Register | Sign In


Understanding through Discussion


EvC Forum active members: 65 (9162 total)
3 online now:
Newest Member: popoi
Post Volume: Total: 915,818 Year: 3,075/9,624 Month: 920/1,588 Week: 103/223 Day: 1/13 Hour: 0/1


Thread  Details

Email This Thread
Newer Topic | Older Topic
  
Author Topic:   Loopy cosmology
subbie
Member (Idle past 1255 days)
Posts: 3509
Joined: 02-26-2006


Message 1 of 9 (501533)
03-06-2009 3:23 PM


I recently stumbled across this article from New Scientist, describing loop quantum cosmology (LQM). It's so far past what I studied in high school 30 years ago that I struggle to even understand the concepts that it discusses. I have no basis whatsoever for judging the merits of the idea, so I am hoping that some of the physics wonks here can edumacate me.
From what I can glean, it sounds quite exciting, in particular the possibility of our universe containing information from previous universes.
Big Bang and Cosmology, por favor.

For we know that our patchwork heritage is a strength, not a weakness. We are a nation of Christians and Muslims, Jews and Hindus -- and non-believers. -- Barack Obama
We see monsters where science shows us windmills. -- Phat

  
AdminNosy
Administrator
Posts: 4754
From: Vancouver, BC, Canada
Joined: 11-11-2003


Message 2 of 9 (501536)
03-06-2009 3:24 PM


Add stuff please, Subbie
Thread moved here from the Proposed New Topics forum.
I've moved this to get the discussion going Subbie.
But, to be fair, I think you should bring some points here and have a go at them (I know this stuff is mind messing ).
It will help those like Son and Cave focus for you a bit. I hope they will help out.
Edited by AdminNosy, : No reason given.

Replies to this message:
 Message 3 by subbie, posted 03-06-2009 3:46 PM AdminNosy has not replied

  
subbie
Member (Idle past 1255 days)
Posts: 3509
Joined: 02-26-2006


Message 3 of 9 (501538)
03-06-2009 3:46 PM
Reply to: Message 2 by AdminNosy
03-06-2009 3:24 PM


Re: Add stuff please, Subbie
Wow, Johnny on the spot there, AdminNosy, thanks.
I guess my first, basic question, is whether LQM is the real thing, or are the guys working on it just a bunch of chuckleheads with too much time on their hands. More specifically, if it's not all utter rubbish, how close could they be to unseating the Big Bang?
The next thing that I'm having the hardest time getting hold of is this excerpt, near the end of the article:
Kiefer cautions that all the predictions of LQC are subject to one big caveat. The predictions of classical cosmology come from solving the equations of general relativity, albeit with certain simplifying assumptions about the universe. Ideally, LQC should be put on the same footing - all its equations should be derived from loop quantum gravity. Instead, Bojowald and others obtained LQC by starting with an idealised universe derived from general relativity and then using techniques from loop quantum gravity to quantise gravity in the model. "From a physicist's point of view, it is fully justified," says Kiefer. "Mathematicians perhaps would not be amused."
Beyond that I am relatively comfortable with my level of understanding of the discussion.

For we know that our patchwork heritage is a strength, not a weakness. We are a nation of Christians and Muslims, Jews and Hindus -- and non-believers. -- Barack Obama
We see monsters where science shows us windmills. -- Phat

This message is a reply to:
 Message 2 by AdminNosy, posted 03-06-2009 3:24 PM AdminNosy has not replied

Replies to this message:
 Message 4 by NosyNed, posted 03-06-2009 3:55 PM subbie has replied
 Message 5 by Dr Jack, posted 03-06-2009 3:59 PM subbie has not replied
 Message 8 by onifre, posted 03-06-2009 5:24 PM subbie has not replied
 Message 9 by cavediver, posted 03-06-2009 6:18 PM subbie has not replied

  
NosyNed
Member
Posts: 8996
From: Canada
Joined: 04-04-2003


Message 4 of 9 (501539)
03-06-2009 3:55 PM
Reply to: Message 3 by subbie
03-06-2009 3:46 PM


