"Apparently no galaxy in space remains where it is. All the galaxies in the universe are moving away from each other at colossal speeds. The further they are away from us, the quicker they move. That means that the distance between the galaxies is increasing all the time."
"I'm trying to picture it."
"If you have a balloon and you paint black spots on it, the spots will move away from each other as you blow up the balloon. That's what's happening with the galaxies in the universe. We say that the universe is expanding."
"What makes it do that?"
"Most astronomers agree that the expanding universe can only have one explanation: Once upon a time, about 15 billion years ago, all substance in the universe was assembled in a relatively small area. The substance was so dense that gravity made it terrifically hot. Finally it got so hot and so tightly packed that it exploded. We call this explosion the Big Bang."
"Just the thought of it makes me shudder."
"The Big Bang caused all the substance in the universe to be expelled in all directions, and as it gradually cooled, it formed stars and galaxies and moons and planets ..."
"But I thought you said the universe was still expanding?"
"Yes I did, and it's expanding precisely because of this explosion billions of years ago. The universe has no timeless geography. The universe is a happening. The universe is an explosion. Galaxies continue to fly through the universe away from each other at colossal speeds."
"Will they go on doing that for ever?"
"That's one possibility. But there is another. You may recall that Alberto told Sophie about the two forces that cause the planets to remain in constant orbit round the sun?"
"Weren't they gravity and inertia?"
"Right, and the same thing applies to the galaxies. Because even though the universe continues to expand, the force of gravity is working the other way. And one day, in a couple of billion years, gravity will perhaps cause the heavenly bodies to be packed together again as the force of the huge explosion begins to weaken. Then we would get a reverse explosion, a so-called implosion. But the distances are so great that it will happen like a movie that is run in slow motion. You might compare it with what happens when you release the air from a balloon."
"Will all the galaxies be drawn together in a tight nucleus again?"
"Yes, you've got it. But what will happen then?"
"There would be another Big Bang and the universe would start expanding again. Because the same natural laws are in operation. And so new stars and galaxies will form."
"Good thinking. Astronomers think there are two possible scenarios for the future of the universe. Either the universe will go on expanding forever so that the galaxies will draw further and further apart--or the universe will begin to contract again. How heavy and massive the universe is will determine what happens. And this is something astronomers have no way of knowing as yet."
"But if the universe is so heavy that it begins to contract again, perhaps it has expanded and contracted lots of times before."
"That would be an obvious conclusion. But on this point theory is divided. It may be that the expansion of the universe is something that will only happen this one time. But if it keeps on expanding for all eternity, the question of where it all began becomes even more pressing."
"Yes, where did it come from, all that stuff that suddenly exploded?"
"For a Christian, it would be obvious to see the Big Bang as the actual moment of creation. The Bible tells us that God said 'Let there be light!