http://orangequark.blogspot.com/2005/04/theories-of-cosmic-acceleration.html

http://preposterousuniverse.blogspot.com/2004/09/dark-energy-equation-of-state.html

Question: Is it fair to say in our mysteriously accelerating and occasionally preposterous Universe that Gravitation as a strictly Attractive force fails to sufficiently explain what’s going on? That there must be a corresponding Repulsion force at play or the whole interpretation goes up in flames? Based on the relatively new evidence we’ve accumulated about not just our expanding universe but a rapidly expanding one, in that it expands at ever greater rates as our conception of Time passes?

Feel free to shoo me away if this is an annoying question.

And you’re right, the denominator of that propagator looks like it is zero otherwise, which is from various points of view: why we redefine the two massless fermions into a single massive one as above, why we amputate diagrams with self-energy corrections to external legs (but that is the same thing I just said in a more general language), and (I think implicitly) what Sean and collaborators were worrying about in the original (three loop) version of the diagram.

Going back to the Higgs insertion, it doesn

http://root.cern.ch/

(the documentation is not great, but search for ‘Feynman’) though of course root itself is a much more sophisticated and general tool. I use it for all of my other plotting needs as well, so it works well for me.

Going back to the Higgs insertion, it doesn’t carry any momentum, and in fact you shouldn’t think of it as a real interaction at all… Think of any number of such insertions as having been already resummed into our definition of the fermion. That language is very useful in order to keep track of fermion chiralities (which in this example is what tells you the whole graph is proprtional to the fermion mass), but it is not really intended for serious calculations.

Oh, and Furry’s theorem works well for QED, but it has loopholes that apply to non-Abelian gauge theories, or a theory with chiral interactions (such as the electroweak theory of the Standard Model).

Tim