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Light from 14 Billion Years Ago

  1. Jan 17, 2010 #1

    dnr

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    I asked this question of a friend.
    We read certain reports that astronomers have “seen” “light” from 500 million years, only, from the creation of the universe. Here is my query.
    1. The universe “started” as an almost infinitely dense singularity.
    2. As this thing expanded, all sorts of interesting things happened, which resulting in stars, galaxies, heavy elements, x-ray background, etc.
    3. As the universe expanded, except for the brief “expansion faster than light” issues, it was expanding at less than the speed of light.
    4. This means than all of the bundles of photons have been expanding at the same rate as the universe.
    5. So, all information about what happened over the 14 billion years of the universe has expanding at the same rate.
    6. This means that there is no “looking” into the past, because this information has been expanding, along with the universe.
    7. The universe then should then be uniform, i.e., the same age, since it has been expanding with “us” since the beginning.
    8. No matter what direction we look in, we should see exactly the same thing.
    9. We should not see stuff from the beginning or 10 billion years ago, because everything has been expanding at the same rate and therefore all bandwidths of information from that source have always been with us.
    10. Stuff expanding in the opposite direction from us, is still moving slower than the speed of light, so we should see it exactly as it is now.
    11. There is no reason that when we look at increasingly sensitive instruments, e.g., the updated Hubble, that we should see anything but current information, i.e., the wavelengths from everything has been traveling with us since the big-bang.
    12. If something happened 10 million years after the beginning of the expansion, then it would look 10+ million years old, but it couldn’t look to be 9 or 8 or 4 or 1 million years old. Those signals arrived long ago.
    13. So, if an event occurred 500 million years after the big-bang, then it got to “us” at 500 million years + plus the travel time at the speed of light after it occurred.
    14. This means that nothing we can see occurred more than expansion rate + the speed of light. If something occurred 500 million years after the big-bang, then that information has been with us since that time, plus the speed of light.
    15. Once this information goes past us, then it is gone. So if something happened 1 second after the big bang, then that information in gone after 1+ seconds. It has passed us by. It is gone. We can’t look into the past and see what happened.
    ---------------------------------------------
    His answer didn't help.

    I used to wonder about this too. I have only worked the answer out mathematically and have never tried to put it into words. Here is my best effort.

    It seems that it must take the galaxy more than 15 billion years to get 15 billion light years away from us. And then it would take another 15 billion years for the light it emits to get back to us. So that would make the universe more than 30 billion years old. But this is not what
    happens. The key to what is really going on is the following property of the universe: As the universe expands, the fabric of space stretches.

    The light we see from a galaxy 15 billion light years away has traveled 15 billion light years. But the galaxy was not 15 billion light years away when it emitted the light. It was, say, one billion light years away. As the light traveled toward us, space stretched due to the expansion of the universe. It did this continuously as the light traveled toward us. By the time the light got to us the galaxy was 15 billion light years away and the light had traveled 15 billion light years. So the galaxy traveled away from us for a little more than one billion years. It then emitted light toward us that took 15 billion years to get to us because the space it traveled through kept stretching and increasing the distance. So the universe is 1 billion years + 15 billion years = 16 billion years old. (These numbers are skewed slightly - the universe is only 13.7 billion years old.)

    According to the standard cosmological model, galaxies formed 400 million years after the big bang. The above galaxy had to form within a billion years of the big bang, so it had plenty of time.

    The universe is very big, with the light from its youth spread throughout. Some of the primordial light from the early universe passed us billions of years ago. Today, primordial light from farther away is passing us.
    ---------------------------------

    Anyone understand the question and have an answer?
     
  2. jcsd
  3. Jan 17, 2010 #2
    The amount of photons emitted(and currently being emitted) was/is almost infinite, so one can argue that space is filled with light of different ages that are constantly interfering with each other, thus we see different things, what we see may not be what is actually so. In fact its a miracle that we can even see space(as black emptiness), with the amount of photons being put out by stars the entire universe should be full of light.

    (A lot of scientists make claims, few are true.)
     
