8 Dark Energy and the Fate of the Universe

What cosmic acceleration means

On very large scales, distances between unbound galaxies increase over time. Measurements show that this expansion is accelerating: the scale factor describing the universe’s size grows at an increasing rate. This does not mean galaxies, solar systems, or atoms are generally expanding; gravity and other forces hold such structures together.

In general relativity, cosmic acceleration depends on the universe’s total energy density and pressure. For a component with density $\rho$, pressure $p$, and equation-of-state parameter $w=p/\rho$, a cosmological constant has $w=-1$. In these units, the combined contents satisfy the condition for accelerated expansion when

Dark energy is the name for the unknown component—or possible change to gravity—used to explain this behavior. It is not the same as dark matter: dark matter attracts gravitationally and helps build cosmic structure, while dark energy is associated with accelerated expansion.

Evidence from cosmic observations

Type Ia supernovae provided the first direct evidence for acceleration. Their peak brightness can be used to estimate distance, while their redshift indicates how the universe expanded while their light traveled. Distant supernovae appeared farther away than expected in a universe whose expansion was only slowing under gravity. Two independent teams reported this result in the late 1990s.

Baryon acoustic oscillations (BAO) provide another distance measure. They are a preferred, measurable scale in the distribution of galaxies, left by sound waves in the early universe. Comparing that scale at different redshifts traces the expansion history.

The cosmic microwave background (CMB) measures conditions in the early universe. Combined with measurements of the later universe, it helps constrain cosmic geometry and expansion history. Weak gravitational lensing and galaxy clustering track the growth of cosmic structure, providing a further test because accelerated expansion slows the growth of large-scale structure.

No single measurement reveals the physical nature of dark energy. The case for it comes from comparing different observations and the assumptions involved in interpreting them.

Leading explanations

The simplest explanation is the cosmological constant, $\Lambda$, which represents a constant energy density of empty space and gives the standard $\Lambda$CDM model. This model fits many observations well. However, quantum theory does not yet explain why the vacuum energy has the small value inferred from cosmology.

In dynamical dark-energy models, such as quintessence, a field changes over time, so the equation-of-state parameter $w$ need not remain at $-1$. A commonly tested description is

where $a$ is the scale factor.

Another possibility is modified gravity: general relativity might need adjustment on cosmic scales, without adding a new dark-energy substance. Tests compare both expansion and structure growth because modified gravity can affect them differently.

Recent results and open questions

Recent results have renewed interest in evolving dark energy, but they do not settle the question. The second data release from DESI, combined with cosmic microwave background data, found a preference for a time-varying model. When supernova data were added, the preference was about $2.8\text{–}4.2$ standard deviations, depending on the supernova sample. The Dark Energy Survey Year 6 analysis, combined with DESI and CMB data, found a preference of roughly $3\sigma$.

These results depend on the data combination and the model. Possible measurement or calibration systematics remain important, especially for supernovae. The results are intriguing evidence, not a confirmed discovery that dark energy changes.

Open questions include what dark energy is, whether its density or pressure changes, whether gravity itself needs revision, and how measurement differences affect inferences about cosmic history. Better distance and structure measurements can help distinguish among the models.

Possible futures of the universe

The universe’s long-term fate depends on how dark energy behaves. These are conditional scenarios, not settled forecasts.

• Big Freeze (heat death): If dark energy remains like a positive cosmological constant, expansion continues to accelerate. Distant galaxies move beyond view, star formation eventually ends, and the cosmos becomes increasingly cold and dilute. This is the usual long-term outcome in the standard model.

• Big Rip: If dark energy becomes increasingly dominant—classically, if its equation-of-state parameter stays below $-1$—expansion could eventually overcome the forces binding galaxies and then smaller structures. This is speculative, not a prediction of standard $\Lambda$CDM.

• Big Crunch: If dark energy weakens enough or becomes negative, expansion could eventually stop and reverse, leading to contraction. Current observations do not establish that this will happen.

If dark energy is constant, the Big Freeze is the expected fate. If its nature changes, other endings remain possible.