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[Marvin_Edwards]

I have this physics problem regarding simple relativity. The current theory is our universe is expanding and that this expansion is accelerating. The evidence, I think, is the red shift of objects moving away from us at great speed. But there is no accounting for this acceleration. One would expect that, at the point of the Big Bang, the energy outward would be greatest earlier and would reduce during the expansion, rather than increase.

But, suppose we're not expanding, but rather contracting. We know that gravity causes objects to accelerate toward a larger center mass. As objects fall from a tower to the Earth they speed up at a constant rate. So, if the universe were collapsing toward the original center mass of the Big Bang, wouldn't it look just like an expansion?

Stars and constellations nearer the mass would be moving away from us, and toward the center, and accelerating faster than objects farther from the center. We would appear to those objects as if we were accelerating away from them, but they would actually be accelerating away from us.

And objects farther from the center mass than us, would also appear to us to be moving away from us at greater and greater speeds, when actually it would be us moving away from them as we fall toward the center mass at greater and greater speed.

Just thinking...

[Angel Trismegistus]

Interesting thought experiment. One question: Does the red shift phenomenon apply to objects moving toward the observer as well as objects moving away from the observer? Your thought experiment needs the red shift both ways, yes?

[Steve3007]

I have this physics problem regarding simple relativity. The current theory is our universe is expanding and that this expansion is accelerating. The evidence, I think, is the red shift of objects moving away from us at great speed.

Red shift is evidence for expansion, but not necessarily for increase in the rate of that expansion (acceleration). The expansion was deduced from the redshift long before it was further concluded that the rate of expansion was increasing.

But there is no accounting for this acceleration.

Apparently it's something to do with a comparison of the velocity, and therefore distance, of distant objects expected by measuring their redshifts and the distance as measured by their intrinsic brightness. Or something like that.

One would expect that, at the point of the Big Bang, the energy outward would be greatest earlier and would reduce during the expansion, rather than increase.

Yes, if we think of it as being like a conventional expansion in which the objects fly outwards ballistically in all directions, and are therefore slowed by their mutual gravitational attraction. We would expect their initial kinetic energy (function of speed) to gradually be converted to gravitational potential energy (function of position).

But, suppose we're not expanding, but rather contracting. We know that gravity causes objects to accelerate toward a larger center mass. As objects fall from a tower to the Earth they speed up at a constant rate. So, if the universe were collapsing toward the original center mass of the Big Bang, wouldn't it look just like an expansion?

Stars and constellations nearer the mass would be moving away from us, and toward the center, and accelerating faster than objects farther from the center. We would appear to those objects as if we were accelerating away from them, but they would actually be accelerating away from us.

And objects farther from the center mass than us, would also appear to us to be moving away from us at greater and greater speeds, when actually it would be us moving away from them as we fall toward the center mass at greater and greater speed.

You seem to be thinking of this expansion/contraction as a conventional form of expansion/contraction, and so as centering on a point, or localized region, in space. Given that assumption, I can see why you would say that objects both closer to the centre and further away from the centre than us would appear to be receding from us. But that would only work for objects positioned along a line which goes through us and through that centre. It wouldn't work for objects in other parts of the sky. Objects off to the side, the same distance from the centre as us, perpendicular to the line between us and the centre, would appear almost stationary, moving very slowly towards us. That's not what's observed. What's observed is that outside our local cluster of galaxies, objects are receding from us no matter which direction in which you look.

Also, as I understand it, the standard model of this phenomenon is that it is not a conventional expansion/contraction like that. It is not an expansion/contraction from/to a particular point in space. It is regarded as an expansion of space itself, so every point in the current universe is at the centre of the Big Bang. It's happening everywhere. Hence the CMBR - the "echo of the Big Bang" - comes from all over space. Or so they say.

[Marvin_Edwards]

You seem to be thinking of this expansion/contraction as a conventional form of expansion/contraction, and so as centering on a point, or localized region, in space. Given that assumption, I can see why you would say that objects both closer to the centre and further away from the centre than us would appear to be receding from us. But that would only work for objects positioned along a line which goes through us and through that centre. It wouldn't work for objects in other parts of the sky. Objects off to the side, the same distance from the centre as us, perpendicular to the line between us and the centre, would appear almost stationary, moving very slowly towards us. That's not what's observed. What's observed is that outside our local cluster of galaxies, objects are receding from us no matter which direction in which you look.

Ah! I thought there was a bug in my logic and that would be it. Thanks! (I hope I can remember it so I don't make the same mistake in the future).

Also, as I understand it, the standard model of this phenomenon is that it is not a conventional expansion/contraction like that. It is not an expansion/contraction from/to a particular point in space. It is regarded as an expansion of space itself, so every point in the current universe is at the centre of the Big Bang. It's happening everywhere. Hence the CMBR - the "echo of the Big Bang" - comes from all over space. Or so they say.

Yeah, that one strains my imagination. The notion of a Big Bang implies a central mass exploding outward. And my notion of space is that it is a measure of the distance between two objects. Locations outside of the scope of the model can be imagined, which is where the notion of infinity comes from.

