Most astronomers now agree that we live in an expanding Universe, as opposed to a static Universe, one which emerged from an explosion of space generally referred to as the Big Bang. At the time of the Big Bang, that is, at the very beginning of time itself, about 15 billion years ago, the Universe was very different to how it is today. Then, the Universe was a small, unimaginably hot, unimaginably dense primordial fireball filled with high-energy radiation. It took until the Universe was about 1 million years old for it to have expanded and cooled sufficiently for clouds of hydrogen and helium to form and to survive, hydrogen and helium being the simplest and most abundant elements in the Universe, and perhaps another 1 billion years for some clouds of hydrogen and helium to collapse under the influence of gravity to form the first galaxies containing the first stars.

Evidence to support Big Bang theory and a Universe which is growing in size comes from studies of the spectra of light from galaxies themselves (certain redshifted spectral lines indicate that most galaxies are moving away from us in all directions with speeds proportional to their distance) and studies of the cosmic microwave background radiation (a predicted relic of the Big Bang which fills the so-called vacuum of space with a corresponding temperature of about 3 degrees above absolute zero or -273ºC).

Whether the Universe will continue to expand or not will ultimately depend upon the amount of matter contained within it. If sufficient, the gravitational attraction between galaxies will cause them to stop moving apart and come back together again. This will most likely end in a Big Crunch from which another Universe might emerge from yet another Big Bang. Alternatively, if insufficient, the gravitational attraction between galaxies will be too weak to prevent them from moving apart. This will most likely end in a Big Freeze with all of the galaxies infinitely far apart and all of the stars within galaxies eventually burning out.

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