General relativity explains the expansion of the universe by describing gravity as the curvature of spacetime and showing how that curvature changes as matter and energy evolve. According to this theory, the universe does not need to expand into an empty space beyond its boundaries. Instead, space itself can stretch, increasing the distances between objects that are not gravitationally bound to one another.
This idea is central to modern cosmology, the scientific study of the universe as a whole. General relativity provides the mathematical framework for understanding how the universe changes over time, why distant galaxies move away from one another, and how the expansion can accelerate. It also helps scientists reconstruct the universe’s history, from its early hot, dense state to its present structure.
What general relativity says about space and time
Albert Einstein published his general theory of relativity in 1915. Unlike Newton’s theory of gravity, which describes gravity as a force acting between objects, general relativity explains gravity through the geometry of spacetime.
Spacetime combines the three dimensions of space with the dimension of time into a single framework. According to general relativity, matter and energy influence the curvature of spacetime, while that curvature determines how matter and light move.
A massive object, such as a star or planet, changes the geometry of the spacetime around it. Other objects follow paths shaped by this geometry. What we experience as gravitational attraction is, in this description, a consequence of curved spacetime.
The theory also applies to the universe on the largest scales. Instead of considering only individual stars, planets, or galaxies, scientists can use Einstein’s equations to describe the geometry and evolution of the entire cosmos.
One important implication is that spacetime is not necessarily fixed or unchanging. Its geometry can evolve. The distances between widely separated galaxies can increase because the underlying space between them expands.
How Einstein’s equations allow the universe to expand
Einstein’s field equations describe the relationship between the geometry of spacetime and the matter and energy within it. In simplified terms, they explain how the distribution of matter, radiation, and other forms of energy determines the curvature and evolution of spacetime.
When scientists apply these equations to the universe as a whole, they can make a useful simplifying assumption: on sufficiently large scales, the universe is approximately homogeneous and isotropic.
Homogeneous means that matter is distributed roughly evenly throughout the universe when viewed over sufficiently large distances. Isotropic means that the universe looks broadly similar in every direction.
These assumptions do not mean that every region is identical. Galaxies, stars, clusters, and cosmic structures create enormous local variations. However, when considered across distances much larger than individual galaxy clusters, the universe is sufficiently uniform for this approximation to work.
Using these assumptions, scientists derive the Friedmann equations, which describe how the universe’s expansion rate changes over time.
One important quantity in these equations is the scale factor. This is a mathematical measure of how the distances between objects moving with the general expansion of the universe change over time.
If the scale factor doubles, the average distance between such objects also doubles. The expansion rate is described by the Hubble parameter, which measures how quickly the scale factor changes relative to its current value.
The Friedmann equations show that cosmic expansion depends on the universe’s contents, including matter, radiation, and dark energy, as well as its spatial curvature.
This is how general relativity connects the physical contents of the universe to its large-scale expansion.
What it means for space itself to expand
When scientists say that the universe is expanding, they do not mean that galaxies are simply flying outward through an otherwise stationary space. They mean that the distance between sufficiently separated, unbound regions of space increases over time.
A useful way to understand this is to imagine points drawn on the surface of an inflating balloon. As the balloon expands, the distance between the points increases, even though the points themselves do not need to move across the rubber surface.
The analogy illustrates how distances can grow because the space connecting objects changes. However, it has limitations. The universe does not need to expand into a surrounding space, and the balloon’s two-dimensional surface is not a literal representation of the universe’s three-dimensional space.
Cosmic expansion also does not mean that everything grows larger. Objects held together by gravity or other forces generally resist the expansion.
For example, the distances between atoms in a solid, the planets in the solar system, and stars within a gravitationally bound galaxy do not increase simply because the universe is expanding. These systems are governed by local physical interactions that dominate over the overall cosmic expansion.
The expansion becomes important primarily across enormous distances, where the gravitational influence of individual structures no longer keeps neighboring galaxies together.
How the expansion of the universe was discovered
The idea that the universe might be expanding emerged from both theoretical physics and astronomical observations.
In the 1920s, astronomers discovered that many distant galaxies showed a redshift in their light. Redshift occurs when light is stretched toward longer wavelengths.
For light traveling through an expanding universe, cosmic expansion stretches its wavelength as it moves through space. The greater the expansion that occurs during the light’s journey, the greater the resulting redshift.
Astronomer Edwin Hubble helped establish an important relationship between the distances of galaxies and their apparent recession speeds. In general, more distant galaxies show greater recession speeds than nearby ones.
This relationship became known as Hubble’s law. It is commonly expressed as:
v = H₀ × d
Here, v represents a galaxy’s recession speed, d represents its distance, and H₀ is the Hubble constant, which describes the present-day expansion rate.
The relationship does not mean that Earth occupies the center of the universe. An observer in another typical galaxy would see distant galaxies receding in much the same way.
This happens because the expansion occurs throughout space rather than originating from a particular central location.
Hubble’s observations provided strong evidence that the universe is expanding, supporting the picture that had emerged from applying general relativity to cosmology.
Why the universe’s expansion changes over time
General relativity does not predict that the universe must always expand at the same rate. Instead, its expansion rate depends on the amounts and properties of the matter and energy present.
Different cosmic components influence expansion in different ways.
Ordinary matter, which includes the material making up stars, planets, gas, and living organisms, contributes to gravitational attraction. Dark matter also contributes gravitationally, even though it does not emit or absorb light in the ordinary way.
In a universe dominated by matter, gravity tends to slow the expansion because matter attracts other matter.
Radiation, including light and other forms of electromagnetic energy, also affects cosmic expansion. Its influence changes as the universe grows because radiation loses energy as its wavelengths stretch.
