# Conformal cyclic cosmology

Conformal cyclic cosmology (CCC) is a cosmological model in the framework of general relativity, advanced by the theoretical physicist Roger Penrose.[1][2][3] In CCC, the universe iterates through infinite cycles, with the future timelike infinity[clarification needed] 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[2] is to connect a countable sequence of open Friedmann–Lemaître–Robertson–Walker metric (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 ${\displaystyle g_{ab}}$  is multiplied by the square of a conformal factor ${\displaystyle \Omega }$  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 is currently believed). 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".

The conformal cyclic cosmology hypothesis requires that all massive particles eventually vanish from existence, including those which become too widely separated from all other particles to annihilate with them. As Penrose points out, proton decay is a possibility contemplated in various speculative extensions of the Standard Model, but it has never been observed. Moreover, all electrons must also decay, or lose their charge and/or mass, and no conventional speculations allow for this.[2]

## 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 their former FLRW counterparts (classically, this corresponds to light-cone structures being preserved under conformal rescaling). 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, thus providing 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 convenient for other aspects of cosmology. 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 (CMB) made by the Wilkinson Microwave Anisotropy Probe (WMAP) and the BOOMERanG experiment contained an excess of concentric circles compared to simulations based on the standard Lambda-CDM model of cosmology, quoting a 6-sigma significance of the result.[5] However, the statistical significance of the claimed detection has since been disputed. Three groups have independently attempted to reproduce these results, but found that the detection of the concentric anomalies was not statistically significant, in that no more concentric circles appeared in the data than in Lambda-CDM simulations.[6][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 introducing a new method they termed the "sky-twist procedure" (not based on simulations) in which WMAP data is directly analysed;[3] in 2015, they published the results of Planck data analysis confirming those of WMAP, including the inhomogeneous sky distribution of those structures.[11]

In a paper published on August 6, 2018 Daniel An, Krzysztof Antoni Meissner, Pawel Nurowski and Penrose presented a continued analysis of the CMB data as it seemed to them that “…anomalous points provide an important new input to cosmology, irrespective of the validity of CCC.” They also suggested that those anomalies could be "Hawking points", remnant signals from the "Hawking evaporation of supermassive black holes in the aeon prior to ours". The original version of their paper claimed that a B-mode location found by the BICEP2 team was located at one of these Hawking points; this claim was removed in a later update.[12] A 2020 analysis claimed that the anomalies were not statistically significant.[13] However, a further 2020 paper from the Penrose group argued for further evidence of anomalies in the CMB that could be explained by Hawking points and not by the "current conventional inflationary picture".[14]

## CCC and the Fermi paradox

In 2015, Gurzadyan and Penrose also discussed the Fermi paradox, the apparent contradiction between the lack of evidence but high probability estimates for the existence of extraterrestrial civilizations. Within conformal cyclic cosmology, the cosmic microwave background provides the possibility of information transfer from one aeon to another, including of intelligent signals within information panspermia concept.[11]

## References

1. ^ Palmer, Jason (2010-11-27). "Cosmos may show echoes of events before Big Bang". BBC News. Retrieved 2010-11-27.
2. ^ a b c Roger Penrose (2006). "Before the Big Bang: An Outrageous New Perspective and its Implications for Particle Physics" (PDF). Proceedings of the EPAC 2006, Edinburgh, Scotland: 2759–2762.
3. ^ a b Gurzadyan, VG; Penrose, R (2013). "On CCC-predicted concentric low-variance circles in the CMB sky". Eur. Phys. J. Plus. 128 (2): 22. arXiv:1302.5162. Bibcode:2013EPJP..128...22G. doi:10.1140/epjp/i2013-13022-4. S2CID 55249027.
4. ^ Cartlidge, Edwin (2010-11-19). "Penrose claims to have glimpsed universe before Big Bang". physicsworld.com. Retrieved 2010-11-27.
5. ^ Gurzadyan VG; Penrose R (2010-11-16). "Concentric circles in WMAP data may provide evidence of violent pre-Big-Bang activity". arXiv:1011.3706 [astro-ph.CO].
6. ^ Wehus IK; Eriksen HK (2010-12-07). "A search for concentric circles in the 7-year WMAP temperature sky maps". The Astrophysical Journal. 733 (2): L29. arXiv:1012.1268. Bibcode:2011ApJ...733L..29W. doi:10.1088/2041-8205/733/2/L29.
7. ^ Moss A; Scott D; Zibin JP (2010-12-07). "No evidence for anomalously low variance circles on the sky". Journal of Cosmology and Astroparticle Physics. 2011 (4): 033. arXiv:1012.1305. Bibcode:2011JCAP...04..033M. doi:10.1088/1475-7516/2011/04/033. S2CID 118433733.
8. ^ Hajian A (2010-12-08). "Are There Echoes From The Pre-Big Bang Universe? A Search for Low Variance Circles in the CMB Sky". The Astrophysical Journal. 740 (2): 52. arXiv:1012.1656. Bibcode:2011ApJ...740...52H. doi:10.1088/0004-637X/740/2/52. S2CID 118515562.
9. ^ DeAbreu, A.; et al. (2015). "Searching for concentric low variance circles in the cosmic microwave background". Journal of Cosmology and Astroparticle Physics. 2015 (12): 031. arXiv:1508.05158. Bibcode:2015JCAP...12..031D. doi:10.1088/1475-7516/2015/12/031. S2CID 119205759.
10. ^ Gurzadyan VG; Penrose R (2010-12-07). "More on the low variance circles in CMB sky". arXiv:1012.1486 [astro-ph.CO].
11. ^ a b Gurzadyan, V.G.; Penrose, R. (2016). "CCC and the Fermi paradox". Eur. Phys. J. Plus. 131: 11. arXiv:1512.00554. Bibcode:2016EPJP..131...11G. doi:10.1140/epjp/i2016-16011-1. S2CID 73537479.
12. ^ Gurzadyan, V. G.; Penrose, R. (2018). "Apparent evidence for Hawking points in the CMB Sky". arXiv:1808.01740 [astro-ph.CO].
13. ^ Jow, Dylan L.; Scott, Douglas (2020-03-09). "Re-evaluating evidence for Hawking points in the CMB". Journal of Cosmology and Astroparticle Physics. 2020 (3): 021. arXiv:1909.09672. Bibcode:2020JCAP...03..021J. doi:10.1088/1475-7516/2020/03/021. ISSN 1475-7516. S2CID 202719103.
14. ^ An, Daniel; Meissner, Krzysztof A.; Nurowski, Paweł; Penrose, Roger (July 2020). "Apparent evidence for Hawking points in the CMB Sky". Monthly Notices of the Royal Astronomical Society. 495 (3): 3403–3408. arXiv:1808.01740. Bibcode:2020MNRAS.495.3403A. doi:10.1093/mnras/staa1343. S2CID 119068764.