Starship Asterisk*

If I'm reading your question correctly I think you're asking about whether or not the Universe is homogeneous?

I think the point is that on the very largest scales the Universe is fairly homogeneous, but clearly a galaxy is very far above the average density of the Universe. This is just because of the large amount of time gravity has had to amplify the small density differences present in the CMB.

One of the greatest successes of the BBT is that you can take the temperature perturbations seen in the CMB and then convert them into a matter density perturbation at the time of matter recombination. If you then take the baryonic to dark matter ratio seen in nearby galaxies, you can work out the amount of mass present at recombination. The amazing thing is if you then the simulate the Universe from recombination to the present using only a starting distribution of mass implied by the CMB and the measured expansion of the Universe, you get something that at redshift 0 (i.e now) that looks remarkably like what we observe.

Fred Hoyle's steady state theory argues for many separate areas of growth rather than an expansion from a big bang like lemaitre and much of modern astrophysics.

My question is about your knowledge of clusters of galaxies like Hercules, Coma, and Virgo being evidence of lack of homogeneity and lack of an isotropic (same every way one looks) universe. When we see clusters and then see voids and then see lonely galaxies or lonely globular clusters or many galaxies in the Hubble deep field, can't we make an assertion for a lack of CMBR like homogeneity and question the isotropic universe?

The basic concept of this question is "Why microscopic CMBR homogeneity being so much discussed as important to establish big bang cosmology but not as much emphasis on macroscopic structural diversity as evidence of concern with big bang cosmology?"


How many different categories of objects do you think we could list?

Such as galaxy, cluster, void, novae, nebulae, star, double star, black hole, planet, asteroid, comet, quasar, brown dwarf, gamma ray burster, gas cloud, supernovae, interstellar gas, star cluster etc . The idea being there is macroscopic diversity.

We're not presently part of any cluster, though over time our own little group will probably get swallowed up by something larger, perhaps the Virgo cluster, though I believe that may be too far away.

The number of red and blue galaxies per cluster is not a sure sign of the clusters age, because something the size of the local group could have fallen in recently, meaning that even an old settled cluster could have quite a few young galaxies, plus there is the problem of galaxies in the fore and background of the cluster being confused as being members.

So, the Virgo Cluster, http://antwrp.gsfc.nasa.gov/apod/ap000220.html, must be of an intermediate age. Although it is dominated by huge ellipticals, plenty of spirals are present, and there is noticable interaction (especially in Markarian's Chain, http://antwrp.gsfc.nasa.gov/apod/ap050316.html), but not to the exent of Hercules.

I would have thought that a greater age would allow time for the galaxies to approach each other to interact or merge. But I guess the galaxies in young clusters that are close enough to interact, do and form these giant ellipticals that sweep out space around them.

I know Andromeda and the Milky Way and their attendant galaxies are part of the local group and that a strong attraction towards the Virgo cluster exists. But is the local group a part of the Virgo cluster? Or are we outside any cluster?

The Hercules cluster is probably younger, the Coma cluster is fairly old and settled, their has been time for interactions to convert the spirals into S0s and ellipticals. The Hercules cluster will probably look pretty similar a few billion years from now, when things have settled down.

There are several differences though, the Herc cluster contains many spiral galaxies but few elliptical. The coma cluster is mostly ellipticals. There are a couple of ellipticals in the Herc cluster that are merging though. I wonder, will they form a larger elliptical or a large spiral???

Is the degree of interaction between member galaxies of the Hercules Cluster unusual? Is there an explanation for this high degree of interaction?

As seen in today's apod http://antwrp.gsfc.nasa.gov/apod/ap070719.html, the Hercules Cluster is full of galaxy mergers and interactions, even though it's not particularly dense or rich in galaxies.

As a contrast, look at the Coma Cluster, http://antwrp.gsfc.nasa.gov/apod/ap060321.html, extremely dense and rich in galaxies, but not much in the way of interactions.