Determining the universe’s size requires defining the boundaries of what is being measured. Scientists distinguish between the observable cosmos—the part accessible to observation—and the total extent of space and time. The measurable size is a fixed number, but the ultimate size is inferred from the universe’s geometry and expansion history.
Defining the Observable Universe
The observable universe represents a spherical region of space centered on Earth from which light has had time to reach us since the Big Bang. This limit is set by the age of the universe and the speed of light, which is the ultimate speed limit for information transfer. The boundary of this region is called the cosmological horizon, and it marks the farthest distance we can possibly see, even with the most powerful telescopes.
The universe is estimated to be approximately 13.8 billion years old, based on observations of the cosmic microwave background radiation. This age might suggest the observable radius is 13.8 billion light-years, but this is a common misconception. Because space itself has been expanding since the light began its journey, the objects that emitted that light are now much farther away.
The current estimated diameter of the observable universe is about 93 billion light-years. This size means that the farthest objects whose light is just reaching us today are presently about 46.5 billion light-years away. This distance represents the physical separation between us and the source of the oldest light, stretched by the intervening expansion of space over billions of years.
Methods for Calculating Cosmic Distance
Determining the vast scale of the observable universe requires a sequence of techniques known as the cosmic distance ladder. No single method is accurate enough to measure distances from nearby stars out to the edge of the cosmos. Instead, astronomers use overlapping methods, where the accuracy of one technique calibrates the next one for greater distances.
For the most extreme distances, two techniques are used to map the universe. The first involves measuring the redshift of light from distant galaxies, which is the stretching of light waves toward the red end of the spectrum as they move away from us. Hubble’s Law relates this observed redshift to the galaxy’s velocity and distance, allowing astronomers to calculate the universe’s expansion rate.
The second technique relies on objects known as standard candles, which have a known intrinsic luminosity. Type Ia supernovae, the explosions of white dwarf stars, are powerful and consistent standard candles visible across billions of light-years. By comparing the known brightness of a supernova to how bright it appears from Earth, scientists use the inverse square law of light to calculate its distance.
The Total Universe and Cosmic Geometry
While the observable universe has a measurable size, the size of the total universe—the entirety of space and time—is unknown. The ultimate size is determined by the overall geometry of space, which is governed by the universe’s total energy density. There are three possibilities for this geometry: positively curved, negatively curved, or flat.
A positively curved universe is finite and closed, like the surface of a sphere; traveling in a straight line would eventually return you to your starting point. A negatively curved universe is open and infinite, resembling a saddle shape. A flat universe, where Euclidean geometry holds true, is also considered infinite in extent.
Current data from the European Space Agency’s Planck satellite, which mapped the cosmic microwave background, suggests the universe is flat to within a small margin of error. This flatness implies that the total universe is likely infinite, or at least vastly larger than the observable part. This conclusion is based on inference from its structure rather than direct measurement.
The Role of Cosmic Expansion in Size
The size difference between the universe’s age (13.8 billion years) and the observable radius (46.5 billion light-years) is explained by the expansion of space itself. Light from the most distant galaxies has traveled for 13.8 billion years, but the space between us and the source has stretched during that time. The expansion of the universe is not constrained by the speed of light, as space itself is growing, not objects moving through space.
Imagine a galaxy emitting light 13.8 billion years ago. As the light journeyed toward us, the fabric of space stretched, carrying the galaxy farther away. By the time the light reaches Earth, the galaxy that emitted it is now at its current distance of about 46.5 billion light-years.