Unseating
if it's not all utter rubbish, how close could they be to unseating the Big Bang?
It won't "unseat" the big bang. That has evidence supporting it having happened. What it will unseat, if it works out, is general relativity in the same way that GR unseated Newtonian mechanics.
GR has a problem. It goes wonky at the beginning because (as I don't understand it ) it allows for an infinitely small space which produces bad answers.
LQC doesn't allow for space to be of arbitrarily small size so it "works" better.
As for you excerpt: we need the real deals here to help but I'll have a go at it.
Ideally the LQG theory (and it's math) should be used as is and just it to see what it produces out the back end. However, to save a LOT of work and/or because LQG isn't "done" yet they used GR and then tweaked the result with some LQG stuff. That isn't so elegant and we know how mathematicians like elegance.
It is, I think, a rough, dodgy math go at it to see what comes out.

This message is a reply to:
 Message 3 by subbie, posted 03-06-2009 3:46 PM subbie has replied

Replies to this message:
 Message 6 by subbie, posted 03-06-2009 4:04 PM NosyNed has not replied

  
Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 5 of 9 (501540)
03-06-2009 3:59 PM
Reply to: Message 3 by subbie
03-06-2009 3:46 PM


Re: Add stuff please, Subbie
I am not a physicist nor do I play one on TV.
I guess my first, basic question, is whether LQM is the real thing, or are the guys working on it just a bunch of chuckleheads with too much time on their hands.
It's about as much of the real thing as is String Theory; which is to say: we simply don't know yet. Until the theory start making predictions we can test nothing will come of it.
More specifically, if it's not all utter rubbish, how close could they be to unseating the Big Bang?
It won't unseat the big bang, although if it works out it does hold promise for saying what happened before the big bang.

This message is a reply to:
 Message 3 by subbie, posted 03-06-2009 3:46 PM subbie has not replied

  
subbie
Member (Idle past 1255 days)
Posts: 3509
Joined: 02-26-2006


Message 6 of 9 (501541)
03-06-2009 4:04 PM
Reply to: Message 4 by NosyNed
03-06-2009 3:55 PM


Re: Unseating
quote:
It is, I think, a rough, dodgy math go at it to see what comes out.
Kind of like the Inconsistent Irrationality that Stile discusses?
quote:
It won't "unseat" the big bang. That has evidence supporting it having happened.
Well, as I didn't understand the article, I thought it was saying that LQM does a better job of explaining the evidence than the BB does. Plus, it seems to me that an implicit part of the BB was that there was nothing before it. Under LQM, there were more (many, an infinite number?) of universes before this one. Wouldn't that in effect unseat the BB?
quote:
What it will unseat, if it works out, is general relativity in the same way that GR unseated Newtonian mechanics.
Wow! Really? I didn't get that at all from that article. Either I missed that completely or the article didn't discuss it.
quote:
GR has a problem. It goes wonky at the beginning because (as I don't understand it ) it allows for an infinitely small space which produces bad answers.
Also, from what I gathered, it allows for infinite density that doesn't sit well, either.

This message is a reply to:
 Message 4 by NosyNed, posted 03-06-2009 3:55 PM NosyNed has not replied

Replies to this message:
 Message 7 by Dr Jack, posted 03-06-2009 4:21 PM subbie has not replied

  
Dr Jack
Member
Posts: 3514
From: Immigrant in the land of Deutsch
Joined: 07-14-2003
Member Rating: 8.7


Message 7 of 9 (501544)
03-06-2009 4:21 PM
Reply to: Message 6 by subbie
03-06-2009 4:04 PM


Re: Unseating
Well, as I didn't understand the article, I thought it was saying that LQM does a better job of explaining the evidence than the BB does. Plus, it seems to me that an implicit part of the BB was that there was nothing before it. Under LQM, there were more (many, an infinite number?) of universes before this one. Wouldn't that in effect unseat the BB?
No. The Big Bang is a collection of ideas about the early universe, begun with the observation that the universe is expanding, and heading back through inflation to 10-41s "after the singularity" (which is really a mathematical extrapolation not a thing physicist think actually existed). This much will remain under quantum loop. Quantum loop adds further details and its mathematics continues to function passed the point were established physics breaks down.
Now, some people think the Big Bang represents the moment of creation of the universe but that's speculation rather than established theory.

