Light/ universe questions from a layman

In summary, the universe is expanding, and although the speed of light remains constant, the images we see today are from a time period relatively recently in the history of the universe.
  • #1
orzos
16
0
I am a bit "obsessed" with cosmology and the universe. Unfortunately, although very intelligent physics is not my strong suit. But- even the most detailed available books cannot always answer my questions and thoughts. With that said I have another question- not exactly a physics one though :) As images from Chandra show a more "vast " universe that the eye can see, is it possible there is some visibilty of a black hole in something we are not seeing so to speak? Also if gravity bends light , and what we see today in fabulous images is from say 400 million years ago- and the universe is expanding, hence smaller then, wouldn't/ shouldn't there be more distortion than images show as galaxies and stars would have been much closer together. Also I understand the expansion and the raisin bread analogy, but with an accelerated expansion at some point wouldnt' the light reach us much "faster" as we are moving towards the former position of where the light left the object even though the object moving away in a certain direction. In other directions we would both be moving away from each other. You can't say we are static even if that is our , and every raisin's perception. Also if Andromeda and our galaxy are on a collision course how is this possible if we are expanding? Thanks for any input on this- my brain hurts :)
 
Space news on Phys.org
  • #2
I just have time to field a couple of these right now:

Last question first. Andromeda and our galaxy are close enough to each other that their mutual gravitational attraction is enough to keep them from separating due to universal expansion. They are part of what is called the local group which consists of a few such galaxies. Galaxies tend to form these clusters and it is these clusters that are receding from each other.

As for the light question, Light has a constant speed for everyone. No matter what our relative velocity towards or away from the source, we always will measure the light's speed as the being the same relative to us. For more on this, look into Einstein's Theory of Relativity.
 
  • #3
I recently watched a show the Theory of Relativity on this with Carl Sagan- but will have to revisit it. I understand that the speed of light stays constant but if light left point A and we approached point A due to expansion as point A then grew further could the speed of expansion allow us to surpass the point where the light left A. Does that make sense? Or, so we see this Hubble deep space field is from the early universe but say this was light captured that was emitted , for arguements sake with the universe at the size of a basketball. Now say the universe is are the size of a swimming pool and our galaxy has traveled past the point that that light was emitted . Although I understand redshift I guess what I am not getting is if we travel past the original point of emission- well how can we see these past emissions. Or was the universe large enough at that time that our galaxy has not reached the "place" where that light may have been omitted. Can you follow that? I'm confusing my self now!
 
  • #4
As Janus has pointed out, gravitationally bound systems like galaxies and solar systems are relatively unaffected by the universe's expansion. For example, the effect of the Sun's gravity on the Earth is much, much larger than the effect of the universe's expansion, and the Earth's orbit is essentially unchanged by the expansion. The expansion really only makes itself apparent over vast, vast distances, like those between clusters of galaxies.

I'm not exactly sure what you mean by some parts of your last post -- "our galaxy has traveled past the point that that light was emitted" just doesn't make any sense to me. From our perspective, all other galaxies are receding from our own, so our galaxy will never cross a "position" in space that used to be occupied by another galaxy in the early universe.

- Warren
 
  • #5
I guess that's the part I am not grasping. If from our perspective all other galaxies are receding- then- from another galaxy's perspective we are in turn receding to them. Well say the universe was much smaller- say the dough for the bread- and the bread is baking our raisin is really moving although not from it's perspective. But the light traveling isn't aware of our perspective. And at some point we can be higher than where we and even the edge of the dough was even though we can't see we are also rising out(in bread terms).

