A universe with a birthday

Why is the night dark?

Here is a riddle that sounds too simple to be a problem. Why is the sky dark at night? Suppose the universe were endless, eternal, and unchanging, with stars scattered through it more or less evenly and shining forever. Then in whatever direction you looked, your line of sight would eventually end on the surface of some star, as every line of sight in a deep forest ends on a tree. Distant stars are fainter, but there are more of them: the number of stars in a thin shell grows exactly as fast as their light fades, so every shell adds the same amount of light, until the nearer stars block the ones behind them. The whole night sky would glow as bright as the surface of a star. Yet the sky is black.

The puzzle is called Olbers’ paradox, after Heinrich Olbers, a Bremen physician and astronomer, who stated it in 1823, though Kepler, Halley, and others had seen it before him. It shows how a simple observation can overturn a belief. What fails is the eternal, unchanging universe of stars that have always shone; space itself may still be endless. In 1848 the poet Edgar Allan Poe, of all people, wrote down the answer in his prose poem Eureka: perhaps the light of the farthest stars has not yet reached us. Then he set it aside, seeing “not even the shadow of a reason” to believe it. Yet that is the modern answer: the stars have not been shining forever. The universe of stars has an age.

The galaxies are running away

Evidence that the universe is changing came from the same telescopes that had found the galaxies. From 1912 Vesto Slipher, at the Lowell Observatory in Arizona, measured the light of spiral nebulae; by 1917 twenty-one of twenty-five were shifted toward the red, as the whistle of a train drops in pitch when it moves away. Their light was being stretched. In 1927 the Belgian priest and physicist Georges Lemaître worked out that this is what an expanding universe should look like, and in 1929 Edwin Hubble, combining his own distances with velocities measured mostly by Slipher, found a rough straight-line rule: the farther away a galaxy is, the faster it is receding. Double the distance, and the speed doubles. His rate was about seven times too high, and his paper did not claim expansion. In 2018 the International Astronomical Union recommended the name Hubble–Lemaître law.

It sounds as though we must be at the center, with everything fleeing from us. We are not. Picture raisins in rising bread dough, or dots drawn on a balloon that is being blown up. From any raisin, all the others move away, and the farther ones move faster. The dough has no center, and neither does the universe. It is not galaxies flying through space; space itself is stretching between them. The activity below lets you stand on any galaxy you like.

If the universe is expanding, run the film backward and everything crowds together into a hot, dense early state: the Big Bang. The English astronomer Fred Hoyle named it in a BBC radio talk in 1949; the name is often called a sneer, but his script suggests he wanted a vivid picture. Hoyle preferred a universe without a beginning, which could expand too, so expansion alone could not settle the matter. The evidence below did, though Hoyle never fully accepted it. In the standard model of cosmology, measurements put that hot, dense state about 13.8 billion years ago: the universe’s birthday, as a figure of speech. Whether anything came before is still open.

The afterglow of the Big Bang

An idea this large needs more than a trend in redshifts. The theory made a prediction. In its first few hundred thousand years the universe was a glowing, opaque fog of hot plasma, and light kept scattering off its free electrons, so no light could travel far. As the universe cooled to about 3,000 degrees above absolute zero, electrons and nuclei joined into neutral atoms, and over a few tens of thousands of years, ending about 380,000 years after the Big Bang, the fog cleared. Light was free to travel, and it has been traveling ever since. In 1948 Ralph Alpher and Robert Herman, working with George Gamow, calculated that this light should still be around us, stretched by the expansion into weak microwaves about five degrees above absolute zero. The prediction was largely forgotten. Sixteen years later, Robert Dicke’s group at Princeton worked it out again and began building a detector.

They were beaten to it by accident. In 1964 two young radio astronomers at Bell Labs in Holmdel, New Jersey, Arno Penzias and Robert Wilson, were calibrating a huge horn-shaped antenna, built to catch signals bounced off Echo, a giant balloon satellite. Their readings kept coming out about three degrees too warm: a faint, steady excess, the same in every direction, day and night, through the seasons. (The famous “hiss” was a meter reading, not a sound.) They checked the electronics. A pair of pigeons had been nesting in the horn, and they cleaned out what Wilson later called “a white material familiar to all city dwellers.” The excess barely changed. Then Penzias heard about the Princeton work and phoned Dicke, whose group explained what Bell Labs had found: the afterglow of the hot early universe. The two groups published side by side in 1965. Today the glow measures 2.7 degrees above absolute zero, cooler than the early predictions: right in kind, not in detail. Penzias and Wilson shared half of the 1978 Nobel Prize in Physics.

