Galaxies and Cosmology
5. The Fate of the Universe
Learning outcomes
- I can describe possible futures for the universe.
- I can explain how expansion influences the universe's future.
- I can compare different cosmological models.
- I can discuss scientific uncertainty in cosmology.
- I can evaluate evidence supporting current models.
What Will Happen to the Universe?
The Universe has been expanding since its early history. Observations also indicate that this expansion is currently accelerating.
But what will happen billions or even trillions of years from now?
The answer depends on several factors, particularly:
- The rate of cosmic expansion.
- The amount and distribution of matter.
- The properties of dark energy.
- How gravity affects cosmic structures.
- Whether the laws of physics continue to behave as current models predict.
Scientists have proposed several possible futures, including the Big Freeze, Big Crunch, and Big Rip.
The Expansion of the Universe
Observations show that distant galaxies are generally moving away from us and from one another on large scales.
This does not mean that Earth is located at the centre of the Universe. Instead, space itself is expanding.
A useful model is dots drawn on the surface of an expanding balloon. As the balloon expands, the distances between the dots increase.
The dots themselves do not need to move across the balloon's surface for their separation to increase.
Similarly, cosmic expansion increases the distances between sufficiently distant, unbound galaxies.
The balloon analogy is useful but imperfect. The Universe is not necessarily expanding "into" some surrounding space, and the two-dimensional balloon surface is only being used to represent three-dimensional space.
Gravity vs Expansion
Two important effects help us understand the future of the Universe.
Gravity
Gravity attracts matter.
It can:
- Hold planets in orbit.
- Bind stars into galaxies.
- Bind galaxies into groups and clusters.
- Slow the separation of matter under some conditions.
Cosmic Expansion
On sufficiently large scales, the Universe is expanding.
This expansion increases the distance between gravitationally unbound regions of the Universe.
The future therefore depends partly on how the expansion evolves compared with the gravitational attraction of matter.
However, observations have revealed another important factor: dark energy.
The Discovery of Accelerating Expansion
During the late 1990s, astronomers studying distant Type Ia supernovae found evidence that the expansion of the Universe was not slowing as expected.
Instead:
The expansion of the Universe is accelerating.
The unknown component associated with this acceleration is called dark energy.
Understanding the nature of dark energy is therefore extremely important when predicting the Universe's future.
Possible Fate 1: The Big Freeze
The Big Freeze, sometimes associated with a future heat death, is currently the leading broad scenario if accelerated expansion continues in a way similar to that predicted by the standard cosmological model.
In this scenario, the Universe continues expanding indefinitely.
Over enormous periods of time:
- Distant gravitationally unbound galaxies become increasingly separated.
- Star formation gradually declines.
- Existing stars eventually exhaust their nuclear fuel.
- Stellar remnants such as white dwarfs, neutron stars, and black holes become increasingly important.
- The Universe becomes progressively colder, darker, and more dilute.
The Big Freeze would not happen suddenly. It would occur over timescales vastly greater than the current age of the Universe.
Eventually, usable energy differences would become increasingly difficult to maintain.
This extremely distant state is often called heat death.
Possible Fate 2: The Big Crunch
Another proposed future is the Big Crunch.
In this scenario, cosmic expansion eventually stops and reverses.
The Universe would begin contracting.
Galaxies would become closer together, and the Universe would become increasingly dense and hot.
Eventually, matter and energy could collapse into an extremely dense state.
Historically, this scenario was considered more plausible if the gravitational attraction of matter were sufficient to reverse cosmic expansion.
However, the discovery of accelerating expansion makes a simple Big Crunch much less consistent with the current standard model.
It cannot be ruled out under every imaginable future model, because the nature of dark energy is not fully understood.
Possible Fate 3: The Big Rip
The Big Rip is a more speculative possibility.
It could occur under certain models in which the effect driving cosmic acceleration becomes sufficiently strong with time.
Expansion would become increasingly rapid.
Eventually, it could potentially overcome increasingly smaller-scale binding forces.
In the most extreme version:
- Galaxy clusters would separate.
- Galaxies could become disrupted.
- Planetary systems could become unbound.
- Eventually, even smaller structures could be affected.
The Big Rip is not currently the standard prediction. It depends on dark energy having particular properties that have not been demonstrated.
Comparing Possible Futures
| Model | Expansion | Long-Term Result | Current Status |
|---|---|---|---|
| Big Freeze | Continues indefinitely | Universe becomes colder, darker and more dilute. | Favoured broad scenario |
| Big Crunch | Stops and reverses | Universe contracts | Not favoured by current standard model |
| Big Rip | Acceleration increases dramatically. | Bound structures eventually disrupted | Possible in some speculative models |
The important point is that these are not simply three equally likely guesses.
Scientific observations allow researchers to determine which models are better supported by evidence.
The Role of Dark Energy
Dark energy is central to our understanding of the Universe's future.
Current observations are consistent with dark energy behaving approximately like a cosmological constant.
The cosmological constant is usually represented by:
Λ
If dark energy behaves like a cosmological constant indefinitely, cosmic expansion should continue accelerating.
This strongly favours a future resembling the Big Freeze rather than a Big Crunch.
However, scientists do not yet know the fundamental physical nature of dark energy.
The Standard Cosmological Model
The most successful current model of the Universe is known as the Lambda Cold Dark Matter model, usually written:
ΛCDM
Here:
- Λ represents dark energy in the form of a cosmological constant.
- CDM means Cold Dark Matter.
