Physics

8.5 - The Origin of the Universe

3 min

note 1 of 4

The expanding Universe points to a beginning

Nearly all galaxies are moving away from one another, and the more distant a galaxy is, the faster it is moving away.

The balloon model

Imagine a balloon with spots painted on it: as it inflates, the rubber stretches and every spot moves further from every other spot.
The balloon's surface represents space itself and each spot represents a galaxy.
Space is stretching and carrying the galaxies apart, so the whole Universe is expanding.

Working backwards

Because galaxies are moving apart now, in the distant past they must have been much closer together.
Following this backwards leads to the conclusion that the Universe began from a single, extremely small region - the starting point for the Big Bang theory.

A small balloon with orange spots close together, an arrow labelled balloon inflates, and a larger balloon with the spots spread apart; labels read spots represent galaxies and surface represents space.

note 2 of 4

The Big Bang theory

The Big Bang theory is the accepted explanation of how the Universe began.
All the matter and energy in the Universe was once concentrated into a single, tiny region that was extremely hot and dense.
It then began to expand rapidly - the Big Bang - and space has been expanding ever since.

The age of the Universe

The theory gives the Universe a finite age.
Working back from the expansion rate gives an age of roughly 14 billion years (about 13.8 billion years).

A cone widening to the right along an arrow labelled time, from the Big Bang at 0 years on the left to galaxies spreading apart at today, 14 billion years, on the right.

note 3 of 4

Microwave radiation from every direction

Detectors pointed anywhere in the sky pick up a faint glow of microwave radiation.
It arrives from every direction at a specific frequency.
This is the cosmic microwave background radiation (CMBR or CMB).
It doesn't come from any single object - it fills all of space.

note 4 of 4

CMBR is stretched-out radiation from the early Universe

CMBR is leftover radiation released shortly after the Universe began.
When first emitted, it was high-energy, short-wavelength radiation.
As space expanded, the radiation was stretched along with it to longer wavelengths, reaching the microwave region today.
Because the Big Bang model predicted this leftover radiation, the CMBR is key evidence supporting the hot Big Bang model.

key terms

TermDefinition

Big Bang theoryThe theory that all the matter in the Universe was once packed into a very small, extremely hot and dense region, and that the Universe has been expanding from it ever since.

Cosmic microwave backgroundFaint electromagnetic radiation filling all of space, left over from the very early Universe.

end of notes

Key points to remember

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9.1 - Change of State (Cooling & Heating Curves)Next up

The expanding Universe points to a beginning

Nearly all galaxies are moving away from one another, and the more distant a galaxy is, the faster it is moving away.

The balloon model

Imagine a balloon with spots painted on it: as it inflates, the rubber stretches and every spot moves further from every other spot.

The balloon's surface represents space itself and each spot represents a galaxy.

Space is stretching and carrying the galaxies apart, so the whole Universe is expanding.

Working backwards

Because galaxies are moving apart now, in the distant past they must have been much closer together.

Following this backwards leads to the conclusion that the Universe began from a single, extremely small region - the starting point for the Big Bang theory.

The Big Bang theory

The Big Bang theory is the accepted explanation of how the Universe began.

All the matter and energy in the Universe was once concentrated into a single, tiny region that was extremely hot and dense.

It then began to expand rapidly - the Big Bang - and space has been expanding ever since.

The age of the Universe

The theory gives the Universe a finite age.

Working back from the expansion rate gives an age of roughly 14 billion years (about 13.8 billion years).

CMBR is stretched-out radiation from the early Universe

CMBR is leftover radiation released shortly after the Universe began.

When first emitted, it was high-energy, short-wavelength radiation.

As space expanded, the radiation was stretched along with it to longer wavelengths, reaching the microwave region today.

Because the Big Bang model predicted this leftover radiation, the CMBR is key evidence supporting the hot Big Bang model.