The Unseen World

By Ctein

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.

Ctein

(*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.

010 by Michael C. Johnston and/or the bylined author. All Rights Reserved.

Comments

[david]

Put me in the minority. To my nose, dark matter has so many problems on local scales that it smells like epicycles. And if a phenomenological theory like MOND can do so well at such scales while assuming that what we see is all there is, then I think there's got to be something real that makes it work.

[Edd Fuller]

So, are you saying that Santa Claus really does exist?

[Richard]

I though "real stuff" only accounted for 5%? "Dark" matter about 25% and "Dark" energy about 70% (70% of what? I'm thinking). Mind you I'm only going by a layman's nodding acquaintance with the subject...

[Archer Sully]

Fritz Zwicky demonstrating the "spherical bastard":

And an article that further refines the concept.

[andy webster]

Of course he does, it's just that we can't see him....

[Hans]

MOND attempts to provide an alternative explanation of galactic dynamics. In that attempt, it seems to model the observations as well as the Dark Matter theory does. But it doesn't appear to account for the bending of light over inter-galactic scales. In that sense, the Dark Matter theory seems to be superior, since it accounts for both observations.

[Kevin Bourque]

"Without electromagnetism, we could not see, hear, taste, and think".

I was having this discussion with someone just the other day. The topic was ghosts and spirits...wasn't it just possible that there was some realm beyond our normal senses? I explained that of course that was possible, but for us to perceive it, it had to interact with us via the four known forces. Otherwise it may as well not exist. Things have to interact before they know about each other. The Invisible Man is, alas, blind.

I sometimes use the galaxy rotation problem as an example of how science works. Clearly, observation does not match theory, so either we need more observational data or the theory needs tweaking. Even if Newton and Einstein are "wrong", they're not dead wrong, as their models are good enough to navigate to Saturn and make GPS work. General Relativity is a refinement to Newtonian gravity, and there may be something else that fits reality even better. Good science is a process of recursive approximation.

I like the word "model" better than "theory". Unfortunately the Creationists have spoiled the milk by repeating "its just a theory" over and over again, implying that "theory" means "wild-ass guess". We should be glad that the Bible has nothing to say about relativity or quantum mechanics.

Looking forward to hearing about dark energy. If there isn't a grindcore band with this name yet, there should be.

[Richard Ward]

Ctien,
What a delightful piece of writing to find on one of my favorite internet sites.
Sincerly, Thank You & The Management for the space to post it.
Richard in Michigan

[Dave Karp]

Fun and mind boggling. Thanks.

[Michael]

Thanks for that. And thanks for sharing so much crunchy, chewy information with us over the years. Delicious!

[Gary K Froehlich]

Hmm, "spherical bastards," I like that. We had a so-called colleague that everyone called Cactus (after all, we are in New Mexico) -- no matter which direction you viewed him from you saw a prick.

[Ken Tanaka]

Nice essay on a mysterious subject, Ctein. Honestly, the concept of the "universe" still has my jaw slackened. Consequently the idea of accounting for the contents of infinity seems, well, ridiculous to me. (Even slightly goofier than accounting for U.S. Government spending.)

Two thoughts more of a noisy nature rather than genuinely informative.

First, it's occurred to me that the accounting for the universe's missing mass is somewhat analogous to trying to account for all of the wealth that evaporated into the 1990's Internet mania or, more recently, the collapsed credit markets. Where did the trillions all go? They must be out there somewhere. I sure don't have them.

Next, your essay brings from my memory a talk that Leon Lederman made during his tenure as head of Fermilab. He described the search for a sub-atomic particle (the quark?) as trying to figure out the game of soccer while not being able to see the ball.

Lederman was such a lively, energetic, and entertaining fellow that used such colloquial, but yet very accurate, concepts to enlist the imaginations his audiences (often school children) into science and math. His analogy certainly aligned my frame of reference for particle physics studies.

[David Dyer-Bennet]

Now you've got me thinking of "spherical bastards of uniform density" :-) .

Strange universe we live in!

[ctein]

Dear david,

First, "dark matter" is a very simple observation-- as I explained, we've known it's been around for 75 years. There's no plausible way to doubt it. The confounding part is what dark matter *IS*. There, you can get great arguments.

