The Big Bang Theory lessons
An expanding universe
Galaxies on a stretching grid. From any home galaxy the others move away at v = H d, while a time slider runs from early on to today.
The Big Bang theory says our universe began in a hot, dense state about 13.8 billion years ago. Ever since, it has been expanding and cooling.
Galaxies are not flying apart through space. Space itself is stretching, and it carries the galaxies along.
This course follows that story from the first split second to the galaxies we see today. It also points out the parts scientists are still working on.
Not an explosion
The Big Bang was not a bomb going off at one spot. It happened everywhere at once, because all of space was squeezed together.
So the universe has no centre that things fly away from. From any galaxy, the others all seem to rush away, just as in the example above.
Atoms, planets and galaxies hold together and do not stretch. Only the space between distant galaxies grows.
Practice: Myth or fact
True or false statements about what the Big Bang theory does and does not say.
Stretched light
Light from a distant galaxy can travel for billions of years to reach us. While it travels, space stretches, and the light waves stretch with it.
A light wave crosses stretching space from a galaxy to us; its wavelength grows by the same factor as space, and its spectral line slides towards the red.
Longer waves look redder, so this is called redshift. We measure it using the dark lines in a galaxy's spectrum, which sit further to the red than they do in a lab.
Redshift: z = \frac{\lambda_{\text{observed}} - \lambda_{\text{emitted}}}{\lambda_{\text{emitted}}}
A redshift of z = 0.1 means every wave arrived 10% longer than it left. Space has stretched by that same 10% since then.
Practice: Line up the lines
Stretch the lab spectrum until its hydrogen lines line up with a galaxy's, then read off the redshift.
Hubble's law
In 1929, Edwin Hubble compared how far away galaxies are with how fast they move away. The farther away a galaxy was, the faster it was leaving.
Hubble's law: \textcolor{#f871b9}{v} = \textcolor{#8b5cf6}{H_0}\textcolor{#3b82f6}{d}
Here v is the speed in km/s and d is the distance in megaparsecs (Mpc). One megaparsec is about 3.26 million light years.
Galaxies plotted by distance and speed lie close to one straight line; each highlighted galaxy gives v divided by d of about 70.
The slope of the line is the Hubble constant, H_0. It is about 70 km/s per megaparsec, so each extra megaparsec adds about 70 km/s of speed.
Practice: Use Hubble's law
Find a galaxy's speed from its distance, or its distance from its speed, with H0 = 70 km/s per Mpc.
How old is the universe?
Run the expansion backwards and every galaxy heads back towards us. One twice as far away moves twice as fast, so they all arrive together.
Galaxies at four distances rewind towards us at v = H0 d and all arrive at the same moment, 1/H0 ago, for three values of H0.
That moment was about \frac{1}{H_0} ago, roughly 14 billion years. The expansion rate has changed over time, and allowing for that gives the best answer: 13.8 billion years.
Nearby galaxies give H_0 near 73, but the early universe points to about 67. Nobody knows why yet, and the gap is called the Hubble tension.
Practice: Rewind the clock
Reason about the age of the universe from the expansion rate, the oldest stars and the Hubble tension.
Cosmic inflation
In its first tiny fraction of a second, space grew far faster than at any time since. This burst is called cosmic inflation.
A size ruler running over 30 powers of ten: a patch far smaller than a proton grows to the size of a grapefruit while the clock runs from 10^-36 to 10^-32 seconds.
Between about 10^{-36} and 10^{-32} seconds, a patch far smaller than a proton grew to about the size of a grapefruit. That is a stretch of at least 10^{26} times.
Inflation explains why the universe looks so smooth and so flat. It is the leading idea, but it has not been confirmed directly yet.
Practice: What inflation explains
Match each thing we observe to the way inflation explains it.
The first minutes
After inflation, the universe was a hot soup of particles. Less than a thousandth of a second in, quarks had joined up into protons and neutrons.
For a few minutes, all of space was hotter than the centre of the Sun. Protons and neutrons fused into the first nuclei, which is called Big Bang nucleosynthesis.
