Why does "dark matter" segregate itself from normal matter?

[SuaSponte]

One thing I've never understood about "dark matter", and have never seen discussed in any article I've read on the subject, is why the stuff appears to segregate itself from normal matter. It appears to huddle on the outskirts of galaxies, which explains why we haven't discovered the stuff until now (if it commingled, we would have noticed that everything was a lot heavier than it should have been, right?), but that doesn't make a lot of sense to me. Can anyone explain?

Thanks,

Sua

[Shagnasty]

I am not an expert but I understand that dark matter and regular matter anihilate each other when they come into contact. They don't exist close together because anytime that happened, things would get destroyed so dark matter can only continue to exist in places where it is far away from regular matter.

[ultrafilter]

Originally Posted by Shagnasty

I am not an expert but I understand that dark matter and regular matter anihilate each other when they come into contact. They don't exist close together because anytime that happened, things would get destroyed so dark matter can only continue to exist in places where it is far away from regular matter.

Are you thinking of antimatter?

[robby]

Originally Posted by Shagnasty

I am not an expert but I understand that dark matter and regular matter anihilate each other when they come into contact. They don't exist close together because anytime that happened, things would get destroyed so dark matter can only continue to exist in places where it is far away from regular matter.

It sounds like you are thinking about antimatter instead of dark matter, Shagnasty.

[Stranger On A Train]

First of all--and let's be absolutely clear on this--nobody really knows what so-called "dark matter", or more appropriately, "missing dark matter" (MDM), is. Technically speaking, most matter is "dark", including yourself, your house, the planet you live on, et cetera; that is to say, it's dark in the sense that it doesn't radiate electromagnetic energy (other than what it absorbs from other sources, like the Sun) and can only be detected by gravitation or the afformentioned absoption and reradiation. The problem is that there is an enormous amount of mass--the majority of it, in fact--that appears to be missing in order for the mechanics of Newtonian and relativistic physics to work on large (galactic and larger) scales.

One set of proposals for this is that MDM is some kind of weakly interacting interstitial matter that flows not just around us but through us, exerting an immeasurably small local gravitational force but comprising such a vast bulk mass that its effects are apparent on large scale structures. There are, in fact, particles like this, called neutrinos which are leptons (elementry particles that don't experience strong nuclear interactions) which lack a charge, so that they also don't experience electromagnetic forces. The Sun vomits an enormous quantity of these as waste products from fusion such that trillions of them are pouring through you every second, but it takes a large and very sophisticated detector to see one even occasionally. However, missing dark matter is most likely not comprised of neutrinos, because while they fit the bill from a non-reactive point of view, the individual particles have a vanishingly small mass and the reactions that produce them send them flying at speeds which proclude them from gathering in structures as small as a mere galaxy, and also we detect no sign of the massive amount of neutrinos we should detect if it were the case that they were around and just moving a lot slower.

There's a significantly more massive complementary family of supersymmetry particles called neutralinos which fit the bill much better; the problem with these is not only that we haven't detected them but they actually are predicted by a hypothesis (supersymmetric extensions to the Standard Model of particle physics) which haven't and can't (with existing technology) be tested. The math works out pleasently enough, but the theory is a chimera, and doesn't get us any closer to validating neutralinos or other exotic matter as comprising MDM.

It's also possible that some of the MDM is ordinary matter that is just less visible than we think it ought be. It can't be dust, because we should be able to detect thick clouds by their absoption, but at least some of it could be brown dwarfs, supermassive free planetary bodies, non-radiating singularities (black holes), et cetera, though it seems unlikely to astronomers and cosmologists that this could make up more than a small fraction of MDM. It could even be something more exotic like cosmic strings, or some form of concentrated gravitational energy we don't know about. Or it could be that there is no MDM and that the gravitational force acts differently over large distances than we expect, or that General Relativity is incomplete, or that there are one or more additional forces that we can't detect on everyday scales. (The latter is seriously unlikely, or would at least require a huge revision in our understanding of relativity and mass particle exchanges, but it's not entirely out of the question.)

