Clumpy Dark Matter in New Simulations

by Paul Gilster on November 22, 2006

More on growth scenarios for interstellar flight soon. But I don’t want to let the recent dark matter news get past us, so a quick nod to the University of California at Santa Cruz, where researchers have run a powerful computer simulation to probe the dark matter halo that evidently surrounds our Milky Way. It’s a further step toward understanding the stuff that makes up 82 percent of the matter in the universe, and that in turns helps us see how the large scale structure of the cosmos has evolved.

Dark matter simulation

Image: Density map of dark matter in a halo the size of the Milky Way galaxy’s dark matter halo. Credit: J. Diemand, UC-SC.

I grew up in a time when it was thought that everything in the cosmos was explicable through gravitational forces produced by objects we could see. From solar systems on up to galaxies, it made sense — and the textbooks did this quite neatly with colorful diagrams — to show how matter found its way into configurations that would turn into celestial objects visible with our telescopes. Now we know the picture was wrong; the situation is far more complex and deeply mysterious.

For if dark matter — whatever it is — so vastly outnumbers matter we can see, then its interactions are crucial for the development of the earliest large-scale structure. Galaxies are inevitably affected by the gravitational wells created by this material, emerging in their distinctive shapes within spherical dark matter haloes. These haloes appear to be much larger than the galaxy they contain, and the new simulations testify as to how clumpy they are, with sub-haloes that themselves exhibit substructure variations. Says Piero Madau (UC-SC):

“We find almost 10,000 subhalos, about one order of magnitude more than in any past simulations, and some of our subhalos exhibit ‘subsubstructure.’ This was expected theoretically, but we have shown it for the first time in a numerical simulation.”

All this despite continuing problems fitting dwarf satellite galaxies — clumpy normal matter — into the dark matter picture. Future gamma-ray detection attempts may provide some answers, though until we have a better read on what dark matter actually is, we’re shooting in the dark.

Centauri Dreams‘ take: The gamma ray connection has continued to bother me — how could missions like GLAST (the Gamma Ray Large Area Space Telescope) have a role to play in detecting dark matter? The best I can do on that is to quote this UC-SC news release:

Astronomers may be able to detect clumps of dark matter within the Milky Way’s halo with future gamma-ray telescopes, but only if the dark matter consists of the types of particles that would give rise to gamma-ray emissions. Certain dark matter candidates–such as the neutralino, a theoretical particle predicted by supersymmetry theory–could annihilate (that is, be mutually destroyed) in collisions, generating new particles and emitting gamma rays.

That’s a big if, of course, but a sound way for investigation to proceed.
The paper is Diemand et al., “Dark matter substructure and gamma-ray annihilation in the Milky Way halo,” accepted for publication in The Astrophysical Journal and available online as a preprint.

Tim Jones November 22, 2006 at 21:02

Hi,

Having read the article, I’m more than a little confused by a statement referring to Dark Matter as being,

‘the stuff that makes up 82 percent of the matter in the universe”

Having recently read that Dark Energy constitutes 70% of the Universe, with physical matter accounting for around 5%, this would only leave Dark Matter able to constitute around 25% of matter in the Universe – so I’m basically saying I don’t understand where the figure of 82% comes from – or am missing something here?

Anyway, thanks for the great website, which seems to be packed full of all sorts of interesting stuff that’s still accessible to a layman such as myself.

Tim

Robin Goodfellow November 22, 2006 at 21:53

Tim, dark energy does not make up any part of the “matter of the universe”. 25% of 30% is 83%.

Tim Jones November 23, 2006 at 6:22

Robin – thanks, sorry for the daft question, which I realised shortly after I’d hit the submit button.

JayPee November 24, 2006 at 6:37

Here is my speculation about the nature of dark energy. We know it is a repulsive force responsible for the expansion of the Universe. We know it was present even at the early stages of the universe’s (latest Nasa press release).. We know the Big Bang initiated the universe expansion, is it then reaonsable to assume that dark energy is the force that powered or drove the Big Bang..i.e. the force behind the big explosion that started the universe.. Furthermore, at a black hole singularity the gravitational force i.e. space-time curvature is infinite.. if we can imagine looking at other side of a black hole, i.e. from outide the 3-dimensional universe, and assuming natural symmetry, the infinite attractive gravity would appear as a repulsive force.. pure specualtion here, at the other end of each black hole is then an emerging small universe..with repuslive dark energy force and matter/energy sucked in from our universe…Could it then be that our big bang universe started as the opposite side of a black hole in another universe or a higher-dimension space domain? I know this is pure speculation but what do you guys think..

