As late as 1920, many leading astronomers believed that the Milky Way was the entire universe. In April of that year two of them, Harlow Shapley and Heber Curtis, met at the National Academy of Sciences in Washington for what became known as the Great Debate. Shapley argued that the faint spiral clouds in the sky were nearby gas within our galaxy. Curtis argued they were "island universes," whole galaxies of their own, enormously far away. Within five years the question was settled, and the universe grew beyond anything either side had imagined.

Our home galaxy

On a dark night the Milky Way is a pale band of light across the sky. Galileo's telescope showed that it is made of countless faint stars. We now know it is a spiral galaxy about 100,000 light-years across, containing somewhere between 100 and 400 billion stars, with the Sun about 26,000 light-years from the center. We see it as a band because we live inside its flat disk and look along it. The Sun takes about 230 million years to orbit the galactic center once. At its heart lies a black hole called Sagittarius A*, about four million times the mass of the Sun, which the Event Horizon Telescope imaged in 2022.

A ruler for the cosmos

Settling the Great Debate required a way to measure huge distances. It came from Henrietta Swan Leavitt, another of the Harvard computers. Studying a type of pulsing star called a Cepheid variable, she found in 1912 that the longer a Cepheid takes to brighten and dim, the more light it truly gives off. That meant that by timing a Cepheid's pulse, astronomers could know its true brightness, and by comparing that with how bright it appears, work out how far away it is. Leavitt had given astronomy a measuring stick for the universe.

Hubble finds other galaxies

In 1923 Edwin Hubble, using the giant 100-inch telescope on Mount Wilson in California, found a Cepheid in the Andromeda "nebula." Excited, he crossed out a label on the photographic plate and wrote "VAR!" for variable. Leavitt's relation put Andromeda far beyond the edge of the Milky Way. His estimate was too small, as we now know Andromeda is about 2.5 million light-years away, but the conclusion was certain: Andromeda is a separate galaxy, and so are the other spiral nebulae. Curtis had been right. Today we estimate the observable universe holds at least hundreds of billions of galaxies.

The universe is expanding

Meanwhile, astronomer Vesto Slipher had measured the spectra of many spiral galaxies and found most of them redshifted, moving away from us at high speed. In 1927 the Belgian physicist and priest Georges Lemaître showed that Einstein's theory of gravity allowed for an expanding universe and that the speeds and distances fit. In 1929 Hubble published his own data showing the pattern clearly: the farther away a galaxy is, the faster it is receding. This relationship is now called the Hubble-Lemaître law.

It does not mean that we sit at the center. Picture raisins in a loaf of bread rising in the oven. As the dough expands, every raisin moves away from every other, and the farther apart two raisins are, the faster they separate. An observer on any raisin would see the same thing. Galaxies are not flying through space away from us; space itself is stretching, carrying the galaxies with it.

The dark side of the universe

In 1933 Fritz Zwicky noticed that galaxies in the Coma cluster moved too fast to be held together by the gravity of their visible stars. In the 1970s Vera Rubin and Kent Ford found that stars in the outer parts of spiral galaxies orbit just as fast as those near the center, when they should move more slowly. Both results point to large amounts of unseen mass, called dark matter. It gives off no light, and despite decades of searching, no one yet knows what it is.

In 1998 two teams measuring distant exploding stars found that the expansion of the universe is not slowing down, as expected, but speeding up. The unknown cause is called dark energy. By current estimates, ordinary matter, everything made of atoms, is only about 5 percent of the universe's contents. Dark matter is about 27 percent and dark energy about 68 percent. Admitting that we do not understand 95 percent of the cosmos is not a failure of science. It is an honest map showing where the unexplored territory lies.

Worked example: estimating the age of the universe

If galaxies are flying apart, run the film backward and they must once have been together. The expansion rate, called the Hubble constant, lets you estimate when.

• The rate: roughly 70 kilometers per second for every megaparsec of distance (a megaparsec is about 3.26 million light-years).
• The logic: a galaxy at distance d moving at speed v needed a time of d ÷ v to get there. Under the Hubble law that time is the same for every galaxy: 1 ÷ the Hubble constant.
• The arithmetic: one megaparsec is about 31 billion billion kilometers. Divide by 70 kilometers per second and you get about 440 million billion seconds, which is about 14 billion years.

This rough estimate lands remarkably close to the best modern value of 13.8 billion years. Astronomers still argue about the exact expansion rate, because different methods give values of about 67 and about 73. That disagreement, the Hubble tension, may be an error or may be a clue to new physics.