10.4 - The Big Bang

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).

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.

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.

The temperature of the background radiation

• The cosmic microwave background matches the radiation emitted by a black body at a temperature of about 3 kelvin (roughly -270°C), so it's also called 3 K background radiation.
• It was first detected in the 1960s by physicists Arno Penzias and Robert Wilson.
• The Big Bang model is currently the only model that successfully explains this radiation.

How the Universe evolved after the Big Bang

As the early Universe expanded and cooled:

1. In the first moments, the Universe expanded extremely rapidly and cooled.
2. As it cooled, neutrons and protons formed.
3. Further cooling let these combine into the first atomic nuclei.
4. Much later, when the temperature had dropped far enough, electrons combined with nuclei to form the first atoms of hydrogen.