The question of how far space extends challenges our intuitive understanding of distance and boundaries. Cosmologists define this expanse as the universe, or spacetime, which encompasses all matter, energy, and the fabric that contains them. The size of the universe we can study is limited by fundamental physics, yet this observable region is likely only a tiny fraction of the whole. Determining the true size and ultimate nature of the cosmos requires examining the limits of observation, the dynamics of expansion, and the geometry of spacetime itself.
Defining the Observable Universe
The “edge” of space we perceive is not a physical wall but a limit imposed by the age of the universe and the speed of light. Since light travels at a finite speed, there is a maximum distance from which light has had time to reach us since the Big Bang began approximately 13.8 billion years ago. This boundary is known as the particle horizon, defining the extent of our observable universe.
If the universe were static, the observable radius would be 13.8 billion light-years, but the expansion of space complicates this calculation. As light from distant galaxies travels toward us, the space it moves through is simultaneously stretching. This means objects that emitted light 13.8 billion years ago are now much farther away than their light travel time suggests.
Accounting for this expansion, the calculated radius of the observable universe is approximately 46.5 billion light-years in every direction from Earth. This results in a spherical region with a diameter of about 93 billion light-years, representing everything we can currently see or theoretically detect. Everything beyond this cosmological horizon exists, but the light from those regions has not yet had sufficient time to traverse the expanding space and reach us.
Cosmic Expansion and the Absence of a Spatial Edge
The universe’s actual size extends far beyond the 93-billion-light-year diameter of our observable region, and its dynamics make the concept of a spatial “edge” meaningless. Since the Big Bang, the space between galaxies has been increasing, an effect known as the metric expansion of space. This expansion is not matter rushing outward into a pre-existing void, but rather the fabric of spacetime itself stretching everywhere simultaneously.
This stretching motion is currently accelerating due to a mysterious force called dark energy, which accounts for approximately 68% of the total mass-energy density of the universe. Dark energy acts as a repulsive gravity, pushing galaxies further apart at an ever-increasing rate. The standard cosmological model, Lambda-CDM, treats dark energy as a constant energy density inherent to space, driving this accelerated growth.
The universe does not require an external void to expand into, which is why a physical boundary or spatial edge cannot exist. If the universe had an edge, one could theoretically look outward, implying an “outside” for the universe to expand into. Instead, the expansion is uniform and happens everywhere within the universe, much like the surface of an inflating balloon stretches without having a center or an edge. This dynamic negates the possibility of a center or a boundary, meaning the full universe is spatially boundless.
Spacetime Geometry and the Ultimate Size of the Universe
The ultimate answer to whether the universe is finite or infinite depends entirely on its global geometry, which is determined by the total density of matter and energy within it. Cosmologists use the density parameter, Omega (Ω), to describe this geometry by comparing the universe’s actual density to a critical density value. If the actual density matches the critical density, Ω equals one, and the universe is considered “flat.”
A flat universe has zero curvature, meaning the rules of Euclidean geometry hold true on the largest scales, and parallel lines will never meet. Such a flat universe is spatially infinite. If the total density is greater than the critical density (Ω > 1), the universe has positive curvature, like the surface of a sphere.
A positively curved universe is spatially finite but unbounded; traveling in a straight line would eventually bring you back to your starting point. Conversely, if the density is less than the critical density (Ω < 1), the universe has negative curvature, resembling the shape of a saddle. A negatively curved, or "open," universe is also spatially infinite. Observations from the Planck satellite, which mapped the cosmic microwave background radiation, have measured the geometry of the observable universe with high precision. These measurements indicate that the universe is extremely close to being flat, with the spatial curvature parameter measured to be near zero. This empirical evidence strongly supports the model of a flat universe, which implies that the total universe is likely infinite in extent. While a minute, unmeasurable positive curvature could still mean the universe is finite but vast, the current best data suggests the boundless nature of the cosmos.