Why dark mater?

Question

They say that 96% of the Universe matter is missing because Stars on the edge of galaxies does not reduce in acceleration like planets in our Solar System does. Then they created this bolony dark energy and dark matter that no one can see or measure!

But you don't even have to go as far as Einstein. Newton's law already explains this - Follow me:
F = - G(m1*m2)/r^2, right!

Newton's law says that:
"If r changes proportionally very little during an object's travel – such as an object falling near the surface of the earth – then the acceleration due to gravity appears very nearly constant". and "If m1 is negligible compared to m2, small masses would have approximately the same acceleration".

Therefore, assuming there's a black hole on the center of the galaxy, it will make the Star's mass negligible since the black hole density is infinite at the singularity. Therefore: acceleration is constant. No need of dark stuff :-)

Since I'm not nearly smart as those guys, What am I missing?

Answer 1

There are three lines of evidence for Dark Matter:
1) the rotation curves of galaxies (including the Milky Way) do not decrease in the expected way;
2) the dynamics of clusters of galaxies do not behave as expected; and
3) computer models of the evolution of structure [formation of galaxies, clusters, stars, etc.] after the Big Bang do not behave as expected.
In each case, what is meant by "expected" is a situation where the gravitational force acting on the objects being studied comes from the objects actually observed to be there. This "expected' situation never occurs: the gravitational force is always stronger than what is expected, given the amount of observable matter. Therefore, the conclusion is that the gravitational force is mostly caused by unobservable (hence "Dark") matter.

Your question refers to the first line of evidence, the rotation of galaxies. You are right to apply Newton's law to this situation. You are right to apply this to the existence of orbits around the Galaxy. The point is, however, that not only do we know the orbits, we also know the velocity of the orbits. These are easily observed using the Doppler shift, and the velocity of the orbits is accurately known (within 1%). By looking at how the velocity of the orbits change with distance to the Galactic center, we know how much gravitational force there is at each radius. We can compare this force with the amount of material we see. Very near the center, the amount of gravitational force is exactly the same as the amount of gravity caused by a black hole that masses a few million times the mass of the Sun. Further out, a few thousand lightyears from the center, the amount of gravitational force is about the same as the amount of stars we see inside that radius, so there is no problem: things are as "expected". Most of the force a few thousand lightyears from the center comes from stars, and only a little from the black hole. Even further out, 100,000 light years from the center, there is still a lot of force, but very few stars, and things are no longer as "expected". Therefore, it is proposed, there is some "Dark Matter" that is causing the force. This happens in almost every galaxy we observe, and so there is evidence for "Dark Matter" in almost every galaxy.

"Dark Matter" may not be so wierd after all. It may simply be a kind of particle (called an "axion") that is predicted to be created in huge quantities by the Big Bang, according to some particle physics theories.

If you want something really wierd, consider "Dark Energy".

Answer 2

What you're saying is that black holes at the center of the galaxies could make the galaxies behave as they do, without the need for gravity from dark matter.

First, the force in your equation is not a function of density or size. Just as your equation says, the force is based on mass of two bodies, 1 and 2; and r. The r in the equation is the distance between the centers of the two bodies 1 and 2. The fact that the black hole has a very small radius doesn't mean anything in that equation. r is the distance between the centers of the two bodies and has nothing to do with how big the two bodies are.

Second they can make a reasonable estimate of the mass that is present in the black holes, from how much radiation is emitted as things spiral down into the black holes.

All the mass we can see and all the mass of the black holes is still not enough to account for how galaxies behave. We need additional mass to explain it, hence "dark matter" which has mass but cannot be seen by us so far.

Answer 3

There are actually a number of reasons why astronomers believe firmly in the existence of dark matter, and not that something is wrong with our understanding of gravity.

It's true that Newton's law of gravity was originally an empirical formulation. It has one enormous strength, however--it works! At least so long as the gravitational field isn't so strong as to put you into the relativistic regime. Ever since it was first formulated, Newton's law has been used to calculate very accurate orbits of the planets. Einstein's general theory of relativity was originally formulated on a much more theoretical basis than was Newton's law. It, too, however, has passed every observational and experimental test to which it has been put.

In addition to passing observational tests, we have good theoretical reasons to believe that gravity should behave in the way described by relativity, based either on the warped spacetime concept of relativity or upon the concepts being developed by attempts to describe gravity using quantum mechanics. We have no reason, therefore, to believe that gravity should behave very differently from the manner described by physical theory.

It is possible to explain at least some of the "dark matter" observations by assuming that gravity doesn't behave quite in the Newtonian or relativistic fashion at large distances, and some attempts have been made to do this. The changes that have to be made to the gravitational laws, however, are pretty arbitrary, and have no good theoretical basis. It seems much simpler, therefore, to believe that the measurements made by astronomers are affected by the presence of unseen matter. After all, it would be incredibly biased to assume that everything in the universe is in a form easily observable by us, and there are any number of candidates for objects or forms of matter that aren't.

Answer 4

The equation you wrote is Newtons law of universal Gravitation and quite different from acceleration due to gravity. We must distinguish between the two.
All the matter you see in the universe namely satellites,planets,stars,galaxies,nebulas,milky way and so on are only 5% of the entire matter in universe.Scientists say rest is all dark matter. It does not reflect light. It is likened to "cosmic glue". We know that all matter in universe is flying away from each other as per Hubble's theories. It is dark matter whose gravitation hold this phenomenon under check. Not much is known about dark matter yet.

Answer 5

First of all, you are missing a "t" from mat(t)er!!

In astrophysics, dark matter refers to matter that does not emit or reflect enough electromagnetic radiation (such as light, x-rays and so on) to be detected directly, but whose presence may be inferred from its gravitational effects on visible matter. Among the observed phenomena consistent with the existence of dark matter are the rotational speeds of galaxies and orbital velocities of galaxies in clusters, gravitational lensing of background objects by galaxy clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter also plays a central role in structure formation and Big Bang nucleosynthesis, and has measurable effects on the anisotropy of the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than is directly observable, indicating that the remainder is dark.

The composition of dark matter is unknown, but may include new elementary particles such as WIMPs and axions, ordinary and heavy neutrinos, dwarf stars and planets collectively called MACHOs, and clouds of nonluminous gas. Current evidence favors models in which the primary component of dark matter is new elementary particles, collectively called nonbaryonic dark matter.

The dark matter component has vastly more mass than the "visible" component of the universe. [1] At present, the density of ordinary baryons and radiation in the universe is estimated to be equivalent to about one hydrogen atom per cubic meter of space. Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space. [2] Some hard-to-detect baryonic matter (see baryonic dark matter) makes a contribution to dark matter, but constitutes only a small portion. [3] [4] Determining the nature of this missing mass is one of the most important problems in modern cosmology and particle physics. It has been noted that dark matter and dark energy serve mainly as expressions of our ignorance, much as the marking of early maps with terra incognita. [2]