    Last edited: Jan 17, 2010
  4. Jan 17, 2010 #3

    Chronos

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    The energy density of the universe has been measured and is nowhere near infinity. In fact, we know the matter-energy density of the universe with great precision, and it is unclear if it is sufficient to reverse expansion, or not.
     
  5. Jan 18, 2010 #4

    Chalnoth

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    Expansion "faster than light" is, in my opinion, really poor language. Expansion isn't a speed, it's a rate. Does it really make sense to say, for instance, 3000 rpm is faster than 30Mph?

    Looking far away is still looking into the past. When we receive a photon in a detector from a distant star, that photon has to first traverse the space between us and that star. That takes time. The further away the star is, the longer it takes. So when we look far away, we are looking at stars that emitted that light much further in the past.

    What we see, then, is a small slice of the universe: just those photons that were emitted at the right time (depending on the distance of the source) to have arrived right now.

    Also, though our universe is highly uniform in all directions on large scales, it isn't completely uniform. The cosmic microwave background, for instance, is the same to one part 100,000 in different directions.

    Most of the stuff we see out there (stuff at very roughly redshift greater than one) is now and always has been receding faster than the speed of light, according to the simplest definition of recession velocity (the definition of recession velocity is actually rather arbitrary).
     
  6. Jan 18, 2010 #5

    marcus

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    Roughly greater than one is right, not to put too fine a point on it. One could safely say that is true for any redshift z > 1.7.

    Dnr, all the questions you are asking are quantitative enough that you had really better learn to use the available online calculators.

    One is called cosmo calculator. If you google "cosmo calculator" you will get
    http://www.astro.ucla.edu/~wright/CosmoCalc.html
    and if you put in 1.7 for z and press calculate you will get the socalled "comoving" distance of something which we can see that has that much redshift.
    It will say 15.54 billion light years. Do you see where it says that? Billion lightyears is abbreviated Gly.

    The "comoving" distance is the actual distance you would measure by timing a radar or light signal if you could freeze the expansion process TODAY. It is a kind of "now" distance, and is what plugs into the Hubble Law:
    v = Hd
    If you put the comoving distance in for "d", then multiplying by H will tell you "v" the rate that the comoving distance is increasing.
    =========================

    I can show you a short cut that will save you doing some arithmetic. If you went to cosmo calculator, you may have noticed 3 important numbers put in, over on the left side, along with the redshift z.
    These are the matter fraction 0.27, and the vacuum energy fraction 0.73, and the Hubble rate 71.
    These numbers help to characterize the standard cosmological model. They are estimated derived from data, for the percentage of matter, the percentage of dark energy or vacuum energy, and the Hubble rate.

    Make a note of these numbers .27, .73, and 71, and google "cosmos calculator".
    This will get you cosmos calculator,
    http://www.uni.edu/morgans/ajjar/Cosmology/cosmos.html
    which is very much the same except it expects you to put in the numbers .27, .73, and 71. (The professor makes you work a bit, for pedagogical reasons, but then, having done that, there is less work to find the recession rate.)
    In this calculator the vacuum energy fraction is called "cosmological constant Lambda"---it's essentially the same thing by a different name, she wants you to put in .73.

    You will get the same distance answer, say for redshift z = 1.7, as you got with the other calculator. But in this case it will tell you the recession rate as well!

    It will say that the distance was increasing at rate 1.02c back then when the light was emitted by the galaxy.

    And it will say that the distance is increasing at rate 1.12c now today when we receive the light.

    Both calculators will say 15.54 billion light years (for redshift z = 1.7). These distances are what you would measure by timing a radar pulse if you could freeze the expansion process at that particular moment (either the moment when the galaxy emitted the light or the present moment today when we receive the light.).

    Most of the galaxies in the observable universe are at redshifts greater than 1.7. And are at greater distance than 15.54 billion ly, on this scale. As a consequence they are currently receding from us at rates greater than 1.12 c.
    And they were receding from us at rates greater than 1.02 c back at the time they emitted the light coming to us now.


    If you have any trouble understanding, you are welcome to ask more questions here. It also helps to play around some with the calculators. And it helps to read the Scientific American article by Charles Lineweaver that I link to in my signature. Great article. A lot of people have found it helpful.
     
