from the question-everything-and-wield-a-pickaxe dept.
Dwarf galaxies are causing a stir in the universe and are calling Dark Matter into question. Two stories on phys.org highlight the problem.
The first is: Map of universe questioned: Dwarf galaxies don't fit standard model
and the second is:
Did Andromeda crash into the Milky Way 10 billion years ago?.
It was thought that Dark Matter was required in order provide sufficient gravitational attraction to overcome the expansion of the cosmos. This is the Lambda-CDM model (i.e. the Standard Model of cosmology). One way to explain the discrepancy lies in MOND (Modified Newtonian Dynamics) which was developed by Mordehai Milgrom.
The Standard Model would predict satellite dwarf galaxies in the Milky Way and Andromeda galaxies to be widely distributed and would have to move in random directions. What the research is finding, instead, is that these satellite dwarf galaxies are located in a huge disk and are all moving in the same direction. This is much like what we see in our own solar system, but on a vastly larger scale.
The proposed solution to this discrepancy is that an earlier collision between the Milky Way and Andromeda galaxies ripped material from the galaxies and threw it a great distance. The observed dwarf galaxies are thought to have been formed in this debris.
(Score: 2) by jasassin on Sunday June 15 2014, @07:39AM
Who makes up this shit?
The first thing that pops into my mind is space dwarf cops who just apprehended some Dark Matter and brought it into the one way mirror interrogation room. After about fifteen minutes of good dwarf cop and bad dwarf cop the Dark Matters sweating and can't take the heat. The good dwarf cop nudges the bad dwarf cop and the bad dwarf cop begrudgingly offers the Dark Matter a cigarette, which the Dark Matter accepts immediately... and another ten minutes of good dwarf cop bad dwarf cop before the Dark Matter finally folds.
jasassin@gmail.com GPG Key ID: 0xE6462C68A9A3DB5A
(Score: 2) by Subsentient on Sunday June 15 2014, @07:47AM
Dwarfs in space, yeah, I think that title was chosen to get the 'wtf is this' clicks.
"It is no measure of health to be well adjusted to a profoundly sick society." -Jiddu Krishnamurti
(Score: 2) by martyb on Sunday June 15 2014, @08:02AM
Wit is intellect, dancing.
(Score: 2) by GreatAuntAnesthesia on Monday June 16 2014, @10:03AM
> Dwarfs in space,
Here you go:
http://www.beardybastards.com/comics/2011/10/10/dwarf-stars/ [beardybastards.com]
No longer updated, sadly.
(Score: 2) by Gaaark on Sunday June 15 2014, @11:44PM
The dark matter no longer exists because 'dwarfs' like Tyrion went all "Kuuh, kuuh, kuuh" on their asses?
Anyone?
Anyone at all?
Shoooosh... tough crowd! :)
--- Please remind me if I haven't been civil to you: I'm channeling MDC. ---Gaaark 2.0 ---
(Score: 2) by rts008 on Sunday June 15 2014, @08:19AM
Not being able to make much sense out of the title, then the even worse summary, I RTFA to get the info.
The doing away with dark matter, and using MOND may work great for modeling the Milky Way - Andromeda behavior, but what do they propose to explain the expansion of the universe?
Aha, that is where those 'Dwarfs in Space' play a part...if there are a lot of dwarfs in space that we did not know about previously, then we could indeed toss the dw- I mean dark matter.
Sheesh, Space Dwarf Physics is much tougher than Quantum Mechanics and String Theory combined!
(Score: 0) by Anonymous Coward on Sunday June 15 2014, @08:42AM
Cosmology, still an ad hoc science.
(Score: 5, Informative) by geb on Sunday June 15 2014, @10:35AM
We have strong evidence that there's dark matter out there. Gravitational lensing allows for direct mapping of mass, and some galaxies clearly show that their light and mass don't match up: http://en.wikipedia.org/wiki/Bullet_Cluster [wikipedia.org]
If you can see that there's matter out there, and it's not emitting visible light... that's dark matter. Not much room for argument. There is room for arguing over what the stuff is, or where it came from, or how it and galaxies interact, or how it affects the formation of galaxies. There's a lot of uncertainty in the details, but unfortunately for fans of MOND, the evidence for their main competition is very strong.
I can't help but see this as clutching at straws. A discrepancy should be investigated of course, but MOND was never very likely even from the start, so reporting this as a new hope for modified gravity is overly optimistic.
(Score: 2) by maxwell demon on Sunday June 15 2014, @11:51AM
Correction: Gravitational lensing allows for direct mapping of spacetime curvature. The conclusion from spacetime curvature to mass is theory dependent.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by geb on Sunday June 15 2014, @01:04PM
Nobody is seriously proposing that empty spacetime might curve near the bullet cluster without matter present. Even the MOND crowd accept that there has to be some kind of mass there.
(Score: 0) by Anonymous Coward on Sunday June 15 2014, @01:31PM
"Spacetime curvature" and "gravity" are the same thing. Lots of spacetime curvature with no visible mass = "dark matter".
(Score: 2) by maxwell demon on Sunday June 15 2014, @04:35PM
But "gravity" and "mass" are not.
That's the hypothesis.
The Tao of math: The numbers you can count are not the real numbers.
