Cosmology · Strong consensus
The Cosmological Constant and Accelerating Expansion
Observations of distant Type Ia supernovae in the late 1990s revealed that cosmic expansion is accelerating, attributed to a form of dark energy modeled mathematically as a cosmological constant, whose value poses one of the most severe unsolved problems in theoretical physics.
The evidence explained
In 1998, two independent teams, the Supernova Cosmology Project led by Saul Perlmutter and the High-Z Supernova Search Team including Brian Schmidt and Adam Riess, measured the brightness of distant Type Ia supernovae, whose intrinsic luminosity is relatively standardized, and found they were dimmer, and thus more distant, than expected in a universe with expansion slowing due to gravitational attraction, indicating instead that cosmic expansion is accelerating, a discovery recognized with the 2011 Nobel Prize in Physics.
The simplest explanation within general relativity is a cosmological constant, a term Einstein originally introduced in 1917 for different reasons (to allow a static universe) and later reportedly regretted, representing a constant energy density inherent to space itself, now generally identified with 'dark energy,' estimated to constitute roughly 68 percent of the universe's total energy content based on combined supernova, cosmic microwave background, and large-scale structure data.
The cosmological constant's observed value poses an extreme theoretical puzzle: naive quantum field theory calculations of the vacuum energy density that should contribute to the cosmological constant predict a value roughly 10^120 times larger than what is observed, a discrepancy physicist Steven Weinberg and others have called the worst prediction in the history of physics, indicating a deep and unresolved tension between quantum field theory and the observed small, positive value of the constant.
Various approaches to this problem exist, including proposed cancellation mechanisms, quintessence models involving a dynamical scalar field rather than a strict constant, and anthropic explanations invoking a multiverse of regions with varying constant values where only some permit observers to exist, an approach associated with Steven Weinberg's own 1987 prediction and later discussed extensively by Leonard Susskind, though these remain speculative and are not empirically confirmed.
The observational reality of accelerating expansion and a small positive cosmological constant is essentially undisputed among cosmologists, while the correct theoretical explanation for its extremely small but nonzero value remains one of the most significant open problems in fundamental physics, with anthropic, dynamical, and yet-undiscovered mechanisms all under active investigation without consensus.
What it does show
- • Type Ia supernova observations, later corroborated by CMB and large-scale structure data, robustly establish that cosmic expansion is accelerating.
- • The simplest theoretical model attributes this to a cosmological constant or dark energy comprising roughly 68 percent of cosmic energy density.
- • This is one of the most well-corroborated observational results in modern cosmology, from multiple independent methods.
What it does not show
- • The observation does not explain why the cosmological constant has its particular small value, a major unsolved theoretical problem.
- • It does not confirm any particular proposed explanation (anthropic selection, quintessence, or otherwise) for that value.
- • It does not resolve the tension between the observed value and quantum field theory's vacuum energy predictions.
Primary sources and literature
- Saul Perlmutter et al., 'Measurements of Omega and Lambda from 42 High-Redshift Supernovae,' Astrophysical Journal 517 (1999)Original Supernova Cosmology Project results.
- Adam Riess et al., 'Observational Evidence from Supernovae for an Accelerating Universe,' Astronomical Journal 116 (1998)Independent High-Z team results.
- Steven Weinberg, 'The Cosmological Constant Problem,' Reviews of Modern Physics 61 (1989)Classic statement of the theoretical puzzle and anthropic proposal.
- Planck Collaboration, 'Planck 2018 Results,' Astronomy & Astrophysics 641 (2020)Precision cosmological parameter measurements including dark energy density.
Where scholars disagree
The observational fact of acceleration is not disputed; the theoretical explanation for the cosmological constant's value remains a major unsolved problem with competing, empirically unconfirmed proposals.
For further study
Primary and secondary sources on both sides of this question. Reading the strongest opposing case is part of the work, not a concession.
The Beginning of the Universe — Alexander Vilenkin (Many Worlds in One)
A cosmologist on why classical spacetime appears to have a beginning, in his own words rather than filtered through apologetics.
The Kalam Cosmological Argument (2 vols.) — Paul Copan and William Lane Craig (eds.)
The scholarly state of the debate on both the philosophical and physical premises.
A Universe from Nothing — Lawrence Krauss
The best-known attempt to make cosmic origins require no cause; note carefully what 'nothing' means in it.
The Big Picture — Sean Carroll
A leading physicist's naturalistic account of reality, including his direct replies to fine-tuning arguments.