If nothing can travel faster than light, how is a 13.8-billion-year-old universe 93 billion light-years across?
If the universe expanded at the speed of light in all directions from a starting point, the radius of this cosmic sphere would be precisely 13.8 billion light-years. To find the total size, you simply double the radius to get the diameter, resulting in a universe that is exactly 27.6 billion light-years across.
However, the reality of cosmology is much stranger and more fascinating. The actual observable universe is vastly larger than 27.6 billion light-years across; it spans roughly 93 billion light-years in diameter.
This massive discrepancy exists because the speed of light is only the absolute speed limit for objects traveling through space. It does not dictate the rules for the expansion of space itself. In the earliest moments of the Big Bang, during a phase known as cosmic inflation, the very fabric of spacetime stretched outward exponentially, at a rate vastly faster than the speed of light. Furthermore, space has continued to expand ever since. When astronomers look at the most distant galaxies, they are seeing light that has traveled for over 13 billion years, but in the meantime, the space between those galaxies and Earth has relentlessly continued to stretch.
So, while a universe strictly bound by the speed of light would measure a tidy 27.6 billion light-years from edge to edge, the physics of a stretching spacetime created a much grander and more expansive cosmos.