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

[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.

 

[GraemeSRC]

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.

In fact the string of marbles along a line from the centre to the edge of the sphere would retain their proportional spacing as you stretched them out. The rate is not less towards the centre. If they were evenly spaced to start with they would remain so.

 

[GraemeSRC]

Just a small final thought before I head off to bed. About the OP's question of whether we can see our past.

In fact that is all we can see. Its simply a question of how old what we see is. From the fact that light does take time to travel from the observed to the observer, however short the distance might be, to the time it takes to process that incoming data to form a perception, it can never be instantaneous. If we were able to look at ourselves in a mirror placed on the moon, which is not entirely unfeasible, we would see ourselves a few seconds ago and the delay would be easily noticeable.

 

[MidgetDwarf]

@Viola. The best way to learn is to put in the effort to do so. What you are doing is called being lazy. You are expecting people to teach you material that took people hundreds, if not more, years to understand in a short interval of time. If you are truly interested in learning. Pick up a mathematics/ introductory physics book, and start learning.

You say you are not formally educated... What is stopping you from becoming acquainted with the field? Hint: the answer to this question is somewhere in my post.

 

[phyzguy]

In fact the string of marbles along a line from the centre to the edge of the sphere would retain their proportional spacing as you stretched them out. The rate is not less towards the centre. If they were evenly spaced to start with they would remain so.

Yes, they remain evenly spaced. But they are getting further apart. There is no center - I never said there was a center. You should imagine that the string of marbles (or ants) is infinitely long. But each marble will see the others as moving away, and see the marbles that are further away moving away faster.

 

[phinds]

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.

Ah, now THERE'S a real problem. You think that because the presenter is a "respected expert" then what he/she says in a pop-science presentation will be real physics. Nothing could be further from the truth. Pop-science is made to sell soap / cars / etc, not to teach anyone actual science. Perfectly respectable scientists will say things in pop-science presentations that would get them hooted out of the room in a real physics discussion, and they know it. "Telling the truth" as you put it often requires much more time and explication that one can possibly fit into short presentations like one hour.

That fact is the bane of our existence here on PF. We are constantly having to correct misconceptions that people like you take away from pop-science presentations. I watched quite literally dozens of such things and read similar books and I found that there are common statements that occur so often that it is quite reasonable to assume that they are correct (even though they are not). When I got her to PF I found out that much of what I "knew" was totally wrong. You are running into the same problem.

My only saving grace was that I really wanted to learn so rather than ask questions at first, based on what I had come to realize were clearly misconceptions, I just read LOTS of posts here, plus some more serious books, and learned enough to make my questions based on a bit more knowledge. I still spend MUCH more time here reading threads than asking questions.

 

[OCR]

I still spend MUCH more time here reading threads than asking questions.

So do I, but I'm going to ask one now. . .

When I got her to PF. . .

Did you miss the "a" an "e" key ? .

 

[phinds]

Did you miss the "a" an "e" key ? . View attachment 247014

Hey, *I'M* the head nitpicker here

It was indeed a slip of figner.

 

[OCR]

Hey, *I'M* the head nitpicker here

. .
. . And, I believe you. . . .

.

 

[Umang Soni]

It's closer. The Sun, for example, is only 8 minutes away. The light from the Sun any older that than is traveling away from us; not towards our telescopes.

absolutely correct ,,,,broadly the light rays which we observe at present is as old as its distance ...now we only can see the past of that light source ( or the subject which is reflecting it ) corresponding to is distance from observer ... it is impossible to see its past ...its like its distance puts limits on the our observation ..

 

[sophiecentaur]

Thank you. I must apologise to Jbriggs444. I have a bad habit of arguing with people when I don't understand something.

IS there any other PF member to whom that doean't apply? Don't worry. @jbriggs444 has a thick skin like (most of) the rest of us.