Can dark matter skew Earth-based gravitational measurements?

[FlexGunship]

I'll be cautious in asking my my question because I'm out of familiar territory. But...

Given the following:

• that dark matter interacts with baryonic matter (exclusively?) via gravity,
• that evidence for dark matter shows that it exists largely near baryonic matter, and
• we are (almost?) certain we're entirely unable to measure it presently...

How do we know dark matter hasn't influenced our current measurements of gravitation on earth?

It's true that we can be confident is our mass-matter relationship understanding, but couldn't our gravity-mass relationship be skewed if we're getting a constant hum of "background gravitation"?

 

[phinds]

dark matter interacts with baryonic matter (exclusively?) via gravity

Dark matter interacts with both baryonic matter AND itself, gravitationally.

that evidence for dark matter shows that it exists largely near baryonic matter

Well, yes, but it may be that you have stated that backwards in the sense that formation of baryonic matter into galaxies is probably PRECEDED by the formation of large regions of dark matter. Not known for sure but what I'm saying is that it might be more correct to say that baryonic matter clumps in the present of large regions of dark matter (which itself, does not clump the same way)

we are (almost?) certain we're entirely unable to measure it presently...

yep.

How do we know dark matter hasn't influenced our current measurements of gravitation on earth?

It's true that we can be confident is our mass-matter relationship understanding, but couldn't our gravity-mass relationship be skewed if we're getting a constant hum of "background gravitation"?

First, dark matter density is so low that even if it had the density around Earth that it has in galactic halos, it would be insignificant.

Second, for reasons that are not understood, it appears that the density of dark matter in the region of our solar system is unexpectedly low. This is a fairly recent discovery and I don't know if any measurements have show that conclusion to be incorrect.

 

[Bill_K]

Dark matter interacts with both baryonic matter AND itself, gravitationally.

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.

 

[phinds]

Depends on what it is, but if the dark matter particle is the neutralino, it can also self-annihilate.

Didn't know that. Thanks.

Earthbound experiments to detect dark matter hope it is a WIMP, meaning it has weak interaction with baryons.

Yeah, I should have pointed that out. Thanks for that as well.

On rereading, I think the "exclusively" that I was responding to might well have been questioning whether gravity is the exclusive interaction. I was responding as thought it was asking if gravitational interaction was exclusively with baryonic matter.

 

[dauto]

I'll be cautious in asking my my question because I'm out of familiar territory. But...

Given the following:

• that dark matter interacts with baryonic matter (exclusively?) via gravity,
• that evidence for dark matter shows that it exists largely near baryonic matter, and
• we are (almost?) certain we're entirely unable to measure it presently...

How do we know dark matter hasn't influenced our current measurements of gravitation on earth?

It's true that we can be confident is our mass-matter relationship understanding, but couldn't our gravity-mass relationship be skewed if we're getting a constant hum of "background gravitation"?

What do you mean by measurement of gravity? Making that point clear is crucial for a sensible answer.

 

[dauto]

Dark matter interacts with both baryonic matter AND itself, gravitationally.



Well, yes, but it may be that you have stated that backwards in the sense that formation of baryonic matter into galaxies is probably PRECEDED by the formation of large regions of dark matter. Not known for sure but what I'm saying is that it might be more correct to say that baryonic matter clumps in the present of large regions of dark matter (which itself, does not clump the same way)



yep.



First, dark matter density is so low that even if it had the density around Earth that it has in galactic halos, it would be insignificant.

Second, for reasons that are not understood, it appears that the density of dark matter in the region of our solar system is unexpectedly low. This is a fairly recent discovery and I don't know if any measurements have show that conclusion to be incorrect.

Can you point to a paper about that lower than expected dark matter density around the Earth? I would like to know more about that.

 

[phinds]

Can you point to a paper about that lower than expected dark matter density around the Earth? I would like to know more about that.

Regrettably I cannot, but the comments about it that I saw were here on this forum, I'm pretty sure. I did a quick forum search and don't see what I was looking for. Sorry.

