Dark matter exerting a gravitational effect on earth?

[albroun]

Dark matter exerting a gravitational effect on earth??

I have a puzzle about so-called dark matter. If it is supposed to supply the additional gravity necessary to explain the rotational velocity of stars in galaxies, and it is also supposed to be everywhere in the universe including here on Earth (albeit as yet undetected) then surely it would be exerting extra gravity here on Earth right now? Yet Einstein's General Relativity, sofar as I know, provides a perfect explanation for gravity here on earth, so the addition of dark matter would mean that there should be a lot more gravity here on Earth than there actually is!

I have also heard of the idea that dark matter exists as a halo surrounding galaxies, so perhaps the theory is that it only exists in certain parts of the universe. But if so, why would we be searching for it in deep mines on earth?

Have I misunderstood something? I am not a physicist and I have no understanding of maths. Thank you.

 

[ibysaiyan]

As far as my knowledge on DM goes,it could be the answer for areas which to our observation is less dense yet speeds appear to be constant.I see where you're getting at but one of the proposed property of DM is that it isn't homogenous /clumps i.e the concentration varies... so I don't see how it would invalidate general relativity for the only thing that I can think of invalidating is the space-time curvature singularity.

The deep mine research which took place in England back in 1999 was done in order to observe Solar neutrino which is thought to be DM candidate.
Regards,
ibysaiyan

 

[albroun]

Aha I wasn't aware of the idea that it was in clumps. Various videos about DM I have seen suggest that millions of DM particles may be passing through our bodies every second, similar to neutrinos. This was what was puzzling me. For if so, why would this not then exert an additional gravitational influence? Perhaps these videos were misleading though they did feature well-known physicists.

 

[Vanadium 50]

I'm afraid dark matter does NOT clump substantially on sub-galactic scales, and solar (stellar) neutrinos are not and have never been a viable dark matter candidate.

The short answer is that there is a pull on the earth, but because dark matter is so diffuse and widely distributed, the net gravitational force is quite small and impossible to detect directly.

 

[Janus]

I have a puzzle about so-called dark matter. If it is supposed to supply the additional gravity necessary to explain the rotational velocity of stars in galaxies, and it is also supposed to be everywhere in the universe including here on Earth (albeit as yet undetected) then surely it would be exerting extra gravity here on Earth right now? Yet Einstein's General Relativity, sofar as I know, provides a perfect explanation for gravity here on earth, so the addition of dark matter would mean that there should be a lot more gravity here on Earth than there actually is!

I have also heard of the idea that dark matter exists as a halo surrounding galaxies, so perhaps the theory is that it only exists in certain parts of the universe. But if so, why would we be searching for it in deep mines on earth?

Have I misunderstood something? I am not a physicist and I have no understanding of maths. Thank you.

Here's the thing, while the total mass of dark matter in the galaxy exceeds that of the visible matter, it is in a huge spherical volume that extends past the disk of the galaxy, while the bulk of the visible matter is confined to the bulge and disk (A much smaller volume). There's more of it, but it has to fill a much larger volume, which decreases its density compared to the density of the visible disk of the galaxy.

Also, the overall density of the Solar system (the mass of the Solar system divided by a volume of a sphere enclosing the planets), is much higher than the average density of the rest of the visible galactic disk. As it works out, if you estimate how much Dark matter is enclosed in that same volume, it works out to be something in the order of 10^20 kg, or about 1/10 the mass of the dwarf planet Ceres. This mass would be spread out over the whole volume of the Solar system, meaning the amount contained within the body of the Earth would be insignificant, considering that the total DM in the solar system is less than 1/10,000 the mass of the Earth to begin with

 

[ibysaiyan]

I'm afraid dark matter does NOT clump substantially on sub-galactic scales, and solar (stellar) neutrinos are not and have never been a viable dark matter candidate.

The short answer is that there is a pull on the earth, but because dark matter is so diffuse and widely distributed, the net gravitational force is quite small and impossible to detect directly.

Yes,I am aware of the complications which most of the neutrinos face including solar neutrino but I have heard of tau neutrino being the potential candidate.The problem with neutrinos is their near massless property gives them the ability to reach nearly relativistic speeds implying hot dark matter under which galaxy formation would't come into shape.

Have I got the right idea?
Regards,
ibysaiyan

 

[Vanadium 50]

You said solar (which I assume you mean stellar) neutrinos. Since the total mass of stellar neutrinos is much much less than the mass of the stars, and the total mass of the stars is less than the mass of the dark matter, it cannot be a viable candidate.

Other sources of neutrinos have, as you point out, other problems as well.

 

[albroun]

... (DM) is in a huge spherical volume that extends past the disk of the galaxy, while the bulk of the visible matter is confined to the bulge and disk (A much smaller volume).

There seems something strangely arbitrary about the dark matter hypothesis - why would it exist there in just those proportions for each and every galaxy? Are there any thoughts as to why it would be distributed in just this way, other than simply as a means of explaining the rotational velocity of the stars in galaxies?