Register | Sign In


Understanding through Discussion


EvC Forum active members: 63 (9161 total)
2 online now:
Newest Member: popoi
Post Volume: Total: 915,585 Year: 2,842/9,624 Month: 687/1,588 Week: 93/229 Day: 4/61 Hour: 0/0


Thread  Details

Email This Thread
Newer Topic | Older Topic
  
Author Topic:   Question About the Universe
Stile
Member
Posts: 4295
From: Ontario, Canada
Joined: 12-02-2004


(5)
Message 31 of 373 (679130)
11-12-2012 3:15 PM
Reply to: Message 29 by Lurkey
11-11-2012 7:17 AM


Cosmological Event Horizon
Lurkey writes:
If there was another me on another earth just over the CH and he shot a satellite at me so that it outpaced my shrinking CH, would i eventually see it blue shift into existence (if i was very patient)?
Or would i never see the satellite because he just cannot chuck it fast enough?...like there is too much expanding universe between me and him 'pushing back' if you know what i mean.
You would never see it. Your CH (Cosmological Horizon) basically represents the point where space is expanding so much that even something travelling at the speed of light will never reach you.
The only way you would ever see it, is if he could "chuck it" faster than the speed of light. Since the speed of light is the speed limit of the universe, this is impossible.
Not only would you never see it, it would also never get to you. In fact, it would only get further and further away from you
I think it's best to describe in terms of units.
Speed of light has the units: m/s (meters per second)
Expansion of the universe has the units: (meters per second per meter)... or the "speed of the expansion" per "distance of space that is expanding"
Here's a (really long) example that kind of shows the math behind it, if that helps:
Example: Think of an ant walking along a rubber band that is being stretched out.
  • Let's say the ant walks at 1 inch/s
  • Let's say that each inch of rubber expands at 0.1 inch/s
  • Let's mark two points on the rubber band that are 3 inches apart.
  • Say the ant starts walking from one point to the other...
    After 1 second:
    -The 3 inches from the ant to the other mark has expanded by 0.3 inches
    -Therefore, the ant had to walk 3.3 inches to the other mark
    -The ant has walked 1 inch
    -Therefore, the ant has not yet reached the other mark, the distance between the ant and the other mark is now 2.3 inches.
    After 2 seconds:
    -The 2.3 inches from the ant to the other mark has expanded by 0.23 inches
    -Therefore, the ant had to walk 2.53 inches to the other mark
    -The ant has walked 1 inch
    -Therefore, the ant has not yet reached the other mark, the distance between the ant and the other mark is now 1.53 inches.
    After 3 seconds:
    -The 1.53 inches from the ant to the other mark has expanded by 0.153 inches
    -Therefore, the ant had to walk 1.683 inches to the other mark
    -The ant has walked 1 inch
    -Therefore, the ant has not yet reached the other mark, the distance between the ant and the other mark is now 0.683 inches.
    After 4 seconds:
    -The 0.683 inches from the ant to the other mark has expanded by 0.0683 inches
    -Therefore, the ant had to walk 0.7513 inches to the other mark
    -The ant has walked 1 inch
    -Therefore, the ant has reached the other mark, and actually walked passed it by 0.2487 inches.
  • Now we're going to do the same thing, but instead of marking two points 3 inches apart, we're going to mark two points 30 inches apart:
    After 1 second:
    -The 30 inches from the ant to the other mark has expanded by 3 inches
    -Therefore, the ant had to walk 33 inches to the other mark
    -The ant has walked 1 inch
    -Therefore, the ant has not yet reached the other mark, the distance between the ant and the other mark is now 32 inches!
    After 2 seconds:
    -The 32 inches from the ant to the other mark has expanded by 3.2 inches
    -Therefore, the ant had to walk 35.2 inches to the other mark
    -The ant has walked 1 inch
    -Therefore, the ant has not yet reached the other mark, the distance between the ant and the other mark is now 34.2 inches!
    ...and it doesn't matter how much longer the ant is walking towards the other mark, it's getting further and further away, he'll never get there, it's impossible.
    It's not so much that there's "too much expanding universe pushing back" just that there's simply "too much expanding universe to overcome at the speed of light."
    The Cosmological Horizon depends on 2 things:
    1. The speed of travel (speed of light vs walking speed of the ant)
    2. The speed of the expansion of the universe (currently about 74.2 km/sec/Mpc for our universe vs 0.1 inches/sec/inch for the ant)
    Our CH is about 46 billion light years (vs 10 inches for the poor ant...).
    The "area of the universe" between you and your keyboard is actually expanding at the exact same rate. It's just that there's so little "space" there that you can easily overcome it's expansion with the movement of your arms (or to even notice). For something billions of light years away... there's just a lot more "space" in between us that's all expanding to overcome.
    Some Dude's Blog on the Speed of the Expansion of the Universe
    Wikipedia - Cosmological Horizon
    Oh, and if you're wondering... the expansion of our universe (74.2 km/sec/Mpc) is about the same as:
    2.4x10^-18 inches/sec/inch, or again written as:
    0.000 000 000 000 000 002 4 inches/sec/inch
    Edited by Stile, : Converting expansion of our universe into inches/sec/inch bitches!!
    Edited by Stile, : Corrected title 'cause it was wrong

