Expansion of space (conceptual question)

[Elwin.Martin]

From what I understand the beginning of the universe was period of rapid expansion.

I've been told that this expansion is not to be thought of as the universe expanding into an empty space and filling things up but space expanding instead?

Is there something off with this and if not how can I better understand what is occurring?

Thank you for your time.

 

[marcus]

... how can I better understand what is occurring?

Google "wright balloon model" and watch the animation.

Imagine that all existence is concentrated in the 2D surface of that balloon and you are a 2D creature in one of the white whirlers (galaxies).

The other galaxies keep getting farther away, and yet there is no edge.

"outside" and "inside" of the balloon surface do not exist because all existence is concentrated entirely on that surface.

All the galaxies are staying in the same place (latitude longitude-wise) but there is more and more distance between them.

The colored wigglers are photons of light. they actually travel from place to place (lat and long-wise).

Personally I'd stop watching when the expansion gets real slow and start the movie over again.

Once you have that in mind, then the challenge to your power of imagination is to picture it in 3D instead of 2D.

If you want to follow my advice, you say how to better understand, I say start by watching the animation a few times. then shut your eyes and imagine that our 3D world is the surface of a hypersphere. then come back and ask more questions here if you want.
It is just one person's advice. might work for you. might not.

 

[Elwin.Martin]

Thanks, I think I understand a little better now.

It's interesting that the speed of light relative to the galaxies remains constant and that the space expands without the galaxies expanding.

How far along in Astrophysics would one go before they could start studying the expansion?
(I would think I'm years from being able to look at the math for it and make sense of it)

Thank you for your time marcus, I regularly enjoy reading through the forum and you tend to give great advice, thanks again for taking the time to post your thoughts.
Elwin Martin

 

[marcus]

Martin,
I can't think of a correct response. It should be possible to understand a lot of stuff about nature and the universe without what you said (basically going for a MS in astrophysics.)

=======================

You got me thinking about teaching physics.

I watched a bit of Walter Lewin:
http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/

It's free on YouTube. Do you know if there is something comparable for Highschool students?
Good HS physics teachers do a lot of good in the world (a good one maybe more than some PhDs do).

You are asking about the fact that geometry is changeable. Distances can expand and contract in response to matter, or to some initial event that plays out. But if this can happen, how do we define distance?

Could an understanding of this (in a modest intuitive way) be incorporated in HS physics or at College Physics level? Since people seem interested in it. I wonder.

At large scale at least, geometry is dynamic--it responds and changes. Why should we expect otherwise? Everything else does. We have no right to insist that geometry should rigid and immutable, nothing else is.

Does this recognition have to wait until one is in graduate school?

Let's say you have done an undergrad physics major Bachelors degree, but you find that you don't have a great affinity for textbook learning. It seems unmotivated and unintuitive to you. Imagine yourself, or someone, like this. Say you want an intuitive understanding of some stuff that comes up in cosmology---early universe stuff, expansion, accelerated expansion---what can you do?

It's all in the Friedmann equations (two: the main one and the acceleration one) and they are just simple ordinary differential equations governing the scalefactor a(t).

And the scalefactor is just a simple factor that plugs into the metric--the distance function that contains the idea of geometry.

why shouldn't---if young people are interested in it---why shouldn't this be made accessible at the HS or freshman level, as an optional unit of some general physics course?

The question bugs me. I really don't know how to respond.

You get Euclid geometry in HS tenth grade, I think. When you are 15 years old.
Suppose they TOLD you at that point that the stiff version is beautiful but only approximately right. That geometry really isn't static.
The beautiful Euclid version is what comes out when gravity is weak enough to be neglected and when the effects of some intense initial conditions have dissipated and spread out enough that they too can be neglected (except over very long distances).

Suppose they told you when you were 15 that it wasn't exact (in very strong gravity or over very large distances) and promised that in two years---if you learned the static Euclid version---and then took calculus as HS Junior---you could choose a 3 week Senior elective, or 6-week elective, and learn about the dynamic Friedmann version.

Would this make any difference. Would people grow up being less puzzled about expansion cosmology?

 

[Elwin.Martin]

I guess it's a matter of having someone explain it in an appropriate manner? The balloon explanation was much appreciated because it gave me something more concrete to visualize and expand with.

While I can read the Friedmann equations and I sort of understand what they are saying, I wouldn't be able to derive them the way they are derived (mostly because I can't understand the Einstein field equations) or even follow it mathematically.

While visualizing the equation might be possible, I'd need to better understand mass density and spatial curvature and the idea that we are viewing things from the perspective of a "perfect fluid" (is that right? if so I don't know what the tensor it's written in means...) and a number of other things Wikipedia informs me that I do not know.

I mean that I could probably spend several hours on wikipedia, I've already spent one trying to go through the information needed to process the Friedmann equation, [I have taken Calculus but Calc I-III where I'm from weren't exactly rigorous and an intuitive understanding (even of something that probably should be simple) was not stressed] I was wondering when I would take a course where material like the FLRW would be presented. I know the idea of changes in metric but I have never studied them and the math seems to be over my head.

I agree that many of the ideas could be presented conceptually and then explained with math later but I'm unsure that all the topics required could be fit in? Maybe I just lack faith in high school (I'm graduating this May and beginning my undergraduate work this June) but most students at my school seem to have trouble on two main tests in Mechanics...vectors and dynamics. I feel fortunate that I didn't struggle with my own basic Mechanics class but I don't know that cosmological ideas could be taught with math because of the need to catch us all up conceptually. Calculus BC is a joke of class for most of the students at my school and that's probably hurting us more than helping us.

