Expansion of our universe (solar system).

Cú Giobach · 17-11-2011 9:17am

rccaulfield wrote: »

Ok, Thank you for that, its becoming clearer now. Its been an eye opening thread for me. My question about the observed expansion to neptune has been answered. One last thing.
Do cosmologists account for the expansion rate between galaxies near to the milky way and earth? I realise these are well within the zone where the expansion is definitely observable(3000mpcs). But as you say the expansion is happening everywhere, however if i take you up right, it cannot be observed at even our galaxies diameter due to local forces and our inability to precisely predict all forces acting at the same time, i.e gravity, etc. But nevertheless it must do something.
E.g. if we can accurately measure the mass and speed of a star on the other side of the galaxy(or a nearby galaxy if you prefer) and all forces are accounted for then the tiny expansion of space between the earth and said star must be accounted for no? Otherwise the perceived position of the star would be out? Or is the expansion simply so tiny that cosmologists are happy to leave things like that to an approximation?

I think where you are going wrong here, is thinking of space as something tangible that is expanding, and consequently exerting a measurable force.
If it worked this way then an object given a "push" in space, like say one of the Voyager probes, would eventually slow down and stop unless energy was continually injected to keep it moving, pushing against the "aether" you could say.

Try not to think of expansion as "space expanding" but rather "distance increasing".
If you move two objects apart the "space" between them has grown ie: the distance has increased, but nothing tangible or new has been created. The recession we see are not things being "pulled along" by expanding space or being "pushed apart" by a force, but a motion away from each other that they always had. (I'm not sure how the speeding up of expansion comes into this model, so ........???)

himnextdoor · 31-12-2011 10:10am

If space is not expanding then what is? And how could light be red-shifted?

In the balloon analogy, all the relative motion of the galaxies can be accounted for by expansion. Stop the expansion and the galaxies will no longer recede. This suggests that the galaxies have little, if any, angular momentum at all with respect to an apparent common centre. And the longer expansion goes on, the less impact gravity will have on the future as galaxies will be too far apart to have any influence on each other. This means any future galactic interaction would be the result of an increasingly rare chance encounter.

All galaxies are flying apart is equivalent to all galaxies are following a path that is perpendicular to the surface of the same sphere.

Consider two galaxies on opposite sides of the Universe; in order for them to recede from each other at the speed of light, the Universe would need to be expanding at half the speed of light.

Or, going back to the balloon and assuming that 'line of sight' is along the circumference, in order for the two most distant galaxies to appear to recede at the speed of light requires that the Universe is expanding at a rate of at least 0.637 c, roughly.

This is actual velocity rather than apparent velocity; the galaxies are moving away from a central point at speed, 0.637 c, and in opposite directions.

We could do a calculation; our galaxy is travelling at 900,000 mph relative to the Cosmic Background Radiation; that's 0.0013 c. So, any galaxy that is receding from us at an apparent speed of light has to occupy a point in the Universe, of a set of points, the produces a red-shift of 100% or one octave, if you like.

If we deny expansion of space then it cannot account for red-shift so all the shift will have to depend on the actual velocity of the other galaxy and its angular position to the hypothetical 'common sphere' with respect to ours.

To obey relativity, the galaxy cannot exceed the speed of light whch means it cannot be travelling allong the same path and in the same direction as ours; it would have to trave at 1.0026 c.

And if the galaxy was travelling in the opposite direction, along the same path, then it would have to travel at 0.9987 c; the lower limit.

This would mean that the Universe would have to be expanding at very nearly twice the speed of light. (A galaxy on the opposite side of the universe is travelling at that speed too, away from a common sphere!)

To be honest, I'm just not comfortable with that. I mean, where does the energy come from? Why are galaxies further away accelerating? And at an apparently increasing rate?

For me, there can only be three reasons why galaxies accelerate; the edge of the Universe is strongly attractive to matter; the centre of the Universe is increasingly repulsive to matter or space interacts with matter.

But maybe they're not accelerating; maybe something else is going on; what if gaxaxies were actually shrinking?

Another alternative would be that we're actually travelling the opposite direction, toward a huge blacke-hole type object at the centre of the Universe.