  • This message is a reply to:
     Message 29 by Lurkey, posted 11-11-2012 7:17 AM Lurkey has replied

    Replies to this message:
     Message 33 by Lurkey, posted 11-14-2012 6:24 PM Stile has seen this message but not replied

      
    Omnivorous
    Member
    Posts: 3973
    From: Adirondackia
    Joined: 07-21-2005
    Member Rating: 6.9


    (1)
    Message 32 of 373 (679609)
    11-14-2012 6:21 PM


    The shape of the universe?
    Lacking the requisite math, I rely on analogies to reach some small degree of understanding about the Big Bang and other cosmological marvels.
    When the pieces come together (determined mostly by being able to rephrase the analogy in a way an expert finds acceptable), it is exhilarating: I often star gaze, and sometimes mull over what I understand about what I see.
    So, to my question and current hope for enlarged understanding: Is the universe more or less spherical? Does the expansion of space following the Big Bang occur in spherical symmetry?
    Or are we lumpish?
    The explanatory diagrams I've seen show the various cosmic epochs as unfolding in a sort of expanding trumpet shape--is this merely a matter of graphical convenience?
    Does the universe have a center away from which everything is accelerating?
    I realized I had not the slightest idea about the answers to these questions the other night after hiking to a nearby mountain top to watching for taurid meteors. They were few, but grand.

    "If you can keep your head while those around you are losing theirs, you can collect a lot of heads."

    Replies to this message:
     Message 37 by Son Goku, posted 11-15-2012 7:27 AM Omnivorous has replied

      
    Lurkey
    Junior Member (Idle past 4078 days)
    Posts: 11
    Joined: 11-03-2012


    Message 33 of 373 (679610)
    11-14-2012 6:24 PM
    Reply to: Message 31 by Stile
    11-12-2012 3:15 PM


