Two theories of the Universe

1. Astronomers agree that the Universe is expanding, which means that the distances between distant galaxies are increasing.
2. Two rival theories were built to explain that expansion, the Big Bang theory and the Steady State theory.
3. Both were serious scientific theories, and both were taken seriously through the middle of the twentieth century because both accounted for the expansion that had been measured.
4. They part company over one question, whether the Universe had a beginning or has simply always been here.

What the Big Bang theory says

1. All of the matter and energy in the Universe began packed into a region that was extremely small, hot and dense.
2. That region began to expand, and space itself has been growing ever since rather than matter flying outwards through empty space.
3. The expansion has spread the same fixed amount of matter through a steadily larger volume, so the Universe has become less dense as it has aged.
4. Expansion also spreads out the energy present at the start, so the Universe has cooled continuously from that first hot state.
5. The theory therefore gives the Universe a finite age, currently taken as about $13.8$ billion years.
6. It also means the Universe looked very different in the past, since everything was closer together, hotter and more crowded.

What the Steady State theory says

1. The Universe has always existed, so there was no starting event and there is no age to calculate.
2. The Universe is still expanding, so this theory agrees with the Big Bang theory that galaxies are moving apart.
3. To stop the expansion thinning everything out, new matter is created continuously in the gaps that open up between the galaxies.
4. That new matter forms new galaxies at the same rate as old ones drift apart, so the average density stays constant.
5. The overall appearance of the Universe is therefore unchanging, which is where the name of the theory comes from.
6. An observer looking at the Universe billions of years ago, or billions of years in the future, would see much the same picture as one looking today.

Comparing the two theories

1. Both theories accept that the Universe is expanding and that distant galaxies are moving away from us.
2. Both also accept that the Earth holds no special place, so an observer in any galaxy would see the other galaxies receding.
3. They differ on the beginning, because the Big Bang theory has a single starting event while the Steady State theory has none.
4. They differ on age, because the Big Bang theory gives a finite age while the Steady State theory makes the Universe infinitely old.
5. They differ on matter, because the Big Bang theory keeps the total amount fixed while the Steady State theory creates more of it as space grows.
6. They differ on density, because the Big Bang theory has it falling with time while the Steady State theory holds it constant.
7. They differ on how the Universe changes over time, because the Big Bang theory says the distant past looked hotter and more crowded while the Steady State theory says it looked the same.
8. Those last differences are what make the two theories testable, since a theory that predicts a changing Universe can be separated from one that predicts an unchanging Universe by observation.