Viopia said: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
Viopia said: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
Viopia said: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?
Viopia said: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?
Viopia said: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.
Viopia said: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?
PeroK said:A lot will have happened in our observable universe that we will never see.
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.phyzguy said: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.
Viopia said: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.
Viopia said:At least we can set a limit to the size of the Universe.
RobertoV said: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.
Viopia said: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.
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 yourViopia said:We are actually at the centre of the Universe, just like everything else is.
phyzguy said: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.
RobertoV said:Am I right in my understanding that we can consider our observation point as a stand still (centre-like), given that all galaxies are moving away from us. And that the relative distance from our observation point to stars in our own Galaxy are not influenced by the expansion?
phyzguy said: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.
Viopia said:I can't understand why we need a scale factor. Hubble discovered that the further away galaxies are, the faster they seem to be retreating from us. The red shift is because the photon energy of the light is diminished because of the velocity of moving apart and is expressed by the broadening of the wavelength. It is because of the link between distance and speed that we know there was a Big Bang. The red shift is not always a measure of distance. For example, Andromeda is 2.5 million light years away from us, but the light is blue shifted because of it's movement towards us. Also, the velocity of light is always the same whether or not it is blue, or red shifted. If light has taken 12.7 billion years to reach us from distant galaxies, then surely they must be 12.7 billion light years away from us. If the scale factor relates to empty space, but not to the matter within that space, I can't see how it has any meaning. I have read about inflation and this also makes little sense to me. It appears that there is far more for me to understand about the expansion of space. I just hope that "scale factors" and "inflation" are not just mathematical artifacts to try and explain things which no one understands.
You are right. I only have a basic knowledge of cosmology but am trying to understand it the best way I can without resorting to complicated mathematics. When you say we see galaxies moving away "uniformly in all directions" do you mean that the gravitational force is slowing down the speed of the galaxies moving apart over time in a uniform way? The word "Uniform" means in a way that is the same in all cases and at all times. You say "This can't be explained by a static universe in which the expansion is due to the velocities of the galaxies, because then the expansion has a center and all observers don't see the same thing". I did not think that the galaxies in the Universe are static. If they were the attractive force of gravity (or the curvature of space-time if you like) would have pulled them all back together a long time ago. The "Big Bang" term suggests that there was an initial velocity to the expansion of the Universe which gravity has been reducing over time. Is this what you mean by the "scale factor". I have read the article about "FLRW" but I cannot understand the mathematics. I still think it is possible to understand a little about the Universe without a good understanding of mathematics even though a full understanding would require mathematical expertise. I have recently learned that the outermost galaxies are accelerating away from us. This makes absolutely no sense to me at all. Are the scientists absolutely sure that this acceleration actually exists? Also, does the Universe have an edge to it?phyzguy said:You need to do some reading on basic cosmology. We see galaxies moving away uniformly in all directions. A fundamental assumption of cosmology is that the universe is homogeneous and isotropic. This means we do not occupy a privileged position, that any observer anywhere in the universe will see all other galaxies moving away uniformly in all directions. This can't be explained by a static universe in which the expansion is due to the velocities of the galaxies, because then the expansion has a center and all observers don't see the same thing. The observations are explained by what is called the Friedmann–Lemaître–Robertson–Walker metric, which is what results when we apply Einstein's equations of General Relativity to the entire universe. A key part of it is there is a scale factor a(t), where all dimensions in the universe are expanding as time goes on. This is what we mean when we say "the universe is expanding." It is not a mathematical artifact, it is a fundamental fact about our universe, and it is backed up by a huge number of observations.
Viopia said:we see galaxies moving away "uniformly in all directions"
Viopia said:Are the scientists absolutely sure that this acceleration actually exists?
Viopia said:Also, does the Universe have an edge to it?