    Re: Cosmological Horizon
    Thanks Stile
    So that sounds reasonable. CH is a bit like earth’s.it’s all about where i stand.the horizon is real, but in a way its an artifact.
    But then the satellite won’t ever catch up! & Ned’s comment:
    The "speed of light" isn't comparable to a speed limit on a highway. It is a fact about the nature of the thing called spacetime.
    Seems all^ makes the CH very real and physical. Like you’ve just defined the edge. There is no shift of position that will ever enable me to see over it (?!)Every star in a whole galaxy’s worth of galaxies could go supernova just over my CH, and it would make zero difference to me, ever.no matter what I did (?!)
    Hey? Are we closed then? But its a horizon!!! If we’re in a pocket, then isn’t every other point in the universe in overlapping pockets? Don’t we shift from one pocket to another just by moving through our own pocket? Oh no! Lost again.
    Say then you and I got together in the early days and each took one end of that rubber band and we glued the poor old ant to the middle..space has expanded since and we’re now over each other’s horizon.Is the band now infinitely stretched? What do you experience if I let go? Where is the ant?
    Sorry if I’m being dense. It fascinates me, but cosmology is like a cupboard of horrors sometimes. You know, open the draw, shout with horror, slam the door shut again. Not the best way to learn, but to be fair you guys throw out some pretty full on things. A horizon that is coming to get me!
    Anyways. Thanks for this too Stile, i WAS wondering:
    Oh, and if you're wondering... the expansion of our universe (74.2 km/sec/Mpc) is about the same as:
2.4x10^-18 inches/sec/inch, or again written as:
0.000 000 000 000 000 002 4 inches/sec/inch
    .
    Take care,
    Lurkey
    ....
    EDIT: since posting, found this which helped a little:
    http://www.mathpages.com/home/kmath666/kmath666.htm
    One benefit of presenting horizons in a spacetime diagram, rather than in a purely spatial diagram, is that it makes clear the transitive nature of the partial ordering of events. In other words, if event b is inside the event horizon of event a, and if event c is inside the event horizon of b, then c is inside the event horizon of a. Lack of clarity on this point sometimes leads to confusion over the fact that galaxy C can be outside the horizon of galaxy A while it is inside the horizon of galaxy B, which is still inside the horizon of A. If horizons were purely spatial, this would seem to imply that C could send a signal to B, which could then relay the signal on to A, and hence C can send a signal to A, contradicting the fact that C is outside the event horizon of A. Needless to say, this reasoning is invalid, because it overlooks the fact that if C is outside the event horizon of A, then, even if B is presently inside the event horizon of A, by the time a signal from C reaches B, the latter must have passed outside of the event horizon of A.
    Edited by Lurkey, : No reason given.

    This message is a reply to:
     Message 31 by Stile, posted 11-12-2012 3:15 PM Stile has seen this message but not replied

    Replies to this message:
     Message 34 by Dr Adequate, posted 11-15-2012 5:47 AM Lurkey has replied
     Message 40 by cavediver, posted 11-15-2012 10:16 AM Lurkey has not replied

      
    Dr Adequate
    Member (Idle past 275 days)
    Posts: 16113
    Joined: 07-20-2006


    (1)
    Message 34 of 373 (679661)
    11-15-2012 5:47 AM
    Reply to: Message 33 by Lurkey
    11-14-2012 6:24 PM


    Re: Cosmological Horizon
    Seems all^ makes the CH very real and physical. Like you’ve just defined the edge.
    No, not really. Imagine an ant jogging as fast as it can on an expanding rubber band. Now, if we know the speed of the ant and the rate of the expansion, then there is a certain point on the band that we could identify and say "the ant will eventually traverse every point on the band up until there, and will never traverse any point on the band beyond there". And yet there is no edge. There's nothing special about that point of the rubber band, because it only exists relative to the point that that particular ant started from. Another ant that started an inch to the right on the rubber band would run right over that point and not even notice that there was anything special about it. Because there isn't.
    And it's the same with us. Physics tells us that there are limits to how much of the universe we can see and explore, and that even if we could travel at the speed of light we could only reach such-and-such a galaxy and no galaxy beyond it. And yet there is no edge. A race of aliens living a billion lightyears away from us could see stars beyond our "edge" and (with sufficiently advanced technology) could cross that "edge", and they would never notice that they'd done anything special, or "crossed an edge", because the edge is only an edge for someone starting from Earth. It is otherwise completely unremarkable.
    As with the ants, the "edge" only marks the point that we can't get to if we start from here. There is nothing special or remarkable about it, it's only the "edge" with respect to the place where we are, not with respect to the Universe.
    Edited by Dr Adequate, : No reason given.

    This message is a reply to:
     Message 33 by Lurkey, posted 11-14-2012 6:24 PM Lurkey has replied

    Replies to this message:
     Message 35 by vimesey, posted 11-15-2012 5:58 AM Dr Adequate has replied
     Message 41 by Lurkey, posted 11-19-2012 7:53 AM Dr Adequate has not replied

      
    vimesey
    Member
    Posts: 1398
    From: Birmingham, England
    Joined: 09-21-2011


    (1)
    Message 35 of 373 (679662)
    11-15-2012 5:58 AM
    Reply to: Message 34 by Dr Adequate
    11-15-2012 5:47 AM


    Re: Cosmological Horizon
    I wonder if that means that we can meet halfway the creationist point of view that the Earth is at the centre of the expanding universe. It's at the centre of the limit of our perception of the expanding universe.

