Dark Matter and the Fate of the Universe

Factual Questions
1

First, let me say that’s a bit late, and my brain is somewhat foggy as a result, so I hope this doesn’t sound too idiotic…

OK. The “Ultimate Fate of the Universe”, as I understand it, pretty much depends upon two things: the rate of expansion and the average density of the universe. With respect to the density, if it is greater than some critical value, gravitational forces will ultimately stop the expansion and contraction will ensue. Similarly, if it is below this critical value, the universe will expand indefinitely.

Now, when we add up all the observable mass in the universe, we get numbers that are only about 10% what is required for even the lowest current estimated expansion rates; some matter is apparently missing if observations are to match calculations (the calculated mass of the observable stuff doesn’t account for all the observable gravitational effects). Thus, “Dark Matter” is postulated to account for this missing matter. However, even with the addition of this Dark Matter, we are still below what is required for the lowest expansion rates (stop me if I’m completely off here!). Scientists appear to be at a loss to explain this.

So…here’s what I’m thinking: what if energy is factored in? My interpretation of Einstein’s famous equation, E=mc[sup]2[/sup], is that energy and mass are basically interconvertable. Or, put another way, the ratio of energy to matter, if we look at the entire universe, is a constant. So, if we could determine the amount of energy out there, couldn’t we determine its effective mass, if it were condensed to matter? And might not this mass account for the missing mass? Granted this is probably way out there, but, hey, I was just wondering :slight_smile:

In a related vein, if energy and mass are interconvertable, is it conceivable that energy can be affected by gravity? Or, perhaps more appropriately, can a sufficiently large concentration of energy have the same effects on space-time as an equivalent mass?

I apologize if these sound idiotic to those of you who actually know something about this stuff (I’m only now getting around to reading Hawking), and hope you take into account my addled brain before you tear me to shreds too harshly :slight_smile:

2

I don’t know about dark matter, but energy certainly can be affected by gravity. Light passing the event horizon of a black hole will be “sucked” in. (That’s why they’re black…hehehehe). Also, galaxies can cause “gravitational lenses” which bend light from surrounding galaxies like a glass lens does.

3

I always thought energy was accounted for in total mass calculations or shown to be so small that it is effectivally 0, but good question why ASS-U-ME. the light from given off by a star millions of light years away must add up to some serious mass.

4

re: Dark Matter - - isn’t its existence postulated on the fact that the observed rotation rate of the galaxy(s) cannot be accounted for by the amount of observable matter in the galaxy(s)? (i.e., not enough observed matter to account for the observed gravity)

Perhaps it also has something to do with the critical density calc (anyone else have some info?)…but it seems that the amount of matter being less than the critical mass coincides with the finding that the universe is flat (eternally expanding).

k2dave - starlight used to be matter (emitted by the fusion of hydrogen), so I think it’s already accounted for.

5

Sounds like you understand the problem very well - I couldn’t have stated it better. I think it’s a bit much to say “Scientists appear to be at a loss to explain this” - as far as I know, there are several theories which seem reasonable enough, but not enough data to choose one over the other. So in that sense it’s still a mystery, but “at a loss to explain” is when you have good data and no theories which fit the data.

As for the energy density of photons - according to the Handbook of Space Astronomy and Astrophysics by M. Zombeck, the energy density of visible light photons is only 0.01 eV/cm[sup]3[/sup]. Cosmic microwave background radiation is a lot more, at 0.4 eV/cm[sup]3[/sup]. There’s a question mark on infrared, says “may be greater than cosmic background.” All the rest are much smaller than these. So we’re talking about 1 eV/cm[sup]3[/sup] total energy density. On the other hand, density of matter is 100 eV/cm[sup]3[/sup]. So as k2save said, it is taken into account, and it is very small.

You can check the book yourself, by the way. The whole book is online. It’s an awesome reference, and I’m not just saying that because it was written by my former advisor.

