Hot answers tagged dark-matter
16
Excellent question!
In short, there are two logical possibilities to explain the data:
There is dark matter and a cosmological constant (standard model)
Gravity needs to be modified
Interestingly, both possibilities have historical precedent:
The discovery of Neptune (by Johann Gottfried Galle and Heinrich Louis d’Arrest) one year after its ...
16
Dark matter can be hot, warm or cold. Hot means the dark matter particles are relativistic (kinetic energy on the order of the rest mass or much higher), cold means they are not relativistic (kinetic energy much less than rest mass) and warm is in between. It is known that the total amount of dark matter in the universe must be about 5 times the ordinary ...
14
Short answer
The question is a bit ambiguous.
If the question is
why do star velocity increase with distance close to the galactic centre ?
the answer is
because their orbit encompass more mass, and this corresponds to a stronger gravity pull.
If the question is
why does their velocity stays constant and does not decrease at big radii, ...
13
No one has discovered it. Dark matter is a proposed explanation to some observed phenomena.
In particular,
Galaxies rotate at a speed that implies they are quite heavy, especially towards the outer edges - but when we look at the mass from stars and interstellar gas, there isn't enough to make them spin the way they do.
Gravitational lensing is a ...
12
Lubos Motl's answer is exactly right. Dark matter has "ordinary" gravitational properties: it attracts other matter, and it attracts itself (i.e., each dark matter particle attracts each other one, as you'd expect).
But it's true that dark matter doesn't seem to have collapsed into very dense structures -- that is, things like stars and planets. Dark matter ...
11
Definitely see the comments on your question. But a very brief outline of the data:
Rotation-curves and galaxy-cluster mass measurements show the detailed distribution of matter in those objects, the amount of mass far exceeds the observed mass ---> most mass is non-observed
Gravitational-lensing searches show that the "dark-matter" constituents must be ...
11
As a general rule, zero mass particles which travel with the velocity of light are not good for dark matter, because dark matter concentrates around gravitational attractors. It has to be particles with some mass that can be at rest in order to stay around a galactic center from the beginning . In addition they have to be controlled by weak interactions, if ...
10
In essence, the only correct answer is, "We don't know."
This is for several reasons. First of all, we don't truly know the actual extent of the universe. Because we don't know if the universe is negatively, positively, or has a flat curvature. If the universe has a negative, or flat curvature then it is indeed infinite, and Andrews answer would be ...
10
Even quiescent black holes tend to show up, through microlensing. Observational tests have put pretty rigorous constraints on a range of black holes masses in the Milky Way, although intergalactic black holes are not as well constrained.
The other problem is figuring out how you make lots of black holes, especially at smaller scales. That's not to say that ...
9
The conventional wisdom about dark matter is that it is likely to be a new kind of particle that is not part of the standard model. Basically, the reason for this is that most of the stable standard model particles interact electromagnetically (and so wouldn't be "dark").
The exception is neutrinos, and for a long time neutrino dark matter was considered ...
9
While it is possible that gravity still needs to be modified, it is looking increasingly unlikely that there ISN'T some form of dark matter. In particular, the observation of the bullet cluster is a tall order for the various modified gravity theories (though, arguably, the extra fields in something like bimetric gravity or TeVeS could be self-coupling in a ...
9
There is a very precise reason why dark planets made of 'ordinary matter' (baryons - particles made up of 3 quarks) cannot be the dark matter. It turns out that the amount of baryons can be measured in two different ways in cosmology:
- by measuring present-day abundances of some light elements (esp deuterium) which are very sensitive to the baryon amount, ...
8
As above answers have mentioned most of the ordinary matter has been considered as candidates and we are fairly certain that there has to be some sort of "dark" matter at work.
Firstly, we take on the phenomenon of gravitational lensing. A very famous example is the Bullet cluster where you can clearly observe the effects of a compact mass acting as an ...
8
It appears Dark Matter (DM) does not radiate any electromagnetic radiation, hence the 'Dark' part of the name. It does however appear to collect together due to gravity, hence the 'Matter' part. It appears to only very weakly interact with other types of matter during 'collisions' and therefore has no method of shedding momentum or energy in a collsion and ...
8
Go out and discover those "other explanations" (and accumulate sufficient supporting evidence, of course) and you can laugh at the dark matter specialists.