    Last edited: Jan 18, 2010
  7. Jan 18, 2010 #6

    dnr

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    Marcus' answer comes very close (for me) to answering the question, i.e., the universe is expanding faster than c. If this is true, whey do we see any light. Everyting is moving apart from each other at > 1c.
    -----------------------------------------------------
    Here is a new answer from my friend, which addresses the same point:

    "When I said "As the universe expands the fabric of space stretches", I meant that the expansion IS the stretching.

    The rate of stretching of space is the Hubble "constant", H. Today H =
    2.5x10^(-18) s^(-1). If H really was constant, then 1/H would be the time it would take for space to stretch by a factor of 2. The fabric of space stretches by factor z+1 during the time it takes for light from a galaxy of redshift z to get to us. We can see galaxies at z=6 today.
    Space therefore stretched by factor 7 during the time light from those galaxy traveled toward us. Space has stretched by factor 1400 since hydrogen atoms formed. So stretching 14 billion light years worth from
    1 billion years out (a stretch by factor 14) is a modest stretch.

    Until dark energy took over the expansion of the universe, the speed of stretching vs. age of the universe was

    H= 2/(3t)

    where t is the age of the universe. Since dark energy took over, the stretching equation has become more complicated, but the above equation is still pretty accurate."

    -------------------------------

    Which leads me to conclude that the universe may be stretching just fast enough (<1c), that, for example, light (phtons) that were emitted 500 million years after the initial creation are just reaching us.
     
    Last edited: Jan 18, 2010
  8. Jan 18, 2010 #7

    Chalnoth

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    There are two huge problems with this.

    1. Expansion isn't a speed. See what I wrote at the start of my post above.
    2. There is no reason to believe that the big bang singularity was an actual starting point for our universe.

    But the really wrong thing there is that there really doesn't appear to be anything all that special about the speed of the expansion. It could be a fair amount faster or slower, and we still would see the light from the CMB just fine.
     
    Last edited: Jan 18, 2010
  9. Jan 18, 2010 #8

    dnr

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    So, back to my original question. How is it then that we can see light now that occured when the expansion of the universe began ~14 billion years ago. Or why can we see light from objects 10 billion light years away? What is it about the rate of expansion, the speed of light and all the objects in the universe that allows us to calculate (redshift and all) that these objects are xxx light years away? Why do we assume these distances?
     
  10. Jan 18, 2010 #9

    DevilsAvocado

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    If it’s the CMB you are wondering about, there’s a very good answer from Wallace in this thread (starts at post #11):
    https://www.physicsforums.com/showthread.php?t=368057
     
  11. Jan 19, 2010 #10

    Chalnoth

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    Photons don't degrade or decay or anything like that. They just keep on going until they hit something. And because our universe has been so extremely transparent since the CMB was first emitted, most of the photons from that time are still zipping around at the speed of light.

    Well, there are various techniques, and different ones are used depending upon what we are measuring the distance to. For most objects out there, the redshift is very easy to measure. For objects like stars, galaxies, and supernovae, we can merely measure the spectrum of light they emit very carefully. In those spectra, we find emission lines that correspond to certain atoms (or, in the case of cooler objects, molecules). We match the pattern of emission lines to the known emission spectra of these various atoms from laboratory experiments here on Earth. Since different atoms tend to have extremely different emission patterns, we can still determine which atoms are emitting what in these far away objects, even though those emission lines have been redshifted. Performing this comparison gives us a very accurate determination of the redshift of these objects.

    For the CMB, we use a slightly different method, but the end result is the same: the redshift of the CMB, like astronomical objects, is very easy to determine to exceedingly high accuracy.

    Of course, a redshift is not, in and of itself, a measure of distance. In order to calculate a distance from a redshift, we have to know how fast our universe has expanded with time. Fortunately, however, there are independent ways to measure distance. With supernovae, for example, we single out a specific type of supernova, Type IA, based upon the spectra of light they emit. These particular supernovae tend to all be right about the same brightness. As a result, we can use them as a sort of "standard candle": if we know the brightness of the object at its source, then we can measure how bright it appears to us to determine how far away it is.