6

I, Mr. Humble 'Lectrician, am probably not worthy to hang on this thread, but I have to ask a silly question on the subject. We are all familiar with the doppler shift and sonic booms and acoustic stuff like that, and I presume that the same sort of thing happens with light, and this is the way we figure out how far away celestial objects are and whether they are moving toward us or away.(“Blue Shift”, I think it’s called) Now I know the Celestial Speed Limit being the speed of light, so no object having any mass can exceed this. However, if two celestial objects were moving toward each other, and each were moving at, say, half the speed of light-Would they not see each other? (I spose you could say the same for two objects moving away from each other at half the speed of light.) Think of it the same way as a plane breaking the sound barrier-you don’t hear it until it PASSES you.
P.S. Be Merciful-and Simple.

7

The speed of light differs from the speed of sound in at least two significant ways. As you said, nothing with mass can achieve or exceed the speed of light. And all observers, no matter what their state of motion, measure the speed of light as the same value relative to themselves.

The first fact means that there is no light analog of a sonic boom or not hearing something approaching you until it’s passed. Both of these require going faster than the speed of the wave, and that can’t happen. So there’s no “light boom” and the two approaching celestial objects will see each other no matter what their speed.

(I think that there’s another reason for no “light boom”; light doesn’t involve vibration of a medium. But the original reason is sufficient).

What gets interesting is when the two approaching celestial objects will see each other. Because everyone measures the speed of light as the same, different people will get different answers. (How this follows may not be immediately obvious, but it does follow; I can try to explain further if you wish). A person (call her A) riding on one of the celestial objects will get one answer, a person riding on the other celestial object (call him B) will get a different answer for when A sees B, and person C who thinks both objects are moving will get another answer. All these answers are correct and consistent; time is plastic.

Oh, and “blue shift” refers to the change in the light from something approaching us, and “red shift” refers to the change in the light form something going away from us. Sometimes they’re Doppler shifts caused by the motion) and sometimes red shifts are not really Doppler shifts; the shift (and the apparent motion) is due to space nd everything in it stretching out.

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Mauve Dog , this subject is way beyond me but you may find this thread interesting(well ,some of the links).

Is the universe accelerating instead of slowing down ?

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Mauve Dog
An amount of energy E has the same gravational effect as a mass of E/c[sup]2[/sup]. When refering to the observable mass, energy is taken into account.

gapcity
If two observers A and B are approaching each other at 1/2 c, and A emits a light signal, A will observe the signal traveling at c relative to A. B will measure the speed of the signal’s speed to be c relative to B. If A looks at B’s meter sticks and clocks as he passes B, A will explain this discrepancy by noticing that B’s meter sticks have contracted in the direction of motion and B’s clocks seem to be running slow. B will conclude the same about A.

The fact that the speed of light is the same for all observers implies that two different observers may disagree when measuring length and/or duration. It also implies that simultaneity is relative; two observers may disagree as to whether two events occurred at the same time or not.

10

This is rather what I was getting at when asking the question, but I wasn’t sure whether such had been taken into account.

On similar lines to my original question (re: the fate of the universe and all that):

Newtonian gravitational theory basically assumed that gravitational forces were instantaneous. Einstein’s General Theory of Relativity states (among other things) that nothing may move faster than the speed of light; his Special Theory states that gravity is the result of mass warping space-time. This would, of course, imply that gravity has a finite speed, if you will.

Consider this example: The nearest star to our sun is Prixima Centauri, about 4 light-years away. Suppose that some aliens (the Vogons or some such) come along and zap Proxima out of existence. We would see that Proxima Centauri is missing until four years after the fact. But, more than that, we wouldn’t notice any gravitational effects until that time either, since nothing can travel faster than the speed of light, including the ‘force’ of gravity. Similarly, if our own sun vanished, not only would we not see that it had happened for about eight minutes, but we wouldn’t go flying off into space until after then as well.

Now, this would seem to have some implications in determining the fate of the universe, since it is therefore not possible for every object in the universe to attract every other object. Distant objects simply wouldn’t have had time to ‘know’ about each other’s existence! This would seem to severely limit the maount of mass that can be brought to bear, so to speak, on the expanding universe question. So, am I really off base with this one, then? Or have I brought myself a step closer to ultimate power (knowledge = power, ultimate knowledge = ultimate power!) with this understanding?

(This is, by the way, something that came to me while reading Hawking’s book, though it was never explicitly stated as such).

