cosmological constant

History

Einstein added the constant to his equations in 1917 so that the model of the Universe would not contract under gravity. After Hubble's discovery in 1929, it was considered unnecessary. But in 1998, observations of supernovae showed that the expansion of the Universe is accelerating—and the cosmological constant returned as the main candidate for dark energy.

How it works

The cosmological constant is equivalent to negative pressure. While ordinary matter and radiation slow down expansion, vacuum energy 'pushes' it, and the more space appears, the stronger this effect becomes—like if from every new cubic centimeter a new spring appeared.

History

Einstein added the constant to his equations in 1917 so that the model of the Universe would not contract under gravity. After Hubble's discovery in 1929, it was considered unnecessary. But in 1998, observations of supernovae showed that the expansion of the Universe is accelerating—and the cosmological constant returned as the main candidate for dark energy.

How it works

The cosmological constant is equivalent to negative pressure. While ordinary matter and radiation slow down expansion, vacuum energy 'pushes' it, and the more space appears, the stronger this effect becomes—like if from every new cubic centimeter a new spring appeared.

History

Einstein introduced Λ in 1917 for a static model of the Universe. In the 1920s, Friedmann and Lemaître showed that solutions with and without Λ lead to expansion. After Hubble's discovery, Einstein abandoned it. At the end of the 20th century, two groups of astronomers (Perlmutter, Riess, Schmidt) discovered acceleration from distant supernovae, which resurrected Λ.

How it works

In Einstein's equations, the cosmological constant adds a term Λ g_{μν}. In a homogeneous and isotropic Universe, it manifests as an effective fluid with equation of state p = -ρ c^2, leading to repulsive gravity. In quantum field theory, the predicted vacuum energy is 120 orders of magnitude larger than observed—this is the cosmological constant problem.

Open problems & fun facts

The cosmological constant problem: the enormous mismatch between the theoretical value of vacuum energy and observations (~10^120 times).; The coincidence of the current densities of dark energy and matter (the coincidence problem).; Is Λ a true constant or a dynamic field (quintessence)?

💡 If the cosmological constant were slightly larger, galaxies could never have formed. Slightly smaller—the Universe would have collapsed back long ago. We live in an amazingly finely tuned cosmos.