If you are still trying to get to grips with the idea that the Universe's expansion is speeding up, check out Schrabback and colleagues' scrutiny of the largest continuous area ever imaged with the Hubble Space Telescope — the COSMOS field (T. Schrabback et al. Astron. Astrophys. in the press; preprint available at arXiv:0911.0053). Their study is not only one of the most comprehensive analyses of the 'weak gravitational lensing' effect that is caused by large-scale structures in this distinctive part of the sky, it also uses this effect to provide independent evidence that the Universe is indeed expanding at an increasing rate.
Evidence for cosmic acceleration has been collected, for example, from analyses of stellar explosions known as type Ia supernovae and from studies of the cosmic microwave background radiation (relic radiation from the Big Bang). But when it comes to testing the Universe's dynamics and its fate, the more evidence the better. Different techniques are subject to different uncertainties, so any independent test is most welcome.
So what is the gravitational lensing effect? Picture a distant object in the sky. If there is nothing between the object and an observer on Earth, the observer sees one image of the object. However, if there is a mass concentration in the foreground — the lens — the object's light is deflected by the gravity of that mass. In the strong regime of this effect, the observer sees multiple images of the background object; in the weak mode, the observer sees a subtle distortion of the object's shape.
In their study, Schrabback et al. combine space-based measurements of the image distortions of more than 400,000 galaxies in the COSMOS field with ground-based estimates of their redshifts to infer the foreground mass — mostly invisible — in this field. They find that the statistical properties of the deduced mass distribution (pictured; white, cyan and green denote distances typically closer to Earth than do orange and red) can be best explained by a cosmological model in which the Universe's expansion is accelerating.
If that isn't enough, the authors also show that the way in which the weak lensing effect changes with redshift is in accord with what is expected from Einstein's theory of general relativity. What remains to be addressed is the nature of the Universe's most bizarre component, dark energy, which is thought to be driving the cosmic acceleration.