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Light/ universe questions from a layman

  1. Jan 5, 2007 #1
    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 :)
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  3. Jan 5, 2007 #2


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    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.
  4. Jan 5, 2007 #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!!
  5. Jan 5, 2007 #4


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    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
  6. Jan 7, 2007 #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: Jan 7, 2007
  7. Jan 7, 2007 #6
    Ok im not 100% sure this relates to your question, because im 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 dont 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. Im 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, lets 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 wont 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
  8. Jan 8, 2007 #7
    Expansion is occuring 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:

    Last edited: Jan 8, 2007
  9. Jan 19, 2007 #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- lets 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 travelled 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: Jan 19, 2007
  10. Jan 19, 2007 #9


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    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 unverse 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.
  11. Jan 19, 2007 #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.
  12. Jan 19, 2007 #11

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    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.
  13. Jan 20, 2007 #12


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    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.
  14. Jan 20, 2007 #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... gotta 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!!!
  15. Jan 20, 2007 #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: Jan 20, 2007
  16. Jan 20, 2007 #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, lets 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 eachother. 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 doesnt do reality justice, but gets accross a basic picture.

    Hope this helps
    Last edited: Jan 20, 2007
  17. Jan 20, 2007 #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!!!!
  18. Jan 20, 2007 #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.

  19. Jan 21, 2007 #18


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    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.
  20. Jan 21, 2007 #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: Jan 21, 2007
  21. Jan 21, 2007 #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!!!
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