But to the OP; if the expansion of space results in a force capable of accelerating a galaxy then it is reasonable to think it should be factored in to any calculations involving the solar system; if the space between the earth and the sun is expanding and the space between earth and Mars is expanding and so on with the other planets, then from the sun's perspective, the planets would all be drifting away slightly. But if the space inside all the masses of the solar system expanded too, then perspective would be maintained.

This would mean that the planets have settled into orbits that took that 'expansion' force into account; that expansion is a variable that was significant to the way material was 'selected' by gravity to occupy certain orbits.

Not taking this force into account gives rise to a built in error to the mathematics of celestial mechanics.

rccaulfield · 02-01-2012 8:50pm

himnextdoor wrote: »

If space is not expanding then what is? And how could light be red-shifted?

In the balloon analogy, all the relative motion of the galaxies can be accounted for by expansion. Stop the expansion and the galaxies will no longer recede. This suggests that the galaxies have little, if any, angular momentum at all with respect to an apparent common centre. And the longer expansion goes on, the less impact gravity will have on the future as galaxies will be too far apart to have any influence on each other. This means any future galactic interaction would be the result of an increasingly rare chance encounter.

All galaxies are flying apart is equivalent to all galaxies are following a path that is perpendicular to the surface of the same sphere.

Consider two galaxies on opposite sides of the Universe; in order for them to recede from each other at the speed of light, the Universe would need to be expanding at half the speed of light.

Or, going back to the balloon and assuming that 'line of sight' is along the circumference, in order for the two most distant galaxies to appear to recede at the speed of light requires that the Universe is expanding at a rate of at least 0.637 c, roughly.

This is actual velocity rather than apparent velocity; the galaxies are moving away from a central point at speed, 0.637 c, and in opposite directions.

We could do a calculation; our galaxy is travelling at 900,000 mph relative to the Cosmic Background Radiation; that's 0.0013 c. So, any galaxy that is receding from us at an apparent speed of light has to occupy a point in the Universe, of a set of points, the produces a red-shift of 100% or one octave, if you like.

If we deny expansion of space then it cannot account for red-shift so all the shift will have to depend on the actual velocity of the other galaxy and its angular position to the hypothetical 'common sphere' with respect to ours.

To obey relativity, the galaxy cannot exceed the speed of light whch means it cannot be travelling allong the same path and in the same direction as ours; it would have to trave at 1.0026 c.

And if the galaxy was travelling in the opposite direction, along the same path, then it would have to travel at 0.9987 c; the lower limit.

This would mean that the Universe would have to be expanding at very nearly twice the speed of light. (A galaxy on the opposite side of the universe is travelling at that speed too, away from a common sphere!)

To be honest, I'm just not comfortable with that. I mean, where does the energy come from? Why are galaxies further away accelerating? And at an apparently increasing rate?

For me, there can only be three reasons why galaxies accelerate; the edge of the Universe is strongly attractive to matter; the centre of the Universe is increasingly repulsive to matter or space interacts with matter.

But maybe they're not accelerating; maybe something else is going on; what if gaxaxies were actually shrinking?

Another alternative would be that we're actually travelling the opposite direction, toward a huge blacke-hole type object at the centre of the Universe.

But to the OP; if the expansion of space results in a force capable of accelerating a galaxy then it is reasonable to think it should be factored in to any calculations involving the solar system; if the space between the earth and the sun is expanding and the space between earth and Mars is expanding and so on with the other planets, then from the sun's perspective, the planets would all be drifting away slightly. But if the space inside all the masses of the solar system expanded too, then perspective would be maintained.

This would mean that the planets have settled into orbits that took that 'expansion' force into account; that expansion is a variable that was significant to the way material was 'selected' by gravity to occupy certain orbits.

Not taking this force into account gives rise to a built in error to the mathematics of celestial mechanics.

I suppose what i've learned in this thread is that the expansion at our solar systems level is like trying to measure the force of gravity between me and my laptop screen here. It's there(we have mass) but other factors and forces present that make the measurement impossible and neglible. But it's still there and falsifiable. Same for the expansion between the earth and neptune.

