The Big Bang model

1. The Big Bang model is the scientific explanation of how the universe began and how it has changed since.
2. According to the model, everything in the universe was once packed into an extremely small, extremely hot and extremely dense region.
3. About 13.8 billion years ago this region began to expand, and the universe has been expanding and cooling ever since.
4. As the universe cooled, the first particles formed, then atoms of hydrogen and helium, and much later the first stars and galaxies.
5. The Big Bang was not an explosion of matter into empty space that already existed, because the model describes space itself expanding.
6. Scientists accept the model because it explains the observations made of the universe today, and each piece of evidence is linked to something the model explains or predicts.

Red-shift and an expanding universe

1. Light from distant galaxies is red-shifted, which shows that they are moving away from us.
2. More distant galaxies have larger red-shifts, which shows that the universe is expanding.
3. If the universe is getting bigger now, then at earlier times the galaxies must have been closer together.
4. Running the expansion backwards in time leads to a moment when everything was squeezed into a tiny region.
5. Squeezing matter into a smaller space raises its density and its temperature, so the early universe must have been very hot and dense.
6. This is the starting state that the Big Bang model describes, so red-shift is evidence that supports the model.
7. Measuring how fast galaxies are moving apart, and how far away they are, also lets scientists work out how long the expansion has been going on, which gives the age of the universe.

Cosmic microwave background radiation

1. The Big Bang model predicts that the young universe was so hot that it was filled with intense electromagnetic radiation.
2. For the first few hundred thousand years, the universe was a hot, glowing gas that the radiation could not pass through.
3. When the universe was about 380 000 years old, it had cooled enough for atoms to form, and the radiation was set free to travel through space.
4. At that time the radiation had short wavelengths in the visible and infrared parts of the spectrum, matching a temperature of about $3000K$.
5. Ever since, the expansion of space has stretched these waves to much longer wavelengths.
6. The radiation now has wavelengths of around a millimetre, which places it in the microwave region of the electromagnetic spectrum.
7. This leftover radiation is the cosmic microwave background radiation, usually shortened to CMBR.
8. The CMBR was discovered by accident in 1965 by Arno Penzias and Robert Wilson, who picked up a faint microwave signal from every direction they pointed their antenna.
9. The CMBR is almost perfectly uniform, arriving with nearly the same intensity from every part of the sky, which shows that it fills the whole universe rather than coming from any single object.
10. Its spread of wavelengths matches the radiation emitted by an object at only about $2.7K$, which is just above absolute zero.
11. The Big Bang model predicted this radiation before it was found, and the measured radiation matches the prediction, so the CMBR is strong evidence for the model.

Big Bang versus Steady State

1. In the middle of the twentieth century, many scientists supported a rival idea called the Steady State model.
2. The Steady State model accepted that the universe is expanding, but it claimed that the universe had no beginning and that new matter was continually created to fill the gaps.
3. Both models could explain the red-shift of distant galaxies, so red-shift alone could not decide between them.
4. Only the Big Bang model includes an early hot, dense stage that would leave behind radiation filling the whole of space.
5. The Steady State model has no early hot stage, so it cannot explain why the CMBR exists.
6. After the CMBR was discovered, most scientists rejected the Steady State model and accepted the Big Bang model.
7. This shows how scientific models develop, because a model is kept while it explains the evidence and is replaced when new evidence cannot be explained by it.