In any case, nobody thinks that MDM, if it exists, is "segregated" from regular matter; merely that we can't see it or observe it in any way except for mass interactions, and that it's gravitational effects on local scales are too small to be detected. If this seems suspiciously like the "luminiferous aether" explaination for the propogation of light from the 19th Century in terms of how vague and insubstantial it is, then perhaps you're right; it's a seriously incomplete hypothesis for a phenomena we still don't really understand, and any physicst or cosmologist worth his diploma will admit such a thing up front.

Stranger

[The Great Sun Jester]

Wow. Nice guess, Stranger.

[Exapno Mapcase]

There was another dark matter thread less than two weeks ago.

Here's another recent thread.

[Pasta]

Collisions of matter on matter affect the distribution of matter in a galaxy. For example, if something is revolving "the wrong way" (i.e., its angular momentum about the core is perpendicular to the total angular momentum of the galaxy), it will repeatedly cross the rotation plane until it hits something which can send it into the plane. At steady state, things are distributed somewhat compactly / squashedly. (This mechanism works well for planetary rings since collisions out of the ring (and into the planet's surface) will eventually rid the ring of misbehaving objects.)

If dark matter is weakly interacting, its distribution will not be heavily influenced by this collisional mechanism, so it can end up, as you say, "segregated".

[Shagnasty]

Originally Posted by ultrafilter

Are you thinking of antimatter?

Oops, I sure was. I really do know the difference in real life

[Anne Neville]

Originally Posted by SuaSponte

One thing I've never understood about "dark matter", and have never seen discussed in any article I've read on the subject, is why the stuff appears to segregate itself from normal matter. It appears to huddle on the outskirts of galaxies, which explains why we haven't discovered the stuff until now (if it commingled, we would have noticed that everything was a lot heavier than it should have been, right?), but that doesn't make a lot of sense to me. Can anyone explain?

This isn't exactly my field of astronomy, but I'll take a whack at this.

Dark matter doesn't avoid the centers of galaxies, but it's harder to observe it there with all the other stuff (normal matter, supermassive black holes, etc) going on in the center of a galaxy.

Most dark matter can't be observed directly- technically, the Earth and all the stuff on it that doesn't emit light is dark matter, and we observe things on Earth, so we can observe some dark matter. But even if all the dark matter were in the form of planets, which it isn't, we can't observe planets around stars in the galactic center or on the far side of the galaxy, let alone in another galaxy. All those extrasolar planets you might have heard about, with the exception of the pulsar planets, are within 200 light-years of the Sun. That's very close compared to the size of the galaxy.

Since we can't look for dark matter directly, we have to find other methods. One method, which is probably what you're thinking of in the OP, is by observing galaxy rotation curves. Those show that the visible matter we see in galaxies can't be all that's there. They also show that the matter in the galaxy must extend out farther from the galactic center than what we see. The current theory is that there is a halo of dark matter (which here just means "something that has mass but we can't see it"- galaxy rotation curves don't tell us anything about the nature of dark matter except that it has mass) extending farther out from the center of the galaxy than the visible matter does.

Because of an electromagnetic phenomenon known as the Poynting-Robertson effect, dust grains in orbit tend to spiral inward. That dust helps in star formation. IANAGalaxy formation expert, but that's my guess as to why the luminous part of the galaxy is so clustered around the galactic center. There's evidence that 85-90% of the mass of the universe is made up of dark matter that doesn't interact electromagnetically, so it wouldn't be subject to Poynting-Robertson drag, and would be less likely to clump at the center of a galaxy.

[Hari Seldon]

I don't know about you other guys but I sure radiate. That's how night vision goggles work. Even at 2.7 deg A, stuff radiates. Sure it is in the microwave range, but it radates. And cold dark matter simply doesn't. It doesn't radiate and doesn't absorb radiation either (two sides of the same coin). As far as is known it doesn't respond to three of the four forces (electromagnetic, strong, and weak), but only to the ultrafeeble force of gravity.

