arXiv subjects

Pekka Teerikorpi

Publications and source records attributed to Pekka Teerikorpi.

16 recordsLinked to original sources

arXiv2022

Quasi-spherical superclusters

Generally the dynamical state of superclusters is poorly known. We study properties of superclusters and select a sample of quasi-spherical superclusters, the dynamics of which can be studied using the $Λ$ significance diagram. We extracted our supercluster sample with an adaptive local threshold density method from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) data and estimated their masses using the dynamical masses for member galaxies and groups. We used topological analysis based on Minkowski functionals and the positions of galaxies and galaxy groups in superclusters. Finally, we highlight the dynamical state of a few exceptional types of superclusters found in this study using the $Λ$ significance diagram. Our final sample contains 65 superclusters in the distance range of 130 to 450 Mpc. Supercluster masses range between $1.1 \times 10^{15} M_{\sun}$ and $1.4 \times 10^{16} M_{\sun}$ and sizes between 25 Mpc and 87 Mpc. We find that pancake-type superclusters form the low-luminosity, small, poor and low-mass end of superclusters. We find four superclusters of unusual types, exhibiting exceptionally spherical shapes. These so-called quasi-spherical systems contain a high-density core surrounded by a relatively spherical density and galaxy distribution.The mass-to-light ratio of these quasi-sphericals is higher than those of the other superclusters, suggesting a relatively high dark matter content. Using the $Λ$ significance diagram for oblate and prolate spheroids, we find that three quasi-spherical superclusters are gravitationally bound at the present epoch. Quasi-spherical superclusters are among the largest gravitationally bound systems found to date, and form a special class of giant systems that, dynamically, are in between large gravitationally unbound superclusters and clusters of galaxies in an equilibrium configuration.

astro-ph.CO↗

arXiv2017

Hubble law: measure and interpretation

We have had the chance to live through a fascinating revolution in measuring the fundamental empirical cosmological Hubble law. The key progress is analysed : 1) improvement of observational means (ground-based radio and optical observations, space missions) ; 2) understanding of the biases that affect both distant and local determinations of the Hubble constant; 3) new theoretical and observational results. These circumstances encourage us to take a critical look at some facts and ideas related to the cosmological red-shift. This is important because we are probably on the eve of a new understanding of our Universe, heralded by the need to interpret some cosmological key observations in terms of unknown processes and substances.

astro-ph.CO↗

arXiv2016

Mattig's relation and dynamical distance indicators

We discuss how the redshift (Mattig) method in Friedmann cosmology relates to dynamical distance indicators based on Newton's gravity (Teerikorpi 2011). It belongs to the class of indicators where the relevant length inside the system is the distance itself (in this case the proper metric distance). As the Friedmann model has Newtonian analogy, its use to infer distances has instructive similarities to classical dynamical distance indicators. In view of the theoretical exact linear distance-velocity law, we emphasize that it is conceptually correct to derive the cosmological distance via the route: redshift (primarily observed) --> space expansion velocity (not directly observed) --> metric distance (physical length in "cm"). Important properties of the proper metric distance are summarized.

gr-qc↗

arXiv2015

Characteristic density contrasts in the evolution of superclusters. The case of A2142 supercluster

The formation and evolution of the cosmic web in which galaxy superclusters are the largest relatively isolated objects is governed by a gravitational attraction of dark matter and antigravity of dark energy (cosmological constant). We study the characteristic density contrasts in the spherical collapse model for several epochs in the supercluster evolution and their dynamical state. We analysed the density contrasts for the turnaround, future collapse and zero gravity in different LCDM models and applied them to study the dynamical state of the supercluster A2142 with an almost spherical main body. The analysis of the supercluster A2142 shows that its high-density core has already started to collapse. The zero-gravity line outlines the outer region of the main body of the supercluster. In the course of future evolution the supercluster may split into several collapsing systems. The various density contrasts presented in our study and applied to the supercluster A2142 offer a promising way to characterise the dynamical state and expected future evolution of galaxy superclusters.

