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Can dark matter skew Earth-based gravitational measurements?

by FlexGunship
Tags: dark, earthbased, gravitational, matter, skew
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nikkkom
#19
May23-14, 07:48 PM
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Quote Quote by FlexGunship View Post
This is still where I'm at (and willing to understand why I'm wrong). Since it'll pull equally in all directions, there's a cancelling effect over large areas with a distributed dark mass.

Or... if it weren't uniformly distributed and it huddled close to other gravitationally significant locales, all it would do is skew you're metrics for determining mass at a distance. You would say: "ah, based on it's orbit, that body must have a mass of 5x1023 kg" when (perhaps, in reality) 0.5% of that mass is not atoms, but dark matter.
Yes, such configuration is not prohibited by laws of physics.
It is prohibited by laws of statistics :)

In order to clump up into a planet-sized body, particles need to concentrate in planet's volume *and then lose part of their energy*. If they don't, they will just fly out of that volume back to infinity.

For normal matter, it's easy. Gas and solids can't penetrate each other - they collide. In doing so, they heat up. Heat gets radiated away, part of the energy is lost, and they can't escape to infinity anymore. Thus, you have accretion process (planetary, stellar, even galactic).

Dark matter can't do that. Even if dark matter particle falls into already existing dense mass (e.g. Jupiter or Sun), it doesn't collide with the mass - it zips through it on a hyperbolic trajectory, and goes back to infinity.

Dark matter *can* form dense object only by having N-body gravitation interactions which exchange kinetic energy in a way that small fraction of dark matter particles gets most of it and flies away, leaving a gravitationally bound "globular cluster" of dark matter particles. Such an event is statistically possible, but exceedingly unlikely: about as likely as cup of water spontaneously boiling because air molecules just happened to give their kinetic thermal energy to the water.

Quote Quote by FlexGunship View Post
If gravity is the sole governing force, then why WOULDN'T it concentrate in areas where electromagnetically bonded gravitational bodies (i.e. the Earth, on a local scale) exist? There's nothing about the Earth that would tend to nudge it out of the way. Instead, you have the last 4.5 billion years of attractive force concentrating dark matter into an orbital ring.
What would *slow down* incoming dark matter particle so that it enters a stable orbit around Earth in order to eventually form a ring? Nothing.

Quote Quote by FlexGunship View Post
The sun is massive. It formed SPECIFICALLY because of gravity. Why wouldn't non-orbital dark matter fall into it?
Sure thing, dark matter probably does fall from infinity into the Sun as we speak. And then it flies right through the Sun and happily continues on its hyperbolic trajectory back to infinity.
phinds
#20
May23-14, 08:02 PM
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Quote Quote by FlexGunship View Post
The sun is massive. It formed SPECIFICALLY because of gravity. Why wouldn't non-orbital dark matter fall into it?
It WOULD. Then it would keep going and end up as far away on the other side as it was far away in the first place. then it would do it again. Regular matter doesn't do that because it collides and thus ends up clumping.

You clearly have not yet gotten your head around the ramifications of the fact that dark matter ONLY interacts gravitationally with ordinary matter.
Bill_K
#21
May24-14, 05:05 AM
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Quote Quote by phinds View Post
You clearly have not yet gotten your head around the ramifications of the fact that dark matter ONLY interacts gravitationally with ordinary matter.
I thought we had decided in posts #3 and #4 above that this was false.
nikkkom
#22
May24-14, 05:18 AM
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Quote Quote by Bill_K View Post
> If it is *uniformly* everywhere in the Solar System, then it will be undetectable even if its density is not low.

Not true, of course. The attractive mass that determines your orbit at radius r is the total mass inside that radius. If the density is uniform, this will increase with r. Consequently there will be an effect on planetary orbits, and especially highly elliptical orbits such as comets.
Here's a thought experiment. An otherwise empty space is filled uniformly with dark matter. We introduce a test particle.

Choose an arbitrary spherical region of space. According to your argument, mass of all dark matter inside that region will attract the test particle, and therefore test particle will orbit that region (or fly on hyperbolic trajectory). I.e. test particle won't fly in a straight line.

Which is obviously ridiculous.

Whis is also why the gravitational effects of dark matter were first noticed in the outer regions of galaxies.
No. Effects were noted because dark matter is *not* uniform on a galactic scale. It's denser towards the center.
Bill_K
#23
May24-14, 07:13 AM
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Quote Quote by nikkkom View Post
Here's a thought experiment. An otherwise empty space is filled uniformly with dark matter. We introduce a test particle.

Choose an arbitrary spherical region of space. According to your argument, mass of all dark matter inside that region will attract the test particle, and therefore test particle will orbit that region (or fly on hyperbolic trajectory). I.e. test particle won't fly in a straight line.

Which is obviously ridiculous.
A more polite word for it is "paradox". It arises from taking a limit in two distinct ways. A standard result from Newtonian gravitation is that for a spherical distribution of matter the gravitational attraction at radius r is toward the center and proportional to the total amount of matter enclosed within that radius. The matter outside that radius does not contribute. This applies to any finite distribution, but, as you point out, not to one which is infinite.

