Three main pieces of evidence for the Big Bang theory are the red-shift of light, cosmic background radiation and types of elements.

Is it possible that a Big Bang never happened? One alternative theory is the Steady State universe. An early rival to the Big Bang theory, Steady State posits continuous creation of matter throughout the universe to explain its apparent expansion. This type of universe would be infinite, with no beginning or end.

The initial singularity is a singularity predicted by some models of the Big Bang theory to have existed before the Big Bang and thought to have contained all the energy and spacetime of the Universe.

Just not to the Big Bang in any way whatsoever. Nothing explodes in the Big Bang. Instead, the Big Bang is described by a hot, dense state that simply expands and cools. That’s it: no explosion or conflagration of any type.

Instead, the collapsing matter reaches an enormous but finite density and rebounds, forming the other side of an Einstein-Rosen bridge, which grows as a new universe. Or, the Big Bang was a supermassive white hole that was the result of a supermassive black hole at the heart of a galaxy in our parent universe.

The universal origin story known as the Big Bang postulates that, 13.7 billion years ago, our universe emerged from a singularity — a point of infinite density and gravity — and that before this event, space and time did not exist (which means the Big Bang took place at no place and no time).

The Big Bang Theory is the leading explanation about how the universe began. At its simplest, it says the universe as we know it started with a small singularity, then inflated over the next 13.8 billion years to the cosmos that we know today.

This is Part 3 in that series.

• Step 1: How It All Started. NASA/WMAP.
• Step 2: The Universe’s First Growth Spurt.
• Step 3: Too Hot to Shine.
• Step 4: Let There Be Light.
• Step 5: Emerging from the Cosmic Dark Ages.
• Step 6: More Stars and More Galaxies.
• Step 7: Birth of Our Solar System.
• Step 8: The Invisible Stuff in the Universe.

Dr Steve Curran, from the School of Chemical and Physical Sciences, answers: That’s an easy one—13.7 billion years ago. The universe is expanding, so all the galaxies are moving apart from each other, and if you extrapolate this backwards they all meet at a point 13.7 billion years ago.