An alternative to the Big Bang theory is the big bounce which hypothesises the universe expands until it can expand no more, before crashing back into one infinitesimally small point in a cycle which repeats for eternity. If the Big Bang is true, it would mean that time as we know it started at a single point 13.8 billion years ago. However, if the Big Bounce theory, which is a cyclic model meaning it is part of a broader cycle, is correct then it would mean that time did not have a beginning, but rather has the ability to flow in all directions and has been doing so for eternity.

As the Universe begins to shrink back into itself, some physicists believe time will also run backwards.

This twin Universe, as Louisiana State University physicist Parampreet Singh puts it, existed before the Big Bang where it was a mirror image of our own cosmos and everything ran in reverse.

Mr Singh said: In the universe before the bounce, all the general features will be the same. It will follow the same dynamical equations, the Einsteins equations when the universe is large.

Further, the matter content will be the same, and it will have the same evolution. Since the pre-bounce universe is contracting, it will look as if we were looking at ours backward in time.

In other words, when the Universe expands, time runs forwards, but when it contracts, time runs backwards.

The Universe expands due to dark energy a mysterious substance in the cosmos which effectively counters the effects of gravity.

However, as the cosmos grows, dark energy will become more sparse, which could theoretically allow gravity to win the battle.

At this point the Universe will begin to contract down to an infinitesimal point.

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Its not clear how exactly to test either of these scenarios.

The Big Bounce theory was first popularised in 1987 when it appeared in German science journals, with physicists using it as a way to explain the problems with the Big Bang, such as where it came from or to remove the problematic singularity.

The only other time in the Universe where a singularity occurs and the laws of physics are thrown out of the window is at the event horizon of a black hole, which is also unexplainable by current scientific methods.

However, other experts believe the Big Bounce theory does not stack up, claiming there is little evidence to support his hypothesis.

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Big Bang Theory shock: Time had no beginning and eventually reverses towards Big Bounce - Express.co.uk