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Originally Posted by coldcreation
Actually, general relativity has passed every test tossed in its direction (except for those related to gravitational waves). It seems that GR is not to blame for the standard model mishaps. Galactic rotational curves being largely flat (e.g.) does not make GR wrong, nor does it mean CDM is responsible.
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In fact, galactic rotation curves leave 2 possibilities:
- GR is wrong
- there is unaccounted-for mass
The only way to make galactic rotation curves agree with General Relativity is to add mass because GR itself cannot be tweaked—it cannot be adjusted. This is something you often disagree with for one reason or another, but it isn't open to interpretation. GR is exactly demanding.
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Furthermore, the predictions of general relativity are fixed; the theory contains no adjustable constants so nothing can be changed. Thus every test of the theory is either a potentially deadly test or a possible probe for new physics. Although it is remarkable that this theory, born 90 years ago out of almost pure thought, has managed to survive every test, the possibility of finding a discrepancy will continue to drive experiments for years to come.
The Confrontation between General Relativity and Experiment (section 7: conclusions)
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General Relativity is composed of non-linear partial differential equations that are extremely difficult to solve for any real-world situation. It is, however, possible to find *exact* solutions to GR:
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The Einstein field equations are nonlinear and very difficult to solve. Einstein used approximation methods in working out initial predictions of the theory. But as early as 1916, the astrophysicist Karl Schwarzschild found the first non-trivial exact solution to the Einstein field equations, the so-called Schwarzschild metric.
-General relativity
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The Schwarzschild metric is an exact solution to GR. It gives exact answers in the setting of a non-rotating spherically symmetric mass. It is, therefore, useful when considering something like a planet or a solar system. There are other such exact solutions and the FLRW metric is one such non-trivial, exact solution.
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The best-known exact solutions, and also those most interesting from a physics point of view, are the Schwarzschild solution, the Reissner-Nordström solution and the Kerr metric, each corresponding to a certain type of black hole in an otherwise empty universe,[41] and the Friedmann-Lemaître-Robertson-Walker and de Sitter universes, each describing an expanding cosmos.
-General relativity
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From a physics standpoint, this has a lot of meaning. The same theory that predicts gravitational effects in our solar system to extraordinary precision also predicts the behavior of the FLRW metric (i.e. a Friedmann universe). Any observation of a homogeneous, isotropic universe that obeys the physics of General Relativity *must* agree with the FLRW metric or General Relativity is proven wrong by example. There is no leeway on this.
Our cosmic observations do indeed agree with the FLRW metric if the makeup of the universe is currently 74% vacuum energy density and 26% mass density as a ratio to the critical density. Either this is not the makeup of our universe, the universe is not homogeneous and isotropic, or FLRW and by extension GR are wrong.
The only Leeway FLRW affords is setting the values (the Omegas) which is the same as declaring what our universe is made of. That's it. You seem to be confusing the setting, measuring, and changing of these parameters with a change to the underlying physics. Notice:
Ned Wright's Javascript Cosmology Calculator
You can set Omega-M and Omega-Vac and get a prediction at a chosen redshift. Pre-1998 cosmology can be solved on this calculator as well as the current Lambda-CDM model, because they use *exactly* the same physics.
~modest
