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Looking back in time

  1. Jul 31, 2011 #1
    How can we see light from 400K years after the big bang? Are we older than this light? Did we travel faster than this light, away from the big bang?
  2. jcsd
  3. Jul 31, 2011 #2


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    Nobody "traveled away from the big bang" :smile:
    That is the Explosion Picture mistake. It is a common misconception.
    Have a look in the FAQ.

    Or read the "Charley" article that I link to in my sig. It is a well-written SciAm piece about some common popular misconceptions people have about the big bang. The first page is blank, so scroll down.

    Expansion of distances is what it is about, not "traveling".
    Traveling involves going somewhere, towards some destination.
    In pure expansion of distances, nobody goes or gets anywhere. Distances just get longer according to a pattern prescribed by the law of gravity (Einst. 1915 equation)

    If you look around in space, there is no point that you can point your finger at and say "there is the big bang and we are traveling away from it."

    There never has been any such point. We and the matter we are made of have NEVER in the whole history of the universe been traveling away from some point. The "big bang" is a stupid newpaper and pop-sci name for the expansion of distances, and for the beginning of that expansion (still a topic for research, may have been a bounce at high density).
    The start of expansion should not be visualized as an explosion. It was a huge disservice to humanity to give it a name suggesting explosion. Please read the "Charley" link and try to get over this mistaken picture.
  4. Jul 31, 2011 #3
    We are limited to detecting radiation from about 400,000 years after the bang, and not earlier, because charged particles before this time disrupted what is now cosmic microwave background...it was absorbed by charged particles on the loose from inflation in the very early universe. The radiation we now detect all around us, coming from everywhere distant, has experienced "cooling"....meaning the expansion of the universe has increased it's relative wavelength...so now it's only about 2.7 degrees K above absolute zero....

    For a good discussion for everyday readers, no heavy math, relating to Marcus' comments, search online for MISCONCEPTIONS ABOUT THE BIG BANG by Lineweaver and Davis in the March 2005 issue Scientific American.

    "If space were not expanding, the most distant object we could see would be about 14B light years away from us, the distance light could have travelled in the 14 billion (B) years since the big bang."... but because the universe is expanding as light travels towards us and light therefore takes longer to get here, we can see out about 46B light years.

    Another good source online is

    Ned Wright UCLA FAQ Cosmology
    Last edited: Jul 31, 2011
  5. Aug 1, 2011 #4
    every object with mass has "gravity" no matter how far away objects are from each other? this seems to imply that all mass pulls all mass towards each other. if time is infinite doesnt this mean that at some time in the very distant future all mass will eventually unite at some point in space? wherever that may be we would expect to observe what we believe as a classical big bang event?

    i myself do not believe our classical big bang theory of all mass at a single point. i believe the big bang was nothing more than an event in time where an exchange of energy occurred, whether from fabrics of parallel universes touching, or just from a crap load of mass coming together, it wasnt all mass at a single point......., and the big bang was not the beginning of time, but rather an event in time....
  6. Aug 1, 2011 #5
    as i have understood it the light 400,000 years after the big bang that we can see is 13 billion three hundred thousand years older than us.We are seeing VERY VERY OLD LIGHT.
  7. Aug 2, 2011 #6
    Currently the rate of expansion of the Universe is accelerating due to Dark Energy. A big "crunch" can only occur if gravitational attraction>force of expansion. It may be beneficial for you to look into possible fates of the universe.

    Believe is a funny word so you should try to avoid it! There are ideas that propose a pre-bang such as LQC, well researched and analysed. Do you have any links for references reagarding these theories of parallel universes etc? Without some good scientific support you may find these ideas dismissed offhand.
  8. Aug 2, 2011 #7
    Cosmo Novice, my bad, i meant to say hypothesis.

    i am trying to simplify the already existing theories which seem odd and overly complex, like big bang where all mass is at a single point which is also the start of time..... that seems a tad odd to me.

    it makes more "sense" to me that the "big bang" was just some event in time, a timeline that is infinite in the past and future. if we can place a time on the big bang event, and we observe distant light that seems older than the big bang event, then it may be probable that not all mass in the universe was at the big bang event.

    so with a notion of time being infinite in the past, and the big bang was just an event in that time, then it is possible we can observe light which is older than the big bang event.
  9. Aug 3, 2011 #8
    The misconception you have is seeing the BB event as an event in a pre-existing space. Space and time are intrinsically linked, which is why the term spacetime is used. The BB event was the beginning of all time/space/energy/mass not the explosion of mass into an already existing space. Essentially as expansion took hold there were areas (roughtly 1 in a 1000) that had slightly higher average density - these spacetime perturbations eventually became the planets/stars/galaxies/clusters that we see today.