    So, with supernovae, for instance, we can measure the distance to them through their brightness, and also measure their redshift. But we know that the distance to them and their redshift are related by the expansion history, and so we use supernovae as a way of measuring how our universe has expanded in time.

    With the CMB, we can make use of a different measurement: the typical size of fluctuations. On the CMB, the typical size of fluctuations is about one degree on the sky. Now, the physics of what went on before the CMB was emitted constrains quite precisely how big those fluctuations actually were, and so this gives us an accurate measurement of how far the CMB actually is.

    This sort of pattern repeats itself with a variety of astrophysical and cosmological phenomena: we have independent ways of measuring redshift and distance. By combining different measures of redshift vs. distance, we obtain accurate measurements of how our universe has expanded in time. All of these measurements combine to give a single, consistent picture of the contents and makeup of our universe.

    And once we have a consistent picture of our universe's contents, we can simply plug in a redshift, and compute a distance, based upon General Relativity. We have confidence that this is a valid calculation because whenever we have an independent way of measuring the distance to an object or the contents of our universe, this calculation still gives the right results (to within the experimental error, of course).
     
  12. Jan 19, 2010 #11

    DevilsAvocado

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    (As a layman, I hope I get this right...)

    Wouldn’t it be more 'accurate' to say - this gives us an accurate measurement of how far out in the CMB we see (in the Hubble volume). Since the CMB is everywhere? And we are continually seeing radiation from different (further) regions each passing moment, right?

    This question puzzled me and I think Wallace gave a very good explanation (between post #11 & #18) in this thread:
    https://www.physicsforums.com/showthread.php?t=368057

    "... Think of this like and long line of soldiers lined up in front of you, each a bit further from you than the rest. If they all fire their guns at you, then you'll be hit by a succession of bullets, each subsequent one originating from a location further away than the previous one.
    ...
    when we look at the CMB, we are continually seeing radiation from different 'objects' each passing moment."
     
    Last edited: Jan 19, 2010
  13. Jan 19, 2010 #12

    Chalnoth

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    Well, yes, that is correct. When we talk about the distance to the CMB, what we really mean is the distance to the part of the CMB that we are seeing now. Twenty years ago, the CMB that we saw was the bit of the CMB that was ever so slightly closer than the bit we see today. Every second we see the photons that were emitted just a tiny bit further out.
     
  14. Jan 19, 2010 #13

    DevilsAvocado

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    Great thanks.
    I was really fumbling out there in the dark for awhile. :redface:
     
  15. Jan 19, 2010 #14

    Chalnoth

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    Given the content of your post that you quoted, let me just state that yes, when we take measurements of the CMB, a major part of the effort of determining what the CMB says about cosmology involves determining how much of the light we observe came from the CMB and how much came from stuff between us and the CMB. This problem is, fundamentally, why WMAP measures the CMB in 5 different frequency bands, and why Planck is measuring it in 9 different bands.
     
  16. Jan 19, 2010 #15

    DevilsAvocado

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    Very interesting. I discussed this topic with Wallace (in the same thread) and came to the conclusion that the Milky Way is the biggest 'problem':

    081015_k_5yr_512.png

    Is there a webpage on Planck yet (pictures/info)? When is the results expected?
     
  17. Jan 19, 2010 #16

    Chalnoth

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    Well, on large scales this is true. But on small scales, point and point-like sources like quasars and H-II regions are the nastiest.

    For now, the only public data from Planck is from the First Light Survey:
    http://www.esa.int/esaCP/SEM5CMFWNZF_index_0.html

    No science results have been released as yet, but the plan is to put the first set of science papers out one year after the first year of observation is finished. So that would be around August-September 2011. I am curious to see whether or not the Planck science team will decide to present some preliminary results earlier, however, such as some early maps (our first survey of the whole sky will be finished in about 3 weeks). I suspect not, as there is a lot of work that needs doing before we're confident enough to really use our results for science.
     
