Talk:Ricci curvature
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How exactly can the Ricci tensor be thought of as a Laplacian of the metric? Certainly, not as the covariant Laplacian ∇*∇ since ∇g = 0 for the Levi-Civita connection, and not as the Hodge-Laplacian (dd* + d*d) which is defined only on differential forms. I can't find a statement like this in any of my Riemannian geometry books. -- Fropuff 18:17, 2004 Oct 8 (UTC)
- it is bit informal, in the exponential coordiantes at one point Ricc_{ij} will be usual laplasian of g_{ij} at this point.
The correct statement is that in harmonic coordinates, the components of the Ricci tensor satisfy
,
where 
is quadratic in the components of g, and also involves first derivatives of these components, and where Δ is the Laplace-Beltrami operator of g. Here the phrase harmonic cordinates means we are using a coordinates chart in which each coordinate xj satisfies Δxj = 0. This formula was used by DeTurck to show that any (Riemannian) Einstein metric is real-analytic in harmonic coordinates, and is also used extensively e.g. in proving Anderson's pre-compactness theorems for the space of Einstein metrics.
70.18.30.252 11:11, 7 August 2006 (UTC) Riemannian geometer
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- BTW, I do not like your edit very much, you made it more formal and therefore less accessible to beginers. In addition you use nonstandard convention, usually Ricci tensor is an opreator on tangent space and not a quadratic form. Tosha 01:16, 10 Oct 2004 (UTC)
I disagree, I think the Ricci tensor is more generally regarded as a bilinear form than an operator on the tangent space. Besides, one needs a metric in order to define it as a operator. The definition of the bilinear form is independent of the metric (i.e. it works for any connection, not just the Levi-Civita connection) and thus more fundamental. -- Fropuff 02:32, 2004 Oct 10 (UTC)
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[edit] Proposed merger
The article on the trace-free Ricci curvature is silly as it stands. It assumes the manifold has dimension 4, but never says so!! In dimension n, the correct formula is 
.
70.18.30.252 11:11, 7 August 2006 (UTC) Riemannian geometer
The article Ricci tensor quite properly focuses on Riemannian geometry. If the merger took place, for stylistic reasons it may not be good to insist on clearly distinguishing between properties relevant to gtr and those irrelevant to gtr. So I'd suggest instead that whoever wanted to merge write an article on Ricci tensor in general relativity and merge the traceless Ricci tensor into that article. If you can't guess what I have in mind for Ricci tensor in general relativity, maybe best leave it to me (but be very patient, it might take 12 months for me to get to this, since there are so many more urgent tasks awaiting me just in the gtr pages.)---CH 08:36, 31 January 2006 (UTC)
- It might be best to just start with a general relativity section in this article. If and when that section grows too large we can move it to its own article. For now I agree that trace-free Ricci tensor should be merged in here. -- Fropuff 16:46, 31 January 2006 (UTC)
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- What fropuff said. --MarSch 13:19, 23 April 2006 (UTC)
[edit] Is this English?
This is one of the most difficult articles on Wikipedia! As Ricci tensors are used to formulate the Einstein field equations, they are of fundamental importance even to laymen. Can someone make this article a little more accessible? At least tell us poor people without graduations in physics/math what a Ricci tensor is? Loom91 18:11, 15 March 2006 (UTC)
One simple way of explaining the Ricci tensor is to say that in geodesic normal coordinates the volume form is
[ 1 - (1/6) Ric_ij x^i x^j + O (|x|^3)] dx^1 ... dx^n
In other words, the Ricci tensor measures whether the volume of a geodesic cone is larger or smaller than what it would be in Euclidean space. I have appended this to the article, though it might be better to move it to the beginning of the article. (However, I will leave it to someone with administrative privileges to decide whether or not this is a good idea.)
I don't know why someone thought they should discuss conformal changes in this article, but in any case the gven formula for the change of the Ricci tensor under conformal change is absolutely incorrect! The leading factor of e^{2f} simply should not be there. The correct formula can, for example, be found at the top of page 59 of Besse's "Einstein Manifolds." --- A Riemannian geometer. 8/5/06
- I agree, but since I am on the verge of quitting WP I am not about to try to fix this. The basic problem is that the Ricci tensor is fundamental in both Riemannian geometry and Lorentzian geometry (particularly for the gtr application), but this background tends to pull the article in somewhat directions from the point of view of notation and what to emphasize. (Buzzwords from Riemannian geometry include Ricci flow and comparison theorems such as the Bishop-Gromov comparison theorem, while buzzwords from general relativity include Raychaudhuri equation. A key point here is the profound implications of positive definite signature, in the case of Riemannian geometry.)
- To (not) answer the question asked, the Ricci tensor is a kind of "trace" of the Riemann tensor (which has various purely geometric interpretations). Therefore, the components of the Ricci tensor can be thought of as a kind of "average" of Riemann components. In the case of Lorentzian manifolds interpreted as models in general relativity, the Riemann and Ricci tensor also acquire some physical interpretations. See for example Gravitational curvature by Theodore Fraenkel, an undergraduate level gtr textbook which offers a particularly extensive discussion of some of the best known interpretations.---CH 19:58, 15 March 2006 (UTC)
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- Yes, this is a nasty article. Oh well. Chris -- don't leave. Just try to invest less time and emotional energy into WP, so you don't have to get angry and feel like quiting. linas 00:44, 16 March 2006 (UTC)
[edit] Joachim Lohkamp's Result
Couldn't get a readable version of his result, but it can't be right as quoted. The Cartan-Hadamard theorem states that any manifold that can be given a metric of nonpositive scalar curvature must have universal cover R^n. S^n (n>2) is a counterexample. But (I believe) the result might be right for AVERAGE curvature.
No, no, no! Lohkamp's theorem is correct as stated. Cartan-Hadamard is about negative SECTIONAL curvature. Scalar curvature is at the other end of the spectrum. The existence of metrics on negative SCALAR curvature on any manifold of dimension > 2 was proved independently by Aubin and by Berard-Bergery in in the mid 1970's. Ricci is harder, but already the existence negative Ricci metrics on any 3-manifold (and in particular, on the 3-sphere) was already proved by Gao and Yau in the early 1980's. This is a good case to try to wrap your mind around, because in dimension 3, the Ricci tensor algebraically determines the sectional curvatures ---- but negative Ricci does not imply negative sectional! ---Riemannian geometer, 8/5/06
[edit] A small erroneous statement
It seems to me that the statement "Because the Levi-Civita connection is torsion-free, the Ricci tensor of a Riemannian manifold is symmetric, in the sense that Ric(ξ,η) = Ric(η,ξ)" is not correct.
In fact, a symmetric linear connection (torsion free) does not have necessarily a symmetric Ricci curvature as the following example shows:
In Rn, let's define a symmetric linear connection (covariant derivative) by the formula:
(\nabla_{A} B) = B'.A + Γ(A,B)
where Γ(A,B)(x) = (A·x)(B·x)u, where · stands for the standard scalar product on Rn and u is a fixed vector.
One can easily compute the Ricci curvature of that symmetric linear connection and find:
Ric(A,B) = (u·x)(A·B) - (A·x)(B·u)
which is not symmetric because of the term (A·x)(B·u).
Hence I would suggest to replace that statement by: "The Ricci tensor of a connection derived from a metric is always symmetric, in the sense that Ric(ξ,η) = Ric(η,ξ)".
Boris Kolev (France)
