Cosmological constant
The cosmological constant, denoted by λ, is a term that Einstein introduced in 1917 into his equations of general relativity. He originally added it to obtain a static universe, in accordance with the prevailing ideas of the time. When Hubble discovered the expansion of the universe in 1929, Einstein removed this constant, calling it the greatest mistake of his career. History, however, proved him wrong: the cosmological constant is now at the heart of modern cosmology.
An error that became a revelation
In 1998, the discovery of the accelerated expansion of the universe— through the observation of distant supernovae—required the cosmological constant to be reinstated. For the equations of general relativity to describe this acceleration, an additional term of érepulsive energy. The cosmological constant plays exactly this role: it represents a vacuum energy, an energy intrinsic tospace itself that exerts a negative pressure and accelerates the expansion.
The Problem of the Cosmological Constant
The measured value of the cosmological constant poses a profound problem. Quantum mechanics predicts that the energy of the vacuum should be enormously greater— by a factor of 10 to the power of 120 — than the observed value. This colossal discrepancy isone of the most significant unsolved problems in theoretical physics, sometimes referred to as the cosmological constant problem or the fine-tuning problem.
Connection to dark energy
The cosmological constant is the simplest interpretation of dark energy. If this constant is indeed a fundamental property of spacetime, its effect is permanent and uniform throughout the universe. Other theoretical models propose a dynamic dark energy that could vary over time—something that future observations will attempt to distinguish.
Did you know?
The Greek letter lambda, used to denote the cosmological constant, has become so iconic that it appears in the name of the standard cosmological model: the Lambda-CDM (Cold Dark Matter) model, which describes the universe in terms of the cosmological constant and cold dark matter.
Frequently Asked Questions
Is the cosmological constant truly constant?
That is what current observations suggest, in accordance with the Lambda-CDM model. But missions such as Euclid and the Vera Rubin Observatory are seeking to detect possible variations in this constant over time, which would indicate dynamic dark energy rather than a true constant.
Why did Einstein regret the cosmological constant?
Because he introduced it for the wrong reasons—to force a static universe—when it could have allowed him to predict the expansion of the universe before Hubble. After the discovery of the expansion, the constant seemed unnecessary. He couldn’t have known that it would reappear in the form of dark energy.
What is the numerical value of the cosmological constant?
Its value is approximately 1.1 × 10⁻⁵² per square meter — an infinitesimal value, which explains why its effects are only evident on a cosmological scale and are completely imperceptible on stellar or galactic scales.