Until then dark matter is the simplest hypothesis on offer that explains multiple observations in one go (galactic rotation curves, cluster dynamics, super cluster dynamics, the bullet cluster, the ...
8
The missing mass problems are several sets of observations that could be explained if there were some matter that has mass (interacts with other matter via gravity) but does not interact with light. The same distribution of this missing mass would explain all of them. All competitors that have been explored fail to explain at least one.
I only partially ...
8
The answer depends on the identity of the dark matter.
In the most widely believed scenario, dark matter is composed of "weakly interacting massive particles" ("WIMP"). The adjective "weak" really means that the particles interact via the weak nuclear force. This pretty much guarantees that they interact with the Higgs boson, too: the WIMPs carry the ...
6
If dark matter emitted very long wave lengths of electromagnetic radiation it would mean it is composed of charged particles. There is no escape from that conclusion. Somebody might propose that dark matter is some strange configuration of charged particles which acts as a very long wavelength antenna. That might be a good model, but there is a hitch with ...
6
The possibility of large dark objects made of normal baryonic matter has been considered. These are called MACHOs.
However, there are various reasons to think that most of the dark matter can't be in the form of MACHOs. From the above wikipedia article (which links to some relevant journal articles):
The Big Bang as it is currently understood simply ...
6
All the matter that we do know to exist (called Baryonic matter) emits some kind of electromagnetic radiation at some frequency. Sometimes it's measured in infrared radiation, because matter, no matter how cold, will still radiate some amount of heat. To the best of our knowledge, it's not actually possible to cool any matter to absolute zero, and it's ...
6
The prevailing theory of dark matter is the Cold Dark Matter (CDM) hypothesis. This hypothesis is favored because it is assumed that the dark matter particles are non-relativistic - i.e. slow moving. Because they are slow moving, they can essentially orbit in and around the original small density fluctuations, making these small density fluctuatuions stable. ...
6
Dark matter doesn't mean that it occludes light. It means that it doesn't interact with light. Therefore, it also can not block light. Light just goes through it as if nothing was there. Dark matter only interacts gravitationally. That's how we know it must be there.
It does not necessarily have to rotate at high speed, but if it doesn't, it will just ...
6
It's unlikely there is an especially high concentration of dark matter in the Solar System.
The principle attribute of dark matter is that it's weakly interacting. It's not just weakly interacting with normal matter, it also interacts only weakly with itself. Trying to get matter to collapse into a ball like the Sun is harder than you might think. The ...
6
First of all, dark matter and dark energy, despite their naming, are two very different concepts. We don't really have any good reason to group them together, other than the fact that both represent things we don't understand. Thus they are not necessarily backed by the same sets of evidence.
Why we believe these things exist
As it happens though, some ...
6
You're right that gravitational lensing, and really any large-scale stuff involving gravity, tells us directly about spacetime rather than the stuff out there.
But spacetime, according to GR, is constrained by the mass, energy, momentum, pressure, and shear. The equation we write down is
$$ G_{\mu\nu} = 8\pi T_{\mu\nu}, $$
which unfortunately means almost ...
6
There are several reasons this model fails, including
A significant portion of the dark matter is known to be gravitationally bound to galaxies and relativistic velocities are far about the escape velocity and the stuff you propose would not remain bound.
The stuff you propose would be hitting the Earth, the sun, the other planets and we'd see it. We do ...
5
A little history because I don't like the formulation of the question title.
Galaxies exhibit a rotational behavior that is inconsistent with the known law of gravitation and the visibly luminous mass distribution of the galaxy. Two ideas present themselves to explain why:
There is some non-visible mass
Gravity doesn't work the way we thought
Well, ...
5
Dark matter surely has to carry a positive mass, and by the equivalence principle, all positive masses have to exert attractive gravity on other masses.
Also, from the viewpoint of phenomenological cosmology, we obviously want dark matter to attract itself. It has to attract visible matter because this is why dark matter was introduced in the first place: ...
5
WIMP's are hypothesized to interact only by the weak force, thus will interact only with the nucleus. The WIMP will hit the nucleus, slow down and change direction. The nucleus will recoil, and introduce vibrations (phonons) into the lattice. Those vibrations create heat which is measured. The crystal must me kept at 10 mK in order to reduce thermal noise. ...
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