Ok after reviewing relativiy- the fact that our galaxies position may have changed realtive to another point in spacetime shouldn't be taken into our context. I guess that is what gets to me- not really confuses- I just don't understand why we should discount the fact that our rasin has moved and therefore could have an effect on what we see. Then again who am I compared to Einstein! Although it is said in examples I read- neither "viewer" is wrong- all calculations were relevant to the events posed in the examples. So we need a person in another galaxies point of view to help me out! :)
 
Last edited:
  • #6
Ok I am not 100% sure this relates to your question, because I am not 100% what youre question is, so ill have a go :)

This also might be rehashing what other guys have already said, but meh :P

Cosmic expansion is a tricky one because its outside our frame of reference, our reality. If you were somehow able to step into a higher dimensional plane you might get a better grasp, but youre stuck here with me instead :P So, think of it like this (this is by now a cliche) You have a deflated baloon and you draw some stuff on it, say two tiny dots. You now inflate the balloon and see that the dots have moved apart, as expected, but due to the infintesimal nature of your dots, they have barely changed. The point is that space is expanding, there is no actual relative motion between the objects, thus relativity and all its confusing glory don't come into it. The thing to remember however is that light moving along also gets stretched and warped, ie redshifted.

On a slight tangent ill raise something i find interesting. I am sure youve heard of olbers paradox and various other cosmological thinking things. Well, one similar one is the horizon problem, which says that assuming the universe is about 15b years old, let's say, we should be able to see 15b light years in every direction, right? As light has had enough time to go that distance. This should also apply to everything, but it doesnt. Why? Because i can see 15b LY to my left, and 15b LY to my right, but they won't be able to see each other, will they? this means at some point the universe underwent MASSIVE expansion, called the inflationary period, whereby the universe expanded faster than the speed of light, and its only just catching up. The reasons for this expansion are a little confusing and are not really down this path of thinking but i can reccomend a good cosmology book if you like.

Hope this helps
-G
 
  • #7
orzos said:
Also I understand the expansion and the raisin bread analogy, but with an accelerated expansion at some point wouldnt' the light reach us much "faster" as we are moving towards the former position of where the light left the object even though the object moving away in a certain direction. In other directions we would both be moving away from each other. You can't say we are static even if that is our , and every raisin's perception. Also if Andromeda and our galaxy are on a collision course how is this possible if we are expanding? Thanks for any input on this- my brain hurts :)

Expansion is occurring at the long cosmological scales and below a certain scale, it is gravity dominated. So a few local galaxies around us we see blue shifted.

For your previous question, just imagine that the light emitting source is at one point of a elastic cord, and the receiver at some other point, at some point of time. Just imagine that the light being emitted is an ant walking the cord. Now what happens is that the cord is being stretched.
The ant walks the cord at a fixed speed, but the distance it must travel grows because we stretch the cord. Which then -- in term of light -- means it gets redshifted.
And the stretching is not constant but increasing, which means an accelerated expansion...

Hope this analogy will help you get the picture.
Just the elastic band stretches -- which depicts space -- the ant and the emitter/receiver are not stretched.

For a cosmological tutorial and FAQ, here is a good one:

http://www.astro.ucla.edu/~wright/cosmolog.htm
 
Last edited:
  • #8
I completely get what you are saying- but back to the balloon analogy. Is there no up and down or say in and out? Are all galaxies in the same plane? If the universe were a sphere And no galaxy were above or below the plane then the balloon analogy makes perfect sense to me. ( well, see below) But say the universe was flat like the cord analogy- at some point that ant will walk past the place the light was emitted. Does this make sense?

Also if the universe is flat- how thick is it? You can see from Deep fields the answer must be very. And we see in all directions not just East or West say.

Then back to the balloon for a sec. For an example- let's call the 2 points where they started A1 and B1. B1 being us and A1 being some other galaxy now billions of light years away. As the balloon inflates the two points move away from each other- let's now say the points are at A2 and B2- although I know they are always moving. If we are seeing "back in time" then here at B2we are seeing the light emitted at the A1 point. Travelling on the plane of the balloon then it would be redshifted when we saw it. But if light is emitted in all directions what about if you drew a line from us now at B2 to the point
A1 where the light was first emitted. Does this make any more sense? It seems so obvious what Iam trying to say but frustrating not to get it across.