astro-ph.CO↗

arXiv2014

Observations of General Relativity at strong and weak limits

Einstein's General Relativity theory has been tested in many ways during the last hundred years as reviewed in this chapter. Two tests are discussed in detail in this article: the concept of a zero gravity surface, the roots of which go back to Järnefelt, Einstein and Straus, and the no-hair theorem of black holes, first proposed by Israel, Carter and Hawking. The former tests the necessity of the cosmological constant Lambda, the latter the concept of a spinning black hole. The zero gravity surface is manifested most prominently in the motions of dwarf galaxies around the Local Group of galaxies. The no-hair theorem is testable for the first time in the binary black hole system OJ287. These represent stringent tests at the limit of weak and strong gravitational fields, respectively. In this article we discuss the current observational situation and future possibilities.

astro-ph.CO↗

arXiv2014

The Hubble diagram for a system within dark energy: influence of some relevant quantities

We study the influence of relevant quantities, including the density of dark energy (DE), to the predicted Hubble outflow around a system of galaxies. In particular, we are interested in the difference between two models: 1) The standard $Λ$CDM model, with the everywhere constant DE density, and 2) the "Swiss cheese model", where the universe is as old as the standard model, but the DE density is zero on short scales, including the environment of the system. We calculate the current predicted outflow patterns of dwarf galaxies around the Local Group-like system, using different values for the mass of the group, the local dark energy density, and the time of ejection of the dwarf galaxies, treated as test particles. These results are compared with the observed Hubble flow around the Local Group. The predicted distance-velocity relations around galaxy groups are not alone very sensitive indicators of the dark energy density, due to the obsevational scatter and the uncertainties caused by the used mass of the group and a range in the ejection times. In general, the Local Group outflow data are in agreement with the local DE density being equal to the global one, if the mass is about $4 \times 10^{12} M_{\odot}$; a lower mass of about $2 \times 10^{12} M_{\odot}$ or less could suggest a zero local DE density. The dependence of the inferred DE density on the mass is a handicap in this and other common dynamical methods. This emphasizes the need to use different approaches together, for constraining the local dark energy density.

astro-ph.CO↗

arXiv2010

The Hubble diagram for a system within dark energy: the location of the zero-gravity radius and the global Hubble rate

Here we continue to discuss the principle of the local measurement of dark energy using the normalized Hubble diagram describing the environment of a system of galaxies. We calculate the present locus of test particles injected a fixed time ago (\sim the age of the universe), in the standard Λ-cosmology and for different values of the system parameters (the model includes a central point mass M and a local dark energy density ρ_{loc}) and discuss the position of the zero-gravity distance R_v in the Hubble diagram. Our main conclusions are: 1) When the local DE density ρ_{loc} is equal to the global DE density ρ_v, the outflow reaches the global Hubble rate at the distance R_2 = (1+z_v)R_v, where z_v is the global zero-acceleration redshift (\approx 0.7 for the standard model). This is also the radius of the ideal Einstein-Straus vacuole. 2) For a wide range of the local-to-global dark energy ratio ρ_{loc}/ρ_v, the local flow reaches the known global rate (the Hubble constant) at a distance R_2 \ga 1.5 \times R_v. Hence, R_v will be between R_2/2 and R_2, giving upper and lower limits to ρ_{loc}/M. For the Local Group, this supports the view that the local density is near the global one.

astro-ph.CO↗

arXiv2006

Non-Friedmann cosmology for the Local Universe, significance of the universal Hubble constant and short-distance indicators of dark energy

Basing on the increasing evidence for the cosmological relevance of the local Hubble flow, we consider a simple analytical cosmological model for the Local Universe. This is a non-Friedmann model with a non-uniform static space-time. The major dynamical factor controlling the local expansion is the antigravity produced by the omnipresent and permanent dark energy of the cosmic vacuum (or the cosmological constant). The antigravity dominates at distances larger than 1-2 Mpc from the center of the Local Group. The model gives a natural explanation of the two key quantitative characteristics of the local expansion flow, which are the local Hubble constant and the velocity dispersion of the flow. The observed kinematical similarity of the local and global flows of expansion is clarified by the model. We demonstrate analytically the efficiency of the vacuum cooling mechanism that allows one to see the Hubble flow so close to the Local Group. Special significance is argued for the 'universal Hubble constant' H_V, depending only on the vacuum density (H_V \approx 60 km/s/Mpc). The model makes a number of testable predictions. It also shows unexpectedly that it is the dwarf galaxies of the local flow with the shortest distances and lowest redshifts that may be the most sensitive indicators of dark energy in our neighborhood.

