Dark energy

Dark energy is a mysterious, proposed form of energy that influences the universe on its largest scales. Its most significant effect is driving the accelerating expansion of the universe. It also plays a role in slowing down the rate at which cosmic structures form. Scientists believe dark energy is a dominant force in our cosmos. In the current observable universe, dark energy accounts for approximately 68% of the total energy density.

Dark matter makes up about 27%, while ordinary baryonic matter—the stuff that makes up stars and planets—accounts for only 5%. Other components, like photons and neutrinos, are nearly negligible by comparison.

To understand how dark energy works, we must look at its unique density. Unlike matter, dark energy is incredibly smooth and uniform. It does not accumulate in galaxies or clusters of stars. Instead, it is thought to be an intrinsic property of space itself. Because it is a property of space, its density remains constant even as the universe expands.

In contrast, the density of ordinary matter decreases as space grows. As more space is created through expansion, more dark energy appears with it. This creates a feedback loop where the increasing amount of space leads to even more expansion. This process acts as a form of gravitational repulsion. This occurs because dark energy exerts a strong negative pressure.

Scientists have proposed several different models to explain what dark energy actually is. The simplest candidate is the cosmological constant, symbolized by the Greek letter Lambda (Λ). This represents a constant energy density that fills space homogeneously. Another possibility involves scalar fields, which are dynamic quantities. These fields, such as quintessence or moduli, can have energy densities that vary across time and space. There are also more complex theories involving interacting dark energy or cosmological coupling. Each of these models attempts to explain the specific way the universe expands and evolves.

The history of these ideas began with Albert Einstein. In 1917, he proposed the cosmological constant to support a static universe model. He wanted to use this constant to balance the inward pull of gravity. Einstein suggested that empty space could act as a source of gravitating negative mass. However, his static model was unstable. If the universe expanded slightly, the vacuum energy would cause even more expansion. If it contracted, it would continue to contract. In 1929, Edwin Hubble observed that the universe was actually expanding. Einstein later called his introduction of the cosmological constant his "greatest blunder."

In the 1980s, researchers explored different types of energy expansion. Alan Guth and Alexei Starobinsky proposed that a negative pressure field drove cosmic inflation. This was an enormous, exponential expansion that occurred during the very early universe. While inflation is a key part of Big Bang models, it happened at a much higher energy density than today's dark energy. It is still unclear how inflation relates to the dark energy we observe now. For a long time, scientists thought dark energy was irrelevant to the modern universe. They focused instead on models containing only matter and dark matter.

Direct evidence for dark energy arrived in 1998 through observations of Type Ia supernovae. These specific stars are useful because they have a constant luminosity, or brightness. This allows astronomers to use them as accurate distance markers. By comparing the distance of these supernovae to their redshift, scientists discovered the expansion was accelerating. This discovery led to the Lambda-CDM model becoming the leading cosmological model. The term "dark energy" was coined in 1998 by Michael S. Turner.

Subsequent experiments, such as the BOOMERanG and Maxima projects in 2000, provided further support. The 2dF Galaxy Redshift Survey in 2001 also confirmed that matter density is only about 30% of the critical density.

Despite these discoveries, the exact nature of dark energy remains a major mystery. It is extremely rarefied, with a mass-energy density of roughly 10⁻²⁷ kg/m³. This low density makes it nearly impossible to detect in a laboratory setting. One major theoretical challenge is the cosmological constant problem. This is a massive disagreement between the observed value of vacuum energy and the theoretical value predicted by quantum field theory. Scientists continue to study the equation of state to understand the relationship between pressure and density. Solving this puzzle is one of the most important goals in modern physics and astronomy.