A show I saw on relativity with Cark Sagan had a car crash and explained with speed and location how at a certain point someone wouldn't see the crash but a car swerving to nothing. That is almost how I think we could be seeing parts of the universe.

I am stilll trying to think of an easy clear cut example of my point, but will read the above tutorial too.

Funkydwarf- you said it exact- cosmic expansion is outside our "reality" so to speak. So if you use the raisin bread analogy and we could stand outside the "universe" we would see that all raisin's are moving away from each other and at a point in time- our raisin would / could travel past where another raisins starting point was if it were outside our starting point in the ball of dough. Or even if the stretching was not perfectly uniform. And at this starting point is where the light was emitted from this other raisin at a point in time say 15 billion years ago. But now today we are in a spot in the bread that is past the starting point and in fact didn't even exist when the dough started cooking. So even for the outer "edge" raisins on the "crust" of the bread they are farther apart but what about the space between the inital point of the raisin and the current point of our raisin. Are we saying light only travels in that one plane- ie. the crust? Because is we are seeing more older galaxies clustered closer aren't we really looking back "inward"at the ball of dough just after it started cooking not along the plane of the crust. So in the aspect of looking inward at the dough we have traveled outward past areas other raisin's have occupied unless and only unless we are on the crust- which we know in reality we are not as there is universe in all directions right?

Please tell me someone understands this after all these examples! And hopefully this makes sense to someone- even if it can be discounted- it is frustrating not getting the point across! Thanks!
 
Last edited:
  • #9
orzos said:
I completely get what you are saying- but back to the balloon analogy. Is there no up and down or say in and out? Are all galaxies in the same plane? If the universe were a sphere And no galaxy were above or below the plane then the balloon analogy makes perfect sense to me. ( well, see below) But say the universe was flat like the cord analogy- at some point that ant will walk past the place the light was emitted. Does this make sense?

Also if the universe is flat- how thick is it? You can see from Deep fields the answer must be very. And we see in all directions not just East or West say.

With the balloon analogy, you have to imagine that we've reduced the universe by one dimension. The skin of the balloon is all that we would be aware of. We could only look in directions that followed the surface. The third dimension, (in and out) would not exist for us except as an abstract concept.
Our three dimensional universe would be the "skin" of a 4 dimensional "hyper-balloon" where "in and "out" lay along the 4th dimension that we cannot directly sense or even really visualize (it would be like asking someone to point in the direction of yesterday).

We use the reduced-dimension balloon analogy since it is something that we can visualize.
 
  • #10
But- we are pointing at the direction of yesterday with the Hubble though- right? So aren't we in a sense including this dimension in dating and sizing the universe? Then wouldn't it alter what we think we are seeing and calculating? Especially as we don't know for sure the shape and dimensions of the universe as a whole.
 
  • #11
orzos said:
I am a bit "obsessed" with cosmology and the universe. Unfortunately, although very intelligent physics is not my strong suit. But- even the most detailed available books cannot always answer my questions and thoughts.

Have you tried the popular book by Weinberg, The First Three Minutes? Some of the cosmology has been overtaken by later developments, but everything he says about the geometrical/physical meaning of the "Big Bang theory", "light cones" in an expanding universe, and so on, is still valid.
 
  • #12
I sense the question relates to the constancy of the speed of light. Try considering this possibility: when space expands, it stretches the wavelength of the photons traveling across it [e.g., redshift]. There is no more logical explanation for the Hubble constant. In that sense, light travels at superluminal velocities as it reaches us from the early universe, but remains constant as viewed by local observers.
 
  • #13
No, not about the speed of light at all. I totally get redshift/blueshift. It's about "position" of where light is emitted in relation to our "position" in space. Not the object moving away or space stretching- but that fact that no matter how you look at it so to speak- when using such large volumes our "position" has changed in the universe even if it isn't relative to us in such a huge universe I can't fathom how it can be discounted. We aren't talking about a train, ball and observer.