astro-ph↗

arXiv2005

The quiescent Hubble flow, local dark energy tests, and pairwise velocity dispersion in a $Ω= 1$ universe

We review the increasing evidence for the cosmological relevance of the cold local Hubble flow. New observations, N-body simulations and other theoretical arguments are discussed, supporting our previous suggestion that the cosmological vacuum or uniform dark energy can have locally observable consequencies, especially a lower velocity scatter in DE dominated regions. The apparent contradiction between the slight dependence of the growth factor on $Ω_Λ$ and the significant influence of dark energy in realistic N-body calculations is clarified. An interesting new result is that in the standard $Λ$ cosmology, gravitation dominates around a typical matter fluctuation up to about the correlation length $r_0$, and we tentatively link this with the high pairwise velocity dispersion on scales up to several Mpc, as measured from galaxy redshift-space correlations. Locally, the smooth Hubble flow on similar scales is consistent with N-body simulations including $Ω_Λ \approx 0.7$ and a low density contrast in the Local Volume, which make it generally vacuum-dominated beyond 1 - 2 Mpc from galaxies and groups. We introduce a useful way to view the Hubble flow in terms of "zero gravity" spheres aound galaxies: e.g. a set of non-intersecting spheres, observed to be expanding, actually participates in accelerating expansion. The observed insensitiveness of the local velocity dispersion to galaxy mass is explained as an effect of the vacuum, too.

astro-ph↗

arXiv2005

Fractal Approach to Large-Scale Galaxy Distribution

We present a review of the history and the present state of the fractal approach to the large-scale distribution of galaxies. Angular correlation function was used as a general instrument for the structure analysis. It was realized later that a normalization condition for the reduced correlation function estimator results in distorted values for both R_{hom} and fractal dimension D. Moreover, according to a theorem on projections of fractals, galaxy angular catalogues can not be used for detecting a structure with the fractal dimension D>2. For this 3-d maps are required, and indeed modern extensive redshift-based 3-d maps have revealed the ``hidden'' fractal dimension of about 2, and have confirmed superclustering at scales even up to 500 Mpc (e.g. the Sloan Great Wall). On scales, where the fractal analysis is possible in completely embedded spheres, a power--law density field has been found. The fractal dimension D =2.2 +- 0.2 was directly obtained from 3-d maps and R_{hom} has expanded from 10 Mpc to scales approaching 100 Mpc. In concordance with the 3-d map results, modern all sky galaxy counts in the interval 10^m - 15^m give a 0.44m-law which corresponds to D=2.2 within a radius of 100h^{-1}_{100} Mpc. We emphasize that the fractal mass--radius law of galaxy clustering has become a key phenomenon in observational cosmology.

astro-ph↗

arXiv2003

The Hubble Flow Why Does the Cosmological Expansion Preserve its Kinematical Identity from a Few MPC Distance to the Observation Horizon?

The problem of the physical nature of the Hubble flow in the Local Volume ($D < 10$ Mpc) stated by Sandage(1986, 1999) is studied. New observational data on galaxy motions and matter distribution around the Local Group and nearby similar systems are described. Dynamical models are discussed on the basis of the recent data on cosmic vacuum or dark energy.

astro-ph↗

arXiv2001

Kinematics of the local universe IX. The Perseus-Pisces supercluster and the Tolman-Bondi model

We study the mass distribution and the infall pattern of the Perseus-Pisces (PP) supercluster. First we calculate the mass of the central part of PP, a sphere with a radius of 15/h Mpc centered at (l,b)=(140.2°,-22.0°), d=50/h Mpc, using the virial and other estimators. We get M_{PP} = 4 -- 7 /h 10^{15} M_{sun}, giving mass-to-light ratio 200 -- 600 h M_{sun} / L_{sun}, and overdensity δ\approx 4. The radially averaged smoothed density distribution around the PP is inputted to the Tolman-Bondi (TB) equations, calculated for different cosmologies: Ω_0 = [0.1,1], Ω_Λ = 1-Ω_0 or 0. As a result we get the infall velocities towards the PP center. Comparing the TB results to the peculiar velocities measured for the Kinematics of the Local Universe (KLUN) Tully-Fisher data set we get the best fit for the conditions Ω_0 = 0.2 -- 0.4 and v_{inf} < 100 km/s for the Local Group infall towards the center of PP. The applicability of the TB method in a complex environment, such as PP, is tested on an N-body simulation.

astro-ph↗

arXiv2000

Why is the Hubble flow so quiet?