    Could there be any greater conceit, than for someone to believe that the universe has to be simple enough for them to be able to understand it ?

    This message is a reply to:
     Message 34 by Dr Adequate, posted 11-15-2012 5:47 AM Dr Adequate has replied

    Replies to this message:
     Message 36 by Dr Adequate, posted 11-15-2012 7:15 AM vimesey has replied

      
    Dr Adequate
    Member (Idle past 275 days)
    Posts: 16113
    Joined: 07-20-2006


    Message 36 of 373 (679670)
    11-15-2012 7:15 AM
    Reply to: Message 35 by vimesey
    11-15-2012 5:58 AM


    Re: Cosmological Horizon
    I wonder if that means that we can meet halfway the creationist point of view that the Earth is at the centre of the expanding universe. It's at the centre of the limit of our perception of the expanding universe.
    No, it doesn't mean that.
    If a looney thought that he was at the center of a flat Earth, would it support his point to explain to him that (in a flat landscape) he must always perceive himself to be at the center of his view of the Earth as limited by his horizon?

    This message is a reply to:
     Message 35 by vimesey, posted 11-15-2012 5:58 AM vimesey has replied

    Replies to this message:
     Message 38 by vimesey, posted 11-15-2012 7:36 AM Dr Adequate has not replied

      
    Son Goku
    Inactive Member


    (1)
    Message 37 of 373 (679672)
    11-15-2012 7:27 AM
    Reply to: Message 32 by Omnivorous
    11-14-2012 6:21 PM


    Re: The shape of the universe?
    Does the universe have a center away from which everything is accelerating?
    Catholic Scientist's analogy is useful here:
    EvC Forum: Size of the universe
    (In fact it's not even a full analogy, it's just making the properties of a four dimensional space/shape, the universe, more obvious by using a two-dimensional space/shape that shares some of those properties)
    So the Big Bang is the North pole and every point in time after that is a circle of constant latitude. A circle of latitude being all of space at one time. As you the circles get larger the further south you go, so the spatial dimensions of the universe increase the further along in time you go.
    Since the circles represent the whole universe at a given time, a point in space at a given time is like a point on a given circle.
    So phrase your question in terms of this analogy:
    The centre of the universe from which everything is accelerating, would be the equivalent of some special point on one of the latitude circles. However there is no special point on these circles that the circles are expanding way from, they are expanding "away" from the North Pole
    Simply as a consequence of being circles on a sphere.
    Similarly the universe is not expanding away from some point where the Big Bang happened. It's just that space is getting larger as you move forward in time (down the sphere).
    Again, if you reverse time the circle shrinks and all points on the circle converge on the North Pole, so the North Pole "happens" at all points on the circle. Similarly the Big Bang happened everywhere.

    This message is a reply to:
     Message 32 by Omnivorous, posted 11-14-2012 6:21 PM Omnivorous has replied

    Replies to this message:
     Message 39 by Omnivorous, posted 11-15-2012 9:35 AM Son Goku has not replied

      
    vimesey
    Member
    Posts: 1398
    From: Birmingham, England
    Joined: 09-21-2011


    Message 38 of 373 (679674)
    11-15-2012 7:36 AM
    Reply to: Message 36 by Dr Adequate
    11-15-2012 7:15 AM


    Re: Cosmological Horizon
    Oh, I wasn't intending to support the looney's point - I was wondering if we could re-frame the looney's terms of reference, so that the looney's opinion more closely resembled reality.
    (Still, if I pursue this, it'll be off-topic, so I'll leave it for the time being).

    Could there be any greater conceit, than for someone to believe that the universe has to be simple enough for them to be able to understand it ?