As for the are the galaxies shrinking instead idea, yea possible but if you think about it for awhile, if that were happening there would be no flying apart at the massive scale we see. They would just sit there in space and appear a little further apart depending on the rate of shrinkage and diameter of galaxies involved. Plus the far side of the galaxy would be shrinking toward us and the near side shrinking away as it gets smaller so that would send different spectra of light our way which isn't seen.

himnextdoor · 02-01-2012 11:10pm

rccaulfield wrote: »

I suppose what i've learned in this thread is that the expansion at our solar systems level is like trying to measure the force of gravity between me and my laptop screen here. It's there(we have mass) but other factors and forces present that make the measurement impossible and neglible. But it's still there and falsifiable. Same for the expansion between the earth and neptune.

As for the are the galaxies shrinking instead idea, yea possible but if you think about it for awhile, if that were happening there would be no flying apart at the massive scale we see. They would just sit there in space and appear a little further apart depending on the rate of shrinkage and diameter of galaxies involved. Plus the far side of the galaxy would be shrinking toward us and the near side shrinking away as it gets smaller so that would send different spectra of light our way which isn't seen.

I think that the spectral changes in the local galaxy due to one side shrinking faster than the other would be virtually indistinguishable from the shifts caused by orbital velocity.

In fact, if it was some kind of super-massive black hole that was drawing us inward, then by the time the gravity of the black hole was able to stretch space to the extent that the amount of red-shift varied significantly across the galaxy then the galaxy would already be near the accretion disk.

I think that we, and most of the galaxies we can observe, are quite a long way from the centre of the Universe and so space is being stretched less.

Also, as the galaxy shrinks, so do our yardsticks; as one metre becomes half a metre, one million light years becomes two; it would look as if the other galaxies were receding. And that is adding to the effect that stretching space has on shift.

An interesting feature of this model is that the centre of the Universe is able to absorb radiation at all frequencies. If it were the closest thing in nature to a perfect black body then we should expect it to radiate thermal energy which should be detectable as microwaves today.

Perhap Cosmic Background Radiation proves the existence of a large central black hole.

Zubeneschamali · 04-01-2012 11:49am

himnextdoor wrote: »

I think that we, and most of the galaxies we can observe, are quite a long way from the centre of the Universe and so space is being stretched less.

There is no centre of the Universe, all parts are uniformly receding from all other parts.

We can tell because all distant galaxies are red-shifted equally in all directions, so if this was the result of everything expanding away from a central point, we would have to be at the centre.

himnextdoor · 04-01-2012 1:12pm

Zubeneschamali wrote: »

There is no centre of the Universe, all parts are uniformly receding from all other parts.

We can tell because all distant galaxies are red-shifted equally in all directions, so if this was the result of everything expanding away from a central point, we would have to be at the centre.

Of course there is a centre of the Universe otherwise the Universe would be infinite. If there was a big bang then there was a point of origin; that would be roughly the centre of mass of the Universe and no matter what the topology space-time is shaped like.

Something that's been puzzling me; apparently, astronomers have located objects in the Universe that are over 13-billion light-years away. But suppose that a billion years ago that there was a star-system like ours with advanced life-forms, like ours, and with the same kind of equipment we use now to view these objects. What would they see? If their telescopes could 'see' 13-billion light-years across space then they would have been able to se the big bang directly wouldn't they?

We just need to 'see' a little further but what if all we see are more stars and galaxies? And what if there are similar objects 14- or 15-billion light-years away from our galaxy in every direction?

Cú Giobach · 04-01-2012 1:46pm

himnextdoor wrote: »

Of course there is a centre of the Universe otherwise the Universe would be infinite. If there was a big bang then there was a point of origin; that would be roughly the centre of mass of the Universe and no matter what the topology space-time is shaped like.

Something that's been puzzling me; apparently, astronomers have located objects in the Universe that are over 13-billion light-years away. But suppose that a billion years ago that there was a star-system like ours with advanced life-forms, like ours, and with the same kind of equipment we use now to view these objects. What would they see? If their telescopes could 'see' 13-billion light-years across space then they would have been able to se the big bang directly wouldn't they?

We just need to 'see' a little further but what if all we see are more stars and galaxies? And what if there are similar objects 14- or 15-billion light-years away from our galaxy in every direction?