Viopia said:You are right. I only have a basic knowledge of cosmology but am trying to understand it the best way I can without resorting to complicated mathematics. When you say we see galaxies moving away "uniformly in all directions" do you mean that the gravitational force is slowing down the speed of the galaxies moving apart over time in a uniform way? The word "Uniform" means in a way that is the same in all cases and at all times. You say "This can't be explained by a static universe in which the expansion is due to the velocities of the galaxies, because then the expansion has a center and all observers don't see the same thing". I did not think that the galaxies in the Universe are static. If they were the attractive force of gravity (or the curvature of space-time if you like) would have pulled them all back together a long time ago. The "Big Bang" term suggests that there was an initial velocity to the expansion of the Universe which gravity has been reducing over time. Is this what you mean by the "scale factor". I have read the article about "FLRW" but I cannot understand the mathematics. I still think it is possible to understand a little about the Universe without a good understanding of mathematics even though a full understanding would require mathematical expertise. I have recently learned that the outermost galaxies are accelerating away from us. This makes absolutely no sense to me at all. Are the scientists absolutely sure that this acceleration actually exists? Also, does the Universe have an edge to it?
Viopia said:The only part of my question remaining unanswered is the following "The "Big Bang" term suggests that there was an initial velocity to the expansion of the Universe which gravity has been reducing over time. Is this what you mean by the "scale factor"?.
This would be very difficult for an ancient civilisation to do. The mirror would also need to be very large. I was thinking that spacetime may curve light back on itself so that, if we looked in the right direction, we may see the light returning from our own milky way when it was only 0.8 billion years old. I think the Universe may need a cylindrical shape for this to happen.DeathByKugelBlitz said:It would be possible, provided a rather large perfect reflector lay in that direction, and was several million/billion years old
Would not the Universe have to be a two dimensional spherical skin, like equally spaced dots on the surface of a balloon being inflated, for it to have a uniform isotropic expansion? Would not a three dimensional Universe be like a million or so marbles all packed as close together as they can be in a spherical shape, and then expanding in a homegeneous way to fill a much larger sphere? This would mean that, after the expansion, the marbles on the surface of the sphere would expand at the same rate, but the marbles in a line along the radius of the sphere would expand at a much smaller rate as they approach the centre of the sphere.phyzguy said:No. The concept of the scale factor in cosmology is not difficult to understand. It just means that the distances between things in the universe are getting larger as time goes on, with all large scale distances being multiplied by a constant factor. One way to think of it is with the "balloon analogy" which was mentioned in an earlier post. Another way to picture it is in one dimension. Imagine a bunch of ants standing on a rubber band that is being continually stretched. The distances between the ants on the rubber band are constantly increasing. Each ant sees all the other ants moving away from it, and the further apart two ants on the rubber band are, the faster they are moving apart. But each ant is standing still on the rubber band, and is not moving with respect to the portion of the rubber band it is standing on. Also, the bodies of the ants themselves are not growing, just like local distances in the universe (the distance from the Earth to the Sun, for example) are not growing. This is the way you should think of it.
phyzguy said: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.[/QUOTE
Do you mean from your comment that the Universe has no edge? Also I have looked up the evidence for the accelleration of the outermost galaxies and think the following may be true. When the light was emitted from the galaxies one billion years after the big bang the Universe was only 1/7 of the size it is now. The red shift would not indicate the speed of the Universe expanding one billion years ago when it eventually hits the Earth 12.7 billion years later, but would indicate the speed of the Universe expanding now because of the decreased rate of expansion due to gravity trying to pull everything back together over the intervening 12.7 billion year period. As the expansion speed has now been reduced from what it was one billion years after the big bang due to the effects of gravity, the red shift (due to velocity) would be less than it would have been 12.7 billion years ago when it hits the objects in our region of space. The amplitude of the emitted light from the galaxies one billion years after the big bang would have actually traveled a distance of 12.7 billion years to catch up with our region of space in the ever expanding Universe. This would not be affected by the difference in expansion velocities between now and one billion years ago and so the amplitude of the light would appear to be less than it should be compared to the red shift. This would mean that the the greater the distance between the galaxies, the greater would be the apparent disparity between the distance denoted by the red shift and the brightness. The result would be that the further away the galaxies are, the dimmer the light would be compared to the distance calculated by the red shift alone. The Hubble constant may appear linear if we measure very close stars from a triangular baseline of only 186 million years, but the difference in the brightness expected from type 1A supernovas would become hugely different at great cosmic distances. It's a bit like shining a light on a moving car after it has passed you when the car starts to apply its brakes. The red shift of the light from the car's perspective is not caused by the average speed of the car, but the speed it is traveling when the light actually hits it. Can you explain where I am wrong in my reasoning please.Grinkle said:@Viopia Look for a post by @phinds and read the balloon analogy insight in his signature link. There is no complicated math in it. I think you would find it helpful.
