Simulation Predicts Clumps of Dark Matter Within Galaxies 131
A team of researchers has simulated the gravitational interaction of dark matter particles over the course of a hypothetical 13.7 billion years. They found that the particles tended to form clumps large enough to assist in the formation of galaxies. The results contradicted observations from previous, smaller studies, but they lent support to an unrelated simulation of how the Milky Way formed. UCSC's press release is also available. Quoting ScienceNews:
"The clumps of dark matter in the simulation have densities that are remarkably similar to densities that a University of California, Irvine research group found when simulating the formation of the Milky Way and its satellite dwarf galaxies, says James Bullock, the astrophysicist who leads the UC-Irvine group and was not involved in the new study. 'This is a remarkable success of the particular model simulated and adds strong support to the idea that the dark matter is made up of particles that are "cold." There are a number of planned experiments aimed at detecting the dark matter that are betting on it being cold, so this is generally good news for the community,' Bullock says. And, [study co-author Piero Madau] notes, larger simulations that might help constrain the nature of dark matter even more are already in the works."
Kahn was right! (Score:3, Informative)
Re:Where's the evidence? (Score:5, Informative)
I Am Not A Physicist, but the problem with MOND is that, well, it explains only one problem of gravity or cosmology, rather: galactic rotation. It fails to explain how spectacularly successful tests of general relativity have been. For example, where does MOND predict frame-dragging? Answer: it doesn't.
MOND is what you get when you have a problem posed to amateur mathematicians and physicists, and they answer that problem (galactic rotation) with the simplest solution (let's just tweak this equation) without considering the fact that their modified theory is inconsistent with well-established theories that currently exist. MOND does not predict certain things that we see in nature, but this isn't seen by proponents. All the proponents see is, hey, it solves this problem. Well, yes, but it causes a whole lot more that you neglect to mention. Chiefly among those is, pretty much every quirk of relativity.
Re:Slightly off topic: Dyson Spheres (Score:5, Informative)
That is a "MACHO," which we have looked for and not seen enough of. MACHOs are unique in that they affect the light behind them (they are opaque and gravitationally lensing) and so while they could account for a lot of matter, we aren't seeing enough lensing and enough holes in the spectrum from "dark stars", areas where the sky is darker from an object blocking light behind it.
And back of the envelope calculations say a dyson sphere wouldn't be anywhere near a black hole's mass, which is what we really would need to find quite a few of in order to find the missing mass.
And this ignores any technical difficulties with actually constructing a dyson sphere.
Re:Where's the evidence? (Score:2, Informative)
First I want to clarify what dark matter is. It is just matter that cannot be seen directly, because it does not emmit light or radiation. But it can be observed by the gravitational effects it has on objects that can be seen. To an alien Saturn would be dark matter. Earth would be dark matter.
So we have two options:
1. We assume that our planets are the only ones in the universe, every other matter is light up and we can observe it by its radiations. Then we have to conclude that there is a fundamental error with Newtons theory of gravity.
2. We assume that Newton is actually a quite accurate model of gravity, and there just exists lots of matter that does not radiate anything we can observe directly.
What does sound more reasonable?
Re:Where's the evidence? (Score:3, Informative)
Re:Dark matter particles are cold? (Score:5, Informative)
Re:ha (Score:3, Informative)
Re:Where's the evidence? (Score:5, Informative)
Say with MOND, why are we so scared to think that perhaps Newtonian mechanics aren't quite enough to calculate with on galactic scales?
Nobody uses MOND for the same reason we don't use epicycles anymore: it's an unnecessary adjustment to an outdated system. Newton is a good approximation for low-speed and low-mass systems, but hasn't been considered perfect during the lifetime of anybody here.
What of a static non-expanding universe and alternate redshift paradigms?
Because it doesn't meet the observations. If you have to throw out everything else that does meet the observations to force fit your pet theory, then you're doing science incorrectly. Also, alternate redshift paradigms? Redshift is a very, very basic thing; it would take a lot of phenomenal evidence to change anything related to it.
I'm just curious as to why dark matter is so widely supported,
Dark matter is supported because it seems likely, fits the data, and doesn't contradict other observations. All it means is that we think there's some more mass out there, and we haven't seen it yet.
is it merely because breaking the standard model makes physicists too uncomfortable?
I get the willies just thinking about it! Or not.
