Dark matter and the cosmic web

I find this extremely confusing : http://news.ucsc.edu/2014/01/cosmic-web.html
But then again, that’s probably true for all things concerning dark matter. Anybody think they can shed some light on this?

Shed some light on dark matter? He-he. … sorry…


In a nutshell, the observation means an independent line of evidence for the existence of dark matter. We seem to have our first glimpse of a structure that was predicted to exist by a computer model of a universe containing dark matter. As you may know, dark matter itself was postulated because the relative motions of galaxies and clusters came out all wrong when their mass was estimated based on the apparent matter they contain.

In addition, this new observation gives some clues about how the dark matter is distributed, and also indicates that there could be more intergalactic gas than we thought up to now, allowing us to refine our models.

The model indicated that dark matter would aggregate in long intersecting “strings” forming a 3D mesh or web pervading the universe. Through gravity, ordinary hydrogen gas collects preferentially around higher concentrations of dark matter, i.e. those “strings”, and the glow the astronomers detected was due to radiation from the quasar knocking electrons about in the gas, similar to what happens in a fluorescent light.

ETA: It’s either that or we’ve got an inside view of God’s brain in operation…


Thanks Mefiante. If I understand this correctly, they detected a gas cloud. And it was bigger than expected. I’m not sure I see the link to dark matter. Is it that the presence of the gas cloud was predicted by dark matter models?

Yes, considerably bigger than it would have been in the absence of dark matter.

Yes — but obviously not that particular gas cloud at that particular point. The model predicted that there should be gas clouds of this type spread around the universe. The one reported was found through the happy coincidence of having a large quasar in its vicinity.


Ok, I think I get this. So dark matter is not spread evenly throughout the universe (which is what I previously thought). It forms this cosmic web with branches and nodes. Ordinary matter like gasses cluster around these nodes due to gravity. So what gives the cosmic web this structure, i.e. what made them suspect it should look like this?

Correct, but there are even greater concentrations where there are galaxies.

(BTW, “… branches and nodes” — spoken like someone comfortable with circuits, closed networks and Kirchhoff’s laws; electrical engineer?)

In the initial state, the dark matter and gas is spread around fairly evenly, but by pure randomness there are small regions where the concentration is higher or lower than average. These are the “seeds” from which galaxies and intergalactic voids grow. Over time, more and more matter is attracted by gravity to the denser areas, and away from the less dense. Simultaneously, the dark matter “strings” are stretched lengthways through the universal spacetime expansion. If the dark matter had initially been perfectly uniformly distributed, these clustering effects wouldn’t happen without some perturbation. Only very slight initial density differences are needed to get the whole separation/coagulation process going quite quickly.


Thanks Mefiante, this makes sense, and ties in with my existing knowledge. Next week we can do dark energy :wink:

BTW, “… branches and nodes” — spoken like someone comfortable with circuits, closed networks and Kirchhoff’s laws; electrical engineer?
Correct guess, but for the wrong reason :). Actually my branches/nodes terminology comes from my recent studying of network theory(the mathematical sort), as I'm attempting to specialize in optimization techniques.

Ah, okay. I was going to try an Eulerian or Hamiltonian path to there next. :wink:


I have heard, long ago, that observing other galaxies is one way of demonstrating the Newton’s laws are universal. But if we have to postulate dark matter, then presumably those galaxies are in fact doing apparently very un-Newtonian things?

Actually, the idea (and fact) that the universe contains billions of galaxies is younger than Einstein’s General Relativity (GR) by about a decade. At that time, it was thought that the Milky Way was the entire universe. I mention this for two reasons. First, Newton’s laws are a limiting case of GR (when the speed of light is assumed to be infinite), and second, Newton’s laws (motion & gravity) were and are splendidly confirmed from planetary considerations (where GR effects are not significant — except for that pesky precession of Mercury’s orbit).

GR is also confirmed by observations of very distant objects (gravitational lensing and the bending of light rays). Their motions relative to one another as well as their rotations in conjunction with their apparent masses and mass distributions are also valuable observations. From careful studies of these it has become clear that the form of the governing GR laws/equations is accurate (perfectly so, within the limits of measurement error), but there’s something funny about the results when the assumed values for some of the parameters are used.

The situation is similar to having, say, a valid population growth model but using US figures to model SA’s population. When you plot the observed and expected profiles, you will find that they have the same shape (i.e. form) but the values don’t coincide.

In this way, the galactic observations likewise confirm the form of the laws we use to model them and they also tell us that there’s a problem with the values we are assuming as inputs. But that doesn’t make them “un-Newtonian” and even less “un-Einsteinian”.


Surely one must remain open to the possibility that there may also be (a) part(s) of the function(s) missing?

I agree, this dark matter hypothesis reminds me a lot of Einstein’s cosmic constant; a little trick to reconcile what we see with what we expect to see. I suppose as science goes it’s the best we have at this point in time.

Yes, that is of course true in principle. In recent years, there has been a resurgence of the controversy over the role, if any, of the cosmological constant of GR in explaining certain observations. The cosmological constant is a term Einstein introduced to preserve a static universe and it was dropped after Hubble’s observations proved that the universe was expanding (and also because it held the universe in a state of unstable equilibrium, a highly unlikely eventuality). Still, in following Occam’s Razor, it makes more sense to posit dark matter/energy than to revise GR since GR is very solidly confirmed by many other lines of evidence. There may come a day when GR is superseded by or subsumed under a more comprehensive theory, just as GR did with Newtonian dynamics.


There is also the possibility that the cosmological principle is false and that the laws of physics are not always and everywhere the same. If they vary systematically in time and space, this would merely mean that our own laws of physics are formally incomplete, a situation that can be remedied by adding the appropriate details once they are known. On the other hand, if the laws change unpredictably in spacetime, this presents an intractable problem even if the changes are gradual and smooth.

That said, it seems extremely unlikely to be the case because our most distant observations are consistent with the cosmological principle being true. For example, it would be an extraordinary coincidence to observe the precise emission/absorption spectra of hydrogen and helium from a galaxy billions of light-years distant if the fusion process going on in those stars is different to what we know. Similarly (and as mentioned in an earlier post), it would be remarkable to observe the relative motions of galaxies to be perfectly in line with the form (I cannot stress this important point enough!) of GR’s equations and yet be the result of a different physics, i.e. more than just erroneous input values. To illustrate more concretely, if Newton’s gravitational law was anything other than (1) linear in the masses of the attracting bodies and (2) inverse square with distance between their centroids, stable planetary orbits would not be possible at all.