Apparently, the results have been up and down. Here's an article that mentions both:

http://www.sci-news.com/astronomy/article00518.html

And here's one that says it is much HIGHER in our solar system:

http://www.universetoday.com/15266/dark-matter-is-denser-in-the-solar-system/

And another that mentions the point of view I expressed but says more recent data says it's wrong:

http://www.forbes.com/sites/alexknapp/2012/08/15/astronomers-detect-dark-matter-near-the-sun/

 

[dauto]

Regrettably I cannot, but the comments about it that I saw were here on this forum, I'm pretty sure. I did a quick forum search and don't see what I was looking for. Sorry.

Apparently, the results have been up and down. Here's an article that mentions both:

http://www.sci-news.com/astronomy/article00518.html

And here's one that says it is much HIGHER in our solar system:

http://www.universetoday.com/15266/dark-matter-is-denser-in-the-solar-system/

And another that mentions the point of view I expressed but says more recent data says it's wrong:

http://www.forbes.com/sites/alexknapp/2012/08/15/astronomers-detect-dark-matter-near-the-sun/

Seems like people are grasping at straws given the absence of actual data.

 

[mfb]

Regrettably I cannot, but the comments about it that I saw were here on this forum, I'm pretty sure. I did a quick forum search and don't see what I was looking for. Sorry.

Apparently, the results have been up and down. Here's an article that mentions both:

http://www.sci-news.com/astronomy/article00518.html

And here's one that says it is much HIGHER in our solar system:

http://www.universetoday.com/15266/dark-matter-is-denser-in-the-solar-system/

And another that mentions the point of view I expressed but says more recent data says it's wrong:

http://www.forbes.com/sites/alexknapp/2012/08/15/astronomers-detect-dark-matter-near-the-sun/

This is certainly an interesting point, but it should be highlighted that all articles discuss very small influences on the scale of the solar system. Compared to that, our Earth is tiny, so the amount of dark matter in our Earth is completely negligible.

 

[phinds]

This is certainly an interesting point, but it should be highlighted that all articles discuss very small influences on the scale of the solar system. Compared to that, our Earth is tiny, so the amount of dark matter in our Earth is completely negligible.

Yeah, one of the articles said that if it is a high density in the solar system it's about equivalent to a small asteroid and utterly negligible.

 

[Bill_K]

Earthbound experiments to detect dark matter hope it is a WIMP, meaning it has weak interaction with baryons.

For those with an interest in clever acronyms, it is worth mentioning two other alternatives that have been proposed: FIMPs (Feebly Interacting Massive Particles) and EWIPs (Extremely Weakly Interacting Particles)!

 

[nikkkom]

It's true that we can be confident is our mass-matter relationship understanding, but couldn't our gravity-mass relationship be skewed if we're getting a constant hum of "background gravitation"?

It's likely that dark matter distribution has characteristic structure scales on the scale of a small galaxy.

This means that on the scales of Solar System or ever a star cluster dark matter has essentially uniform density. There are no "small denser clouds" of it around.

 

[FlexGunship]

Dark matter interacts with both baryonic matter AND itself, gravitationally.

Yes, I did mean to convey that. I only meant that it interacts exclusively via gravity (i.e. not EM).

First, dark matter density is so low that even if it had the density around Earth that it has in galactic halos, it would be insignificant.

Second, for reasons that are not understood, it appears that the density of dark matter in the region of our solar system is unexpectedly low. This is a fairly recent discovery and I don't know if any measurements have show that conclusion to be incorrect.

Comprising ~85% of total matter (http://en.wikipedia.org/wiki/Dark_matter), you'd expect that dark matter would be the DOMINANT source of gravity. Although, this assumes that dark matter's distribution roughly matches that of "regular" matter.

It's likely that dark matter distribution has characteristic structure scales on the scale of a small galaxy.

This means that on the scales of Solar System or ever a star cluster dark matter has essentially uniform density. There are no "small denser clouds" of it around.

Again, this might be the smoking gun that address my question. But it seems to beg the question (in the actual use of the term: http://en.wikipedia.org/wiki/Begging_the_question).

My Question: "Couldn't dark matter skew our understanding of "atomic" (normal matter) gravitation?"
Your "No, because it's uniformly everywhere."

