Included in the standard cosmological model, Einstein's theory of general relativity (and its solutions) is the framework allowing today to describe the parameters and the evolution of the Universe. However, Einstein did not always buy into the idea of a dynamic universe. This is why, two years after having published his theory, he added a cosmological constant to it, making it possible to obtain a static universe. A few years later, astronomers Lemaître and Hubble would prove him wrong. Although this is a parameter introduced on an ad hoc basis , the cosmological constant returned to center stage with the discovery of accelerating expansion in 1998.
Albert Einstein introduced the cosmological constant, which he called the "universal constant," in 1917 as a way to balance certain calculations in his theory of general relativity. At the time, physicists thought the Universe was static — neither expanding nor contracting — but Einstein's work suggested that gravity invariably imparted momentum to the Universe.
Thus, to agree with his own vision of the Universe (which was also that of the scientific consensus), Einstein inserted an ad hoc factor , denoted by the Greek letter lambda (Λ), in its results, keeping the cosmos motionless. Yet just over a decade later, American astronomer Edwin Hubble noticed that galaxies were moving away from us, indicating that the Universe was expanding. Einstein later described lambda as his "biggest mistake".
Hubble observations negated the need for a cosmological constant for decades, but that changed when astronomers examining distant supernovae in the late 1990s discovered that the cosmos was not only expanding, but expanding. was accelerating. They named this mysterious force "dark energy".
In the 1920s, Russian physicist Alexander Friedmann developed an equation, now called Friedmann's equation, which describes the properties and evolution of the Universe from the Big Bang. By reusing Einstein's constant and introducing it into Friedmann's equations, the researchers were able to model the cosmos correctly — that is, with an accelerated rate of expansion.
This version of Friedmann's equation now forms the backbone of contemporary cosmological theory, known as the standard cosmological model ΛCDM (Lambda CDM, where CDM stands for cold dark matter), based on the FLRW metric (Friedmann-Lemaître -Robertson-Walker).
However, today the nature of lambda is uncertain. Most physicists consider it to be interchangeable with the concept of dark energy, but this does not shed more light on the matter, as the nature of dark energy is also unknown. One potential explanation for the cosmological constant lies in the realm of modern particle physics.
Experiments have confirmed (notably via the Casimir effect) that empty space is permeated by countless virtual particles that constantly arise and disappear. This relentless action creates what is known as "vacuum energy", inherent in the fabric of spacetime. But relating vacuum energy to the cosmological constant is not straightforward. Based on their observations of supernovae, astronomers estimate that dark energy should have a low to moderate value, just enough to keep everything in the Universe away for billions of years.
Yet when cosmologists try to calculate how much energy should come from the motion of virtual particles, they arrive at a result 120 orders of magnitude larger than supernova data suggests. To add to the puzzle, some researchers have proposed that the cosmological constant is not a constant at all, but that it changes or fluctuates over time. This theory is called "the quintessence", and some projects like the Dark Energy Survey , are currently making precise observations to find clues.
