. In my first such, "Keeping the X in Xmas," I explained the physics of photography from first principles, how electromagnetism makes up our existence and our entire awareness. Without electromagnetism, we could not see, hear, taste, and think. Or even touch—it's the repulsion between electrons in our fingertips and the things that we handle that let us feel and grasp them. That's electromagnetism at work. Without it, we would be like ghosts, our hands passing intangibly through whatever we reached for (never mind that without electromagnetism, we wouldn't even be here).
Such ghosts do exist and most of you will have heard of them: neutrinos. Very small, very lightweight particles that don't respond to electromagnetism. The universe is lousy with them. Trillions of them pass through your body every second, and you never notice them, because they hardly ever notice you.
Neutrinos don't have a lot of mass (why they have any at all is an interesting and unanswered question) and even though there are more of them than you can shake a star at, they don't make up a large fraction of the universe. They are bit-part players.
Well, that's a little unfair to neutrinos. Most of the time they are bit players, but if it weren't for neutrinos, there wouldn't be supernovae, and if there weren't supernovae, there wouldn't be anything distributing heavy elements (meaning stuff heavier than hydrogen and helium) throughout the universe, and that would sure make it a lot harder for planets and people to exist. In the normal daily routine, though, neutrinos are minor players.
But, what would a universe be like where such ghostly entities weren't minor players? I can answer that, but let me lay down some background first:
For 75 years, astronomers have been dealing with the mystery of dark matter. What do astronomers mean by "dark?" Just what the rest of us would: matter that we can't see against the blackness of space. So how do we even know it's there?
Back in the 1930s, the infamously irascible* Fritz Zwicky noticed that stars and galaxies were moving too fast to be gravitationally bound to each other. We can approximately estimate the masses of galaxies by looking at how much light they emit and knowing how much an average star emits. We can measure velocities by measuring Doppler shifts in spectral lines. Fritz looked at a bunch of this data and calculated that if the galaxies had anything close to their observed, visible mass, then they should just fly apart. The error wasn't small; Fritz estimated that the galaxies had to have 100 times as much mass as we could see to hold everything together.
Over the years, that ratio shrank but it still remained very large. Meanwhile alternative theories were tested and fell by the wayside. There was good evidence that gravity worked more or less the same way, even on galactic scales. With ever better observational equipment, we were able to look at galaxies in more and more detail, and we did find more and more astronomical objects (most recently, another gazillion red dwarfs), but nowhere near enough to balance the equation.
There are still some respectable astronomers who think we can account for the missing mass in ordinary ways or with revised theories of gravity, but they're in the minority.
Meanwhile entirely different lines of evidence were pointing to a big strangeness. It's surprisingly easy (on the graduate student level) to calculate how massive the universe should be, based on its observed composition. The Big Bang starts out as this amorphous glob of energy that matter eventually "condenses" out of in a simple and predictable way. One straightforward calculation is how changing the amount of energy in the initial fireball changes the mix of different kinds of matter that you get out. You might think of it as being like some kind of self-baking cake batter that heats the oven and cooks itself. Adding more batter doesn't just make a bigger cake, it make the oven get hotter for longer, so the cake doesn't bake the same.
This Hubble Space Telescope composite image shows dark matter (in blue) surrounding the galaxy cluster ZwCl0024+1652 (pinkish-white). This is not an artist's illustration—this is a computer-generated photograph of the dark matter superimposed on a normal photograph of the cluster. Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)
Anyway, those simple big-bang calculations match the observed elemental composition of the universe extremely well and also match the subatomic physics that we see in particle accelerators. It is an extraordinary and grand synthesis of many separate lines of study. There is only one small problem:
Put all the pieces of data together into one coherent picture, and there's nowhere near enough ordinary matter to account for all the stuff in the universe. In fact, the best calculations today say that there is over five times as much really weird stuff that we've never seen, called "nonbaryonic matter," as there is the ordinary "baryonic matter" that we are made of and familiar with. Put another way, the entire universe that we directly observe and more-or-less understand is only about 15% of the matter in the universe. We are very much in the minority.
Which answers the question I posed earlier, which is what would a universe look like if it was composed mostly of ghostly matter? The answer is that it would look like the one we're living in.
We still don't know what this weird stuff (or dark matter) is. Many theories, no answers. Neutrinos make up part of that weird stuff, but as I previously mentioned, they can only account for a small fraction of it. Evidence accumulates; we can observe dark matter indirectly. Why? Because it is matter and so it exerts a gravitational effect. Gravity bends light. By looking at distant galaxies (the sky is lousy with them) we can see distortions in the shapes of those galaxies and the patterns they are distributed in. Those distortions are caused by ripples in space-time, and those ripples are caused by gravity, and where there is gravity there is mass. Just as you can get a sense of where the ripples are in a pane of glass by how the scene that you see through it is distorted, it's possible to take those observations of distant galaxies, throw a whole bunch of computers at them, and calculate what the pattern is for the intervening mass that is distorting them.
The result is that we are managing to produce computer-generated "photographs" of matter that we can't even see. How cool is that!? Not to mention counterintuitive. What we see when we look at those pictures is stuff that behaves like cold dark matter that doesn't interact electromagnetically. Ghost stuff. A whole shadow universe, and it's over five times bigger than our own.
It's a small shock, finding out that 85% of the matter out there is "Something Else." But, we've known that something was wrong for 75 years, we just lacked the details. The thing is...
That's not even close to the whole story. It turns out that shadow matter universe is still just a minority component of the entire universe. And no one expected that.
There's where I'll go next week. If you think it's been a weird ride so far, hang on tight; it's going to get a lot stranger.
(*The man is famous for once describing his long-suffering colleagues as "spherical bastards" because they were bastards no matter how you looked at them.)
Ctein's far-flung weekly column appears on TOP on Wednesday mornings.
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Original contents copyright 2010 by Michael C. Johnston and/or the bylined author. All Rights Reserved.