Matter In the Universe, At Present


This is the universe at present.  You’re seeing not the light but the dark — you knew that most of the universe’s matter is dark, right? — and its motions.  Not until the far right end are you seeing the light.  Go ahead and clickety click all over this; zoom it in and out, drag it around. It’s a computer simulation based on and ending with observed reality.  But it doesn’t read from left to right; it’s less the chronology of the universe’s life than a series of its aspects.  It’s so beautiful you might faint.
The universe is a foam of blank bubbles surrounded by skins of matter.  The matter in the skins is mostly dark.  Nobody knows what the dark matter really is – some kind of particle.  But dark matter feels gravity and so it’s pulled, and so it moves.  This is a simulation of that motion: the gravity of the matter in the skins is pulling the dark particles; they’re streaming away from blank voids, their streamlines meeting other streamlines and forming a running foam, a network of walls.  The streamlines and walls look bright but they’re made of dark matter so they’re not shining; think of them instead as fast and dense.  It’s a map of gravity and motion.

Within those walls are more filaments of dark matter, also flowing toward and meeting other filaments, on smaller and smaller scales. They feel each others’ gravity, they pile up on top of each other, forming a finer and finer fractal foam. Streaming away from the voids, they make the voids bigger. You see this happening not so well in this picture but better in the picture at the top of the post — in the first five frames that are set below the picture, like a predella below a painting.

The young scientists who are the simulators have taken out the dark matter’s motion.  What you see clearly now are the walls and filaments that have pulled the dark matter out of the voids and into themselves.  The finer and finer filaments have gotten richer, and the poor voids are poorer.  Gravity being what it is — utterly relentless — it has concentrated some of the dark matter into dense ridges and some into little spots, into ridges and peaks of density.

The simulators have put the motions back in, so you can see how the dark matter is emptying out of voids and falling into those ridges and peaks.  In fact, those streamlines show the whole history of those particles’ motion, getting redder as they approach the present.  I’m not confident that I understand this picture. But you can almost feel the pull into the knots and nodes.  And the matter has emptied the voids to such an extent that the universe begins to look divided into nothingness and something.

The simulators have taken the motions back out again.  This is the foamy pattern of dark matter that cosmologists call the universe’s structure, the cosmic web:  huge, yawning voids; and the fuzz of dark matter still falling into filaments and piling up into ridges of density, into density peaks like cities and small towns.  You’d think you were in an airplane looking down, but you’re not, you’re on this planet looking up. This is how the static universe really looks.

But all we see of it is this.  This is real data; they’re real superclusters, real clusters, real galaxies.  They’re made of regular matter, which is a small trace of all matter and which has all along been traveling with the dark.  It has collected into atoms and eventually into stars, so it shines.  It looks like bright little islands poised on nets of mountain ranges invisible under the dark water.

It looks lonely.  No, it doesn’t.  It looks like it’s full of courage and faith in the endless dark mountains that are its foundation.


I first saw the picture — a kind of moving poster — at the top of the post because it won first place in Science magazine’s 2011 International Science & Engineering Visualization Challenge and was on the magazine’s cover.  Here’s the journal paper explaining it all.

The picture was made by Miguel Angel Aragón-Calvo, with the help of Julieta Aguilera and Mark SubbaRao of the Adler Planetarium.  It is used here with their kind permission. Aragón-Calvo is at Johns Hopkins University as an assistant research scientist.  He’s job-hunting.  Wish him luck.


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5 thoughts on “Matter In the Universe, At Present

  1. Ann, such a nice essay! Dark matter is very strange stuff, and we have to be careful that just because we have given it a name does not mean we understand it. We are funny in that way – once we give a name to our ignorance, we are much more comfortable with it and it begins to become “obvious.”

    But it is not. I was at a conference once where an optical astronomer, as I am, was explaining some really messy observations of the probability that for any galaxy, there is another galaxy 10 megaparsec away, or 8 megaparsecs, or any distance actually more than a few megaparsecs (5 million light years). Her data were messy, there were systematic errors, she had to make large corrections – it was a tough thing to measure!

    A theoretician was in the audience who did one of the simulations of the formation of structure in the Universe as described above. He told her that she should use his computer output and measure the distances because she would get a lot cleaner result. She (and I ) was puzzled because that is the thing you want to measure. The theoretician continued explaining that his data could be used to measure these distances between galaxies.

    It became clear to us that he was actually confusing the real universe with the one that comes out of a computer. The universe is what is up there in the sky, not what is on a computer. His models do all sorts of cool stuff and give us experimentalists ideas on what to measure – to find new stuff, or to test his models.

    But the model is never the real thing.

    Or is it? At what point does a model become reality? If I can trace the evolution of particles to incredible accuracy, then is that reality? We seem to be crossing that fuzzy frontier now, with so much access to technology. How much of me is modeled on what information is given to me on tv, internet, and the like? And that then becomes my reality. Of course, th e models ultimately must conform to what you measure, but some of what we measure is told to us by the modelers. We are getting to the point where theory cannot be tested by experiment in astronomy, and what is “real” becomes hard to understand.

  2. Richard Feynman said, “Physicists are fond of saying ‘These are the starting conditions now what happens next?'” I think the point that Nicholas makes is a good one. First we must get the starting conditions right i.e. the factual basis, the data, must be known, but data is often “messy” and not amenable to a unique fit or unique modeling.

    What you can get, if you are not careful, is a feedback loop between the corrupted and derogated (massaged)data and the strong model. I hope that astronomers don’t allow the data to be impacted the way the modeller suggested. That is how you corrupt science.

    I have documented the extreme effort made by Eddington to confirm general relativity with the Sobral and Principe Eclipse data in 1919. Never mind that the data was read to perhaps 100-1000 X the precision that the conditions and equipment allowed, that dubious “data” is still quoted as Gospel by top scientists.

    Strong modesl corrupt weak men and women. I fear astronomers will see these simulations and “interpret” their “facts” differently. There is a real danger that astronomers will now come up with, as Sciama has pointed out, the “right” answers.

    It is a slippery slope when theory drives data.

  3. As a totally ad hoc idea what if dark matter and matter repel each other i.e. it is a push rather than a pull? Dark matter would then repel matter out of the voids and create the walls.

    MOND still gives you a better fit to the shape of your galaxies yet preserves dark matter as the cause of the large-scale structures. In other words the more matter is around the less dark matter is around.

    This would also avoid the need for dark energy and gives you accelerating universal expansion.

  4. Nick and Richard both, you’re above my pay grade in these matters. Theory always scares me a little — so many things are possible!

  5. Richard,

    We have a very good idea of the matter density of the Universe, from the abundances of hydrogen, deuterium, helium, and lithium. We can’t have much more “baryonic” (ie normal) matter without screwing up the hydrogen to helium ratio that comes out of the big bang. That is one of the reasons we don’t think dark matter is mini black holes.

    Dark matter is totally mysterious, and we need to be careful in stating what is “obvious.” One particularly hard thing to do is to get different types of matter to repel. It just does not fit into General Relativity. That does not mean it is wrong, just that there will be lots of weird consequences if you suddenly have matter repelling itself. We need to keep our minds open. But these speculations are right now beyond my grasp to test. The big question is – is dark energy the same as a cosmological constant? It sure looks that way.

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