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Thread: Looking back in time....

  1. #1 Looking back in time.... 
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    Hi All,
    First post so I am sorry if this has been asked before...

    There is one thing I don't get about astronomy and cosmology.

    Astronomers talk about looking at events that happened almost 14 billion years ago and just after the big bang.

    Assuming they are looking near the central point in the universe, how did we get so far away from there that we are only now just seeing light that has travelled for almost 14 billion years?

    This is bazaar to me considering we are not the furthest thing out in the universe.

    What am I missing? I asked this question on another forum (not science specific) and I got so many difference answers it confused me more. Feel free to dumb it down a bit

    @georgeTmaxwell
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  3. #2  
    Forum Radioactive Isotope skeptic's Avatar
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    George

    About the only thing I know of that can go faster than light is the expansion of the universe. The basic rule says " no cause and effect relationship can propogate faster than light". However, objects within the universe that are far apart do not exchange even photons, and so there is no cause and effect relationship. Thus they can move apart at greater than light speed. This is not normal movement of course, since it derives from the expansion of space itself, and is not simply things flying apart.

    When you realise that the universe's expansion is faster than light, being able to look back 14 billion years is no longer so strange.


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  4. #3  
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    Quote Originally Posted by skeptic View Post
    GeorgeAbout the only thing I know of that can go faster than light is the expansion of the universe. The basic rule says " no cause and effect relationship can propogate faster than light". However, objects within the universe that are far apart do not exchange even photons, and so there is no cause and effect relationship. Thus they can move apart at greater than light speed. This is not normal movement of course, since it derives from the expansion of space itself, and is not simply things flying apart.When you realise that the universe's expansion is faster than light, being able to look back 14 billion years is no longer so strange.
    Thanks for that but it just leads me to another question. If the universe expands faster than light, how can light catch up?Or did at some stage in the expansion, did the speed slow down?gTm.
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    Forum Junior brane wave's Avatar
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    The expansion is believed to have been very fast early in the universes history ,then it slowed down...but now appear to be speeding up.
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    Hello and welcome to the forum!

    Let me try to give a few answers:

    One misconception is that there is a centre of the universe. As much as we know, there isn't. As a result, it does not matter, where you are in the universe. There is always a natural limit up to where or when you can look. Remember that the speed of light is not infinite. So, it takes time for any photon or image to be transported over large distances. So, if you see something that is 10 billion lightyears away, you see as it was 10 billion years ago. This is true also for a non-expanding universe.

    Now, include expansion. As was already mentioned, there is no limit to the rate of the expansion (calling it speed is not a good idea), because there is nothing that moves relative to the metric of the universe. As a result, the distance between two objects increases. The rate is given by the Hubble constant that tells you by how much two objects at a given distance move away from each other due to expansion motion. The currently accepted value is about 70 km/s per Mpc (1 pc = 3.086 x 10^16 m). I repeat, this is not a real motion of these objects - they are just drawn along with the expansion. This also shows you that the apparent recession speed depends on the distance. The farther away, the more the expansion affects its apparent motion. Now imagine a distance where the recession speed reaches the speed of light. This is the natural limit up to which we can look, because no photon that is emitted beyond that distance will ever reach us. But this is not necessarily the size of the universe. It can be much bigger. It is just as far as we can look. And this is true for all spots in the universe.

    Therefore, the reason why the universe seems to look different at very large distances from us is not that we are at a special location from where we can see these things. The only reason is that we see the universe as it was at an early stage of its evolution. Remember: distance = age
    Last edited by Dishmaster; July 28th, 2011 at 04:03 PM.
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    Here are a couple of things I wrote a few years back, that might help here.

    Let's make a model.

    Now to model an expanding space we need to assign coordinates within that space. For the moment, forget about any edges to that space, we don't need edges, we just need coordinates in order to measure the expansion of space. Galaxies come later, so for now just imagine a 3 dimensional grid. At each grid intersection we will assign a coordinate, a point, a dot. Let's say each intersection point is 1 meter apart.

    Put yourself on a point somewhere in this space. Whatever axis you look along you see neighbouring points 1, 2, 3, 4, 5 etc meters away, receding off into the distance. Then we introduce some expansion. Let's say the space grows to 10 times its original size in 1 second! That seems fast perhaps, but this is just a model with easy numbers. The key thing to remember is that the grid expands with the space.

    So, here we are, still sitting on our point (but it could havebeen any point!) 1 second later. Now lets look along an axis. We see those neighbouring points are now 10, 20, 30, 40, 50 etc meters away. The space increased to 10 times its original size, and so did the distance between each intersection point on that grid.

    Our nearest neighbouring point has receded from 1 to 10 meters in 1 second, so it has receded at 9 meters per second. The next point away has receded from 2 to 20 meters in 1 second, so that point receded at 18 meters per second. The fifth point has moved from 5 to5 0 meters away in 1 second, so that one has receded at 45 meters per second. The further away you look, the faster a point will seem to have receded!

