Simpler solution than "dark matter" to cosmological prob?

Great Debates
1

An LA Times article this morning set off a (super?) string of thought. So I’m going to show off my ignorance here at the SDMB. Because ignorance is what it’s all about.

There is, apparently, a pressing problem in cosmology having to do with the fact that our galaxy has more gravity than surveys by astronomers think it ought to. The solution most in favor seems to be that about 90% of the galaxy’s mass is due to what is called “dark matter”–basically, stuff that is (so far) invisible and undetectible.

Two ideas immediately occurred to me. As they occurred to ME, they must be obvious; which means they’ve been considered and rejected many times over the years. Could someone tell me the basis upon which they have been discounted?

(1) In measuring the behavior of the rest of our own galaxy, not to mention the more distant objects in the cosmos, we are looking back in time between 16,000 and 147,000 years, with most of the “accessible” galaxy nearer the larger figure. Other galaxies are, obviously, much much further “back in time”. So wouldn’t one solution to the problem be, that the gravitational constant G has actually decreased in value (relative to some other constants) such that the out-of-date universe we “see” appears to us way too massive?

(2) Or–we might fiddle around, not with G, but with the exponent. Instead of dropping off uniformly with the square of the distance, maybe the drop off is a shade less. At distances like those within our solar system, that shade might be far too slight to be detectible; but at interstellar or intergalactic distances, it could be significant. (And of course it could be a nonlinear function.)

If these notions are nonstarters, please tell me why–and what experimental evidence counts against it. (I mean, invisible matter isn’t exactly “pretty”, either.)

2

Well if the gravitational constant G has changed drastically within such a short period of time, we would expect to see a very definte record of it on Earth (for example in the fossil record) and also we would probably expect to still be able to measure a decrease. Also I imagine the oobservational evidence would be different for higher values of G.

I think the effects of (2) would defintely be obervable. BTW gravitational force with distance already is a non-linear function, being related to the square of the distance.

3

It’s also a lot easier to accept the idea that we just can’t see most of the matter in the universe than that the fundamental constants of the universe aren’t actually constant.

Unexplained phenomena require new hypotheses… but scientists have effectively internalized Occam’s Razor.

4

A little off-topic, but what do you suppose the alternative to “dark matter” is? I mean, if you could look at the sky with eyes capable of receiving more bandwidths on the infrared spectrum, it stands to reason that you’d see a lot more out in the heavens. Perhaps this dark matter is merely invisible to our current level of technology, and we have to partially theorize it in order for it to fit into our grand scheme? Of course, I believe there’s some truth to the theory, maybe the stuff isn’t as physical as it’s being portrayed as. Perhaps it’s just energy with mass, and in truth we really know next-to-nothing about how the four forces interact with matter.

/Glaring ignorance of the physical laws surrounding the stuff…

5

It seems to me that if the gravitational constant had changed over time, the observed behaviour of distant stars would also be very different. But when we look at stars of a certain mass, we see the same types of processes nad spectra and such that we see with closer stars. That would indicate that the various physical processes behave in about the same way.

Why is it hard to accept dark matter? It makes perfect sense that the universe would be full of all kinds of stuff that we can’t see because it’s spread thinly or too small to fuse and emit light. Look at reflection nebulae: A star explodes, and a reflection of its light appears on a gigantic field of dust light years across. We see that reflection, but if the origina star hadn’t been there, would we know about that cloud of dust?

6

My current understanding (which I admit is somewhat weak) is that everyone is looking for Dark Matter to be:

a) WIMPs, i.e. unseen, unfelt particles that pass through us and everything else continually that have mass but don’t interact with normal (baryonic) matter. My understanding is that there isn’t even a weak theory on what these particles may be.

b) MACHOs, basically lotsa dark massive rocks out there that are difficult to detect. I recall that there was a study of gravitational lensing that seemed to detect quite a few of these occluding ‘nearby’ (galactic) objects.

