Center of the universe the origin of the big bang?

marcus

You're asking about distance growth speeds at various "lookback times". It's kind of interesting. Distances have been growing all along, for the whole 13.7 billion years. But for the first 7 billion years or so their growth was decelerating. Then around the 7 billion year mark the growth curve started getting steeper.
You can see it on this plot. The heavy solid line is the standard model one. You can see it starting off convex (getting less steep) and then around -6 or -7 (around 6 or 7 billion year lookback) it gets concave and the slope starts steepening.
http://ned.ipac.caltech.edu/level5/M...s/figure14.jpg

The deceleration and acceleration of distance growth is a fairly subtle effect so its good to get an idea of the distance growth rates themselves. Here's a table if you want to look at a few speeds of distance growth at various times in the past. http://physicsforums.com/showthread....79#post4022179
Keep in mind that these are not like speeds of ordinary motion. In a uniform pattern of expanding distances nobody gets anywhere---everybody just gets farther apart. So we not talking about travel speeds. Distance expansion can easily be faster than the speed of light, indeed for large distances it typically is.

hubble_bubble

On the Caltech graph I am assuming that the redshift axis is for another graph. Otherwise we would be getting future blue shifts.

hubble_bubble

If we had an anomaly which was spherical and was expanding at a rate faster than the surrounding universe then compaction would shrink the space in that area to accommodate this stretching. At what point would this shrinkage be equivalent to a black hole. I am not saying this is possible but if we think of black holes as space compaction maybe it would bring some new ideas.

hubble_bubble

Also we assume that objects would be torn apart by the gravitational forces in black holes but the universe stretches and we aren't torn apart. It may be that inside a black hole to the observer it is like a 13 billion year big crunch. Also inside a black hole doppler redshift I would assume would appear larger at greater distances.

marcus

That's an interesting guess! But actually the redshift scale belongs there as an alternative to the regular scale on the left side of the plot.

The point is we don't receive light from the future. Our galaxy sends light to the future.
So for us to send them a normal wavelength (say for example 1 meter) wave we would have to first compress it.

Across from scalefactor 1.5 there should be written
- 0.333

because we have to shorten the wavelength by a third in order for it to get to those people.
(They are living at a time when distances are 50% larger than now.)

Across from scalefactor 2.0 on the plot, very near the top, should be written
- 0.5
because those people are living at a time when distances are TWICE what they are now.
So in order to make sure they receive a normal size wavelength we have to shorten what we send them by half---we have to reduce the wavelength by 50%

When you are thinking redshift you are always thinking from the perspective of the present moment, what we receive from the past, or send to the future.

If you use the letter a(t) for the scalefactor at time t, then the formula for z is

1+z = a(now)/a(then) = a(us)/a(other people)

If they live when distances are half what they are now, then 1+z = 2, so z = 1
If they live when distances are twice what they are now, then 1 + z = 0.5 so z = - 0.5
f they live when distances are 1.5 what they are now, then 1 + z = 0.666 so z = - 0.333
I'm just repeating what I said earlier but with a formula.

I'm glad you had a look at the scalefactor curve at the Caltech site.
http://ned.ipac.caltech.edu/level5/M...s/figure14.jpg
It is from a valuable article by Charley Lineweaver. For another Charley article, try the SciAm link in my signature. It is also enlightening and well illustrated.

hubble_bubble

Let's take the ripples in a pond scenario. I am at point a somewhere in a pond let's call me object A. Two other objects B and C are at remote distances. Object C is twice as far away from me as object B. If ripples are started at both points B and C at the same time. Object C's ripple takes twice as long to reach me. If I measure the diameters of the received ripples these will have been magnified by a significant amount. Neither object B or C have moved away from me but some amplifying factor, the outspreading wave, has increase the magnitude I perceive. This of course is not an analogy that can be directly applied to the universe, but if we didn't know the proper cause and effect of this ripple phenomena with any certainty we could apply all sorts of theoretical models to it and make them sound reasonable.

hubble_bubble

Thank you for bearing with me. I like to be devil's advocate sometimes.

hubble_bubble

I know that the supernova studies showed objects as more distant than expected, but if those objects are further away due to a stretching of spacetime then we should perceive an increase in object size. Matter is not independent of spacetime and surely must be affected by this stretching. Are those distant galaxies larger than expected? Has anyone tested for this?

marcus

matter-to-matter couplings tend to be vastly stronger than matter-geometry coupling.
So objects the size of galaxies hold themselves together at a stable size. The effect of geometry (Hubble-law distance growth) is not noticeable at that scale.

Personally I never talk about "stretching of spacetime". Or about "stretching of space". I don't think of space as a material. If you use the balloon analogy as a help, ignore the rubber. Pretend the rubber isn't there and that the picture is meant only to illustrate evolving geometry. The galaxies are not moving, they are only all getting farther apart. All existence concentrated on the infinitely thin sphere, no inside or outside, or rubber .

It takes some concentration to think in terms of an evolving geometry (rather than material being deformed stretched/compressed within a fixed geometry). But the concentration gets you closer to what GR is really saying.

"Dadurch verlieren Zeit und Raum den letzten Rest von physicalische Realität". In other words "it's not a material, don't think of it that way!"

hubble_bubble

Something struck me about the ripples analogy. What if the 7 billion year old deceleration is a shock wave?

hubble_bubble

If we imagine a singularity which will have an event horizon. Say this singularity suddenly ejects it's matter for some unknown reason. The gravity waves propagating at the speed of light would be holding back this escaping material under immense pressure until the event horizon was reached. The matter would just be expressed as energy at this point and no particles would exist. Because this matter has no chance to expand significantly until reaching the event horizon you would possibly get a following shock wave instead of one with a leading edge. This could be the 7 billion year old deceleration.

marcus

You posted #27 while I was still working on #26 so you may have missed what I said due to my slow typing:

===quote===
matter-to-matter couplings tend to be vastly stronger than matter-geometry coupling.
So objects the size of galaxies hold themselves together at a stable size. The effect of geometry (Hubble-law distance growth) is not noticeable at that scale.

Personally I never talk about "stretching of spacetime". Or about "stretching of space". I don't think of space as a material. If you use the balloon analogy as a help, ignore the rubber. Pretend the rubber isn't there and that the picture is meant only to illustrate evolving geometry. The galaxies are not moving, they are only all getting farther apart. All existence concentrated on the infinitely thin sphere, no inside or outside, or rubber .

It takes some concentration to think in terms of an evolving geometry (rather than material being deformed stretched/compressed within a fixed geometry). But the concentration gets you closer to what GR is really saying.

"Dadurch verlieren Zeit und Raum den letzten Rest von physicalische Realität". In other words "it's not a material, don't think of it that way!"
==endquote==
Shockwaves are something that happen to material thought of WITHIN A FIXED GEOMETRY. that's not what we're trying to understand.
GR is about the evolution of geometry itself. It is our theory of geometry (explaining why it is approximately Euclidean in some cases and not in other cases.) Hopefully it will soon be improved on, people are working on a quantum version of geometry theory. But GR where it applies is amazingly accurate in its predictions so one has to take it seriously and accept it as the best we can do so far.

It says please do not persist in using material analogies
Geometry has its own distinctive equation that it evolves according to, and according to which it is influenced by matter.