Don't over-complicate it. It means (maybe among other things) that no matter what direction we look from anywhere on Earth or in orbit around earth, we see basically the same thing in terms of recession speed vs distance of observed galaxies.
Yes.
Not as far as any model I have read about postulates. If the universe is closed, it no more has an edge than does the surface of the earth. If it is open, then it is infinite in extent.
That's not how expansion works. We do not live in a three dimensional Newtonian/Euclidean universe where an initial explosion flung everything away and attractive gravity is slowing things down. We live in a four dimensional space time where the spatial distance between events on the world-tubes for a pair of typical widely separated objects "now" is greater than it was "then" when judged using "co-moving" coordinates.Viopia said:the speed of the Universe expanding now because of the decreased rate of expansion due to gravity trying to pull everything back together over the intervening 12.7 billion year period.
Viopia said:Do you mean from your comment that the Universe has no edge?
It is not that a notion of an edge cannot be modeled. It is that an edge beyond what we can see with no evidence extending to what we can see is scientifically irrelevant. Occam's razor slices it away cleanly. A model without an edge has fewer free parameters than a model with one. Fewer parameters is a good thing.Grinkle said:No one has come up with a geometry for the universe that has an edge and is also consistent with our cosmological observations as far as I know (and I don't know much of what there is to know regarding cosmology, so someone may correct me in this).
Viopia said:Would not the Universe have to be a two dimensional spherical skin, like equally spaced dots on the surface of a balloon being inflated, for it to have a uniform isotropic expansion? Would not a three dimensional Universe be like a million or so marbles all packed as close together as they can be in a spherical shape, and then expanding in a homegeneous way to fill a much larger sphere? This would mean that, after the expansion, the marbles on the surface of the sphere would expand at the same rate, but the marbles in a line along the radius of the sphere would expand at a much smaller rate as they approach the centre of the sphere.
OK. Please substitute "curved spacetime" instead of gravity.jbriggs444 said:That's not how expansion works. We do not live in a three dimensional Newtonian/Euclidean universe where an initial explosion flung everything away and attractive gravity is slowing things down. We live in a four dimensional space time where the spatial distance between events on the world-tubes for a pair of typical widely separated objects "now" is greater than it was "then" when judged using "co-moving" coordinates.
Now you have the problem that we live in an epoch of accelerating expansion. Co-moving distances in this curved spacetime are not increasing more slowly over time. They are increasing more rapidly.Viopia said:OK. Please substitute "curved spacetime" instead of gravity.
If you read my post which ended with the words ("The result would be that the further away the galaxies are, the dimmer the light would be compared to the distance calculated by the red shift alone. The Hubble constant may appear linear if we measure very close stars from a triangular baseline of only 186 million years, but the difference in the brightness expected from type 1A supernovas would become hugely different at great cosmic distances. It's a bit like shining a light on a moving car after it has passed you when the car starts to apply its brakes. The red shift of the light from the car's perspective is not caused by the average speed of the car, but the speed it is traveling when the light actually hits it"). the conclusion may be that the outermost galaxies are not accelerating and there is no need for dark energy. This is why I asked what is wrong with my reasoning. As yet I have received no reply.jbriggs444 said:Now you have the problem that we live in an epoch of accelerating expansion. Co-moving distances in this curved spacetime are not increasing more slowly over time. They are increasing more rapidly.
