Dark energy

In physical cosmology and astronomy, dark energy is a mysterious kind of energy that influences the largest parts of the universe. Its most important effect is to make the universe expand faster and faster. Before scientists discovered this, they thought the pull of matter would slow the universe's growth, but dark energy does the opposite.

Dark energy makes up about 68% of everything in the observable universe today. Even though its density is extremely low—much less than the amount of ordinary matter or even dark matter—it spreads out evenly everywhere, so it has a big effect overall.

The first clues that dark energy exists came from studying bright explosions called Type Ia supernovae. These explosions have the same brightness, so scientists can use them to measure distances very accurately. By comparing these distances with how fast these supernovae are moving away (their redshift), researchers found that the universe's expansion is actually accelerating. This surprising discovery led scientists to look for explanations, and dark energy is the leading idea they have today.

Exactly what dark energy is still remains one of the biggest mysteries in science. Some theories suggest it could be something called the cosmological constant, a steady energy that fills space the same everywhere. Other ideas involve changing energy fields like quintessence, but so far, no one knows for sure.

History of discovery and previous speculation

The "cosmological constant" is a simple idea for what dark energy might be. It is a term that can be added to the Einstein field equations from general relativity. This idea was first suggested by Einstein to make the universe stay still and not expand or contract. Later, we learned that the universe is actually expanding, as shown by Edwin Hubble in 1929.

Alan Guth and Alexei Starobinsky suggested in 1980 that a force like dark energy could have caused a very fast expansion of the universe right after the Big Bang. In 1998, observations of exploding stars called supernovae showed that the universe's expansion is speeding up, giving strong evidence for dark energy. This led to the Lambda-CDM model becoming the most popular explanation. Further studies with tools like the WMAP satellite helped confirm these ideas and measure important properties of the universe more accurately.

Nature

The nature of dark energy is still a mystery. Scientists believe it is very smooth and does not clump together like matter. Unlike ordinary matter, dark energy does not seem to interact through any of the basic forces except gravity. Because it is so spread out, it is very hard to detect in labs. Even though dark energy is very thin, it makes up about 68% of everything in the universe because it fills all of space evenly.

One idea is that dark energy comes from the energy of empty space itself. This energy should create a pulling force, helping to push the universe apart. Scientists are still trying to understand exactly what dark energy is and how it works. They need careful measurements of how fast the universe is expanding to learn more about it. Recent studies suggest that the amount of dark energy might be changing slowly over time.

Evidence of existence

The evidence for dark energy comes from three main sources. First, measurements of how far away objects are in space, compared to how fast they are moving, suggest the universe has been expanding more in recent times than in the past. Second, theories need an extra kind of energy—not made of matter—to explain why the universe appears flat. Third, studies of the patterns in the mass of the universe support the idea that dark energy exists.

In 1998, observations of certain exploding stars called Type Ia supernovae showed that the expansion of the universe is accelerating. This important discovery was recognized with the 2011 Nobel Prize in Physics awarded to Saul Perlmutter, Brian P. Schmidt, and Adam G. Riess. Since then, many other observations, including studies of the cosmic microwave background and the way galaxies are arranged, have supported the idea that dark energy makes up about 66% of the universe, with the rest being dark matter and ordinary matter.

Theories of dark energy

Dark energy is a mysterious force that makes the universe expand faster and faster. Scientists are trying to understand it by looking at different ideas. One simple idea is called the cosmological constant, which says that empty space has a tiny amount of energy that pushes everything apart. This energy is related to mass by Einstein's famous equation, E = mc².

Another idea is called quintessence, where a special kind of energy field moves and changes over time. This field can make the universe expand in different ways than the cosmological constant. Scientists are still studying whether quintessence exists and how it might work. There are also other theories that try to explain dark energy by linking it to dark matter or changing the rules of gravity itself. All these ideas help scientists learn more about the universe and its expansion.

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 exist, and that what looks like its effects could be explained by changes to our understanding of how gravity works or by looking at the universe in new ways. For example, one idea is that we might live in a part of space that is emptier than average, which could make the universe seem to expand faster than it really is.

Another idea is that dark energy could be linked to black holes. Some theories suggest that as the universe grows, black holes also grow in a way that could mimic the effects of dark energy. However, many experts are still not sure if these ideas fully explain what we see in the universe.

Implications for the fate of the universe

Cosmologists believe the universe's expansion began speeding 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, the amount of dark matter decreases faster than dark energy.

Depending on what dark energy is, the future of the universe could look very different. If dark energy stays the same, 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 visible. Other ideas suggest dark energy might grow so strong it could tear apart galaxies, stars, and even atoms in an event called the Big Rip. There are also ideas that the universe might eventually stop expanding and collapse in a Big Crunch, or even go through cycles of expanding and collapsing.

In philosophy of science

Some scientists think that dark energy is added to explain things we see in space, but it doesn't give us new information to test. One scientist, David Merritt, says this idea might not be able to be proven wrong, like many theories need to be. Not all scientists agree with him, though.