  18. Jan 19, 2010 #17

    DevilsAvocado

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    Wow! What a resolution!
    317iko8.png
    This will be like waiting for Santa Claus10! :smile:
    So you are actually working on this as a pro? Cool! :cool:
     
  19. Jan 19, 2010 #18

    Chalnoth

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    Indeed :smile: I work in data analysis in general, and do a few different things in conjunction with the Planck collaboration. But this is a bit far outside of the topic at hand...
     
  20. Jan 19, 2010 #19

    DevilsAvocado

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    :cool: This is very cool! :cool:
    Insider information in the pipeline from the Planck satellite! Let’s stay in contact buddy! :wink:
     
  21. Jan 19, 2010 #20

    Chalnoth

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    Haha, sorry, not gonna happen. There's a privacy policy that all scientists working on Planck sign, so basically all I can do is point you to public information, and perhaps answer a few general questions that any expert in the field of CMB data analysis would know.

    Of course, once the science papers are out, this privacy policy will be finished, and all information will be available.
     
  22. Jan 19, 2010 #21

    dnr

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    I have read and re-read with great interests all the posts that have been spawned from my original post (some of which are now getting way off-topic).

    None of this, however, has answered my original question. CMB, expansion rates, etc., are all extrememly interesting, but I think I understand all of this well enough to say that I do not feel that my original question has been answered. Let me try it in a different way.

    1. Photon is 1 unit from observer
    <)----> o Photon, t=0 (event just occured, observer does not detect it)
    2. Photon arrives at position of observer
    <)----> o Photon, t=>t+1 (observer detects event)
    3. Photon is past position of observer
    o Photon <)----> t=>t+2 (observer can no long can detect photon)

    So, let's say 1 unit is 500 million light-years and we are at "1." We are 500 million light-years from the occurance of the event. Now let's say we are at "3" and that we look 14 billion years back in time and claim to "see" the event. The problem is that the photons from the event went past our observation position at "2" 14 billion years ago.

    No matter how fast the rate of expansion of the universe, the relative position of the observer and the photon have not changed. Do the same photon just keep coming by and we can look at their atomic composition, CMB, red-shift, etc., and based on our proposed history of the universe, say that they came from a particular time? This would seem to yield an answer which says that all matter in the universe is always detectable by any observer.

    (Hum, I think I just anwered my own question. When we observe these photons, which are now 14 billion light years from us, we can determine what the universe was like at that point, because we have a model which tells us what things from that time should look like.)
     
  23. Jan 19, 2010 #22

    Chalnoth

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    Er, what? That's completely wrong. Photons are zipping around at the speed of light, while observers like ourselves are effectively stationary with respect to the average expansion. This statement of yours makes no sense whatsoever, and I can't see how it follows at all from what you wrote before.
     
  24. Jan 19, 2010 #23

    D H

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    This is the heart of your misunderstanding. A photon moves at c relative to the objects in the local space through which the photon is moving.

    Suppose that some superluminal (near-infinite speed) particle exists that enables an observer to "see" photons. Suppose some light source is pulsed on and then off and these superluminal particles are used to track the progress of that pulse of emitted photons over time. As time progresses and the photons go ever further from the emitter, the emitter would eventually "see" that distance between the emitter and the photon pulse is growing at be more than ct, where t is the time since the light source was pulsed on and off. The reason is that the photons are moving at c relative to the space they are in. The photons ride along with the expansion of space.

    While such superluminal particles don't exist, the mathematics does. For a starter, assume the Hubble Constant truly is a constant (but see [thread=241266]this thread[/thread]).

    Suppose at some time t0 some emitter pulses a (very powerful) light source on and off. At that time, some object is located at a distance d0 from the emitter. When, if ever, will that light pulse reach the target object?

    Some nomenclature:
    • [itex]t[/itex] Time, as measured by the emitter.
    • [itex]t_0[/itex] The time at which the light source was pulsed.
    • [itex]d_0[/itex] The distance between the emitter and target at time [itex]t_0[/itex].
    • [itex]H_0[/itex] Hubble's constant, assumed here to be a true constant.
    • [itex]d(t)[/itex] The distance between the emitter and target at time [itex]t[/itex].
    • [itex]x(t)[/itex] The distance between the emitter and the photons at time [itex]t[/itex].

