##### The Hubble Hunter's Guide
Measurements of the Hubble constant, and more generally measurements of the expansion rate and distances over the interval $0 < z < 1$, appear to be inconsistent with the predictions of the standard cosmological model ($\Lambda$CDM) given observations of cosmic microwave background temperature and polarization anisotropies. Here we consider a variety of types of departures from $\Lambda$CDM that could, in principle, restore concordance among these datasets, and we explain why we find almost all of them unlikely to be successful. We single out the set of solutions that increase the expansion rate in the decade of scale factor expansion just prior to recombination as the least unlikely. These solutions are themselves tightly constrained by their impact on photon diffusion and on the gravitational driving of acoustic oscillations of the modes that begin oscillating during this epoch -- modes that project on to angular scales that are very well measured. We point out that a general feature of such solutions is a residual to fits to $\Lambda$CDM, like the one observed in Planck power spectra. This residual drives the modestly significant inferences of angular-scale dependence to the matter density and anomalously high lensing power, puzzling aspects of a data set that is otherwise extremely well fit by $\Lambda$CDM.
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 Lloyd Knox (edit) Marius Millea (edit)
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###### Tweets
jterning: RT @LloydEKnox: New paper out with @cosmic_mar: https://t.co/xCX838HbSA, The Hubble Hunter's Guide. We qualitatively explore potential co…
LloydEKnox: Thanks, @LoverdeMarilena and @nu_phases. Only took us 9 months to get it on to the arXiv! https://t.co/xCX838pAu0 https://t.co/CNUt3pJfTS
yvonnenobis: RT @LloydEKnox: New paper out with @cosmic_mar: https://t.co/xCX838HbSA, The Hubble Hunter's Guide. We qualitatively explore potential co…
LloydEKnox: I agree with @CyrRacine. From a model-building perspective, the BAO data are very constraining. I include here a related figure, in part as advert for our latest paper: https://t.co/xCX838HbSA https://t.co/gGiQS0QmLV https://t.co/dlkjL5cNjl
LloydEKnox: New paper out with @cosmic_mar: https://t.co/xCX838HbSA, The Hubble Hunter's Guide. We qualitatively explore potential cosmological solutions to the Hubble constant problem and the challenges they face. We also speculate about the origin of some curiosities in the Planck data.
SeshNadathur: Today I have been spending some very enjoyable time reading The Hubble Hunter's Guide by @LloydEKnox and @cosmic_mar (https://t.co/lOcKcME1EZ) – both fun and useful!
AddisonGraeme: Enjoyed reading this new paper by @LloydEKnox and @cosmic_mar, particularly the "mini-review" style walking through different attempts at resolving H0 in previous papers. Liked discussion on what *doesn't* work and why. Definitely recommending to students. https://t.co/kA0nlgsc12
jrdmb: New paper: The Hubble Hunter's Guide, by @LloydEKnox and @cosmic_mar "As a guide to ourselves and others, we...consider the broadest possible set of potential cosmological solutions to reconcile distance ladder, BAO and CMB observations." https://t.co/UDuw0n8WGL #cosmology