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| natural inflation | |
|---|---|
| Name | Natural inflation |
| Field | Cosmology |
| Introduced | 1990s |
| Proponents | [Freese, Frieman, Olinto] |
| Related | [Axion, Pseudo-Nambu–Goldstone boson, Cosmic inflation] |
natural inflation
Natural inflation is a model of cosmic inflation that posits the inflaton field arises from a pseudo-Nambu–Goldstone boson with a periodic potential. It aims to reconcile a technically flat potential with symmetry protection, using shift symmetries associated with spontaneous breaking of a global symmetry. The model links ideas from high-energy particle physics, early-universe cosmology, and quantum field theory to produce a sustained accelerated expansion in the very early universe.
Natural inflation was proposed to address fine-tuning problems in early-universe models by invoking a naturally flat potential protected by an approximate global symmetry. The mechanism draws on concepts developed in studies of spontaneous symmetry breaking, anomaly physics, and axion-like fields in particle theory. Proponents argued that the periodic form of the inflaton potential can produce sufficient e-folds of expansion while generating primordial perturbations consistent with primordial nucleosynthesis and large-scale structure formation.
The model builds on the framework of spontaneous symmetry breaking first formalized in analyses of the Nambu–Goldstone theorem and later extended in the context of pseudo-Nambu–Goldstone bosons (pNGBs). Its motivation connects to work on the Peccei–Quinn mechanism, axion phenomenology, and models of symmetry protection in quantum field theory. Theoretical ingredients trace through developments in grand unified theories explored at CERN, model-building methods employed at SLAC National Accelerator Laboratory, and symmetry considerations discussed in seminars at Princeton University and Harvard University. The shift symmetry that protects the inflaton potential is analogous to symmetries used in constructions at Massachusetts Institute of Technology and California Institute of Technology for stabilizing scalar masses. Radiative corrections and naturalness arguments draw on techniques established by researchers associated with Stanford University and Fermilab.
The characteristic potential is periodic, often written in a cosine form motivated by instanton effects studied in nonperturbative analyses at Institute for Advanced Study. Dynamics of the inflaton field in this potential are analyzed using the slow-roll approximation developed in seminal work at Observatoire de Paris and refined in lectures at University of Cambridge. Key parameters include the symmetry-breaking scale f and the potential amplitude, which connect to scales discussed in the context of National Aeronautics and Space Administration missions probing the cosmic microwave background. The requirement for sufficient inflation ties into calculations first formalized in studies of horizon and flatness problems at University of Chicago and treatments of reheating explored at Yale University.
Predictions include a nearly scale-invariant spectrum of scalar perturbations and a tensor-to-scalar ratio that depends sensitively on the symmetry-breaking scale f. These signatures are framed within the perturbation theory developed by researchers at Max Planck Institute for Astrophysics and Institut d'Astrophysique de Paris. Predictions for the scalar spectral index and tensor amplitude are compared to parameterizations used in analyses from Planck Collaboration and forecasts prepared by teams at Jet Propulsion Laboratory. The model can yield observable primordial gravitational waves under parameter choices related to studies at LIGO Laboratory and proposals discussed at Kavli Institute for Cosmology.
Constraints on parameters come from measurements of the cosmic microwave background anisotropies and polarization reported by Planck Collaboration, WMAP, and ground-based experiments associated with Atacama Cosmology Telescope and South Pole Telescope. Large-scale structure surveys conducted by teams at Sloan Digital Sky Survey and Dark Energy Survey further restrict viable regions of parameter space. Searches for primordial B-mode polarization motivated by groups at BICEP/Keck Array provide limits on the tensor-to-scalar ratio relevant to the model. Constraints from primordial nucleosynthesis calculations performed by researchers at University of California, Berkeley and timing of the recombination epoch studied by National Radio Astronomy Observatory also inform model viability.
Extensions include multi-field realizations, aligned axion scenarios inspired by constructions at University of Oxford and trans-Planckian decay constant mechanisms explored by theorists at Imperial College London. Embeddings in string-theory frameworks draw on work from Institute for Theoretical Physics, Utrecht and compactification studies at Rudolf Peierls Centre for Theoretical Physics. Clockwork and alignment mechanisms connecting multiple pNGBs were developed in collaborations involving researchers at Perimeter Institute and University of Tokyo. Hybrid constructions that combine features of assisted inflation have been investigated in contexts associated with Rutgers University and University of Michigan.
Critiques focus on the plausibility of super-Planckian symmetry-breaking scales and control of higher-dimension operators, concerns extensively debated in seminars at CERN and workshops hosted by Kavli Institute for Theoretical Physics. Embedding the required shift symmetry in ultraviolet-complete theories such as string theory remains challenging; debates occur among groups at Institute for Advanced Study and Caltech. Open issues include the sensitivity to quantum gravity corrections discussed in forums at Perimeter Institute and the connection to low-energy axion searches pursued by experiments at ADMX and CAST. Further empirical tests are expected from upcoming missions and collaborations at European Space Agency and instrument teams at South Pole Telescope.