    The distance between the emitter and target grows exponentially with time due to the expansion of the universe:

    [tex]d(t) = d_0 e^{H_0(t-t_0)}[/tex]

    Note that the time derivative of [itex]d(t)[/itex] is

    [tex]\dot d(t) = H_0 d(t)[/tex]

    Another way to write this is

    [tex]H_0 = \frac{\dot d(t)}{d(t)}[/tex]

    In other words, a ratio of a velocity and a distance. This is why you will see Hubble's constant expressed as 71 (km/s)/megaparsec. If you think about it for a bit, this is really just an inverse time constant written in a somewhat funny way.


    The photons are moving at c with respect to the local space they are traversing. The distance between the observer and that local space is expanding. Thus with respect to the emitter, the photons are moving at

    [tex]\dot x(t) = H_0x(t) + c[/tex]

    The generic solution to this differential equation is

    [tex]x(t) = ae^{H_0t} - \frac{c}{H_0}[/tex]

    where a is some arbitrary constant. At time t0, the distance between the emitter and the photons was zero. This means that

    [tex]x(t) = \frac{c}{H_0}\left(e^{H_0(t-t_0)}-1\right)[/tex]

    The photons will hit the target at time t1 when the distance between emitter and photon pulse equals with the distance between the emitter and target, i.e. when x(t1)=d(t1) or

    [tex]\frac{c}{H_0}\left(e^{H_0(t_1-t_0)}-1\right) = d_0 e^{H_0(t_1-t_0)}[/tex]

    Solving for t1,

    [tex]t_1 = t_0 - \frac 1 {H_0}\,\ln\left(1-\frac{d_0H_0}c\right)[/tex]

    Note that
    • That logarithm is negative, so the interception time t1 is after t0, as it should be.
    • The intercept time is imaginary if [itex]d_0\ge c/H_0[/itex]. If the emitter and target are separated by more than this critical distance the photons will never reach the target.
    • As the initial separation approaches this critical distance, the intercept time grows toward infinity. It will take light a *long* time to reach objects that are just within this observability limit.
     
  25. Jan 19, 2010 #24

    DevilsAvocado

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    (I promise to be a nice on-topic guy after this. o:))
    It’s cool anyway. Just to know there is chance to talk to 'the pioneers at the frontline' of our knowable universe makes me happy. PF is an absolutely wonderful place in this way. For many years – when trying to talk to friends and relatives about the Universe, one of three things usually happened:

    1) Anger – "Why not talk about all the problems here on Earth instead!?"
    2) Illness – "Please! Stop talking about the universe, I feel dizzy... I wanna throw up!"
    3) Accusation – "Ehh... are you sure you’re not drunk..."
    :biggrin:

    Thank god for PF! ... or should I say – Thank Einstein! ...
     
  26. Jan 19, 2010 #25

    DevilsAvocado

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    Just to straighten things out: You wrote the questions 1 to 15 in post #1, and your (science) friend wrote the rest, right?

    1. The universe “started” as an almost infinitely dense singularity.
    Yes that’s the theory. Note that it has not been proven, since General Relativity is not compatible with Quantum Mechanics, and at the Planck epoch QM is dominant (and GR breaks down). Also note that universe might be spatially infinite and we only see our 'local' BB.

    2. As this thing expanded, all sorts of interesting things happened, which resulting in stars, galaxies, heavy elements, x-ray background, etc.
    Yes. If you refer to CMB as the "x-ray background", it could be doubtful... since it’s microwave radiation…

    3. As the universe expanded, except for the brief “expansion faster than light” issues, it was expanding at less than the speed of light.
    Wrong. There is no speed limit on the expansion of space.

    4. This means than all of the bundles of photons have been expanding at the same rate as the universe.
    Wrong. Photons always travel at the speed of light 299,792,458 meters per second (in vacuum).

