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Author Topic:   Did the expansion rate of the universe exceed lightspeed?
Explorer
Junior Member (Idle past 5868 days)
Posts: 24
From: Sweden
Joined: 02-24-2008


Message 1 of 86 (458026)
02-26-2008 8:55 PM


I have read from some books during the years that the expansion rate of the universe at the moment of its birth was several times greater than the speed of light. As normal matter can’t travel faster than the speed of light this is remarkable. The explanations I have read is that the normal laws of physics just wasn’t there in the beginning, allowing things to go superluminal.
Does anyone know if the expansion rate of the universe did exceed light speed and by how much? What could it mean if the birth of the universe allowed superluminal speeds? What would happen if we accelerate something beyond the speed of light?
Timeline of the big bang:
Graphical timeline of the Big Bang - Wikipedia
Apparently the definition "Big Bang" is often referred to BOTH the hot dense phase in the beginning of the universe and the expansion-theory of the universe itself.
Big Bang - Wikipedia
This message promoted from message 3 of the "Proposed New Topic" (PNT) version. - Adminnemooseus
Edited by Adminnemooseus, : See above.

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 Message 3 by Percy, posted 03-01-2008 11:28 AM Explorer has not replied
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Explorer
Junior Member (Idle past 5868 days)
Posts: 24
From: Sweden
Joined: 02-24-2008


Message 2 of 86 (458631)
03-01-2008 10:31 AM


A clarification:
If the laws of physics didn't exist (as we know them today) in the very birth of the universe then my early question about the expansion rate answers itself (for an unknown cause/pre-law things were allowed to go superluminal). But I am interested in if the expansion rate AFTER the formation of modern laws could have exceeded light speed due to the enormous energy from the birth of the universe. I have read (somewhere) that it would acquire a source of infinite energy to exceed light speed for normal matter... isn’t that just what the birth of the universe is described like? As having infinite energy in the beginning?
cavediver... you seem to know this kind of stuff! Please drop a line in here to give your views on this!

  
Percy
Member
Posts: 22388
From: New Hampshire
Joined: 12-23-2000
Member Rating: 5.2


Message 3 of 86 (458644)
03-01-2008 11:28 AM
Reply to: Message 1 by Explorer
02-26-2008 8:55 PM


You're talking about inflation, an hypothesized period during which the early universe grew in size at an enormous rate often described as exceeding the speed of light. Inflation is the expansion of space itself, which is not limited by the speed of light. Inflation has nothing to do with the motion of matter and energy through space, which can't exceed the speed of light.
We don't really know what was happening to the universe when it was younger than T=10-43 seconds. Before that time the density of the universe was greater than the Planck density, which means there was more energy than could fit into the space available under physical laws as currently understood.
But we have a pretty good idea about what happened after T=10-43 seconds, and we're pretty sure the physical laws we know and love today were already in force, to the extent we comprehend them. Cavediver can correct me if I'm wrong, but I believe T=10-43 seconds marks the beginning of inflation, during which the size of the universe doubled roughly every 10-34 seconds. By T=10-32 seconds inflation was over and the universe had doubled in size at least 100 times.
Since physical laws were in force during expansion, no matter or energy traveled faster than the speed of light. But as I said earlier, inflation is often described as a period in the early universe where space (not matter or energy) expanded faster than the speed of light. In other words, the volume of space expanded faster than the speed of light, not the matter and energy within that space.
Cavediver will, I hope, correct me if I've got this wrong, but I take issue with the description that space expanded faster than the speed of light. While it is true that some parts of the inflationary universe were receding from each other at relative speeds greater than light, this is also true today. Space itself is not expanding that fast today, and it was not expanding that fast during inflation. A doubling in size every 10-34 seconds means, for example, that two points in space separated by a single Planck distance, which is 1.6 10-35 meters, would in a mere 10-34 seconds become separated by twice that distance. Dividing the change in distance by the change in time yields 0.16 meters/second, which is far less than the speed of light.
--Percy

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 4 of 86 (458661)
03-01-2008 1:24 PM
Reply to: Message 1 by Explorer
02-26-2008 8:55 PM