I recently read something that cold dark matter doesn't even respond to itself. There appears to be no clumping (except from gravity).

[The Great Sun Jester]

You can observe my mass when you pry it from my cold dar ... ok, nevermind.

[Chronos]

As far as is known it doesn't respond to three of the four forces (electromagnetic, strong, and weak), but only to the ultrafeeble force of gravity.

Whatever it is, it probably responds to the weak force, and might even respond to the strong force. That is to say, of the theoretical particles which have been proposed to explain dark matter, most of them are subject to the weak force (I can't actually think of any examples which don't, except perhaps microscopic black holes), and a few of them are subject to the strong force as well. But neither of the nuclear forces can have any significant effect over astronomical distances, as gravity and electromagnetism can.

[figure9]

Stranger on a train Thanks for getting this picture stuck in my head. "The Sun vomits an enormous quantity of these...." Now I'm always going to think of sunlight as solar-spew. I can't wait to go outside and feel the sun barf on my face!

[Little Nemo]

It's self-imposed segregation. Dark matter just feels more comfortable socializing with other dark matter.

[Chief Pedant]

Originally Posted by Stranger On A Train

If this seems suspiciously like the "luminiferous aether" explaination for the propogation of light from the 19th Century in terms of how vague and insubstantial it is, then perhaps you're right; it's a seriously incomplete hypothesis for a phenomena we still don't really understand, and any physicst or cosmologist worth his diploma will admit such a thing up front.

Stranger

Stranger,

This ezzack thing has been bugging my undereducated mind for years.

Is there any train of thought that gives not just a density concept to space, but instead of that density being a constant K, lets space vary in density, and lets space be smooth rather than particulate or quantized? (I'm using "space" in very much layman's terms; whatever the substrate is within which we observe energy and particles and the like.)

In such a construct gravity would be a property of the relative density of space, rather than the more-often applied analogy of warping (with the natural path of a particle or light beam flowing down a density gradient...), all space contributes to the overall mass of the universe, dark matter clusters are areas where space is simply more dense, and Newton gets the last laugh arguing that without an aether there is no propagation of light. MDM is simply space itself, but with a quality closer to a paste of varying density rather that particles of any kind.

[Lumpy]

If dark matter exists in the form of particles that don't interact with electromagnetism, then why haven't they all collapsed into black holes? Electron repulsion is pretty much why there is any such thing as matter, instead of just a universe of gravity holes.

[vix]

Originally Posted by Stranger On A Train

In any case, nobody thinks that MDM, if it exists, is "segregated" from regular matter; merely that we can't see it or observe it in any way except for mass interactions, and that it's gravitational effects on local scales are too small to be detected.

Well, here's my problem; dark matter is supposed to be, what, 95% of the matter in the universe, but its gravitational effects on local scales are too small to be detected?! If 95% of the mass of the solar system was dark matter, the dark matter would mass more than the Sun. Wouldn't that affect planetary orbits, or otherwise cause effects we would surely notice? And if dark matter isn't in the solar system, why not?

Sua

[Exapno Mapcase]

Think of the earth. You would be weightless at the earth's center because there would be equal amounts of mass in every direction pulling at you. The net effect is zero.

If the solar system were filled with dark matter in an equal mist everywhere, the effect would be similar. The pulls would cancel out and the orbits would be unaffected.

However, it's not known whether dark matter actually fills all space, including between planets in the solar system, or whether it mostly forms a halo surrounding the galaxy. The latter is more likely and makes the question of dark matter's behavior inside the solar system moot.

[Exapno Mapcase]

Originally Posted by Lumpy

If dark matter exists in the form of particles that don't interact with electromagnetism, then why haven't they all collapsed into black holes?

Black holes are unusual formations, created only when an extreme concentration of matter congregates. Most black holes we know of are at the center of galaxies, where the highest densities of matter exist.

