Cosmology

Big Bang Theory

Definition

The Big Bang Theory states that:

• the universe began from an extremely hot, dense state,

• then expanded rapidly,

• and continues expanding today.

It was an expansion of space, not a normal explosion.

Key Ideas

Expansion of the Universe

Galaxies are moving away from each other.

Farther galaxies move away faster.

This suggests:

• the universe was once much smaller and denser.

Redshift and Blueshift

Redshift

When a galaxy moves away:

• light stretches,

• wavelength becomes longer,

• spectrum shifts toward red.

This is called redshift.

Blueshift

When a galaxy moves toward us:

• wavelength becomes shorter,

• spectrum shifts toward blue.

This is called blueshift.

Most galaxies show redshift → universe is expanding.

Edwin Hubble

Hubble’s Discovery (1929)

Edwin Hubble discovered:

• distant galaxies move away faster,

• expansion rate is proportional to distance.

This is called Hubble’s Law.

Georges Lemaître

Georges Lemaître proposed:

• the universe was once compressed into a tiny dense state,

• expansion began from this original state.

Evidence for the Big Bang

1. Expansion of the Universe

Observed through redshift and Hubble’s Law.

2. Abundance of Light Elements

Helium

About:

• 25% helium,

• 75% hydrogen.

Helium abundance supports the Big Bang because:

• stars alone cannot explain so much helium.

Deuterium

Deuterium:

• formed in early hot universe,

• cannot survive long inside stars.

Its existence supports Big Bang nucleosynthesis.

Big Bang Nucleosynthesis

Definition

Formation of light elements shortly after the Big Bang.

Produced:

• hydrogen,

• helium,

• deuterium,

• small amounts of lithium and beryllium.

Main scientist:
George Gamow

Cosmic Microwave Background Radiation (CMB)

Definition

Faint leftover radiation from the Big Bang.

Discovered accidentally by:

• Arno Penzias

• Robert Wilson

Properties:

• comes from all directions,

• extremely uniform,

• oldest observable light in universe.

COBE Satellite

NASA launched:

• Cosmic Background Explorer (COBE).

Purpose:

• map cosmic background radiation.

Result:

• radiation matched blackbody spectrum predictions.

Blackbody

Definition

A perfect absorber and emitter of radiation.

The cosmic background radiation behaves like blackbody radiation.

Age of the Universe

Evidence:

• radioactive dating,

• oldest stars,

• expansion rate.

Estimated age:

• around 13–14 billion years.

Dark Matter

Definition

Invisible matter detected through gravity.

Properties:

• does not emit light,

• not normal baryonic matter,

• not antimatter.

Possible composition:

• exotic particles such as axions.

Universe composition:

• ~27% dark matter.

Dark Energy

Definition

Unknown force causing accelerated expansion of universe.

Universe composition:

• ~68% dark energy.

Effects:

• pushes galaxies apart,

• accelerates expansion.

Universe Composition

Approximate composition:

• 68% dark energy

• 27% dark matter

• 5% normal matter

Fate of the Universe

1. Big Crunch

Expansion stops and reverses.
Universe collapses inward.

Largely ruled out.

2. Big Chill

Universe expands forever.
Stars die out.
Universe becomes cold and dark.

3. Big Rip

Expansion accelerates so much that:

• galaxies,

• stars,

• planets,

• atoms

are torn apart.

Depends on behavior of dark energy.

Steady State Theory

Definition

Universe:

• always existed,

• always looks the same,

• matter continuously created.

Supported by:

• Fred Hoyle

• Hermann Bondi

• Thomas Gold

Mostly rejected after discovery of CMB.

Lambda-CDM Model

Current Accepted Cosmological Model

Components:

• Lambda (λ) = dark energy

• CDM = Cold Dark Matter

Explains:

• universe expansion,

• structure formation,

• observable universe evolution.

Important Vocabulary

Most Important Points to Memorize

1. Universe is expanding.

2. Redshift supports expansion.

3. Cosmic background radiation strongly supports Big Bang.

4. Big Bang nucleosynthesis explains helium and deuterium abundance.

5. Dark energy accelerates expansion.

6. Universe composition:

   • 68% dark energy

   • 27% dark matter

   • 5% normal matter.