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Register a new userThe KBC void and Hubble tension contradict $Lambda$CDM on a Gpc scale $-$ Milgromian dynamics as a possible solution
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The KBC void is a local underdensity with the observed relative density contrast $delta equiv 1 - rho/rho_{0} = 0.46 pm 0.06$ between 40 and 300 Mpc around the Local Group. If mass is conserved in the Universe, such a void could explain the $5.3sigma$ Hubble tension. However, the MXXL simulation shows that the KBC void causes $6.04sigma$ tension with standard cosmology ($Lambda$CDM). Combined with the Hubble tension, $Lambda$CDM is ruled out at $7.09sigma$ confidence. Consequently, the density and velocity distribution on Gpc scales suggest a long-range modification to gravity. In this context, we consider a cosmological MOND model supplemented with $11 , rm{eV}/c^{2}$ sterile neutrinos. We explain why this $ u$HDM model has a nearly standard expansion history, primordial abundances of light elements, and cosmic microwave background (CMB) anisotropies. In MOND, structure growth is self-regulated by external fields from surrounding structures. We constrain our model parameters with the KBC void density profile, the local Hubble and deceleration parameters derived jointly from supernovae at redshifts $0.023 - 0.15$, time delays in strong lensing systems, and the Local Group velocity relative to the CMB. Our best-fitting model simultaneously explains these observables at the $1.14%$ confidence level (${2.53 sigma}$ tension) if the void is embedded in a time-independent external field of ${0.055 , a_{_0}}$. Thus, we show for the first time that the KBC void can naturally resolve the Hubble tension in Milgromian dynamics. Given the many successful a priori MOND predictions on galaxy scales that are difficult to reconcile with $Lambda$CDM, Milgromian dynamics supplemented by $11 , rm{eV}/c^{2}$ sterile neutrinos may provide a more holistic explanation for astronomical observations across all scales.
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We present a full-fledged analysis of Brans-Dicke cosmology with a cosmological constant and cold dark matter (BD-$Lambda$CDM for short). We extend the scenarios where the current cosmological value of the BD-field is restricted by the local astrophysical domain to scenarios where that value is fixed only by the cosmological observations, which should be more natural in view of the possible existence of local screening mechanims. Our analysis includes both the background and perturbations equations in different gauges. We find that the BD-$Lambda$CDM is favored by the overall cosmological data as compared to the concordance GR-$Lambda$CDM model, namely data on distant supernovae, cosmic chronometers, local measurements of the Hubble parameter, baryonic acoustic oscillations, Large-Scale Structure formation and the cosmic microwave background under full Planck 2018 CMB likelihood. We also test the impact of Strong and Weak-Lensing data on our results, which can be significant. We find that the BD-$Lambda$CDM can mimic effective quintessence with a significance of about $3-3.5sigma$ c.l. (depending on the lensing datasets). The fact that the BD-$Lambda$CDM behaves effectively as a Running Vacuum Model (RVM) when viewed from the GR perspective helps to alleviate some of the existing tensions with the data, such as the $sigma_8$ excess predicted by GR-$Lambda$CDM. On the other hand, the BD-$Lambda$CDM model has a crucial bearing on the acute $H_0$-tension with the local measurements, which is rendered virtually harmless owing to the small increase of the effective value of the gravitational constant with the expansion. The simultaneous alleviation of the two tensions is a most remarkable feature of BD-gravity with a cosmological constant in the light of the current observations, and hence goes in support of BD-$Lambda$CDM against GR-$Lambda$CDM
We present the first simulations within an effective theory of structure formation (ETHOS), which includes the effect of interactions between dark matter and dark radiation on the linear initial power spectrum and dark matter self-interactions during non-linear structure formation. We simulate a Milky Way-like halo in four different dark matter models and the cold dark matter case. Our highest resolution simulation has a particle mass of $2.8times 10^4,{rm M}_odot$ and a softening length of $72.4,{rm pc}$. We demonstrate that all alternative models have only a negligible impact on large scale structure formation. On galactic scales, however, the models significantly affect the structure and abundance of subhaloes due to the combined effects of small scale primordial damping in the power spectrum and late time self-interactions. We derive an analytic mapping from the primordial damping scale in the power spectrum to the cutoff scale in the halo mass function and the kinetic decoupling temperature. We demonstrate that certain models within this extended effective framework that can alleviate the too-big-to-fail and missing satellite problems simultaneously, and possibly the core-cusp problem. The primordial power spectrum cutoff of our models naturally creates a diversity in the circular velocity profiles, which is larger than that found for cold dark matter simulations. We show that the parameter space of models can be constrained by contrasting model predictions to astrophysical observations. For example, some models may be challenged by the missing satellite problem if baryonic processes were to be included and even over-solve the too-big-to-fail problem; thus ruling them out.
