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Register a new user$H_0$ Tension, Swampland Conjectures and the Epoch of Fading Dark Matter
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Motivated by the swampland dS conjecture, we consider a rolling scalar field as the source of dark energy. Furthermore, the swampland distance conjecture suggests that the rolling field will lead at late times to an exponentially light tower of states. Identifying this tower as residing in the dark sector suggests a natural coupling of the scalar field to the dark matter, leading to a continually reducing dark matter mass as the scalar field rolls in the recent cosmological epoch. The exponent in the distance conjecture, $tilde{c}$, is expected to be an $mathcal{O}(1)$ number. Interestingly, when we include the local measurement of $H_0$, our model prefers a non-zero value of the coupling $tilde{c}$ with a significance of $2.8sigma$ and a best-fit at $tilde{c} sim 0.3$. Modifying the recent evolution of the universe in this way improves the fit to data at the $2sigma$ level compared to $Lambda$CDM. This string-inspired model automatically reduces cosmological tensions in the $H_0$ measurement as well as $sigma_8$.
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We study the cosmological effects of two-body dark matter decays where the products of the decay include a massless and a massive particle. We show that if the massive daughter particle is slightly warm it is possible to relieve the tension between distance ladder measurements of the present day Hubble parameter with measurements from the cosmic microwave background.
We constrain and update the bounds on the life-time of a decaying dark matter model with a warm massive daughter particle using the most recent low-redshift probes. We use Supernovae Type-Ia, Baryon Acoustic Oscillations and the time delay measurements of gravitationally lensed quasars. These data sets are complemented by the early universe priors taken from the Cosmic Microwave background. For the maximum allowed fraction of the relativistic daughter particle, the updated bounds on the life-time are found to be $tau > 9, rm{Gyr}$ and $tau >11,rm{Gyr}$ at $95%$ C.L., for the two-body and many-body decay scenarios, respectively. We also comment on the recent proposal that the current two-body decaying dark matter model can provide resolution for the $H_0$-tension, by contrasting against the standard $Lambda$CDM model. We infer that the current dark matter decaying scenario is unlikely to alleviate the $H_0$-tension. We find that the decaying dark matter is able to reduce the trend of the decreasing $H_0$ values with increasing lens redshifts observed in the strong lensing dataset.
We investigate the possibility of phantom crossing in the dark energy sector and solution for the Hubble tension between early and late universe observations. We use robust combinations of different cosmological observations, namely the CMB, local measurement of Hubble constant ($H_0$), BAO and SnIa for this purpose. For a combination of CMB+BAO data which is related to early Universe physics, phantom crossing in the dark energy sector is confirmed at $95$% confidence level and we obtain the constraint $H_0=71.0^{+2.9}_{-3.8}$ km/s/Mpc at 68% confidence level which is in perfect agreement with the local measurement by Riess et al. We show that constraints from different combination of data are consistent with each other and all of them are consistent with phantom crossing in the dark energy sector. For the combination of all data considered, we obtain the constraint $H_0=70.25pm 0.78$ km/s/Mpc at 68% confidence level and the phantom crossing happening at the scale factor $a_m=0.851^{+0.048}_{-0.031}$ at 68% confidence level.
It has been suggested that late-universe dark matter decays can alleviate the tension between measurements of $H_0$ in the local universe and its value inferred from cosmic microwave background fluctuations. Decaying dark matter can potentially account for this discrepancy as it reshuffles the energy density between matter and radiation and as a result allows dark energy to become dominant at earlier times. We show that the low multipoles amplitude of the cosmic microwave background anisotropy power spectrum severely constrains the feasibility of late-time decays as a solution to the $H_0$ tension.
Phantom dark energy can produce amplified cosmic acceleration at late times, thus increasing the value of $H_0$ favored by CMB data and releasing the tension with local measurements of $H_0$. We show that the best fit value of $H_0$ in the context of the CMB power spectrum is degenerate with a constant equation of state parameter $w$, in accordance with the approximate effective linear equation $H_0 + 30.93; w - 36.47 = 0$ ($H_0$ in $km ; sec^{-1} ; Mpc^{-1}$). This equation is derived by assuming that both $Omega_{0 rm m}h^2$ and $d_A=int_0^{z_{rec}}frac{dz}{H(z)}$ remain constant (for invariant CMB spectrum) and equal to their best fit Planck/$Lambda$CDM values as $H_0$, $Omega_{0 rm m}$ and $w$ vary. For $w=-1$, this linear degeneracy equation leads to the best fit $H_0=67.4 ; km ; sec^{-1} ; Mpc^{-1}$ as expected. For $w=-1.22$ the corresponding predicted CMB best fit Hubble constant is $H_0=74 ; km ; sec^{-1} ; Mpc^{-1}$ which is identical with the value obtained by local distance ladder measurements while the best fit matter density parameter is predicted to decrease since $Omega_{0 rm m}h^2$ is fixed. We verify the above $H_0-w$ degeneracy equation by fitting a $w$CDM model with fixed values of $w$ to the Planck TT spectrum showing also that the quality of fit ($chi^2$) is similar to that of $Lambda$CDM. However, when including SnIa, BAO or growth data the quality of fit becomes worse than $Lambda$CDM when $w< -1$. Finally, we generalize the $H_0-w(z)$ degeneracy equation for $w(z)=w_0+w_1; z/(1+z)$ and identify analytically the full $w_0-w_1$ parameter region that leads to a best fit $H_0=74; km ; sec^{-1} ; Mpc^{-1}$ in the context of the Planck CMB spectrum. This exploitation of $H_0-w(z)$ degeneracy can lead to immediate identification of all parameter values of a given $w(z)$ parametrization that can potentially resolve the $H_0$ tension.
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