Whatever dark matter is, it doesn't congregate. It's widely dispersed and possibly moving at a high speed. It has extremely low density. This makes it highly unlikely that it would collect at all, let alone to such an extreme extent that a black hole would form.

[Chronos]

Most of the matter in the Solar System is ordinary matter. Most of the matter in the Galaxy is dark matter. These two statements are not in conflict with each other. Remember, there's a lot of space between solar systems, and there's dark matter in those spaces, too.

[Squink]

Originally Posted by Exapno Mapcase

Whatever dark matter is, it doesn't congregate.

That's because, whatever it is, dark matter has a tiny collisional cross section, both with normal matter and with itself.
Like neutrinos passing through the earth, the stuff is extremely difficult to slow down.

[Stranger On A Train]

Originally Posted by Chief Pedant

Is there any train of thought that gives not just a density concept to space, but instead of that density being a constant K, lets space vary in density, and lets space be smooth rather than particulate or quantized? (I'm using "space" in very much layman's terms; whatever the substrate is within which we observe energy and particles and the like.)

In such a construct gravity would be a property of the relative density of space, rather than the more-often applied analogy of warping (with the natural path of a particle or light beam flowing down a density gradient...), all space contributes to the overall mass of the universe, dark matter clusters are areas where space is simply more dense, and Newton gets the last laugh arguing that without an aether there is no propagation of light. MDM is simply space itself, but with a quality closer to a paste of varying density rather that particles of any kind.

I'm not really sure how to answer this question. Attempts to formulate a theory of gravity that is congruent with quantum field theory assume that spacetime is some kind of granular matrix, but nobody has gotten to far with this. Newtowian mechanics and General Relativity assume that space is continuous (except at singularities, where GR can't say much of anything other than, "Hmmm...interesting...I'll get back to you on that,") but the underlying assumption of GR is that the presence of mass warps the otherwise "flat" plenum of spacetime. In order to get GR to work with a stationary universe--that is, one that doesn't collapse upon itself via its own gravitational attraction--Einstein had to throw in the cosmological constant to the Einstein Field Tensor, which after the discovery of Hubble expansion, he felt to be the biggest mistake of his career. However, we now believe that the rate of expansion of space (rate of change of the ill-named Hubble constant) increases, and that something like the cosmological constant is an appropriate modificaiton to the EFT.

What causes this rate of change is unclear, but is popularly and seemingly oxymoronically dubbed dark energy. If it exists, it is, in fact, "dark" because it acts with a negative pressure. While this would intuititively seem like it would pull things together, the negative pressure in this sense means that spacetime, when not "compressed" by the pressence of mass-energy (which has positive pressure) it tends to push away from itself, thus causing space to expand and objects that are not otherwise strongly gravitatationally bound to each other to move away at a rate proportional to their distance. In a sense, space is "denser" in the presense of mass. It's possible that there could be other causes besides the presence of mass-energy that would cause this "density" of space to be nonhomogeneous, but this would require substantial modifications to General Relativity as we understand it, or additional forms of mass-like energy fields that we've currently observed no sign of. Many people have put forward various theories involving phantom energy, quintessence, et cetera, but none have yet uniquely satisfied the conditions we attribute to "dark energy" and "[missing] dark matter", or provided significant insight into why the universe behaves the way it does.

Originally Posted by vix

Well, here's my problem; dark matter is supposed to be, what, 95% of the matter in the universe, but its gravitational effects on local scales are too small to be detected?! If 95% of the mass of the solar system was dark matter, the dark matter would mass more than the Sun. Wouldn't that affect planetary orbits, or otherwise cause effects we would surely notice? And if dark matter isn't in the solar system, why not?

