The Cosmic Web

Galaxies are not distributed uniformly across space but are found grouped together in ‘clusters’ of galaxies; these clusters are themselves grouped together into larger structures. The distribution of galaxies and clusters is sometimes refered to as the ‘cosmic web’ because its many strands and connections resemble a spider’s web.


Galaxy cluster Abel 2029. Left: X-ray emission shows the gas. Right: visible light shows the galaxies. (NASA/CXC/SAO)

A galaxy cluster is the largest single object in the universe. The force of gravity holds the individual galaxies in orbit around one another so that they stay grouped together. Also contained within the cluster is a large mass of hot (ionized) hydrogen gas, which is a strong emitter of X-rays. However, the total amount of mass in galaxies and gas does not provide enough force to hold the cluster together. There must be another source of gravity within the cluster and this hidden mass is known as ‘dark matter’ (because it doesn’t emit any light or radiation). What is it? We don’t know – the physical nature of this dark matter is one of the greatest outstanding mysteries in all of physics.

As part of the Spitzer Wide-area Infrared Extragalactic legacy survey (SWIRE), we have used the Spitzer Space Telescope to map an area of sky 250-times larger than the full moon. This map is being used to measure the positions of several million galaxies and determine how the cosmic web of galaxies and clusters has changed over time. From these measurements we hope to deduce whether the galaxies (which we can see) are found at the same locations as the dark matter (which we can’t see, but theory tells us must be there). This relationship between galaxies and dark matter, known as ‘bias’, is a key factor in understanding the structure of the universe itself.