First, size (space) and age (time) are measured in different units. However, you certainly fell into this article because you thought: if the universe is 13.8 billion years old and the speed of light is constant, how is it possible that the observable universe has a radius of 46.5 billion light years?
The distance light determines the observable universe has traveled since the Big Bang, approximately 13.8 billion years ago. In theory, this would suggest a radius of 13.8 billion light years. However, the observable universe is about 46.5 billion light years in radius. This discrepancy arises from the expansion of the universe.
The cosmic expansion of the universe
The universe has been expanding since the Big Bang. This expansion is not just the movement of galaxies through space but the stretching of space itself. As a result, light from distant galaxies has to travel ever greater distances to reach us. Even as light travels toward us, the space it moves through expands, effectively increasing the distance it needs to travel — the best evidence of this phenomenon lies in the existence of cosmic microwave background radiation.
Furthermore, immediately after the Big Bang, the universe underwent a brief, rapid expansion known as cosmic inflation, this inflationary period extended far beyond the visible horizon, making many of the universe’s galaxies invisible. The after-effects of this rapid expansion mean that the observable universe includes regions of space much further away than a simple calculation of the age and speed of light would suggest.
It is important to note that this expansion does not imply that anything is moving faster than the speed of light. The universe’s expansion is a change in the metric of space, not a movement through space.
The concept of metric expansion is crucial here. It’s not just that objects are moving away from each other; the metric that defines distance in the universe is changing. The very fabric of space is stretching, which allows distances to become greater than the travel time of light would imply. The metric expansion of the universe is a fundamental concept in cosmology that explains how the universe has evolved and grown since its beginnings in the Big Bang.
One of the first and most significant observations supporting metric expansion was made by Edwin Hubble, who discovered in 1929 that distant galaxies are moving away from us at speeds proportional to their distance. This is encapsulated in the Hubble-Lemaître Law, which states that the radial velocity of a galaxy is directly proportional to its distance from us. This relationship is a direct consequence of metric expansion: more distant objects are moving away more quickly.
In general relativity, the metric tensor describes the geometry of spacetime. This geometry is not static; it changes over time. The metric expansion of the universe refers to changes in this metric tensor, increasing the separation of parts of the universe.
“But the Big Bang is just a theory”
It is worth remembering that the expansion of the universe is not “just a theory”; it is a fact proven by observing the redshift of distant objects, such as quasars. Thousands of observations and measurements carried out over decades, already proven laws such as Hubble’s law (above), as well as a series of mathematical theorems from cosmology, such as the Friedmann–Lemaître–Robertson–Walker metric and the curvature tensor of Riemann, give us models of how the Big Bang and the expansion of the universe occurred — and still occurs.
But, of course, there is still a lot to be discovered! The problems of dark matter and dark energy are just two of the biggest open problems in physics that directly interfere with current cosmological theories.