There is no centre to the universe just like there is no centre on the surface of a sphere.

There are lots more galaxies further than 14 billion light years away, think of our light horizon of 13.7 billion light years like the horizon at sea, it is not an actual "barrier" or edge and is like a bubble surrounding you. If you were to travel 13.7 billion light years from Earth you would still see 13.7 billion light years around you, but where the Earth would be, would now be your horizon.

himnextdoor · 05-01-2012 2:22am

Cú Giobach wrote: »

There is no centre to the universe just like there is no centre on the surface of a sphere.

And from where on the surface of the sphere did the big bang occur?

If the centre of the sphere of the Universe has no relationship with the surface of the sphere, the area of which is constantly increasing, then gravity will lose out against expansion at a galactic scale.

Is that true?

The thing is, if we imagine that the matter in the Universe has formed a spherical shell which is expanding away from the centre of origin then it would appear that since we can see the same density of matter in all six directions, to the front, to the rear, to the left, to the right, up and down, the shell must be at least 26-billion light-years thick!

It must be at least 26-billion light-years to the origin of the big bang because objects that are 13-billion light-years away from us in opposite directions must be 26-billion light-years away from each other. This means that the Universe is sat on the surface of a sphere whose radius is more than 26-billion light-years.

There are objects that are 26-billion light-years away from the origin of the big bang; how is this possilble?

Cú Giobach · 05-01-2012 9:04am

himnextdoor wrote: »

And from where on the surface of the sphere did the big bang occur?

If we are imagining a flat 2 dimensional universe wrapped around into a sphere that expanded from a point, then every point on the surface is where expansion started, and the actual "place" expansion started (the centre of the sphere) is no longer in that universe.
Move up one dimension to a 3 dimensional universe wrapped around in a higher dimension and the point of origin of our universe is both everywhere and also no longer in this universe.

If the centre of the sphere of the Universe has no relationship with the surface of the sphere, the area of which is constantly increasing, then gravity will lose out against expansion at a galactic scale.

Is that true?

Well the expansion is accelerating.

The thing is, if we imagine that the matter in the Universe has formed a spherical shell which is expanding away from the centre of origin then it would appear that since we can see the same density of matter in all six directions, to the front, to the rear, to the left, to the right, up and down, the shell must be at least 26-billion light-years thick!

It must be at least 26-billion light-years to the origin of the big bang because objects that are 13-billion light-years away from us in opposite directions must be 26-billion light-years away from each other. This means that the Universe is sat on the surface of a sphere whose radius is more than 26-billion light-years.

There are objects that are 26-billion light-years away from the origin of the big bang; how is this possilble?

You seem to be confusing our "light horizon" with the actual size of the universe. The Universe could be infinite in size but because light has only had 13.7 billion years to travel, then no matter where you are you will only be able to observe yourself at the centre of a bubble with a radius of 13.7 billion light years, trying to look further than that and you would be trying to see something before it was there.
Also the Universe didn't expand from the start at exactly the speed of light making it 26 billion light years across in 13 billion years.

The thing is we just don't know what shape the Universe is. Imagining it is a sphere would be like someone on our hypothetical two dimensional universe wrapped into a sphere believing that their universe is a flat circle, because they can't comprehend a sphere in 3 dimensions.

Think about this, if we can see something at 13 billion light years distant then the light has been travelling for 13 billion years to reach us, in all that time the object has been moving away from us and is much further away now than when the light left it, as a general consensus over 40 billion ly's .

Zubeneschamali · 05-01-2012 10:46am

himnextdoor wrote: »

The thing is, if we imagine that the matter in the Universe has formed a spherical shell which is expanding away from the centre of origin

If we imagine that, we are barking up the wrong tree.

There is no centre. The Universe is not a 3D sphere.

himnextdoor · 05-01-2012 11:43am

Cú Giobach wrote: »

If we are imagining a flat 2 dimensional universe wrapped around into a sphere that expanded from a point, then every point on the surface is where expansion started, and the actual "place" expansion started (the centre of the sphere) is no longer in that universe.
Move up one dimension to a 3 dimensional universe wrapped around in a higher dimension and the point of origin of our universe is both everywhere and also no longer in this universe.

