Quantum Gravity in Cosmological Models

How different cosmological approaches handle the challenge of quantum gravity

Quantum Gravity in Conformal Cyclic Cosmology (CCC)

Classical Conformal Geometry Over Quantum Gravity

In CCC, transitions between aeons are conformally smooth and treated as a classical geometric process rather than a quantum gravitational one. The conformal rescaling of the metric allows the infinite future of one aeon to become the Big Bang of the next, avoiding the need for a quantum gravity description at the singularity.

Role of Gravitational Waves and Gravitons

In the late stages of an aeon, gravitational waves (and eventually gravitons) dominate as matter decays. Although gravitons are quantum particles, their collective behavior is described classically in CCC, akin to how a classical gas emerges from quantum atoms.

Black Hole Evaporation and Information Loss

CCC requires information loss in black holes to maintain low entropy at the start of each aeon. Fermionic matter (which carries information) is assumed to be irretrievably lost during black hole evaporation, while bosonic radiation (conformally invariant) crosses aeons.

Alternative Quantum Gravity Approaches

S

String Theory

L

Loop Quantum Gravity

C

Causal Dynamical Triangulation

String Theory Approach

String theory posits that fundamental particles are vibrations of one-dimensional strings. It naturally incorporates gravity through graviton particles and requires extra spatial dimensions. String theory aims to be a complete theory of quantum gravity but lacks experimental verification.

Loop Quantum Gravity

LQG quantizes space itself, suggesting spacetime has a discrete structure at the Planck scale. It represents space as networks of loops (spin networks) and provides a framework where singularities like the Big Bang may be resolved through quantum bounce mechanisms.

Key Comparisons

Aspect	CCC Approach	Standard Quantum Gravity
Singularity Resolution	Conformal rescaling (classical)	Quantization of spacetime (quantum)
Information Preservation	Loss of fermionic information allowed	Unitarity required
Role of Inflation	Replaced by previous aeon's expansion	Often incorporated or required
Empirical Predictions	Hawking points in CMB (disputed)	Extra dimensions/particles (not observed)

Challenges in Quantum Gravity

The Measurement Problem

How does the classical world emerge from quantum description? This is particularly challenging for spacetime itself in quantum gravity theories.

Background Independence

A true theory of quantum gravity should not depend on a pre-existing spacetime background, but should generate spacetime from more fundamental entities.

Experimental Verification

The extremely high energies required to test quantum gravity predictions make direct experimental verification currently impossible, though indirect evidence is sought.

Conclusion

Different cosmological models handle quantum gravity in distinct ways:

• CCC largely sidesteps the need for a full quantum gravity theory by relying on conformal geometry and classical transitions between aeons
• String theory attempts to unify all forces including gravity through higher-dimensional strings
• Loop quantum gravity focuses on quantizing spacetime itself rather than particles

Each approach faces significant challenges, particularly regarding empirical verification and reconciliation with quantum mechanics. While CCC offers an innovative way to avoid quantum gravity issues at cosmological transitions, it requires controversial assumptions about information loss that conflict with standard quantum mechanics.

This HTML presentation summarizes how different cosmological models handle the challenge of quantum gravity.

Based on current theoretical physics understanding and cosmological research.