At the heart of the standard cosmological model lies Einstein's general relativity, which describes the universe's parameters and evolution. Initially, Einstein resisted the idea of a dynamic universe. Just two years after publishing his theory in 1917, he introduced the cosmological constant to achieve a static cosmos. Yet, astronomers Georges Lemaître and Edwin Hubble soon proved him wrong. Despite being an ad hoc addition, the constant made a triumphant return in 1998 with the discovery of the universe's accelerating expansion.
Albert Einstein introduced the cosmological constant—his "universal constant"—in 1917 to balance general relativity's equations. Prevailing wisdom held that the universe was static, neither expanding nor contracting. However, Einstein's theory implied gravity would cause collapse, so he added the Greek letter lambda (Λ) to maintain equilibrium. A decade later, Edwin Hubble's observations revealed galaxies receding from us, confirming expansion. Einstein later called lambda his "biggest blunder."
Hubble's findings sidelined the constant for decades, until late-1990s observations of distant supernovae revealed not just expansion, but accelerating expansion. Scientists dubbed this enigma "dark energy."
In the 1920s, Alexander Friedmann derived an equation describing the universe's evolution from the Big Bang. Incorporating Einstein's lambda into Friedmann's equations accurately modeled this acceleration.
This updated framework underpins the ΛCDM model (Lambda Cold Dark Matter), based on the Friedmann-Lemaître-Robertson-Walker (FLRW) metric, forming the cornerstone of modern cosmology.
Today, lambda's essence remains elusive. Many equate it with dark energy, though both concepts lack full explanation. Insights from particle physics offer clues: experiments like the Casimir effect confirm that "empty" space teems with virtual particles popping in and out, generating vacuum energy woven into spacetime.
Linking this vacuum energy to lambda proves challenging. Supernova data suggest a modest value sufficient for billions of years of gentle repulsion. Yet, quantum calculations yield a value 120 orders of magnitude larger—a profound discrepancy.
Complicating matters, some propose lambda isn't constant but varies over time, as in "quintessence" models. Projects like the Dark Energy Survey are probing these ideas with unprecedented precision.