So... if it's uniformly everywhere... couldn't it skew out understanding/measurement of "atomic" gravitation?

Or, asked another way: given that we can't directly observe dark matter, how do we factor it out of local gravitation calculations? Or has it always been implicitly included?

 

[Bill_K]

Or, asked another way: given that we can't directly observe dark matter, how do we factor it out of local gravitation calculations? Or has it always been implicitly included?

The http://cdms.berkeley.edu/index.html at Berkeley maintains a dark matter FAQ, which http://cdms.berkeley.edu/Education/DMpages/FAQ/question36.html on this question:

DARK MATTER IN THE SOLAR SYSTEM

Dark matter should have gravitational effects on the planets orbits and on space probes, but we are so far unable to detect them. This is not surprising, however, because they are hidden by bigger effects: the gravitational pulls of the sun and planets are much, much larger.

The average density of dark matter near the solar system is approximately 1 proton-mass for every 3 cubic centimeters, which is roughly 6x10^-28 kg/cm³. The actual density might be a little lower or higher, but this is the right order of magnitude.

Based on this number, we can work out the total mass of dark matter within the radius of Earth's orbit around the sun: for an orbital radius of 100 million km, we get a total of 2.3x10¹² kg of dark matter within the Earth's orbit. This sounds like a lot, but the sun's mass is 2x10³⁰ kg. All of that dark matter only weighs 10^-18 as much as the sun does, so we cannot detect the tiny pull of dark matter upon the Earth's orbit. The same story is true all over the solar system: the gravitational pulls of the sun and planets are always much larger than that of the dark matter.

 

[mfb]

Comprising ~85% of total matter (http://en.wikipedia.org/wiki/Dark_matter), you'd expect that dark matter would be the DOMINANT source of gravity. Although, this assumes that dark matter's distribution roughly matches that of "regular" matter.

This assumption is not true.
To clump on small scales similar to regular matter, dark matter would have to interact via the electromagnetic or strong force, and then it would not be "dark" any more.

 

[nikkkom]

So... if it's uniformly everywhere... couldn't it skew out understanding/measurement of "atomic" gravitation?

Or, asked another way: given that we can't directly observe dark matter, how do we factor it out of local gravitation calculations? Or has it always been implicitly included?

If it is *uniformly* everywhere in the Solar System, then it will be undetectable even if its density is not low.

 

[Bill_K]

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.

This is also why the gravitational effects of dark matter were first noticed in the outer regions of galaxies.

 

[FlexGunship]

This assumption is not true.
To clump on small scales similar to regular matter, dark matter would have to interact via the electromagnetic or strong force, and then it would not be "dark" any more.

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.

Is there a fifth fundamental force, that causes it to re-disperse?

If it is *uniformly* everywhere in the Solar System, then it will be undetectable even if its density is not low.

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.

Running this same test at home, (keeping in mind that dark matter could not influence a laboratory scale because it cannot exert force on the scale) the apparent mass of iron on a scale (in the presence of Earth's gravity + dark matter gravity) would vary from that of iron without the dark matter present.

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.

Imagine the following two scenarios:

1. A comet in orbit around the sun as we understand it with mass 2x10³⁰ kg of ordinary matter
2. The same comet in an identical orbit around the sun with a concentration of 1% dark matter (which is constantly falling towards the gravitational center of the sun) instead of regular matter

What test would you perform to differentiate these two scenarios?

If your response is: "Well, we know the mass of the sun because we know what it's made of and how much of it there is. And there's nothing significant missing for dark matter to make up." I'd argue that's a tautology; you're assuming the conclusion as a premise.

The sun is massive. It formed SPECIFICALLY because of gravity. Why wouldn't non-orbital dark matter fall into it? Why wouldn't dark matter in our orbit tend to fall to the gravitational epicenter like all of the other matter that originally formed the Earth?

Only Cavendish's ORIGINAL experiment to determine the constant of gravitation would be unaffected by dark matter in this way.

 

[nikkkom]

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.

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.

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]

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]

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]

> 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]

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]

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]

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:

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]

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]

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]

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]

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]

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.