Chronos addressed this adequately, but let me expand upon it with this illustration: think of walking through a rainstorm. You get wet, and you can see where the "wetness" is coming from; it's raindrops that fall from the sky, because the old man up in the clouds is sad. (That's the rationale that a friend extends, and I'm not going to rain on her parade by contradicting her.) However, when you walk through a dense fog you also get wet, only you can't see where the wetness comes from, because it's made of droplets too small to see that are suspended in the air. Similiarly, we can directly observe and feel gravity from normal matter, which visibly "clumps" together in the form of stars, planets, moons, comets, et cetera. This clumped up matter creates very localized gravitational fields with strong gradients. Our hypothetical weakly-interacting interstitial "[missing] dark matter", however, is all spread out, and so at least in localities small enough for us to directly observe, the gravitational effects are very small and the forces therefrom mostly cancelled out; it's a constant field, and so there's no gain or loss of potential energy in moving from one point to another. Over larger (galactic) scales, it might stick together and "clump" becuase the sum of the mass attraction over that size of a body has a net positive force, but the net resultant force for any small region inside is virtually nil.

Does this sound like voodoo? Well, it probably should; we know that there is something wonky either about the amount of mass we can observe and the difference between the effect it should have and the effects we see. The two lines of explanation involve either postulating some kind of heretofore unknown mass or other energy fields that aren't manifest on everyday scales, or that there is something dreadfully, horribly, possibly fatally wrong with General Relativity. GR has served us well for nigh on a century now, and is, next to quantum electrodynamics and natural selection, one of the most strongly validated quantitiative theories in Nature. Plus there are scores of textbooks which include detail descriptions of Special and General Relativity, and modifying it now would require huge typesetting changes, whereas throwing in some extra unseen mass is merely a footnote or an extra appendix. On the balance, it's much easier to cope with the idea of magic fairy dust than throwing away a long and ardently established general principle of physics, especially one that works so well in virtually every other application. (Well, except for the breakdown between GR and quantum mechanics, but that's all the fault of those QM people who can't even figure out exactly where their fundamental particles are.)

So, in short, it's because normal matter gloms together in big concentrations of mass which have dramatic, measureable gravitational effects on local spacetime, while "[missing] dark matter" (whatever the hell it is) is all spread out like a fine mist, a very indistinct haze that only shows up when you're flying above.

Stranger

[Stranger On A Train]

Originally Posted by Exapno Mapcase

Black holes are unusual formations, created only when an extreme concentration of matter congregates. Most black holes we know of are at the center of galaxies, where the highest densities of matter exist.

I missed this before, but I found this interesting when I first came across it. While the average density of the galactic core is very high, the density of a (hypothetical) supermassive singularity inside of it (based upon a volume calcluated from the circumference of the event horizon) is quite low, probably only a few grams per cubic meter. This is still much higher than the interstellar medium, but much lower than sea-level air pressure. The collection of mass in one space, however, is enough to create a gravitational field from which the escape velocity is >c. Weird.

MDM, because it doesn't interact electrostatically (again, hypothetically) can't stick together, and thus can't form things like atoms, molecules, planets, cricket balls, et cetera. So two MDM particles might influence one another and even "orbit" each other but they're not going to glob together. For large masses of MDM moving at high speeds (>solar escape speed), they could all collectively orbit a common center in a big massively interacting cloud but won't form "solid" objects the way normall matter will. Depending on the parameters you give it, you can easily make it form structures the size and with the dynamic behavior of the Milky Way or other galaxies. Whether there actually is such a thing and what it is composed of are still standing questions in cosmology and astrophysics.

Stranger

[Chief Pedant]

Originally Posted by Stranger On A Train

In a sense, space is "denser" in the presense of mass. It's possible that there could be other causes besides the presence of mass-energy that would cause this "density" of space to be nonhomogeneous, but this would require substantial modifications to General Relativity as we understand it, or additional forms of mass-like energy fields that we've currently observed no sign of.

Stranger

Thanks. This is really what I'm trying to get my neurons around. Since Quantum Mechanics and GR don't agree, it doesn't bother me that GR will get modified when we (I am not including myself here ) figger it out. They are obviously both insufficient explanations.

