The act of looking out into the cosmos is inherently a form of time travel. Because light moves at a finite speed, the greater the distance an object is from Earth, the longer its light takes to reach our detectors. When astronomers measure distance, they use the light-year, which represents the distance light travels in one Earth year, approximately 5.88 trillion miles. Observing a galaxy 100 million light-years away means we are seeing it as it existed 100 million years ago. This fundamental principle establishes that the furthest we have seen is synonymous with the earliest moment in the universe’s history that light can reveal.
Defining the Observable Universe
The maximum physical distance we can theoretically see defines the boundary of what astronomers call the Observable Universe. This region is a perfect sphere centered on Earth, encompassing all matter from which light has had time to reach us since the beginning of cosmic expansion. Although the universe is estimated to be about 13.8 billion years old, the edge of this observable sphere is not 13.8 billion light-years away. The current estimate places the diameter of the Observable Universe at approximately 93 billion light-years.
This massive discrepancy between age and size is a direct consequence of the continuous expansion of space itself. The space between galaxies has been stretching since the light began its journey toward us, carrying the distant light source further away over billions of years. For an object whose light traveled for 13.8 billion years, the object itself has moved to a current distance, or proper distance, of about 46.5 billion light-years from us in the expanding cosmic fabric. This proper distance defines the Observable Universe’s radius. The current size represents the distance to the objects now, even though the light we observe left them when they were much closer.
The Limit of Earliest Light
The actual visual limit of the universe is set not by a theoretical distance, but by a physical barrier of light that represents the furthest time we can see. This barrier is the Cosmic Microwave Background Radiation (CMBR), a faint, uniform glow of residual heat filling all of space. The CMBR is the oldest light we can detect, originating from the epoch when the universe was only about 380,000 years old. Before this point, the universe was an extremely hot, dense, and opaque plasma of charged particles, similar to a perpetual fog.
In this early state, electrons and protons were not yet bound into neutral atoms, and free electrons constantly scattered photons, preventing light from traveling more than a very short distance. This constant scattering made the early universe effectively impenetrable to light, much like looking into a dense cloud on Earth. As the universe expanded, it cooled rapidly, eventually reaching a temperature of about 3,000 Kelvin. This cooling allowed electrons and protons to combine for the first time to form stable, neutral hydrogen and helium atoms in an event called recombination.
The formation of neutral atoms dramatically reduced the number of free electrons, causing the universe to suddenly become transparent to light. The photons that were finally released to travel freely at that moment have been traveling ever since, stretched into the microwave portion of the electromagnetic spectrum by the expansion of space. This light, the CMBR, forms the “surface of last scattering,” and represents the absolute visual horizon; we cannot see light that originated any earlier because it was trapped in the opaque plasma.
Measuring the Immense Distances
The determination of these immense cosmic distances relies heavily on a phenomenon known as redshift, which is the primary tool for mapping the most distant reaches of the cosmos. Redshift occurs because the expansion of the universe stretches the wavelength of light as it travels through space. This stretching shifts the light toward the red end of the spectrum, which has longer wavelengths. The amount of redshift is directly proportional to the distance of the light source, meaning that the farther away a galaxy is, the more its light has been stretched.
For the most distant galaxies, the light originally emitted in the ultraviolet or visible spectrum is redshifted so severely that it arrives at Earth as faint infrared light. Astronomers use powerful instruments, such as the James Webb Space Telescope and the Hubble Space Telescope, to capture this faint, stretched light. The most accurate method for measuring redshift involves spectroscopy, where scientists analyze the specific patterns of light absorption and emission, known as spectral lines, within a galaxy’s light.
By comparing the observed position of these spectral lines to where they should be in an unshifted spectrum, scientists calculate the precise redshift value. This value, often denoted as z, is then fed into cosmological models to calculate the distance and the look-back time. These measurements allow scientists to chart the structure of the universe and identify the current record holders for the most ancient and distant observed galaxies.