# A pretty big question

## Main Question or Discussion Point

Hi I'm 16 this is my first post here, something just struck me, I know nothing about physics and alot of things but I guess this is the right place to ask this kind of thing and I'm intrigued to know what peoples thoughts are on this....

If it takes 90% of estimated time to see the earliest observable galaxy, its next position in the sky, the galaxy should also project an image of it consisting of its light at a later time, this should cause it to create a streak of its own image across the sky, unless it was coming towards us or moving very jerkily and fast at intervals over long distances. These streaks would not necessarily be straight. These streaks would not be huge but enough to make the object untrue to its original shape or size. Which leads onto whether galaxies are actually moving away from each other, if they were surley this streak would be visible in a radial direction facing away from a certain point relying on the assumption there is a centre to the observable universe. I suppose this contradicts the big bang theory and the notion of the observable universe expanding but i just cant get my head round it.

Danger
Gold Member
Welcome to PF, Trisss.
There is no 'centre' of the universe, if it is indeed infinite but bounded as it appears. More precisely, any point can be considered the centre. No matter where you start, you can travel in any direction and eventually end up where you started.
I don't get your reference to the time factor as regards light propagation. Think of a garden sprinkler; it puts out only a set number of droplets which sweep outward regardless of how fast it cycles.

Welcome to PF, Trisss.
There is no 'centre' of the universe, if it is indeed infinite but bounded as it appears. More precisely, any point can be considered the centre. No matter where you start, you can travel in any direction and eventually end up where you started.
I don't get your reference to the time factor as regards light propagation. Think of a garden sprinkler; it puts out only a set number of droplets which sweep outward regardless of how fast it cycles.
ok due to an extremely limited knowledge of stuff like this. big bang theory states that all the galaxies are moving away from each other and from where the big bang occurred?

this is completely hypothetical but what i mean is say light from the sun takes 8 minutes to reach earth and it moved in any direction to a new position except from straight towards us or away from us. We would see both the light it produced from its original position and the new positon at different times. My question is why would it not appear that instead of a circular shape we see after it had changed position, we would see an oval due to light from both positions showing... however thinking about it this could only happen where an object was so massive and of a particular shape that we would see light both from its original and new position where it no longer existed at the same time... i think i have answered the question for myself there.

How can you know that you would end up in the same place if we kept going?

Danger
Gold Member
we would see light both from its original and new position where it no longer existed at the same time... i think i have answered the question for myself there.
I believe that you have. The photons from the 8 or 9 minute previous position are the ones that arrive on Earth. The photons from 'now' arrive in the same place after the Earth has left that observing point. That's not really a scientific explanation, but the best that I can come up with right now.
I'll leave it up to one of the astronomy or astrophysics experts to deal with your 'centre of the universe' question. I'm not qualified.

I believe that you have. The photons from the 8 or 9 minute previous position are the ones that arrive on Earth. The photons from 'now' arrive in the same place after the Earth has left that observing point. That's not really a scientific explanation, but the best that I can come up with right now.
I'll leave it up to one of the astronomy or astrophysics experts to deal with your 'centre of the universe' question. I'm not qualified.
I don't think anyone is really qualified (as no one has made a trip around the universe . That doesn't normally stop speculation though :)

I'm personally of the opinion that the universe is potentially infinite in every direction... however due to the speed of light, the observable universe is much smaller (only what can be observed, ironically , with earth at it's centre. If we were to observe the universe from Mars right now (for a given value of 'right now') we would see a slightly different observable universe and the centre of that universe would be Mars. Every point is the centre of it's own observable universe, which fits with either the bounded universe that bends back on itself or the infinite universe theories :)

chroot
Staff Emeritus
Gold Member
We can only see photons when they enter our eyes, or strike our camera sensors. There are certainly rays of light streaming past Pluto, but unless those rays can be diverted somehow so they arrive at Earth, we will not be able to observe them. If there were a lot of gas and dust around to scatter the light, you'd be able to see a galaxy's glow from any direction, like a flashlight in a dense fog. Since intergalactic space is virtually entirely empty, though, light rays from distant galaxies essentially go in perfectly straight lines.