The model also includes ordinary matter and radiation.
ΛCDM successfully explains a remarkably wide range of observations.
Evidence Supporting the Current Model
Scientists do not choose cosmological models simply because they sound reasonable. Models must agree with observations.
Several major lines of evidence support the current cosmological picture.
1. Galaxy Redshifts
Light from distant galaxies is generally redshifted.
The relationship between galaxy distance and recession provides strong evidence that the Universe is expanding.
2. Type Ia Supernovae
Distant Type Ia supernovae can be used to measure cosmic distances.
Observations of these explosions provided important evidence that cosmic expansion has been accelerating.
3. Cosmic Microwave Background
The cosmic microwave background (CMB) is radiation remaining from the early Universe.
Its temperature variations contain information about the Universe's early conditions, geometry, and composition.
Measurements of the CMB provide strong constraints on cosmological models and support a Universe containing ordinary matter, dark matter, and dark energy.
4. Large-Scale Structure
Galaxies are not distributed randomly.
They form enormous patterns including:
- Clusters
- Filaments
- Voids
- Superclusters
Together these structures form what is sometimes called the cosmic web.
Computer models containing dark matter and dark energy can reproduce many important features of this observed structure.
Why Doesn't Expansion Pull Apart the Solar System?
Cosmic expansion mainly affects the Universe on very large scales.
Gravity is strong enough to keep many smaller systems gravitationally bound.
For example:
- The Moon remains bound to Earth.
- Earth remains bound to the Sun.
- Stars remain bound within galaxies.
- Many galaxies remain bound within groups or clusters.
Therefore, the expansion of the Universe does not simply cause everything to grow farther apart.
Scientific Uncertainty
Science does not require absolute certainty.
Instead, scientists develop models that best explain the available evidence and continually test them against new observations.
Our current cosmological model explains a large amount of evidence, but important questions remain.
For example:
- What exactly is dark matter?
- What exactly is dark energy?
- Is dark energy truly constant?
- Will accelerated expansion continue forever?
- Are our theories of gravity complete on the largest scales?
These questions mean that predictions about the extremely distant future contain scientific uncertainty.
Models Can Change
A scientific model is not a permanent statement that can never change.
New observations can:
- Strengthen a model.
- Require modifications.
- Reveal limitations.
- Support an alternative model.
For example, before the discovery of accelerating expansion, scientists considered different future scenarios based heavily on whether gravity would eventually stop cosmic expansion.
Evidence for dark energy significantly changed that discussion.
This is an important feature of science:
Scientific explanations change when new evidence requires them to change.
Evaluating the Evidence
Suppose we compare two claims:
Claim A: The Universe will eventually collapse in a Big Crunch.
Claim B: The Universe will continue expanding indefinitely.
Current evidence supports Claim B more strongly.
Why?
Observations indicate that:
- The Universe is expanding.
- The expansion is currently accelerating.
- Measurements of the CMB, supernovae, and large-scale structure broadly agree with a model containing dark energy.
Therefore, continued expansion is currently more consistent with observations.
However, this conclusion assumes that the properties of dark energy do not change dramatically in the future.
How Certain Are We About the Universe's Fate?
Scientists can make increasingly accurate measurements of the Universe, but predicting its state trillions of years into the future necessarily involves assumptions.
The current evidence strongly supports continued expansion.
Therefore:
Big Freeze is currently the leading broad scenario.
But science remains open to new evidence.
Future observations could improve our understanding of dark energy and potentially change predictions about the ultimate fate of the Universe.
Did You Know?
The Universe is approximately 13.8 billion years old, but some predictions for its distant future involve timescales enormously longer than this.
Some stellar remnants may persist for trillions of years, while black holes could survive for vastly longer periods before gradually losing energy through processes such as Hawking radiation.
The future history of the Universe could therefore be unimaginably longer than the period in which stars like the Sun actively shine.
Key Terms
- Cosmic expansion – the increase in distance between widely separated, gravitationally unbound regions of the Universe.
- Dark energy – the unknown component associated with accelerating cosmic expansion.
- Big Freeze – a scenario in which expansion continues and the Universe becomes increasingly cold and dilute.
- Heat death – a possible extremely distant state with little usable free energy available for physical processes.
- Big Crunch – a proposed scenario in which cosmic expansion reverses into contraction.
- Big Rip – a hypothetical scenario involving increasingly extreme accelerated expansion.
- Cosmological constant (Λ) – a possible description of dark energy with approximately constant energy density.
- ΛCDM model – the current standard cosmological model involving a cosmological constant and cold dark matter.
- Cosmic microwave background – ancient radiation originating from the early Universe.
- Redshift – an increase in the observed wavelength of light, important in studying cosmic expansion.
Key Takeaways
- The future of the Universe depends strongly on how cosmic expansion changes over time.
- Observations indicate that the Universe's expansion is currently accelerating.
- The Big Freeze, Big Crunch, and Big Rip represent different possible cosmological futures.
- Current evidence most strongly supports continued expansion leading broadly toward a Big Freeze.
- The ΛCDM model is currently the standard model used to describe the evolution of the Universe.
- Evidence comes from galaxy redshifts, Type Ia supernovae, the cosmic microwave background, and large-scale structure.
- Dark energy plays a major role in determining the Universe's long-term future.
- Scientists still do not understand the fundamental nature of dark matter or dark energy.
- Cosmological predictions contain uncertainty and may change as new evidence and better measurements become available.