I'm familiar with Professor Lewin's OCW videos :) He's the reason I trust myself with my basics though I did have a good teacher, she was just busy with the students who were really struggling in the course.

I'm unaware of any really good resources for conceptual physics on a high school level, I looked long and hard for anything I could find and ended up settling on using Halliday/Resnick's discussion questions and discussing them with my teacher whenever she had free time.

I'm not sure I understand entirely what you mean by the realization that geometry changes on large scales but I am assuming it has to do with relativity and the idea of a curved space (? I'm guessing). I think the ideas could be introduced in an honors physics course or a special conceptual class, that would make a great elective physics course.

In my state, we no longer offer Geometry and honestly, I tested out of it when it was still available to move onto "higher" algebra. Luckily my Calc III/Linear Algebra instructor has taken a random day or two do discuss things like hyperbolic geometry (he teaches ARML-level geometry at a math summer camp). I suppose students probably would be able to understand the changes in metric(the way the conversion factor "plugs-in" reminds me of the Jacobian right now) if we were taught it but I'm still not sure where we would fit in the math understanding first

Martin,

I can't think of a correct response. It should be possible to understand a lot of stuff about nature and the universe without what you said (basically going for a MS in astrophysics.)

=======================

You got me thinking about teaching physics.

I watched a bit of Walter Lewin:
http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/

It's free on YouTube. Do you know if there is something comparable for Highschool students?
Good HS physics teachers do a lot of good in the world (a good one maybe more than some PhDs do).

You are asking about the fact that geometry is changeable. Distances can expand and contract in response to matter, or to some initial event that plays out. But if this can happen, how do we define distance?

Could an understanding of this (in a modest intuitive way) be incorporated in HS physics or at College Physics level? Since people seem interested in it. I wonder.

At large scale at least, geometry is dynamic--it responds and changes. Why should we expect otherwise? Everything else does. We have no right to insist that geometry should rigid and immutable, nothing else is.

Does this recognition have to wait until one is in graduate school?

Let's say you have done an undergrad physics major Bachelors degree, but you find that you don't have a great affinity for textbook learning. It seems unmotivated and unintuitive to you. Imagine yourself, or someone, like this. Say you want an intuitive understanding of some stuff that comes up in cosmology---early universe stuff, expansion, accelerated expansion---what can you do?

It's all in the Friedmann equations (two: the main one and the acceleration one) and they are just simple ordinary differential equations governing the scalefactor a(t).

And the scalefactor is just a simple factor that plugs into the metric--the distance function that contains the idea of geometry.

why shouldn't---if young people are interested in it---why shouldn't this be made accessible at the HS or freshman level, as an optional unit of some general physics course?

The question bugs me. I really don't know how to respond.

You get Euclid geometry in HS tenth grade, I think. When you are 15 years old.
Suppose they TOLD you at that point that the stiff version is beautiful but only approximately right. That geometry really isn't static.
The beautiful Euclid version is what comes out when gravity is weak enough to be neglected and when the effects of some intense initial conditions have dissipated and spread out enough that they too can be neglected (except over very long distances).

Suppose they told you when you were 15 that it wasn't exact (in very strong gravity or over very large distances) and promised that in two years---if you learned the static Euclid version---and then took calculus as HS Junior---you could choose a 3 week Senior elective, or 6-week elective, and learn about the dynamic Friedmann version.

Would this make any difference. Would people grow up being less puzzled about expansion cosmology?

 

[bcrowell]

I'm unaware of any really good resources for conceptual physics on a high school level, I looked long and hard for anything I could find and ended up settling on using Halliday/Resnick's discussion questions and discussing them with my teacher whenever she had free time.

Hewitt's Conceptual Physics is pretty good IMO. Might be too low level for you, though.

 

[phinds]

It's interesting that ... and that the space expands without the galaxies expanding.

Elwin Martin

Elwin, I'd judge from your 3rd post in this thread that you know more about all this stuff than I do, but I think I have it right that the reason that the galaxies (and galactic clusters too, for that matter) don't expand is pretty simple --- they are gravitationally bound, meaning that the local gravity overcomes the effects of both the expansion and the acceleration of the expansion.

Paul

 

[Elwin.Martin]

@phinds
from the very little popular science I've heard recently (I've yet to do any sort of legitimate investigation into the matter), it would seem that appropriate clusters of dark matter formed the gravitational...sinks? I'm not sure what to call them that allowed for galaxies to stay together. So basically what you said sounds great to me! (hopefully someone more considerably more knowledgeable than either of us can comment?)

Wikipedia appears to be of a similar position (http://en.wikipedia.org/wiki/Dark_matter#Structure_formation)

Can anyone point me in the direction of information on Dark Matter that I can read without getting to deep? Wikipedia's article seems like a good generalization but I'd like to learn something more detailed. It seems that it is difficult to find an in-between for the article that covers a publication and the actual publication.

http://www.nature.com/nature/journal/v445/n7125/full/nature05497.html
vs.
http://www.newscientist.com/article/dn10903-dark-matter-mapped-in-3d-for-first-time.html

@bcrowell
I've heard good things about the Hewitt book as a conceptual reference, I think I'll try it out and let you know what I think. The majority of the text covers material I've already looked at but it can't hurt to see if it gives unique explanations to things I already (think I) understand. Thanks for the advice!