Dark energy

In physical cosmology and astronomy, dark energy is a mysterious kind of energy that affects the universe on very large scales. Its biggest effect is to make the accelerating expansion of the universe. This means the universe is growing faster over time, instead of slowing down. Dark energy also makes it harder for stars and galaxies to come together into bigger groups.

According to the lambda-CDM model, which is the most popular way we describe our universe, dark energy makes up about 68% of all the mass and energy in the universe today. This is much more than dark matter, which is about 27%, and ordinary matter, which is only 5%. Even though dark energy is so important, it is very weak. Its density is about 7×10⁻³⁰ g/cm³, which is much lower than the density of ordinary matter in galaxies.

The idea of dark energy started with observations of Type Ia supernovae. These supernovae have a consistent brightness, so they can be used like cosmic rulers to measure distances. By comparing these distances with how fast the supernovae are moving away (measured by their redshift), scientists found that the universe's expansion is actually speeding up. Before this discovery, many scientists thought the pull of gravity would slow the universe's expansion. Since then, many other observations have supported the existence of dark energy, but exactly what it is remains one of the biggest mysteries in science.

History of discovery and previous speculation

The "cosmological constant" is an idea about what dark energy might be. It is a term added to the equations that describe how the universe works. Einstein first thought of this idea to keep the universe still, but later learned the universe was actually growing.

Later, scientists like Alan Guth and Alexei Starobinsky suggested that a force like dark energy might have made the universe expand very quickly right after it began. In the 1990s, observations showed that the universe’s expansion is speeding up, which supports the idea of dark energy.

Nature

The nature of dark energy is still a mystery. It is thought to be very smooth and does not interact with anything except gravity. Even though it is very thin — about 10⁻²⁷ kg/m³ — it fills all of space and makes up most of the universe.

One idea is that dark energy comes from the energy of empty space itself. This energy should create a pulling force, which can push the universe to expand faster. However, there is a puzzle: theories predict this energy should be much stronger than what we actually see. Dark energy must have a special kind of pressure to explain why the universe is spreading out at an accelerating pace. This special pressure works like a kind of "gravitational repulsion" that helps push everything apart.

See also: Friedmann equations

Evidence of existence

We have three main reasons to think dark energy exists. First, we see that objects far away in space look redder than they should. This tells us the universe has grown more in recent times than before. Second, theories say we need a special kind of energy, not made of matter, to explain why the universe looks flat. Third, studies of big changes in how much matter there is across the universe also support this idea.

In 1998, scientists looked at certain exploding stars called Type Ia supernovae. They found that the universe is expanding faster now than before. This big discovery won a Nobel Prize in 2011. Since then, many more observations, like studies of the oldest light in the universe and how galaxies are arranged, have agreed that dark energy makes up about two-thirds of the universe. These observations help scientists learn how the universe has changed over time and how dark energy affects its growth.

Theories of dark energy

Dark energy is a mysterious force that scientists are still trying to understand. One simple idea is that dark energy is a special energy that exists in empty space, called the cosmological constant. This energy would change how gravity works, making the universe expand faster over time.

Another idea is called quintessence. In this idea, a special kind of energy field causes the expansion. This field can change over time and space, unlike the cosmological constant. Scientists are still looking for evidence to support these ideas, and there are many theories about how dark energy might behave.

Main article: Cosmological constant

Further information: Equation of state (cosmology)

Main article: Quintessence (physics)

Alternatives to dark energy

Some scientists think that dark energy might not be real. They believe we might explain what we see by changing our ideas about gravity or the universe. For example, some theories change Einstein’s idea of general relativity. These theories suggest we might not need dark energy at all. But most experts still believe dark energy is real. They think these new ideas do not match what we observe as well.

Other ideas suggest that our place in the universe might make it seem like things are expanding faster. For example, if we are in an emptier area of space, it might look like the universe is expanding quickly. There are also ideas that black holes might act in ways that look like dark energy. But these ideas are still being studied. Most scientists think more work is needed before they can replace the idea of dark energy.

Implications for the fate of the universe

Cosmologists believe that the expansion of the universe began to speed up about 5 billion years ago. Before this, the universe was slowing down because of the pull of matter. Dark energy, which we don’t fully understand, eventually became more powerful than dark matter. This means that as space grows, dark matter spreads out and weakens, but dark energy stays strong.

Scientists have different ideas about what might happen in the far future. If dark energy stays constant, galaxies far from us will move away so fast that their light will never reach us. Only galaxies in our local group, including the Milky Way, will stay together. Over time, everything else will disappear from our view. Some other theories suggest dark energy could grow even stronger.

In philosophy of science

Some scientists think that dark energy is just a guess to explain what we see in space. They say it doesn’t tell us anything new and can’t be proven wrong, based on the ideas of a scientist named Karl Popper. But not all scientists agree with this view.