Does the model then account for the motion of other galaxies as a triangle, with three lines: one from us to the center, one from the other galaxy to the center and a third line between us and the other galaxy? That would assume an expansion from the center.

Or is there no center in the standard model? And if there is no center, then we must discard the Big Bang model.

[Steve3007]

One question: Does the red shift phenomenon apply to objects moving toward the observer as well as objects moving away from the observer?

No. Objects moving towards an observer are blue shifted.

Ah! I thought there was a bug in my logic and that would be it. Thanks! (I hope I can remember it so I don't make the same mistake in the future).

I guess you can see it if you imagine a load of dots falling towards a centre and then imagine yourself as one of those dots. The phenomenon that you described, of seeing dots both closer and further away from the centre than you, along a radial line, getting further away from you, is the tidal effect. It's the reason why they say that if you fell into a black hole (feet or head first) you'd get torn apart lengthways before reaching it. It's also the reason for the "Roche limit" and why Jupiter's moon Io is so volcanic.

Does the model then account for the motion of other galaxies as a triangle, with three lines: one from us to the center, one from the other galaxy to the center and a third line between us and the other galaxy? That would assume an expansion from the center.

No, because (the model says) there is no centre within 3D space, analogous to the fact that the centre of the earth doesn't exist on the earth's surface.

Or is there no center in the standard model? And if there is no center, then we must discard the Big Bang model.

Yes, no centre. I think a lot of people certainly think, for various reasons, that the name of the model is misleading. But I guess we shouldn't judge a book by its cover.

[Marvin_Edwards]

I guess you can see it if you imagine a load of dots falling towards a centre and then imagine yourself as one of those dots. The phenomenon that you described, of seeing dots both closer and further away from the centre than you, along a radial line, getting further away from you, is the tidal effect. It's the reason why they say that if you fell into a black hole (feet or head first) you'd get torn apart lengthways before reaching it. It's also the reason for the "Roche limit" and why Jupiter's moon Io is so volcanic.

Always learning something new here. I looked up the "Roche limit" and it is about two large bodies and the ability of the larger one to pull apart the smaller one if it gets too close. The tidal effect is similar to the Moon's effect on the Earth's tides. The Moon has enough pull to affect the tides but not enough to pull the ocean's off the face of the Earth. (Thank goodness!)

I seem to recall that there would also be a time-dilation effect as an object accelerates as it is pulled into a black hole.

Does the model then account for the motion of other galaxies as a triangle, with three lines: one from us to the center, one from the other galaxy to the center and a third line between us and the other galaxy? That would assume an expansion from the center.

No, because (the model says) there is no centre within 3D space, analogous to the fact that the centre of the earth doesn't exist on the earth's surface.

Or is there no center in the standard model? And if there is no center, then we must discard the Big Bang model.

Yes, no centre. I think a lot of people certainly think, for various reasons, that the name of the model is misleading. But I guess we shouldn't judge a book by its cover.

I seem to recall images of the visible universe being a large egg-shaped object, an oval rather than a circle. But still, there would be a theoretical center to the object from which the expansion in any direction would be measured, would there not? If I were to "expand" a balloon, it would start as a small circle and become bigger equally in all directions. But the expansion from one side to the center would be only half as fast as the expansion from one side to the other side, because the other side is going in the opposite direction.

By the way, I have this theory that if the expansion is universal, that is so say, expanding between atoms in the same fashion as expansion between galaxies, then we're all becoming "airheads", which would explain a lot.

[Steve3007]

I looked up the "Roche limit" and it is about two large bodies and the ability of the larger one to pull apart the smaller one if it gets too close. The tidal effect is similar to the Moon's effect on the Earth's tides. The Moon has enough pull to affect the tides but not enough to pull the ocean's off the face of the Earth. (Thank goodness!)

Yes. In your thought experiment at the start of this topic you correctly pointed out that objects both nearer than you to the centre of gravity towards which everything is falling and further away from it than you are moving away from you. So if you imagine you and both those sets of objects as being parts of a single extended object (like a moon or the earth) you can see that the object experiences a force which is trying to tear it apart. As you said, in the case of the gravitational force that the moon exerts on the earth, luckily it doesn't succeed!

I seem to recall that there would also be a time-dilation effect as an object accelerates as it is pulled into a black hole. ...

I seem to recall images of the visible universe being a large egg-shaped object, an oval rather than a circle. But still, there would be a theoretical center to the object from which the expansion in any direction would be measured, would there not? If I were to "expand" a balloon, it would start as a small circle and become bigger equally in all directions. But the expansion from one side to the center would be only half as fast as the expansion from one side to the other side, because the other side is going in the opposite direction.

