Ask Ethan: Does cosmic inflation violate energy conservation? | by Ethan Siegel | Starts With A Bang! | Jun, 2025

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When the Universe undergoes a period of cosmic inflation, the fabric of space — which has a tremendously high energy density — expands at an exponential rate, creating new space (also full of large amounts of energy) with each moment that elapses. This appears to violate the conservation of energy, and in fact energy is not conserved in this scenario. (Credit: Ben Gibson / Big Think / Adobe Stock)

A few physical quantities, in all laboratory experiments, are always conserved: including energy. But for the entire Universe? Not so much.

Here in our ever-changing Universe, there are a few things we can rely on remaining constant and unchanged. From place-to-place and moment-to-moment, the laws of physics don’t change; they remain constant, and apply equally across the entire span of the Universe. The values of the fundamental constants — the parameters that determine the masses of particles and the strength of forces — don’t change with time or across space. And in every experiment we’ve ever performed, we’ve seen that certain quantities are always conserved: the total amount that you start with always equals the total amount that you end with.

Many conserved quantities abound: electric charge, linear momentum, angular momentum, and, of course, energy. The conservation of energy is maybe the best known of the conservation laws, and applies to objects on Earth just as well as it applies to the Solar System, the stars, and the entire Milky Way. But if you go way, way back in cosmic history, would you find that, just possibly, energy isn’t conserved after all? That’s what Michael Genovese wants to know, asking:

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