What I'm trying to get at is the assumption that space(time) is smooth--homogeneous--absent it being warped by mass-energy. It seems to me standard putative explanations revolve around the notion of some "dark" something doing the warping which in turn affects the behaviour of particles and light in spacetime.

I think I'm trying to figure out whether there is a train of thought that says the substrate of space itself is inhomogeneous. If space has a "pressure"--a density, so to speak--and K is not a constant but varies independently of whether mass-energy is warping it, it seems to me we can stop looking for dark matter. What changes is our perception of what space is. A few of my synapses are lobbying for a different paradigm from the one in which matter and energy are actors on a totally flat stage that only warps where they happen to be playing.

The mass-energy distribution is plenty lumpy. Why do we assume space itself has to be smooth unless mass-energy is distorting it? The aberrencies which trigger the search for dark matter are predicated on the assumption that the K of space is the same in that region, and therefore there must be some dark matter warping it. Didn't we make the same mistake (again; not me personally ) assuming time was everywhere the same?

This is only one of many idiotic questions that keep me up at nite so don't think I'm asking you to waste your time actually answering it.

[essell]

Originally Posted by Stranger on a Train

The Sun vomits an enormous quantity of these as waste products from fusion such that trillions of them are pouring through you every second, but it takes a large and very sophisticated detector to see one even occasionally.<mega snip>

Sophisticated ? I thought it involved filling mines with lots and lots of very big tanks of water ?

[Stranger On A Train]

Originally Posted by essell

Sophisticated ? I thought it involved filling mines with lots and lots of very big tanks of water ?

Here is a brief description of the The Sudbury Neutrino Observatory. Here is a picture of the detector with it's massive number of photomultiplier tubes glommed onto it, looking like something out of a James Cameron movie. The occasional decay event is recorded and interpreted to figure out what it was, where it come from, and how much energy it had. There's a lot more than just filling up an underground bore with water.

Stranger

[Chronos]

Quoth Stranger on a Train:

Plus there are scores of textbooks which include detail descriptions of Special and General Relativity, and modifying it now would require huge typesetting changes, whereas throwing in some extra unseen mass is merely a footnote or an extra appendix.

Changing the theory wouldn't be so bad, either, if we knew what we had to change it to. But the best anyone's come up with in that direction is Modified Newtonian Gravity, or MOND, and nobody knows of a way to fit that into the realm of relativity (even in the Newtonian regime of low speeds, there are some problems with it).

While the average density of the galactic core is very high, the density of a (hypothetical) supermassive singularity inside of it (based upon a volume calcluated from the circumference of the event horizon) is quite low, probably only a few grams per cubic meter.

"Black hole" and "singularity" are not synonymous. A singularity is what's in the center of a black hole, and does not include the horizon. If you're going to talk about the density of a singularity, you're stuck with saying that it's infinite (or at least, if it's finite, current theories give us no way of saying how big it is).

Quoth essell:

Sophisticated ? I thought it involved filling mines with lots and lots of very big tanks of water ?

It does, but there's a lot more to it than that. You've got to go to insane lengths to shield those tanks from any other form of radiation (things like lead from the ballast of 500-year-old shipwrecks, since lead fresh out of a mine will itself be too radioactive). And then you have to also detect whatever effect the neutrinos are having. This might mean, for instance, finding a half-dozen atoms of argon, in a tank that holds millions of gallons.

[Pasta]

Continuing Stranger's SNO example: Just to get to the detector clean room, you must strip naked, shower, and put on special clothing, lest you bring in any dust that could add radioactive contaminants that would ruin the detector's ability to pick out the neutrino events. The detector needs 10k designed and regularly calibrated electronics channels and photomultiplier tubes such that you can tell to within about one nanosecond the exact time that a single photon arrived at the face. (Even the glass in the photomultiplier tubes is special low-radioactivity glass. $1000+ a pop for those babies.) Or how about understanding the electromagnetic shower properties of heavy water (and the optical photon propagation properties of heavy water) such that you can distinguish a 3 MeV gamma versus a 3 MeV electron. These things are why the first SNO result took 10 years to come out.

