| Cosmology | |
|---|---|
| Name | Cosmology |
| Focus | Origin and evolution of the Universe |
| Subdiscipline | Physical cosmology, quantum cosmology |
| Related | Astrophysics, General relativity, Particle physics |
Cosmology Cosmology is the scientific study of the origin, evolution, structure, and large-scale dynamics of the Universe. Within the context of Quantum physics, cosmology investigates how quantum phenomena at microscopic scales influence cosmic history, from the Big Bang through cosmic inflation to present large-scale structure. This intersection is central to understanding phenomena such as primordial fluctuations, particle creation, and attempts to quantize gravity.
Cosmology as a discipline overlaps with General relativity for macroscopic dynamics and with Quantum field theory for microscopic processes. The scope within quantum physics includes study of the inflaton in models of cosmic inflation, quantum generation of density perturbations as described by Mukhanov–Sasaki equation, and particle production mechanisms studied in early universe scenarios. Key institutions and collaborations advancing this interface include ESA missions (e.g., Planck), the NASA WMAP mission, and ground-based projects such as the BICEP experiments and the Atacama Cosmology Telescope.
Foundational questions address how quantum mechanics applies to the whole Universe and how classical spacetime emerges. Foundational frameworks include canonical quantization approaches developed by Paul Dirac and Bryce DeWitt and path-integral methods inspired by Richard Feynman. Seminal theoretical contributions include the Bekenstein–Hawking entropy for black holes, the Hawking radiation derivation by Stephen Hawking, and the Bunch–Davies vacuum prescription for inflationary initial states. Foundational debates involve the role of decoherence by environments like cosmic background radiation, advocated in work by Wojciech Zurek, and interpretations such as the many-worlds interpretation proposed by Hugh Everett III.
The early Universe hosts phenomena where quantum effects are dominant. During reheating and preheating after inflation, perturbative and nonperturbative particle production is modeled with quantum field theory in curved spacetime techniques and lattice simulations by groups at institutions like CERN and Perimeter Institute for Theoretical Physics. Quantum tunneling processes underlie models such as false vacuum decay first analyzed by Sidney Coleman and Frank De Luccia (Coleman–De Luccia instantons). Baryogenesis mechanisms—e.g., electroweak baryogenesis and leptogenesis—invoke CP violation and out-of-equilibrium quantum processes, with constraints informed by results from the Large Hadron Collider and neutrino experiments like Super-Kamiokande.
Quantum field theory on curved backgrounds formalizes particle creation by expanding universes and black hole evaporation. Key formal developments include the Unruh effect and the algebraic approach to quantum fields developed by researchers such as Rudolf Haag and Robert Wald. Applications in cosmology include calculation of the primordial power spectrum from vacuum fluctuations and backreaction studies that assess quantum stress-energy contributions to the Friedmann equations. Renormalization techniques adapted for curved spacetime trace back to work by Leonard Parker and Nicholas Birrell and the use of adiabatic regularization, point-splitting, and effective action methods.
Quantum cosmology seeks a quantum description of the whole Universe. Prominent formalisms include canonical quantum gravity (the Wheeler–DeWitt equation developed by Bryce DeWitt and John Wheeler) and path-integral proposals like the Hartle–Hawking state by James Hartle and Stephen Hawking. Approaches attempting to embed cosmology in a UV-complete theory include string theory landscapes studied by Leonard Susskind and Andreas Albrecht, and loop quantum gravity proposals with cosmological models advanced by Martin Bojowald and Abhay Ashtekar, yielding loop quantum cosmology effects such as bounce scenarios. These frameworks address issues like initial singularities, time in quantum gravity, and the origin of classical spacetime.
Quantum-origin predictions produce observable signatures. Primordial scalar and tensor perturbations lead to anisotropies in the cosmic microwave background (CMB) measured by Planck and WMAP, and targeted by polarization experiments like BICEP2 and Keck Array to detect primordial gravitational waves. Non-Gaussianity constraints from CMB and large-scale structure surveys (e.g., Sloan Digital Sky Survey) test models of interacting inflatons and alternatives such as ekpyrotic universe scenarios. Particle relics from early quantum processes motivate searches for axions and WIMPs in experiments like ADMX and LUX-ZEPLIN. Precision tests of quantum field effects in curved spacetime also inform constraints on semiclassical backreaction and modified gravity candidates.
Major open problems include constructing a consistent theory of quantum gravity that yields testable cosmological predictions, resolving the cosmological singularity and the cosmological constant problem, and explaining the small value of dark energy observed by surveys such as Dark Energy Survey. Research directions emphasize observational probes (next-generation CMB missions, 21-cm cosmology, gravitational-wave observatories like LIGO/LISA), theoretical development in string cosmology and loop quantum cosmology, and interdisciplinary work linking quantum information theory (e.g., entanglement entropy studies by Mark Van Raamsdonk) to spacetime emergence. Collaborative efforts between experimental consortia, national labs (e.g., Fermilab), and universities aim to close the gap between quantum foundations and cosmological observation.
Category:Physical cosmology