(Score: 1) by boristhespider on Sunday June 15 2014, @10:19PM
Quite.
To expand significantly on this, no-one is seriously suggesting that MOND is the answer to everything -- it clearly isn't. It's arbitrary, it's phenomenology and, most damning, it does not in and of itself permit a covariant generalisation meaning that it cannot be applied on a cosmological scale. Equally damning on an observation level, MOND cannot explain the dynamics of galactic clusters, which may or may not imply that MOND is only a good description of the dynamics of virialised systems.
On the other hand, MOND works admirably, astonishingly well in galaxies. No-one knows *why* it works, but with a single parameter, the acceleration below which MONDian effects kick in and gravity hits a floor, it can fit a vast array of galactic types far more straightforwardly (and with far fewer parameters) than a CDM model can.
What this suggests, to people who discuss and research MOND as well as to interested observers, is that MOND is symptomatic of something else. It is obviously not fundamental, and it is obviously not working on cluster scales, but it is obviously a successful phenomenological theory on roughly kiloparsec scales. The question is then how to find a theory that might boil down to MOND in that kind of regime but does not suffer its other restrictions: its arbitrary nature, its phenomenological nature, the gross mismatch with observations on megaparsec (cluster) scales, and its inapplicability on gigaparsec (cosmological) scales.
My point is that the only ways found to sensibly do this are so-called scalar/vector/tensor theories. General relativity is a "tensor" theory: the gravitational field is described by a metric tensor which is the only dynamic field and which describes geometry (essentially, it gives the four-dimensional Pythagoras law). Something like Brans-Dicke theory is a "scalar/tensor" theory: dynamics are governed by a scalar field in addition to the metric tensor, which can be interpreted as a time- and space-dependent Newton constant (so that G in the Newton's law, F=GMm/r^2 becomes G(x,y,z,t)). But to get something like MOND you go the whole hog and chuck in a vector field, too. The interpretation of this is, to be frank, rancid, and the earliest semi-successful attempt to get a covariant MOND, Bekenstein's model known as TeVeS (for TensorVectorScalar), is an ugly, ungainly, unmotivated mess built by hand to reduce to MOND in the appropriate limits. However, it was soon found (by Bekenstein, amongst others; similar models have been proposed by Moffat for quite some time beforehand) that a much nicer description can be found if one instead works in a model that is described by *two* metric tensors. The scalar and vector in scalar/vector/tensor gravity then do nothing more than describe the differences between those two metrics. This allows you a far more flexible approach, and a much, much better way to motivate a theory since it is clean and is no more (or less) ad hoc than the motivation for GR itself, and is currently the focus of a lot of theoretical effort.
The important point though is that from the point of view of an observer, depending on how you set up that other metric and what it does -- essentially, does normal matter couple to that metric alone? to both metrics? do photons couple to the same metric as matter? and so forth; there are ways to limit the choices based not least on ensuring we don't catastrophically destroy causality, but there is still significant freedom -- the differences with normal dynamics will show up *if interpreted, as we typically do on cluster scales, through a mix of Newtonian and Einsteinian gravity*, as extra mass.
It is not known whether a theory could bounce enough curvature in the metric photons propagate on to reproduce observations of something like the bullet cluster in a setup that isn't painfully artificial, but proofs of concept certainly exist, and a more detailed analysis of the bullet cluster in the context of TeVeS, which is itself regarded as nothing more than a well-explored toy, suggests that while TeVeS on its own cannot reproduce the signal, a small amount of massive neutrinos, broadly consistent with existing limits on neutrino mass from other sources (sum of the mass of the three species less than around a couple of eV), can reconcile the theory with the signal. Since neutrinos *are* massive, and since we currently do not know their combined mass, particularly in the absence of cosmological data and if we're dealing with a fundamental change in the underlying theory of gravity we absolutely *cannot* use limits on the neutrino mass from cosmology), this is at the least a good suggestion that setups like the bullet cluster can, in fact, be entirely explicable within the context of a covariant generalisation of MOND and a plausible background of massive neutrinos... and that a significant proportion of the effect is in fact from the modifications to gravity, it is at least not implausible that similar effects can be observed.
(Score: 0) by Anonymous Coward on Monday June 16 2014, @03:53AM
Dark Matter (the 'weird missing mass' not the invisible normal matter) and Dark Energy are figments of people's imagination.... ok, that sounded harsh. Let's call them "best guesses" or a hypothesis.
And here is the problem. Media and general public hear these things and think that "Dark Matter" or "Dark Energy" exists. They have never been observed. They have never been measured. They have never been created in any lab. All we see is their supposed effects. But these effects could be from other things we don't understand - gravity. We haven't been able to quantize gravity, observe its charge, reflect it, or otherwise manipulate it. We know as much about gravity as we knew about electricity before Faraday or Maxwell (for example, we didn't know that static charge and magnetism are all very much related). We know almost nothing about the nature of gravity. Who is to say that "Dark Energy" and/or "Dark Matter" are simply not some gravitational effect on a larger scale?? Might as well start talking about String Theory as something real too.
Concepts like Dark Matter are at cutting edge of cosmology, not something measured or observed. Please stop talking about it like it is. It's not something that has been nailed down unlike either Anthropogenic Global Warming or Evolution. Yes, I'm serious.