What is Degeneracy of Dark Matter?

[priced02]

Hi everyone - I'm having some trouble with the real concept behind "degeneracy" of dark matter.

I've heard of degeneracy before in maths (eg. a 'point' is a degenerate 'circle'), and in physics (eg. energy level degeneracy in atom shells), but I don't quite see how either of these work when we're talking about dark energy. From what I understand, in this context degeneracy is a 'bad' thing - but is it anomalous to error, arbitrary-ness, or the maths/physics definition?

 

[Wallace]

Degeneracy dosn't mean 'bad' as such, but you will need to be a bit more specific in your question since there are several things you could be talking about in your question. Can you give an example of where the term was used that confused you?

 

[priced02]

Some examples

From "The Dark Degeneracy: On the number and nature of dark components" - Martin Kunz.

"We use that gravity probes only the total energy momentum tensor to show how this leads to a degeneracy for generalised dark energy models. Because of this degeneracy, Omega-m cannot be measured."

"Gravity therefore only constrains the total w(z). Any further freedom, like sub-dividing the dark EMT into dark matter and dark energy, or introducing cou-
plings between the dark constituents, cannot be directly measured and will introduce degeneracies."

 

[Wallace]

Okay, so what that means in that context is that there are many different models for dark energy and dark matter that look the same observational, so by degenerate Kunz means that you can't distinguish them hence you cannot know which theory is correct. I.e. do you have this much stuff that behaves in this way or a different amount that behaves in a different way per unit density such that the effect on the expansion is the same?

I remember reading that paper when it was first published and thinking that they didn't show well enough how the same degeneracy exists in structure data as exists in supernovae data. It's very exist to construct a variety of models that give the same a(t) (which is all that SN probe) but that have different structure statistics.

In the end though model degeneracies is an issue for dark energy (and cosmology generally) at the moment, since there is no micro physical theory or direct detection in the lab. There is essentially infinite freedom to make up phenomenological models without having to have a rigorous physical theory to explain the reasons behind the behavior.