Tim Jones November 24, 2006 at 20:25

What is most interesting to me is that dark matter was present at the Big Bang, to be followed by dark energy kicking in several billion years later – with the Universe having been tentatively re-dated to something like 15% older than previous estimates, the lag between dark matter and the eventual appearance of dark energy at 9 billion years ago, means there could have been a gap of 7 or 8 billion years. I don’t know if that revised figure would alter the calculations of an expanding universe that would eventually have collapsed in on itself i.e was the timing of the dark energy event an exact phenomenon that could only have happened at that time to be effective, or was there a degree of leeway?

The injection of dark energy might be viewed as a solution to a problem, which poses the slightly odd question of whether the Universe ‘knew’ it had to fix the gravitational problem or die, or even if there was some external ‘knowledge’ that fixed the problem for unknown reasons – I hesitate to say so that life like us could one day flourish, but maybe one of Michio Kaku’s proposed Type 10 civilisations intervened on our, and/or others’ behalf, for reasons best known to themselves.

Some might contend that the injection of dark energy could be ascribed to a God/Creator rescue act, but I think an entity like that would, or should, be able to construct a perfect Universe from day 1, without the need for further intervention, and it is this original imperfection that to me would initially argue against such a situation.

But either way, the dark energy event has the ‘feel’ of being artificial and sentient in aspect, rather than something like a mathematical inevitability (which it may well have been, I guess). It’s just very strange that dark matter and dark energy are so far separated in Time, a clue that might indicate two separate origins for the two distinct phenomena.

I was wondering if it is known whether dark energy is a force within that pushes the Universe outward, or something outside, or on the the very periphery, that’s pulling the Universe out? And also, is the scale at which it works a constant – although the Universe itself is being pulled apart, our planets, ourselves and atoms are seemingly immune from runaway expansion – but will this always be the case, or is considerately self-regulating?

Regarding JayPee’s previous comment regarding black holes, I think there are some astounding discoveries to be made regarding what a black hole actually generates – could dark matter be a residual effect from black holes in other universes – it also occurred that maybe a previous universe did collpase in on itself, but instead of destroying all the matter in that Universe, some of it survived in the form of dark, or ghost matter, which eventually helped kick-start our universe, rather than Big Bang emanating from some putative zero point – after all, although it’s known that at the time suggested, the Universe did indeed go through a very hot and energetic fluid expansion, this Big Bang might have been an episode rather than a pilot show for the cosmic drama that was to unfold.

I know these are all broad generalisations, and as I have no maths, physics or astronomy I obviously can’t back any of them up, but I was wondering if any of this strikes a chord with anyone out there, or whether all or or any of these points have already been discussed or summarily dismissed?

Thanks, Tim

Administrator November 26, 2006 at 15:15

Tim, these are fascinating conjectures indeed. And I think the answer is that when we get this far into such a poorly understood phenomenon (dark energy’s effects are only just now becoming understood, and just what it is remains a mystery), conjecture is all we have to go on until we get better data. The idea of dark energy as the solution to a problem makes for some intriguing scenarios including, as you say, intervention by one of Kaku’s conjectural advanced civilizations. No way to prove of disprove such a thing at present, but I like energetic and interesting ideas like this. We must hope that some of the upcoming work on dark energy (including the JDEM mission discussed here previously) can help to lift the veil on some of these mysteries. If not, then we’ve still developed some interesting science fiction plots!

Oh, and that earlier JDEM story is:

http://www.centauri-dreams.org/?p=908

Tim Jones November 26, 2006 at 15:39

Paul, thanks for your reply to my post – and also for the JDEM link – I know NASA have had to cancel a few science missions due to budgetary constraints, so I hope JDEM and other projects are allowed to keep going, although I think ESA and other agencies should also be addressing these issues, if they aren’t doing so already. Best, Tim.

leslie england November 29, 2006 at 7:47

As we look out through our own milky way halo toward other spiral galaxies’ halos, shouldn’t we be able to at least detect something evidencing such elusive dark matter and energy, other than mere stellar motion mismatched with visible mass? I mean, we have the best vantage point both from inside/out and outside/in. What fills the space between the Milky Way and Andromeda? Nearly emptiness and vacuum, right? A couple of photons? Just having a harder time understanding this, more so even than black holes, event horizons, or the singularity. Does Andromeda lense anything lurking behind it? Do ellipticals have the same halos around their overall older stars? Thank you.

Administrator November 29, 2006 at 8:30

Yes, dark matter and especially dark energy are tricky to get a handle on, to say the least! Consider this: galaxies are evidently surrounded by a halo of dark matter, which played a role in their creation. Yet we can’t see it near the Milky Way or between our galaxy and Andromeda or anywhere else. So when you ask what fills the space between us and Andromeda, it’s a harder question to answer now than it would have been, say, fifty years ago. Dark energy is likewise not visible to us and perhaps only detectible through very specific experiments, though even there we’re not sure how to go about it. So a great deal of work is going to go into this, and it’s hugely important when you consider what a tiny percentage of the universe we can actually observe with our telescopes.

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