What happens when a universe comes to an end?

Every physical system ends at some point, even universes. This prospect raises all sorts of issues. How will our universe end? What comes after?

It is widely accepted the universe started just under 14 billion years ago, in an event now referred to as the “Big Bang.” Since then, it has been expanding. Until recently we thought this expansion is probably due to a big kick given to everything back at the “Big Bang.”

Since every body in the universe is gravitationally pulling at every other body, we would expect the expansion to be gradually slowing down. That raised the possibility the expansion would slow and be brought to a stop, after which everything would start falling back together, eventually coming together in an event referred to as "The Big Crunch".

Then, at some future time, a new “Big Bang” would happen. This was a convenient solution, in that it neatly explained what came before and what comes after. There would be just a never-ending series of “bangs” and “crunches,” with brand-new universes existing between them.

However, this convenient idea has been demolished by the discovery that the universe's expansion is not slowing down as expected, it is speeding up. Everything is being pushed outwards by a mysterious force, which for want of a better term, we call "Dark Energy". Ostensibly, the universe will keep expanding indefinitely, and the future fate of the universe becomes an important and fascinating question.

Stars are the Swiss Army knives of the universe. They provide light and heat, and manufacture the elements needed to make planets and people as by-products of their energy production. The fuel they need to function is hydrogen. Huge quantities of this were made back at the beginning of the universe. However, the universe is a closed system. That primordial hydrogen is all we are going to get to fuel existing stars and to make new ones.

There is still, fortunately, a tremendous amount of unused hydrogen out there, so the supply of stars and raw materials for planets and people can continue for a long time yet. Ffor sure there is a point, fortunately far off in the future, when the hydrogen will all have been used up. The existing stars will run out of fuel, cool off and go out. This leaves one other source of energy, gravity.

When the last stars go out there will still be black holes and other high-gravity objects. These don't care whether the stars they pull in and swallow are shining or just cold cinders. As they are pulled, torn apart and swallowed, a colossal amount of energy is released. This includes light, heat, x-rays and radio waves. These bursts of energy won't do much to warm the inhabitants of freezing planets because flow of energy is so variable. They would freeze for most of the time and then get fried by bursts of high-energy radiation.

Finally, at some point in the remote future, all sources of energy will have been exhausted and everything in the universe will be at the same very low temperature, close to absolute zero (-273 C). The situation will be rather like a dam with the same water level on both sides. There will be no usable energy for anything. We call this state "heat death". The universe will be dead.

An idea attracting interest is that our universe is just one member of the "multiverse", which has countless universes forming in it, like bubbles in a cosmic foam, forming, growing and then dissipating. There is a chance we can see if this is the case. Even though we cannot see out of the universe, if our cosmic bubble is touching another, like bubbles in the foam of a bubble bath, we might be able to see the area of contact. It might be reassuring to know that the end of our universe does not mean the end of everything.

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• Saturn rises around 9 p.m. and Jupiter around 11 p.m. Venus rises in the dawn twilight.

•The Moon will be new on Sept. 14.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory, near Penticton.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.