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Originally Posted by coldcreation
Indeed, this situation seems to require the introduction of non-baryonic matter (a hypothetical form of matter not made of electrons, protons, neutrons, quarks, etc.). It is a well-known that much of the mass in the universe is invisible to our telescopes. Planets, rocks, asteroids, brown dwarfs, often called “massive astrophysical compact halo objects” (MACHOS), objects with very little (or no) surface luminosity. But these objects may be insufficient to appease the necessary constraints.
Only 20% of the dark matter in our galaxy is in the form of MACHOs.
True, then, there appears to be an additional budgetary problem, but I wouldn't count on something nonbaryonic until it can be demonstrably tested experimentally that such a bizarre form of material exists. As you know, I argue that there is no such thing. Like eather of the 19th century, I suppose we'll have to live with it for a while, until an alternative quantitative solution (aside from MOND) emerges that does away with the untenable concept. (There already exists a qualitative scenario  ). |
Well said. I don't see this as an unreasonable position. Non-baryonic dark matter is not an easy pill to swallow. It seems, though, more and more evidence is coming in that is consistent with non-baryonic and cold dark matter—like dark matter gravitational lensing. Meanwhile, more and more candidates for baryonic dark matter are ruled out as telescopes get better at looking for them.
The indirect evidence is speaking pretty loudly.
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Originally Posted by coldcreation
The difference is that there is no need for CDM or DE within solar system dynamics (GR works). The same cannot be said of a Friedmann universe, where GR needs to be supplemented liberally.
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Well, I would prefer GR work at all scales. As far as the solar system goes, I don't think the presence of dark energy or dark matter would really change the motion of the planets.
The cosmological constant part of gravity (dark energy) increases linearly with distance while the term responsible for gravitational attraction decreases quadratically.
This means there is virtually no contribution from the cosmological constant over small distances. The second term above is very nearly zero with a small r (r is distance). Over larger and larger distances the cosmological constant becomes a larger and larger factor. As r increases the first term above tends toward zero while the second becomes increasingly larger. So, dark energy which has a very noticeable effect on the evolution of the universe may have no noticeable effect on the motion of the planets. The following paper comes to that conclusion doing all the proper calculations:
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In this note we have discussed the possibility of constraining the cosmological constant  , in a general relativistic framework, with Solar System observations in view of the latest results in planetary orbit determinations. Contrary to what claimed by some authors, it turns out that it is not possible to get useful bounds on from such local scale tests.
http://arxiv.org/PS_cache/gr-qc/pdf/0602/0602095v2.pdf
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Also, non-baryonic CDM might not show up as gravitational effects in the solar system. Dark matter doesn't interact with ordinary matter so it wouldn't necessarily clump together with normal matter on scales so small. Dark matter would only affect the orbit of the planets if the concentration of dark matter in our solar system was greater than the concentration in the background of the galaxy. And, that is not the case:
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In this paper we have worked out the effects that a local excess of dark matter in our Solar System over the galactic background would induce on the orbits of the planets... The comparison with the latest data show that the upper bounds obtainable from the mean longitudes and the perihelia are of the order of 10^−20 g cm−3 and 10^−19 g cm−3, respectively.
http://arxiv.org/PS_cache/gr-qc/pdf/0602/0602095v2.pdf
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So, General Relativity can be consistent with our observations in the solar system as well as galactic and cosmic observations
if dark matter and dark energy do indeed exist. If they do *not* exist then we would expect solar system observations to be the same, but galactic and cosmic observations would be somewhat different that we have observed.
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Originally Posted by coldcreation
That doesn't mean GR is wrong. It could simply be that the FLRW metric is not the metric of choice when it comes to describing the universe. If the latter is the case, then there is our leeway.
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But, there can't be two solutions describing the same physical situation that give two different answers. That would be like solving GR and finding the surface gravity on earth should be g and solving it a different way and finding the surface gravity should be twice g. As far as I know, General Relativity (including any exact solutions to GR) give exact answers. I don't think we can just find a new metric, or try to solve GR differently.
A great example of this are the first two models of cosmology based on GR—that of de Sitter and Einstein himself. Einstein's metric had matter, was spatially closed, and had a cosmological constant. De Sitter's had no matter or radiation pressure, was spatially open, and had a cosmological constant. These two metrics described two very different situations, but the later development of Friedmann's metric (FLRW) could include both situations. In fact, it was shown that Einstein's and de Sitter's universe were two examples of a family of universes described by FLRW. It is now generally accepted that our universe is turning into a de Sitter universe as described by wiki here:
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Because our Universe has entered the Dark Energy Dominated Era a few billion years ago, our universe is probably approaching a de Sitter universe in the infinite future. If the current acceleration of our universe is due to a cosmological constant then as the universe continues to expand all of the matter and radiation will be diluted. Eventually there will be almost nothing left but the cosmological constant, and our universe will have become a de Sitter universe.
de Sitter universe - Wikipedia, the free encyclopedia
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Such a universe can be described with the de Sitter metric or the FLRW metric. Where they describe the same thing, they are the same. So, I don't think it's a matter of finding a new metric.
It seems, at this point, we either need to accept dark energy and dark matter as plausible or we need to figure out what has gone wrong with our cosmic solutions to general relativity.
Also, let me be clear—we're talking about the finer points of a big bang model here. The integrity of "the big bang" (i.e. the primordial atom) is not IMHO in jeopardy. The evidence for a big bang persist even if ΛCDM ends up being completely broken. If a person uses math to model a car crash and the model ends up being wrong that doesn't mean the car crash didn't happen. The broken glass and skid marks and whatnot are evidence of the crash with or without the model.
~modest
