# Dark matter

1. Oct 17, 2009

### mviswanathan

Could the effect of "Dark Matter" be explained if in the formaul for Gravitational attraction:
F = GMm/d^x
Where x is considered 2 is wee bit less (may be 1.98 or some thing)

2. Oct 17, 2009

### Vanadium 50

Staff Emeritus
No. This is one of the first things that was checked, and it doesn't match observations.

3. Oct 17, 2009

### Chronos

I have difficulty [or disappointment] believing this is still in doubt, mvis, too many scientists have worked too long and hard to miss on this one. Dark matter is definitely out there. The only question is, what it is.

4. Oct 17, 2009

### mviswanathan

Well, Thanks.
Is there is any derivation to arrive at the value of x as 2?

5. Oct 17, 2009

### Vanadium 50

Staff Emeritus
It's a measured quantity.

6. Oct 17, 2009

### mviswanathan

Thanks again.
It is one of the many strange things that is equal to integer 2

7. Oct 17, 2009

### Jonathan Scott

There are alternative theories to Dark Matter which involve modification to the gravitational force law.

The most well-known of these is MOND (Modified Newtonian Dynamics), which initially had spectacularly great success with accounting for galactic rotation curves, but there are various other situations where MOND opponents claim that dark matter is a better explanation. MOND is however theoretically somewhat unscientific and arbitrary, and various attempts have been made to produce a more theoretically consistent equivalent, such as TeVeS or MOG/STVG. Although these theories are more consistent, the modifications are so far somewhat arbitrary and not very appealing compared with the simplicity of GR.

The main practical feature of MOND and of these other theories is that they all effectively give an extra acceleration which instead of being proportional to $M/r^2$ is proportional to the square root of that, $\sqrt{M}/r$.

Although this extra term is very weak and only has a significant effect on galactic scales, the MOND theory postulates that it is somehow cut off below certain absolute accelerations, presumably simply because otherwise this effect could in theory be detectable within the solar system. (It's actually on a similar scale to the well-known Pioneer Anomaly, but doesn't match it particularly convincingly). At least MOG does away with the concept of "absolute" acceleration, effectively replacing this with a Machian concept of acceleration "relative to the fixed stars".

This idea of different effects occurring below some limit acceleration sound very implausible to me, because for example even if a star is accelerating only weakly overall, its component particles are all subject to huge accelerations due to its own gravity. Similarly, what about a double star system (which is very common) or a star and its planets? In the fringes of a galaxy, these would be accelerating around one another at far more than the MOND threshold acceleration, yet somehow MOND is expected to apply if the overall system is accelerating below the threshold limit.

However, the fit between MOND predictions and the measured properties of galactic rotation curves are very impressive.

I personally suspect that one possible explanation is that the MOND acceleration term does occur in nature, but that there is no acceleration cut-off. Instead, this effect could be present even in the weak case, and could be observed within the solar system and possibly even in the laboratory, but in the absence of experiments designed specifically to test for it, it has been ignored or treated as a component of systematic error, because it varies more weakly with r than the standard force. (I'm aware of experiments to search for variation of the force with higher powers of 1/r, but not with lower powers of 1/r than the standard 1/r2. I'm also aware that attempts to measure G in the laboratory have been giving surprisingly variable results. If anyone knows of any experiment which specifically rules out this MOND effect at the laboratory level, I'd be very interested to hear about it).

8. Oct 17, 2009

### twofish-quant

One thing that makes me go hmmmmmmm......

Is that all of the dark matter stuff has something to do with missing mass, whereas the Higgs boson (the one remaining missing particle of the standard model) has something also to do with mass. All this is start smell like ether......

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