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Cosmology

Cosmology

The Age and Size of the Universe

Cepheids

The top-ranked scientific justification for building Hubble was to determine the size and age of the Universe through observations of Cepheid variables in distant galaxies. This scientific goal was so important that it put constraints on the lower limit of the size of Hubble's primary mirror.

Cepheids are a special type of variable star with very stable and predictable brightness variations. The period of these variations depends on physical properties of the stars such as their mass and true brightness. This means that astronomers, just by looking at the variability of their light, can find out about the Cepheids' physical nature, which then can be used very effectively to determine their distance. For this reason cosmologists call Cepheids 'standard candles'.

Several groups of astronomers have used Hubble to observe Cepheids with extraordinary results. The Cepheids have then been used as stepping-stones to make distance measurements for supernovae, which have, in turn, given a measure for the scale of the Universe. Today we know the age of the Universe to a much higher precision than before Hubble: around 15 billion years.

Supernovae

Hubble's sharp vision means that it can see exploding stars, supernovf that are billions of light years away and difficult for other telescopes to study.

Most scientists today believe that the expansion of the Universe is accelerating. This result came from combined measurements of remote supernovf with most of the world's top-class telescopes, including Hubble, and it was a very surprising one. For many years cosmologists have discussed whether the expansion of the Universe would stop in some distant future or continue ever more slowly. From the new results it seems clear that the expansion is nowhere near slowing down. In fact, due to some mysterious property of space itself (called vacuum energy), the expansion is accelerating and will continue forever.

Hubble has given these supernovae measurements an added precision, mostly due to its high resolution. From the ground an image of the supernova usually blends in with the image of its host galaxy. Hubble can distinguish the light from the two sources and thus measure the supernova directly.

Gustav A. Tammann
Astronomer, University of Basle

"We certainly live in exciting times. Hubble has made enormous progress possible within cosmology. Today we have a much more unified cosmological picture than was possible even five years ago when people were talking of 'the cosmology in crisis'. We have seen a dramatic change from misery to glory!"

Bruno Leibundgut
Astronomer, European Southern Observatory (ESO)

"Hubble gave us the distance measurements of the first four supernovae that made us realise something was wrong with our present understanding of the Universe. Even though the definite proof that the Universe is accelerating came later, we could not reconcile our Hubble observations with a Universe where the expansion is slowing down."

The Composition of the Universe

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."

Last Update: 1 September 2019
28-Mar-2024 08:43 UT

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