In the current framework, the standard parametrization of our Universe is the so-called Lambda Cold Dark Matter ({Lambda}CDM) model. Recently, Risaliti & Lusso (2019) have shown a ~4{sigma} tension with the {Lambda}CDM model through a model-independent parametrization of a Hubble Diagram of supernovae Ia (SNe Ia) from the JLA survey and quasars. Model-independent approaches and independent samples over a wide redshift range are key to testing this tension and any possible systematics. Here we present an analysis of a combined Hubble Diagram of SNe Ia, quasars, and gamma-ray bursts (GRBs) to check the agreement of the quasar and GRB cosmological parameters at high redshifts (z>2) and to test the concordance flat {Lambda}CDM model with improved statistical accuracy. We build a Hubble diagram with SNe Ia from the Pantheon sample (Scolnic et al. 2018), quasars from the Risaliti & Lusso (2019) sample, and GRBs from the Demianski et al. (2017a) sample, where quasars are standardised through the observed non-linear relation between their ultraviolet and X-ray emission and GRBs through the correlation between the spectral peak energy and the isotropic-equivalent radiated energy (the so-called Amati relation). We fit the data with cosmographic models consisting of a fourth-order logarithmic polynomial and a fifth-order linear polynomial, and compare the results with the expectations from a flat {Lambda}CDM model. We confirm the tension between the best fit cosmographic parameters and the {Lambda}CDM model at ~4{sigma} with SNe Ia and quasars, at ~2{sigma} with SNe Ia and GRBs, and at >4{sigma} with the whole SNe Ia+quasars+GRB data set. The completely independent high-redshift Hubble diagrams of quasars and GRBs are fully consistent with each other, strongly suggesting that the deviation from the standard model is not due to unknown systematic effects but to new physics.
We investigate the $H_0$ tension in a range of extended model frameworks beyond the standard $Lambda$CDM without the data from cosmic microwave background (CMB). Specifically, we adopt the data from baryon acoustic oscillation, big bang nucleosynthesis and type Ia supernovae as indirect measurements of $H_0$ to study the tension. We show that the estimated value of $H_0$ from indirect measurements is overall lower than that from direct local ones regardless of the data sets and a range of extended models to be analyzed, which indicates that, although the significance of the tension varies depending on models, the $H_0$ tension persists in a broad framework beyond the standard $Lambda$CDM model even without CMB data.
We compare the properties of galaxies that form in a cosmological simulation without strong feedback to observations at z=0. We confirm previous findings that models without strong feedback overproduce the observed galaxy baryonic mass function, especially at the low and high mass extremes. Through post-processing we investigate what kinds of feedback would be required to reproduce observed galaxy masses and star formation rates. To mimic an extreme form of preventive feedback (e.g., AGN radio mode) we remove all baryonic mass that was originally accreted via hot mode from shock-heated gas. This does not bring the high mass end of the galaxy mass function into agreement with observations because much of the stellar mass in these systems formed at high redshift from baryons that originally accreted via cold mode onto lower mass progenitors. An efficient ejective feedback mechanism, such as supernova driven winds, must reduce the masses of these progenitors. Feedback must also reduce the masses of lower mass z=0 galaxies, which assemble at lower redshifts and have much lower star formation rates. If we monotonically re-map galaxy masses to reproduce the observed mass function, but retain the simulations predicted star formation rates, we obtain fairly good agreement with the observed sequence of star-forming galaxies but fail to recover the observed population of passive, low star formation rate galaxies. Suppressing all hot mode accretion improves agreement for high mass galaxies but worsens the agreement at intermediate masses. Reproducing these z=0 observations requires a feedback mechanism that dramatically suppresses star formation in a fraction of galaxies, increasing with mass, while leaving star formation rates of other galaxies essentially unchanged.
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