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History of the universe

The end of the universe

What future awaits a universe in which distant galaxies keep moving farther apart? From a future with little starlight to models of a contracting universe, explore the conditions that different endings require.

The end of the universe asks where expansion may lead

On very large scales, the universe is expanding. Distances generally grow between faraway galaxies that are not bound together by gravity. If this change continues without end, what might the distant future look like? Space with little starlight could keep expanding, or conditions that change the expansion could lead to a very different future. Studying the end of the universe means exploring how far these changes may go.

Stellar death explores what a star leaves behind when it loses the way it used to shine. Understanding the universe's eventual state requires knowing about the expansion of space as well as stellar lifetimes. Here, an end does not necessarily mean that time stops one day. It can also mean a future that keeps changing while approaching a particular state.

Light from distant objects reveals how expansion has changed

Light from a distant galaxy takes a long time to reach us. As space expands, the light's wavelength also stretches. Astronomers study both how much it has stretched and the distance to the object to find out how expansion changed while the light was traveling.

Some types of supernovae can be used to measure distances because their original brightness can be estimated. Combining observations of these supernovae, galaxies, and other objects indicates that the universe entered a period of speeding expansion during its recent history. This is called accelerated expansion. Here, recent refers to a stretch of cosmic history far longer than a human lifetime.

Dark energy is the name for an unidentified component included in models of the universe to explain accelerated expansion. One explanation assumes that the amount of dark energy in a fixed volume of space—its density—stays constant even as space expands. A positive cosmological constant describes this property. Scientists also study explanations in which dark energy changes over time, so whether its properties will persist into the future is a separate question.

In heat death, differences that can power change fade away

Leave a cup of hot water in a room, and heat passes from the water to its surroundings. As the water and the room approach the same temperature, there is less scope for heat to flow between them. A temperature difference can run a heat engine, but once that difference disappears, the engine can no longer do work that way.

The universe also contains differences that allow energy to flow and cause change, such as those between hot stars and the cold space around them. Heat death is the name for a state in the very distant future in which almost none of these usable differences remain. The key is not that all energy suddenly disappears. It is that obtaining energy to keep producing light or driving other changes becomes difficult.

In a model where a positive cosmological constant persists, the universe keeps expanding. Groups of galaxies that are not bound together move farther apart, and we can consider a future in which no new energy sources appear for a very long time. Researchers study the universe's long-term approach toward heat death under this assumption. The cup example shows only the role of temperature differences; it is not a miniature universe that also includes gravity and expanding space.

A Big Rip requires conditions that can pull even bound structures apart

Even in an expanding universe, the Solar System and the Milky Way are held together by gravity. Expansion in the cosmological-constant model described above does not by itself stretch the distance between the Sun and Earth at the same rate as the universe as a whole. The motion of bound objects and expansion on very large scales need to be considered separately.

The Big Rip assumes different properties for dark energy. In a representative theoretical model, dark energy's density keeps increasing as the universe expands, and its effect grows without limit within a finite time. As a result, even gravitationally bound galaxies and planetary systems come apart. If the model is followed further, more tightly bound structures such as atoms also become unable to hold together.

Hypothetical dark energy with these properties is called phantom dark energy. For a Big Rip to occur, the required behavior must persist in the necessary way. Discovering accelerated expansion alone does not predict a Big Rip. This is a theoretical scenario showing the consequences of particular conditions.

In a Big Crunch, expansion stops and becomes contraction

We can also consider a future in which distances that have been growing on very large scales start to shrink. If expansion stops and the universe begins to contract, matter and light reach an increasingly dense, hot state. This ending is called the Big Crunch. Both its scale and its cause differ from one galaxy swallowing neighboring galaxies.

This scenario requires conditions that turn expansion into contraction. In some theoretical models, a change in dark energy's properties allows the universe to contract again after its current accelerated expansion. Such research tests the conditions under which expansion could reverse, rather than establishing what will happen.

