Big Bang as accepted model

The Big Bang account, stage by stage

Everything starts here: all the matter in the universe in one tiny region of space, incredibly dense and incredibly hot.

Drag the scrubber (or press play) and watch what each stage needs before the next one can happen.

Predict, then check

Two galaxies, one question. Commit first, then look.

Galaxy A is fairly close to us. Galaxy B is much further away. Whose absorption lines show the greater redshift?

What you need to know

• The Big Bang theory says all the matter in the universe once sat in one tiny region of space, incredibly dense and incredibly hot.
• That region expanded rapidly. It was not an explosion of existing matter, and the temperature fell as it expanded.
• As it cooled, electrons, protons and neutrons formed, then atoms. Gravity pulled atoms together into stars and galaxies.
• Have a goQuick sort: which forms first, the atoms or the electrons, protons and neutrons? And what has to happen before the next stage?

  Electrons, protons and neutrons form first. Atoms form as the universe cools further.

  Atoms are formed from those particles as they cool further, so the particles have to come first.

• Redshift means the wavelength of light increases because a star or galaxy is moving away from us.
• Light from nearly all galaxies is redshifted, and the faster a galaxy moves away, the bigger the shift in its absorption lines.
• Hubble's law says a galaxy's distance from the Milky Way is directly proportional to its velocity away from us.
• Have a goGalaxy Zed is 3 times as far from us as galaxy Zip. Zed insists it is moving away at exactly the same speed as Zip. Is Zed right?

  No. Zed is moving away 3 times as fast as Zip.

  Distance is directly proportional to velocity, so tripling the distance triples the velocity.

• Nearly every galaxy is moving away from every other galaxy, so the Milky Way is not special and there is no centre.
• Galaxies are spreading apart over time, so they must have been closer together in the past.
• Have a goImagine playing a film of the galaxies spreading apart backwards. What happens to the gaps between them?

  The gaps shrink: the galaxies get closer together.

  If galaxies are spreading apart over time, they must have been closer together in the past.

• Cosmic microwave background radiation (CMBR) comes from the hot early universe, stretched into microwaves, and arrives from every direction.
• The steady state model could not explain the CMBR, so scientists rejected it in favour of the Big Bang theory.

The big picture

The Big Bang theory says the universe started in a tiny, incredibly hot, dense region that expanded and cooled, forming particles, atoms, stars and galaxies. It is the accepted model because the galaxies are moving apart (redshift and Hubble's law) and the cosmic microwave background radiation fits it.

⚠ Watch out

Picturing the Big Bang as matter exploding outwards from a centre that we are near. It was a tiny region of space expanding rapidly, and nearly every galaxy moves away from every other, so there is no centre.

Memory hook

Three clues, one story: galaxies rushing apart, an age of about 14 billion years when they were all together, and a microwave glow arriving from every direction.

Check yourself

In two sentences: why is the CMBR evidence for the Big Bang theory and a problem for the steady state model?