cosmological constant (Λ)

The cosmological constant, written with the Greek letter Lambda, is an extra term Einstein added to his field equations representing an energy belonging to empty space itself. Where ordinary matter and energy pull things together, this term acts like a steady, built-in pressure that pushes space to expand. Crucially, it is a property of the vacuum: every cubic metre of empty space carries the same fixed amount of it, so as space grows, its total influence grows with it.

Einstein introduced Lambda in 1917 for a reason that turned out to be wrong. He wanted a static, unchanging universe, and added the term to balance gravity's inward pull and hold the cosmos still. When astronomers discovered that the universe is in fact expanding, the balance was unnecessary, and Einstein reportedly called Lambda his greatest blunder. For decades it was simply set to zero and forgotten.

Then in 1998 two teams measuring distant exploding stars found that the expansion of the universe is not slowing down as gravity should cause, but speeding up. A small positive Lambda fits this beautifully, reviving the discarded term as a description of what we now call dark energy. Honestly, though, naming it is not the same as explaining it: why empty space carries this particular tiny energy, and why it dominates the cosmos today, remains one of the deepest unsolved puzzles in physics.