Science › Big Bang & cosmos
The cosmological constant (Λ)
1917· Theory
Well supportedStrong evidence or math, but not fully tested.
A single fixed number in Einstein's equations: an energy built into every cubic meter of empty space that pushes space apart.
See it on the Science map →How it works
Einstein added it in 1917. Its energy density never thins as space grows, and it carries negative pressure, a kind of tension, which in general relativity repels instead of attracts. Matter dilutes as space expands; Λ does not, so it eventually dominates and expansion accelerates without end, eventually doubling distances about every 12 billion years. Today's value equals the mass of about four hydrogen atoms per cubic meter, tiny, yet across all of space it outweighs all matter about two to one.
Why it matters
It is the simplest explanation of dark energy and the Λ in ΛCDM. If it is truly constant, it fixes the universe's far future: ever-faster expansion.
Think of it like
Like a fixed charge per square foot: add floor area and the total grows with it, while a fixed number of tenants (matter) gets spread ever thinner. Eventually the per-foot charge dominates.
How we know
Supernovae (1998), the afterglow and galaxy surveys all require it or something like it. Until 2024, every test found dark energy constant within a few percent, as Λ demands.
Still a mystery
Why is it so small, far below the naive vacuum-energy estimate, yet not zero? Nobody has a convincing answer. And DESI hints that it may not be constant at all.
Connections (4)
▸The cosmological constant (Λ) is the simplest explanation of Dark energyWell supported
The relationship
In general relativity, pressure gravitates as well as mass. Ordinary pressure adds pull; strong enough negative pressure reverses it into push. A cosmological constant's pressure is exactly the negative of its energy density, which is also what keeps its density fixed as space grows. That is just what dark energy looks like so far, and it needs only one number, which Einstein's equations already allowed.
How the link was found
Before the supernovae, Lawrence Krauss and Michael Turner, and separately Jeremiah Ostriker and Paul Steinhardt, argued in 1995 that the data already favored a cosmological constant.
Is it proven?
Until 2024, all measurements of dark energy's pressure-to-density ratio gave about −1, the constant's value, within a few percent. DESI's hint is the first notable departure.
Open questions and mysteries
If it is the constant, why is its value so small? If it is not, what replaces it? Either answer likely needs physics beyond what we know.
▸Albert Einstein introduced, then regretted, The cosmological constant (Λ)Proven
The relationship
Applying his new theory to the whole universe in 1917, Einstein assumed, as nearly everyone then did, that it was static. But matter's gravity would make a static universe collapse. He added Λ, a gentle cosmic repulsion, to balance it. In 1930 Arthur Eddington showed the balance was unstable, like a pencil on its tip. After Hubble's data, Einstein dropped the term in 1931.
How the link was found
Einstein's paper 'Cosmological considerations in the general theory of relativity', February 1917. Willem de Sitter found that same year a universe with Λ and no matter, later understood to be expanding.
Why it's proven
Hubble's 1929 data ended the static universe. The 1998 supernovae revived Λ itself: the term was legitimate, but its original purpose was wrong.
Open questions and mysteries
Did Einstein call it his 'biggest blunder'? The phrase comes only from George Gamow's later recollections, and historians still debate it. A 1931 manuscript, identified in 2014, shows Einstein briefly tried using Λ in a steady-state universe.
▸DESI (2024–25): is dark energy weakening? challenges The cosmological constant (Λ)Speculative
The relationship
A true cosmological constant has the same density at every era, so the expansion history it produces is fixed once the amount of matter is known. DESI measures distances in seven slices of cosmic time spanning 11 billion years. If dark energy's density has changed at all, Λ is ruled out as its explanation, and the Λ in ΛCDM must be replaced.
How the link was found
DESI's first-year results (April 2024) first raised the challenge; the three-year results (March 2025) strengthened it.
Is it proven?
Not yet decisive. George Efstathiou argued in 2024 that calibration differences between supernova samples could drive much of the signal. DESI's own distances alone sit closer to a constant.
Open questions and mysteries
Will DESI's full five-year data, plus Euclid and the Rubin Observatory, push the signal to 5σ or erase it? The answer decides between constant and changing dark energy.
▸ΛCDM: the standard model of cosmology includes The cosmological constant (Λ)Well supported
The relationship
In ΛCDM, dark energy is assumed to be exactly Einstein's constant: unchanging and smooth, with no new particles or fields. In the standard six-number version, Λ is not even one of the six. Space is taken as flat, so Λ is simply whatever fills the gap between the matter measured and the total that flatness requires, about 68%.
How the link was found
Ostriker and Steinhardt called the mix of Λ and cold dark matter the 'cosmic concordance' in 1995; the 1998 supernovae and WMAP (2003) made it standard.
Is it proven?
Supernovae, the afterglow and galaxy surveys agree on about 68% if dark energy is constant. Allowing it to vary gave no improvement until DESI's 2024–25 data.
Open questions and mysteries
If Λ must be swapped for changing dark energy, is ΛCDM revised or replaced? The name itself would no longer fit.