trisss said:
this is completely hypothetical but what i mean is say light from the sun takes 8 minutes to reach earth and it moved in any direction to a new position except from straight towards us or away from us. We would see both the light it produced from its original position and the new positon at different times. My question is why would it not appear that instead of a circular shape we see after it had changed position, we would see an oval due to light from both positions showing...
If you made a photograph with an incredibly long exposure time, say, a thousand years, you would actually be able to see a smear, indicating the movement of some galaxies as they march steadily across the sky. At any given instant, though, the image of the galaxy is crystal clear.

- Warren

DaveC426913
Gold Member
ok due to an extremely limited knowledge of stuff like this. big bang theory states that all the galaxies are moving away from each other and from where the big bang occurred?
Yes on one. No one two.

The theory states that the Big Bang occurred everywhere. The Big Bang is the expansion of space. All points in the universe are the centre.

MATLABdude
I saw a good analogy a while back...

Take a balloon, and put a bunch of dots on it with a marker before blowing it up. Now blow it up. The dots get further apart on the surface of the balloon the more you blow it up.

Now imagine those dots represent galaxies, and you can see how galaxies get further and further apart without needing a common center. Keep in mind that this is just an analogy, and the fact that a balloon is a 3D object and not a 2D one may, or may not have ramifications to the structure of the universe. (Also, I'm just an engineer, not a physicist, or astrophysicist)

DaveC426913
Gold Member
I saw a good analogy a while back...

Take a balloon, and put a bunch of dots on it with a marker before blowing it up. Now blow it up. The dots get further apart on the surface of the balloon the more you blow it up.
It is important to add some detail to this analogy. In the ideal case, the balloon has no spigot i.e. no centre*. No spot on the balloon is special; all points are equal. No point is "the centre from which all other points expand". All dots are moving away from each other, but they are not moving away from any central point.

*For those nitpickers, it is possible to build this to prove the point - seal the spigot and let the balloon expand by heating it.

Danger
Gold Member
*For those nitpickers, it is possible to build this to prove the point - seal the spigot and let the balloon expand by heating it.
Or bake a ball of raisin muffin dough.

It is important to add some detail to this analogy. In the ideal case, the balloon has no spigot i.e. no centre*. No spot on the balloon is special; all points are equal. No point is "the centre from which all other points expand". All dots are moving away from each other, but they are not moving away from any central point.

*For those nitpickers, it is possible to build this to prove the point - seal the spigot and let the balloon expand by heating it.
all points can be moving way from each other from no central point but all points in relation to their previous positions could be, the point would be the center of the inside of the balloon all points would move further away from this point as it inflates. Also if space wasn't infinite it would have a middle point. Am I thinking about this the wrong way?

Danger
Gold Member
the point would be the center of the inside of the balloon all points would move further away from this point as it inflates.
I believe that where you're getting messed up with the balloon analogy is that a balloon is 2D surface, whereas space is 3D (or 4D if you want to include the time factor). That's actually why I introduced the muffin idea. As the dough rises, all of the raisins get farther away from each other in every direction. You can set off from any raisin, in any direction, and following the curvature of the muffin you will end up on the same raisin that you started from. Once again, this is probably not a world-class scientific explanation.

MATLABdude
all points can be moving way from each other from no central point but all points in relation to their previous positions could be, the point would be the center of the inside of the balloon all points would move further away from this point as it inflates. Also if space wasn't infinite it would have a middle point. Am I thinking about this the wrong way?
As Danger says, the universe would be like the surface of the balloon in this analogy. So what about the inside of the balloon? Unfortunately, that's where the analogy breaks down. There is no 'inside of the universe' (that's a popular mechanism in sci-fi... Instead of going around the outside of the balloon, you simply tunnel through the balloon to get from point A to point B on the surface of the balloon; doesn't work however).

EDIT: Doesn't work when the balloon is the universe. Actually, doesn't work for balloons either, since they tend to pop!

