2dF QSO Redshift Survey (2QZ)

One of my main projects in recent years has been the 2dF quasar redshift survey, which produced the largest ever homogeneous quasar survey, with 23,000 redshifts (the Sloan Digital Sky Survey now has more QSOs but is not yet complete). The survey only covers part of the sky: there are two surveyed regions, each of which is a slice through the universe, and one of those slices is shown here.

Hopefully you can see this video (created by Edd Edmondson). The picture below and the video show the largest three-dimensional structures ever seen in the universe. The overall wedge shape is just the area covered by the survey. The red blobs are where there is an overdensity of QSOs from the 2QZ survey, blue regions are where there's an underdensity (a "void"). The scale is shown on the picture below, which is a 2D slice through the survey. The surveyed region extends more than 2000 Mpc, about two-thirds the size of the observable universe. The structures that can be seen may be up to 500 Mpc (1500 million light years) across but correspond to enhancements in density of less than 30 percent: these are huge structures that are just starting to collapse under their own gravity over a period of about ten billion years.

There has been debate for decades about whether such large structures exist and can be detected in the distribution of QSOs. I can remember coming back on a plane from La Palma in 1989 with Paul Mitchell convinced that we'd found the largest structures in the Universe (we hadn't, it was just noise). It has taken a huge survey such as 2QZ to obtain a large enough statistical sample to enable structures to be seen. And now we can quantify what it means: the excess numbers found are just what is expected from the standard model for large-scale structure in the universe, the "biased Lambda cold dark matter model".

One of the other, related, main quantitative results of this study is the ability to measure the spatial power spectrum of the distribution of quasars on the largest scales yet probed by any redshift survey. Because quasars can be seen at significant cosmological distances, we can also measure how the strength of clustering has varied with cosmic epoch. Surprisingly, to a naive observer, things were more strongly clustered in the past than they are now, despite the growth of structure by gravitational instability. This result tells us how the bias of massive galaxies, relative to the underlying dark matter distribution, evolves with cosmic time.