Thanks.
I googled "Are superclusters expanding" and found an abstract from a 2002 paper at
http://arxiv.org/abs/astro-ph/0204121.
I don't get the details of their abstract . . .
We investigate the dynamical state of superclusters in Lambda cold dark matter ($\Lambda$CDM) cosmological models, where the density parameter $\Omega_0=0.2-0.4$ and $\sigma_8$ (the rms fluctuation on the $8h^{-1}$Mpc scale) is $0.7-0.9$. To study the nonlinear regime, we use N-body simulations. We define superclusters as maxima of the density field smoothed on the scale $R=10h^{-1}$Mpc. Smaller superclusters defined by the density field smoothed on the scale $R=5h^{-1}$Mpc are also investigated. We find the relations between the radially averaged peculiar velocity and the density contrast in the superclusters for different cosmological models. These relations can be used to estimate the dynamical state of a supercluster on the basis of its density contrast. In the simulations studied, all the superclusters defined with the $10h^{-1}$Mpc smoothing are expanding by the present epoch. Only a small fraction of the superclusters defined with $R=5h^{-1}$Mpc has already reached their turnaround radius and these superclusters have started to collapse. In the model with $\Omega_0=0.3$ and $\sigma_8=0.9$, the number density of objects which have started to collapse is $5 \times 10^{-6}h^3$Mpc$^{-3}$. The results for superclusters in the N-body simulations are compared with the spherical collapse model. We find that the radial peculiar velocities in N-body simulations are systematically smaller than those predicted by the spherical collapse model ($\sim 25$% for the $R=5h^{-1}$Mpc superclusters).
. . . but what I take from it is that
according to their simulations, most superclusters are expanding as they state that their model suggests that 'Only a small fraction of the superclusters defined with . . . has already reached their turnaround radius and these superclusters have started to collapse.' I note that this is a simulation that suggests a result, not an observation that confirms a result.
Also, my comment about superclusters being the largest structure in the universe is debatable according to the wikipedia article at
http://en.wikipedia.org/wiki/Supercluster
On the one hand
Once thought to be the largest structures in nature, superclusters are now understood to be subordinate to enormous walls or sheets, usually called "filaments", sometimes called "super cluster complexes", "walls" or "sheets", that can span a billion light-years in length, more than 5% of the observable universe.
On the other hand
According to some astronomers, no clusters of super clusters (“hyperclusters”) are known; the existence of structures larger than superclusters is debated (see Galaxy filament). Interspersed among super clusters are large voids of space in which few galaxies exist. Even though superclusters are the largest structures confirmed, the total number of superclusters leaves possibilities for structural distribution; the total number of super clusters in the universe is believed to be close to 10 million.
Just posting this in interests of accuracy.
Thanks.
I googled "Are superclusters expanding" and found an abstract from a 2002 paper at [url]http://arxiv.org/abs/astro-ph/0204121[/url].
I don't get the details of their abstract . . .
[quote]We investigate the dynamical state of superclusters in Lambda cold dark matter ($\Lambda$CDM) cosmological models, where the density parameter $\Omega_0=0.2-0.4$ and $\sigma_8$ (the rms fluctuation on the $8h^{-1}$Mpc scale) is $0.7-0.9$. To study the nonlinear regime, we use N-body simulations. We define superclusters as maxima of the density field smoothed on the scale $R=10h^{-1}$Mpc. Smaller superclusters defined by the density field smoothed on the scale $R=5h^{-1}$Mpc are also investigated. We find the relations between the radially averaged peculiar velocity and the density contrast in the superclusters for different cosmological models. These relations can be used to estimate the dynamical state of a supercluster on the basis of its density contrast. In the simulations studied, all the superclusters defined with the $10h^{-1}$Mpc smoothing are expanding by the present epoch. Only a small fraction of the superclusters defined with $R=5h^{-1}$Mpc has already reached their turnaround radius and these superclusters have started to collapse. In the model with $\Omega_0=0.3$ and $\sigma_8=0.9$, the number density of objects which have started to collapse is $5 \times 10^{-6}h^3$Mpc$^{-3}$. The results for superclusters in the N-body simulations are compared with the spherical collapse model. We find that the radial peculiar velocities in N-body simulations are systematically smaller than those predicted by the spherical collapse model ($\sim 25$% for the $R=5h^{-1}$Mpc superclusters). [/quote]
. . . but what I take from it is that [i][b]according to their simulations[/b][/i], most superclusters are expanding as they state that their model suggests that 'Only a small fraction of the superclusters defined with . . . has already reached their turnaround radius and these superclusters have started to collapse.' I note that this is a simulation that suggests a result, not an observation that confirms a result.
Also, my comment about superclusters being the largest structure in the universe is debatable according to the wikipedia article at
[url]http://en.wikipedia.org/wiki/Supercluster[/url]
On the one hand
[quote]Once thought to be the largest structures in nature, superclusters are now understood to be subordinate to enormous walls or sheets, usually called "filaments", sometimes called "super cluster complexes", "walls" or "sheets", that can span a billion light-years in length, more than 5% of the observable universe. [/quote]
On the other hand
[quote]According to some astronomers, no clusters of super clusters (“hyperclusters”) are known; the existence of structures larger than superclusters is debated (see Galaxy filament). Interspersed among super clusters are large voids of space in which few galaxies exist. Even though superclusters are the largest structures confirmed, the total number of superclusters leaves possibilities for structural distribution; the total number of super clusters in the universe is believed to be close to 10 million. [/quote]
Just posting this in interests of accuracy.