Ok the cornell site has helped a bit with getting the balloon analogy and the perpendicular I was talking about is really a fourth dimension( if it exists) in our reality. And the actual stretching of space and not movement of galaxies through space. But-----back to raisin's as you stretch the dough between raisin's the "dough directly around the raisin doesn't change- I get that- the raisin hasn't moved -but- due to the stretching the raisin is now in a different place -in the dough- from the place it started correct?

Or am I thinking too much stretching? If this is all after inflation has the stretching been "minimal" enough in universe terms that - the raisin's have not yet moved far enough that their places in the dough could have been stretched to the point they have changed their place in the bread?

Ok from UCLA- quote

The Hubble law defines a special frame of reference at any point in the Universe. An observer with a large motion with respect to the Hubble flow would measure blueshifts in front and large redshifts behind, instead of the same redshifts proportional to distance in all directions. Thus we can measure our motion relative to the Hubble flow, which is also our motion relative to the observable Universe. A comoving observer is at rest in this special frame of reference. Our Solar System is not quite comoving: we have a velocity of 370 km/sec relative to the observable Universe. The Local Group of galaxies, which includes the Milky Way, appears to be moving at 600 km/sec relative to the observable Universe.

Yipppeeeeee- homologous expansion! I am figuring it out! This is what I mean. A space time diagram explains my point exactly- which leads to Light Cones- oh my... got to keep reading. I think maybe I need to go get a cosmology degree! This seems to be kinda deep to be pondering for a stay at home mom- albeit one with wasted intelligence:) !

This is what I was saying...

The grey hyperbolae show the surfaces of constant proper time since the Big Bang. When we flatten these out to make the previous space-time diagram, the worldlines of the galaxies get flatter and giving velocities v = dDnow/dt that are greater than c. But in special relativistic coordinates the velocities are less than c. We also see that our past light cone crosses the worldline of the most distant galaxies at a special relativistic distance x = c*to/2. But the Hubble law distance Dnow, which is measured now, of these most distant galaxies is infinity (in this model). Furthermore, this galaxy with infinite Hubble law distance and hence infinite Hubble law velocity is visible to us, since in this model the observable Universe is the entire Universe. The relationships between the Hubble law distance and velocity (Dnow & v) and the redshift z are given below.


Got it, got it, space time and world line.

So I think what I am trying to say in a simple form is is at some point we will comove pass the beginning point of a world line for another galaxy's light emission X years ago.

Or- using light cones, our spacetime position "now" puts us at a place "inside" past light cones of galaxies which is why we see the past universe right?

Here we go-about the media not talking about redshift..

Distance is defined as the spatial separation at a common time. It makes no sense to talk about the difference in spatial positions of a distant galaxy seen 9.1 billion years ago and the Milky Way now when galaxies are moving.

My point has to do with this spatial position and from where that galaxy was 9.1 billion years ago and we are now and how this affects what we are really seeing now and if in an inflating universecould we have surpassed the worldline for a light emission from this galaxy.

Oh I hope someone is getting this...but I still am reading that site and it has been almost 3 hours and I still haven't fully satisfied my question.

Ok I'll end this post but this is the right line of thought sorry for all the random thoughts but it is coming together.

Can you guys get me yet? I am finally getting to my point in "real" cosmological exlpanations I think. Intersecting world lines and light cones hold my answer I think.

That balloon lady quote above was sort of wrong as the perpendicular to the balloon is time and the line I was drawing from point A1 and A2 to B1 and B2 are the light cones and/or world lines!
 
  • #14
Ok sorry this brings me to another sort of related question. The Hubble deep space pics collected light for quite awhile to get those pics. If there is a light cone and world line for each instant of light emission- how the heck is that instant caught. Wouldn't be a jumbled mess of light cone intersections? And since we are comoving nad not necessarily in the same direction as what we see based on light cone diagram there are not only areas of the universe we cannot see , are there are also areas that will move out of our line of vision so to speak? I wish I had a good simulator for this.

