Can we see our past?

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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?
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PeroK
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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
Not directly. We can see other parts of the Milky Way only up to thousands of years ago.
 
phyzguy
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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
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.
 
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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
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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?
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.
 
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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?
 
PeroK
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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?
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.
 
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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.
 
PeroK
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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.
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.
 
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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.
 
phyzguy
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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?
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.
 
Grinkle
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A lot will have happened in our observable universe that we will never see.
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).
 
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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.
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.
 
PeroK
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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.
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.
 
Grinkle
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At least we can set a limit to the size of the Universe.
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.
 
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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.
 
PeroK
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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.
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.
 
phyzguy
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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.
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.
 
davenn
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We are actually at the centre of the Universe, just like everything else is.

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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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.
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?
 
PeroK
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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?
Essentially, yes. There is the concept of cosmological "comoving" coordinates, which describe how everything on a large scale is swept along by the Hubble flow - i.e. everything is moving apart through expansion. But, on a smaller scale - up to superclusters of galaxies! - things can have additiional motion relative to each other and relative to the Hubble flow. The Earth orbiting the Sun, the Sun orbiting the galactic centre, the Milky Way and Andromeda galaxies on collision course etc.

You have two factors, therefore, in terms of how we observe the Cosmos from Earth. This might be interesting:

https://en.wikipedia.org/wiki/Local_Group
 
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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.
 
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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.
 
phyzguy
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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 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.
 

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