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B Can we see our past?

  1. Dec 5, 2018 at 1:59 PM #1
    The Milky Way is about 13.5 billion years old. The Hubble deep field observation could see galaxies which existed only a billion years after the Big Bang. If the Hubble telescope was pointed in the right direction, could it see the Milky Way when it was only 0.8 billion years old?
    Participants: ibix
     
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  3. Dec 5, 2018 at 2:08 PM #2

    PeroK

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    Not directly. We can see other parts of the Milky Way only up to thousands of years ago.
     
  4. Dec 5, 2018 at 2:09 PM #3

    phyzguy

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    No. The light that left the Milky Way billions of years ago has now propagated billions of light years out into space and is no longer observable by us.
     
  5. Dec 5, 2018 at 2:34 PM #4
    I can see why the previous posts gave the answers they did. Put my question another way. Was the "young" 0.8 billion year old Milky Way one the galaxies which existed only a billion years after the Big Bang? If so, why is the Milky way different from the other galaxies which the Hubble deep field telescope can see which existed only a billion years after the Big Bang?
     
  6. Dec 5, 2018 at 2:37 PM #5

    PeroK

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    It's closer. The Sun, for example, is only 8 minutes away. The light from the Sun any older that than is travelling away from us; not towards our telescopes.
     
  7. Dec 5, 2018 at 2:54 PM #6
    This must mean that the Hubble deep field observation cannot see all of the galaxies which existed only a billion years after the Big Bang. There must have been many, many more galaxies than the huge amount observed in a very small region of the sky. Would this vast amount of early universes be more than the current estimate of Universes which exist now?
     
  8. Dec 5, 2018 at 3:05 PM #7

    PeroK

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    A lot will have happened in our observable universe that we will never see. In order to see something 13 billion years old, it must have been about 13 billion light years away at that time. A galaxy closer than that can only be seen from a more recent time. 10 billion years, 5 billion years, 2 million years (in the case of the Andromeda galaxy) and 8 minutes (in the case of the Sun). You can only look back in time by looking a long distance away as well.
     
  9. Dec 5, 2018 at 3:34 PM #8
    Yes, I can see what you mean. We are actually at the centre of the Universe, just like everything else is. I'm not trying to be argumentative, but I've had a confusing thought. If we look out in all directions we can see galaxies 12.7 billion light years out in any direction we look. It's as thought they are imprinted on the inside of a sphere with a radius of 12.7 billion light years, and we are at the centre of the sphere. This seems to indicate that the Universe was at least 25.4 billion light years in diameter only a billion years after the Big Bang. I thought that the Universe was much smaller than this only a million years after the Big Bang. I promise not to ask you any further questions if you can please explain to me why this is not so.
     
  10. Dec 5, 2018 at 3:48 PM #9

    PeroK

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    The universe is possibly infinite, we only know about the observable universe.

    One complicating factor is that the universe is expanding. This means that if something is, say, 5 billion light years away today, it will take longer than 5 billion years for the light to reach us, as space is expanding while the light is travelling towards us.
     
  11. Dec 5, 2018 at 4:27 PM #10
    Thank you for explaining that. Einstein's light speed speed limit is not compromised because space is expanding rather than the velocity of light being less than c. At least we can set a limit to the size of the Universe. The diameter of the "sphere" I was talking about can only be uncreased to a maximum of 13.7 billion light years. This is because the only thing that happened 13.7 billion years ago was the Big Bang. Nothing existed before 13.7 billion years ago and so there would be nothing to see.
     
  12. Dec 5, 2018 at 4:48 PM #11

    CWatters

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  13. Dec 5, 2018 at 5:45 PM #12

    phyzguy

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    I don't understand your question. Yes, there was a young Milky Way which existed 0.8 billion years after the Big Bang. We believe it is a typical galaxy, so if we had a snapshot of it from that time, it would look similar to the galaxies seen in the Hubble deep field. Why do you think it is different? Your question seems to me like asking why an adult looks different from a class full of children.
     