The explanations I have read is that the normal laws of physics just wasn’t there in the beginning, allowing things to go superluminal.
No, that's just nonsense
Does anyone know if the expansion rate of the universe did exceed light speed and by how much?
It did and it still does - but this is misleading. Percy has already explained this, but i will repeat:
Space itself is expanding - there is no actual motion. You can tell this because you are experiencing no acceleration as the Universe expands: everything appears to be moving away from you and you seem to be at the centre of the expansion. But if you chat with someone in the one of the galaxies that is moving away, they will tell you that they also are not accelerating, and it is they, not you, that are at the centre of the expansion! So even though this galaxy is moving away at ever increasing speed, it is not actually moving! The intervening space is simply expanding. It is quite possible that the amount of expansion increases the relative distance between two galaxies faster than a light ray can close that distance. This is where the expansion can be said to be greater than the speed of light, in a bastardisation of terminology. There are galaxies that we can see today (as they were billions if years ago) that we can never travel to, no matter if we travelled at the speed of light. They are forever lost to us.

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Replies to this message:
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fallacycop
Member (Idle past 5520 days)
Posts: 692
From: Fortaleza-CE Brazil
Joined: 02-18-2006


Message 5 of 86 (458662)
03-01-2008 1:29 PM
Reply to: Message 3 by Percy
03-01-2008 11:28 AM


Cavediver will, I hope, correct me if I've got this wrong, but I take issue with the description that space expanded faster than the speed of light.
And you have good reasons to take issue with such a description.
The expanssion of the universe and the speed of light are measured with different physical units and, therefore, there is no meaningfull way to compare the two.

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 6 of 86 (458664)
03-01-2008 1:41 PM
Reply to: Message 3 by Percy
03-01-2008 11:28 AM


While it is true that some parts of the inflationary universe were receding from each other at relative speeds greater than light, this is also true today.
Certainly
Space itself is not expanding that fast today, and it was not expanding that fast during inflation.
Actually, it was - so fast that fast doesn't even begin to capture it!
doubling in size every 10-34 seconds means, for example, that two points in space separated by a single Planck distance, which is 1.6 10-35 meters, would in a mere 10-34 seconds become separated by twice that distance. Dividing the change in distance by the change in time yields 0.16 meters/second, which is far less than the speed of light.
Yep, and in ten of those 10^-34 seconds, ~five billion Planck times, the points would be separated by one thousand Planck lengths, so around 60m/s. And in thirty of those 10^34 seconds, 15 billion Planck times, the points would be separated by 1 billion Planck lengths, still well below the speed of light.
But in 100 doublings, which is fifty billion Planck times, the points would now be separated by 2^100 = 10^30 Planck lengths!!! So a mere 10^19 times fast than the speed of light Never undersetimate the power of exponentiation...
Edited by cavediver, : No reason given.

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Replies to this message:
 Message 7 by Chiroptera, posted 03-01-2008 1:47 PM cavediver has replied
 Message 17 by Straggler, posted 03-02-2008 6:39 AM cavediver has replied

  
Chiroptera
Inactive Member


Message 7 of 86 (458666)
03-01-2008 1:47 PM
Reply to: Message 6 by cavediver
03-01-2008 1:41 PM


Hey, cavediver, here is a question from someone who is stuck in flat-space-time-special-relativity mode:
What does it even mean that two distant galaxies are separating at a speed greater than the speed of light? Does it really come out of GR that the relative "speed" (in the sense of the distance between them) can be greater than the speed of light?

If I had a million dollars, I'd buy you a monkey.
Haven't you always wanted a monkey?
-- The Barenaked Ladies

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 8 of 86 (458672)
03-01-2008 2:17 PM
Reply to: Message 7 by Chiroptera
03-01-2008 1:47 PM


Does it really come out of GR that the relative "speed" (in the sense of the distance between them) can be greater than the speed of light?
Sure. There are going to be pairs of galaxies for which causal communication is possible, and pairs for which it is impossible. In our accelerating FLRW space-time, the former evolve into the latter. But you can have exactly the same scenario in flat space-time and SR. If I start accelerating away from you, there will come a time after which I am out of causal contact with you. I will be asymptoting to a particular null ray radiating from your world line, and no null rays emiited after that asymptote will intersect my trajectory. I will always be 'below' them (on a classic space-time diagram). We need a sketch pad add-in... Percy???

