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Relativity and Cosmology

This is a Blog on relativity and cosmology for engineers and the like. You are welcome to comment upon or question anything said on my website (http://www.relativity-4-engineers.com), in the eBook or in the snippets I post here.

Comments/questions of a general nature should preferably be posted to the FAQ section of this Blog (http://cr4.globalspec.com/blogentry/316/Relativity-Cosmology-FAQ).

A complete index to the Relativity and Cosmology Blog can be viewed here: http://cr4.globalspec.com/blog/browse/22/Relativity-and-Cosmology"

Regards, Jorrie

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Age and Size of the Universe

Posted July 10, 2007 1:55 PM by Jorrie

I received the following question by email and thought others might also be interested in discussing it.

"It is widely acknowledged that the age of the universe is 13.5-14.0 billion years.

It is suspected that the boundary of the observable universe is further away, say 45 billion light years.

How can these two things co-exist in the domain of Big Bang and expansion? It seems that the boundary of the expansion has needed to move away at an average of around 3-3.5 times the speed of light for this to happen!"

Just after the end of the hypothesized inflation epoch, space itself was expanding so fast that two hypothetical light sources[1] at opposite ends of the present observable universe would have separated by an apparent 'speed' of larger than 1024c. Since they were not moving through space, there is no relativistic problem with that. This superluminal recession speed rapidly diminished as the inverse square law gravity retarded it.

The best models say those sources were only meters apart then, but today they are around 90 billion light years (ly) apart, 45 billion ly each way, with us in the center. It took the sources some 13.7 billion years to achieve that separation, for an average apparent recession speed (from us) of ~3.3c, as stated correctly in the question.

Now for a crucial bit of required insight: when the light that we today (hypothetically) observe from those sources were emitted,[2] they were only meters from our location (as stated above). However, due to the spatial expansion history, it took that light 13.7 billion years to cover the distance to us, moving precisely at c through local space.

For most of the 13.7 billion years, the photons were actually receding from us and it was only in the last 5 billion years or so that they actually made headway against the 'current' of expanding space. Remember that unlike a real current, expanding space 'flows' more rapidly the farther from us in any direction. Near us, there is no observable 'expansion current' today, but when the universe was young, even nearby 'currents' were strong.

I hope this answers the questions to some extent. Further questions are welcome and I will answer what I can, as long as we can stick to science and avoid philosophy.

-Jorrie

[1] There were no discreet light sources then, only super hot plasma everywhere and the tiny universe was not transparent to light. For ease of comprehension of the issue at hand, let's pretended that there were light sources and that the universe was transparent from the beginning.

[2] The farthest we can observe optically is the CMB, which is the first photons that could travel freely - when the 'fog' of the original plasma 'lifted'. This happened some 380 thousand years after the big bang.


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#101
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Re: Age and Size of the Universe

08/26/2007 11:52 PM

1. Where did you get this quote from?

2. Were you ever exposed to the so-called "Balloon Analogy"?

Here, take a look.

There has always been a fringe facet to mainstream science called "The Anthropocentric View", which takes the vast universe and nature in general, as creations, 'meant' to serve the existence of man.

It was finally abolished in the early nineteen century, although with many attempts to revive it, from mystical or religious points of view.

Also see reference to the so called "Anthropic Principle", which takes in our observation on nature, considerable account, of our limitations to grasp, anything beyond our given knowledge and data processing capability, into our overall view, concerning observation in general.

A good book to tackle these issues from both philosophical and scientific points of view, is Richard Dawkins' "The Blind Watchmaker".

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#102
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Re: Age and Size of the Universe

08/26/2007 11:56 PM

It's late for me so I have not viewed your post thoroughly but as I always enjoy yuor entries I assure you that I will when time allows.

In the meanwhile, I took the quote from a statement posed in the original question.

Come now Yuval. One must read them all. Through all the little bits.

cr3

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#103
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Re: Age and Size of the Universe

08/27/2007 12:02 AM

Admittedly, as much as I try to deny it, my memory is not as all that. You cannot really expect me, at 54, to recall everything said here, to the letter.

I'm lucky to remember where I put my keys last, especially, out of my appartment...

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#104
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Re: Age and Size of the Universe

08/27/2007 12:22 AM

I remarked to coworker that reads SCI-AM religiously, that we haven't got the TOE yet, we have yet to completely understand quantum physics, and now were rushing ahead to explain multi-verses. Walk before you can crawl!

I assume that this has something to do with the most fundamental force in our known Universe - "Publish or Perish."

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#105
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Re: Age and Size of the Universe

08/27/2007 12:31 AM

It must have been that "Publish or Perish", to give rise to the notorious 'bacteria from mars' (and Clinton's blabber about it), exactly as the 'we are in the center of the universe' in explanation to Hubble's red-shift initial findings.

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#106
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Re: Age and Size of the Universe

08/27/2007 1:27 AM

Hi CR3.

"The best models say those sources were only meters apart then, but today they are around 90 billion light years (ly) apart, 45 billion ly each way, with us in the center."

I think you quoted from some previous post of mine. Read in context, this simply said that we are in the center of our observable universe. We have no information indicating we are in the center of the Universe - in fact the 'best models' today say that there is no center of the Universe, because it is either infinite or it is closed and finite, yet unbounded like the surface of a sphere, just it's a 4-d sphere.

Jorrie

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#109
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Re: Age and Size of the Universe

08/27/2007 12:38 PM

Thank you so much.

And you are correct regarding the extraction of text. The "observable universe" statement, although seeming a bit redundant or rhetorical, does in fact, clear the statement nicely.

However, now I have to spend a week swishing your last statement around in my fishbowl!

obliged,

cr3

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#107
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Re: Age and Size of the Universe

08/27/2007 11:55 AM

As I understand it, we are at the center of the visible universe. It continues beyond our ability to see, but our visual limit is 45B in any direction. Tom C.

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#108
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Re: Age and Size of the Universe

08/27/2007 12:02 PM

Yes, "observable universe". Next time I'll read all the latest posts before I bark off a response.

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#110
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Re: Age and Size of the Universe

08/27/2007 12:42 PM

Hi guitarhunter.

'Visible' and 'observable' here mean more or less the same thing anyway.

Jorrie

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#111
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Re: Age and Size of the Universe

08/27/2007 11:05 PM

Hey Jorrie,

After the grilling you've gotten here, wanna join us physics jocks at the Jacuzzi for a Texas-size ribeye and a few drinks 'on the house?' You da man!

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#112
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Re: Age and Size of the Universe

08/28/2007 4:07 AM

Hi -e. Sounds great. When/where?

I guess the equivalent of your 'Texas size ribeye' will be a 'Blue Bull steak' in my valley. The Blue Bulls are our provincial rugby team here in Pretoria and it gave a name to the biggest, meanest steak in town.

So if I can't come that-a-way, maybe you can join a few engineers this side for a Blue Bull and a few Black Labels (that's beer over here).

Jorrie

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