The consensus I recall is that neither WIMPs nor MACHOs can do the job of explaining the missing Dark Matter, and that it’s probably some combination of the two. Or some other candidate entirely.

All of this is gathered from several popular books that are probably at least 3-4 years out of date regarding Dark Matter, so pardon any out-of-date summaries.

Does anyone care to update the MACHO v WIMP consensus, esp regarding possible WIMP particle candidates?

Sam: I agree the changing constants idea is weak. The articles I’ve read seem to say that this had been discussed and discarded, except re the now-ressurected “cosmological constant”, which may become greater over very large timespans. Or not, and it’s all a figment of the properties of gravity that aren’t understood.

Lastly: there is no spoon.

7

If gravity were variable over such a short timescale, the Sun(which is in a delicate balancing act between gravitation and light pressure) would have been ** much** brighter or dimmer in the comparitvely recent past, an event which would show up in the Antarctic ice for a start.

I can’t really choose myself between the various dark matter candidates, as any choice might be blown out of the water by the next bit of research to be published, so here is my list of serious and not so- serious candidates which have been put forward in the past.
(Some categories may overlap)
Neutrinos
Axions
Photinos
Wimps
Machos
Black holes
Dyson Spheres
neutron stars
monopoles
cosmic string
domain walls
textures
the gravity leaking from adjacent branes or universes
mirror matter

Sci-fi worldbuilding at
http://www.orionsarm.com/main.html

8

Thanks, all.

I’m not gonna go to any trouble to defend the Dickerson Cosmological Hypothesis; but I am going to ask a few follow-up questions. (and hope that some of the original posters will come back to this thread and choose to respond further…)

(Anyone care to give the formula to determine the average distance between a selected point within a disk, and the rest of the disk?)

On testing for G-change in fossils: I suppose what is envisioned is that leg bones would have to be much thicker to fight-off stronger gravity, etc. So we might say something like this–any diminution of “G” over the last half-billion years would have to be at the low end. Would somebody care to estimate a “threshold of detectibility” based on paleo-evidence? Could we detect by examining, say, pleiseosaur fossils whether gravity was 130% of present back in their heydey? How about 200%?

But the real issue is the distance of the objects we use to determine that our galaxy is overmuscled G-wise. Is it purely the speed of rotation? Or does it have to do with the arrangement of distant objects billions of years away?

Well, my first option looks like a loser, but what about the exponent idea? I think I’ve read that equations are classified according to “orders” having to do with whether they have nothin’ beyond adding-subtracting-multiplying-dividing (first order) or include exponentiation or, er, rooting (second order): and then there are yet higher complexities (third order, etc). So maybe I have in mind that the standard grav formula needs a little high-level goosing that makes no easily-detectible difference at less than, say, distances like the galactic radius.

Aren’t there graph-curves that are “as close as you like” to X = Y^2 near the origin, then begin breaking loose rather radically somewhat further out?

These ideas may be silly. But wasn’t there a quasi-respectable suggestion a decade or two back that gravitation might turn mildly repulsive at inter-galactic distances?

I have nothing at all against “dark matter” in its various forms. But I can’t help thinking of Antony Flew and his “invisible gardener” parable. In the absence of fairly convincing evidence of its existence, will we go for decades or centuries trying to shore up the hypothesis? (There were increasingly complicated attempts to rescue the luminiferous aether before the concept was dumped.) “Constants” are called CONSTANTS because of the role they play in mathematical calculations, not because one absolutely must believe that they do not ebb and flow over large (to us) periods of time. Nature provides many examples of measurable entities than undergo periodic variation.

Well, this is not a religious thread. Make your comments; I’ll shut up eventually.

9

Why are you trying to find evidence of gravitational change in the leg-bones of Dinosaurs? We have much better markers than that.

We can see into the past today, by looking at distant stars. If their gravitational constant were different, we’d know it. The behaviour of the star would be very, very different. Stars of the same size would be more massive, and/or hotter. I imagine the emission spectra would be different. I’m sure there are plenty of other things we could look for.