So the big bang was simply a separation of all the points in space. All points behave as a bubble, expanding away leaving a central void. But why did the points separate? If it is because they are repulsive to each other in nature then why doesn't the central void fill up with space in order to reduce the expansion pressure? That would also make the Universe spherical; a cosmically comfortable shape and still intimately bound to the origin of the big-bang.

Cú Giobach wrote: »

Well the expansion is accelerating.

Why would it do that?

Cú Giobach wrote: »

You seem to be confusing our "light horizon" with the actual size of the universe. The Universe could be infinite in size but because light has only had 13.7 billion years to travel, then no matter where you are you will only be able to observe yourself at the centre of a bubble with a radius of 13.7 billion light years, trying to look further than that and you would be trying to see something before it was there.
Also the Universe didn't expand from the start at exactly the speed of light making it 26 billion light years across in 13 billion years.

How can the Univers be infinite in size and expanding?

We shouldn't be able to see anything older than 13.7 billion years; such objects would precede the big bang. there may well be objects that are 40 billion light-years away but it will be a long time before we can see them.

But we do see galaxies that existed 13.1 billion years ago which means it must have been formed only a few hundred million years after the Universe came into existence. If only we could see a little further perhaps at a slightly longer wavelength; imagine finding a 15-billion year-old galaxy. Then we'd have to think, wouldn't we?

Cú Giobach wrote: »

The thing is we just don't know what shape the Universe is. Imagining it is a sphere would be like someone on our hypothetical two dimensional universe wrapped into a sphere believing that their universe is a flat circle, because they can't comprehend a sphere in 3 dimensions.

But it is reasonable to suppose that it is a sphere; explosion effect space within a sphere.

Cú Giobach wrote: »

Think about this, if we can see something at 13 billion light years distant then the light has been travelling for 13 billion years to reach us, in all that time the object has been moving away from us and is much further away now than when the light left it, as a general consensus over 40 billion ly's .

Travelling in what direction though? They must be travelling away from a common centre, on divergent paths, in order to appear to be separating equally from each other. This would mean that their relative angular positions don't change very much or else it would appear that many galaxies were on converging paths. This is not what we observe. In the balloon analogy this equates to the galaxies being painted into fixed positions more or less equidistant from their neighbours.

The galaxies are moving perpendicular to the surface of the sphere, away from a common centre. But toward what? Why are they accelerating?

Or are they? Perhaps it is simply the case that those galaxies with higher velocity started out with more energy; perhaps the fastest galaxies are the ones the were formed first and were 'launched' by a force that reduced as the central mass decreased. Perhaps there is no acceleration at all and maybe all the galaxies are travelling at different but constant speeds that is proportional to their age.

It seems simpler that way.

Cú Giobach · 05-01-2012 11:58am

himnextdoor wrote: »

So the big bang was simply a separation of all the points in space. All points behave as a bubble, expanding away leaving a central void. But why did the points separate? If it is because they are repulsive to each other in nature then why doesn't the central void fill up with space in order to reduce the expansion pressure? That would also make the Universe spherical; a cosmically comfortable shape and still intimately bound to the origin of the big-bang.

Why would it do that?

How can the Univers be infinite in size and expanding?

But it is reasonable to suppose that it is a sphere; explosion effect space within a sphere.

Travelling in what direction though? They must be travelling away from a common centre, on divergent paths, in order to appear to be separating equally from each other. This would mean that their relative angular positions don't change very much or else it would appear that many galaxies were on converging paths. This is not what we observe. In the balloon analogy this equates to the galaxies being painted into fixed positions more or less equidistant from their neighbours.

The galaxies are moving perpendicular to the surface of the sphere, away from a common centre. But toward what? Why are they accelerating?

Or are they? Perhaps it is simply the case that those galaxies with higher velocity started out with more energy; perhaps the fastest galaxies are the ones the were formed first and were 'launched' by a force that reduced as the central mass decreased. Perhaps there is no acceleration at all and maybe all the galaxies are travelling at different but constant speeds that is proportional to their age.

It seems simpler that way.

Much of what you ask here has been already been explained in this thread.

