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Author Topic:   Expansion of the Universe
Jefferinoopolis
Junior Member (Idle past 4090 days)
Posts: 19
Joined: 09-27-2010


Message 1 of 31 (605344)
02-18-2011 12:48 PM


I've been following the "Where did the matter and energy come from?" thread. And it touched on the expansion of the universe.
My understanding of this is that things aren't actually moving away from each other but space is actually expanding. We know this because objects that are farther away are moving away from each other faster than closer objects.
The analogy that made me see this the best was putting two sets of marks on an elastic band. Two close together and two further apart. As you stretch the elastic the change in the position of the closer holes is much less than the change in the farther holes.
Do I understand this correctly and does this mean that objects like planets are expanding dimensionally? I'm sure that the size of material objects, even stars, is so small that it would be impossible to measure but over billions of years would there be a mearurable expansion in something the size of a planet?

Replies to this message:
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AdminSlev
Member (Idle past 4641 days)
Posts: 113
Joined: 03-28-2010


Message 2 of 31 (605346)
02-18-2011 5:23 PM


Thread Copied from Proposed New Topics Forum
Thread copied here from the Expansion of the Universe thread in the Proposed New Topics forum.

  
slevesque
Member (Idle past 4641 days)
Posts: 1456
Joined: 05-14-2009


Message 3 of 31 (605349)
02-18-2011 5:30 PM
Reply to: Message 1 by Jefferinoopolis
02-18-2011 12:48 PM


I've been following the "Where did the matter and energy come from?" thread. And it touched on the expansion of the universe.
My understanding of this is that things aren't actually moving away from each other but space is actually expanding. We know this because objects that are farther away are moving away from each other faster than closer objects.
I don't think the 'from each other' part is correct. Objects farther away move faster away from us then closer objects, but not from each other.
But don't forget that what we measure isn't speed, it is redshift.
Do I understand this correctly and does this mean that objects like planets are expanding dimensionally? I'm sure that the size of material objects, even stars, is so small that it would be impossible to measure but over billions of years would there be a mearurable expansion in something the size of a planet?
The universe expand, but things the size of planets and stars don't because gravity keeps them together.

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 Message 4 by Percy, posted 02-18-2011 5:45 PM slevesque has replied

  
Percy
Member
Posts: 22392
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 4 of 31 (605351)
02-18-2011 5:45 PM
Reply to: Message 3 by slevesque
02-18-2011 5:30 PM


slevesque writes:
I don't think the 'from each other' part is correct. Objects farther away move faster away from us then closer objects, but not from each other.
On a scale above millions of light years, everything is retreating from everything else.
--Percy

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Replies to this message:
 Message 7 by slevesque, posted 02-18-2011 6:45 PM Percy has replied

  
Taq
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Posts: 9973
Joined: 03-06-2009
Member Rating: 5.7


Message 5 of 31 (605352)
02-18-2011 5:54 PM
Reply to: Message 1 by Jefferinoopolis
02-18-2011 12:48 PM


Do I understand this correctly and does this mean that objects like planets are expanding dimensionally? I'm sure that the size of material objects, even stars, is so small that it would be impossible to measure but over billions of years would there be a mearurable expansion in something the size of a planet?
You also need to factor in gravity. Yes, space is expanding everywhere, including the space within planets and stars. However, this expansion is very, very small for short distances. Over these short distances gravity is able to easily overwhelm the force of expansion. Keep in mind, this is for the current expansion rate.
If my understanding is correct, the expansion rate is accelerating. There might very well be a time in the future when the expansion rate is high enough to overcome the gravity within a planet. This is called the "Big Rip".
Edited by Taq, : No reason given.

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jar
Member (Idle past 394 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 6 of 31 (605353)
02-18-2011 5:56 PM
Reply to: Message 5 by Taq
02-18-2011 5:54 PM


Taq writes:
Do I understand this correctly and does this mean that objects like planets are expanding dimensionally? I'm sure that the size of material objects, even stars, is so small that it would be impossible to measure but over billions of years would there be a mearurable expansion in something the size of a planet?
You also need to factor in gravity. Yes, space is expanding everywhere, including the space within planets and stars. However, this expansion is very, very small for short distances. Over these short distances gravity is able to easily overwhelm the force of expansion. Keep in mind, this is for the current expansion rate.
If my understanding is correct, the expansion rate is accelerating. There might very well be a time in the future when the expansion rate is high enough to overcome the gravity within a planet. This is called the "Big Rip".
Gravity is one thing, but what about the much stronger forces that operate over even shorter differences?

Anyone so limited that they can only spell a word one way is severely handicapped!

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slevesque
Member (Idle past 4641 days)
Posts: 1456
Joined: 05-14-2009


Message 7 of 31 (605356)
02-18-2011 6:45 PM
Reply to: Message 4 by Percy
02-18-2011 5:45 PM


And yet andromeda is 2,5 million light years away from us, and it is coming towards us

This message is a reply to:
 Message 4 by Percy, posted 02-18-2011 5:45 PM Percy has replied

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 Message 10 by Percy, posted 02-18-2011 10:05 PM slevesque has replied

  
JonF
Member (Idle past 168 days)
Posts: 6174
Joined: 06-23-2003


Message 8 of 31 (605375)
02-18-2011 8:30 PM
Reply to: Message 7 by slevesque
02-18-2011 6:45 PM


slevesque writes:
And yet andromeda is 2,5 million light years away from us, and it is coming towards us
Andromeda's our next door neighbor, just a stone's throw away. To see the effects of spatial expansion you need to look at things that are far away.

