I'm currently reading "Many Worlds in One: The Search for Other Universes" by Alexander Vilenkin to entertain my curiosity in cosmology and metaphysics while applying to medical school. Anyhow, the current chapter is discussing George Gamow and his thoughts on the nature of the universe at the exact moment of and immediately following the Big Bang. One sentence struck me as curious when it said "the most eventful part of the fireball history, marked by a rapid succession of exotic particle populations, occurred during the first second of its existence." How is this second measured? Is it a second as we would perceive it here on Earth, or would this first second last on the order of an infinite number of years relative to Earth seconds due to the rate at which space was expanding during the first second (measured in "Big Bang seconds")?