This message is a reply to:
 Message 6 by subbie, posted 03-06-2009 4:04 PM subbie has not replied

  
onifre
Member (Idle past 2951 days)
Posts: 4854
From: Dark Side of the Moon
Joined: 02-20-2008


Message 8 of 9 (501554)
03-06-2009 5:24 PM
Reply to: Message 3 by subbie
03-06-2009 3:46 PM


Re: Add stuff please, Subbie
Hi subbie,
More specifically, if it's not all utter rubbish, how close could they be to unseating the Big Bang?
I'm not an expert by any means on this. I was however recently talking to a post-grad at school about LQG and, according to him, most of the unifying theories, which LQG is, are very far away from becoming accepted theories. I don't know how much that bit helps but that's how he felt on the matter.
With that said, as I understand it, what LQG does, theoretically, is remove the BB singularity as the point of origin. GR tells us that the universe 'began' as a singularity - a single point of infinite density - but GR does not explain the quantum structures of spacetime which limits how tightly matter can be compressed/concentrated and how strong gravity can become.
LQG, from what I've read, basically says that the orignal dense state of the BB singularity came about when a pre-existing universe collapsed under the force of gravity, causing the density to increase to the point that the force of gravity becomes repulsive causing space to expand at an accelerating rate - cosmic inflation. In LQG this process is infinite, this is known as the Big Bounce.
So, if, when, LQG/Big Bounce is accepted the Big Bang will no longer be considered the point of origin. Don't worry we can still use BB it'll just mean Big Bounce and not Big Bang.
However, this is mostly what I've gather from laymen books, magazines and talking to a few people that I consider to be very well knowledged in physics. An expert opinion is needed because I'm assuming I screwed up a few things.
Edited by onifre, : spelling

"I smoke pot. If this bothers anyone, I suggest you look around at the world in which we live and shut your mouth."--Bill Hicks
"I never knew there was another option other than to question everything"--Noam Chomsky

This message is a reply to:
 Message 3 by subbie, posted 03-06-2009 3:46 PM subbie has not replied

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


Message 9 of 9 (501562)
03-06-2009 6:18 PM
Reply to: Message 3 by subbie
03-06-2009 3:46 PM


Re: Add stuff please, Subbie
LQG is a quantum gravity approach that arises from a distinguished and successful *classical* approach to General Relativiy. It is a natural way of tackling the quantisation of gravity, and I think it succeeds in certain areas. However, as per my early postgrad thinking, it does single out gravitation in way that now feels decidedly lacking. Its major competitor, string theory, works in a very different way - quantisation of gravitation is secondary to actually understanding gravitation's role in the scheme of things. The previous (failed) attempt at a theory of everything, SUperGRAvity, arises naturally in the context of string theory, which brings together all of the fields of nature. As such, I see LQG as a calculational approach rather than a theory revealing a more unifying picture of reality.
Instead, Bojowald and others obtained LQC by starting with an idealised universe derived from general relativity and then using techniques from loop quantum gravity to quantise gravity in the model.
And this is a very important drawback. The interactions of the other fields, which are considered negligible at the classical scale, are kept negligible in the LQG process. Should they become important, as a number of us think they will, then LQG is seriously lacking. This is why string theory appears the more plausible approach to some of us. Of course, LQG's drawbacks are no evidence for string theory! And I fully support work in both areas - we still seem a long way from any definite statement regarding quantum gravity and a ToE, and negative results are almost as important as positive results in terms of defining the constraints of our "final" theory. I have zero patience, and considerable scorn, for those "academics" that insist that ST or LQG are long past their sell-buy dates.

This message is a reply to:
 Message 3 by subbie, posted 03-06-2009 3:46 PM subbie has not replied

  
Newer Topic | Older Topic
Jump to:


Copyright 2001-2023 by EvC Forum, All Rights Reserved

™ Version 4.2
Innovative software from Qwixotic © 2024