Radioactive Decay Reaction Used to Date Rocks¹

Parent

Daughter

Half-Life (billions of years)

Samarium-147

Neodymium-143

106

Rubidium-87

Strontium-87

48.8

Thorium-232

Lead-208

14.0

Uranium-238

Lead-206

4.47

Potassium-40

Argon-40

1.31

The Universe’s History

Cosmic Inflation

Big Bang and Nucleosynthesis

Recombination

Dark Ages

First Stars

Reionization

When?

13.8 bya

a fraction of a second later…

380,000 years later

next 200 million years

200 million years +

1 by +

What?

Universe expanded! (>speed of light!)

Energy transfers to matter/light, atomic start to form (H, He, Li, Be), too hot for electrons to bond with nuclei

Electrons bond with the nuclei, light can now travel because electrons are no longer scattering them

Dark b/c new H gas absorbing the light that is being produced. Still mostly H and He

First stars/galaxies (Thanks gravity!)

UV light breaks up the newly formed H gas into protons and elections (ions!)

The Universe’s History

Cosmic Inflation

Big Bang and Nucleosynthesis

Recombination

Dark Ages

First Stars

Reionization

When?

13.8 bya

a fraction of a second later

380,000 years later

Up to 200 million years later

200 million years plus

Up to 1 billion years later

What?

Universe expanded

energy transfers to matter/light, atomic nuclei form but too hot for electrons to attach

electrons combine with atomic nuclei, new nuclei created light (cosmic microwave background radiation)

Newly formed hydrogen gas absorbs all the light…

​

most of the universe is H and He

First stars/galaxies! (Thanks Gravity!)

UV light breaks up the H gas into protons and electrons (ions!)

The Universe’s History

Cosmic Inflation

Big Bang and Nucleosynthesis

Recombination

Dark Ages

First Stars

Reionization

When?

3.8 bya

3.8 bya

​

​

​

3.7 bya

What?

The universe expanded, lots of energy

Energy becomes matter/light,

first elements

Cools enough that electrons can stick to atoms, emit first light “cosmic glow” (microwaves)

New hydrogen atoms are absorbing all the light

Cooler denser regions become first stars, clump to form first galaxies (Go Gravity!)

UV radiation breaks up H gas into protons and electrons (so we can now see through things)