You seem to be trying to visualise the universe as some sort of explosion within a space, yet the creation of the universe was the creation of space.
Don't get confused with the 2d universe wrapped into a sphere analogy, in that there is no "space" inside the sphere, all that exists is the surface.
We call what seems to be causing the expansion to accelerate "dark energy" because we don't understand it (yet).

We shouldn't be able to see anything older than 13.7 billion years; such objects would precede the big bang. there may well be objects that are 40 billion light-years away but it will be a long time before we can see them.

But we do see galaxies that existed 13.1 billion years ago which means it must have been formed only a few hundred million years after the Universe came into existence. If only we could see a little further perhaps at a slightly longer wavelength; imagine finding a 15-billion year-old galaxy. Then we'd have to think, wouldn't we?

These two comments are a bit odd, in one you correctly state we cant see anything further than 13 odd billion ly and then state we might see something further

No matter what the wavelength of light it still travels at the speed of light (the hint is in the name).
Read this. .

himnextdoor · 05-01-2012 1:07pm

Zubeneschamali wrote: »

If we imagine that, we are barking up the wrong tree.

There is no centre. The Universe is not a 3D sphere.

I'm not sure how you can know that.

How can the Universe be expanding in every direction except away from the origin of the big bang? If it wasn't expanding in that direction then there must be lots of galactic collisions practically all of which are out of view of our telescopes; are all the galaxies being accelerated toward each other on the other side of the Universe?

If that was the case then our galaxy must be in an optimal place in order to avoid such a catastrophe. We're lucky, huh?

If we consider our galaxy as being stationary then we would observe that all other galaxies were moving away from us and might determine that they are heading toward some boundary. But if you choose another galaxy to be the centre of the Universe then you find that all the other galaxies are headed toward that same boundary. In this part of the Universe everything seems to be moving toward a common edge and away from a central point.

If the Universe is like the surface of a sphere then that picture would change; as the galaxies cross an equator the will begin to get closer to each other. If one of these galaxies is chosen as the centre of the Universe it would appear that the Universe is contracting toward that point. There would be other points of view where there was an uneven distribution of galaxies; on the one side the Universe appears to be expanding and on the other it appears to be contracting.

In any event, there is a point where everything eventually meets and that point can be considered the centre of the Universe regardless of the shape of the Universe. It would be the largest black hole in the Universe and everything would be doomed to fall into it.

Surely we may as well consider the Universe as being spherical in shape, with a huge central black hole.

That is even if we don't consider the Universe expanding outward, the galaxies moving away from a common centre.

But a thermodynamic model would be very satisfying. A medium of zero density and zero energy containing a point whose density is greater than zero and with energy greater than zero could reduce the Universe to terms of pressure equalisation and heat distribution and would be spherical by nature.

That would be nice.

Zubeneschamali · 05-01-2012 1:29pm

himnextdoor wrote: »

I'm not sure how you can know that.

The distant galaxies we see, where we can get a measurable cosmological red shift, all seem to be moving away from us, in every direction.

If the Universe was as you are imagining it then some galaxies on the opposite side of the centre would be moving away twice as fast, while galaxies on our side would be traveling at much the same speed as us.

That is not the case. We know for a fact that they all appear to be moving away from us symmetrically in all directions. The only way this could fit with your model is if by some ridiculous coincidence we just happen to be exactly at the centre of the entire Universe.

In fact, the same thing would be seen from any other galaxy: all the others are moving away symmetrically, because the galaxies are not moving through a 3D universe away from a central point: space itself is expanding, and all the distances between all the galaxies are growing.

himnextdoor · 05-01-2012 1:53pm

Cú Giobach wrote: »

These two comments are a bit odd, in one you correctly state we cant see anything further than 13 odd billion ly and then state we might see something further
No matter what the wavelength of light it still travels at the speed of light (the hint is in the name).
Read this. .

That's rather my point; a galaxy that appears 13 billion light years away may be billions of light years further away now but if we found an older object then we might have to re-think the big bang model.

I mean, if we observed a galaxy that was 15 billion light years away wouldn't that cause a bit of a stir?

