More than a century ago, Albert Einstein applied his general theory of relativity to the structure of the Universe. In 1917, he added a cosmological constant to the equations, denoted by the Greek letter lambda, to describe a static Universe, as it was then believed to be.
This addition worked: the constant created an effect that counteracted the gravitational attraction of matter. However, Edwin Hubble soon discovered the expansion of the Universe, and Alexander Friedmann showed that Einstein's equations naturally describe a dynamic cosmology.
Einstein abandoned lambda and later called its introduction his "greatest blunder." A model without the constant became the foundation of the Big Bang theory.
In 1998, two groups of astronomers discovered that the expansion of the Universe is not slowing down, but accelerating. There turned out to be too little matter to explain the observed behavior, and the simplest explanation once again became the cosmological constant—now in the role of dark energy.
Thus arose the ΛCDM model, where lambda denotes dark energy and CDM stands for cold dark matter. The model relies on a small number of parameters within the framework of general relativity and successfully describes the history of expansion, the cosmic microwave background, baryon acoustic oscillations, galaxy growth, and large-scale structure.
ΛCDM has become one of the most tested models in science. And yet, it appears it is not final.
The discovery of acceleration brought Einstein's idea back to the center of modern cosmology, but it left questions about the nature of dark energy and the possible limitations of the current picture of the world.
