Hubble Hullabaloo and String Cosmology

Abstract

The discrepancy in measurements of the Hubble constant indicates new physics in dark energy, dark matter, or both. Drawing inspiration from string theory, we explore possible solutions to overcome the $H_0$ problem. We investigate the interplay between the cosmological determination of $ΔN_{\rm eff}{}$ and $Z'{}$ searches at the LHC Run3.

Figures (6)

• Figure 1: The Hubble constant as a function of the publication date. The solid lines indicate the evolution of the mean measurements and the shaded regions span values within one standard deviation of the mean. The blue color represent values of $H_0$ determined in the nearby universe with a calibration based on the Cepheid distance scale applied to SNe Ia. The first measurement is from the Hubble Key Project Freedman:2000cf, the next two measurements are from the SH0ES group Riess:2009puRiess:2011yx, the fourth measurement is from the Carnegie Hubble Program which used mid-infrared data to recalibrate the data from the Hubble Key Project Freedman:2012ny, and the last three measurements are also from the SH0ES group Riess:2016jrrRiess:2018bycRiess:2019cxk. The brown color indicates derived values of $H_0$ based on the $\Lambda$CDM model and measurements of the CMB. The first five measurements are from WMAP Spergel:2003cbSpergel:2006hyKomatsu:2008hkKomatsu:2010fbHinshaw:2012aka, the next two are from the Planck mission Ade:2013zuvAde:2015xua, then there is the estimate from the dark energy survey + baryon acoustic oscillations + BBN Abbott:2017smn, and the last point is also from the Planck mission Aghanim:2018eyx. The red color indicates local $H_0$ measurements with a calibration based on the tip of the red-giant branch distance scale applied to SNe Ia Jang:2015Jang:2017dxnFreedman:2019jwvFreedman:2020dne. Adapted from Freedman:2019jwv.
• Figure 2: The CMB spectrum ${\cal C}_l \equiv l(l + 1)C_l$ as observed by Planck Aghanim:2018eyx, the South Pole Telescope (SPT) Calabrese:2013jyk, and the Atacama Cosmology Telescope (ACT) Louis:2016ahn. The angular variations of the CMB power spectrum are consequence of the dynamics of sound waves in the photon-baryon fluid. On large scales (region I), the fluctuations are frozen and we directly see the spectrum of the initial conditions. At intermediate scales (region II), we observe the oscillations of the fluid as captured at the moment of last-scattering. Finally, on small scales (region III), fluctuations are damped because their wavelengths are smaller than the mean free path of the photons. This figure is courtesy of Daniel Baumann and has been published in Baumann:2018muz.
• Figure 3: Variation of the CMB spectrum ${\cal C}_l \equiv l(l + 1)C_l$ as a function of $N_{\rm eff}$ for fixed $\theta_*$. This figure is courtesy of Daniel Baumann and has been published in Baumann:2018muz.
• Figure 4: Rescaled posterior distributions of $h$ (due to marginalization over additional free parameters) with different choices of $N_{\rm eff}$ in the $\Lambda$CDM 6 parameter fit of Vagnozzi:2019ezj. The rescaled posterior distribution of $h$ for the best fit ($\tilde{c} = 0.3$) of the fading dark matter model is indicated with the dashed curve Agrawal:2019dlm.The shaded areas indicate the $1\sigma$ and $2\sigma$ regions as determined by SH0ES Riess:2019cxk.
• Figure 5: Left. The parametrization of the entropy density given in (\ref{['soverT']}) (dashed line) superposed on the result from high statistics lattice simulations Bazavov:2009zn (solid line). Right. Comparison of $g_* (T)$ obtained using (\ref{['gdet']}) (dashed line) and the phenomenological estimate of Laine:2006cpSteigman:2012nb (solid line).