The light from 13 billion year old galaxies should be red-shifted almost to the extent of the Cosmic Background Radiation; it's not that much younger than the 'last scattering' is it? You would imagine that there were some super energetic photons released at that time so those photons should be more visible today. And yet the view of these galaxies show them against a dark sky; surely such distant galaxies could only be seen at longer wavelengths comparable to the background radiation. Why aren't these galaxies swamped by noise from the last scattering?

It just seems a bit odd.

Cú Giobach · 05-01-2012 2:02pm

himnextdoor wrote: »

That's rather my point; a galaxy that appears 13 billion light years away may be billions of light years further away now but if we found an older object then we might have to re-think the big bang model.

I mean, if we observed a galaxy that was 15 billion light years away wouldn't that cause a bit of a stir?

And if an elephant floated over my house we might have to rethink gravity.

himnextdoor · 06-01-2012 3:31am

Cú Giobach wrote: »

And if an elephant floated over my house we might have to rethink gravity.

Case closed then.

himnextdoor · 08-01-2012 10:22am

Zubeneschamali wrote: »

The distant galaxies we see, where we can get a measurable cosmological red shift, all seem to be moving away from us, in every direction.

Yes.

Zubeneschamali wrote: »

If the Universe was as you are imagining it then some galaxies on the opposite side of the centre would be moving away twice as fast, while galaxies on our side would be traveling at much the same speed as us.

Sort of. The galaxies adjacent to ours have a similar velocity away from the central point and indeed you could think of the galaxies as being an arrangement of 'shells' that relate to their radial velocities; outer shells moving faster than inner shells.

So, for adjacent galaxies there are two components to their relative velocity; they are on divergent paths and that increases their distance at a certain rate and space expansion contributes another mechanism for increasing distance.

For galaxies in different shells; the radial velocity reflect the energy inparted by the big bang itself, the galaxies closest to the edge constitute the earliest dissipation of energy and are travelling at a contant speed away from the centre plus of course, space expansion between us and them gives a second component to their apparent recession.

Galaxies further from the centre than our are moving faster than ours because they started out faster; subsequent generation started their journeys with less energy.

A sphere shaped Universe would look exactly as we observe to be if a big bang occurred at the actual centre.

Similarly, if a kettle boils it releases steam from the spout which forms a cloud that is roughly spherical. If we consider the cloud as a Universe and each water molecule to be a galaxy then some sentient being observing his 'Universe' from inside his 'galaxy' would conclude that he exists in a Universe that behaves like ours and that every molecule could consider irself the central point. The water molecules do recede away from each other.

(Imagine his consternation though when his Universe ends up as condensation on some cosmic kitchen window.)

Zubeneschamali wrote: »

That is not the case. We know for a fact that they all appear to be moving away from us symmetrically in all directions. The only way this could fit with your model is if by some ridiculous coincidence we just happen to be exactly at the centre of the entire Universe.

We don't know for a fact that distant galaxies are receding at an accelerating rate; we haven't had time to register changes in velocity in any individual galaxy over time. Like I said, they may travel at constant speed. And what if it's actually slowing down; we wouldn't detect that either.

Imagine that you had a catapult that fired projectiles made from its elastic. The first time it is fired it will impart a certain amount of energy into the missile at the expense of some mass. The velocity of the projectiles will be related to energy of the elastic which is diminishing on every shot. Eventually, of course, the elastic would break.

From the perspective of the projectiles in flight and assuming they don't stop, the missiles are receding from each other. And each one could consider itself the central point of something. Especially if you add a little expansion.

Suppose you fired the catapult straight up and rapidly from the earth. Again, from the point of view of any one projectile for a certain period of time, expansion seems to occur in the space between them. But gravity wins; eventually the projectiles slow down, and then fall toward the earth.

Perhaps the galaxies are still expending that initial burst of energy. The scary thought is though, that equally the galaxies are on their way down; they would still follow divergent paths due to expansion. At least until the central point curves their paths toward the centre.

Theoretically, our Universe could be in the creation phase or the destruction phase.

Don't forget, the galaxies would slow at the same rate; some would fall from a higher orbit than others in order to satisfy conservation. We would see what we see. At any stage. Until destruction. And the it's 'Back on the merry go round'.

Zubeneschamali wrote: »

In fact, the same thing would be seen from any other galaxy: all the others are moving away symmetrically, because the galaxies are not moving through a 3D universe away from a central point: space itself is expanding, and all the distances between all the galaxies are growing.

