Starship Asterisk*

Hello Chris and neufer,'

Grear stuff! Thank you both.

Hello again,

So as a follow-up it then seems as the expansion rate has out distanced the gravitational attraction over time in spite of the mass-concentration of the Shapley Supercluster, what speeds! Is there anything you can add WRT the area called the Great Attractor? Maybe it's speed relative to the Shapley and us, expansion might say that the GA is faster than us but slower than the Shapley or am I getting too picky here? Where's the Shapley going anyway, just "out there" somewhere?

astrolabe wrote:So as a follow-up it then seems as the expansion rate has out distanced the gravitational attraction over time in spite of the mass-concentration of the Shapley Supercluster,

For us way out in the 'far suburbs' yes. For 'downtown' Shapley, itself, perhaps not (at least quite yet).

astrolabe wrote:Is there anything you can add WRT the area called the Great Attractor? Maybe it's speed relative to the Shapley and us, expansion might say that the GA is faster than us but slower than the Shapley or am I getting too picky here? Where's the Shapley going anyway, just "out there" somewhere?

The "Original Great Attractor" located in the Centaurus Supercluster is not nearly "as great" as we thought it was.

The "New Great Attractor" located in the more distant Shapley Supercluster is presumably centered on the SS itself and is the primary cause for the SS concentration over the life of the universe. Shapley is receding from us primarily because the space between us is expanding.

For now Shapley is probably going nowhere... including into 'space expansion dispersal' thanks to it's own self gravity and/or the "New Great Attractor" which is holding it together.

astrolabe wrote:So as a follow-up it then seems as the expansion rate has out distanced the gravitational attraction over time in spite of the mass-concentration of the Shapley Supercluster, what speeds! Is there anything you can add WRT the area called the Great Attractor? Maybe it's speed relative to the Shapley and us, expansion might say that the GA is faster than us but slower than the Shapley or am I getting too picky here? Where's the Shapley going anyway, just "out there" somewhere?

A couple of points. First, the Great Attractor may not exist- that is, it may not be any physical structure. Its existence is inferred from the motion of surrounding clusters, and that motion may be the result of how things got mixed up in the early universe. Second, the apparent attraction is very, very small. The effects on the motion seen in other clusters is nearly in the noise compared with the motion of expanding space. It isn't as if gravity is dominating expansion, as it does over local scales (such as between the Milky Way and M31); cosmological expansion is still dominant over the distance scales the Great Attractor affects.

Hello neufer and Chris,

Once again, many thanks for staying with it. Very sensible answers and logic that is understandable to boot. I'm grateful to be a member of this Forum during a time when so much in the way of knowledge, coupled with all the images, is so available to us all.

You all are aware that the CMBR dipole is produced by our motion relative to the distant background and must be subtracted out, along with some other stuff, to produce this,

http://antwrp.gsfc.nasa.gov/apod/ap050925.html

right? The dipole will be different depending on where you are in the visible universe. The background, however, will be the same for everyone.

Speaking of Capt. Cook: he could estimate his position on the surface of the Earth because he had the Sun, stars, and a good clock to provide an external reference. If you think about it, we're kind of in the same boat as Ptolemy was with respect to the Universe. He saw a constant star field overhead and, lacking an external reference, could only conclude that Earth was at the center of the Universe. Copernicus came along somewhat later and figured out how to transform observations to Solar coordinates and the picture we understand today began to emerge. On the scale of the Universe, we can measure position and velocity of things we see around us but only relative to our own vantage point. We lack an external reference against which to map the ultimate structure of the Universe.

zbvhs wrote:You all are aware that the CMBR dipole is produced by our motion relative to the distant background and must be subtracted out, along with some other stuff, to produce this,

http://antwrp.gsfc.nasa.gov/apod/ap050925.html

right? The dipole will be different depending on where you are in the visible universe. The background, however, will be the same for everyone.

The dipole is presumed to be caused by our motion relative to our view of the CMB, although there may be other explanations (but certainly, none now that seem as good).

However, as previously noted, it is a mistake to imagine the CMB as a common background, as if it were some fixed sphere around the Universe. Every observer from his own position in the Universe sees his own, different, CMB. Modern theory tells us that each should see a structurally similar background, but certainly not identical. The random fluctuations we observe in our CMB will look different to somebody in a different place in the Universe, because they are seeing a different background.