A Brief History of Time | Study Guide

Stephen Hawking

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A Brief History of Time | Chapter 3 : The Expanding Universe | Summary



Stephen Hawking points out in this chapter that humans have been aware of the Milky Way and other visible celestial features since ancient times. However, our understanding of these bodies was slow in coming. It wasn't until 1750 when observations indicated that the band of stars in the sky was one branch of a disk-shaped cluster of stars, and the objects in orbit around them, a galaxy. Sir William Herschel later made a detailed survey of visible stars that agreed with this model. However, the picture of our solar system as one of many in a great cluster of stars revolving around a spinning center caught on only in the early 20th century. The concept of a wider universe gained greater acceptance in 1924, when Edwin Hubble went on to demonstrate that ours was only one of many galaxies.

In the 1920s both nearby and distant stars were studied. Types of stars and galaxies can be told apart from one another by charting their light spectra (bands of colors in the light they emit, just as a rainbow represents the band of all visible light from violet to red). By measuring the relative brightness of these distant objects, their thermal (or temperature) spectra can also be determined. But astronomers found the spectra of very distant objects confusing—the spectra were so strange the stars seemed to be composed of elements never observed on Earth—until astronomers recognized familiar patterns and shifted the spectra toward the red end of the scale (called a red shift). Because the spectral lines were simply uniformly out of place (for example, as if we found all of the buildings in a major city shifted 50 miles south with the entire grid of roads left in place), the shifted spectra showed compositions similar to stars in our own galaxy.

The only available explanation for this red shift has been that these objects are all moving away from us at a rapid rate, suggesting that the universe itself is expanding. Furthermore, it appeared to be expanding at the same rate in all directions, an effect verified in the 1960s by measurements of microwave radiation. In a case of situational irony, both Sir Isaac Newton's theory of gravity and Albert Einstein's theory of relativity could predict an expanding universe, but in both cases the idea was dismissed as being attributed to flaws in the models.

The idea that the universe is expanding suggests (as Hawking explains it) that in the past, all the objects that are moving away from each other now must once have been much closer together. The implication is, according to Hawking, that the universe had a beginning and may have an end at some future time. These suppositions were supported by Russian mathematician Alexander Friedmann, who predicted in 1922 that Hubble's observations would be correct, taking into account two fundamental assumptions. One assumption was that the universe looks the same in every direction around us (in accounting for very distant objects). The other assumption was that the expansion of the universe would look the same when viewed from any galaxy other than our own.

These two assumptions led to three Friedmann models of origin-to-end for the universe. One model describes an arc by which, at a given point of expansion, the entire process reverses until all matter converges at the end, the so-called big crunch. Hawking explains it as one in which "space is bent in on itself, like the surface of the earth." The second model states that the expansion continues indefinitely, "bending space the other way like the surface of a saddle." The third model shows that with a balanced rate of expansion, space is infinitely flat.

Hawking continues with his own entry into the study of the big bang theory, which was built on the work of Roger Penrose. Penrose's work showed that a collapsing star can create a singularity. Hawking took Penrose's model and reversed the time, proposing it as a model for how a beginning—the big bang—might have happened. Penrose and Hawking jointly released a paper in 1970, arguing and convincing many that the big bang theory is true. However, Hawking has since changed his position and argues that no singularity existed at the big bang. He argues that quantum mechanics, or the behavior of subatomic particles, can offer a different explanation.


This chapter traces how the popular and scientific conceptions of the universe have changed, particularly during the 20th century. It is notable that even scientists have been reluctant to accept radical changes. One may see this reticence as commendable, reflecting professional skepticism of a scientist. On the other hand, one could also see it as a sort of professional self-defense—trying not to attract criticism for what initially seemed like outlandish predictions.

This chapter also highlights the fact that whether a model is accepted rejected sometimes depends on political and religious matters more than on science. For example, whereas many scientists resisted the big bang theory in part because it smacked of divine intervention, the Roman Catholic Church accepted the theory precisely because they saw it as in accordance with scripture. Similarly, the Penrose–Hawking singularity-based model for the big bang met with resistance because of both political and aesthetic reasons.

Another interesting tidbit in this chapter is found in the section where Hawking refers to his own PhD thesis. Had his ALS progressed more quickly, he might not have continued his studies. But as it was, two years after his diagnosis, Hawking was still feeling relatively well and looking to marry, so he had ample personal motivations to continue his studies and start a career as a researcher.

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