No, it's because they are. And you can't discount shrinking galaxies which might affect the value of the cosmological constant.

Zubeneschamali · 09-01-2012 10:29am

himnextdoor wrote: »

Sort of. The galaxies adjacent to ours have a similar velocity away from the central point and indeed you could think of the galaxies as being an arrangement of 'shells' that relate to their radial velocities; outer shells moving faster than inner shells.

As I said, if this were true, then the galaxies on the far side of the centre would be receding at double speed.

There is no such effect. This is a fact.

You are completely wrong.

himnextdoor · 10-01-2012 6:21am

Zubeneschamali wrote: »

As I said, if this were true, then the galaxies on the far side of the centre would be receding at double speed.

There is no such effect. This is a fact.

You are completely wrong.

How do you know they're not?

I recognise though that we can't locate the centre until we locate the edge then it will all make perfect sense.

Cú Giobach · 10-01-2012 8:26am

himnextdoor wrote: »

I recognise though that we can't locate the centre until we locate the edge then it will all make perfect sense.

Your ideas make me think of this.

Zubeneschamali · 10-01-2012 10:14am

himnextdoor wrote: »

How do you know they're not?

Have you even heard of red shift?

The fact is that we see uniform red shift in every direction, just as if we were right at the centre of expansion. Astronomers have spent a lot of time measuring red shifts and distances. Look it up.

The two competing theories to explain it were the steady state universe, in which new space is continually being created, forever, and the big bang in which space appears at a singularity and then expands forever, or expands and then contracts back to a singularity.

Neither of these actual theories has a centre to the Universe, and neither bears any resemblance to your crayon drawings.

himnextdoor · 11-01-2012 9:50am

Zubeneschamali wrote: »

Have you even heard of red shift?

The fact is that we see uniform red shift in every direction, just as if we were right at the centre of expansion. Astronomers have spent a lot of time measuring red shifts and distances. Look it up.

The two competing theories to explain it were the steady state universe, in which new space is continually being created, forever, and the big bang in which space appears at a singularity and then expands forever, or expands and then contracts back to a singularity.

Neither of these actual theories has a centre to the Universe, and neither bears any resemblance to your crayon drawings.

And yet entire clusters of galaxies collide; I bet occupants of those galaxies don't consider themselves at the centre.

If red-shift were uniform then galaxies wouldn't collide.

At some stage the Universe was the size of a pea, then a beach-ball; both these things have a physical centre - where did the centre go?

Like I said before, a boiling kettle produces a cloud that behaves just like the Universe; you cannot tell from studying the movement of the particles of steam relative to each other where the centre of origin is located, which in this case is the spout of the kettle.

That's what complicates it; we've been out of the kettle for so long that we have lost sight of the spout. But it continues to contribute energy to the motion and therein lies a clue; expansion is faster away from the direction of the spout.

Now, back to my colouring book.

Myrddin · 11-01-2012 10:36am

Because there are galaxies outside of our light horizon, more than 13.7 billion light years away, in every direction...this obviously means the universe is 'larger' than 13.7 billion light years. If it's only 13.7 billion years old, is this how the theory of inflation explains how the size is bigger than the age [in a crayola nutshell of course]?

Maximus Alexander · 11-01-2012 3:00pm

EnterNow wrote: »

Because there are galaxies outside of our light horizon, more than 13.7 billion light years away, in every direction...this obviously means the universe is 'larger' than 13.7 billion light years. If it's only 13.7 billion years old, is this how the theory of inflation explains how the size is bigger than the age [in a crayola nutshell of course]?

The observable universe has been calculated to have a radius of about 46 billion light-years. Objects that can be seen at 13.7 billion ly would have been at a distance of about 42 million ly at the time that the light was emitted. Due to the expansion of space it has taken the light 13.7 billion years to reach us. Their actual distance at this point would be approximately 46 billion ly, so 46 billion ly is the distance that we can see in any direction.

The actual size of the universe appears to be larger than this since there is no evidence that any light has had time to circumnavigate the universe. Nobody can say with certainty how much larger it may be.

Expansion of our universe (solar system).

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