what we see

[michel123456]

You must know this representation of the expansion of the universe. It is published in several places on the web.

In fact, this representation is a Space-Time diagram. WMAP is the point where we are today as observers of the Universe.

 

 

_The Big Bang happened 13,7 billions years ago, in the past. The past is on the left side of the diagram. The present time is where the WMAP sattelite is pictured. The future is on the right side. Time goes from left to right.

 

_Each slice of the diagram represents space at a specific period. 3d space has been reduced to a 2d surface.

 

What part of the diagram are we looking at when focusing with our telescopes?

Roughly, it is represented in the diagram below.

 

 

_I put on the diagram the light cone of the observer: Earth's light cone. It is a simplified representation of course.

 

_the events outside the light cone are not visible. Point B is not visible.

 

Some example, to be more clear:

Point A is let's say the event of a star exploding today at a distance of 12 billions of light-years from us. We will see the explosion in 12 billions of years in the future.This event is invisible to us, today.

 

Point B is a galaxy that happened to explode 8 billions of years ago, and 10 billions of lightyears away. We will see this explosion in 10-8=2 billions years in the future. Point B (event B) is invisible to us.

 

Point C sits upon the light cone. Event C is visible from us in our telescopes.

 

Resuming, what we see in our telescopes is something like this:

 

 

That makes the diagram pretty empty isn't it?

Edited November 13, 2011 by michel123456

[imatfaal]

i don't know - we see enough to start understnading the basics and begin unravelling the intricacies. And not to put too fine a point on it; we see enough that we have gained enough comprehension for posters on internet fora to be discussing what we see.

 

Great post, and I like that diagram even if it has come in for some criticism.

 

BTW in a universe that is expanding and that expansion is accelerating - something happening 12 Glyr away will not be seen on earth (or on Sol's glowing ember) for well over 12 Gyr.

[Mystery111]

You know, before I was well-aquainted with relativity, I actually predicted a dark age. I called it the ''dark belt'' - I noted it's existence because there was a part of expansion I had read from pop science book that light never came out of the vacuum until quite late. Using that I fathomed the beginning of the universe would be dark to us, named it a dark belt in similar contentions to the green belt found round large cities

 

 

Sorry, back to topic guys, wavered off a bit there.

[MigL]

Not quite the same thing, Mystery111.

The dark age happened after radiation decoupled from matter. At roughly 4000 deg the universe was no longer filled with a plasma where electrons had too much energy to stick to nucleii ( what we now see as the CMB ), but by stable Hydrogen and Helium atoms ( with few others ). It naturally took some time for these atoms to come together to form suns that radiate energy, and so, there was a period of darkness. The event you are considering would have happened much earlier, before the inflationary period.

[Mystery111]

Not quite the same thing, Mystery111.

The dark age happened after radiation decoupled from matter. At roughly 4000 deg the universe was no longer filled with a plasma where electrons had too much energy to stick to nucleii ( what we now see as the CMB ), but by stable Hydrogen and Helium atoms ( with few others ). It naturally took some time for these atoms to come together to form suns that radiate energy, and so, there was a period of darkness. The event you are considering would have happened much earlier, before the inflationary period.

 

Respect, I was as knowledgable as morgs....

 

 

.... obvously nowadays I can conduct things to a better intellect.

[Airbrush]

Respect, I was as knowledgable as morgs....

 

 

.... obvously nowadays I can conduct things to a better intellect.

 

You seem to be very knowledgeable. Please share with us your credentials. Are you a published scientist or professor?

 

What does this mean: "Respect, I was as knowledgeable as morgs..."?

 

What does it mean to "conduct things to a better intellect."?

 

I guess knowledgeable people are hard to follow.

Edited November 14, 2011 by Airbrush

[michel123456]

So nobody seems really impressed by the fact (I see that as a fact) that we are looking at a small part of the big picture.

We can open our eyes wide, create new telescopes, invent new technology, but what we can see is restricted by the laws of physics. And although we know there are things outside of reach (see the first diagram), we are still searching...

[Iggy]

So nobody seems really impressed by the fact (I see that as a fact) that we are looking at a small part of the big picture.

I don't mean insult, but it is a little banal.

 

Your OP is very nicely illustrated, but I think it can be totally summarized "if you weren't looking at something that happened then you missed it". I don't think that would surprise most people. Nobody would expect that every time you look at the moon you'll see an apollo spacecraft landing there.

[michel123456]

So you feel fine with my empty diagram.

