Who proved that an empty space can expand and how?

 In 1915, Albert Einstein’s own math proved that empty space can expand. But he found the concept so profoundly absurd that he altered his equations to hide the discovery.

To force the universe to remain static, Einstein added a mathematical fudge factor to his theory of general relativity.

The first to mathematically prove that empty space could expand was Russian physicist Alexander Friedmann in 1922, followed independently by Georges Lemaître, a Belgian Catholic priest, in 1927. While Friedmann derived the expanding universe from Einstein's equations, Lemaître took the crucial next step. He proposed that the stretching of space itself would cause distant galaxies to appear to be moving away from Earth, and he calculated the rate at which this expansion was happening.

Lemaître’s mathematics provided the theory, but the empirical proof came in 1929 from American astronomer Edwin Hubble. Using the massive 100-inch Hooker telescope at the Mount Wilson Observatory in California, Hubble observed the light from distant galaxies. Building on earlier spectral measurements by astronomer Vesto Slipher, Hubble discovered a clear correlation: the farther a galaxy was from Earth, the faster it appeared to be receding. Its light was "redshifted"—meaning the light waves were physically stretched to longer wavelengths as they traveled across the expanding fabric of space.

The mechanism of this expansion is entirely counterintuitive. It is not an explosion of matter moving outward into a pre-existing empty void. The void itself is growing. The classic analogy is a loaf of raisin bread rising in an oven. As the dough expands, every raisin moves farther away from every other raisin, even though none of them are moving relative to the dough immediately surrounding them.

In the cosmos, galaxies are the raisins. They are largely stationary within their local regions of space, held together by their own gravity. It is the empty space between them that is stretching. Because this metric expansion happens everywhere evenly, a galaxy that is twice as far away has twice as much expanding space between it and the observer, causing it to recede twice as fast.

Edwin Hubble at the Mount Wilson 100-inch telescope in California, the instrument he used to confirm the distance-redshift relationship of galaxies in 1929. Photo by NOIRLab is licensed under CC BY 4.0.

How large is the universe assuming it expanded at the speed of light since the beginning?

 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.