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30+ Commonly Held Concepts About Cosmology

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30+ Commonly Held Concepts About Cosmology 30+ Commonly held Concepts about Cosmology Big bang 1. The big bang is unavoidable, as proven by well-known “singularity theorems.” 2. Big bang nucleosynthesis, the successful theory of the formation of the elements, requires a big bang (as the name implies). 3. Extrapolating back in time, the universe must have reached a phase (the Planck epoch) in which quantum gravity effects dominate. 4. Time must have a beginning. 5. In an expanding universe, space expands but time does not. 6. The discovery that the expansion of the universe is accelerating today means that the universe will expand forever into the future. 7. There is no natural mechanism that could cause the universe to switch from its current cosmic acceleration to slow contraction. 8. With the exception of the bang, cosmology probes gravity in the weak coupling limit (in contrast with black hole collisions that probe gravity in the strong coupling limit). Smoothness, flatness and inflation 9. Inflation naturally explains why the universe is smooth (homogeneous), isotropic and flat. 10. You can understand how inflation flattens the universe by imagining an inflating balloon. The more it inflates, the flatter it gets. 11. Inflation is the only known cosmological mechanism for flattening the universe. 12. Inflation is proven because we observe the universe to be homogeneous, isotropic and flat. 13. Inflation is equivalent to a “nearly de Sitter” universe. 14. The parameters ns, r, / etc. extracted from the CMB measurements are “inflationary parameters.” 15. Inflation predicts a nearly scale-invariant spectrum of CMB temperature variations. 16. Inflation is the only mechanism for generating super-Hubble wavelength modes (which is essential for explaining the CMB). 17. Quantum fluctuations produced on sub-Hubble scales become frozen in amplitude when they evolve to super-Hubble scales. 18. The value of r (the ratio of tensor to scalar modes in the CMB) depends on the scale of inflation. 19. Inflation is easily compatible with string theory & quantum gravity. Bouncing and cyclic cosmology 20. You can understand how contraction creates a flatness problem by imagining a deflating balloon. The more it deflates, the more curved it becomes. 2 21. Slow contraction (which requires p/ >> 1) results in sound speeds cs >1, a violation of special relativity. 22. Slow contraction requires exotic microphysics that cannot be explained with scalar fields and potentials. 23. Bouncing models rely on string theory, branes & extra dimensions. 24. Contraction inevitably leads to chaotic mixmaster dynamics that destroys homogeneity. 25. There do not exist examples of well-behaved classical bounce solutions. 26. Modifications of gravity needed to have a bounce are ruled out by LIGO measurements. 27. Bouncing models generate non-gaussianity at levels inconsistent with CMB observations. 28. Bouncing models require a big crunch. 29. Cyclic models are analogous to a balloon that undergoes regular periods of inflation and deflation that periodically change its size from large to small and back to large again. 30. The only currently known distinctive outcome of bouncing models (compared to big bang inflation) is the absence of tensor modes. 31. Contraction in a bouncing or cyclic model draws any previously existing black holes (stars, galaxies, etc.) together in an ultradense mass prone to collapse to a singularity. 32. Cyclic models are impossible because they violate the second law of thermodynamics. 33. Information from one cycle cannot pass through a bounce to the next cycle. 34. Bouncing and cyclic models are incompatible with string theory and quantum gravity. .
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