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  #1  
Old 13-09-2017, 07:55 AM
RetEng
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Decelerating Universe ?

Accelerating Universe has been much publicized in recent times.
Yet as far as we can see in distance and back in time from here-and-now,
redshifts increase. Looks more like DEceleration to me.
What am I missing here ?
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  #2  
Old 13-09-2017, 08:08 AM
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Googling on subject and looking at Wikipedia in detail maybe?
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  #3  
Old 13-09-2017, 10:16 AM
N1 (Mirko)
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Hey, where's the fun in that? Let's try our hand at a simple answer (and assume we all agree that the universe is expanding, the question is just whether that expansion is steady or speeding up/slowing down):

Suppose the expansion doubles all distances over a given time. So the galaxy that is 1 unit away at the start will wind up 2 units away at the end. The one that's 10 units away, 20 units and so on.

So:

The further an object is away, the faster it will be receding from us. 1 unit vs. 10 units in the same amount of time in the above example, with correspondingly stronger redshift. So the further away the more redshift, even if the expansion does not change in speed.

That alone would help explain why looking further away (and necessarily back in time) results in greater redshift being observed.

Except:

The above would result in a linear distance/redshift chart, but that's not what's being observed. Instead, an additional amount of distance travelled is being seen* (for a certain amount of redshift), and the more distant the object is, the stronger the deviation from the straight line. That's because during the time the light has taken to reach us, acceleration of the expansion has had time to add more distance on top of what what you'd expect anyway from a non accelerated expansion. If it was decelerating, you'd end up with less distance than what a constant rate suggests.

It's worth noting that the light we see isn't just a snapshot of the object we are looking at, or its (radial) speed, but also the product of the various effects that light has been subjected to on its way here. So we are not just seeing what the distant object looked like when the light was emitted, but also what happened to the space in between since then.

*All of that would not be possible without knowing how far away the object in question is. Luckily there are certain types of object that are a fairly predictable brightness at the source. By putting that against the brightness being seen from here, one can work out the distance. So the distance observed by redshift and the distance observed by apparent brightness don't agree in the way they should if there was no acceleration. Instead, their relationship is consistent with accelerated expansion.

I could be wrong though

Last edited by N1; 13-09-2017 at 11:58 AM.
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  #4  
Old 13-09-2017, 02:30 PM
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Quote:
Originally Posted by N1 View Post

I could be wrong though
Hmmm. I wonder whether the is some effect on light travelling cosmological distances we have not yet discovered yet.... If not, its going to get bloody cold!
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  #5  
Old 13-09-2017, 02:46 PM
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eddiedunlop (Martin)
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I think what RetEng means is that galaxies closer to us in time and space have smaller redshifts than more remote galaxies, suggesting that the rate at which the universe expands has decreased in more recent times.
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  #6  
Old 13-09-2017, 04:06 PM
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Originally Posted by eddiedunlop View Post
I think what RetEng means is that galaxies closer to us in time and space have smaller redshifts than more remote galaxies, suggesting that the rate at which the universe expands has decreased in more recent times.
Hmmm... possibly slowing down and leading to the grand finale, which of course is the reversing process and ultimately total implosion and the next 'big bang'. There's nothing to say the last big bang was the first hey?

Just a thought.
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  #7  
Old 13-09-2017, 04:17 PM
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Quote:
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Hmmm... possibly slowing down and leading to the grand finale, which of course is the reversing process and ultimately total implosion and the next 'big bang'. There's nothing to say the last big bang was the first hey?

Just a thought.
Totally agree. The Universe is reborn, over and over again. A Big Bang followed by an immense period of time during which the Universe expands and then slows its expansion and begins to contract....leading to another big bang and another rebirth. This makes perfect sense to me.

