In 1964 two radio astronomers at Bell Labs in New Jersey, Arno Penzias and Robert Wilson, could not get rid of a faint hiss in their large horn-shaped antenna. It came from every direction, day and night. They checked their equipment, and even cleaned out the droppings of pigeons nesting inside the horn. The hiss remained. Only after a phone call to physicists at nearby Princeton did they understand what they had found: the afterglow of the birth of the universe. They shared the 1978 Nobel Prize in Physics.

From an expanding universe to a beginning

If the universe is expanding, then in the past it was smaller, denser and hotter. In 1931 Georges Lemaître pushed the idea all the way back and proposed that the universe began from a single "primeval atom." The rival Steady State theory, championed by the astronomer Fred Hoyle, held that the universe had no beginning and that new matter appeared continuously to fill the gaps as it expanded. On BBC radio in 1949, Hoyle described the idea he opposed as a "big bang." The name stuck.

The Big Bang theory says that about 13.8 billion years ago the universe was in an extremely hot, dense state and has been expanding and cooling ever since. It is important to understand what the theory does not say. It does not describe an explosion of matter into empty space from a single point. The expansion happened everywhere at once, and space itself has been stretching. The theory also does not explain what, if anything, came before or caused the hot dense state. That remains an open question.

Three pillars of evidence

Scientists accept the Big Bang because several independent lines of evidence point to it.

• Expansion: galaxies are receding from one another, as Hubble and Lemaître showed.
• The cosmic microwave background: for about 380,000 years the early universe was a glowing fog of hot gas that light could not travel through. When it cooled enough for atoms to form, the light was set free. Expansion has stretched that light into faint microwaves with a temperature of just 2.7 degrees above absolute zero, exactly the kind of signal the Big Bang predicted. In 1948 physicists Ralph Alpher and Robert Herman had predicted it, and Penzias and Wilson found it by accident.
• The light elements: in its first few minutes the universe was hot enough to fuse hydrogen into helium, but cooled too quickly to make much else. The theory predicts that about a quarter of ordinary matter by mass should be helium, with traces of deuterium and lithium. Observations of the oldest stars and gas clouds match.

Satellite missions such as COBE, WMAP and Planck later mapped the microwave background in great detail, revealing tiny ripples of slightly hotter and cooler regions. Those ripples were the seeds from which gravity later built galaxies.

Why is the night sky dark?

Here is an old puzzle with a cosmic answer. If the universe were infinite, unchanging and filled with stars, then every line of sight would eventually end on a star, and the whole night sky should blaze as bright as the Sun. This is called Olbers' paradox. The Big Bang resolves it. The universe has a finite age, so light from very distant stars has not had time to reach us, and the stretching of space dims and reddens the light that does arrive. The darkness between the stars is evidence that the universe had a beginning.

A brief history of everything

After the microwave background was released, the universe entered a "dark age" with no stars. Within roughly the first few hundred million years, gravity pulled gas together into the first stars and galaxies. The James Webb Space Telescope has spotted galaxies shining less than 300 million years after the Big Bang, some brighter than many astronomers expected, which is pushing researchers to refine their models of how quickly galaxies formed. Generations of stars lived and died, enriching gas with heavier elements. About 4.6 billion years ago, one cloud of that enriched gas collapsed to form the Sun and Earth.

Worked example: the cosmic calendar

Carl Sagan made the vast timescale easier to grasp by squeezing the history of the universe into a single calendar year. Using 13.8 billion years, each day stands for about 38 million years.

• January 1, midnight: the Big Bang.
• The first week of January: the first stars begin to shine.
• About September 1: the Sun and Earth form.
• December 30: an asteroid strikes and the last of the non-bird dinosaurs die out, 66 million years ago.
• December 31, about 11:48 p.m.: anatomically modern humans appear.
• The last 11 seconds before midnight: all of recorded history, from the first writing to today.

Every king, war, invention and work of art fits into the final seconds of the cosmic year. Seeing ourselves on that calendar is not meant to make us feel small, but to show how recent, and how precious, the human story is.