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Is it practically possible to look back in time?

  1. Oct 2, 2011 #1
    Yes, I have heard the theory – judging by the distance we know how old the coming picture is. However I don’t understand why if we are looking at an object that is 17 bln light years away we assume that we see it as it was 17bln years in the past, since the speed of the expansion of the universe is higher than the speed of light and it depends on how close the object is to the center.
    Second – I don’t understand why we assume that the objects furthest from us are the ones that are most distant in time as the origin (big bang) must be somewhere in the center.
    And last - why do we think that we can almost see the big bang when it seems to me that we need to be very fortunate to live in just the right time frame that would allow us to do so, as there will need to be some great alignment between the speed of light and the speed of the expansion of the universe and the time frame that we live in.
    Thank you!
     
  2. jcsd
  3. Oct 2, 2011 #2

    Chronos

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    1] The big bang had no center, it happened everywhere in the universe [which was quite tiny at the time].
    2] The big bang is believed to have occured about 13.7 billion years ago.
    3] The initial inflationary epoch of the universe occured much faster than light. It takes time for photons then emitted to traverse the vast distances created by inflation [and the subsequent expansion era].
     
  4. Oct 2, 2011 #3
    Chronos,
    Thank you, but that does not answer my questions directly.
    1. I am refering to the current center of the univrse (since it is inflating - there must be a center that remains at the same position)
    2. I don't understand why you included this line :) (about the occurnce of the big bang)
    3. In the third line you are basically stating sth that I have based my questions on.
    Don't get me wrong I am very happy to recieve your reply, I just want to recieve direct answers.
    Than you!
     
  5. Oct 3, 2011 #4
    Sobadzhiev, what Chronos actually tell you is that your two first questions are wrong.
    Today accepted theory says that the Big Bang had no center, or one can considers every point of Universe as the center of BB. Without a center your questions are pointless.

    About third question: Chronos told you about the initial inflation because it is the reason we can see the BB moment (in fact we can't see nothing before 380.000 years after BB, see http://en.wikipedia.org/wiki/Timeline_of_the_Big_Bang#Photon_epoch" of Universe?
     
    Last edited by a moderator: Apr 26, 2017
  6. Oct 3, 2011 #5

    Drakkith

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    There is no point in space that you could claim is the "center of the universe", nor has there ever been. If you were 10 billion light years from Earth you would have a different observable universe from Earth and be able to see things that we could not see from Earth, while not being able to see other things that we can see from Earth. AKA you would see stuff further on "your side" of the universe, but since you were so far away, you would not see as far as we do on "our side" of the universe. Hope that makes sense.

    Also, we will never be able to see the very beginning of the universe, as it took about 380,000 years from the beginning of the universe for matter to form into neutral atoms and allow photons to propagate without being absorbed and re-emitted.
     
  7. Oct 3, 2011 #6
    This is sound from Drakkith, although I would amend this slightly and rephrase this as:

    "there is no one point in space that you could claim is the "center of the universe""

    and this is because all points are similarly the center of the spatial universe and edge of the temporal universe.

    Isotropy negates the validity of the concept of "centre". Wherever I stand on the face of the Earth I will never be in the center, this is a 2-D analogue of a 3-D space which can be extended further ignoring limitations and using a bit of thoughtfulness!

    Hope this helps emphasize Drakkiths point, what Drakkith says is true there is just a minor difference of Semantics which can sometimes be important.
     
  8. Oct 3, 2011 #7
    Because Universe are expanding since its beginning we can say that now the Observable Universe has more "points" than had 13,3 billion years ago. In other words, we can say that now some "points" exists that did not exist in the past.

    This is way I don't like (and I don't use) the expression "all points of Universe can be considered as center of Universe". Instead I use the phrase "Universe has no center at all". IMHO
     
  9. Oct 3, 2011 #8

    Drakkith

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    Just realize that each point is the center of it's own observable universe, but this is not the same as the whole universe.
     
  10. Oct 3, 2011 #9

    Chronos

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    We will never see photons emitted before t~380,000 years [behind the surface of last scattering]. Neutrinos and gravity waves are a different matter. We could theoretically 'see' back almost to the original big bang itself by these methods - which is one of the reasons scientists are preoccupied with detecting gravitational waves. The problem is neutrinos and gravity waves are very hard to detect.
     
  11. Oct 4, 2011 #10
    I guess I need to get myself familiar with more information on this matter.
    But just to clarify - as a center of the universe I did not mean the center of the observable universe, but the point that is situated at the same distance from all the edges (if we assume there are edges) of the universe. For example if the Earth was the universe - the center would be the very center of Earth's core. Since the universe is inflating - common logic suggests that there is a point that remains at the same position while everything else is moving further and further.
     
  12. Oct 4, 2011 #11

    Drakkith

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    That is one of the most common misconceptions about the universe that there is! According to current models, there is no edge and no center! Or to be more precise, current models do not require the universe to have an edge of a center. Do we KNOW that it doesn't? Nope.

    And if you think "common logic" is broken pretty badly in cosmology, take a look at Quantum Mechanics.
     
  13. Oct 4, 2011 #12
    You are imagining it wrong. If the Earth was the universe - if you want to use the analogy - you don't take it as 3D but as 2D, e.g. only the surface. For the sake of the analogy we will ignore leaving the surface on the Z-axis, and regard it only having X- and Y-Axis. The point being, you can move in any of these two directions as long as you want, you will never reach any kind of edge. Eventually, you return to your starting point if you don't change directions ever after starting.

    You also must not imagine that the universe has an edge that is moving ahead. Space itself expands, the room between points is expanding, there is no leading edge. If the universe doubles in size, every distance you could measure from any point to any other also doubles. Google for "expanding universe raisin bread analogy" or "expanding universe raisin balloon analogy". The point you are looking for, the point that is the same distance away from any point of the edge - it simply does not exist. You are probably imagining that the universe is expanding "into something", into some kind of super-darkness or super-void. That is not true, there isn't anything (as far as we know) to expand "into".

    The center of the observable universe is quite well defined - it is of course the Earth, from our point of view.
     
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