Cosmological Constant Problem

The cosmological constant problem is the most severe numerical discrepancy in theoretical physics: quantum field theory predicts that the energy density of the vacuum should be approximately 10¹²⁰ times larger than the value of the cosmological constant Λ inferred from cosmological observations. That is, theory and observation disagree by 120 orders of magnitude — the largest such discrepancy in the history of science. No proposed resolution — supersymmetry, the anthropic principle, string landscape arguments — has achieved consensus, and several require accepting that our theories are not explaining a phenomenon but merely parameterizing our ignorance of it.

The problem has two components that are often conflated: the old cosmological constant problem (why is Λ not enormous, given that QFT predicts it should be?) and the new cosmological constant problem (why does the observed value of Λ produce cosmic acceleration only now, at the current epoch of the universe — a "coincidence" with no known explanation, called the coincidence problem). Resolving the first without addressing the second would still leave the universe's timing unexplained.

The cosmological constant problem is a fine-tuning problem of extraordinary severity. Any physics that cannot answer it is not yet a complete physics — it has successfully described the universe while refusing to explain why the universe is describable in those terms.