A Brief History of Time: Chapter 3 Summary & Analysis

On a moonless night the brightest objects are most likely Venus, Mars, Jupiter, and Saturn. There are also many stars, similar to our sun, just farther away. But they are not fixed; they all move relative to each other, which we can see because these stars are relatively close to us. As the earth moves round the sun we these stars move against the background of more distant stars; the nearer the stars, the more they seem to move.

Proxima Centauri is the nearest star and is four light-years, or 23 million million miles, away. Other visible stars are mostly within a few hundred light-years, while the sun is just eight light minutes away. These stars are concentrated in a band we call the Milky Way. Even in 1750, astronomers thought this must be because we are in a spiral galaxy. Sir William Herschel confirmed this some decades later, but the idea only gained traction in the 1900s.

In 1924, Hubble defined our modern understanding of the universe when he showed that ours was not the only galaxy, proving there were many others, with lots of space in between. To measure how far away they were, he measured their luminosity, which is affected by their distance from us.

Hubble worked out distances to nine galaxies this way, showing how there are hundreds of thousands of millions of them. Our galaxy is 100,000 light-years wide and rotating. Our sun is an average star among one of the galaxy's spiral arms. We've come a long way from thinking we were the center of the universe.

Newton discovered that by using a prism we can measure the different colors of the light spectrum. By using a telescope and a prism, we can see the light make up of whatever star or galaxy focused on. We can in turn tell a light's temperature from its spectrum. Missing colors indicate what chemicals are in each star.

When in the 1920s scientists looked at stars in other galaxies, they had the same missing colors as similar, closer stars, but they were all shifted toward the red end of the light spectrum. The Doppler effect tells us that as something moves away from us, each wavelength will be longer, while if it is approaching us each wave would reach us more quickly. As light is a wave, its wavelength will lengthen as an object moves away from us, and our eyes see longer wavelengths as red light.

As Hubble catalogued the galaxies and their distances from us, he found most galaxies were red-shifted, meaning they were moving away from the earth. Indeed, the red shift is proportional to the galaxies’ distance from the earth, meaning the further it is the faster it is moving away. As such, the universe must be expanding.

This was such a great intellectual revolution that people wondered how it had not been thought of before. Newton should have guessed it, as otherwise the universe would have contracted under the influence of gravity. But as the universe is expanding, this cancels out that gravitational pull. If the universe is expanding beyond what gravity can balance, it could expand forever, like a rocket bursting out of the atmosphere and continuing through space instead of falling back to earth.

People could have realized the universe was expanding from Newton's theory of gravity, but everyone at the time seemed set on believing in a static universe. Even Einstein overlooked this idea in his general theory of relativity. Instead he thought up a kind of anti-gravitational force he called the cosmological constant.

Yet a Russian physicist called Alexander Friedmann tackled this head on. He assumed first that the universe looked uniformly the same in every direction on the large scale, and second that this would be true from wherever you looked in the universe. This alone suggested the universe is not static, and he suggested it before Hubble's landmark discovery.

In nearby space, the universe does not look uniform, but further out it does. This was further backed up by two American physicists, Arno Penzias and Robert Wilson. They tested a very sensitive microwave detector, which picked up a lot more background noise than expected. After checking their equipment, then taking measurements in all directions as the earth traveled around the sun, they determined it was coming, uniformly, from the whole universe. They had confirmed Friedmann's first assumption.

At the same time, two other American physicists, Bob Dicke and Jim Peebles, were setting themselves up to look into microwave radiation to examine George Gamow's idea the early universe was hot and glowed brightly. They said it was so long ago that energy would now have red-shifted to become microwave radiation. Penzias and Wilson heard about this, and saw they had already found the evidence. The latter won the Nobel Prize in 1978 for their work, which seems hard on those who suggested the theories in the first place.

If the universe looks the same in every direction, it should look the same from any other point in the universe too. We argue this on the basis of modesty; we cannot prove it yet. In Friedmann's model, the universe is expanding, like a balloon with every point expanding from every other point. The further away two points are, the faster they'll be expanding, just as Hubble found. But despite his accuracy, Friedmann's work was not known in the West until Howard Robertson and Arthur Walker did similar work.

Three models obey Friedmann's assumptions, though he only suggested one himself. In the first, gravity can slow and eventually stop the expansion. Finally, the universe will contract again. In the second, gravity can slow the expansion a little, but not halt it, until it steadies to a constant rate. In the third, the gravity is just below where it can stop the expansion, so the universe continues to expand forever, but at an increasingly slow rate.

In the first model, or theory, the universe is finite, yet does not have a boundary. Gravity bends space around itself, so space is curved, like the surface of the earth. When combining the general theory of relativity with quantum mechanics (as discussed later), space and time can be finite without a boundary. But this doesn't mean you could travel right around the universe back to where you started; it would collapse again before you managed it. You'd have to travel faster than light, and you can't. In the second model, the universe is bent like a saddle, so it is infinite in space. In the third model, space is flat and infinite.

To know which model fits best, we need to know the universe's rate of expansion and average density, to determine if gravity will slow or stop the expansion. We know from the Doppler effect that galaxies are moving away from us at a rate of 5 to 10 percent every billion years. Our estimate for density is even more vague. What we can see and measure is less than one percent of the mass required to halt the expansion. Even what we cannot see but think is there would not add up to enough. Right now, it seems the universe will expand forever, but even if it did recollapse, it would most likely be billions of years after humans had died out anyway.

All the Friedmann models start out with a beginning where the space between everything was zero—the universe was infinitely dense and curved. Laws of science break down at this point. This big bang singularity means any previous events would not have any meaning to us now. In a sense, then, time began with the big bang.

While the church liked the big bang model because it leaves room for God, many dislike the idea. Hermann Bondi, Thomas Gold, and Fred Hoyle proposed the steady state theory, where matter spontaneously comes into being in the gaps between expanding galaxies. A group of astronomers headed up by Martin Ryle looked into radio waves from different galaxies, and found variations that disproved the theory. Penzias and Wilson’s earlier discovery also suggested the universe had been denser in the past, further disproving the theory.

Russian scientists Evgenii Lifshitz and Isaac Khalatnikov also tried to disprove the big bang. They said that as galaxies do not move directly away from each other, perhaps they were simply nearby at the beginning, not in a singularity. They created many more models, and found that there were examples for both sides, retracting an earlier statement that said there were many more scenarios in which there was no big bang. They did show that a big bang was possible under the general theory of relativity.

In 1965, Roger Penrose showed that stars can collapse in on themselves to become singularities, in this case, black holes. While Penrose only talked about stars, a young Stephen Hawking saw the relevance this had for the big bang theory. After surviving longer than expected after being diagnosed with Lou Gehrig’s disease, Hawking took the research matter up for his PhD. He suggested that if the universe is infinite and expanding too fast to recollapse, it should have started at a singularity. Penrose and Hawking’s resulting paper in 1970 faced opposition. But the math held out, and now everyone tends to assume there was a big bang. However, Hawking himself has since changed his mind, when taking quantum mechanics into account.

Over the millennia, our understanding has changed significantly. Penrose and Hawking’s work showed that Einstein’s general theory of relativity is only a partial theory. It breaks down at the beginning of the universe. When the universe was squeezed into infinite density, quantum mechanics comes into play. As such, their focus turned from the massive to the miniscule.