1.1 The Big Bang
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What is the Big Bang?
The enormous explosion thought to have created the universe.
How long ago did the Big Bang occur?
About 13.7 billion years ago.
What came into existence at the moment of the Big Bang?
All subatomic particles such as protons, neutrons, and electrons.
What was the temperature at the moment of the Big Bang?
Approximately 10³² kelvin.
What was the density at the moment of the Big Bang?
Approximately 10⁹⁶ g/cm³.
What was the universe’s temperature one second after the Big Bang?
About 10¹⁰ kelvin.
What happened to neutrons in the early universe?
They decayed into protons and electrons.
What was the half-life of neutron decay after the Big Bang?
Around 10 minutes.
Which two elements formed first as the universe cooled?
Hydrogen and helium.
Why was hydrogen more likely to form than helium?
It is the simplest element—with one proton and one electron.
What proportions of elements were formed after the Big Bang?
About 75% hydrogen and 25% helium.
What percentage of today’s universe is hydrogen and helium?
99%.
What percentage of the universe is made of elements heavier than helium?
About 1%.
What gases accumulate to form stars?
Hydrogen and helium.
What causes gas clouds to contract during star formation?
Gravitational attraction.
At what temperature does hydrogen burning begin?
About 10⁷ kelvin.
What is hydrogen burning?
Fusion of hydrogen atoms to form helium and release energy.
What is the reaction for hydrogen burning?
4¹H → ⁴He + 2e⁺ + 2ν + energy.
At what temperature does helium burning begin?
About 10⁸ kelvin.
What element forms when three helium atoms fuse?
Carbon (¹²C).
Up to which element can stars produce elements through fusion?
Iron-56 (⁵⁶Fe).
Why does fusion stop at iron-56?
Heavier elements require energy to form instead of releasing it.
What is binding energy per nucleon?
The energy holding the nucleus together per nucleon.
Why does fusion release less energy for heavier elements?
Binding energy per nucleon peaks at iron.