Re:Where's the evidence? (Score:3, Informative)
Our models need to account for 90% more mass than empirical evidence proves exists. This only applies when considering galactic scales. Either our models are wrong, or there is a large mass of hidden matter that violates all the known properties of matter except one.
I believe our models are wrong, and this study is flawed. Don't get the willies, just try to broaden your perspective.
Re:Where's the evidence? (Score:4, Informative)
Why the obsession dark matter?
How about because MOND completely and utterly fails to describe the Bullet cluster [wikipedia.org]?
Re:Where's the evidence? (Score:4, Informative)
Dark matter is inferred from a number of observations and calculations, including excess rotation speed of stars around galactic centers, excess speed of members of clusters of galaxies, and lensing of background galaxies not associated with luminous matter. While one may fiddle with MOND to possibly fit the first two phenomena (for a specific case, moreover --- one adjustment to Newton's laws had better account for *all* rotation curves), the last group of observations really seem to argue strongly against any reasonable form of MOND.
If you google the Bullet Cluster image and description, you'll note that the blue region (most lensing of background) is tracing the highest density of matter, where the pink traces the density of luminous (here, X-ray cluster gas) matter. It is clear that there is a huge component of matter not identified with luminous matter. To account for this with MOND, it is not enough to increase the *strength* of the gravitational field, but also now the *direction*, since we observe the lensing in a direction roughly perpendicular to the axis of the cluster. Writing down such a gravitational law to account for this case would make any astrophysicist uncomfortable indeed.
Now, especially since we can work backwards and calculate the distribution of dark matter needed to cause each observation, dark matter seems to be the *much* simpler explanation. Besides, we already have examples of matter that interact only gravitationally (and possibly weakly) --- neutrinos. As hard as it is to detect them, there may well be others we don't know about yet.
Re:Gravity from "elsewhere"? (Score:4, Informative)
Other than the large amount of seemingly "dark matter"...
There's nothing non-classical (i.e., quantum) about the behavior of dark matter. At least nothing we've observed so far. It's just matter, maybe like a neutrino but heavier. You don't need to appeal to quantum entanglement or anything exotic to explain it.
If we have a pair of entangled particles, they can THEN be separated by (apparently) quite any distance in classical space and nevertheless remain entangled.
In theory, yes. In practice, they need to be extremely isolated from everything else in the universe to remain entangled. That's why it's so difficult to maintain long-range entanglement for purposes like quantum encryption. You may think space is "empty", but it just takes a single photon to ruin the entanglement.
This is why my hypothesis considers that they may actually not be physically separate at all - they are "right beside each other" through twisted "extra" dimensions, just in some way that we currently do not have the ability to measure or understand particularly well.
Now you're mixing up two different ideas, quantum mechanics and extra dimensions. If they're interacting through other dimensions, fine, but you don't need entanglement to explain that either.
This leads on to the idea of "dark matter" being gravity from "nearby" objects that are classically quite distant, but in reality quite "close".
All the above problems notwithstanding, it's easy to come up with more. Even if you ignore the fact that these particles can't plausibly remain entangled, you have to explain how all these distant particles got entangled with each other in the first place. (If you're tempted to say "the Big Bang", you really can't ignore the previous fact given the extreme temperatures involved. Maintaining entanglement requires no outside interaction with other particles at any time over the intervening 14 billion years.) Furthermore, entangled particles interact with each other, but that interaction in general doesn't look anything like the vector or tensor field theories that give rise to what we think of as "forces" such as gravity or electromagnetism. (Why should this purported "entanglement force" act like gravity anyway?) And there's no explanation for the astrophysical observations like how entanglement with distant particles can simulate the effects of a spherical cloud of dark matter enclosing a disc galaxy.
Basically, it seems like a very bizarre hypotheses that, all the physical evidence against it aside, doesn't even seem to have much to commend it over the alternatives. It's not like the idea of there being weakly interacting particles out there is so crazy; plenty of particle theories predict them for completely independent reasons including possibly the Standard Model, and there are other weakly interacting (though less massive) particles out there already, like neutrinos.
Re:Where's the evidence? (Score:5, Informative)
Say with MOND, why are we so scared to think that perhaps Newtonian mechanics aren't quite enough to calculate with on galactic scales? Why do they think MOND is for cranks and crackpots?
