Does This Make Sense--About Dark Matter...?

Question

Greetings,

Irrespective of some of the other previous questions about dark matter, I was wondering considering that WIMPs (Weakly Interactive Massive Particles) were soley suggested because they rectified some of the problems with how much the universe actually weighs because of calculations...SOOO...

My question is this: IF gravity is such a deciding factor for figuring out the total mass of the universe AND Newtonian Physics still describes how we calculate the total mass of the universe, THEN how, if at all, does Relativity effect the supposed idea of Dark Matter?

IOW, Someone explained to me that in order to calculate the total mass of the universe you had to use Newtonian Physics, I wanted to know if Modern Physics could describe it (the total mass of the universe) better?

wussup starry-eyed (someone called me that once, too cute!!) anyway, from what i know, which isn't much, neutrinos have such a relatively small mass, in order for your theory to work the whole universe would have to made up of them. AND we can detect neutrinos and we can't detect WIMPs... still thinking about this... sounds like I may have to turn to the philosophers for help tho.

thanks!

jeez mathematician, you're a fun sucker. thanks for sucking the fun out of a completely innocent question--fun-sucker!

wussup raymond! wow hombre, you're pretty thoughrough!!! i appreciate it, and unlike mr. mathematician man... i like your answers. clear, concise, logical, and helpful. thanks for giving me some other stuff to read too... i'll have to check out those references you gave me...

stay cool :)

Answer 1

Newtonian gravity does not take into account the increase of mass for moving objects. Since objects can obtain a substantial fraction of the speed of light relative to each other, this is a factor that should be taken into account.

However, Newton's gravity gives a very precise approximation of the true picture.

Don't count the neutrino out as a candidate for a wimp. It is NOT a massive particle, but there are a bunch of them and new experiments suggest some mass. Without mass, the neutrinos could not change from one type to another type as they have been suggested (almost proven) to do. This alone could account for the missing mass and their mutual gravity would create clouds of neutrinos around nuclear reactions in stars and galaxies.

Answer 2

The total mass (or energy) of the universe is difficult to even define in a general relativistic context. The idea would be to have something like the conservation of mass-energy be valid, and this is true 'locally' because of properties of the stress energy tensor. The problem is that there is no way to 'integrate' the local conservation law into a 'global' conservation law unless the spacetime has very particular properties (asymptotic freedom) which the universe as a whole does not have. Essentially, the total energy of a system can be defined if the system is 'isolated', but the universe doesn't qualify as an isolated system (there is stuff 'at infinity').

As for dark matter, the relativistic equations describing how light is deflected as it goes by matter also lead to the conclusion that there is a large amount of matter (actually mass/energy) around galaxies and in galactic clusters that we don't see. So, while Newtonian physics is a good enough approximation when modeling velocity curves, the full relativistic treatment comes to the same conclusions.

Answer 3

I almost feel like making a top ten list of the many ways we try to make sense of the dark matter thing.

I guess WIMPs were suggested because we already had some (neutrinos) and theories predicted they should exist, could exist or, at least, were not totally forbidden from existing. And, if they did exist, they could explain so many things. MACHOs were also suggested. I guess since the WIMP acronym already existed, we could not avoid countering it with MACHO.

For very large entities (like galaxies), things are not moving very fast. Newtonian physics should give us pretty good results.

Calculations were also done using "modern physics", based on the idea that if you had that much mass (like a galaxy's worth), then the presence of mass itself (or the huge amount of gravitational energy -- same thing) would itself increase gravity (says so in General Relativity).

Mass determinations were done using various factors for the quantity of neutral, cold hydrogen. It is very difficult to detect hydrogen (cold HI -- pronounced H-one) so we use other stand-ins (like CO) and multiply by some factor to estimate the quantity of hydrogen. Our factors could be wrong (hydrogen being the most abundant element, a small mistake means large quantities). This could mean that the missing mass (dark matter) is simply normal, baryonic matter that we cannot detect and that we are not very good at estimating. We try to refine our estimations, but we all accept that we could be wrong -- it's just that a lot of different methods would have to be wrong at the same time...

Some people have even suggested that gravity does not work exactly the way we have always thought, and that for large distances, maybe it does not vary as the square of the distance, but by some power ever so slightly different from 2. Does not show up at short distance (like within the solar system) but makes a big difference over large galaxies or between galaxies. Look up MOND.

Yet, in most cases, the differences are small enough that almost everyone agrees that "something's missing from the big picture"

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Neutrinos are "massive" in the sense that their mass is not zero. It may be very, very small, but it is not zero. They are definitely "weakly interactive" and they are particles.


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Fast moving objects do not increase in mass. Their energy content increases (to the energy represented by their mass -- what we used to call rest-mass -- the kinetic energy due to speed is added) and it is the total energy content that determines an object's effect on the curvature of space (a.k.a. gravity)

Answer 4

We can't know how relativity,or anything for that matter,will affect an unknown.We do not know what dark matter is,everything concerning dark matter and energy is purely hypothetical at this point