B Can We See Our Past? - Milky Way & Hubble Telescope

[phyzguy]

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"?.

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.

 

[DeathByKugelBlitz]

It would be possible, provided a rather large perfect reflector lay in that direction, and was several million/billion years old

 

[Viopia]

It would be possible, provided a rather large perfect reflector lay in that direction, and was several million/billion years old

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.

 

[Viopia]

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.

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.

 

[Viopia]

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

@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.

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.

 

[jbriggs444]

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.

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.

 

[Grinkle]

Do you mean from your comment that the Universe has no edge?

Short answer is yes, that is what I mean.

Longer answer is I have never seen any description of what an edge looks like or means in the context of a hypothesized geometry for the universe. I have seen discussions of a spherical universe, an infinite flat universe, an infinite hyperbolic shaped (positive curvature) universe, and I think an infinite negative curvature universe that is not closed, so not a sphere but still with negative curvature. All of those descriptions lay some claim to not being contradicted by our current cosmological observations, but the curved versions keep needing to have less and less curvature to maintain that claim, AFAIK. None of those descriptions contains an edge. 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).

 

[jbriggs444]

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).

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.

If we think in terms of an ant on a tablecloth, the table could be infinite and flat. It could be round and sharply cut off with the ant in the center. It could be round and sharply cut off with the ant offset from the center. It could be spherical with the ant on top. It could have ruffled edges. There could be a fire burning on the rims. The cloth might be on a loom that weaves new cloth at one end while it rots away at the other. As long as the edges are beyond what the ant can see and as long as the cloth is flat as far as the ant can see, no hypothetical edge need enter into its tablecloth model.

 

[phyzguy]

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.

No. You're still thinking of the marbles as moving in a static 3D space. Try thinking instead of static marbles in a space which is expanding. Then every marble sees all of the marbles moving away from it, with marbles that are further away moving away faster. It is really impossible to visualize in 3D, which is why we focus on the mathematics.

 

[Viopia]

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.

OK. Please substitute "curved spacetime" instead of gravity.

 

[jbriggs444]

OK. Please substitute "curved spacetime" instead of gravity.

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.

 

[Viopia]

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.

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.

 

[phyzguy]

@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]

@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.

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.

 

[anorlunda]

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.

You're also ignoring the advice of others to go study some basic cosmology before posting questions here. Just asking questions is a very poor learning strategy. That is why schools organize courses, and have textbooks, rather than giving students 4 years to ask random questions. First, you listen to the teacher's lecture, later you ask questions.

 

[lomidrevo]

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.

Then you should really improve your math skills. If you want to have any serious understanding of physics (not only cosmology, but any other area) you cannot avoid math. The common language is just not sufficient to describe the universe. You don't have to get at a level of a strictly rigorous math, many physics textbooks provide an introduction to the math required to understand the topic. You just have to begin with a textbook appropriate to your current level of knowledge and step-by-step get to the more advanced topics. If you are seriously interested, you would be surprised how quickly you can progress. It just requires time (as any other skills/hobbies).

 

[Viopia]

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 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]

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...

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]

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.

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.

 

[anorlunda]

I don't mind a bit of "dumbing down" as line as they are telling me the truth.

We do mind. Note there is a difficulty tag on these threads B/I/A. Sometimes we have to say, "Sorry, there is no B level answer to that question." We prefer that rather than dumbing down.

You should be aware of what PF is. Our mission statement says:

Our goal is to provide a community for people (whether students, professional scientists, or hobbyists) to learn and discuss science as it is currently generally understood and practiced by the professional scientific community.

 

[Viopia]

We do mind. Note there is a difficulty tag on these threads B/I/A. Sometimes we have to say, "Sorry, there is no B level answer to that question." We prefer that rather than dumbing down.

You should be aware of what PF is. Our mission statement says:

Thank you. I am pleased. I will try to limit any questions I have to things I have been unable to find out for myself.

 

[Viopia]

I am still waiting.

 

[anorlunda]

It depends on who "we" is.

If you mean yourself. That you could get on a rocket, travel faster than light, then look back and see a dim view of yourself; the answer is no. The reason why should be obvious.

If you mean beings on a distant planet seeing light from Earth that left Earth long ago, then yes.

 

[Viopia]

I am sorry if I have upset anyone. You must consider that I am only an average person who is trying to understand what I have become interested in even though I have no formal education in these subjects. I have to rely on things I can understand, like YouTube videos and TV documentaries. I am upset that I am now banned from posting on your site. l meant no harm. I have decided that I will concentrate more on my music than science because I believe I will never understand physics, cosmology or astrophysics. They are just too complicated. It is best we all concentrate on what we are best suited for. Best wishes, Chris.

 

[gneill]

I am upset that I am now banned from posting on your site.

I can see no evidence in the Mentor/Moderator action logs of your being banned from posting at PF, or even being excluded from this particular thread. May I inquire how you came to that conclusion?

 

[jbriggs444]

I can see no evidence in the Mentor/Moderator action logs of your being banned from posting at PF, or even being excluded from this particular thread. May I inquire how you came to that conclusion?

He or she picked up a thread ban over here.

 

[gneill]

He or she picked up a thread ban over here.

Ah! Thank you. I somehow missed that. Shame on me

@Viopia : That is merely a thread ban (one particular thread), not a ban from posting at PF. I was looking for evidence of a more egregious sort of rule infraction that might have resulted in a temporary or permanent ban from posting.

 

[Viopia]

Thank you. I must apologise to Jbriggs444. I have a bad habit of arguing with people when I don't understand something. He made a very reasonable comment about the origin of a video I had posted and I have to agree that he was right. You are all very knowledgeable about your subjects and I should listen more and avoid jumping to conclusions. It is hard to change one's personality, but I will try. I am pleased I misunderstood the ban and can still post questions. Thanks again for your understanding.

 

[GraemeSRC]

There's a good insight here:

https://www.physicsforums.com/insights/inflationary-misconceptions-basics-cosmological-horizons/

Two points, however, have to be made.

It's all right to want to try to understand things without too much mathematics, if you yourself have little mathematical knowledge. But, mathematics is the language of physics and any ideas that mathematics somehow is not a valid explanation of "reality" are misplaced.

If you have limited knowledge of a subject then it is also invalid to interpret your lack of understanding as a general problem with the theory. I, for example, can speak no Russian, but I cannot then doubt that Russians are actually speaking their own language, and start to believe that no one can speak Russian.

PF is a place to learn, but your ability to learn will be severly limited if you interpret your lack of understanding as a reason to doubt mainstream physics.

The Russian analogy only holds if there are things one can say in Russian that are not translateable into English. Which I doubt.

 

[gneill]

The Russian analogy only holds if there are things one can say in Russian that are not translateable into English. Which I doubt.

That makes no sense. The analogy made no reference to translation between languages. Please be more circumspect before posting.