Chapter 16: The Expanding Universe

A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the metricseen on the left.

It took a heroic effort to measure the expansion rate of the universe. Hubble learned the difficulty of the measurement when he confused two types of Cepheid variable stars and got an expansion rate that was eight times too high. Since the inverse of the expansion rate is the age of the universe, his inferred age was just two billion years, younger than the age of the Earth! This was a crisis in astronomy for a while until the problem was understood. It took half a century and an effort involving dozens of astronomers and hundreds of nights of telescope time to measure the expansion rate to an accuracy of better than 10%. The past decade has seen the accuracy get to 5%, but it took an enormous effort involving dozens of telescopes and hundreds of astronomers.

Scatter plot of fit of redshifts to Hubble's law.

As cosmology developed in the 1960s it was known that the gravity of all the galaxies in the universe should act to retard or slow down the initial expansion rate. The Hubble constant is not actually a constant; it's a parameter that has varied over cosmic time. It is the tug of all the galaxies on each other that has slowed down the expansion, and that should be observable by extending the Hubble relation — the linear relationship or graph between galaxy distance and redshift — to larger distances and so larger lookback times. Remember that distant light is old light, so higher redshift galaxies are seen as they were back in time, when the expansion rate should have been higher. In the 1970s and 1980s, astronomers used large telescopes to try and trace the expansion history of the universe. Doing so would give a measure of the matter content of the universe. By the mid-1980s the answer seemed clear; the change in the expansion rate looking back in time was very small, indicating not nearly enough mass in the universe to overcome the initial expansion. The universe was destined to expand forever.

Distant Galaxies in the Hubble Ultra Deep Field

The most distant galaxies whose light we can gather are at a distance of 46 billion light years. This sounds wrong — how can there be galaxies this far away in a universe only 13.8 billion years old? The answer is that the universe was expanding faster than light for much of its early history. That is, the distance between any two points in expanding space-time was increasing faster than light speed. So there are regions of the universe we have never seen because the light has not been able to reach us in the entire history of the universe. Due to deceleration, some of those regions came into view. As the universe got older, more and more distant regions became observable. But recently the expansion rate has been increasing and some galaxies whose light we see now will in the future be removed from view. The universe is stranger and more amazing than we could possibly imagine!