The cosmological vacuum, which is perfectly uniform, dominates by density over all the forms of cosmic matter. It makes the Universe be actually more uniform than it could be seen from the visible picture of the highly non-uniform matter distribution, especially inside the observed cell of uniformity (100-150 Mpc). This uniformity reveals itself in the structure of the Hubble matter flow which extends over a giant range of cosmic space scales -- from few Mpc to a thousand Mpc, -- preserving its kinematical identity. According to Sandage (1999), this flow is mysteriously regular and quiet even deep inside the cell of uniformity. An answer we propose to the question in the title above is as follows: This is most probably because the flow is dynamically controlled by the cosmological vacuum. An additional conjecture of cosmological intermittency, that addresses a complex statistical structure of initial chaotic perturbations, is also suggested in this context.

astro-ph↗

arXiv2000

The local Hubble flow: a manifestation of dark energy

Our local environment at $r<10$ Mpc expands linearly and smoothly, as if ruled by a uniform matter distribution, while observations show the very clumpy local universe. This is a long standing enigma in cosmology. We argue that the recently discovered vacuum or quintessence (dark energy (DE) component with the equation of state $p_Q = w ρ_Q c^2$, $w \in [-1,0)$) from observations of the high-redshift universe may also manifest itself in the properties of the very local Hubble flow. We introduce the concept of the critical distance $r_Q$ where the repulsive force of dark energy starts to dominate over the gravity of a mass concentration. For the Local Group $r_Q$ is about 1.5 Mpc. Intriguingly, at the same distance 1.5 Mpc the linear and very "cold" Hubble flow emerges, with about the global Hubble constant. We also consider the critical epoch $t_Q$, when the DE antigravity began to dominate over the local matter gravity for a galaxy which at the present epoch is in the local DE dominated region. Our main result is that the homogeneous dark energy component, revealed by SNIa observations, resolves the old confrontation between the local Hubble flow and local highly non-uniform, fractal matter distribution. It explains why the Hubble law starts on the outskirts of the Local Group, with the same Hubble constant as globally and with a remarkably small velocity dispersion.

astro-ph↗

arXiv1999

Lemaître-Tolman-Bondi model: fractality, bang time, and Hubble law I. Initial conditions and compatibility of density and velocity laws

We start a systematic study of the Lemaître-Tolman-Bondi (LTB) model as applied to the large scale structure and its evolution. Here we study three possible initial conditions of the LTB models which are asymptotically FRW at large scales: bang time, fractal density (with fractal dimension D=2), and velocity law. Any two of these determine the third one. Fractal density and simultaneous bang time provide a quantitative estimate for the scale beyond which the deflection from the linear Hubble law is small. This border may be identified with the zero-velocity surface. For fractal density and linear Hubble law it is shown that the bang time is necessarily non-simultaneous.

gr-qc↗

arXiv1998

On the fractal structure of galaxy distribution and its implications for cosmology

Two fundamental empirical laws have been established in the analysis of galaxy space distribution. First, recent analyses have revealed that the three dimensional distribution of galaxies and clusters is characterized by large scale structures and huge voids: such a distribution shows fractal correlations up to the limits of the available samples. This has confirmed the earlier de Vaucouleurs power-law density - distance relation, now corresponding to a fractal structure with dimension $D \approx 2$, at least, in the range of scales $ \sim 1 ÷200 Mpc$ ($H_0 = 55 km/sec/Mpc$). An eventual cut-off towards homogenization has not been yet identified. Second, since Hubble's discovery, the linear redshift-distance law has been well established within $200 Mpc$ and also much deeper. The co-existence of these laws within the same scales is a challenge for the standard cosmology where the linear Hubble law is a strict consequence of homogeneity of the expanding universe. This puzzle is now sufficiently strong to raise doubts for the standard cosmology.

astro-ph↗