Here is a quote from theJuly 19 by the Sloan Digital Sky Survey
http://www.sdss.org/news/releases/20030721.darkenergy.html
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SDSS collaborator [Andrew] Connolly said if the depth of the gravitational well decreases while the photon travels through it then the photon would exit with slightly more energy. "If this were true then we would expect to see that the cosmic microwave background temperature is slightly hotter in regions with more galaxies. This is exactly what we found."
[Albert] Stebbins added that the net energy change expected from a single concentration of mass is less than one part in a million and researchers had to look at a large number of galaxies before they could expect to see the effect. He said that the results confirm that dark energy exists in relatively small mass concentrations: only 100 million light years across where the previously observed effects dark energy were on a scale of 10 billion light years across. A unique aspect of the SDSS data is its ability to accurately measure the distances to all galaxies from photographic analysis of their photometric redshifts. "Therefore, we can watch the imprint of this effect on the CMB grow as a function of the age of the universe," Connolly said. "Eventually we might be able to determine the nature of the dark energy from measurements like these, though that is a bit in the future."
"To make the conclusion that dark energy exists we only have to assume that the universe is not curved. After the Wilkinson Microwave Anisotropy Probe results came in (in February, 2003), that's a well-accepted assumption," [Ryan] Scranton explained. "This is extremely exciting. We didn't know if we could get a signal so we spent a lot of time testing the data against contamination from our galaxy or other sources. Having the results come out as strongly as they did was extremely satisfying."
The discoveries were made in 3,400 square degrees of the sky surveyed by the SDSS.
"This combination of space-based microwave and ground-based optical data gave us this new window into the properties of dark energy," said David Spergel, a Princeton University cosmologist and a member of the WMAP science team. "By combining WMAP and SDSS data, Scranton and his collaborators have shown that dark energy, whatever it is, is something that is not attracted by gravity even on the large scales probed by the Sloan Digital Sky Survey.
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In case anyone wants to see a preprint of their technical article:
http://arxiv.org/astro-ph/0307335
"Physical Evidence of Dark Energy" Scranton et al.
Ryan Scranton quoted here is team leader of the project. A photon gains energy by passing thru a gravity well while the well is getting shallower according to what is called the "Integrated Sachs-Wolfe" effect. It gains energy going down into the well and then does not have to give up all of it while coming out, so it has some left. Which the dudes measured.
