Dark Energy/Matter: Why Does It Make Sense?

[kindlin]

Why does it make more sense to imagine so much dark energy and dark matter to make things work (cosmologically speaking). Why can't gravity change? It just seems to make a lot more sense to alter one theory, than to start imagining all of this extra none-interacting material, which, oh , btw, is affected by gravity, and oh, btw, space itself is full of dark energy that's flinging everything away! muahaha

 

[Dale]

This is a pretty common thing in the history of science. Whenever you have a model which has been experimentally confirmed to work well in a wide variety of known circumstances then when you find a deviation you first look for unknown sources in the existing model before looking for a new model.

For example, both Neptune and Pluto were discovered because of deviations from the predictions of Newtonian gravity. Similarly, the neutrino was discovered because of deviations from the predictions of conservation laws.

For the specific case of GR, we know that it works well in a wide variety of circumstances, and we also know that we don't have an independent way to measure the total energy and matter content of the universe. Therefore it makes much more sense to assume that our experimentally validated model works and use that model to estimate the unknown portions of the matter and energy content of the universe than it would make to assume that the unknown portions are exactly 0 and make a new model to fit that assumption.

The difference between dark matter/energy and the previous examples is simply that we have not yet discovered the corresponding Neptune and neutrino.

 

[shahbaznihal]

Also, alterations to our understanding of gravity on cosmological scales hasn't been very promising. It is not sufficient to explain all the discrepancies.

 

[Chalnoth]

Why does it make more sense to imagine so much dark energy and dark matter to make things work (cosmologically speaking). Why can't gravity change? It just seems to make a lot more sense to alter one theory, than to start imagining all of this extra none-interacting material, which, oh , btw, is affected by gravity, and oh, btw, space itself is full of dark energy that's flinging everything away! muahaha

Lots and lots of physicists have tried to modify gravity in such a way that it can explain our observations. So far, all such attempts have failed. Here is one example of a rather visceral observation which is quite a strong demonstration of the existence of dark matter:
http://blogs.discovermagazine.com/cosmicvariance/2006/08/21/dark-matter-exists/

To be fair, some people who have been working with modified gravity models have also been able to fit their model to the above cluster. But they could only do so by adding a fourth type of neutrino (i.e., a different kind of dark matter).