Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userTesting Lambda CDM cosmology at turnaround: where to look for violations of the bound?
166
0
0.0
(
0
)
Authors
Dimitrios Tanoglidis
Ask ChatGPT about the research
No Arabic abstract
In $Lambda$CDM cosmology, structure formation is halted shortly after dark energy dominates the mass/energy budget of the Universe. A manifestation of this effect is that in such a cosmology the turnaround radius has an upper bound. Recently, a new, local, test for the existence of dark energy in the form of a cosmological constant was proposed based on this turnaround bound. Before designing an experiment that, through high-precision determination of masses and turnaround radii, will challenge $Lambda$CDM cosmology, we have to answer two important questions: first, when turnaround-scale structures are predicted to be close enough to their maximum size, so that a possible violation of the bound may be observable. Second, which is the best mass scale to target for possible violations of the bound. Using the Press-Schechter formalism, we find that turnaround structures have in practice already stopped forming, and consequently, the turnaround radius of structures must be very close to the maximum value today. We also find that the mass scale of $sim 10^{13} M_odot$ characterizes turnaround structures that start to form in a statistically important number density today. This mass scale also separates turnaround structures with different cosmological evolution: smaller structures are no longer readjusting their mass distribution inside the turnaround scale, they asymptotically approach their ultimate abundance from higher values, and they are common enough to have, at some epoch, experienced major mergers with structures of comparable mass; larger structures exhibit the opposite behavior. We call this mass scale the transitional mass scale and we argue that it is the optimal for the purpose outlined above. As a corollary result, we explain the different accretion behavior of small and larger structures observed in already conducted numerical simulations.
rate research
Read More
The homogeneous, isotropic, and flat $Lambda$CDM universe favored by observations of the cosmic microwave background can be described using only Euclidean geometry, locally correct Newtonian mechanics, and the basic postulates of special and general relativity. We present simple derivations of the most useful equations connecting astronomical observables (redshift, flux density, angular diameter, brightness, local space density,...) with the corresponding intrinsic properties of distant sources (lookback time, distance, spectral luminosity, linear size, specific intensity, source counts,...). We also present an analytic equation for lookback time that is accurate within 0.1% for all redshifts $z$. The exact equation for comoving distance is an elliptic integral that must be evaluated numerically, but we found a simple approximation with errors $< 0.2$% for all redshifts up to $z approx 50$.
We investigate the observational viability of a class of cosmological models in which the vacuum energy density decays linearly with the Hubble parameter, resulting in a production of cold dark matter particles at late times. Similarly to the flat Lambda CDM case, there is only one free parameter to be adjusted by the data in this class of Lambda(t)CDM scenarios, namely, the matter density parameter. To perform our analysis we use three of the most recent SNe Ia compilation sets (Union2, SDSS and Constitution) along with the current measurements of distance to the BAO peaks at z = 0.2 and z = 0.35 and the position of the first acoustic peak of the CMB power spectrum. We show that in terms of $chi^2$ statistics both models provide good fits to the data and similar results. A quantitative analysis discussing the differences in parameter estimation due to SNe light-curve fitting methods (SALT2 and MLCS2k2) is studied using the current SDSS and Constitution SNe Ia compilations. A matter power spectrum analysis using the 2dFGRS is also performed, providing a very good concordance with the constraints from the SDSS and Constitution MLCS2k2 data.
This paper is an extension of the paper by Del Popolo, Chan, and Mota (2020) to take account the effect of dynamical friction. We show how dynamical friction changes the threshold of collapse, $delta_c$, and the turn-around radius, $R_t$. We find numerically the relationship between the turnaround radius, $R_{rm t}$, and mass, $M_{rm t}$, in $Lambda$CDM, in dark energy scenarios, and in a $f(R)$ modified gravity model. Dynamical friction gives rise to a $R_{rm t}-M_{rm t}$ relation differing from that of the standard spherical collapse. In particular, dynamical friction amplifies the effect of shear, and vorticity already studied in Del Popolo, Chan, and Mota (2020). A comparison of the $R_{rm t}-M_{rm t}$ relationship for the $Lambda$CDM, and those for the dark energy, and modified gravity models shows, that the $R_{rm t}-M_{rm t}$ relationship of the $Lambda$CDM is similar to that of the dark energy models, and small differences are seen when comparing with the $f(R)$ models. The effect of shear, rotation, and dynamical friction is particularly evident at galactic scales, giving rise to a difference between the $R_{rm t}-M_{rm t}$ relation of the standard spherical collapse of the order of $simeq 60%$. Finally, we show how the new values of the $R_{rm t}-M_{rm t}$ influence the constraints to the $w$ parameter of the equation of state.
We determine the relationship between the turnaround radius, $R_{rm t}$, and mass, $M_{rm t}$, in $Lambda$CDM, and in dark energy scenarios, using an extended spherical collapse model taking into account the effects of shear and vorticity. We find a more general formula than that usually described in literature, showing a dependence of $R_{rm t}$ from shear, and vorticity. The $R_{rm t}-M_{rm t}$ relation differs from that obtained not taking into account shear, and rotation, especially at galactic scales, differing $simeq 30%$ from the result given in literature. This has effects on the constraint of the $w$ parameter of the equation of state. We compare the $R_{rm t}-M_{rm t}$ relationship obtained for the $Lambda$CDM, and different dark energy models to that obtained in the $f(R)$ modified gravity (MG) scenario. The $R_{rm t}-M_{rm t}$ relationship in $Lambda$CDM, and dark energy scenarios are tantamount to the prediction of the $f(R)$ theories. Then, the $R_{rm t}-M_{rm t}$ relationship is not a good probe to test gravity theories beyond Einsteins general relativity.
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
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
