“You might as well go to bed, Mom. We have so much to talk about.”
The Big Bang
... we too are stardust. . .
Hilde settled herself comfortably in the glider beside her father. It was nearly midnight. They sat looking out across the bay. A few stars glimmered palely in the light sky. Gentle waves lapped over the stones under the dock.
Her father broke the silence.
“It’s a strange thought that we live on a tiny little planet in the universe.”
“Yes ...”
“Earth is only one of many planets orbiting the sun. Yet Earth is the only living planet.”
“Perhaps the only one in the entire universe?”
“It’s possible. But it’s also possible that the universe is teeming with life. The universe is inconceivably huge. The distances are so great that we measure them in light-minutes and light-years.”
“What are they, actually?”
“A light-minute is the distance light travels in one minute. And that’s a long way, because light travels through space at 300,000 kilometers a second. That means that a light-minute is 60 times 300,000—or 18 million kilometers. A light-year is nearly ten trillion kilometers.”
“How far away is the sun?”
“It’s a little over eight light-minutes away. The rays of sunlight warming our faces on a hot June day have traveled for eight minutes through the universe before they reach us.”
“Go on...”
“Pluto, which is the planet farthest out in our solar system, is about five light-hours away from us. When an astronomer looks at Pluto through his telescope, he is in fact looking five hours back in time. We could also say that the picture of Pluto takes five hours to get here.”
“It’s a bit hard to visualize, but I think I understand.”
“That’s good, Hilde. But we here on Earth are only just beginning to orient ourselves. Our own sun is one of 400 billion other stars in the galaxy we call the Milky Way. This galaxy resembles a large discus, with our sun situated in one of its several spiral arms. When we look up at the sky on a clear winter’s night, we see a broad band of stars. This is because we are looking toward the center of the Milky Way.”
“I suppose that’s why the Milky Way is called ‘Winter Street’ in Swedish.”
“The distance to the star in the Milky Way that is our nearest neighbor is four light-years. Maybe that’s it just above the island over there. If you could imagine that at this very moment a stargazer is sitting up there with a powerful telescope pointing at Bjerkely—he would see Bjerkely as it looked four years ago. He might see an eleven-year-old girl swinging her legs in the glider.”
“Incredible.”
“But that’s only the nearest star. The whole galaxy— or nebula, as we also call it—is 90,000 light-years wide. That is another way of describing the time it takes for light to travel from one end of the galaxy to the other. When we gaze at a star in the Milky Way which is 50,000 light-years away from our sun, we are looking back 50,000 years in time.”
“The idea is much too big for my little head.”
“The only way we can look out into space, then, is to look back in time. We can never know what the universe is like now. We only know what it was like then. When we look up at a star that is thousands of light-years away, we are really traveling thousands of years back in the history of space.”
“It’s completely incomprehensible.”
“But everything we see meets the eye in the form of light waves. And these light waves take time to travel through space. We could compare it to thunder. We always hear the thunder after we have seen the lightning. That’s because sound waves travel slower than light waves. When I hear a peal of thunder, I’m hearing the sound of something that happened a little while ago. It’s the same thing with the stars. When I look at a star that is thousands of light-years away, I’m seeing the ‘peal of thunder’ from an event that lies thousands of years back in time.”
“Yes, I see.”
“But so far, we’ve only been talking about our own galaxy. Astronomers say there are about a hundred billion of such galaxies in the universe, and each of these galaxies consists of about a hundred billion stars. We call the nearest galaxy to the Milky Way the Andromeda nebula. It lies two million light-years from our own galaxy. That means the light from that galaxy takes two million years to reach us. So we’re looking two million years back in time when we see the Andromeda nebula high up in the sky. If there was a clever stargazer in this nebula—I can just imagine him pointing his telescope at Earth right now—he wouldn’t be able to see us. If he was lucky, he’d see a few flat-faced Neanderthals.”
“It’s amazing.”
“The most distant galaxies we know of today are about ten billion light-years away from us. When we receive signals from these galaxies, we are going ten billion years back in the history of the universe. That’s about twice as long as our own solar system has existed.”
“You’re making me dizzy.”
“Although it is hard enough to comprehend what it means to look so far back in time, astronomers have discovered something that has even greater significance for our world picture.”
“What?”
“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