A slight weakening of dark energy does not always lead to a Big Crunch. The matter and energy in the universe and the geometric properties of space jointly determine expansion. The standard model with a persistent positive cosmological constant does not predict this contraction. There is not yet evidence to treat a Big Crunch as our universe's certain final scene.

New observations test the conditions shaping the future more closely

DESI research released in 2025 investigated the history of expansion using the distribution of many galaxies. DESI data alone are well described by the standard cosmological-constant model. When combined with light from the early universe and supernova data, however, the analysis showed a preference for an explanation in which dark energy changes over time. The strength of that preference also depended on which datasets were combined.

This result is a clue that changes in dark energy need further investigation. It does not establish that dark energy changes, nor does it determine how it would change in the future or how the universe will end. Remaining observational errors and the assumptions used in the analysis must be examined, along with whether new data show the same tendency.

Presenting heat death, the Big Rip, and the Big Crunch side by side does not make them equally likely. A positive cosmological constant is an assumption in the standard model used for comparison with observations, while a Big Rip and future contraction require additional conditions. These three scenarios are not an exhaustive list of every possible future, either.

Just as research on the Big Bang uses old light to explore the universe's past, research on its future begins with the light we have received. Observations tell us which conditions fit the universe well. Whether those conditions persist into a future that has not yet arrived remains an open question.

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How different conditions change the universe's eventual state

Select a scenario to compare its required conditions, expected changes, and questions for observations to test.

Heat death

Required conditions: Consider a future in which a positive cosmological constant persists, expansion continues, and new usable energy sources are not replenished for a very long time.

Differences that can drive change, such as those between hot and cold regions, diminish. The two circles becoming alike represents this reduction in differences, not the disappearance of all energy.

This explanatory model shows conditional changes from left to right. In heat death, the two circles represent regions at different temperatures. In the Big Rip, the dots and ring represent a bound structure. In the Big Crunch, the dot pattern represents matter spread through space. Arrows indicate the direction of change; the drawings do not calculate actual sizes, temperatures, or elapsed times.

Heat death

Required conditions
Consider a future in which a positive cosmological constant persists, expansion continues, and new usable energy sources are not replenished for a very long time.
Expected changes
Differences that can drive change, such as those between hot and cold regions, diminish. The two circles becoming alike represents this reduction in differences, not the disappearance of all energy.
Observations and open questions
A positive cosmological constant is part of the standard model compared with observations. Whether its properties persist into the distant future, and which energy sources can be used for how long, are separate questions.

Big Rip

Required conditions
Assume conditions like those in a representative phantom dark energy model: density keeps rising during expansion, and its effect grows without limit within a finite time.
Expected changes
Gravitationally bound galaxies and planetary systems come apart. If the model is followed further, smaller bound structures also become unable to hold together. The broken ring represents the loss of binding, not a physical shell.
Observations and open questions
This ending does not follow from observations of accelerated expansion alone. The required properties of dark energy and whether they persist have not been established.

Big Crunch

Required conditions
Conditions must allow expansion to stop and become contraction. Some theoretical models examine changes in dark energy's properties. A slight weakening of dark energy alone is not enough.
Expected changes
Distances shrink on very large scales, and matter and light reach a denser, hotter state. The whole dot pattern becoming more compact represents contraction, not everything falling toward one particular galaxy.
Observations and open questions
The standard model with a persistent positive cosmological constant does not predict this contraction. Hints that dark energy changes do not by themselves establish that the universe will contract.

Presenting these three scenarios does not imply that they are equally likely. A positive cosmological constant is an assumption of the standard model; a Big Rip and future contraction require additional conditions. The edges or middle of the drawing do not represent a boundary or center of the universe.

The heat-death drawing begins with two circles of different sizes and colors, then shows two of the same size and color to convey a decreasing temperature difference. In the Big Rip drawing, dots gathered inside a ring scatter and the ring breaks. In the Big Crunch drawing, gaps throughout the dot pattern shrink. The conditions and explanations for every scenario are also available in the text below.