The universe has been expanding and cooling since the Big Bang. As it cooled, its particles could combine into more complex structures. A simplified timeline:• t = 0: the Big Bang. The universe is extremely hot and dense, and space and time begin.
• t ≈ 10−35 s: a very rapid expansion (inflation). The universe is a hot 'soup' of fundamental particles: quarks, leptons and photons, with particles and antiparticles constantly created and annihilated.
• t ≈ 10−6 s: the universe has cooled enough for quarks to combine into hadrons, such as protons and neutrons. Slightly more matter than antimatter survives the annihilations.
• t ≈ 100 s to 20 minutes: protons and neutrons fuse to form helium nuclei (and a little deuterium and lithium). After this the universe is too cool and spread out for fusion, leaving about 75% hydrogen and 25% helium by mass.
• t ≈ 380 000 years: the temperature falls to about 3000 K, so nuclei capture electrons to form neutral atoms. Photons can now travel freely; this radiation is seen today as the CMB.
• t ≈ 200 million years: gravity pulls denser regions of gas together to form the first stars and galaxies. Fusion in stars, and supernovae, begin to make heavier elements.
• t ≈ 9 billion years: the Solar System forms from a cloud of gas and dust containing these heavier elements.
• t ≈ 13.8 billion years: the present day. The CMB has cooled to 2.7 K and the expansion of the universe is accelerating.The pattern: as the universe cooled, particles no longer had enough energy to break apart hadrons, then nuclei, then atoms, so each could form in turn.
• t ≈ 10−35 s: a very rapid expansion (inflation). The universe is a hot 'soup' of fundamental particles: quarks, leptons and photons, with particles and antiparticles constantly created and annihilated.
• t ≈ 10−6 s: the universe has cooled enough for quarks to combine into hadrons, such as protons and neutrons. Slightly more matter than antimatter survives the annihilations.
• t ≈ 100 s to 20 minutes: protons and neutrons fuse to form helium nuclei (and a little deuterium and lithium). After this the universe is too cool and spread out for fusion, leaving about 75% hydrogen and 25% helium by mass.
• t ≈ 380 000 years: the temperature falls to about 3000 K, so nuclei capture electrons to form neutral atoms. Photons can now travel freely; this radiation is seen today as the CMB.
• t ≈ 200 million years: gravity pulls denser regions of gas together to form the first stars and galaxies. Fusion in stars, and supernovae, begin to make heavier elements.
• t ≈ 9 billion years: the Solar System forms from a cloud of gas and dust containing these heavier elements.
• t ≈ 13.8 billion years: the present day. The CMB has cooled to 2.7 K and the expansion of the universe is accelerating.The pattern: as the universe cooled, particles no longer had enough energy to break apart hadrons, then nuclei, then atoms, so each could form in turn.