Clusters of galaxies and large-scale structures

Research in this area explores the cosmic web and massive structures called clusters of galaxies.

The cosmic web

If we look out from our Milky Way on large enough scales, we see that the Universe is not random. Galaxies have a web- or foam-like structure that we call the cosmic web. In this structure, vast filaments intersect to form nodes where matter is highly concentrated and corresponding voids where there is very little matter.

Through numerical simulations, we can show that this large-scale structure forms from an initially smooth, random distribution of particles as the attractive force of gravity combats the expansion of the Universe.

The image above shows the large-scale distribution of galaxies mapped out by the Sloan Digital Sky Survey, looking out from the Milky Way at the centre of the circle. “Redshift” corresponds to distance, with the outer part of the circle showing galaxies around two billion light-years away. The light left these galaxies about 12 billion years ago.

Find out more about the Sloan Digital Sky Survey.

Also see this flythrough on YouTube.

Clusters of galaxies

At the intersections of the filaments of the cosmic web are massive structures called clusters of galaxies. These can contain hundreds of galaxies—but that’s only a tiny fraction of the mass that we know is there due to gravitational lensing effects.

Light from background galaxies is redirected by the gravitational fields of clusters, showing that around 85% of a cluster’s mass must be made up of dark matter. The remaining 15% is mostly made up of hot gas—around a million degrees Celsius—that emits in the X-ray spectrum and that interacts with the photons of the cosmic microwave background to produce a signal known as the Sunyaev-Zel’dovich (SZ) effect.

The long arc and other elongated features are background galaxies that appear distorted because their light is redirected by the gravitational field of the cluster—a bit like looking through the bottom of a wine glass. (Credit:NASA, ESA, the Hubble SM4 ERO Team, and ST-ECF. NASA Astronomy Picture of the Day.

This image shows galaxies in the background, hot gas observed in X-ray (pink) and dark matter distribution inferred from lensing (blue). Credit: X-ray: NASA/CXC/CfA M. Markevitch et al.

Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al.

Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al. https://apod.nasa.gov/apod/ap170115.html