We think in a linear fashion, but the ultimate reality is probably cyclical in nature....there was no beginning and there is no end to this process.....it just goes on and on, ad infinitum.
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  #8  
Old 13-09-2017, 07:18 PM
deanm (Dean)
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I can see how matter might do this 'Big Bang..Bang..Bang', ongoing thing except for: how do you get lost (i.e. radiated) energy back into the system?
Otherwise the system eventually decays to zero anything.
Dean
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  #9  
Old 13-09-2017, 10:06 PM
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sharpiel (Les)
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I can see how matter might do this 'Big Bang..Bang..Bang', ongoing thing except for: how do you get lost (i.e. radiated) energy back into the system?
Otherwise the system eventually decays to zero anything.
Dean
If all the energy is confined to a single closed system (one universe) then is it possible to lose any energy at all?
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  #10  
Old 14-09-2017, 07:29 AM
N1 (Mirko)
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If all the energy is confined to a single closed system (one universe) then is it possible to lose any energy at all?
I would think not. However energy can turn into matter (at least theoretically) and vice versa.

http://www.nature.com/nphoton/journa...n.2014.95.html

No idea what would trigger the reversal of expansion into contraction though

Last edited by N1; 14-09-2017 at 07:42 AM. Reason: added link
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  #11  
Old 14-09-2017, 12:34 PM
julianh72 (Julian)
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This makes perfect sense to me.
My understanding of cosmology and quantum mechanics etc is limited, but what I do understand suggests that if an idea "makes perfect sense", it is almost certainly not in accordance with modern theory!
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  #12  
Old 14-09-2017, 03:39 PM
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My understanding of cosmology and quantum mechanics etc is limited, but what I do understand suggests that if an idea "makes perfect sense", it is almost certainly not in accordance with modern theory!
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  #13  
Old 14-09-2017, 05:20 PM
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I like a steady state universe whatever that state may be I am not sure.

I like the artist impressions of a slice of the universe which gallaxies making up strands all interlaced..they look as they are part of a system...but only a tiny tiny slice of part of it all.
alex

alex
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  #14  
Old 14-09-2017, 06:37 PM
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Originally Posted by xelasnave View Post
I like a steady state universe whatever that state may be I am not sure.

I like the artist impressions of a slice of the universe which gallaxies making up strands all interlaced..they look as they are part of a system...but only a tiny tiny slice of part of it all.
alex

alex
What makes up the rest of the pie then?
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  #15  
Old 14-09-2017, 07:17 PM
tonybarry (Tony)
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Quote:
Originally Posted by RetEng View Post
Accelerating Universe has been much publicized in recent times.
Yet as far as we can see in distance and back in time from here-and-now,
redshifts increase. Looks more like DEceleration to me.
What am I missing here ?
The missing link is that you are moving, as well as the objects you are looking at.

https://en.wikipedia.org/wiki/Metric_expansion_of_space

When we look at nearby galaxy clusters (e.g. the Virgo cluster ~ 50 million light years, receding at 0.4% c) we see doppler red shifts.

When we look at ancient galaxy clusters (e.g. Abell 2744 ~ 1.2 gigaparsec, 0.3c) we see expansion red shifts.

https://en.wikipedia.org/wiki/Abell_2744

Intuitively, you imagine the following:- Stuff that's far away (and hence long ago) is moving away really fast. Stuff that's nearby (and hence recent) is moving relatively slowly away. Therefore, we're decelerating.

And indeed, we sort of are co-aligned with respect to our Local Group (Andromeda, etc) but we are no longer able to see the stuff that is not local with us. So yes, the nearby stuff is at the same velocity as us, but the far away is gone forever. The evidence of expansion has fled. All that's left is the nearby. Only in the past is it there for us to see.

Regards,
Tony Barry
Penrith, Sydney
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  #16  
Old 14-09-2017, 09:05 PM
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doppler (Rick)
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And because space and time (past and into the future) extends out forever there might be other universes out there that have experienced big bangs as well.
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  #17  
Old 15-09-2017, 07:06 AM
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On the question on the role of energy in the Universe, the conservation of energy or how energy is lost and gained so that total energy is a constant is a property of a static Universe.
An expanding Universe where space-time is not static is not conserved.
Particles moving through space-time are energy conserved but the expanding space time itself is not conserved.

The non conservation of energy for expanding space time is important as it explains certain aspects of Cosmology such as why every object in the Universe is causally connected, to the formation of protons, neutrons and various light nuclei in the very early history of the Universe.

Regards

Steven
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  #18  
Old 15-09-2017, 02:24 PM
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What makes up the rest of the pie then?
more pie.

alex
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