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 ====== 1.3. Synthesis of elements ======
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 Immediately after the Big Bang, the universe was a blazing furnace of energy where temperatures were too extreme for even simple atoms to survive. Within microseconds, the cosmos cooled enough for fundamental **quarks** to combine into **protons** (made of two "up" and one "down" quark) and **neutrons** (made of one "up" and two "down" quarks). However, the path to building heavier elements was blocked by the "deuterium bottleneck." For several minutes, intense radiation acted like a cosmic hammer, instantly shattering **deuterium**—a "heavy" hydrogen isotope made of one proton and one neutron—as soon as it formed. This fragile stepping stone was vital for cosmic evolution but could not endure the heat until the temperature fell below 1 billion Kelvin. Roughly two minutes after the start of time, the radiation finally eased, allowing deuterium to survive and triggering the next stage of creation.