The Hot Big Bang Model and Cosmic Evolution

Key Insight

The generally accepted history of the universe begins with the hot big bang model, where the universe had zero size and was infinitely hot. As it expanded, the temperature rapidly decreased; by one second after the big bang, it had fallen to about 10000000000 degrees, a temperature comparable to H-bomb explosions. At this stage, the universe consisted mainly of photons, electrons, neutrinos, their antiparticles, and some protons and neutrons. As cooling continued, electron/antielectron pairs annihilated, leaving few electrons. Neutrinos and antineutrinos, due to their weak interaction, did not annihilate and are theorized to still exist today, providing a potential test for this hot early stage, although their low energy makes direct observation challenging. If neutrinos possess a small mass, as some experiments suggest, they could constitute a form of dark matter.

Around 100 seconds after the big bang, the temperature had dropped to 1000000000 degrees, similar to the hottest stars. At this temperature, protons and neutrons combined due to the strong nuclear force to form atomic nuclei: deuterium (one proton, one neutron), then helium (two protons, two neutrons), and small quantities of heavier elements like lithium and beryllium. Calculations suggest approximately a quarter of the initial protons and neutrons were converted into helium nuclei, with the remaining neutrons decaying into protons, forming ordinary hydrogen. This model also predicted that residual radiation from the early, hot universe would still be present, cooled to a few degrees above absolute zero (-273°C), a prediction famously confirmed by Penzias and Wilson in 1965. This agreement, along with the correct explanation for the universe's helium abundance, reinforces confidence in the hot big bang picture back to about one second after its inception.

Within a few hours of the big bang, the production of elements ceased. For the subsequent million years, the universe continued to expand and cool without significant events. When the temperature fell to a few thousand degrees, electrons and nuclei combined to form atoms. Regions with slightly higher density experienced slowed expansion due to stronger gravitational attraction, eventually halting expansion and initiating collapse. This collapse, combined with rotational effects from external matter, led to the formation of disklike spiral galaxies, while non-rotating regions became oval elliptical galaxies. Within these galaxies, hydrogen and helium gas collapsed further, heating up to ignite nuclear fusion reactions, thus forming stars like our sun. More massive stars consume their fuel faster, then collapse to dense states like neutron stars or black holes, often expelling outer layers in supernova explosions that enrich galactic gas with heavier elements. Our sun, a second or third-generation star, formed about 5000000000 years ago from such enriched gas, with a small amount of heavier elements aggregating to form planets like Earth. Earth initially cooled from a hot state and developed an atmosphere from emitted gases, which, through the evolution of primitive, self-reproducing macromolecules in the oceans that consumed hydrogen sulfide and released oxygen, gradually became the life-sustaining atmosphere we observe today, leading to the development of higher life forms.