[Exapno Mapcase]

Originally Posted by Chief Pedant

Didn't we make the same mistake (again; not me personally ) assuming time was everywhere the same?

I'm missing something. Time is everywhere the same. GR says it's perceived differently when different reference frames come together, but time always flows at 1 sec/sec for every observer.

[Chief Pedant]

Originally Posted by Exapno Mapcase

I'm missing something. Time is everywhere the same. GR says it's perceived differently when different reference frames come together, but time always flows at 1 sec/sec for every observer.

Apologies for a hastily-written analogy. I was not intending to sum up GR in that sentence. I was referring to the fact that my tick is not your tock if we are moving at different speeds relative to one another, but what I was trying to convey is the kind of paradigm shift that occurs when science overturns a base assertion--"Time is absolute and not relative to a frame of reference" e.g.

In any case, the assumption I'm interested in is that space iself--the background matrix in which mass-energy performs--is uniform. If the universe is expanding, is the substrate that is space which is created by this expansion consistent everywhere? I understand the notion that it has any "density" (for lack of a better term) at all is a questionable concept. Indeed if whatever space is is the same everywhere, such a property might be completely unmeasurable. Particles and energy then exist within a matrix that is completely smooth and everywhere the same. But if the matrix of space , so to speak, varies regionally (indepently of the presence of mass-energy) then the (local) behaviour of particles and energy can vary. Instead of finding dark matter particle carriers for the missing mass, we find regional variations in space itself are what account for what we were assuming was the presence of dark matter. What we thought were regions of dark matter concentrations affecting gravitational fields were instead regions of inhomogeneity of space itself.

[Lumpy]

I've always read that General Relativity somehow implies that the laws of physics have to be uniform throughout the observable universe, though I don't understand why. If so, that would pretty much eliminate the inhomogenous space theory Chief Pedant was wondering about.

[tbdi]

Originally Posted by figure9

I can't wait to go outside and feel the sun barf on my face!

As far as the neutrino barf goes there's no need to go outside, it's hitting your face quite well inside. In fact, at night and it'll be hitting your face right through the transparent (to neutrinos anyway) earth.

[Exapno Mapcase]

Originally Posted by Lumpy

I've always read that General Relativity somehow implies that the laws of physics have to be uniform throughout the observable universe

I believe that is an assumption, not something that emerges out of GR. So it should be possible for conditions to differ. There are hypotheses that rely on this, such as the ones that postulate that constants aren't constant over time.

But I simply don't get whatever it is that Chief Pedant is trying to say. Dark matter is supposed to be pretty much everywhere. If space itself varies locally within and surrounding galaxies then everything needs to be rewritten and the effects would ripple across every measurement we take. Nothing would be fixed or calculable.

Since this is obviously not the case, the underlying invariance of space seems to be unquestionable. I wish he'd try to explain how there can be such a disruption to space-time that it accounts for 23% of the universe but leaves every measurement of light passing through it totally untouched.

[Chief Pedant]

Originally Posted by Exapno Mapcase

I believe that is an assumption, not something that emerges out of GR. So it should be possible for conditions to differ. There are hypotheses that rely on this, such as the ones that postulate that constants aren't constant over time.

But I simply don't get whatever it is that Chief Pedant is trying to say. Dark matter is supposed to be pretty much everywhere. If space itself varies locally within and surrounding galaxies then everything needs to be rewritten and the effects would ripple across every measurement we take. Nothing would be fixed or calculable.

Since this is obviously not the case, the underlying invariance of space seems to be unquestionable. I wish he'd try to explain how there can be such a disruption to space-time that it accounts for 23% of the universe but leaves every measurement of light passing through it totally untouched.