11

I think it more pertains to the overall influence of matter on the totality of space (not matter attracting matter). i.e., gravity needs to slow the expansion of space itself in between galaxies, not prevent other galaxies from getting farther away buy direct gravitational attraction.

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Ok, first off, the minor quibble: Mauve Dog got the special and general theories of relativity mixed up. Special deals with speeds, general deals with gravity. He got the details right, though.

As to dark matter: There’s two phenomena you can be referring to with that term. First, we have galactic dark matter, which we infer from the motion of stars in galaxies. We know for an absolute fact that that exists, and it’s probably relatively normal stuff, just dark. Then, there’s cosmological dark matter. You need a certain density for a flat Universe, which theorists like. If you take all the visible matter + all the galactic dark matter, you’re still only at about 10% of the necessary density. One may reasonably ask, of course, who cares what theorists like, but lately, the observationalists have started to agree on this point: The Universe is flat, so we need cosmological dark matter. Currently, the best candidate is the cosmological constant, which isn’t exactly matter, and may be vacuum energy (what you’ve really got when you’ve got nothing), but regardless of just what you consider it to be, it can do the job of helping to flatten out the Universe. Current estimates are that approximately 30% of the flattening is due to ordinary matter (larger than the number for galactic dark matter because it’s not all in galaxies), and 70% is due to the cosmological constant, which makes theorists and observationalists happy.

What’s interesting, is that while the cosmological constant has the same effect as matter on the curvature, it has the opposite effect on the ultimate fate of the Universe. A flat universe, or even one with a slight positive (closed) curvature, with a positive cosmological constant, will eventually go into a phase of exponential expansion, never to return or slow. What’s really wild, is it looks as if our own Universe may have already entered this phase.

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Chronos: “We know for an absolute fact that that exists, and it’s probably relatively normal stuff, just dark.”

Nothing new to say, but I always found it interesting that while I tend to think of dark matter as this mysterious, exotic stuff, you, me, and the entire Earth are all comprised of dark matter. Pretty mundane.

14

Some dark matter is mundane. But some might be exotic (for example, the hypothesized, but yet-to-be-discovered Weakly Interacting Massive Particles - WIMPS)

15

Scientific American quietly tucked away a little article in the August edition which could have explosive ramifications.

Here is a link to a news item that speaks to it. Unfortunately, http://www.SciAm.com isn’t posting this article online.

As a math idiot, I can only paraphrase what the article appears to imply (I haven’t read the full article yet). It would appear that if one postulates gravity working in a multidimensional fashion, the problem of dark matter might just prove to be no problem at all. According to this theory, gravity exerts a force of similar strength to the other three forces, but it is dissipated through multidimensional space.

Having said that, I’d sure appreciate the opinion of someone who knows what he/she is talking about regarding this article. Any takers?

16

Yeah, I saw that article, too, but I haven’t had a chance to read it yet. Folks have been proposing similar stuff for years, but what would really have explosive ramifications, is if someone proposes an experiment or obseration to test it. I’ll read up on what they have to say and report back hopefully tomorrow.

As for exotic dark matter, on the galactic scale, MACHOs (Massive Compact Halo Objects; Jupiter-like things) account for at least 30% of the dark matter, last I read, and the results are consistent with them being most or all of it. We can’t rule out WIMPs, but we don’t seem to need them, so Occam’s Razor applies. On the cosmological scale, however, normal baryonic matter just can’t cut it, and you can’t get much more exotic than vacuum energy.

17

Having still not read the article, but having now read several articles about the article, the theory looks good on the surface (to an idiot like me who failed physics and chemistry in college). It basically proposes that the force of gravity projects itself into “looped” or “folded” dimensions where the other forces do not act. Thus, if space folds on itself and almost touches, gravity can have an effect on that part of folded space adjacent to the source. Alternatively, other universes that do not otherwise interact with our own could be trading gravity forces.

The neat part about the theory is that it appears to be testable by experiments that are already in progress or in the works, by measuring the force of gravity at distances below one milimeter. Should we discover that G is not constant at extremely close distances, as this theory predicts, we could find ourselves onto something very big, indeed.