The cosmological constant accounts for the effects of a. dark matter. b. the Big Bang. c. dark energy. d. gravity.

To understand the cosmological constant, we need to delve into the concept of dark energy. Dark energy fills the universe and makes up about 68% of its total energy. Instead of attracting objects like gravity, dark energy has a repulsive effect. This mysterious force is believed to be causing the accelerated expansion of the universe.
Scientists believe dark energy might be linked to the cosmological constant. This constant was originally added by Einstein to his equations to allow for a static universe. However, when it was discovered that the universe is expanding, the meaning of the cosmological constant changed. It is now associated with dark energy, helping to explain why the expansion is speeding up rather than slowing down.

Albert Einstein's theory of General Relativity is crucial for understanding how the cosmological constant fits into the bigger picture. This theory describes how mass and energy in the universe interact with space and time. In simple terms, massive objects like planets and stars cause a curvature in space-time, which we perceive as gravity.
In 1917, Einstein introduced the cosmological constant into his equations of General Relativity. At the time, it was believed the universe was static and unchanging. To match this belief, Einstein added this 'fudge factor' to balance the gravitational pull and make his equations consistent with a stationary universe.
Later, when it became clear that the universe is expanding, Einstein regretted this addition, calling it his greatest blunder. However, modern cosmology has found that the cosmological constant is, in fact, crucial for explaining the observed acceleration in the universe's expansion.

The concept of universe expansion is essential to modern cosmology. Observations show that galaxies and other celestial bodies are moving away from each other, implying that the universe is expanding. This discovery was first made by Edwin Hubble in 1929. Initially, it was assumed that the expansion would slow down over time due to gravitational forces.
However, more recent observations suggest that the expansion is actually accelerating. This acceleration can be explained by the presence of dark energy, which counteracts the force of gravity. The cosmological constant is a term that can represent this dark energy in equations.
Understanding universe expansion helps shed light on many cosmic phenomena, including the fate of the universe. If the acceleration continues, the universe could keep expanding indefinitely, potentially ending in a 'Big Freeze' where stars and galaxies drift apart and the universe becomes cold and dark.