Let me caution you against trying to get a dullard to explained an incompetently-framed question born of ignorance...but

It is not my impression that dark matter "is supposed to be pretty much everywhere." My impression is that dark matter is an explanation for gravitational perturbations where there is not enough ordinary matter to account for the perturbation. Although in one sense it is "everywhere" its distribution is not assumed to be perfectly smooth anymore than ordinary matter is equally distributed in every cubic meter of space. Dark matter is postulated precisely because we observe regional variations in space unaccounted for by ordinary matter.

Although the principle of gravitational force applies everywhere, the distribution of gravitational fields is not everywhere the same, and GR (as my 3 neurons understand it) ascribes that "lumpy" distribution--those regional curvatures of space-time--to the effect of mass from particles (which is why we assume in the first place that there must be some dark matter warping space-time in those regions where there is insufficient ordinary matter to explain the observed gravitational effects). Implicit within this construct is the assumption that the underlying matrix of space itself is gravitationally neutral, or flat, or uniformly dense, or whatever term conveys that notion.

We do, in fact, see extreme "disruptions" across regions of space. Gravitational fields do vary locally, and light passing through those fields is definitely affected by them. We currently ascribe those disruptions only to the presence of mass-containing particles curving an otherwise uniformly flat substrate and never to an inhomogeneity of space-time substrate itself. I'm wondering if there is any train of thought that space-time itself has inhomogeneities independent of the presence of mass-containing particles.

And until we bring GR and quantum mechanics under one theory, I guess I do think everything needs to be rewritten. Hopefully, for me, rewritten at the sixth-grade level...

jethro

[toadspittle]

Originally Posted by Chief Pedant

Let me caution you against trying to get a dullard to explained an incompetently-framed question born of ignorance...but

It is not my impression that dark matter "is supposed to be pretty much everywhere." My impression is that dark matter is an explanation for gravitational perturbations where there is not enough ordinary matter to account for the perturbation. Although in one sense it is "everywhere" its distribution is not assumed to be perfectly smooth anymore than ordinary matter is equally distributed in every cubic meter of space. Dark matter is postulated precisely because we observe regional variations in space unaccounted for by ordinary matter.

Although the principle of gravitational force applies everywhere, the distribution of gravitational fields is not everywhere the same, and GR (as my 3 neurons understand it) ascribes that "lumpy" distribution--those regional curvatures of space-time--to the effect of mass from particles (which is why we assume in the first place that there must be some dark matter warping space-time in those regions where there is insufficient ordinary matter to explain the observed gravitational effects). Implicit within this construct is the assumption that the underlying matrix of space itself is gravitationally neutral, or flat, or uniformly dense, or whatever term conveys that notion.

We do, in fact, see extreme "disruptions" across regions of space. Gravitational fields do vary locally, and light passing through those fields is definitely affected by them. We currently ascribe those disruptions only to the presence of mass-containing particles curving an otherwise uniformly flat substrate and never to an inhomogeneity of space-time substrate itself. I'm wondering if there is any train of thought that space-time itself has inhomogeneities independent of the presence of mass-containing particles.

And until we bring GR and quantum mechanics under one theory, I guess I do think everything needs to be rewritten. Hopefully, for me, rewritten at the sixth-grade level...

jethro

Chief: I think the problem you're going to run into is that, while we see these odd galaxy rotational curves, they show that, at least, the effect across the whole of the galaxy is homogeneous. So if space itself is what is weird, then it's uniformly weird in exactly the place that we're seeing all the mass.

If your hypothesis were correct (that space is altering the behavior of matter, rather than the other way around), then we should see parts of some galaxies flying off in strange directions for no apparent reason--as if the galaxy hit a giant cosmic pothole--when those parts encounter a spatial discontinuity. But we don't. We see galaxies sticking together, and the strange effect that creates the need for the MDM explanation correlates pretty well to where the mass is. If the two aren't related, then that's a pretty big coincidence. (Either that, or every single galaxy in the universe has already fallen into a cosmic pothole--or billiards pocket--and now we can't get out.)