[imatfaal]

I am not sure that I agree with your summation of the facts - I agree with the facts themselves. But we are seeing the whole picture that is available to us - and the fact that which is not available to us is forever denied, in my mind at least, is easily accepted. I see it as part of the human psyche that we understand that some things are beyond our capability of vision- that we only glimpse a fraction of spacetime continuum does not annoy/perturb me in the manner that we may not be sending many more spacecraft to the moon, the planets and their moons (and almost certainly no more men) really does make me angry.

[Iggy]

So you feel fine with my empty diagram.

I'm not sure how you mean by fine, but I think it accurately depicts the observations we can currently make.

 

The only technicality that I could add is that the past light cone wouldn't have straight lines. The curved bell shape of the original image is actually a forward pointing light-cone. A past light cone starting from WMAP would have the same bell shape like the diagram about half way down this page.

 

That is a very small technicality though. I very much agree with the OP.

Edited November 15, 2011 by Iggy

[michel123456]

I am not sure that I agree with your summation of the facts - I agree with the facts themselves. But we are seeing the whole picture that is available to us - and the fact that which is not available to us is forever denied, in my mind at least, is easily accepted. I see it as part of the human psyche that we understand that some things are beyond our capability of vision- that we only glimpse a fraction of spacetime continuum does not annoy/perturb me in the manner that we may not be sending many more spacecraft to the moon, the planets and their moons (and almost certainly no more men) really does make me angry.

 

That is not the point. Angry or disappointed, who cares.

 

The fact is that we see only a small part of the Universe.

The Universe is the first full picture. What we see is the empty picture.

Or, to say it otherwise, a large part of the Universe is there in front of our eyes but we cannot see it.

[Iggy]

The fact is that we see only a small part of the Universe.

The Universe is the first full picture. What we see is the empty picture.

Or, to say it otherwise, a large part of the Universe is there in front of our eyes but we cannot see it.

Now you've got me echoing imatfaal, it would be tough to agree with the summation. We don't currently see most of the events which have happened in the universe, but that's not quite the same as saying "we see only a small part of the universe".

 

Using the same analogy, we don't see most events that happened on the moon but that doesn't really translate to "we only see a small part of the moon" or "what we see is an empty moon"

[Klaynos]

What do you think we would see if we could see everything in the universe? If the speed of light was much much much higher...

[imatfaal]

That is not the point. Angry or disappointed, who cares.

 

The fact is that we see only a small part of the Universe.

The Universe is the first full picture. What we see is the empty picture.

Or, to say it otherwise, a large part of the Universe is there in front of our eyes but we cannot see it.

 

I was responding to how I felt about that that diagram

 

We do see the whole universe - the constraint of a finite speed of light just determines the particular form of the picture. If the speed of light was much slower or faster we would see a qualitatively similar universe - we see a snapshot in time of the universe as we do of anything else; I cannot look at my hands typing at my keyboard as they were last week nor how they will be a week into the future. The finite speed of light changes how that selection is made over distances but does not affect the necessity that a selection be made.

[michel123456]

What do you think we would see if we could see everything in the universe?

 

Good question, what do you mean with "everything"? Every event that happened at any time at any place? I guess it would be a very blurred image.

 

If the speed of light was much much much higher...

That would change the angle of the cone. With SOL infinite the cone would be flat as a disk and we would observe the present time. all the rest would still be invisible. And for SOL equal to zero, the cone would transform into a line and we would observe ourself in our past only. Again, all the rest would be invisible.

 

(...) If the speed of light was much slower or faster we would see a qualitatively similar universe - we see a snapshot in time of the universe as we do of anything else; I cannot look at my hands typing at my keyboard as they were last week nor how they will be a week into the future. (...)

 

Exactly. You cannot see your own past because your past lies INSIDE the light cone. You cannot look at your hands in the future because that is OUTSIDE of the light cone. An observer in the future can look at you (smile), the same as we are looking to the past. But you can't see him.

Edited November 15, 2011 by michel123456

[Klaynos]

Good question, what do you mean with "everything"? Every event that happened at any time at any place? I guess it would be a very blurred image.

 

 

That would change the angle of the cone. With SOL infinite the cone would be flat as a disk and we would observe the present time. all the rest would still be invisible. And for SOL equal to zero, the cone would transform into a line and we would observe ourself in our past only. Again, all the rest would be invisible.

 

I was referring to the case where the cone is a flat disk. It wouldn't so much be a blurred image as an incredibly bright one... I've not described this very well.

