Hubble - greatest discoveries
Composition of the Universe:
How Hubble studied what the Universe is made of, and came to some startling conclusions
All over the Universe stars work as giant reprocessing plants taking light chemical elements and transforming them into heavier ones. The original, primordial, composition of the Universe is studied in such fine detail because it is one of the keys to our understanding of processes in the very early Universe.
Astronomers investigated the nature of the gaseous matter that fills the vast volume of intergalactic space. By observing ultraviolet light from a distant quasar, which would otherwise have been absorbed by the Earth's atmosphere, scientists found the long-sought signature of helium in the early Universe. This was an important piece of supporting evidence for the Big Bang theory. It also confirmed scientists' expectation that, in the very early Universe, matter not yet locked up in stars and galaxies was nearly completely ionised (the atoms were stripped of their electrons). This was an important step forward for cosmology.
Today astronomers believe that around three quarters of the mass of the Universe consists of dark matter, a substance quite different from the normal matter that makes up the familiar world around us. Hubble has played an important part in work intended to establish the amount of dark matter in the Universe and to determine where it is.
The riddle of what the ghostly dark matter is made of is still far from solved, but Hubble's incredibly sharp observations of gravitational lenses have provided stepping stones for future work in this area. Dark matter only interacts with gravity, which means it neither reflects, emits or obstructs light. Because of this, it cannot be observed directly. However, Hubble studies of how clusters of galaxies bend the light that passes through them lets astronomers deduce where the hidden mass lies. This means that they are able to make maps of where the dark matter lies in a cluster.
One of Hubble's big breakthroughs in this area is the discovery of how dark matter behaves when clusters collide with each other. Studies of a number of these clusters have shown that the location of dark matter does not match the distribution of hot gas. This strongly supports theories about dark matter: we expect hot gases to slow down as they hit each other and the pressure increases. Dark matter, on the other hand, should not experience friction or pressure, so we would expect it to pass through the collision relatively unhindered. Hubble and Chandra observations have indeed confirmed that this is the case.
A 3D map of the dark matter distribution in the Universe
In 2007 an international team of astronomers used Hubble to create the first three-dimensional map of the large-scale distribution of dark matter in the Universe. It was constructed by measuring the shapes of half a million galaxies observed by Hubble. The light of these galaxies travelled – until it reached Hubble – down a path interrupted by clumps of dark matter which deformed the appearance of the galaxies. Astronomers used the observed distortion of the galaxies shapes to reconstruct their original shape and could therefore also calculate the distribution of dark matter in between.
This map showed that normal matter, largely in the form of galaxies, accumulates along the densest concentrations of dark matter. The created map stretches halfway back to the beginning of the Universe and shows how dark matter grew increasingly clumpy as it collapsed under gravity. Mapping dark matter distribution down to even smaller scales is fundamental for our understanding of how galaxies grew and clustered over billions of years. Tracing the growth of clustering in dark matter may eventually also shed light on dark energy.
More intriguing still than dark matter is dark energy. Hubble studies of the expansion rate of the Universe have found that the expansion is actually speeding up. Astronomers have explained this using the theory of dark energy, as a sort of negative gravity that pushes the Universe apart ever faster. Studies of the rate of expansion of the cosmos suggests that dark energy is by far the largest part of the Universe's mass-energy content, far outweighing both normal matter and dark matter. While astronomers have been able to take steps along the path to understanding how dark energy works and what it does, its true nature is still a mystery.
This is one of nine articles highlighting some of the greatest discoveries made by the Hubble Space Telescope. Read more in the articles linked from the right-hand menu.