Re-labelling Newtonian intuitions with different terminology does not restore those intuitions to correctness.
I'm sorry you are checking out of this thread. I am new to Cosmology and my lack of understanding may appear that I am not listening to what people are telling me. Cosmology is far more complicated than I thought it would be. I am interested in it however, and perhaps I may be able to make more valuable contributions to this site after a few years when I understand more. My lack of mathematical skills will always be a disadvantage to me but I will not let this put me off. Thank you so much for your contribution. I will be sorry to see you leave. Best wishes, Chris.phyzguy said:@Viopia You seem unwilling to listen to what people are telling you. I for one am checking out of this thread. Good luck in your attempts to understand cosmology.
Viopia said:I'm sorry you are checking out of this thread. I am new to Cosmology and my lack of understanding may appear that I am not listening to what people are telling me.
Viopia said:I'm sorry you are checking out of this thread. I am new to Cosmology and my lack of understanding may appear that I am not listening to what people are telling me. Cosmology is far more complicated than I thought it would be. I am interested in it however, and perhaps I may be able to make more valuable contributions to this site after a few years when I understand more. My lack of mathematical skills will always be a disadvantage to me but I will not let this put me off. Thank you so much for your contribution. I will be sorry to see you leave. Best wishes, Chris.
I would like to make a confession. I can see the frustration in you replies to my questions and think you ought to know that I am not formally studying cosmology or physics. My interests in physics was aroused when I saw some YouTube videos by the ex-NASA physicist Tom Campbell about the double slit experiments in relation to his simulated Universe theory. This led me to watch YouTube videos regarding quantum physics leading to the quantum field theory and cosmology etc. My knowledge is totally based on TV documentaries and YouTube videos about science and I have discovered it is easier to ask questions than to plough through textbooks which would take ages, and include mathematics which I may not understand. The reason I want you to know this is that your "physics pages" may not be designed for questions from people like me. If you do not want to answer questions from someone who is not formally educated in these subjects I will understand. I have found a very good YouTube video entitled "The accelerating Universe: Nobel Laureate Brian Schmidt" which I found very easy to understand and answered most of my questions. He even used terminology, which I was criticised for, like the pulling force of gravity. It therefore appears that even though Newton law has since been superseded by Einstein's theory of general relativity it continues to be used as an excellent approximation of the effects of gravity in most applications.jbriggs444 said:Now you have the problem that we live in an epoch of accelerating expansion. Co-moving distances in this curved spacetime are not increasing more slowly over time. They are increasing more rapidly.
Re-labelling Newtonian intuitions with different terminology does not restore those intuitions to correctness.
TV and YouTube pop-science typically becomes understandable by "dumbing down" the content. Even when the portrayals are not outright false, they tend not to be viable starting points from which to reason further.Viopia said:I would like to make a confession. I can see the frustration in you replies to my questions and think you ought to know that I am not formally studying cosmology or physics. My interests in physics was aroused when I saw some YouTube videos by the ex-NASA physicist Tom Campbell about the double slit experiments in relation to his simulated Universe theory. This led me to watch YouTube videos regarding quantum physics leading to the quantum field theory and cosmology etc. My knowledge is totally based on TV documentaries and YouTube videos...
Many of these videos are made by respected experts in their fields, like the Nobel Laureate Brian Schmidt" in the YouTube video I mentioned. Why would they want to give false information? It is easy to find out who the authors of these videos are, and if they are reputable scientists. I don't mind a bit of "dumbing down" as line as they are telling me the truth.jbriggs444 said:TV and YouTube pop-science typically becomes understandable by "dumbing down" the content. Even when the portrayals are not outright false, they tend not to be viable starting points from which to reason further.
Viopia said:I don't mind a bit of "dumbing down" as line as they are telling me the truth.
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