ESA Science & Technology03-Aug-2005 11:29:43

Galaxies and the Expanding Universe

Big Bang

The moment that our Universe came into being, commonly referred to as the Big Bang, occurred around ten billion years ago. In a fraction of a second, our Universe expanded from the size of a grapefruit to a size equivalent to the current Earth.

At the time of the Big Bang, it is widely believed that matter and antimatter were created in almost equal numbers. Antimatter consists of antiparticles: identical partners of elementary particles with opposing charges. Since they are opposites, when matter and antimatter meet, they destroy each other. Since we see plenty of matter around us everyday, scientists have concluded that at the moment of the Big Bang, there was an excess of matter over antimatter. Particle physicists conduct experiments in order to find out whether antiparticles are indeed the exact opposite of particles, recreating the environment of the beginnings of the Universe in laboratories called €˜particle accelerators€™.

We can still detect the existence of antimatter today in the form of positrons, the antiparticle of an electron.

As the Universe expanded, it cooled sufficiently to allow electrons and protons to combine into hydrogen atoms. As the temperature fell further, nucleosynthesis of the light elements occurred: into deuterium (D), helium-3, and helium-4. It is thought that around a quarter of the Universe is made up of helium-4.

Figure 1: Universe Evolution

The Big Bang model also gives us an explanation of how stars and galaxies came to be formed. After about 10 000 years, the temperature of matter had cooled down so much that the energy density of the Universe began to be concentrated in massive particles. Inevitably, gravitational forces between these massive particles came into play, forcing them together into million solar mass stars. Scientists estimate that these enormous structures eventually exploded to form the myriad of stars we have today, which in turn are continually subjected to the forces of gravity in order to form galaxies and galactic clusters.

Many of our questions about the beginnings of Universe will be addressed by ESA's Planck mission, due to be launched in 2007. Planck will be studying the most ancient radiation in the Universe, known as the 'cosmic microwave background'. These observations will allow scientists to look back at the dawn of time, in order to find out more about how individual galaxies and galactic clusters formed.

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