Mapping the peculiar velocities of stars

All things dark are all the rage is cosmology at the moment. There is dark matter—a type of matter that only weakly interacts with light. And dark energy—the label used to denote the observed increase in the rate of expansion of the universe. Our knowledge of what dark matter is and what dark energy denotes is woefully inadequate, opening up a theoretician's paradise. There are all sorts of models out there and, in the case of dark energy, they all have to fit one data point, making it kind of trivial to obtain a good result. In the meantime, astronomers are scrabbling around—in, yes, the dark—figuring out how to obtain more precise measurements of the increasing acceleration of the universe. HangZhou Night Net

In particular, there are a set of models that predict that the distribution of dark energy is not uniform, meaning that measurements of the velocity of stars at different distances and directions should be able to tell theoreticians whether barking up this particular tree is worthwhile. However, there is a problem: it is quite difficult to measure these velocities. Locally, astronomers use Type Ia supernova as references for distance and speed, but the further away the supernovas are, the weaker the signal, and the more significant confounding sources of noise become.

One source of noise is gravitational lensing, which causes an apparent change in the brightness of the supernova, resulting in an incorrect distance calculation. A pair of Chinese astronomers have now examined the problem and showed that the signature of gravitational lensing can be removed.

A gravitational lens will often smear the image of the star into an arc shape, depending on the relative location of the star, the lens, and the telescope. The behavior of the lens is relatively static and its influence can be calculated in two dimensions by examining the correlations between points on the image and calculating the spatial frequencies of those correlations—dark matter can be observed through this method.

However, this 2D power spectrum does not allow a correction to be made for the distance and velocity of the star. To do that, the researchers performed the correlation and power spectrum calculations in 3D. The supernova light has most of its power along the line of sight, while the lens power spectrum remains 2D and at right angles to the line of sight. This effectively separates out the contribution of the lens, allowing researchers to correct for gravitational lensing.

So, this seems like a pretty obscure bit of research to put on Nobel Intent, but I think it is important to show these slightly less sexy parts of the scientific process. Should dark energy models with a non-isotropic distribution of dark energy prove correct, measurements derived from observations of Type Ia supernova will play a critical role in confirming them. Before that can happen, these sorts of problems need to be solved.

To give you some insight into how important issue is to the astronomy community, during the time this paper was being written and going through peer review, four other papers on the topic were published or accepted for publication, presenting other ways to solve the same problem.

Physical Review D, 2008, DOI: 10.1103/PhysRevD.78.023006