    And the view would be the same, whatever viewpoint you choose in the grid! There is no "centre" of expansion, no origin point within that grid - the whole thing, the whole space has expanded from something where the spaces between things were really small to something where the spaces between things are much larger. The expansion of that space has carried matter and energy along for the ride.

    Remember I said the grid of points receded off into the distance.. well a point that was initially 33,000,000 meters away will have moved away to 330,000,000 meters in 1 one second, meaning that it has receded at 300,000,000 meters per second - the speed of light! Any point initially more distant than 33,000,000 meters away from another point will have receded from that point faster than the speed of light. That is the distance were an object recedes at light speed in this "little" model of expansion. If you look at a point that has receded at the speed of light, then from that point, the point you are on has receded at the speed of light. But no object would be moving through space faster than light, no photon would ever overtake another photon, it all just gets carried along by the cosmic flow.

    Now I know this is a very simple model, dealing with a simple 10 times expansion in 1 second. This might seem very different from a universe where the rate of expansion was slowing from immense speed and then starting to accelerate, but if you start your grid very small and apply different rates of expansion to that grid, incrementally, over different lengths of time, to simulate slowing it down and then speeding it up, when you look at the end result it is essentially the same. (Whenever there is a change in the rate of expansion, it is the rate of expansion for the whole grid that changes).

    You might be asking how useful this model actually is. Well you can substitute different distance measures and time-scales if you like but the principle remains. If you sprinkle galaxies throughout the grid and then expand that grid such that the galaxies move with the expansion, you would find that galaxies interact gravitationally with their near neighbours. The further apart galaxies are when they form, the less the gravitational attraction between them. If they are less than a certain distance apart, the galaxies will move towards each other and cluster together, but if there is enough distance they will be moved apart by the expansion of the universe.

    We end up with clusters of gravitationally-bound galaxies and increasing distance between the centres of those clusters, in a universe where there is no "origin point" or centre of expansion. The whole thing was the origin point and we have no way of knowing how much larger than our observable part of it the whole thing is. We don't even know if it has an edge, and it doesn't actually need one, mathematically. It is not quite as simple as saying "if it has an overall shape, it must have a centre", unfortunately.
    Last edited by SpeedFreek; July 28th, 2011 at 04:08 PM. Reason: formatting: added missing spaces
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    The observable universe:

    Imagine the beginning of time. If light were around, it would take time to reach you, but this is the beginning of time so no light has had time to move yet! Right at the beginning, your observable universe has no size at all! As time moves forward, any light that exists will move at the speed of light. Suddenly you might see a small distance all around you, as light starts coming in from different directions.

    After a year, you would be able to see 1 light-year in all directions. After 100 years therefore, your observable universe would be a sphere, 100 light-years in radius. After 13.7 billion years, your observable universe would be 13.7 billion light-years in radius, as you receive light that has been travelling for 13.7 billion years.

    Oh, if only it were that simple! The problem comes when considering that the universe is expanding. At the start of things our observable part of the universe was very small and the universe was expanding incredibly fast, much faster than light. Also, light could not move freely until around 370,000 years after the Big-Bang. Before that, photons were frequently interacting with other particles and atoms didn't exist as everything was very hot and mixed up!

    But at 370,000 years in, the universe had been expanding and the temperature had cooled enough for atoms to form in a flash of light (the universe finally became transparent and photons first moved freely throughout it). These photons filled the universe at that time, have been passing this way ever since, and we still receive these photons today. They are now stretched into microwaves (by the expansion of the universe) and are known as the Cosmic Microwave Background Radiation (CMBR). All the CMBR was emitted at once, nearly 13.7 billion years ago.

    As all this was happening, the universe was expanding. When we worked out how much we thought those CMBR photons had been "stretched" by the expansion, it told us how much bigger the universe is today, than it was when those photons were emitted. We estimate that, when the CMBR was emitted, our observable universe was around 42 million light-years in radius, around 1100 times smaller than it is today!

    Hang on though! Didn't I earlier imply that, 370,000 years after the BB, our observable universe would be 370,000 light-years in radius? Well, that radius, based on the time that light takes to travel, is not actually a useful measure of distance at all! In an expanding universe like ours, it is a measure of time elapsed only. When astronomers say the universe is 13.7 billion light-years in radius they are not giving you a distance through space, they are giving you a distance through time, known as the light-travel time. The "actual" distance across an expanding universe, known as the comoving distance, is a different thing entirely (although at distances closer to today, they are essentially the same).

    The CMBR photons we receive today have been travelling for nearly 13.7 billion years, but they were emitted at a proper distance of only 42 million light-years away, all that time ago. The reason they have taken so long to reach us is that the universe is expanding, putting more distance in between those CMBR photons and their eventual "targets". Near the beginning of time, if a coordinate point in space was only a few centimetres from another, and those points moved apart with the expansion, then only 370,000 years later those points in space were 42 million light-years apart - that's how fast the universe was expanding, early on!

