ACARA v9 CONTENT DESCRIPTION “describe how the big bang theory models the origin and evolution of the universe and analyse the supporting evidence for the theory”
Builds on earlier work on light, waves and the structure of the solar system, and on the idea that scientific models are judged by how well they fit evidence. Here those ideas come together in cosmology: the big bang is the leading scientific model for how the whole universe began and developed, and it is tested against several independent observations.
What the big bang theory describes
The big bang theory is the leading scientific model for the origin and development of the universe. It describes a universe that began extremely hot and dense and has been expanding and cooling ever since. This is a model about how the universe itself developed over time: how space stretched, how it cooled, and how atoms, stars and galaxies came to exist. It is built from physics and tested against what we observe in the sky.
The expanding universe
Space itself stretches over time. As you run time forward, every galaxy moves away from every other one, with no special centre.
Early on, the galaxies sit close together on a small patch of space.
The universe is expanding
When astronomers measure galaxies across the sky, they find that almost all of them are moving away from us, and from each other. The space between galaxies is stretching, so the universe is getting larger over time. Running this idea backwards leads to a much smaller, hotter, denser past, which is exactly what the big bang model describes.
Redshift: distant galaxies recede faster
The further away a galaxy is, the faster it moves away, and the more its light is stretched to longer, redder wavelengths. Pick a distance to compare.
Light from a receding galaxy is stretched to longer wavelengths, an effect called redshift. More distant galaxies recede faster, so their light is shifted more. This steady relationship between distance and recession speed is the kind of pattern first measured by Edwin Hubble.
Redshift and the speed of galaxies
The evidence for expansion comes from redshift. Light from a galaxy moving away is stretched to longer, redder wavelengths. The further away a galaxy is, the faster it recedes and the larger its redshift. This steady link between distance and speed was first measured by Edwin Hubble, and it is one of the central pieces of evidence for an expanding universe.
Cosmic microwave background: the leftover glow
Faint microwave radiation reaches us from every direction. It is heat left over from when the whole universe was hot and dense. Reveal it to see the even glow.
The cosmic microwave background is radiation released when the early universe first cooled enough for light to travel freely. It reaches us from every direction and is remarkably even, with only tiny variations. Finding this predicted glow was strong support for the big bang model.
The leftover glow of the early universe
The big bang model predicted that the hot early universe should have left behind a faint radiation that still fills space today. This cosmic microwave background was later detected reaching us evenly from every direction, just as predicted. Its discovery, and how smooth it is, gave the model some of its strongest support.
Light elements: mostly hydrogen and helium
The big bang model predicts that the early universe made mostly hydrogen and helium. Pick an element to see its share of ordinary matter.
About three quarters of ordinary matter is hydrogen, the lightest element, formed in the first minutes.
The amounts of light elements
In its first minutes the universe was hot enough to build atomic nuclei. The model predicts that this should have produced mostly hydrogen and a good fraction of helium, with only traces of heavier elements. When astronomers measure the make-up of old, distant gas, the amounts match the prediction closely. Heavier elements such as carbon and iron were built later, inside stars.
A cosmic timeline
Step from a hot, dense start through expansion and cooling to the first atoms, then stars and galaxies. The colours cool as the universe expands.
The universe begins extremely hot and dense, packed into a tiny region.
Putting the evidence together
No single observation proves the big bang on its own. Its strength is that several independent lines of evidence all point the same way: the universe is expanding, distant galaxies are redshifted, a leftover microwave glow fills the sky, and the measured amounts of light elements match the prediction. Because the model fits all of these at once, scientists accept it as the best current explanation for how the universe began and developed.
Quick self-check
1. What does the expansion of the universe tell us about distant galaxies?
2. What happens to the light from a galaxy that is moving away from us?
3. What is the cosmic microwave background?
4. Which elements did the early universe form in the greatest amounts?
5. Why is the big bang theory accepted by scientists?