Who's this "they"? There is a really unpleasant meme running around Slashdot that reactionary scientists scorn and mock anything that isn't mainstream. In reality, MOND is not a mainstream theory (and for good reason), but it's still discussed seriously. Google around for Sean Carroll's presentation a year or two ago on MOND vs. dark matter (can't remember when that was when he was at U. Chicago or Caltech). This [cosmicvariance.com] is a good summary of his for why dark matter is likely to exist.
What of a static non-expanding universe and alternate redshift paradigms? Are they not just as feasible as exotic matter that only interacts gravitationally?
No, not even remotely. They are vastly less plausible than even MOND, which has problems of its own. But since this is about dark matter and its competitors, I'll stick to those.
I'm just curious as to why dark matter is so widely supported, is it merely because breaking the standard model makes physicists too uncomfortable?
Yeah, it's because theoretical physicists hate new theories. No theoretical physicist ever got fame and tenure by coming up with a new theory. They gotta stick to the old ones to survive.
Seriously, tone down the paranoia. Dark matter also breaks the Standard Model by introducing new kinds of particles. (Well, unless you think it's axions, which are arguably part of the Standard Model). It's not like nobody has ever thought of an alternative gravity theory before. You can carpet a small moon with all the alternative gravity theories out there; scalar-tensor gravity, vector-tensor gravity, conformal gravity, chiral gravity, supergravity, and so on.
The simple facts are that it's really hard to muck around with gravity in a way that simultaneously agrees with observations on tabletop, stellar, planetary system, galactic, and cosmological scales. With MOND it's easy to reproduce galactic rotation curves, but not much else. MOND also contradicts relativity; it's strictly Newtonian. There has been an attempt to correct that in the form of TeVeS (tensor-vector-scalar gravity). But TeVeS requires you to introduce two new gravitational fields, plus couple them together in just the right way, and even then it's far from certain whether it can explain dark matter evidence on all different scales (galaxies, clusters, cosmology, etc.). Furthermore MOND has difficulty explaining rather direct evidence of dark matter like the Bullet Cluster. Even if MOND is correct, it seems likely that you still need dark matter to explain everything.
After all that, MOND looks far more ad hoc than just postulating the existence of a new kind of particle, especially since most of the new particle theories out there predict some kind of dark-matter like particle anyway for completely independent reasons. It's not like weakly interacting particles are terribly bizarre in the first place; neutrinos are dark matter, although they're too light to be most of the dark matter. The main difference between most of the dark matter and neutrinos is mass, and what's so odd something weak like a neutrino, only heavier? Such a particle, predicted by many theories, can (unlike MOND) simultaneously explain all the astrophysical phenomena which point towards dark matter.
Re:Where's the evidence? (Score:5, Informative)
The blind dedication to the dark matter cause seems superfluous to the realisation that our local models of physics don't scale galacticaly.
There is no "blind dedication to dark matter". And in fact, not scaling properly is exactly what's wrong with MOND. If you apply it to galaxies, it doesn't work for cosmology, and so on.
Re:Where's the evidence? (Score:3, Informative)
The Standard Model arguably includes axions, which were postulated to solve the strong-CP problem in quantum chromodynamics. They are dark matter candidates (although I think astrophysical constrains favor SUSY.) However, like the Higgs boson, they have not yet been observed.
Re:Dark matter particles are cold? (Score:1, Informative)
(and this 'Cold' refers to something like mean velocity of the particles, not temperature)
I understood that to be the definition of temperature.
Re:Where's the evidence? (Score:3, Informative)
We actually have a better chance of detecting the exotic particles than the rogue planets, so some people are focusing on that
It's the other way around. Dark matter in the form of brown dwarves and such has actually been detected through gravitational microlensing observations. That's how we know that most of the dark matter isn't brown dwarves and other compact astrophysical bodies; it's there, but there isn't enough of it to explain the necessary gravitational effects.
Re:Where's the evidence? (Score:3, Informative)
Since MOND does revert to GR in high Gravity situations so it will support frame dragging.
Don't confuse MOND and TeVeS. MOND is MOdified Newtonian Dynamics. It doesn't revert to GR or any relativistic theory in high gravity situations; in fact, it grows more different from those theories in strong gravity. TeVeS is a relativistic theory which has MOND as a particular limit (and Newtonian dynamics as another). It probably approaches GR in some other limit (decoupling of the scalar and vector fields?), but whether it agrees with all observations in strong gravity situations is, as you say, unknown. It doesn't automatically agree with GR, so unless someone has performed the frame dragging calculation specifically, we can't say how much frame dragging it may predict.