# Difference between Dark Energy and Cosmological Constant

1. Sep 22, 2009

Can someone remind me what the difference between the Cosmological Constant ($\Lambda$) and Dark Energy (DE)? Doesn't DE (with w = -1) show up in the Einstein's field equations in exactly the same way as $\Lambda$ does?

(In case your an expert, I was trying to understand the conclusion of http://arxiv.org/PS_cache/arxiv/pdf/0909/0909.3853v1.pdf" [Broken] paper)

Last edited by a moderator: May 4, 2017
2. Sep 23, 2009

### Chalnoth

"Dark Energy" basically means the (currently unknown) cause of the observed accelerated expansion. The cosmological constant is a specific proposal for what dark energy might be.

Dark energy with w = -1 is considered to be the exact same thing as the cosmological constant.

What they're saying in the conclusion there is that there's a tentative signal that may indicate that the correct explanation for the accelerated expansion may be modified gravity, as opposed to something like the cosmological constant or a scalar field.

Last edited by a moderator: May 4, 2017
3. Sep 23, 2009

I don't think the author is claiming a detection of modified gravity. In the conclusion she says, "The COSMOS data gives the first signal that dark energy might be a modification to GR, rather than $\Lambda$." I'm guessing she's saying that the equation of state parameter, w, evolves with redshift, which is inconsistent with $\Lambda$.

4. Sep 23, 2009

### Chalnoth

No, she's using a different parameterization of gravity itself, not of some matter/energy with w different than -1. Weak lensing constrains possible deviations from General Relativity by carefully tracing the growth of structure, as there are specific relations that GR must follow, whether or not there is some form of matter out there with w < -1/3 (w < -1/3 is required to explain the observed acceleration).

5. Sep 23, 2009

### Wallace

Exactly right. There are relationships that must hold between the distance history and structure formation if GR is correct. This study is probing those relationships, not w itself as such.

There is a slight problem that some people use the term 'dark energy' to mean 'all models than can be described by some function w(a) within GR' while others use it more broadly to mean 'whatever is causing the apparent acceleration'. The author is this paper is clearly using the latter definition when saying "The COSMOS data gives the first signal that dark energy might be a modification to GR, rather than $$\Lambda$$".

It is a very interesting paper that is sure to provoke similiar studies with other data sets, but I suspect there won't be many people convienced by the results just yet.

6. Sep 24, 2009