Accelerating expansion of the universe

The accelerating expansion of the universe is the observation that the universe is expanding at an increasing rate. This phenomenon was first discovered in the late 1990s through observations of distant Type Ia supernovae, which appeared dimmer than expected, indicating that they were farther away than their redshift suggested. This discovery led to the proposal of a mysterious form of energy, known as dark energy, which is thought to permeate all of space and drive the acceleration.

Background

The concept of an expanding universe was first proposed by Edwin Hubble in the 1920s, when he observed that distant galaxies were moving away from us, leading to the formulation of Hubble's law. According to this law, the velocity at which a galaxy recedes from us is proportional to its distance. This was initially interpreted as evidence for a universe that was expanding uniformly.

However, the discovery of the accelerating expansion suggested that the dynamics of the universe were more complex than previously thought. The acceleration implies that there is a repulsive force acting against the gravitational pull of matter, which would otherwise slow down the expansion.

Dark Energy

Dark energy is the term used to describe the unknown force driving the accelerating expansion. It is estimated to constitute about 68% of the total energy density of the universe. The nature of dark energy is one of the biggest mysteries in cosmology. Several theories have been proposed to explain dark energy, including the cosmological constant introduced by Albert Einstein in his equations of general relativity, and various quintessence models that suggest a dynamic field responsible for the acceleration.

Observational Evidence

The primary evidence for the accelerating expansion comes from observations of Type Ia supernovae, which serve as "standard candles" for measuring astronomical distances. These supernovae have a known intrinsic brightness, allowing astronomers to determine their distance based on their observed brightness. The unexpected dimness of these supernovae at high redshifts provided the first direct evidence for acceleration.

Additional evidence comes from measurements of the cosmic microwave background radiation, the large-scale structure of the universe, and baryon acoustic oscillations. These observations are consistent with a universe dominated by dark energy.

Theoretical Implications

The accelerating expansion of the universe has profound implications for the ultimate fate of the universe. If the acceleration continues indefinitely, the universe may end in a "Big Freeze" or "Heat Death", where galaxies move away from each other, stars burn out, and the universe becomes cold and dark.

Also see

• Cosmology
• Dark matter
• General relativity
• Hubble's law
• Big Bang