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Feb3-09, 12:33 AM
Sci Advisor
P: 4,805
Quote Quote by Peter Watkins View Post
In 1929 it was observed that, with rare exception, galaxies in all directions are exhibiting degrees of red-shift that increase with distance. This information alone is sufficient to, (a), describe the structure of the universe, (b), state that collapse is inevitable, and, (c), predict that the rate at which galaxies separate will increase. This being the case, why was the "dark energy" theory ever put forward? It is totally unnecessary.
The point is that gravity, when combined with a model for the contents of the universe, provides a very specific prediction for the relationship between redshift and distance (among a great many other things which I won't go into here...). If we assume that normal matter plus dark matter are the only contents in the universe, we get the wrong answer. Clearly this means one of two things:
1. Our model of gravity is incorrect a very large distances (potential solution: modified gravity).
2. Our understanding of the contents of the universe is incorrect (potential solution: dark energy).

Both are being investigated.