  14. Dec 5, 2018 at 7:41 PM #13

    Grinkle

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    If the universe is infinite in extent, then looking far enough out I'd expect we could observe light from galaxies similar to our own all the way to as soon after the surface of last scattering such galaxies formed - is that not correct?

    It makes sense to me that events which occurred outside our light cone we can never observe (I think this is what you are saying).
     
  15. Dec 6, 2018 at 6:18 AM #14
    PeroK understands my question. If you ask PeroK he will explain my question to you. Basically it seems that whatever direction we look we can only ever see past events. The further we look in light years, the younger the Universe appears to us because it has taken so long for the light to reach us. We can now see galaxies which appear to us as though they are only a billion years after the Big Bang. If we look up, these young galaxies are 12.7 billion light years away from us. If we look in the opposite direction, these galaxies also look as though they are 12.7 billion light years away us. This means that the distance between the the two observations is 25.4 billion light years. This seems a rather large distance when the Universe was only one billion years old. This is the only bit of my question I don't yet understand, but I will give it a lot of thought.
     
  16. Dec 6, 2018 at 6:37 AM #15

    PeroK

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    There was a period of rapid inflation in the Big Bang Theory:

    https://en.wikipedia.org/wiki/Inflation_(cosmology)

    However, if the universe is infinite, then it has always been infinite; and hence was infinite as far back towards the big bang as you go.
     
  17. Dec 6, 2018 at 8:25 AM #16

    Grinkle

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    Making more wordy what PeroK says in post 15 -

    There is a limit to the size of the observable universe. Its hard for me to visualize an infinite-in-extent universe going from very very dense to less dense. The size of our present day observable universe was about that of a softball pre-inflation, according to some popular science characterizations I have read and you can find if you google. Softball or grain of sand or 1km sphere is not the main point - the main point being to picture the softball size early universe as a small piece of similar really dense stuff that is infinite in extent, all of which expands / inflates. Today we are limited observing that blown-up primordial softball, but its still surrounded by an infinite expanse of similarly blown up stuff, if the universe is infinite in extent.

    If the cosmic background radiation were ever to suddenly cease, I think (I may be wrong) that would indicate that the universe is not infinite in extent, or at least that not all of it expanded like our neighborhood appears to have.
     
  18. Dec 7, 2018 at 7:22 AM #17
    What puzzles me in this debate: where do we suppose that our Galaxy is in relation to where the Big Bang happened = is the BB where the centre is and are all galaxies moving away from that centre? Hence is a younger Galaxy closer to that (virtual) centre. Because if this is true, it makes a difference in which direction you observe the cosmos. Not all galaxies move away from the observer (we) at the same speed; some are moving away from the centre in the direction that we we do, some at the other side of the (virtual) centre, opposite of ours at double speed; maybe my perception of the big bang as a the creation of a big sphere is wrong. I don’t know.
     
  19. Dec 7, 2018 at 9:55 AM #18

    PeroK

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    The big bang was a fast expansion of all space. It had no centre. It happened everywhere. There are many threads on here about it.
     
  20. Dec 7, 2018 at 10:19 AM #19

    phyzguy

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    Note that a period of rapid expansion like inflation is not needed to understand your issue. It is simply a consequence of the fact that the universe is expanding. As you said, we see young galaxies in all directions whose light has taken 12.7 billion years to reach us. These galaxies have a redshift z of about 6. But the universe is expanding, and this is characterized by a scale factor, usually denoted by a. We take a=1 today, and we can write that a = 1/(1+z). So the scale factor of the universe 12.7 billion years ago when that light was emitted was only 1/7. So the universe was only 1/7 as big at that time. So those galaxies were much closer to us (and to each other) when the light was emitted. If you want to calculate how much closer they were, we have to agree on what distance measure you want to use, because there are numerous ways to define distance in an expanding universe.
     
  21. Dec 7, 2018 at 4:14 PM #20

    davenn

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    not quite .... We are at the centre of our observable universe. You move a billion lightyears in "xxx" direction and you are now at the centre of your
    observable universe from that point.
    The universe has no centre.
     
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