That background is smooth almost to perfection, but not quite. In 1990 the COBE satellite showed that the glow has almost exactly the spectrum of pure heat radiation, and in 1992 it found ripples in it of about one part in a hundred thousand. Ripples like these are tiny, but they are the seeds: slightly denser patches in the early fog pulled in more matter and grew into galaxies. John Mather, for the spectrum, and George Smoot, for the ripples, shared the 2006 Nobel Prize in Physics. COBE’s blurry view caught only the largest; WMAP and Planck mapped the finer ones so exactly that, within the standard model, they gave us the universe’s age and what it is made of.

A universe that speeds up

By the 1990s astronomers knew that the universe is expanding, and they asked the next question: is it slowing down? Gravity pulls every galaxy toward every other one, the way gravity slows a laser pointer that you toss in the air. So the expansion ought to be braking. Two independent teams set out to measure how much: one led by Saul Perlmutter in California, the other by Brian Schmidt from Mount Stromlo in Australia. Both used a kind of exploding star, the Type Ia supernova, as a standard candle: their peak brightness is nearly the same, and the small differences can be corrected from how fast each one fades. In 1998 both teams reported the same shocking result: distant supernovae were fainter than expected for a slowing universe. The expansion was not slowing down. It was speeding up. Perlmutter, Schmidt, and Adam Riess of Schmidt’s team shared the 2011 Nobel Prize in Physics.

Something is pushing space apart. We call it dark energy, and we do not know what it is. In the standard model, the universe’s total energy content is about 68 percent dark energy, about 27 percent dark matter, an unseen material that holds galaxies together, and only about 5 percent ordinary atoms. Everything in this course, and everything you have ever seen, is that last 5 percent. What dark energy does will decide the future. If it stays constant, the usual expectation, the galaxies will drift ever farther apart and the stars will slowly burn out, a “big freeze.” If it grows stronger, it could eventually tear galaxies, stars, and atoms apart in a “big rip.” As of 2026, results from the DESI galaxy survey hint that dark energy may instead be weakening, which would make a rip less likely and, in some models, could allow a distant “big crunch.” The hints have softened and are not proof; DESI’s full results are due around 2027. Cycles of collapse and rebirth remain a minority view. All of this hangs on something we do not understand.

Telescopes and China

If you list the people behind these discoveries, from Galileo to Hubble to Penzias and Wilson, almost all of them worked in Europe, North America, or Australia. That is awkward for China, whose astronomers had kept some of the world’s longest and most careful records of the sky: the “guest stars” (客星 kèxīng) of A.D. 185 and 1054, the second of which became the Crab Nebula, and the superb calendar of Guo Shoujing in the 1200s. Telescopes reached Beijing quickly: Adam Schall’s book on them, written with Li Zubai, appeared in 1626, only seventeen years after Galileo’s. They were soon put to work on the imperial calendar, yet China took little part in the discoveries that followed.

Why not? One famous explanation goes by the name of the Needham question, after the historian Joseph Needham: why did modern science arise in Europe rather than China, which had been far ahead? One common answer, which you met in Course 2, is that Chinese astronomy served the court, the calendar, the farming year, and the reading of omens, and was not asked to look for general laws of nature. In this course’s reading, when the need was met, the work stopped and never pushed toward the instrument that could look deeper. Another answer is simply about instruments. The power of a telescope depends mostly on its mirror: the bigger the mirror, the more light it collects, and so the fainter and farther the objects it can see. The aperture of the largest telescopes has grown steadily for four centuries. China’s two largest general-purpose optical telescopes for many years, at Xinglong (2.16 meters, 1989) and Lijiang (2.4 meters, 2008), are about the size of the Hooker telescope that Hubble used in 1923.