 

But imagine the universe, infinite in extent. If the speed of light was such that everything that has been emitted has had time to reach everything else the flux of photons would be truly immense it'd be fantastically high... Our very limited view is a good thing.

[michel123456]

I didn't say the contrary. It is a good thing from the moment we understand profoundly that it is a limited view.

 

Now you've got me echoing imatfaal, it would be tough to agree with the summation. We don't currently see most of the events which have happened in the universe, but that's not quite the same as saying "we see only a small part of the universe".

 

Think about it twice.

You agree that we don't currently see most of the events which have happened in the universe, but that causes no trouble to your scientific conscience.

 

Think that all the small points pictured on the first diagram are gravitationaly connected to the few we see in the empty picture by a mesh of cones emanating from each one of them. An observer on any of these points sees another universe than ours, at the exception of a few objects at the intersection of his cone and ours. He sees planets and galaxies that we may never see in all humanity. Planets & stars that exist but that we cannot see. Doesn't that blow your mind?

Edited November 15, 2011 by michel123456

[MigL]

You are missing one important point, michel123456. The only things in the universe that can affect us or be affected by us ( not quite but close enough ) are the things we can see. Not only light but all forces and information propagate at c. That light cone you posted and seem disappointed with is the observable universe, anything outside it would make no difference even if it ceased to exist.

[michel123456]

You are missing one important point, michel123456. The only things in the universe that can affect us or be affected by us ( not quite but close enough ) are the things we can see. Not only light but all forces and information propagate at c. That light cone you posted and seem disappointed with is the observable universe, anything outside it would make no difference even if it ceased to exist.

 

It is not true and it is provable.

 

 

In this diagram, time is upright, space is horizontal. Point A is where you are. Point B is outside your light cone, you cannot see point B. Point C (event C) is at the intersection of both light cones, which means that point C is gravitationaly connected to points A & B. If point B came to magically disappear you would see an influence on point C.

 

 

The same upon a stolen diagram from Ned Wright's tutorial page.

 

Of course the life line of B crosses our light cone, which means we are observing B's past. But that is another story.

Edited November 16, 2011 by michel123456

[Iggy]

I didn't say the contrary. It is a good thing from the moment we understand profoundly that it is a limited view.

 

 

 

Think about it twice.

You agree that we don't currently see most of the events which have happened in the universe, but that causes no trouble to your scientific conscience.

 

Think that all the small points pictured on the first diagram are gravitationaly connected to the few we see in the empty picture by a mesh of cones emanating from each one of them. An observer on any of these points sees another universe than ours, at the exception of a few objects at the intersection of his cone and ours. He sees planets and galaxies that we may never see in all humanity. Planets & stars that exist but that we cannot see. Doesn't that blow your mind?

I would say that an observer inside or on our past lightcone sees the same stuff (the same mass), but that the mass is doing different things. Someone outside our past light cone would genuinely see different mass, but that mass isn't causally connected to us so it wouldn't matter if it vanished.

 

Scrolling past the OP's image again, I noticed I misspoke,

 

The curved bell shape of the original image is actually a forward pointing light-cone.

The outside of the bell-shaped cone of the original image is, I think, two comoving observers rather than a forward pointing light cone. my bad.

[michel123456]

I would say that an observer inside or on our past lightcone sees the same stuff (the same mass), but that the mass is doing different things. Someone outside our past light cone would genuinely see different mass, (...)

 

I disagree, how did you come to such a conclusion?

 

1_you stated "an observer inside our past lightcone sees the same stuff (the same mass)"

 

here below the diagram for an observer inside our past lightcone (point D). His lightcone is smaller than us, point D sees a smaller universe. Since you agreed that what he sees is on the surface of the light cone, he sees a universe that we cannot see, IOW a different universe. This "other universe" is made from a portion of the same "stuff" we are currently observing.

 

 

 

2_you stated "an observer on our past lightcone sees the same stuff (the same mass)"

 

here below the diagram for an observer on our past lightcone. Point E has a lightcone smaller than us, point E sees a smaller universe. He sees a part of the same stuff that we currently do, because the left part of the lightcone coincides with ours, but the right part of his lightcone is different. IOW he sees a different, smaller universe, made partly of the same stuff we are seeing today from point A.

 

3_you stated "Someone outside our past light cone would genuinely see different mass,"

here below the diagram for an observer out of our past lightcone. Point B has a lightcone that has the same perimeter than us, but if you putted point B elsewhere it could be differently . Point B sees a similar universe, but the image that he receives is completely different because what he sees is on the surface of his lightcone. The only point that he sees the same with us is point C. He sees a part of the same stuff than we do, and part of stuff that we don't actually see. But we will see it in the future. If I understand correctly, there is always an overlap between both lightcones, no matter where you put point B.