Sixteen protons and neutrons cool from 10 billion K: deuterium survives at about 3 minutes, helium forms, and the mass ends up 75% hydrogen and 25% helium.
There was about one neutron for every seven protons, and almost every neutron ended up in helium. That left about 75% of the mass as hydrogen and 25% as helium, just as we measure in the oldest gas.
Practice: Count the helium
Given the ratio of neutrons to protons, work out the percentage of the mass that ends up in helium or hydrogen.
The first light
For 380,000 years the universe was a glowing fog. Free electrons scattered light in every direction, so it could not travel far.
Photons zigzag between free electrons until the temperature falls to about 3000 K; electrons join nuclei to make atoms and the light flies straight.
As the universe cooled to about 3000 K, electrons joined nuclei to make the first atoms. Atoms let light pass, so the fog cleared and the light flew free.
This moment is called recombination. The light set free then is the oldest light we can see.
The cosmic microwave background
That first light still arrives from every direction. Space has stretched about 1100 times since it left, so its waves are now microwaves.
This glow is the cosmic microwave background (CMB). Its temperature has dropped from about 3000 K to just 2.7\ \text{K}.
A map of the whole sky in microwaves; boosting the contrast ten thousand times reveals hot and cold spots a few parts in 100,000 apart.
The CMB is almost exactly the same in every direction. Its tiny hot and cold spots are the seeds that later grew into galaxies.
Practice: Stretch the glow
Stretch the 3000 K glow until it reaches a target temperature, watching its peak wavelength slide from infrared to microwaves.
Dark ages and the first stars
After the CMB was released, there were no stars yet. This long, dark stretch of time is called the cosmic dark ages.
Gravity pulled gas into the denser spots, and the first stars lit up 100 to 200 million years after the Big Bang. Their ultraviolet light then split the hydrogen gas around them, clearing the way for starlight.
Galaxies switch on across a field of hydrogen gas and blow clear bubbles that grow and merge, with a timeline from the CMB to 1 billion years.
The James Webb Space Telescope has spotted galaxies from less than 300 million years after the Big Bang. There are more bright ones than many models expected, so astronomers are rethinking how fast galaxies grew.
Practice: Order the story
Put events from the history of the universe in order, earliest first.
Dark matter
Stars near the edge of a galaxy orbit much faster than its visible matter can explain. Something unseen must add extra gravity.
Stars orbit a galaxy at three distances; with visible matter alone the outer ones should slow down, but the measured speeds stay high until a dark matter halo is added.
We call it dark matter. It does not give off or block light, but its gravity holds galaxies and clusters together.
Light bent around galaxy clusters and the pattern in the CMB tell the same story. Yet no dark matter particle has been detected, despite decades of searching.
Dark energy
In 1998, astronomers measured distant exploding stars and found a surprise. The expansion of the universe is speeding up, not slowing down.
The size of space over time: gravity slows the expansion at first, then dark energy takes over and it speeds up; a universe without dark energy keeps slowing.
Whatever drives this is called dark energy. Its push started to win over gravity about 6 billion years ago.
Dark energy makes up about 68% of the universe and dark matter about 27%. Everything made of atoms is only about 5%.
Practice: The cosmic budget
Complete a table of what the universe is made of: each part's share and what it does.
The evidence
Three main clues support the Big Bang. Galaxies are moving apart, a cool microwave glow fills space, and the oldest gas is about a quarter helium.
The theory predicted these clues rather than just fitting them afterwards. The CMB was predicted in 1948 and found by accident in 1965.
No rival idea explains all three together. That is why almost every cosmologist accepts the Big Bang.
Practice: Sort the evidence
Sort observations by which clue to the Big Bang they belong to: expansion, light elements or the cosmic glow.
What we still don't know
Nobody knows what dark matter or dark energy actually is. Together they make up about 95% of the universe.
Results from the DESI survey hint that dark energy may be weakening over time, but they are not yet certain. The Hubble tension is also still open, and either one could point to new physics.
We also do not know what powered inflation, or whether anything came before it. New telescopes and surveys are testing these questions right now.