10

Although G is hard to measure accurately, it’s not that hard to get limits on how it varies.

People started speculating about different exponents within Newton’s lifetime, so this isn’t exactly a new suggestion. It’s not precisely equivalent, but the current usual alternative to dark matter is MOND. This framed in terms of a difference given different accelerations rather than a different dependence on distance, but it’s duly similar. Anyway, there are at least two lines of empirical evidence against MOND.
Over and above the observational data, an r[sup]-2[/sup] dependence at large distances does follow rather naturally from GR.

11

Of all of eburacum’s candidates, this is the one I find most fascinating.

All of the other forces (electrostrongweak) act only in “our” universe, at “our” time.

Mind bomb of the day: Gravity acts through time as well as space. The increased gravitational force observed in our galaxies comes from the same galaxy in its past and future!!.

The first time someone explained that to me I almost dropped the joint.

12

Integrate over all possible positions and divide by the number of positions for a disc of radius R (thus area 2[symbol]pi[/symbol]R^2, given that any point will be a fraction of this (r/R) from the centre?

13

Could someone verify whether gravity acting through time as well as space is indeed true, or even better, proven mathematically and/or experimentally?

14

Well the speed of gravity is believed to be the same as the speed of light, but this has not been properly confirmed (there have been arguments about what a recent experiment that apeared to prove this actually proved).

I’m not sure what exactly Sentient meat is referring to, perhaps adavanced waves?

15

Apologies, the idea of “leaky gravity” is a mere hypothesis at present, with very little sound evidence to support it - I was merely relaying a cool, dark supposition to brighten up a dull Monday.

The Universe’s Unseen Dimensions (Scientific American, August 2000).

16

(The figure halfway down the above shows a “folded” membrane in which the effects of gravity can travel “forward in time” while light has to go the long way round.)

17

I was just about to say that i thought the brane theory didn’t mean gravity travelling though time, but i see now it sort of does in that gravity is effectively travelling faster than light.

This was one the most amazing theories i’ve ever heard about too. What got me was the fact that we might be inches away from say the andromeda galaxy, or a galaxy that looks like its billions of light years away.

Although now i think about it, wouldn’t it allow faster than light communication, and therefore be impossible because it could create paradoxes?

18

As I understand it (and I’m way more comfortable with Special compared to General, Relatively speaking) the c in vacuo limitation on communications is true for a given curvature of spacetime, but where different curvatures exist then a situation is possible whereby an object or wave does travel “faster than light” compared to an observer in flat spacetime. (General relativists please feel free to comment on which orifice you believe I am talking out of.)

This is the theory behind “warp” space, “wormholes” and the like. They are possible in the sense that certain formulations do not violate Einstein’s equations. However, neither does a naked, unaided human flying to the moon or a craft which can fly through the sun: It simply may not be physically possible.

19

Well, I can’t explain things any simpler, but if there was a concept in modern physics I’d nominate for the “ether” award, it would be dark matter. It shows all the classic signs of a paradigm shift.

20

Some cosmological questions for clarification:

  1. What scale does dark matter appear to operate on? Is it only at galactic scales (the observed motion of galaxies), or also at sub-galactic scales (the movement/behavior of parts of stars, nebulae, the solar system)? If it’s only galactic, where (literally, as where in space) would the dark matter need to be–concentrated around the galactic core, concentrated around heavenly bodies, dispersed evenly throughout the galaxy, dispersed evenly through the voids of intergalactic space, too?

  2. What scale does the expansion of space appear to operate on? Just between galaxies? I thought that, inside glaxies, no such expansion has been detected. What’s the critical distance? Aren’t the Local Group galaxies gravtiationally bound, and not expanding away from each other? Could all of this indicate that gravity, like, say, the strong nuclear force (weak at close range, strong at farther distances, nonexistent at even farther distances), behaves differently at different distances? Could gravity become a repellent force at some crucial distance?
    Also, as an aside–if we think space is expanding, why don’t we think that time is expanding as well?