Danger
Gold Member
Actually, doesn't work for balloons either, since they tend to pop!
:rofl:

DaveC426913
Gold Member
I believe that where you're getting messed up with the balloon analogy is that a balloon is 2D surface, whereas space is 3D (or 4D if you want to include the time factor). That's actually why I introduced the muffin idea. As the dough rises, all of the raisins get farther away from each other in every direction. You can set off from any raisin, in any direction, and following the curvature of the muffin you will end up on the same raisin that you started from. Once again, this is probably not a world-class scientific explanation.
IMO, the muffin analogy is worse than the balloon analogy; it misses the point. While the muffin analogy is a 3D model, the problem with it is that the muffin has a centre from which all raisins do recede. This defeats the entire purpose of the explanation, since it doesn't show the reader how something can expand but not have a centre of expansion.

IMO, the muffin analogy is worse than the balloon analogy; it misses the point. While the muffin analogy is a 3D model, the problem with it is that the muffin has a centre from which all raisins do recede. This defeats the entire purpose of the explanation, since it doesn't show the reader how something can expand but not have a centre of expansion.
yeah thats pretty much why i could'nt get my head round it.

I still cant understand how everything could be moving further apart from everything without moving away from a particular point though. If everything is getting further apart this must mean that at one time everything was closer and closer and so on until it reaches a point. This seems to be like the kind of thing that is more observable in day to day life and for me a more plausible explanation.

anyone got an analogy that doesn't involve imagining something is not there?

No matter where you start, you can travel in any direction and eventually end up where you started.
That is only one of three possible shapes of the universe predicted by GR according to the cosmological principle

Further, these three models require a great leap of faith by assuming that the entire universe is homogeneous and isotropic beyond the observable range, and the only "evidence" for those properties are that the observable universe appears approximately so.

I have long been pointing out that this is an illogical argument because it is an over-extrapolation of observations. A simplified form of the argument makes this more obvious. Question: "does the universe has an edge beyond our range of observation?" answer, "we can't see an edge, therefore there is no edge." Obviously the fact that we can't see an edge is not really a convincing argument that there is no edge beyond the range that we can see, especially when it forces complex mathematical descriptions that could otherwise be much more simple and intuitive.

Furthermore, recent evidence suggests is that even the locally observable universe is not homogeneous and isotropic. In other words, the hypersphere model of the universe you are referring to is more faith based than theory based at this point.

DaveC426913
Gold Member
yeah thats pretty much why i could'nt get my head round it.

I still cant understand how everything could be moving further apart from everything without moving away from a particular point though. If everything is getting further apart this must mean that at one time everything was closer and closer and so on until it reaches a point.
That's why the balloon analogy explains this best. Everything was closer in the past, and indeed it did originally arise from a single point. But the key is that it is not a particular point at some location on the balloon. Every location on the balloon was originally at the centre. No ponit is special. i.e. there is no point in our current universe that can be considired "the centre".

That's why the balloon analogy explains this best. Everything was closer in the past, and indeed it did originally arise from a single point. But the key is that it is not a particular point at some location on the balloon. Every location on the balloon was originally at the centre. No ponit is special. i.e. there is no point in our current universe that can be considired "the centre".
yeah that was how i thought of it, the point may not be special but it would still be the place everything started moving away from as i guess area of empty space or whatever it is it will not move

DaveC426913
Gold Member
yeah that was how i thought of it, the point may not be special but it would still be the place everything started moving away from as i guess area of empty space or whatever it is it will not move
No!!:grumpy:

Look at the balloon analogy.

The universe is represented by the surface of the balloon. More accurately, our 3D universe is represented by the 2D surface of the balloon.

Alternately, you could think of our universe as the surface of a 3D hyper-sphere seen in 4 dimensions. But that's hard to do, so throw away one dimension and we'll just look at a 2-dimensional universe seen in 3-dimensions.

But remember, our universe is the surface of the balloon, not its volume.