Ok I have to go play a mindless game for a bit I am driving myself crazy!
 
Last edited:
  • #15
Lets see here

Hi Orzos, it seems like your right on the verge of figuring some things out and that is always exciting.

Let me try to clear up a couple things for you here. First thing, let's think about the raisin + dough analogy and some of its shortcomings. The raisins are NOT moving through the dough, the dough is just expanding and so the volume of dough between the raisins is getting bigger. The confusion is that in the model the raisins are moving through real 3D space because of the doughs motion and so one raisin can pass the real space point that another raisin once occupied. However notice that no two raisins will move through the dough to occupy the same piece of dough.

The raisin model would correspond to the real universe if dough = space and every galaxy was at rest with respect to the Hubble flow. This is not the case in reality, but its an easy model to think about. The velocity that a galaxy has once the Hubble flow is subtracted is called its peculiar velocity. Peculiar velocity is the only reason that some galaxies close to us are blue shifted instead of redshifted.

Now in this model where the galaxies are all at rest with respect to the Hubble flow, no galaxy can ever get closer to any other galaxy.They are all eternally expanding away from each other. Also since they are not moving through space at all, but only being carried apart by the expansion of space, one galaxy will never approach the point in space that another galaxy is (or was) in.

I think your difficulty was that you were picturing a background static space that this expansion is happening in (which the raisin in dough model leads people to believe sometimes)

Now, light cones. A star turns on (starts shining) at time t=0, x=a. Its light cone at any future time includes all the places light would have had time to travel to. Imagine the standard axes (time as the vertical and 1D space as the horizontal) Imagine the space time points where two stars beging shining (t1=0, x1=0; t2=0, x2=a) Each light cone is an upside down triangle with a vertex at the star. At some point in time later then when they turn on, the cones will intersect. Now imagine the world lines of the stars as vertical lines in the diagram cutting each light cone in half. Eventually, the light cone of one star will intersect the world line of the other star. At this point in space time, observers orbiting one star will be able to see the other star. This is also the first point in time that the two stars can have any causal connection. In an expanding universe the lightcones take on the funnel shape because as time goes on the expansion of the universe stretches the light cones. That last bit is meant to be only qualitative and doesn't do reality justice, but gets across a basic picture.

Hope this helps
-G
 
Last edited:
  • #16
"I think your difficulty was that you were picturing a background static space that this expansion is happening in (which the raisin in dough model leads people to believe sometimes)"

Yes this is what I was doing. I still haven't fully resolved it in my mind- but- all I read early today helped. Ialso happened to get a new book the other day called The Curious History of Relativity so I delved into that this evening. Not knowing "the" answers drives me crazy. I sincerely hope in my lifetime some answers to the universe are found!
 
  • #17
for more about light cones and the expanding universe then you ever wanted to know check this paper out. Its directed at pros, but you can understand the points it makes even if the explanations might be difficult. It also shows that even professional astrophysicists get this stuff confused.

http://arxiv.org/abs/astro-ph/0310808
 
  • #18
Conservation is the name of the game. The total energy content of the universe remains fixed. Expansion dilutes everything . . . gravity, photonic energy, etc. Gravity tries to pull it all back into the initial singularity, but is losing the battle. . . at least according to observational evidence.
 
  • #19
Allright Allday! If this were a contest you would be a winner! That was a great explanation and example! I have had up to advanced calculus in math so it wasn't too hard to read and comprehend. Even if it has been 13 years since I have had any real use for it in my day to day life!

Along with the idea that I was using a "backround space" for the raisin bread to be expanding in- although I am still debating that one in my head- this has helped alot. It just seems that that "backround space" is time- the fourth dimension- because the stretching of space causes all galaxies to now have a new "location" in this "backround space" or in time. Say, take a snapshot of the raisin bread dough, then take another snapshot of it 2 hours later in the oven. Due to time and stretching the raisin's are in another location in this "backround space"- or time. Well maybe back to the light cones, worldlines and spacetime- plotting it shows the point at where the light is emitted and the point at which we receive it. Off to re read that Abstract. One of those links before mentioned if you should even include/ prove a fourth dimension- but now after all this I can't see how anyone can not include it in any professional test, observation, theory, etc. This whole discussion also made how time is a fourth dimension so easily understandable which on another thread seemed more complicated!