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jar
Member (Idle past 394 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 9 of 31 (605376)
02-18-2011 8:33 PM
Reply to: Message 8 by JonF
02-18-2011 8:30 PM


JonF writes:
slevesque writes:
And yet andromeda is 2,5 million light years away from us, and it is coming towards us
Andromeda's our next door neighbor, just a stone's throw away. To see the effects of spatial expansion you need to look at things that are far away.
Andromeda is also part of the Local Group.

Anyone so limited that they can only spell a word one way is severely handicapped!

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Percy
Member
Posts: 22392
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 10 of 31 (605381)
02-18-2011 10:05 PM
Reply to: Message 7 by slevesque
02-18-2011 6:45 PM


slevesque writes:
And yet andromeda is 2,5 million light years away from us, and it is coming towards us
There's no clear line of demarcation because with increasing distance the effects of gravity diminish gradually while those of expansion increase gradually. The larger the scale the more easily expansion will win out over gravity.
I was just responding to your statement that, "Objects farther away move faster away from us then closer objects, but not from each other." All observers in the universe would see precisely the same thing that we see from our vantage point here on Earth, that the further away an object is the faster it is retreating. This is because all points in space are retreating from all other points in space, but what's contained in that space, such as light and matter, will have their own motions and will attract each other gravitationally.
--Percy

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 Message 7 by slevesque, posted 02-18-2011 6:45 PM slevesque has replied

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 Message 11 by slevesque, posted 02-18-2011 11:27 PM Percy has replied

  
slevesque
Member (Idle past 4641 days)
Posts: 1456
Joined: 05-14-2009


Message 11 of 31 (605387)
02-18-2011 11:27 PM
Reply to: Message 10 by Percy
02-18-2011 10:05 PM


There's no clear line of demarcation because with increasing distance the effects of gravity diminish gradually while those of expansion increase gradually. The larger the scale the more easily expansion will win out over gravity.
I was just responding to your statement that, "Objects farther away move faster away from us then closer objects, but not from each other." All observers in the universe would see precisely the same thing that we see from our vantage point here on Earth, that the further away an object is the faster it is retreating. This is because all points in space are retreating from all other points in space, but what's contained in that space, such as light and matter, will have their own motions and will attract each other gravitationally.
Unless I'm understanding all this wrong, then what you are saying here is exactly what I said earlier.
The OP said ''objects farther away move away faster from each other then objects closer away'' and I took this as meaning from our point of view, if we observe 4 objects, the two close far ones will move away from each other faster then the two closer ones from each other.
This is the kind of misunderstanding you get when the reference frames aren't clearly identified

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Percy
Member
Posts: 22392
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 12 of 31 (605417)
02-19-2011 12:42 PM
Reply to: Message 11 by slevesque
02-18-2011 11:27 PM


slevesque writes:
The OP said ''objects farther away move away faster from each other then objects closer away'' and I took this as meaning from our point of view, if we observe 4 objects, the two close far ones will move away from each other faster then the two closer ones from each other.
Along a single line of sight this would be true, trivially true in fact. It wouldn't be possible for it to be any other way.
--Percy

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 Message 13 by NoNukes, posted 02-19-2011 5:40 PM Percy has replied

  
NoNukes
Inactive Member


Message 13 of 31 (605441)
02-19-2011 5:40 PM
Reply to: Message 12 by Percy
02-19-2011 12:42 PM


quote:
Along a single line of sight this would be true, trivially true in fact. It wouldn't be possible for it to be any other way.
It isn't true at all. Assuming that the expansion is isotropic, then space between the pair of points expands at a rate proportional to the current separation of the pairs of points. The distance from us to the pair of points is not relevant.

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cavediver
Member (Idle past 3644 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 14 of 31 (605443)
02-19-2011 6:19 PM
Reply to: Message 13 by NoNukes
02-19-2011 5:40 PM


It isn't true at all. Assuming that the expansion is isotropic, then space between the pair of points expands at a rate proportional to the current separation of the pairs of points. The distance from us to the pair of points is not relevant.
And actually it is, observationally, but opposite to what has been suggested. Distant pairs of objects will be observed to expand away from each other more slowly than similarly separated pairs that are closer, as the more distant pair are being more red-shifted.

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Percy
Member
Posts: 22392
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 15 of 31 (605464)
02-20-2011 1:55 AM
Reply to: Message 13 by NoNukes
02-19-2011 5:40 PM


Hi NoNukes,
I realize now that there was no way to tell what I was responding to. In the paragraph I quoted there was the OP excerpt that Slevesque quoted, and then there was his interpretation of it. I should have just quoted the OP excerpt instead of the entire paragraph, because that's what I was responding to. I didn't address Slevesque's comment because it seemed like a post hoc rationalization of what he wrote earlier, and he shifted the context to observed recession speed, which has a relativistic component, as Cavediver noted. Of course, this must also sound like a post hoc rationalization.
But let me repeat what you said, because I think it's the important point about the expansion of space. It seemed like it was getting muddled earlier in the thread, and that was my original reason for posting. Changing your wording only slightly:
Space between any two pair of points expands at a rate proportional to their separation distance.
--Percy

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