I'm not sure I understand your post very well.

First, you'll have to spell out MOND for me.

Second, by "problems on a local scale" do you mean why we don't see gravitational anomalies within the solar system or on earth itself? That's easy. The average total density of matter in the universe is very, very low. We can be sitting in a thin soup of dark matter and never notice. It's only when you get up to the galactic scale that the overall gravitational effect becomes so obvious, where it screws with the orbits of the stars and star clusters.

Why aren't there dark matter stars or at least large enough planets to mess with our solar system? Well, in the "normal matter" scenario, we'd have to say, "Just lucky, I guess." Though there could be lots and lots of dark matter asteroids or dwarf planets (a.k.a plutons) and we'd not notice-- the gravity effects would be too small.

But in the current "nonbaryonic matter" (NB) situation, which the overwhelming majority of science now believes, I must emphasize,it's less luck. Primordial matter condenses to form stars and planets in part because frictional forces dissipate gas velocities and because the electromagnetic force binds it into dust particles, which collide and stick.

So far as we can figure, NB matter would be nearly frictionless. So, other than large scale clumping due to gravity, what would slow down the primordial stuff enough that it could condense? Nothing we know of.

So, NB matter actually makes the local problems less, umm, problematical. We'd not expect to see a big dark-matter planet.

OTOH, if we ever do find a dark matter star or large planet somewhere, we'll have some 'splainin' to do to figure out how it formed.

pax / Ctein

[Christian]

That was very cool to read. Thank you, Ctein. Very very

[ctein]

Dear David,

OK, did my own homework-- am up to speed on MOND.

For laypeople-- MOND is a theory that says that Newton's Laws of gravity aren't accurate under certain conditions of very low acceleration. It hasn't been experimentally disproven, and it does explain Zwicky's observations.

Problem is, MOND doesn't model colliding galaxies well without more tweaking.

It can't explain the universal-scale observations of structure without even more ad-hoc tweaking.

It can't account for the mapping of space-time ripples via light bending that show humongous amounts of stuff we can't see is there (relativistic light-bending's well established).

It can't explain why both Big Bang theory and particle physics experiments agree that normal matter can only be a small component of our universe.

It can't explain the dark matter 'photographs' of colliding galaxies that show that their dark matter haloes are essentially frictionless.

In other words, it's a nice, simple mechanical theory that explains one nice and simple mechanical problem. It's a plausible explanation for one tree in a forest. For those who look at all the trees in the forest, though, it fails to explain the forest.

The general problem with most non-Newtonian/non-Einsteinian gravity theories is that they predict small effects. Which would be hugely important to our understanding of the universe, should they be true, and there are plausible nN/nE theories that are still being investigated. But without lots of fiddling they can't explain the elephant in the living room-- their effects are simply too small.

pax / Ctein

[Steve]

The X in Christmas is not an unknown it's Jesus. Interesting article. What I get out of it is the more science learns about the Universe the more we realize how much we don't know. Maybe scientists should look at the historical, prophetic and manuscript evidence in the Bible. Think of the incredible faith you need to believe that the universe appeared from an amorphous glob of energy!

[Peter Morgan]

"Good science is a process of recursive approximation", perhaps, but there's always the global problem of whether you've found the top of the highest mountain. Recursive approximation finds the top of the mountain you happen to be on, the global problem needs you to take a leap to somewhere very low in the hope that you might find a higher mountain. Then you have to persuade other people that you really have found somewhere higher. There is also the question of whether how high you are is really what you're looking for; would it be rational to look for better mountains to be blown off? Some of the best Scientists take leaps and are not heard of again. Lee Smolin is one of the best known Scientists who uses the analogy of valley crossing.

The same process of something more than approximation happens repeatedly in the development of new camera technology, and elsewhere, of course.

[Peter Hovmand]

Guess I really like supernovae, since without them there wouldn't be any cameras either ...

[Ed Kirkpatrick]

Very interesting! Thank you. I have to wonder about the infinite void of space beyond the expanding visible edge of our Universe and about all those neighboring Universes maybe laying outside our view still. Sort of like marbles in a jar, we are all there but can't see one another... yet.
And if you keep backing up the point of your perspective into this infinity, pretty soon you might be looking back at our worlds resembling a structure that looks very much what our sub-atomic world looks like to us. Please forgive my laymanesque attempt to describe something I have always been able to see clearly and draw but not put into words so much.

