# Conformal cyclic cosmology

(Redirected from Conformal Cyclic Cosmology)

The conformal cyclic cosmology (CCC) is a cosmological model in the framework of general relativity, advanced by the theoretical physicists Roger Penrose and Vahe Gurzadyan.[1][2][3] In CCC, the universe iterates through infinite cycles, with the future timelike infinity of each previous iteration being identified with the Big Bang singularity of the next.[4] Penrose popularized this theory in his 2010 book Cycles of Time: An Extraordinary New View of the Universe.

## Basic construction

Penrose's basic construction[5] is to connect a countable sequence of open FLRW spacetimes, each representing a big bang followed by an infinite future expansion. Penrose noticed that the past conformal boundary of one copy of FLRW spacetime can be "attached" to the future conformal boundary of another, after an appropriate conformal rescaling. In particular, each individual FLRW metric $g_{ab}$ is multiplied by the square of a conformal factor $\Omega$ that approaches zero at timelike infinity, effectively "squashing down" the future conformal boundary to a conformally regular hypersurface (which is spacelike if there is a positive cosmological constant, as we currently believe). The result is a new solution to Einstein's equations, which Penrose takes to represent the entire Universe, and which is composed of a sequence of sectors that Penrose calls "aeons".

## Physical implications

The significant feature of this construction for particle physics is that, since bosons obey the laws of conformally invariant quantum theory, they will behave in the same way in the rescaled aeons as in the original FLRW counterparts. (Classically, this corresponds to the fact that light cone structure is preserved under conformal rescalings.) For such particles, the boundary between aeons is not a boundary at all, but just a spacelike surface that can be passed across like any other. Fermions, on the other hand, remain confined to a given aeon. This provides a convenient solution to the black hole information paradox; according to Penrose, fermions must be irreversibly converted into radiation during black hole evaporation, to preserve the smoothness of the boundary between aeons.

The curvature properties of Penrose's cosmology are also highly desirable. First, the boundary between aeons satisfies the Weyl curvature hypothesis, thus providing a certain kind of low-entropy past as required by statistical mechanics and by observation. Second, Penrose has calculated that a certain amount of gravitational radiation should be preserved across the boundary between aeons. Penrose suggests this extra gravitational radiation may be enough to explain the observed cosmic acceleration without appeal to a dark energy matter field.

## Empirical tests

In 2010, Penrose and Vahe Gurzadyan published a preprint of a paper claiming that observations of the cosmic microwave background made by the Wilkinson Microwave Anisotropy Probe and the BOOMERanG experiment showed concentric anomalies which were consistent with the CCC hypothesis, with a low probability of the null hypothesis that the observations in question were caused by chance.[6] However, the statistical significance of the claimed detection has since been questioned. Three groups have independently attempted to reproduce these results, but found that the detection of the concentric anomalies was not statistically significant, in the sense that such circles would appear in a proper Gaussian simulation of the anisotropy in the CMB data.[7][8][9]

The reason for the disagreement was tracked down to an issue of how to construct the simulations that are used to determine the significance: The three independent attempts to repeat the analysis all used simulations based on the standard Lambda-CDM model, while Penrose and Gurzadyan used an undocumented non-standard approach.[10]

In 2013 Gurzadyan and Penrose published the further development of their work, also introducing a new method, the sky-twist transformation (not using simulations).[3]