This is also why the gravitational effects of dark matter were first noticed in the outer regions of galaxies.
No. Effects were noted because dark matter is *not* uniform on a galactic scale. It's denser towards the center.
The original presumption was that the matter in a galaxy was concentrated towards the center. If that were true the orbital speed in outer parts should fall off. Zwicky observed that the speed remained approximately constant. His conclusion was that there was a large amount of nonvisible matter in the halo, which is spherical and extends well beyond the spiral.

The distribution of dark matter is nonuniform on an intergalactic scale, but approximately uniform within a galaxy. However this last point is actively being studied. For example, there may be a shallow central core.

The halo may be oblate.

Also, "Recent computer simulations have shown that the halo is surprisingly clumpy, with relatively dense concentrations of dark matter in gravitationally bound ‘subhalos’ within the halo."
nikkkom
#24
May24-14, 07:45 AM
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Quote Quote by Bill_K View Post
The distribution of dark matter is nonuniform on an intergalactic scale, but approximately uniform within a galaxy.
Yes, that is exactly how I understand it too.
Bill_K
#25
May24-14, 09:50 AM
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Quote Quote by Bill_K View Post
Depends on what it is, but if the dark matter particle is the neutralino, it can also self-annihilate.

Earthbound experiments to detect dark matter hope it is a WIMP, meaning it has weak interaction with baryons.
Should mention a third possibility. It has been suggested that dark matter is made of light sterile neutrinos. Light meaning a mass in the keV range, and sterile meaning they don't participate in the weak interactions. How, then, could these be detected?

From another thread:
Quote Quote by nikkkom
Neutrinos _can_ interact with photons. Since neutrinos participate in weak interaction, they have quantum corrections in a form of W boson loops. And those particles, being charged, interact with with photons.
The proposed new neutrino flavor eigenstate would be sterile, but it would mix ever so slightly with the other flavors. Thus its mass eigenstate would not be entirely sterile, and could decay weakly, by emitting a virtual W, which in turn would emit an X-ray photon, and we would scan the skies looking for X-rays.
mfb
#26
May24-14, 10:15 AM
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Quote Quote by nikkkom View Post
Here's a thought experiment. An otherwise empty space is filled uniformly with dark matter. We introduce a test particle.

Choose an arbitrary spherical region of space. According to your argument, mass of all dark matter inside that region will attract the test particle, and therefore test particle will orbit that region (or fly on hyperbolic trajectory). I.e. test particle won't fly in a straight line.

Which is obviously ridiculous.
This looks like a paradox, but it has a simple solution in GR: if your whole universe is filled with matter, it contracts. There is no orbit with a single particle, but two test particles would still move together faster than without the dark matter - or get different orbits.

You don't need inhomogeneous distributions. A finite ball with constant density (like a galactic halo) gives the same effect - two test masses feel tidal gravity which looks like an attraction between them.
FlexGunship
#27
May25-14, 09:07 AM
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Quote Quote by nikkkom View Post
In order to clump up into a planet-sized body, particles need to concentrate in planet's volume *and then lose part of their energy*. If they don't, they will just fly out of that volume back to infinity.
I think I'm good with this; makes sense. It's not quite intuitive.

I guess this is because, now second-nature, image of mass accreting gravitationally includes the constant bumping of particles into each other leading to an averaging in their momentum over time.

Is this also true of orbital bodies which have cleared their orbits of material? Do they ultimately collide with co-orbital matter? Or is that operation purely gravitational? (Keeping in mind that clearing an orbit can mean accreting the matter or flinging it out of the orbit.)
nikkkom
#28
May25-14, 03:14 PM
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Quote Quote by FlexGunship View Post
Is this also true of orbital bodies which have cleared their orbits of material? Do they ultimately collide with co-orbital matter?
Yes, they do.
Orodruin
#29
May25-14, 06:47 PM
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Quote Quote by FlexGunship View Post
If gravity is the sole governing force, then why WOULDN'T it concentrate in areas where electromagnetically bonded gravitational bodies (i.e. the Earth, on a local scale) exist? There's nothing about the Earth that would tend to nudge it out of the way. Instead, you have the last 4.5 billion years of attractive force concentrating dark matter into an orbital ring.
In this scenario there is no mechanism (apart from the very weak gravitational interaction) for the dark matter to get rid of a significant amount of energy in order to become gravitationally bound to the standard matter.
mfb
#30
May26-14, 03:38 PM
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Quote Quote by FlexGunship View Post
Is this also true of orbital bodies which have cleared their orbits of material? Do they ultimately collide with co-orbital matter? Or is that operation purely gravitational? (Keeping in mind that clearing an orbit can mean accreting the matter or flinging it out of the orbit.)
This is purely gravitational (apart from collisions of course), but they don't have to collide. Objects close to two lagrange points can be in a stable co-orbit, see Trojan (astronomy) at Wikipedia.


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