    5. So, all information about what happened over the 14 billion years of the universe has expanding at the same rate.
    Yes, at the speed of light. Please note though; the information (light) has not expanded – it’s the 'fabric of space' that expands... and space itself never carries any information (vacuum)...

    6. This means that there is no “looking” into the past, because this information has been expanding, along with the universe.
    Wrong. We can see the CMB from less than 400,000 years after the Big Bang. Note that we don’t see the CMB exactly as it was 400,000 years after BB, the light has been stretched out, but the 'information' and photons are there, sorry here (now).

    7. The universe then should then be uniform, i.e., the same age, since it has been expanding with “us” since the beginning.
    'Odd' question... we are in the universe, and there is no center. The Cosmological Principle stipulates that the universe is homogeneous (of the same or similar nature) and isotropic (uniform in all directions).

    8. No matter what direction we look in, we should see exactly the same thing.
    Yes and no, on the very large scale we see almost the same thing (pattern). I.e. the Sloan Digital Sky Survey sees a uniform pattern of galaxy clusters in all directions, and The Hubble Space Telescope sees many different forms of galaxies and nebulas on another scale, in all directions...

    9. We should not see stuff from the beginning or 10 billion years ago, because everything has been expanding at the same rate and therefore all bandwidths of information from that source have always been with us.
    Wrong. We can see the CMB from less than 400,000 years after the Big Bang. The most distant astronomical object observed as of 2009 is a gamma ray burst, from a collapsed star when the universe was approximately 600 million years old.

    10. Stuff expanding in the opposite direction from us, is still moving slower than the speed of light, so we should see it exactly as it is now.
    'Odd' question... almost all 'stuff' is moving away from us (except the local group). No 'object' in space can move faster than the speed of light. Expansion of space itself has no speed limit. Now is an 'impossible' word to use in the universe, since there is no universal 'Now'. What we see now (here) is how the object looked like when the photons left the object, billions of years ago.

    11. There is no reason that when we look at increasingly sensitive instruments, e.g., the updated Hubble, that we should see anything but current information, i.e., the wavelengths from everything has been traveling with us since the big-bang.
    See previous 10.

    12. If something happened 10 million years after the beginning of the expansion, then it would look 10+ million years old, but it couldn’t look to be 9 or 8 or 4 or 1 million years old. Those signals arrived long ago.
    We can’t see stars/galaxies 10 million years after the beginning of the expansion. The first formation of stars occurred around 400 million years after BB.

    13. So, if an event occurred 500 million years after the big-bang, then it got to “us” at 500 million years + plus the travel time at the speed of light after it occurred.
    Yes... Space(time) is curved and the light also has to travel 'upstream' the expansion of space. The crucial thing though, is how far the 'event' was from us when it occurred. If it was too far initially, it might never reach us...

    14. This means that nothing we can see occurred more than expansion rate + the speed of light. If something occurred 500 million years after the big-bang, then that information has been with us since that time, plus the speed of light.
    Wrong. Photons never 'freeze'. See previous 13.

    15. Once this information goes past us, then it is gone. So if something happened 1 second after the big bang, then that information in gone after 1+ seconds. It has passed us by. It is gone. We can’t look into the past and see what happened.
    We can’t see anything before the Recombination (377,000 years after BB), but we can look into the past after the Recombination.


    (As a layman I expect the pros to correct me if and when I’m wrong.)

    Pictures have always helped me to understand this complex matter, and I think you could use the picture below to understand how a light ray (red line) can travel an effective distance of 28 billion light years (orange line) in just 13 billion years. This also reveals the fact that observable universe = calculated visible universe.
    300px-Embedded_LambdaCDM_geometry.png
    For more info, read http://en.wikipedia.org/wiki/Expanding_universe#Understanding_the_expansion_of_space" on Wikipedia.


    Another good example is this one, from http://www.astro.ucla.edu/~wright/photons_outrun.html" [Broken].
    cphotons.gif
    The animation shows the expansion of space and the evolution of the galaxy density, positions and the photon positions within a '78 billion light year box'. Each black dot represents a galaxy, and the two green dots are galaxies emitting red photons.
     
    Last edited by a moderator: May 4, 2017
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