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Register a new userDiscovery of a Supercluster at $zsim$0.91 and Testing the $Lambda$CDM Cosmological Model
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The $Lambda$CDM cosmological model successfully reproduces many aspects of the galaxy and structure formation of the universe. However, the growth of large-scale structures (LSSs) in the early universe is not well tested yet with observational data. Here, we have utilized wide and deep optical--near-infrared data in order to search for distant galaxy clusters and superclusters ($0.8<z<1.2$). From the spectroscopic observation with the Inamori Magellan Areal Camera and Spectrograph (IMACS) on the Magellan telescope, three massive clusters at $zsim$0.91 are confirmed in the SSA22 field. Interestingly, all of them have similar redshifts within $Delta zsim$0.01 with velocity dispersions ranging from 470 to 1300 km s$^{-1}$. Moreover, as the maximum separation is $sim$15 Mpc, they compose a supercluster at $zsim$0.91, meaning that this is one of the most massive superclusters at this redshift to date. The galaxy density map implies that the confirmed clusters are embedded in a larger structure stretching over $sim$100 Mpc. $Lambda$CDM models predict about one supercluster like this in our surveyed volume, consistent with our finding so far. However, there are more supercluster candidates in this field, suggesting that additional studies are required to determine if the $Lambda$CDM cosmological model can successfully reproduce the LSSs at high redshift.
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Inspired by the recent conjecture that the universe has transitioned from AdS vacua to dS vacua in the late universe made via graduated dark energy, we extend the $Lambda$CDM model by a cosmological `constant ($Lambda_{rm s}$) that switches sign at certain redshift, $z_dagger$, and name it as $Lambda_{rm s}$CDM. We discuss the construction and theoretical features of this model, and find out that, when the consistency of $Lambda_{rm s}$CDM with the CMB data is ensured, (i) $z_daggergtrsim1.1$ is implied by the condition that the universe monotonically expands, (ii) $H_0$ is inversely correlated with $z_dagger$ and reaches $approx74.5~{rm km, s^{-1}, Mpc^{-1}}$ for $z_dagger=1.5$, (iii) $H(z)$ presents an excellent fit to the Ly-$alpha$ measurements provided that $z_daggerlesssim 2.34$. We further investigate the model constraints by using the full Planck CMB data, with and without BAO data. We find that the CMB data alone does not constrain $z_dagger$ but CMB+BAO dataset favors the sign switch of $Lambda_{rm s}$ providing the constraint: $z_dagger=2.44pm0.29$ (68% CL). Our analysis reveals that the lower and upper limits of $z_dagger$ are controlled by the Galaxy and Ly-$alpha$ BAO measurements, respectively, and the larger $z_{dagger}$ values imposed by the Galaxy BAO data prevent the model from achieving the highest local $H_0$ measurements. In general, $Lambda_{rm s}$CDM (i) relaxes the $H_0$ tension while being fully consistent with the TRGB measurement, (ii) removes the discrepancy with the Ly-$alpha$ measurements, (iii) relaxes the $S_8$ tension, and (iv) finds a better agreement with the BBN constraints of physical baryon density. We find no strong statistical evidence to discriminate between the $Lambda_{rm s}$CDM and $Lambda$CDM models. However, interesting and promising features of $Lambda_{rm s}$CDM provide an upper edge over $Lambda$CDM.
In this work we discuss a general approach for the dissipative dark matter considering a nonextensive bulk viscosity and taking into account the role of generalized Friedmann equations. This generalized $Lambda$CDM model encompasses a flat universe with a dissipative nonextensive viscous dark matter component, following the Eckart theory of bulk viscosity. In order to compare models and constrain cosmological parameters, we perform Bayesian analysis using one of the most recent observations of Type Ia Supernova, baryon acoustic oscillations, and cosmic microwave background data.
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.
Telescopes to be launched over the next decade-and-a-half, such as JWST, EUCLID, ATHENA and Lynx, promise to revolutionise the study of the high redshift Universe and greatly advance our understanding of the early stages of galaxy formation. We use a model that follows the evolution of the masses and spins of supermassive black holes (SMBHs) within a semi-analytic model of galaxy formation to make predictions for the Active Galactic Nucleus (AGN) luminosity function at $zgeq7$ in the broadband filters of JWST and EUCLID at near-infrared wavelengths, and ATHENA and Lynx at X-ray energies. The predictions of our model are relatively insensitive to the choice of seed black hole mass, except at the lowest luminosities ($L_{mathrm{bol}}<10^{43}mathrm{ergs^{-1}}$) and the highest redshifts ($z>10$). We predict that surveys with these different telescopes will select somewhat different samples of SMBHs, with EUCLID unveiling the most massive, highest accretion rate SMBHs, Lynx the least massive, lowest accretion rate SMBHs, and JWST and ATHENA covering objects inbetween. At $z=7$, we predict that typical detectable SMBHs will have masses, $M_{mathrm{BH}}sim10^{5-8}M_{odot}$, and Eddington normalised mass accretion rates, $dot{M}/dot{M}_{mathrm{Edd}}sim0.6-2$. The SMBHs will be hosted by galaxies of stellar mass $M_{star}sim10^{8-10}M_{odot}$, and dark matter haloes of mass $M_{mathrm{halo}}sim10^{11-12}M_{odot}$. We predict that the detectable SMBHs at $z=10$ will have slightly smaller black holes, accreting at slightly higher Eddington normalised mass accretion rates, in slightly lower mass host galaxies compared to those at $z=7$, and reside in haloes of mass $M_{mathrm{halo}}sim10^{10-11}M_{odot}$.
We present the results from a multi-band optical imaging program which has definitively confirmed the existence of a supercluster at z = 0.91. Two massive clusters of galaxies, CL1604+4304 at z = 0.897 and CL1604+4321 at z = 0.924, were originally observed in the high-redshift cluster survey of Oke, Postman & Lubin (1998). They are separated by 4300 km/s in radial velocity and 17 arcminutes on the plane of the sky. Their physical and redshift proximity suggested a promising supercluster candidate. Deep BRi imaging of the region between the two clusters indicates a large population of red galaxies. This population forms a tight, red sequence in the color--magnitude diagram at (R-i) = 1.4. The characteristic color is identical to that of the spectroscopically-confirmed early-type galaxies in the two member clusters. The red galaxies are spread throughout the 5 Mpc region between CL1604+4304 and CL1604+4321. Their spatial distribution delineates the entire large scale structure with high concentrations at the cluster centers. In addition, we detect a significant overdensity of red galaxies directly between CL1604+4304 and CL1604+4321 which is the signature of a third, rich cluster associated with this system. The strong sequence of red galaxies and their spatial distribution clearly indicate that we have discovered a supercluster at z = 0.91.
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