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Conformal Cyclic Cosmology

Conformal Cyclic Cosmology. LeCosPA Quantum Cosmology Group Meeting 13 July 2011. Arrow of Time as a Problem in Cosmology. Why did the entropy start out so low? Question about initial condition of the universe. 2 of 13. Cyclic Models at a Glance. Bojowald, 2006. 4 of 13.

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Conformal Cyclic Cosmology

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  1. Conformal Cyclic Cosmology LeCosPAQuantum Cosmology Group Meeting13 July 2011

  2. Arrow of Time as a Problem in Cosmology • Why did the entropy start out so low? • Question about initial condition of the universe. 2 of 13

  3. Cyclic Models at a Glance

  4. Bojowald, 2006. 4 of 13

  5. Against Cyclic Models “If you try to invent a cosmology in which you straightforwardly replace the singular Big Bang by a smooth Big Bounce continuation into a previous spacetime, you have one of two choices: either the entropy continues to decrease as we travel backwards in time through the Bang, or it changes direction and begins to increase. Sadly, neither makes any sense.” Sean Carroll [http://blogs.discovermagazine.com/cosmicvariance/2007/07/02/against-bounces/] Scenario 1: Second Law holds indefinitely to the past, increasing in each cycle. Thus entropy is infinitely low in the past. Scenario 2: Entropy was decreasing in the previous universe. But why is the middle entropy so slow? 5 of 13

  6. CCC in a Nutshell • Early Time: Particles are massless. Conformal geometry. • Late Time: If particles are again massless, we again only have conformal geometry. Identify as early time of the next aeon. • Decay of mass at late time: Maybe not impossible, since mass is not Casimir operator in de Sitter space. [but Penrose has no proof that it is possible either!] More problems later. 6 of 13

  7. Addressing the Second Law Penrose believes that QM has to be modified in the presence of strong gravity – unitarity is lost, i.e. information is truly lost in black holes. If so, phase space is greatly reduced in late time at the end of each aeon. The zero of entropy is reset so that the new aeon has low entropy with respect to the reduced phase space. 7 of 13

  8. Comment: AdS/CFT and holographic principle seem to imply no information loss. Given the successes of AdS/CFT, I find it hard to accept this proposal of Penrose. 8 of 13

  9. Problems Personally, I think CCC has too many “wishful thinking” [perhaps too strong a phrase]: 1. Need mechanism for eventual decay of mass. 2. Even if particles all become massless at the very late time, how do we deal with quantum fluctuations? Since Penrose is willing to wait for eternity, even more complicated things will fluctuate out of equilibrium. 3. De Sitter space meta-stability? Sean Carroll’s comment: While the early and late universes are both smooth, they are very different in other obvious ways, such as the energy density. What causes the low-density late universe to come alive into something like the high-density early universe? Something like that happens in the Steinhardt-Turok cyclic universe, but in order to make it happen you need to specify some particular matter fields with very specific dynamics. This isn’t a trivial task; there are things you can try, but they generally are plagued by instabilities and singularities. I don’t see where Penrose has done that, so I’m not even sure what there is to be criticized. 9 of 13

  10. Sean Carroll’s Comment Continues Regardless, I am highly skeptical of cyclic cosmologies no matter what flavor they come in. The most obvious empirical fact about our observable universe is its temporal asymmetry — the early phase is very different from the late phase, even though no such difference is to be found in the fundamental laws of physics. Our goal should be to explain that asymmetry. But cyclic cosmologies simply extend it over an infinite number of cycles, without any explanation. If you took a typical state of the universe today and played it backwards in time, you wouldn’t expect to get anything like these cyclic cosmologies; it would just collapse into a mess. What you would need to do is argue that this kind of behavior arises robustly from a wide variety of possible initial conditions. If you need some special conditions, fine — but you’re not doing any better than the ordinary Big Bang. [http://blogs.discovermagazine.com/cosmicvariance/2010/12/07/penroses-cyclic-cosmology/] 10 of 13

  11. So What to Do? • In view the previous discussions, I don’t think CCC is worth putting too much time into. • Inflation or not, the problem remains that initial condition of the universe is extremely fine-tuned with low entropy. This needs to be addressed. • My personal hunch is global topology might be important. 11 of 13

  12. Topology in Cosmology Alice Universe: http://iopscience.iop.org/0264-9381/14/9/010 12 of 13

  13. Topology in Cosmology Example: hep-th/0408164v2 13 of 13

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