    Then those CMBR photons were emitted throughout the universe and in the case of the ones we detect today, the space they were travelling through was receding from this point in space so fast that, to us, it was as if the photons themselves were receding from us too! The gradual deceleration of the expansion allowed those photons to eventually start making actual progress towards us, from our point of view (they were always travelling away from their original origin point). By the time they found themselves in regions ofspace where an object was receding from us slower than light, they were 5.7 billion light years away from this point in space, and the universe was around 4.5 billion years old! (This is when those photons crossed into our Hubble Sphere as it was at that time).

    13.7 billion years after they were originally emitted, 9.1 billion years after they found themselves in space that was receding from us only sub-luminally, we receive those CMBR photons that were only emitted 42 million light-years away. And the real mind-bender is that we think that the original emission point is now over 46 BILLION light-years away. The edge of our observable universe, the most distant point from which we have received CMBR photons, is 46 billion light years away and continues to recede from us. That "edge", known as the surface of last scattering, was receding from this point in space at over 50 times the speed of light when those CMBR photons were emitted, it is still receding at around 3 times the speed of light today and we assume there are galaxies there now, but all we see is the radiation emitted from there, long ago.

    The other mind-bender is that the whole universe is probably larger than our observable universe. After a fraction of a second, when our observable universe only had a radius of 10cm, there may well have been the same thing happening 20cm away. When the CMBR was emitted, and our observable universe was only 42 million light-yearsin radius, there might have been CMBR emitted 80 million light-years away, or much further away than that. Today, when we think our observable universe has a radius of 46 billion light-years and we assume, as galaxies formed in these parts, that there would be galaxies throughout, there could be galaxies whose own observable part of the whole universe is totally separate from ours, galaxies that are 100s of billions of light-years away, outside of our bservable part of the universe but still a part of our universe nonetheless.
    Last edited by SpeedFreek; July 28th, 2011 at 04:16 PM.
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    Thanks Dishmaster - thats a great start.

    and Thanks too SpeedFreek. It's beginning to take shape now. Without having to know the numbers, the concept makes sense but as I read more into the second post, it got into the WTF land. Will have to re-read, and re-read

    Speeding up, slowing down, no center - I see why the further we look the harder it is to understand.

    I have always understood that the BB started as a singularity - a point in nothingness, which I assumed was the center as everything expanded outward from that point.

    I obviously have a lot to learn.

    gTm
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    Quote Originally Posted by georgeTmaxwell View Post
    I have always understood that the BB started as a singularity - a point in nothingness, which I assumed was the center as everything expanded outward from that point.
    Perhaps it might help to consider that the whole universe was contained in that singularity, and the universe expanded within itself, rather into pre-existing space. If the singularity expanded into the universe as it is today, then everyone in the universe can trace the expansion back towards themselves.

    The Big-Bang happened right here. And the same is true, everywhere in the universe.
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  11. #10  
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    Quote Originally Posted by georgeTmaxwell View Post
    Thanks too SpeedFreek. It's beginning to take shape now. Without having to know the numbers, the concept makes sense but as I read more into the second post, it got into the WTF land. Will have to re-read, and re-read
    Just imagine that, when the CMBR was released, each intersection on the grid was 42 million light-years apart. The grid has expanded such that those intersections are now 46 billion light-years apart. All the matter that was within 42 million light-years of here, in each direction, when the CMBR was released, is now spread out across a distance of 46 billion light-years.

    The CMBR has been hitting us here ever since, all emitted at the same time but coming in from increasing distance as time goes on. The CMBR from that next intersection point is just reaching us now, having passed all the other matter on the way, whilst the distance between things (at the largest scales) has been increasing during the time that CMBR has been travelling.

    As the CMBR headed towards us, galaxies formed, but the rate of expansion was still so fast that the regions of space those galaxies were in were receding from here at multiples of the speed of light, meaning that the CMBR we detect today was still receding too, along with the light emitted in this direction from those galaxies at that time. It took 4.5 billion years before the combination of distance travelled and the deceleration of expansion allowed all that light to pass galaxies that were only receding from us at the speed of light. After that, the light found itself in regions of the universe receding from here at less than the speed of light.

    And as all that light travelled, passing galaxy after galaxy on its way towards us, it formed what is known as our past light-cone. It formed the image of the universe that we see at the present moment.

    In the future, we expect to be detecting CMBR that was originally emitted at a greater distance than the CMBR we detect today. It will have passed all those same galaxies, a little further on in their lives, on its way towards us. That CMBR was released from a place currently more distant than 46 billion light-years. The observable universe is growing, with time.

    But, with the onset of the acceleration of the expansion of the universe, there comes a limit to how far we will eventually be able to see - a limit on the ultimate size of our observable universe.
    Last edited by SpeedFreek; July 29th, 2011 at 06:08 PM.
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    Thanks again. You have been explaining terms I have heard of but not understood. I will keep digesting. gTm.
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