The chemical composition of the Universe and the physical nature of its constituent matter are topics that have occupied scientists for centuries. From its privileged position above the Earth's atmosphere Hubble has been able to contribute significantly to this area of research.

All over the Universe stars work as giant reprocessing plants taking light chemical elements and transforming them into heavier ones. The original, so-called 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.

Helium in the early Universe

Shortly after the First Servicing Mission successfully corrected the spherical aberration in Hubble's mirror a team led by European astronomer Peter Jakobsen 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, they 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.

Quasar lighthouses

This investigation of helium in the early Universe is one of many ways that Hubble has used distant quasars as lighthouses. As light from the quasars passes through the intervening intergalactic matter, the light signal is changed in such a way as to reveal the composition of the gas. The results have filled in important pieces of the puzzle of the total composition of the Universe now and in the past.

Dark Matter

Today astronomers believe that close to 95% of the mass of the Universe consists of dark matter, a substance quite different from the normal matter that makes up atoms and 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 its composition. The riddle of the ghostly dark matter is still far from solved, but Hubble's incredibly sharp observations of, for instance, gravitational lenses (see pages 32-33) have provided stepping stones for future work in this area.

Peter Jakobsen
ESA astronomer, NGST Study Scientist

"I believe that we now have a good understanding of the amount and composition of 'normal' matter of the Universe. By looking further and further back in time we are now beginning to piece together the history of this matter since it emerged from the Big Bang and eventually collapsed to form the stars and galaxies that we see in the present day Universe. Hubble has played a very important part in unravelling this history. With the Next Generation Space Telescope we hope to reach back to even earlier times and see the very first stars turn on."

The Age and Size of the Universe