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B How can we create a map of the universe?

  1. Dec 17, 2016 #1
    Hello Everyone,

    This is my first post, and I don't have any formal background in cosmology. I'm in the middle of reading a book called 'The Exploration of Outer Space' by Sir Bernard Lovell and I find myself falling in love with this topic!

    I have a question that has gnawed at my head for a long time: How can we create a map of the universe when the light, radio waves, x-rays, etc. that we measure with our measuring instruments here on earth (and in orbit) are actually measuring an extremely large spectrum of time?

    When we create a map here on earth, we are able to place and associate the various parts of a map with each other because we can prove and validate that those things are actually next to each other (by visiting those places), and because of our observation that the most basic fundamentals of physics hold true for us (New York is not going to suddenly fly off into another dimension!).

    However for the various electromagnetic waves that we are measuring here on earth, some represent cosmological entities that existed 14 billion years ago (the farthest reaches of the CMB), and some that existed 8 minutes ago (the sun). It doesn't seem logical to me to try to 'paste' all of these representations together in a single map because whatever the universe looked like (in a particular region) 14 billion years ago has most likely change dramatically in the intervening period. (E.g. Maybe the outer rim of the currently visible universe is starting to come back towards us - just for the sake of argument!)

    It seems to me that if we try to piece together all the measurements we make here in our region of the universe in a given moment in time, that we will have a map that is warped over a period of 14 billion years.

    Thank you in advance for helping an aspiring astronomer! :smile:

    Andrew
     
  2. jcsd
  3. Dec 17, 2016 #2
  4. Dec 17, 2016 #3
    Thank you!

    And thanks for the link to Quora. That's also a nice site full of interesting questions and answers.

    Best Regards,

    Andrew
     
  5. Dec 17, 2016 #4

    Drakkith

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    We have no choice. We are limited by the speed of light and have to map what we can observe, which is obviously more and more outdated as we look further away. But that's also the beauty of it. The finite speed of light means that we can see almost the entire history of the universe just by looking out into space.
     
  6. Dec 17, 2016 #5
    Technically any map created of the earth would be warped over a period of time due to plate techtonics. As I understand it, the only possible near point-in-time maps of the universe are CMB and presumably CvB and GWB eventually. Check out part 1 and 2 of A Poor Man's CMB Primer by @bapowell.
     
  7. Dec 17, 2016 #6
    I did go through the article both part I and II, but I am not sure it answers the OP's original question. The article like most discussions in respect with most efforts trying to explain the 4 dimensional nature of the universe by using the surface of the Earth analogy. I mean you sort of think about it that way but ultimately it's almost impossible trying to visualize 4 dimensions using a three dimension analogy.
    The article in part 3 and 4 use mathematics to talk about the CMB and I think it's probably the only way to understand it.
    I posted a thread awhile back asking how can one create a 4 dimensional navigation device? For example, if you were to travel in a straight line, assuming the universe is close, finite, and does not expand faster than the speed of light, will you come back to where you started? In order to travel in a "straight line" in a 4 dimension space-time, how would you do it?
     
  8. Dec 17, 2016 #7
    I think I recall that discussion, something about a spacetime compass? My understanding is the physics community does not accept the universe as a closed/finite system.
     
  9. Dec 17, 2016 #8
    Hi Andrew.
    I think you will find that GR (General relativity), is the best model we have so far.
     
  10. Dec 17, 2016 #9

    mfb

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    Typical relative velocities (apart from the global, uniform expansion of space) are a few hundred kilometers per second. About 0.1% of the speed of light. If we map a structure 100 million light years across 10 billion light years away, in the worst case we get a 10% uncertainty (0.1*10 billion light years = 10 million light years). If we observe many galaxies at the same time, we can do even better, because we can average over them. We don't know where exactly each galaxy will be today, but we have a good approximation. And ultimately it does not matter for making maps: we just record where they were in the past.
     