I live near Johns Hopkins- I think I should look into further school there - this has really been fascinating!
 
Last edited:
  • #20
In the Feb 2007 Astronomy magazine there is an article -Bob Berman's Strange Universe talking about shadows. At the end he talks about the lack of shadows- more specifically that Astronomers from the Wilkinson Microwave Anisotropy Probe observatory published analyses of data showing that the CMB does not show shadows from many galaxy clusters. He says- no shadows, how could thid be. He goes on to say the most logical answer is that the CMB originates from space that's nearer than the galaxies-EUREKA! That's what Iam saying - those galaxies have moved farther out past the point where the light was emitted . He questioned it could mean throwing out Big Bang, nopt necessary , or if the galaxies all give off the 2.725 Kelvin temp to fill in- it makes no sense to him- it makes absolute perfect sense- when you take "backround space"/ time- as the dimension the raisin bread is expanding in!
 
  • #21
Hi Orzos, glad to see you're understanding more. I've got to make something clear though. There is no "background static space" in the real universe. The expanding space IS space and that's what we live in. Let's try another explanation that might help.

The CMB exists everywhere in the universe because it was emitted from every point in the universe at a time about 13 billion years ago. It is the black body emmission of the plasma that occupied the entire universe when it was in a much denser state. The region that emmitted the CMB photons that are detected ON EARTH is a shell that is concentric with the Earth at a distance that is changing. As time passes we have to look farther out to see the fixed time of CMB emission. Therefore the distance to the region that emitted the CMB that we see on Earth is constantly receding from us. That is why the CMB will become more and more redshifted as time goes on.

Also there were no galaxies when the CMB was emitted. Every galaxy observed was formed after the emission of the CMB and so has to be closer to us than the region that emitted the CMB we see.

ill have to take a look at that astronomy article. The Sunyev Zeldovich effect is like a shadow that galaxies leave on the CMB so I am not sure what sort of shadow the writer is referring to. If the CMB were emitted from regions closer to Earth than some galaxies than that means the universe was very clumpy when it was emitted. It would be very difficult to reproduce the smoothness of the CMB from such a group of emmitters that were clumpy.

cheers
-gabe
 
Last edited:
  • #22
The paper is probably this one:

The Sunyaev-Zel'dovich effect in a sample of 31 clusters - a comparison between the X-ray predicted and WMAP observed CMB temperature decrement
http://arxiv.org/astro-ph/0510160 [Broken]

I do not understand the whole paper and I would like to read the opinion of someone working in this field. To my eyes there are too many uncertainties. They seam to base the work on the basic assumption of the validity of the isothermal beta model for all of their 31 clusters. I have the impression that a small deviation from that assumption might question the results.
 
Last edited by a moderator:
  • #23
Ok, I know there is no backround static space- but- using spacetime coordinates and the recession of galaxies then the "position" of everything would change constantly on a spacetime coordinate graph so to speak.

As for the CMb- using an analogy I guess I was theorizing using the crust of the bread instead of all the dough as the emitter of the CMB.

I did a search after reading the astronomy mag using WMAP findings and galaxy shadow after reading an abstract off of WMAP's site.

Here is just one simple link- not the paper but about the question

http://www.physorg.com/news76314500.html

A quote-

If that were the case, the background microwave radiation reaching Earth today would have traveled billions of light years through space from the furthest edges of the universe.

This is why I was using the crust of the bread. - the edges of the universe .

I have to go work on this one!
 
