# Tag Info

22

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 ...

19

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 ...

18

The show you watched seems to get two concepts mixed up: Supersymmetry and Dark Matter. The existence of Dark Matter is strongly hinted at by comsological and astrophysical considerations. It is the easiest explanation for several observations we make in the universe. Supersymmetry on the other hand provides a candidate particle. The lightest ...

15

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, ...

14

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 ...

13

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, ...

13

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 ...

13

Dark matter would affect planetary motion, but the influence of dark matter on planets in our solar system is too small to detect even currenlty, due to the low concentration of dark matter compare to ordinary matter in our solar system. See Constraints on Dark Matter in the Solar System. The density of dark matter is very low, $<~10^{-19} grams/cm^3$, ...

12

I think the problem with matter that only interacts gravitationally is that it's hard to get it all to stay in one place. Nebula slowly form stars and planets in part because of collisions between particles lead to larger particles, which tend to attract further particles. But particles that just wizz right through each-other can't coalesce without violating ...

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

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 ...

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

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 ...

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 ...

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

A Goldstone boson is a generic type of particle formed when symmetries are spontaneously broken. If you want to suggest that dark matter is a Goldstone boson then that says very little unless you suggest a specific model with a symmetry to be broken. When exact symmetries are broken you get a massless Goldstone boson (except in a few special circustances, ...

9

The answer is because dark-matter has relatively constant density, as has been given explicitly in another answer. Then, it logically follows that the impact on the Milky Way due to this low density. To show this step, I will establish a figure of merit. $$FOM = \frac{M_{dark}}{M_{normal}}$$ That is, the ratio of dark matter within the area of ...

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 ...

8

I feel that exactly the opposite should be the case; that is, dark matter halo should be inside the galaxy rather than outside. Your feeling is entirely correct, and actually agrees with dark matter theories. Your only mistake is in thinking that the dark matter halo of those theories is only surrounding the galaxy; it's also inside the galaxy, and is ...

8

The cosmologically relevant light is the cosmic microwave background (CMB), not radiation from stars. The energy density of the CMB is about $10^{-13}$ J/m3. This is of the same order of magnitude as the energy density of starlight within our galaxy, but most of the universe is intergalactic space where the density of starlight is much lower. The average ...

7

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 ...

7

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: ...

7

Actually, on a dark night, the fraction of the sky that is light is pretty negligible. That's what it means to be a dark night ;-) It's actually not hard to get an estimate of the density of light in the universe. Let's say that "light" includes photons of all wavelengths (not just visible light) for simplicity. A straightforward way to do it is to point a ...

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

Physically, if you look at the low level of CMB anisotropy and the BAO power spectrum, you need some sort of mass that interacts gravitationally but is decoupled from the photons before recombination (i.e. - dark matter). Otherwise there would be no way to seed the galaxy formation that we see occuring at lower redshifts. That is to say, dark matter needs to ...

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