The Big Bang theory predicts that fluctuations in the density of matter in the first moments of the universe caused most matter to condense into a web of tangled filaments. This hypothesis has been supported by numerical simulations, which suggest that the universe is structured as a 'cosmic web' of filaments at the nodes of which there are massive galaxy clusters. These extended and diffuse filaments are mainly made up of dark matter(2).
A filament was identified for the first time in July 2012(3). Now, an international team including French researchers from LAM and the Centre de Recherche Astrophysique de Lyon (CNRS/ENS Lyon/Université Lyon 1), has analyzed another cosmic filament, this time in three dimensions. This achievement is all the more remarkable as such filaments are extremely extended and diffuse, which makes them very hard to detect. This new approach enabled the researchers to determine the mass density of the filament and compare it with simulations.
To do this, the team combined high-resolution images of the galaxy cluster MACSJ0717 and of the neighboring field, obtained by the Hubble space telescope, with images from the ground-based Subaru (NAO) and Canada-France-Hawaii (CFHT) telescopes, and then with spectroscopic data on galaxies in the cluster, from the Keck and Gemini observatories.
This technique enabled the team to locate thousands of galaxies within the filament, and to measure most of their speeds. By combining the positions and speeds of all these galaxies, the astronomers were able to reveal the three-dimensional shape of the filament, as well as its orientation: it extends over a length of around 60 million light years behind MACSJ0717, and is almost perfectly aligned with our line of sight. This is therefore an exceptional structure, even on astronomical scales: if the measured mass can be taken to be representative of filaments near giant clusters, then cosmic filaments should make up over half the mass of the universe, which is considerably more than predicted by theorists.
The forthcoming space telescope, the NASA/ESA/CSA James Webb Space Telescope(4), will be a powerful tool whose very high resolution will play a key role in detecting cosmic filaments.
© NASA, ESA, Harald Ebeling (University of Hawaii at Manoa) & Jean-Paul Kneib (LAM)
This image is available from the CNRS photo library, firstname.lastname@example.org
1. The Hubble space telescope is an ESA/NASA international cooperation project.
2. Dark matter makes up around three quarters of the total matter in the universe. It cannot be observed directly since it neither emits nor reflects light. In addition, it can pass through other matter without friction (it is said to be collisionless). Dark matter interacts exclusively through gravity. Its presence can be detected by its gravitational effects, for example on the rotation rate of galaxies, or on the path of light, according to the Theory of General Relativity.
3. Nature, J. Dietrich et al. 'A filament of dark matter between two clusters of galaxies', 4 July 2012.
4. The James Webb Space Telescope (JWST) will be the successor to the Hubble space telescope, which it is expected to replace in 2018. The mission is led by NASA, with the participation of Europe, under the responsibility of the European Space Agency (ESA), and Canada, through the Canadian Space Agency (CSA). The telescope will have 7 times more light gathering power than Hubble and will be dedicated to observing the universe at infrared wavelengths (wavelengths of 1 to 27 micrometers).
Mathilde Jauzac, Eric Jullo, Jean-Paul Kneib, Harald Ebeling, Alexie Leauthaud, Marceau Limousin, Richard Massey, Johan Richard. Monthly Notices of the Royal Astronomical Society, October 2012. View web site