Edited November 17, 2011 by michel123456

[Iggy]

1_you stated "an observer inside our past lightcone sees the same stuff (the same mass)"

yes, and I should be more specific to avoid confusion. An observer in or on our past lightcone cannot see mass that is unobservable to us.

 

The mass may be doing something different when (s)he observes it, but it is the same mass. Before responding to your specific points, let me give an example so that we're on the same page.

 

An observer in our current location 6 billion years ago would be in our current past light cone. They would see a nebula that will one day collapse into our sun. They don't see our sun doing what it is currently doing. They see the same mass, but it hasn't yet collapsed into a star. It is the same mass, but doing something different, as I put it.

 

here below the diagram for an observer inside our past lightcone (point D). His lightcone is smaller than us, point D sees a smaller universe. Since you agreed that what he sees is on the surface of the light cone, he sees a universe that we cannot see, IOW a different universe. This "other universe" is made from a portion of the same "stuff" we are currently observing.

Every world line that crosses D's past light cone also crosses ours. This means that all of the mass which D saw, we currently see.

 

2_you stated "an observer on our past lightcone sees the same stuff (the same mass)"

 

here below the diagram for an observer on our past lightcone. Point E has a lightcone smaller than us, point E sees a smaller universe. He sees a part of the same stuff that we currently do, because the left part of the lightcone coincides with ours, but the right part of his lightcone is different. IOW he sees a different, smaller universe, made partly of the same stuff we are seeing today from point A.

Again, each world line crossing E's past light cone also crosses ours. We see all of the mass that E saw.

 

3_you stated "Someone outside our past light cone would genuinely see different mass,"

here below the diagram for an observer out of our past lightcone. Point B has a lightcone that has the same perimeter than us, but if you putted point B elsewhere it could be differently . Point B sees a similar universe, but the image that he receives is completely different because what he sees is on the surface of his lightcone. The only point that he sees the same with us is point C. He sees a part of the same stuff than we do, and part of stuff that we don't actually see. But we will see it in the future. If I understand correctly, there is always an overlap between both lightcones, no matter where you put point B.

B can observe mass which we cannot observe because there are world lines crossing his light cone which doesn't cross ours. The would be at the extreme bottom left of his past light cone.

 

If the same world line crosses the past light cone of two different observers then both observers see the same 'stuff' or the same 'mass'. The mass may be doing something different at the time of each observation (different events), but it is the same mass (same world line).

[michel123456]

yes, and I should be more specific to avoid confusion. An observer in or on our past lightcone cannot see mass that is unobservable to us.

 

The mass may be doing something different when (s)he observes it, but it is the same mass. Before responding to your specific points, let me give an example so that we're on the same page.

 

An observer in our current location 6 billion years ago would be in our current past light cone. They would see a nebula that will one day collapse into our sun. They don't see our sun doing what it is currently doing. They see the same mass, but it hasn't yet collapsed into a star. It is the same mass, but doing something different, as I put it.

Perfectly correct.

 

 

Every world line that crosses D's past light cone also crosses ours. This means that all of the mass which D saw, we currently see.

Yes, but we see more than that.

 

Again, each world line crossing E's past light cone also crosses ours. We see all of the mass that E saw.

Yes, but we see more than that.

 

 

B can observe mass which we cannot observe because there are world lines crossing his light cone which doesn't cross ours. The would be at the extreme bottom left of his past light cone.

Yes

 

 

If the same world line crosses the past light cone of two different observers then both observers see the same 'stuff' or the same 'mass'. The mass may be doing something different at the time of each observation (different events), but it is the same mass (same world line).

 

(emphasis mine)

Yes. We are in almost complete agreement.

I word it differently saying that they see different things.

I should have stated they see different events.

Anyway, the image in the telescope is different.

 

AND there are no 2 positions on the diagram that see the same thing (event).

Edited November 17, 2011 by michel123456

[Iggy]

Yes, but we see more than that.

Right. When I originally said "I would say that an observer inside or on our past lightcone sees the same stuff (the same mass), but that the mass is doing different things.", I didn't mean to imply that (s)he saw all of the same mass, but my wording was sloppy in that.

 

Yes. We are in almost complete agreement.

 

I word it differently saying that they see different things.

I should have stated they see different events.

Anyway, the image in the telescope is different.

 

AND there are no 2 positions on the diagram that see the same thing (event).

+1