The Cosmological Constant Problem (1989): Weinberg's Famous Review

The cosmological constant problem, set out by Nobel laureate Steven Weinberg in a famous 1989 review, asks why the energy of empty space is so tiny compared with what quantum physics naturally predicts. Written nearly a decade before dark energy was discovered, it remains the deepest unsolved puzzle about it.

The question before the paper

Einstein’s general relativity allows a cosmological constant: a fixed energy density belonging to space itself. Quantum field theory, which describes particles, says empty space is not truly empty. It seethes with fluctuations that should carry energy, and that energy should act exactly like a cosmological constant. Natural estimates of its size come out enormous, while astronomy showed the real value is either zero or extremely small. Something had to cancel almost all of it. What, and why so precisely?

What the paper found

This is a review, so its result is a map of the problem rather than a measurement.

• A sharp statement of the mismatch. Weinberg laid out why vacuum energy should be large in quantum theory, and why observations demand it be small.
• A survey of proposed fixes, including supersymmetry, adjustment mechanisms in which some field dynamically cancels the vacuum energy, ideas from quantum cosmology, and anthropic arguments.
• No accepted solution. None of the approaches solved the problem convincingly.

How he did it

Weinberg wrote the review from the standpoint of a particle theorist. He collected the relevant pieces of quantum field theory and cosmology, explained what each proposed mechanism would have to achieve, and identified where each one fails or remains unproven.

Why it mattered

Weinberg also discussed a line of reasoning from his own 1987 paper. If the cosmological constant varies from region to region in a much larger universe, galaxies, and therefore observers, can only form in regions where it is small. That anthropic argument gives an upper limit on its value, and it suggested the constant might be small but not exactly zero.

Nine years later, supernova surveys found exactly that: cosmic expansion is accelerating, as expected from a small positive cosmological constant, a result confirmed by the Supernova Cosmology Project. The question changed from “why is it zero?” to “why is it so small, but not zero?”

What we know now

That question is still unanswered, and it now has a name: dark energy. Observations have also opened a new possibility Weinberg’s framing allows for. If dark energy is not a constant but changes with time, as DESI’s 2025 results from the Dark Energy Spectroscopic Instrument tentatively suggest, it may be a dynamical field rather than the energy of the vacuum. See is dark energy weakening? for the current state of that test.

Who wrote it

Steven Weinberg was a particle theorist who shared the 1979 Nobel Prize in Physics for the theory that unifies the weak and electromagnetic interactions. He wrote this review alone. His review is one of the most cited theoretical papers on dark energy, with about 3,900 citations in Semantic Scholar as of October 2026.

Read the original

Weinberg, S. (1989). “The cosmological constant problem.” Reviews of Modern Physics 61, 1. doi:10.1103/RevModPhys.61.1. The journal version requires a subscription. His 1987 anthropic paper is at doi:10.1103/PhysRevLett.59.2607.