During the period of inflation, the universe may have briefly expanded at \(10^{30}\) (a million trillion trillion) or more times the speed of light. Why did this ultra-rapid expansion not violate Einstein's special theory of relativity, which says that neither matter nor communication can travel faster than the speed of light?

Special relativity, proposed by Albert Einstein in 1905, changed our understanding of space and time. One of its key principles is that nothing can travel faster than light in a vacuum. This speed limit is approximately 299,792,458 meters per second. Special relativity also introduced the concept that time and space are intertwined in what we call spacetime. This theory has been confirmed by many experiments and is a cornerstone of modern physics.
In special relativity, the speed of light acts as an ultimate speed limit. No object with mass can reach, let alone exceed this speed. This is why you often hear that faster-than-light travel is impossible. The theory ensures that cause precedes effect, maintaining the logical flow of events.
Even though special relativity imposes limits on speed, it does not say anything about the expansion of space itself. This distinction opens the door for understanding how rapid cosmic inflation is possible without breaking Einstein's theory.

The concept of the expanding universe is central to cosmology. Edwin Hubble first observed in the 1920s that galaxies are moving away from us, suggesting that the universe is expanding. This discovery led to the Big Bang theory, the idea that the universe started from an extremely hot and dense state and has been expanding ever since.
The expansion of the universe means that space itself is getting larger. Imagine dots on a balloon. As the balloon inflates, the dots move away from each other, not by traveling across the balloon's surface but because the surface itself is expanding. Similarly, galaxies are moving apart because the space between them is expanding.
This expansion is uniform on large scales, meaning it occurs everywhere in the universe. Importantly, it is not about galaxies rushing through space; instead, space itself is stretching. This subtlety is why the rapid expansion during inflation can occur without violating special relativity.

The metric expansion of space is a way to describe how distances between objects in the universe change over time. In general relativity, Einstein's extension of special relativity, the geometry of space is dynamic and can evolve. The metric is a mathematical way to measure distances in this flexible, changing geometry.
During cosmic inflation, a period thought to have occurred just after the Big Bang, the metric expansion was extraordinarily rapid. Space expanded so quickly that distances between points increased exponentially.
This rapid expansion does not mean galaxies or particles were traveling faster than light. Instead, it means the space between them was growing at such a rate. Think of it like dots on a rubber band being stretched. The dots aren't moving along the band faster than any speed limit; the band itself is expanding.

• Key Point 1: The metric expansion of space allows space itself to grow, creating more distance between objects without them moving through space.
• Key Point 2: Inflation expanded space much faster than the speed of light, but this did not involve any violation of relativistic speed limits.

The notion of metric expansion reconciles the rapid growth of the universe with special relativity's speed limits, providing a clear understanding of how inflation works.