  • #24
Ok after some thought and a few drawings- I am going to disagree with you Allday. Let me explain. First let me ask if CMB was emitted everywhere- how and why and what "region" are we only seeing? Now to my thoughts- First there is no "backround space" . CMB does exist everywhere in space but not spacetime. That's the distinction. Let's use a sphere for example since so much in the universe is sphere / circle like.

Get 3 pieces of paper and a black marker.

Take a circle say fifty cent size and color it black. Now I know this will create a center of the universe- but hey this is just for sake of argument. Now draw a circle about tennis ball size and draw a few lines of X's across it- representing galaxies. Now draw a circle bigger than a softball. Use the same pattern of X's you used but space them out in all directions some to account for expansion. Now add some U's for "newer" galaxies and make sure to include one by the "center" of our universe.

So- each one of these x's or u's represent a a galaxy. Our galaxy would be a U if not on a later "slice" of spacetime. Maybe do a fourth one- same concept and add some M's- picking one to be the Milky way. Lay them atop one another smallest on the bottom- and try to picture them as spheres insisde each other. ( Use 2 at a time if needed to see through the paper)Since space is expanding and stretching.

So the "center" fifty cent piece size is the CMB emitted in the entire universe at it's size at that spacetime 13 billion years ago.

Now the second sphere is say 1 million years later and look at that- some of those galaxies have moved- in SPACETIME- outside of where the CMB was emitted. Some galaxies have formed outside of the spacetime where the CMB was emitted- even if it was emitted from the whole universe at that time.
Some new ones have formed inside the spacetime.

By looking at CMB- we are essentially looking back in time right? How do you not have the factor of spacetime in the equation. Because we now occupy an area of spacetime not existing at the spacetime the CMb was emitted even though we are in the "space" it was emitted. Maybe I canm figure out how to do one of those Minkowski space graphs for this and see what happens. Nevermind- just reread some and I'll never be able to do it. I'll have to start looking for a physicist! My little drawings are not 4D or 3D of course but the closest I can come right now to put it on paper. And it is so non scientific it has a ton of faults- but it gets confusing dealing with spacetime! Then again a baking loaf of raisin bread isn't the most scientific analogy either! :)
 
  • #25
Orzos, let's take a look at your model and see what's going on here. First thing let's consider the finite 2-D model of the universe that you have on paper. One thing to realize is that the way you are drawing it, each circle represents the whole universe at one instant of time (which is impossible to observe from earth), but let's develop that idea. Also realize that there is nothing outside the first circle that you draw. The universe just ends there. Let's suppose that we are just looking at a chunk of the universe and are neglecting what is outside the circle. Then we still have to realize that nothing outside the circle (except photons) will cross into the circle and nothing inside the circle (except photons) will cross outside of it as it expands. Also what exists out side that chunk is similar at all times to what is inside the chunk (from our view on high (see below) Anywho ...

The first quarter size circle is the instant of CMB emission, time = 0 for us. Next trade the paper for rubber. You were expanding the universe on a background static space again and that's not good. Instead, stretch the rubber so the circle is the size of a tennis ball. The CMB is still everywhere in the universe, but the universe has gotten bigger and the CMB has cooled a bit. Stretch some more and you have galaxies starting to form. They form everywhere as the universe cools and so they are distributed evenly in the circle now. They are still bathed in the CMB which is still everywhere, but has cooled even more. We keep stretching for 13 billion years and we end up eventually with a universe where the CMB is about 3 degrees Kelvin and galaxies are roughly evenly distributed throughout the whole universe.

Now put yourself at the center of this universe and make an observation. It is important to remember that looking down on the paper we are seeing the entire universe at one instant. When we make the observation outward from the center we have to take into account the finite speed of light. Say light travels a milimeter every billion years in this universe. Here is the big question.

If the CMB light we observe at the center has been traveling at the speed of light for 13 billion years where on the circle did it have to come from?

The answer is at least 13 mm away right? It is actually more because during the trip the rubber has expanded. Light from points closer to the center gives us information on the universe AFTER the CMB was emitted and light from points further from the center hasn't had time to reach the center since the start of the universe.

