E11 Quantum Cosmology

Topic

At the heart of this model lies quantum cosmology—the attempt to quantize gravity itself in order to describe the universe at the instant prior to Planck time.

This field relies on the Wheeler-DeWitt equation, which serves as a Schrödinger equation for the entire universe but features a notable peculiarity: it contains no explicit time variable. This gives rise to the so-called "problem of time" in quantum cosmology—the difficulty of describing evolution without a temporal reference against which that evolution can occur.

Within this same quantum framework, quantum fluctuations of the inflaton field—as they cross outward beyond the horizon during inflation—stretch until they behave like classical fluctuations; this stretching causes these originally quantum fluctuations to become the very source of the primordial spectrum of perturbations observed in the universe today.

Bekenstein-Hawking entropy, meanwhile, links gravity, thermodynamics, and information by connecting a black hole's entropy directly to the area of ​​its horizon, such that a larger horizon necessarily implies greater entropy.

Two main approaches currently compete to fully describe this array of phenomena: loop quantum gravity and string theory; neither has yet yielded an observational prediction that would allow them to be distinguished from one another. Thus, from the quantum cosmology with which this document begins to the two currently indistinguishable approaches with which it concludes—encompassing the equation governing the entire universe, the quantum origin of the primordial spectrum, and the entropy linking gravity, thermodynamics, and information—quantum cosmology demonstrates that the physics of the smallest scales (the quantum world) is indispensable for understanding the physics of the largest scales (the universe as a whole).

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