    This message is a reply to:
     Message 36 by Dr Adequate, posted 11-15-2012 7:15 AM Dr Adequate has not replied

      
    Omnivorous
    Member
    Posts: 3973
    From: Adirondackia
    Joined: 07-21-2005
    Member Rating: 6.9


    Message 39 of 373 (679693)
    11-15-2012 9:35 AM
    Reply to: Message 37 by Son Goku
    11-15-2012 7:27 AM


    Re: The shape of the universe?
    Son Goku writes:
    Similarly the universe is not expanding away from some point where the Big Bang happened. It's just that space is getting larger as you move forward in time (down the sphere).
    Again, if you reverse time the circle shrinks and all points on the circle converge on the North Pole, so the North Pole "happens" at all points on the circle. Similarly the Big Bang happened everywhere.
    Thank you. That is quite helpful.

    "If you can keep your head while those around you are losing theirs, you can collect a lot of heads."

    This message is a reply to:
     Message 37 by Son Goku, posted 11-15-2012 7:27 AM Son Goku has not replied

      
    cavediver
    Member (Idle past 3634 days)
    Posts: 4129
    From: UK
    Joined: 06-16-2005


    (1)
    Message 40 of 373 (679696)
    11-15-2012 10:16 AM
    Reply to: Message 33 by Lurkey
    11-14-2012 6:24 PM


    Re: Cosmological Horizon
    If we’re in a pocket, then isn’t every other point in the universe in overlapping pockets?
    Yes
    Don’t we shift from one pocket to another just by moving through our own pocket?
    No - each "pocket" as you call it is defined by a point in space *and* time. You are not free to move to any other pocket, only those that can be reached by you travelling at less than the speed of light.
    Someone in a neighbouring pocket has areas of the Universe within his horizon that are not within yours. He can potentially reach these regions - you cannot. But for him to do so, he needs to set off *now*, otherwise those regions will be lost to him. By the time you manage to get to where he is now, those regions will be long gone. Make sense?
    Oh, and just to correct some common confusion - we're not talking about the Cosmological Horizon (or Particle Horizon) here but the (Cosmological) Event Horizon.

    This message is a reply to:
     Message 33 by Lurkey, posted 11-14-2012 6:24 PM Lurkey has not replied

    Replies to this message:
     Message 43 by Omnivorous, posted 03-31-2013 12:16 PM cavediver has not replied

      
    Lurkey
    Junior Member (Idle past 4078 days)
    Posts: 11
    Joined: 11-03-2012


    Message 41 of 373 (680343)
    11-19-2012 7:53 AM
    Reply to: Message 34 by Dr Adequate
    11-15-2012 5:47 AM


    Re: Cosmological Horizon
    Thanks everyone
    yeah CD, just by themselves all the names are confusing enough!
    Physics tells us that there are limits to how much of the universe we can see and explore, and that even if we could travel at the speed of light we could only reach such-and-such a galaxy and no galaxy beyond it. And yet there is no edge.
    No information flow. No casual contact. This just sounds so much like an edge!
    But i know, i know... that's^ my problem. I think i'm making some headway but its taking me ages...one thing leads to another, you know?
    Hopefully i'll get there one day.
    By the way, I like the idea that the event horizon is our universe's ultimate light cone.
    Also the whacky one where we're living in an inside out black hole
    Hey and i'll cheer any extremely brave and patient person who cares to try and dumb this down for me:
    http://ned.ipac.caltech.edu/...ineweaver/Figures/figure1.jpg

    This message is a reply to:
     Message 34 by Dr Adequate, posted 11-15-2012 5:47 AM Dr Adequate has not replied

      
    Dogmafood
    Member (Idle past 339 days)
    Posts: 1815
    From: Ontario Canada
    Joined: 08-04-2010


    Message 42 of 373 (694953)
    03-31-2013 10:56 AM


    Madly off in all directions
    The expansion of space is detectable by the red shifting of light. This is expansion along the vector that the light is travelling. Can we see the expansion of space orthogonal to that vector?
    To put it another way, would 2 streams of photons that are parallel at their sources remain parallel across big space?

