The Big Bang: Evidence for an Expanding Universe

Remember from Lessons 5 and 8

You learned that the farther away we look, the further back in time we see, and that the universe contains hundreds of billions of galaxies. Now for the biggest question of all: where did the universe come from?

A sound to start

Have you heard an ambulance siren as it rushes past? The pitch is higher as it comes toward you and lower as it moves away. This is called the Doppler effect. Light does the same thing. That simple idea, applied to galaxies, changed our understanding of the entire universe.

What is the Big Bang theory?

The Big Bang theory says that about 13.8 billion years ago, the universe was extremely hot and dense, and it has been expanding and cooling ever since.

Two common misunderstandings:

• It was not an explosion in space, like a firework going off in an empty room. It was the expansion of space itself, everywhere at once.
• There is no centre that everything is flying away from. Every galaxy sees the others moving away, like dots on an inflating balloon.

The Big Bang theory describes what happened after the universe began expanding. Science does not yet have a tested answer for what came before, or why it began.

Tool: reading the light of galaxies

When light from a star or galaxy passes through a spectroscope, it spreads into a spectrum of colours. Elements in the stars absorb certain exact colours, leaving dark lines, like a barcode. Each element has its own barcode, which is how we know what the Sun and distant stars are made of.

In the early 1900s, astronomers including Vesto Slipher noticed that the barcode lines in most galaxies were shifted toward the red end of the spectrum. By the Doppler effect, light from an object moving away is stretched to longer wavelengths: redshift. (Light from an object moving toward us is shifted toward blue: blueshift.)

Evidence 1: Redshift and the expanding universe

• In 1927, Belgian physicist and priest Georges Lemaître proposed that the universe is expanding.
• In 1929, Edwin Hubble combined galaxy redshifts with distance measurements and found that the farther away a galaxy is, the faster it is moving away. This is Hubble's law (today often called the Hubble–Lemaître law). His distances relied on the method discovered by Henrietta Swan Leavitt, who found how to use a type of pulsating star (Cepheid variables) to measure distance.
• If all galaxies are moving apart now, then in the past they were closer together. Run the movie backward and everything was once packed into an extremely hot, dense state.

The balloon model: draw dots on a balloon and blow it up. Every dot moves away from every other dot, and dots that started farther apart separate faster. No dot is the centre. That is what we observe with galaxies.

Evidence 2: The cosmic microwave background (CMB)

The Big Bang theory predicted that the hot early universe would have filled space with light. As the universe expanded, that light would stretch and cool until today it would be faint microwaves, coming from every direction.

In 1964–65, Arno Penzias and Robert Wilson, working with a radio antenna in the United States, found a faint, steady microwave 'hiss' coming from every direction in the sky. It was the cosmic microwave background: light released when the universe was about 380 000 years old, now cooled to about −270 °C (2.7 degrees above absolute zero). They won the 1978 Nobel Prize in Physics.

Canadian connection: Winnipeg-born physicist James Peebles worked out, in the 1960s and later, what the CMB should be like and what it reveals about the universe's history and contents. He shared the 2019 Nobel Prize in Physics for this work.

Evidence 3: The amounts of hydrogen and helium

The theory predicts that in the first few minutes, the universe was hot enough for fusion to make helium from hydrogen, but cooled too quickly to make much else. It predicts a universe of roughly three-quarters hydrogen and one-quarter helium by mass. That is what astronomers measure in the oldest stars and gas clouds.

Evidence 4: Distant galaxies look young

Because looking far away means looking back in time, the most distant galaxies show the universe when it was young. Telescopes such as the James Webb Space Telescope (which includes Canadian-built instruments) see that very distant galaxies are smaller and less developed, just as an evolving, expanding universe predicts.