This argument can be repeated for any time greater than t=0
 
  • #26
I agreer 100% with your first paragraph. But- when you take this as the 3d rubber ball analogy- at the point of CMB there was nothing outside of the ball- but of course lots of matter inside. Then after it stretches -spacetime- alters the location of all the matter inside and at the "outer edge" of the universe in spacetime coordiantes ( underlined) so even though the CMB was emitted within the matter it was not emitted at our location in spacetime nor did our location in spacetime even exist in our universe at the time of emission. So if you stand in the center of the ball you can't say where on in the sphere it was emitted but you would be able to say where in spacetime it was emitted if plotted out( which I do not have the means to do- but sure wish I could right now!) If you could plot a transparent CMB ball on a 3d graph then overlay the larger sphere ( representing today) graphed onto the same graph- whereas the outward motion of the sphere's growth would be the 4th dimension- time. Any galaxies formed outside this imaginary spacetime line of the first "cmb ball" would cast a shadow- inside would not. But I just stumped myself as if the universe is flat I have to think of how a galaxy could form on the other side of this imaginary line. Or could it somehow be on the other side of the sphere- and we are seeing it "through" the CMB sphere so to speak. Gotta think on this one. But again since looking at the CMB is looking at how it existed 13b years ago- we have to look back at how the universe existed then and face our current location in spacetime coordiantes is really actually outside the oringinal small rubber ball in that area we agreed nothing crosses in and out of. And it hasn't it just expanded into that "area" of spacetime using the 4th dimension- time.
 
  • #27
I was just reading in that relativity book mentioned above about Minkowski space and I am not sure a sphere is the best analogy for my point I think as with light cones there is only a forward / future and back / past in visibility. I see your point that the space we inhabit is simply stretched space from the same space the CMB was emitted in- but the spacetime we inhabit did not exist at the time of CMB as the expanding of space causes everything to move outward on a spacetime coordinate graph and also pushes the "edge" of space out into that nonexisting outer backround space- time- if you will. So even though there is nothing outside of our universe nor do we interact with anything outside some imaginary edge- time stretches this edge ever outwards creating new spacetime coordinates as it goes. Same "space" just new coordinates in spacetime. Sort of like how planets orbits are really helix's when time is factored in. A tube shape may be better suited with the center or even one end being CMB- let me think about this.
 
Last edited:

1. What is light?

Light is a form of electromagnetic radiation that is visible to the human eye. It is a type of energy that travels in waves and is made up of particles called photons. Light can be produced by various sources, such as the sun, fire, or artificial light bulbs.

2. How does light travel?

Light travels in a straight line and at a very high speed, about 299,792,458 meters per second, which is the speed of light in a vacuum. When light encounters a medium, such as air or water, its speed can change and it can also be refracted or reflected.

3. What is the relationship between light and color?

Color is a perception created by our brains in response to different wavelengths of light. When white light from the sun or a light source hits an object, certain wavelengths are absorbed and others are reflected. The reflected wavelengths determine the color that we see.

4. How does light affect our daily lives?

Light plays a crucial role in our daily lives. Not only does it provide us with the ability to see and navigate our surroundings, but it also affects our mood, sleep patterns, and overall health. Light is also used in various technologies, such as communication, medicine, and energy production.

5. What is the universe made of?

The universe is made up of a variety of elements, including matter, energy, and dark matter. Matter is made up of atoms, which are composed of protons, neutrons, and electrons. The remaining 95% of the universe is made up of dark matter and dark energy, which are currently not fully understood by scientists.

Similar threads

Replies
6
Views
368
Replies
19
Views
2K
Replies
9
Views
1K
  • Cosmology
Replies
11
Views
1K
Replies
8
Views
1K
Replies
5
Views
1K
Replies
23
Views
1K
Replies
5
Views
1K
  • Cosmology
Replies
5
Views
1K
Back
Top