Dark energy has a different effect. It is the name given to whatever is responsible for the observed acceleration of cosmic expansion. Unlike ordinary matter and dark matter, dark energy appears to have a gravitational effect that promotes accelerated expansion on cosmic scales.
The simplest explanation is Einstein’s cosmological constant, a term that can be included in the field equations. It corresponds to a constant energy density associated with space itself.
As the universe expands, the density of ordinary matter decreases because the same amount of matter occupies a larger volume. Radiation density decreases even faster because its energy is also reduced by cosmic redshift.
If dark energy is a cosmological constant, however, its energy density remains approximately constant as space expands.
Consequently, dark energy becomes increasingly important relative to matter and radiation. This provides an explanation for why the universe’s expansion, which was slowing during earlier cosmic history, eventually began accelerating.
The role of the cosmological constant and dark energy
Einstein originally introduced the cosmological constant into his equations while investigating whether a static universe could be mathematically possible.
At the time, the prevailing scientific expectation was that the universe was unchanging on the largest scales. Einstein’s additional term could counteract the gravitational attraction of matter and produce a static cosmological model.
However, observations and theoretical developments later established that the universe is expanding. The static-universe model was no longer necessary.
In the late 1990s, observations of distant Type Ia supernovae provided evidence that cosmic expansion is accelerating rather than slowing down. These supernovae serve as useful distance indicators because their intrinsic brightness can be estimated.
The discovery changed scientists’ understanding of the universe’s long-term evolution and brought the cosmological constant back into serious consideration.
Today, dark energy is commonly associated with the cosmological constant in the standard model of cosmology. This model successfully explains many observations, including the expansion history and the large-scale structure of the universe.
However, scientists do not yet know the fundamental physical nature of dark energy. It may be a constant property of empty space, or it could involve a more complicated process that changes over time.
General relativity provides a mathematical framework for describing its gravitational influence, but it does not by itself establish what dark energy fundamentally is.
Why the expansion does not have a center
One common misunderstanding is that the universe must be expanding outward from a central point, much like an explosion.
The Big Bang was not an explosion of matter into preexisting empty space. It describes the expansion of space itself from an earlier state in which the observable universe was much hotter and denser.
Because space expands throughout the universe, there is no special central location from which all galaxies move outward.
An analogy involving dots on an inflating balloon helps explain this idea. Every dot moves farther from the others as the surface expands. An observer located at any dot would see other sufficiently distant dots moving away.
Similarly, observers in different galaxies can see the large-scale universe expanding around them without occupying a privileged position.
The analogy should not be taken literally, because the balloon has an outside and a center in its surrounding three-dimensional space. The universe does not require an equivalent external space or central point.
The Big Bang also does not necessarily describe the absolute beginning of everything. It describes the evolution of the observable universe from an extremely early hot, dense condition. What, if anything, preceded that condition remains an open scientific question.
How cosmic expansion affects light and distance
The expansion of space has important consequences for how astronomers observe distant objects.
When light travels through an expanding universe, its wavelength stretches along with space. This phenomenon is known as cosmological redshift.
The amount of redshift provides information about how much the universe has expanded since the light was emitted.
Astronomers use this information, along with measurements of brightness and other properties, to estimate distances and investigate the history of cosmic expansion.
There is also an important distinction between the distance to an object when it emitted the light we observe and its distance today.
Because the universe continued expanding while the light traveled toward Earth, a distant galaxy may now be much farther away than it was when its light began its journey.
For extremely distant objects, the recession speed associated with cosmic expansion can exceed the speed of light. This does not violate special relativity, which prohibits objects from traveling through their local space faster than light. The apparent recession results from the expansion of the space between distant locations.
This distinction is essential for understanding the observable universe and the limits of what astronomers can detect.
What general relativity reveals about the universe’s future
General relativity allows scientists to investigate possible futures for the universe by examining how its expansion responds to matter, radiation, dark energy, and spatial curvature.
If dark energy behaves like a cosmological constant, the universe is expected to continue expanding at an accelerating rate.
Over extremely long periods, distant galaxies that are not gravitationally bound to our local group will become increasingly separated. Their light will become more strongly redshifted, and many will eventually become unobservable from our region of the universe.
Meanwhile, gravitationally bound structures will remain intact. The solar system, the Milky Way, and the Local Group of galaxies will not simply be pulled apart by ordinary cosmic expansion under the standard cosmological picture.
Other possible futures depend on the nature of dark energy. If its properties differ significantly from those of a cosmological constant, the expansion history could evolve differently.
For example, certain hypothetical forms of dark energy could produce increasingly rapid expansion with consequences for gravitationally bound structures. Other models could lead to a different long-term expansion rate.
At present, the simplest model involving a cosmological constant remains consistent with a wide range of observations. Nevertheless, the ultimate fate of the universe depends on physical questions that scientists are still investigating.
Why general relativity remains essential to modern cosmology
General relativity provides the foundation for understanding the universe’s large-scale structure and evolution. It explains how matter and energy influence spacetime and how the geometry of spacetime changes as the universe expands.
The theory connects astronomical observations, such as galaxy redshifts and supernova distances, with mathematical descriptions of cosmic history. It also explains why the expansion rate changes and how different forms of energy influence that change.
Its success does not mean that every aspect of cosmology is fully understood. Scientists still investigate the nature of dark energy, the properties of dark matter, the earliest moments of cosmic history, and the relationship between general relativity and quantum mechanics.
Nevertheless, the central insight is well established: the universe is not expanding through a fixed background of space. Space itself evolves.
By describing that evolution through the geometry of spacetime, general relativity turns cosmic expansion from a simple observation about distant galaxies into a fundamental explanation of how the universe changes over time.
