| quantum cosmology | |
|---|---|
| Name | Quantum cosmology |
| Era | Contemporary physics |
| Main interests | Quantum description of the universe |
| Notable ideas | Wheeler–DeWitt equation, Hartle–Hawking state |
quantum cosmology
Quantum cosmology is the study of the application of quantum mechanics and quantum field theory to the universe as a whole, seeking a quantum description of cosmological spacetimes and their origins. It matters in Quantum Physics because it attempts to reconcile general relativity with quantum principles, addressing questions about the initial singularity, the origin of structure, and the arrow of time.
Quantum cosmology emerged from efforts to quantize gravity and to understand the early universe within a single framework. Key historical contributions include the canonical quantization program of John Archibald Wheeler and Bryce DeWitt leading to the Wheeler–DeWitt equation, and path-integral approaches advocated by Richard Feynman and later adapted by James Hartle and Stephen Hawking. Developments in the mid-20th century at institutions such as Princeton University, Cambridge University, and CERN paralleled advances in particle physics and cosmology. The subject draws on work by researchers including Andrei Linde, Alan Guth, Paul Steinhardt, Abhay Ashtekar, and Martin Rees, and intersects with programs like string theory and loop quantum gravity.
Foundationally, quantum cosmology rests on proposals for a theory of quantum gravity. Canonical approaches lead to constraints like the Hamiltonian constraint and the Wheeler–DeWitt equation; covariant approaches use the Feynman path integral for metrics and fields. Prominent frameworks invoked in quantum cosmology include loop quantum gravity (with work by Carlo Rovelli and Lee Smolin), string theory (with contributions from Edward Witten and the AdS/CFT correspondence), and semiclassical methods combining quantum field theory in curved spacetime developed by N. D. Birrell and P. C. W. Davies. The role of the Planck scale and the Planck epoch is central, since quantum cosmology addresses regimes where quantum fluctuations of spacetime are non-negligible.
Quantum cosmology represents the universe by a quantum state, often a wavefunctional Ψ[h_{ij}, φ] over geometries and matter fields. The Wheeler–DeWitt formalism formalizes this as a constraint equation on Ψ, paralleling the Schrödinger equation without an external time parameter. Alternative representations use density matrices or path-integral amplitudes. Important conceptual issues include the problem of time (debated by Karel Kuchař and Christopher Isham), the role of observables in diffeomorphism-invariant theories, and the interpretation of the wavefunctional within frameworks such as the Copenhagen interpretation, many-worlds interpretation (notable proponents like Hugh Everett III), and relational perspectives.
Proposed boundary conditions aim to specify a unique or natural cosmological quantum state. The Hartle–Hawking state (no-boundary proposal) uses a Euclidean path integral to define a regular initial condition; it was introduced by James Hartle and Stephen Hawking. The Vilenkin tunneling proposal (associated with Alexander Vilenkin) proposes a tunneling wavefunction from "nothing". Debates compare predictions for inflationary initial conditions, the sign choices in semiclassical expansions, and implications for the cosmological constant problem studied by figures like Steven Weinberg. Other approaches include the density matrix proposals of Don Page and considerations of decoherence by Wojciech Zurek.
To obtain concrete predictions, practitioners often truncate degrees of freedom to minisuperspace models, reducing the infinite-dimensional superspace to a finite set of variables such as the Friedmann–Lemaître–Robertson–Walker (FLRW) scale factor and homogeneous scalar fields like the inflaton. Minisuperspace quantization makes the Wheeler–DeWitt equation tractable and enables semiclassical approximations. Calculations often reference specific potentials (e.g., chaotic inflation potentials by Andrei Linde), and engage techniques from semiclassical gravity and WKB theory. Numerical and analytical studies have been performed at institutions including University of Cambridge and Harvard University.
Quantum cosmology provides a setting to derive or constrain cosmic inflation and the origin of primordial perturbations. The choice of initial quantum state affects predictions for the spectrum of cosmic microwave background (CMB) anisotropies measured by missions such as COBE, WMAP, and Planck. The generation of primordial perturbations via quantum fluctuations of the inflaton field ties quantum cosmology to quantum field theory in curved spacetime and to observational programs in astrophysics and observational cosmology. Models inspired by string cosmology and brane cosmology (e.g., work by Gabriele Veneziano and Joseph Polchinski) seek to embed inflationary scenarios within a fundamental quantum-gravitational framework.
Connecting quantum cosmology to data faces conceptual and technical obstacles: defining probabilities without a fixed external time, regulating path integrals over geometries, and dealing with measure problems in the multiverse or eternal inflation scenarios (studied by Alex Vilenkin and Alan Guth). Observable imprints could include specific signatures in the CMB, non-Gaussianities probed by experiments like Planck and future surveys, or relics such as primordial gravitational waves sought by instruments like BICEP and LIGO. Theoretical challenges motivate interplay with laboratories and collaborations at SLAC National Accelerator Laboratory, Perimeter Institute for Theoretical Physics, and national science agencies. Progress often relies on conservative theoretical synthesis, aiming for stable, testable proposals compatible with established pillars of physics.
Category:Quantum gravity Category:Cosmology Category:Foundations of physics