Author Topic: Questions from the Big Rip episode  (Read 2292 times)

Offline DanP

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Questions from the Big Rip episode
« on: February 22, 2013, 09:24:23 AM »
Ben:

I listen to these shows probably three to four times each--there's just so much good stuff that I'm totally fascinated.  Sound waves creating the structure of the universe--that blows me away!  Anyway, questions from the episode.

Recombination--approximately, how big was the universe at this time, and how long did recombination last?

Inflation--What was the approximate size of the universe at the beginning and end of inflation?  I've heard it went from pea size to softball size, or earth size to solar system size.  What is the current thinking?

Expansion of space--I always thought space got larger because the distant galaxies where zooming out--going where no galaxies have gone before--and the universe got bigger that way.  The way it sounds, though, is space itself is growing.  So, is the distance between the earth and moon growing, the space between nuclei and electrons growing?  The space between me and my neighbor growing (this would be a highly relevant line of thought)?  I understand the further away, the faster space is growing, but does it affect us right here, too?

Thanks! 
Not a scientist--I just love science.

Offline bn

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Re: Questions from the Big Rip episode
« Reply #1 on: February 22, 2013, 09:58:52 AM »
Hey dan.

Recombination--approximately, how big was the universe at this time, and how long did recombination last?

this type of cosmology isn't really my field of research. I'll send mike to this thread. maybe he'll know. *shrug*

Inflation--What was the approximate size of the universe at the beginning and end of inflation?  I've heard it went from pea size to softball size, or earth size to solar system size.  What is the current thinking?


again, i'll try to make mike answer your question.
but incidentally. the universe is infinitely wide (and tall, and long) and always has been.
the space between points is just increasing/decreasing. So  it wasn't ever really the size of a pea.

rather, a volume which starts out the size of a pea (in the really really energy dense early universe_ will get stretched to enormous size over the course of inflation. 

Expansion of space--I always thought space got larger because the distant galaxies where zooming out--going where no galaxies have gone before--and the universe got bigger that way.  The way it sounds, though, is space itself is growing.  So, is the distance between the earth and moon growing, the space between nuclei and electrons growing?  The space between me and my neighbor growing (this would be a highly relevant line of thought)?  I understand the further away, the faster space is growing, but does it affect us right here, too?

Thanks! 


So, yes and no. as with so many things, it's a matter of scale. specifically, the local gravitational attraction (or electrostatic force or strong force or whatever) is much stronger than the phenomena involved in the expansion of the universe. I think it only really kicks in on scales that are extragalactic.

lets play the zoom-out game.
imagine that i'm using a magnet to lift a paperclip.
in spite of the fact that the earth has gravity, the magnet's effect on the paperclip is stronger than the earth's gravity.

now zoom out.

even though the sun is huge and heavy, the earth's gravity is much stronger on me than the sun's gravity.

now zoom out.

even though the galaxy is huge and heavy, and has a super giant black hole in the center, the sun's gravity is much stronger on us than the center of the galaxy.

now zoom out.

even though the local group of galaxies is made up of other enormous galaxies, the milky way's gravity is still much stronger on us than these other galaxies.

and zoom out again.

even though the universe is expanding, the local group of galaxies' attraction is still much stronger on us.


so.
the effect is there. but it's very weak compared to all the other things pushing us around.

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Offline zerogravitas

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Re: Questions from the Big Rip episode
« Reply #2 on: March 03, 2013, 12:03:41 PM »
Hi dan - Ben was able to get me to reply, eventually.

Quote
Recombination--approximately, how big was the universe at this time, and how long did recombination last?

Well, that depends on what you mean by "big".  If you mean how wide was the light horizon at reionization, which is probably the best way to judge, the Universe was about 60 Billion light years across.  Recombination lasted a relatively short time - about 115,000 years, which is almost an eyeblink in cosmological terms.

Quote
Inflation--What was the approximate size of the universe at the beginning and end of inflation?  I've heard it went from pea size to softball size, or earth size to solar system size.  What is the current thinking?