    Although it may make "sense" it does not fit evidence or models of current cosmology. The universe is isotropic on the large scale - the only break in uniformity of the CMB is the 1in1000 due to matter dense pertubations. The BB was the beggining of our entire U, therfore any mass not at the BB would not be part of our U. While I understand your reasons for looking pre BB, the way in which you are looking is currently flawed - the classical model cannot be extended all the way back; as you may know t=0 is not something physicist will discuss, but LQC pushes the model back pre BB without needing to "break" the current model.

    Time in the classical sense can never be pushaed back further than BB and the opaqueness of the early Universe (prior to the last scattering) prevent us seeing all the way back. Essentially the CMB is the limit of what we can see :)
    Last edited: Aug 3, 2011
  10. Aug 3, 2011 #9
    How can the BB have been the begining of Space itself? If nothing existed before that, isn't nothing just and endless empty void? hmmm.that is a mind boggler!
  11. Aug 3, 2011 #10
    Thats the point!

    Space (the distance between masses) is an intrinsic part of our universe, its is not a "void" or a "nonspace". Essentially it is part of the metric of our spacetime.

    This is NOT to say that the universe is expanding into a "void", its is the essence of reality that is expanding. Both to the OP and in response to JLA I would definetely reccomend some of the stickies on this forum as reading - some very helpful posts.

    The Big Bang began in all points of spacetime simultaneously - this is easily seen looking at isotropy of the U on a largescale.

    Just some food for thougt:

    Our Milky Way galaxy is roughly where it started as a matter dense pertubation in the spacetime manifold after the BB event - since then all the other matter dense regions (now other galaxies) have "moved" away from us due to expansion. From their perspective we are the ones who have "moved". For large gravitational bound systems the kinematic motion can be negligeble.

    Hope this helps.
  12. Aug 3, 2011 #11


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    An "endless empty void" is still space. The BB cosmology has the size of the universe at zero at the t=0 event. The BB didn't "fill" space it created space.

    It is not just that "nothing existed before that" but that the very concept of "before that" is invalid. It's like saying "nothing is norther than the north pole". North is defined as toward the north pole.

    Here "before" is by definition closer in time to the BB event....

    Now we can speculate on continuations of time beyond the BB as e.g. cyclic universes and such but since the predictive validity of our theories breaks down before the t=0 singularity we can't really say anything meaningful until we can improve our theory. That's one of the big payoffs of the current quest for a GR QM unification.
  13. Aug 3, 2011 #12
    As I understand it physicists will not discuss t=0, only t>0 has any real meaning as t=0 is mathematical nonsense. It is the fact that a singularity makes no sense that theory is looking to pre bang models or a unification of GR and QG.

    That being said only t>0 and more specifically t>planck has real meaning. If I am incorrect in my assumptions please correct me as I am relatively new to this area.

  14. Aug 4, 2011 #13
    We see light from 400,000 years after the Big Bang because that's how long it took to travel to Earth. 400,000 years after the Big Bang, its light source came into existance, and since then the photons have been travelling at the speed of light towards Earth. By the time they arrive (now), they are a bit "out of date", and show the object they came from as it was in the early stages of the Universe. By now, the object itself will have almost certainly moved in poisition, changed in appearance or disappeared altogether.
    If you mean "we" as humans or the Earth, we are not older than the light, because the photons are about 13 billion years old (the Earth is about 4 billion).
    We didn't travel away from the Big Bang faster than the luminescant object, because we didn't travel away from the Big Bang at all: nothing did. The Big Bang was a simultaneous force that occured everywhere: everything created by the Big Bang was and is the universe.
  15. Aug 26, 2011 #14
    i have a hard time with this one.

    BB is ~13.7 billion yrs old

    light emitter UDFy-38135539 is ~13billion lt yrs away. how does a light emitter get to a distance of 13billion lt years away in just ~700 million yrs ??? a light emitter is of course mass, which we believe was part of the singularity of space-time we know of as the BB.

    this suggests that the galaxy itself formed ~700 million years after the BB but would have to be traveling at the speed of light. how else could it get to that distance?

    and if it take 13 billion years for this emitter light to reach us this suggests that the galaxy itself (from that location) has traveled away for the same amount of time making its distance now 26 billion lt yrs away (older than the BB)!. how is this explained, is there a time dilation?
    Last edited: Aug 26, 2011
  16. Aug 27, 2011 #15


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    There is a huge amount of time dilation between 'now' and the surface of last scattering - around a factor of 1100.
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