  11. Dec 17, 2016 #10
    Before we continue the discussion, I would like to see if we can agree on this. The OP's original question is about the map of the universe whereas the CMB is the map of the cosmic background radiation so there might be subtle differences between the two although they correlate in some way.

    But let get back to the Earth surface analogy, let's assume the people of earth surface trying to make a CMB map, there would be two possible scenario's (at least as far as this analogy goes):
    1. The earth surface is so large, all the far away radiation has not reached the observer.
    2. The earth surface is small enough that all the light has reached the observer.
    In scenario 1, I can imagine the parallel between the earth surface CMB vs. our CMB.
    In scenario 2, how would the people of the earth surface see the earth since the radiation has made a complete round trip?

    Let's assume the universe is small enough(but we know this is not the case) that light has enough time to travel a complete round trip,
    how would the CMB map look like? For example, if an observer tried to draw the CMB map right after the big bang where the universe
    was small enough, how would the CMB map look like?
     
  12. Dec 17, 2016 #11

    mfb

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    We live in scenario 1, otherwise we would see the same CMB patterns at multiple places in the sky.

    The CMB map, seen by some observer, changes over time. The small-scale fluctuations change faster than the large-scale fluctuations, and if we would have a full record we could make a 3D map instead of a 2D map. In a billion years, we'll see different fluctuations because we see the light from matter further away (something like ~43 million light years instead of ~42) at the time the CMB formed.
     
  13. Dec 17, 2016 #12
    Could you elaborate more on this? So at different angles of observation (I guess that is all available to us), we will see the same pattern?

    On the same topic, assume some other observer from a different part of the universe, would he/she eventually come up with the same CMB map?
    I am using the same Earth surface analogy, that is anyone on the Earth surface would see the similar radiation patterns?
     
  14. Dec 18, 2016 #13
    I would like to return to the original question that was posed and ask An other question in return: What would be the meaning of zich a map?
    The problem is causality, we always talk about the speed of light as being the limit but actually what we should say is the speed of causality, what can effect you OR what you can effect on. Given that knowledge, what would the purpose be of such a map?
     
  15. Dec 18, 2016 #14

    Drakkith

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    The purpose of such a map is to increase our knowledge about the universe. Without a map we cannot understand the formation and evolution of the universe nor how its laws work on larger scales.
     
  16. Dec 18, 2016 #15
    Ok but of you dat understand its formation and evolution wouldn't that imply that there needs to be interaction possible, in the past present or future. What does this mean if you are looking at pieces of the universe that aren`t causally linked anymore?
     
  17. Dec 18, 2016 #16

    mfb

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    Our own place, 13 billion years ago, looked like the most distant places we see today, 10 billion years ago it looked like structures a bit closer to us and so on. By understanding how they worked we can see how the universe close to us (where we see the current stage but not the earlier ones) evolved over time.
    Hard to find good references as other observations rule out that the universe is so small. This blog article discusses it a bit.
     
    Last edited: Dec 18, 2016
  18. Dec 18, 2016 #17
    Would this map be one that could aliens 1 billion light years away could also use? Just wondering how Sagan and co mapped the Earth on the golden record for aliens to find us.
     
  19. Dec 18, 2016 #18

    mfb

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    With some inaccuracy and with some missing regions, but in principle, yes.
    The spacecrafts won't even leave our galaxy, and with the current precision the global map looks identical from everywhere in our galaxy (~100,000 light years across).

    The golden record is completely different from the map of the observable universe. It shows the directions (from Sun) to 14 pulsars and the galactic center. It is meant to look different for different positions: an alien civilization finding the drawing could determine where Earth is based on those directions.
     
  20. Dec 18, 2016 #19
    I think it had a way of indicating where the Sun is relation to uniquely identifiable pulsars and other stars
     
  21. Dec 19, 2016 #20

    Chalnoth

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    Yes, this is definitely true. Fortunately, however, most of those distortions are very gradual and easy to measure, while the stronger ones (due to gravitational lensing from massive galaxies/galaxy clusters) are rare.
     
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