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 9 of 86 (458674)
03-01-2008 2:21 PM
Reply to: Message 5 by fallacycop
03-01-2008 1:29 PM


And you have good reasons to take issue with such a description.
The expanssion of the universe and the speed of light are measured with different physical units and, therefore, there is no meaningfull way to compare the two.
I disagree - I think the scanarios I have discussed with Percy and Chiroptera provide such meaningful comparisons. I admit, you have to choose your scenario carefully, and you cannot make any blanket statement regarding the two phenomena.

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Explorer
Junior Member (Idle past 5868 days)
Posts: 24
From: Sweden
Joined: 02-24-2008


Message 10 of 86 (458676)
03-01-2008 2:40 PM
Reply to: Message 9 by cavediver
03-01-2008 2:21 PM


Thanks Percy and cavediver for your answers. Much to take in but I think I got the most important parts right. And it does make a whole lot of sense. But if the space itself continue to grow like this.. is there any other outcome than a forever-going expansion? How come all the theories of possible "big crunch" scenarios? Or does that simply only apply to MATTER?
Edited by Explorer, : No reason given.

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fallacycop
Member (Idle past 5520 days)
Posts: 692
From: Fortaleza-CE Brazil
Joined: 02-18-2006


Message 11 of 86 (458683)
03-01-2008 3:29 PM
Reply to: Message 9 by cavediver
03-01-2008 2:21 PM


To make meaningfull statements in these scenarios, you had to stipulate some scale that was not intrinsicaly included in the original question. The scale chosen in the discussion with pecy was the plank scale. The scale chosen in the discussion with Chiroptera was the distance between two carefully chosen galaxies.
Edited by fallacycop, : grammar

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fallacycop
Member (Idle past 5520 days)
Posts: 692
From: Fortaleza-CE Brazil
Joined: 02-18-2006


Message 12 of 86 (458684)
03-01-2008 3:33 PM
Reply to: Message 10 by Explorer
03-01-2008 2:40 PM


In some scenarios, depending on how much matter is present in the universe, the expansion may eventually halt and turn around into the Big Crunch.

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 13 of 86 (458685)
03-01-2008 3:41 PM
Reply to: Message 11 by fallacycop
03-01-2008 3:29 PM


Yes, very true. Which is why I was objecting to your absolutist "no meaningful way"

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fallacycop
Member (Idle past 5520 days)
Posts: 692
From: Fortaleza-CE Brazil
Joined: 02-18-2006


Message 14 of 86 (458689)
03-01-2008 4:27 PM
Reply to: Message 13 by cavediver
03-01-2008 3:41 PM


But then it is always possible (as long as there is some expansion at all) to find an scale large enough (unless the universe is finite) for the expansion to be fater then the speed of light.
How is that a meaningfull statement?

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 15 of 86 (458693)
03-01-2008 5:11 PM
Reply to: Message 14 by fallacycop
03-01-2008 4:27 PM


But then it is always possible (as long as there is some expansion at all) to find an scale large enough (unless the universe is finite) for the expansion to be fater then the speed of light.
Of course
How is that a meaningfull statement?
How is it not meaningful? Unless you are thinking of a statement such as "during the inflationary epoch, space-time expanded much faster than the speed of light", which I agree that although true, is rather misleading, because as you state, you can (typically) always find a length scale at which space is expanding "faster than light".
The point is:
1) two objects can have a perceived recession "velocity" caused by the expansion of space that is less than c.
2) this "velocity" can exceed c (though of course not be directly perceived) for two sufficiently separated objects.
3) This in no-way contradicts the usual limits of the speed of light nor any part of Special Relativity.
No part of 1), 2), and 3) is meaningless.

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