[ggl]

MOND (Modified Newtonian dynamics) is a theory that has yet to be confirmed. Also WIMPs (weakly interacting massive particles) have yet to be discovered. We'll see. There is also a recent paper that recalculates the rotation curves of galaxies by numerically solving the mass distribution in a rotating disc, leading to results that are close to observable data and also consistent with Newton's Law.

But I have to admit that the dark matter theory sounds really cool and Ctein's piece was a very nice read.

[david]

By local scale, I meant on the level of one galaxy--why doesn't it fly apart? Just what dark matter was dreamed up for, of course. There MOND just seems too good to be chance. There are hundreds of rotation curves from all types of galaxies which MOND correctly predicts based _only_ on the distribution of visible matter. That's remarkable for a theory with one free parameter. Contrast with dark matter, where you can arrange the dark matter halos in almost any way you like. Yet somehow the halos always seem to be arranged so that MOND works...

Until someone can explain why dark matter arranges itself in such a constrained way, I think there's a least one big reason to doubt it.

[Aravind]

The more I think about it, the more I laugh - with apologies to whoever said that first ...
"The Universe is a figment of its own imagination".

[Jim Hayes]

If you enjoy keeping abreast of news on astronomy, the MIT Technology Review newsletter and Futurity.com, produced by a consortium of universities to publicize their research, are good reads.

This showed up on the MIT newsletter recently:

Astronomers Find First Evidence Of Other Universes
Our cosmos was "bruised" in collisions with other universes. Now astronomers have found the first evidence of these impacts in the cosmic microwave background

http://www.technologyreview.com/blog/arxiv/26132/?nlid=3888

That should shake up a few people!

As an astronomy student in the early 60s who helped develop the first data acquisition and reduction system for astronomical photometry, I'm always amused at the sophistication of today's work, like that visual simulation of dark matter. In those days, it was a big deal to get a mainframe to calculate the dynamics of a galaxy using a few dozen points and show how spiral galaxies develop. It might take a few hours of computation!

[Simon Griffee]

A great article, thank you Ctein!

This year I read Star Maker by Olaf Stapledon—warmly recommended. It can also read online here: http://ebooks.adelaide.edu.au/s/stapledon/olaf/star/

[David Bostedo]

"...pretty soon you might be looking back at our worlds resembling a structure that looks very much what our sub-atomic world looks like to us."

Ed - If you're getting at what I think you are (and my apologies if you aren't), that's a very popular and clever possibility - that our entire universe may be something else's atomic scale. Unfortunately, that arises from some grossly simplified views of what the atomic world is like and there's really no chance that our world is anything like the sub-atomic realm, either physically, or in terms of behavior.

[Jim Hayes]

Discussions of dark matter always remind me of the epicyclic orbits of celestial bodies in a geocentric solar system model - the problem may be simply that we're looking at the data using the wrong assumptions. The problem is we don't know yet what the correct viewpoint is.
Astronomy is the ultimate bootstrap science; especially on a cosmological scale.
Perhaps the idea put forth in the MIT article I mentioned above provides a clue - we're but one "universe" in a larger space, and the physics of this new system offers a solution.
BTW, while studying Astronomy as an undergraduate, I got a minor in Philosophy - seemed quite complementary!

[ctein]

Dear Jim and others,

Discussions of the nature of dark matter are indeed a wide-open arena.

But when you can make photographs of it, it ain't epicycyles.

Also, the whole idea of epicycles was to make minor corrections to a incorrect model using circles. Dark matter isn't minor-- it's the dominant component.

Now, if we were composed of dark matter, it'd be a possible arguing point, whether that minor component called "baryons" was really there or was just a bad tweak on the existing model. But from our side, it ain't a tweak.

Using the simplest and most direct definition of dark matter-- stuff we can't see that exerts gravitational force-- its existence is inarguable.

Now, WHAT it is... that's what the next couple of decades of physics hopes to find out.

The fact we haven't found any on earth has no bearing on the question of its existence. Helium was first found in the sun; it took another 25 years to find it on earth. The universe is a big place.

pax / Ctein