The balloon starts off tiny (ideally, it starts off as a point particle, with a surface area of zero).
So, as the balloon grows, its surface grows; all points on its surface move away from each other.
But there is no point on the surface of the balloon that can be considered "the centre". Or, more accurately: all points on its surface are the centre.

The balloon surface has no centre. No empty space, no unmoving place on it that is in any way different than any other point.

No!!:grumpy:

Look at the balloon analogy.

The universe is represented by the surface of the balloon. More accurately, our 3D universe is represented by the 2D surface of the balloon.

Alternately, you could think of our universe as the surface of a 3D hyper-sphere seen in 4 dimensions. But that's hard to do, so throw away one dimension and we'll just look at a 2-dimensional universe seen in 3-dimensions.

But remember, our universe is the surface of the balloon, not its volume.

The balloon starts off tiny (ideally, it starts off as a point particle, with a surface area of zero).
So, as the balloon grows, its surface grows; all points on its surface move away from each other.
But there is no point on the surface of the balloon that can be considered "the centre". Or, more accurately: all points on its surface are the centre.

The balloon surface has no centre. No empty space, no unmoving place on it that is in any way different than any other point.
im pretty sure i understand what you're saying but i see the center as the point at which the points on the balloon are closest, that place still exists regardless of whether there is stuff there or not?

chroot
Staff Emeritus
Gold Member
Here's a simple way to understand things.

Make a transparency with some dots on it, randomly if you like. Label several of the dots with letters, A, B, C, etc.

Photocopy that transparency onto another transparency, but magnify it by 10% or 20%.

Lay the two transparencies on top of each other, so that the dots labeled A line up. Notice that from A's perspective, every other dot seems to have moved further away.

Now lay the two transparencies on top of each other, so that the dots labeled B line up. Notice that from B's perspective, every other dot seems to have moved further away.

Repeat as necessary, until you reach the conclusion that every dot always sees itself as the center of the expansion -- which, of course, means that there is no center at all.

This analogy readily generalizes to three dimensions.

- Warren

DaveC426913
Gold Member
im pretty sure i understand what you're saying but i see the center as the point at which the points on the balloon are closest, that place still exists regardless of whether there is stuff there or not?
Where is this? All points on the balloon are equidistant from their neighbors. There is no location where points are closest.

You seem to be thinking that there is a centre that is somehow invisible otherwise indistinguishable from everywhere else - but that technically there's still a centre in there somewhere.

Let's throw away a dimension for a moment.

A plain, flat circle (like on a piece of paper) starts off microscopically small - a point if you will.
The circle then expands.
As it does so, we see that it has one thousand tiny notches on it, all equidistant.
The circle expands to a full six inches in diameter. All one thousand points are equidistant from each other around the perimeter of the circle.

Now, would you say that one point on the circle's perimeter is special? That one point is the beginning from which all the other points expanded?

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Where is this? All points on the balloon are equidistant from their neighbors. There is no location where points are closest.

You seem to be thinking that there is a centre that is somehow invisible otherwise indistinguishable from everywhere else - but that technically there's still a centre in there somewhere.

Let's throw away a dimension for a moment.

A plain, flat circle (like on a piece of paper) starts off microscopically small - a point if you will.
The circle then expands.
As it does so, we see that it has one thousand tiny notches on it, all equidistant.
The circle expands to a full six inches in diameter. All one thousand points are equidistant from each other around the perimeter of the circle.

Now, would you say that one point on the circle's perimeter is special? That one point is the beginning from which all the other points expanded?
if the circle expands this means the notches are further apart from each other than they were before the expansion took place. The size of the first circle when the tiny notches are at equal distance from each other but closer together in relation to the expanded circle would be the center of the expanded circle its dots would still be at an equal distance from each other in both cases. I just cant see it any other way than that

now I've said that i've just suddenly got it ha I can see why it took alot of explaining I'd find it really difficult to explain. But if it has an edge/s it can have a center that is how i was thinking of it... this theory would work only if the universe was infinite right?

DaveC426913
Gold Member
this theory would work only if the universe was infinite right?
No. In fact, it requires the universe to not be infinite. Just like the circle and hte balloon are not infinite.