The idea is that the centre of the expansion/contraction is not within 3D space, analogous to the fact that the centre of the earth is not on its surface. So if we think of this idea as just an abstract model and not "real", then that centre doesn't "really" exist. But the trouble with this stuff is that I think it's more advanced than people realize, thanks to it featuring frequently in popular science shows, videos and books. I studied physics to degree level but only briefly and superficially was General Relativity, on which theories of Cosmology rely, covered. That's a subject for more advanced theoretical physics courses and postgraduate work. So my knowledge of it is pretty much as hazy as anybody else who's read a popular book on it. ("ABC Of Relativity" by Bertrand Russell was one of the first, and, as you'd expect from Russell, it's very good). I think one of the valuable lessons learnt from studying a subject is the extent of what you don't yet know about it. Perhaps just as valuable as what you actually learn.

By the way, I have this theory that if the expansion is universal, that is so say, expanding between atoms in the same fashion as expansion between galaxies, then we're all becoming "airheads", which would explain a lot.

. Yes, I've heard people float that idea before. I don't know the answer to it! I do know that on a relatively local scale (the scale of our local galactic neighbourhood) things aren't receding from us. The Andromeda galaxy and the Milky Way are falling together and will collide in about 5 billion years. I guess that's something to look forward to.

[Marvin_Edwards]

I looked up the "Roche limit" and it is about two large bodies and the ability of the larger one to pull apart the smaller one if it gets too close. The tidal effect is similar to the Moon's effect on the Earth's tides. The Moon has enough pull to affect the tides but not enough to pull the ocean's off the face of the Earth. (Thank goodness!)

Yes. In your thought experiment at the start of this topic you correctly pointed out that objects both nearer than you to the centre of gravity towards which everything is falling and further away from it than you are moving away from you. So if you imagine you and both those sets of objects as being parts of a single extended object (like a moon or the earth) you can see that the object experiences a force which is trying to tear it apart. As you said, in the case of the gravitational force that the moon exerts on the earth, luckily it doesn't succeed!

I seem to recall that there would also be a time-dilation effect as an object accelerates as it is pulled into a black hole. ...

I seem to recall images of the visible universe being a large egg-shaped object, an oval rather than a circle. But still, there would be a theoretical center to the object from which the expansion in any direction would be measured, would there not? If I were to "expand" a balloon, it would start as a small circle and become bigger equally in all directions. But the expansion from one side to the center would be only half as fast as the expansion from one side to the other side, because the other side is going in the opposite direction.

The idea is that the centre of the expansion/contraction is not within 3D space, analogous to the fact that the centre of the earth is not on its surface. So if we think of this idea as just an abstract model and not "real", then that centre doesn't "really" exist. But the trouble with this stuff is that I think it's more advanced than people realize, thanks to it featuring frequently in popular science shows, videos and books. I studied physics to degree level but only briefly and superficially was General Relativity, on which theories of Cosmology rely, covered. That's a subject for more advanced theoretical physics courses and postgraduate work. So my knowledge of it is pretty much as hazy as anybody else who's read a popular book on it. ("ABC Of Relativity" by Bertrand Russell was one of the first, and, as you'd expect from Russell, it's very good). I think one of the valuable lessons learnt from studying a subject is the extent of what you don't yet know about it. Perhaps just as valuable as what you actually learn.

By the way, I have this theory that if the expansion is universal, that is so say, expanding between atoms in the same fashion as expansion between galaxies, then we're all becoming "airheads", which would explain a lot.

. Yes, I've heard people float that idea before. I don't know the answer to it! I do know that on a relatively local scale (the scale of our local galactic neighbourhood) things aren't receding from us. The Andromeda galaxy and the Milky Way are falling together and will collide in about 5 billion years. I guess that's something to look forward to.

Did not know that Russell wrote a book on relativity! I ordered the Kindle version to take a look at when I get a round tuit. I read some of his writings in my younger days. In my later days I found some of his writing less useful. But I can't recall which books I was reading. That would have been in my teens and twenties, and I'm in my seventies now.

The center is theoretical, as you say. But it could be theoretically measured as an internal point equidistant from all of the current edges at a given point in time.

[Steve3007]

Did not know that Russell wrote a book on relativity! I ordered the Kindle version to take a look at when I get a round tuit.

I hope you enjoy it. Considering it was first written in 1925 (I think) it's very insightful. But, of course, Russell was an insightful guy, and he knew it. I read somewhere that when someone said "only three people in the world understand General Relativity" he paused for a moment and then said "I'm just trying to think who the other two could be". Or something like that.

The center is theoretical, as you say. But it could be theoretically measured as an internal point equidistant from all of the current edges at a given point in time.

Interesting thing to ponder. By analogy we could imagine a world of 2D flat-landers living on a sphere. They can't point to the centre of the sphere but they can deduce from the fact that they appear to live in an unbounded world with a finite amount of space that it might exist. But their philosophers might argue as what constitutes existence, and whether a postulated by unreachable 3rd dimension meets the criteria.

If they assume that they're living on a sphere, and they know that the circumference of a sphere is pi times its diameter then they can measure the distance they travel in a straight line before coming back to the same point, and thereby calculate the diameter of this sphere that some of their philosophers will tell them only exists in their minds.