Let me answer a slightly different question, which is how big the change in the size of the Universe was during inflation.    If you work through the math, the change is *at least* 10^78 in volume, or 10^26 along any linear dimension.  I don't know if you're familiar with exponential notation, but that's 1 with 26 zeros after it, or 100 million billion billions.  Which is a lot.  So that means a region the size of say an atomic nucleus, which has a characteristic scale of 10^-15m, would have inflated up to 10^26 * 10^-15 = 10^11m, which is about the radius of the orbit of Venus.  Or going the other direction in bigness, it's similar to inflating something the size of a beach ball into the size of the observable universe today.

The reason I'm ducking the pea analogy is that we don't really know how big the "Universe" is in absolute terms - it depends on your definition of "universe".  So we always talk about the "size of the observable universe", or some example size, etc.   

Quote
Expansion of space--I always thought space got larger because the distant galaxies where zooming out--going where no galaxies have gone before--and the universe got bigger that way.  The way it sounds, though, is space itself is growing.  So, is the distance between the earth and moon growing, the space between nuclei and electrons growing?  The space between me and my neighbor growing (this would be a highly relevant line of thought)?  I understand the further away, the faster space is growing, but does it affect us right here, too?

Well, the short answer is: yes!  The thing about the Universal expansion is that it's happening everywhere equally at any given time.  The reason you don't notice it - and why you are still so unfortunately close to your neighbour - is that the force of the expansion is so weak that you need a large volume of space - say the distance between galaxies - to notice it very much.  On scales we're used to - day to day stuff on earth, or even within our own galaxy - the forces of electromagnetism and gravity are very strong compared to the cosmological expansion so we don't notice them.

The most clear analogy of cosmological expansion I have heard of is to first paint little dots on an uninflated balloon.  Now blow up the balloon.  You'll notice that the distance between the dots increases as you add air.  What's happening?  We'll it's not that the dots are moving into new space exactly, it's that the space the dots live on (the 2d surface of the balloon) is expanding, so all the dots are getting further from each other as the space they live on gets larger.  In this analogy, your question is equivalent to "don't the dots themselves get bigger?" - they do, but the difference between real life and the analogy is that in real life there's extra stuff happening that keeps the dots themselves the same size.

So the point of the big rip is that if at some point you tune the expansion parameter to a large enough value, or allow it to accelerate, at some point it's no longer small compared to the other forces and begins to dominate.  That's when you start getting the moon moving further from the earth as space carries it away, or you further from your neighbour, or even at really extreme times electronics getting stripped from atoms themselves.  The bad news is that, according to our best measurements, the expansion seems to be accelerating, though at a rate where we don't have to worry about it for billions and billions of years.  But in a way that's good news, because now they're saying we live in a region of metastable vacuum energy anyway  http://cosmiclog.nbcnews.com/_news/2013/02/18/17006552-will-our-universe-end-in-a-big-slurp-higgs-like-particle-suggests-it-might?lite, so at some unpredictable point in the future the Universe will tunnel to its ground state and none of this will matter. 

If that's not a good reason to sit in your hammock with a drink, I don't know what is. 

Offline Lynx Cat

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Re: Questions from the Big Rip episode
« Reply #3 on: March 05, 2013, 11:01:09 AM »
Man, this whole metastability thing gives me the heebie jeebies. It could theoretically happen any moment for some random reason, or it could never happen, and I guess it's pretty much impossible to predict... and would be also impossible to survive. I wonder what would it look like, though, if it happened, say, in a distant galaxy - would we be able to detect it in advance? At least long enough to throw ourselves one helluva huge final party?
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Offline DanP

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Re: Questions from the Big Rip episode
« Reply #4 on: March 07, 2013, 08:14:21 AM »
Thanks Ben and zerogravitas!  I really appreciate the thoughtful and thorough answers.  zerogravitas, your explanation of inflation was a huge help in understanding the incredible scale of expansion--never knew it was that vast.  Lastly, thanks for the interesting link from the CosmicLog.  I guess I'm not too concerned about the Big Rip.  I draw comfort from the thought that the earth will have been burnt to a crisp long before that happens. 

Again, thank you both for taking time out of your busy schedules to answer my questions!

Dan
Not a scientist--I just love science.

Offline bn

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Re: Questions from the Big Rip episode
« Reply #5 on: March 07, 2013, 02:22:22 PM »
by the way.
zerogravitas is mike zemcov, the titanium physicist.  ;)
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