    Replies to this message:
     Message 44 by Stile, posted 04-01-2013 9:13 AM Dogmafood has replied

      
    Omnivorous
    Member
    Posts: 3973
    From: Adirondackia
    Joined: 07-21-2005
    Member Rating: 6.9


    (1)
    Message 43 of 373 (694958)
    03-31-2013 12:16 PM
    Reply to: Message 40 by cavediver
    11-15-2012 10:16 AM


    Re: Cosmological Horizon
    At long last, "You can't get there from here" is no joke.

    "If you can keep your head while those around you are losing theirs, you can collect a lot of heads."

    This message is a reply to:
     Message 40 by cavediver, posted 11-15-2012 10:16 AM cavediver has not replied

      
    Stile
    Member
    Posts: 4295
    From: Ontario, Canada
    Joined: 12-02-2004


    Message 44 of 373 (695002)
    04-01-2013 9:13 AM
    Reply to: Message 42 by Dogmafood
    03-31-2013 10:56 AM


    Re: Madly off in all directions
    My Guess:
    Prototypical writes:
    Can we see the expansion of space orthogonal to that vector?
    No.
    would 2 streams of photons that are parallel at their sources remain parallel across big space?
    Yes.
    Movement is relative.
    Light is only moving in one direction. Therefore, if the two rays/streams are parallel, there is no movement "toward" or "away" from each other.
    Therefore, we can consider them as if they are stationary with respect to each other (as far as their "parallel-ness" is concerned).
    Therefore, they will "grow apart" as the space in between them expands. However, they will remain parallel.
    They will never overlap. Or cross.
    ...or, that's my guess, anyway.
    Maybe if there was somehow a constant stream of two light rays running parallel (not sure how this could happen naturally, but let's just say it did).
    We could measure the distance between them near the source and get some value.
    And measure the distance between them at some great distance away from their source and get a larger value?
    ...or, perhaps, this is all impossible because of a light ray's (or lazer's) natural tendency to diffuse out like a flashlight beam (get wider the further from the source...) anyway. Therefore, it might not be possible to measure the distance between two beams since they wouldn't stay as a straight, solid "beam" of light. Therefore, the two parallel streams would overlap at some point. Like two conical sources of light... like a car's headlights.
    So maybe I won't answer your questions at all, and just talk about some stuff for a bit.
    Chew on that!

    This message is a reply to:
     Message 42 by Dogmafood, posted 03-31-2013 10:56 AM Dogmafood has replied

    Replies to this message:
     Message 45 by NoNukes, posted 04-01-2013 12:24 PM Stile has seen this message but not replied
     Message 46 by Dogmafood, posted 04-02-2013 6:54 AM Stile has replied

      
    NoNukes
    Inactive Member


    Message 45 of 373 (695014)
    04-01-2013 12:24 PM
    Reply to: Message 44 by Stile
    04-01-2013 9:13 AM


    Re: Madly off in all directions
    Light is only moving in one direction. Therefore, if the two rays/streams are parallel, there is no movement "toward" or "away" from each other.
    Only in flat space... In non Euclidean space parallel lines may intesect (for example lines of longitude on the surface of a sphere). Or parallel lines may diverge in a space with a different curvature.

    Under a government which imprisons any unjustly, the true place for a just man is also in prison. Thoreau: Civil Disobedience (1846)
    I would say here something that was heard from an ecclesiastic of the most eminent degree; ‘That the intention of the Holy Ghost is to teach us how one goes to heaven, not how the heaven goes.’ Galileo Galilei 1615.
    If there is no struggle, there is no progress. Those who profess to favor freedom, and deprecate agitation, are men who want crops without plowing up the ground, they want rain without thunder and lightning. Frederick Douglass

    This message is a reply to:
     Message 44 by Stile, posted 04-01-2013 9:13 AM Stile has seen this message but not replied

      
    Newer Topic | Older Topic
    Jump to:


    Copyright 2001-2023 by EvC Forum, All Rights Reserved

    ™ Version 4.2
    Innovative software from Qwixotic © 2024