Recession velocity

[StringJunky]

The further away a receding galaxy is from us the faster it recedes. Is the increasing rate of recession with distance due to spacetime curvature; much like, analogously, when you see the sun setting...it apparently 'drops' quicker towards the horizon? I can't get my head around why the recession velocity increases with distance. I have no problem understanding expansion itself from any given point.

[imatfaal]

No it is a simple artifact of uniform background expansion (which is what is happening) rather than movement through space (which isnt).

 

 

Image One is an unstretched band with a black central line (the origin) and red, blue and black dots on either side. The dots are spaced at 1 cm intervals

 

 

 

Image Two is that band stretched over a ruler - the stretching was uniform and took one second (for the sake of argument)

 

 

Following based on view from BLACK LINE Origin

You will notice that the red dots have moved about 12mm (10->22) in that time - ie 12mm/s average speed. The blue dots have moved 25mm (20->45) - ie 24mm/s average speed. The black dots have moved 35mm (30->65) - ie 35mm/s. The marks further from the Origin have moved further in the same amount of time - ie they have a higher speed relative to the origin.

 

BUT The ratios of distances between adjacent marks has stayed the same Black->Blue->Red->Origin->Red->Blue->Black - were originally all the same as each other, and they are all still the same as each other (before they were 10mm now they are all 23-25mm). All gaps between adjacents have remained in the same ratio - ie the expansion is uniform.

 

Hope that helps.

[zapatos]

Also note that had imatfaal chosen to describe what happens from any of the red, blue, or black dots, instead of from the central line, the results would have been exactly the same.

 

No matter where you are on that rubberband (or in the universe) you would see the same average speeds, with those galaxies closest to you receding the slowest, and those galaxies furthest from you receding the fastest.

[Airbrush]

The further away a receding galaxy is from us the faster it recedes. Is the increasing rate of recession with distance due to spacetime curvature; much like, analogously, when you see the sun setting...it apparently 'drops' quicker towards the horizon? I can't get my head around why the recession velocity increases with distance. I have no problem understanding expansion itself from any given point.

 

I think the velocity is simply directly proportional to the distance. Distant superclusters that are twice as far away as a nearby supercluster will be receding at twice the speed.

Edited October 22, 2012 by Airbrush

[StringJunky]

Thanks for putting that together imatfaal. I appreciate it. I replicated your demo to help me see what was happening and I think I see what you mean. I took on board what Airbrush said and done this graphic...is it correct?

 

Edited October 23, 2012 by StringJunky

[michel123456]

No it is a simple artifact of uniform background expansion (which is what is happening) rather than movement through space (which isnt).

 

 

Image One is an unstretched band with a black central line (the origin) and red, blue and black dots on either side. The dots are spaced at 1 cm intervals

 

 

 

Image Two is that band stretched over a ruler - the stretching was uniform and took one second (for the sake of argument)

 

 

Following based on view from BLACK LINE Origin

You will notice that the red dots have moved about 12mm (10->22) in that time - ie 12mm/s average speed. The blue dots have moved 25mm (20->45) - ie 24mm/s average speed. The black dots have moved 35mm (30->65) - ie 35mm/s. The marks further from the Origin have moved further in the same amount of time - ie they have a higher speed relative to the origin.

 

BUT The ratios of distances between adjacent marks has stayed the same Black->Blue->Red->Origin->Red->Blue->Black - were originally all the same as each other, and they are all still the same as each other (before they were 10mm now they are all 23-25mm). All gaps between adjacents have remained in the same ratio - ie the expansion is uniform.

 

Hope that helps.

Right.

Except that the dots are not dots anymore, they expanded together with the rubber band.

What you should have done in order to represent the standard model is to stick pennies on the rubber band. Better, it should have been pennies with rubber band collated between them (no rubber between the pennies, or rubber not expanding beneath the penny).

 

Here below what I understand:

 

In this analogy, the penny is a galaxy cluster.

What maintains the penny from expanding is what we call gravity.

The material the rubber band is made of is unknown (as far as I know)

The force that makes the rubber band expand is unknown (as far as I know)

What glues the pennies to the rubber is unknown (as far as I know)

and BTW what gravity is made of is also unknown (as far as I know)

 

It is believed that the pennies don't move very much along the rubber band, they roughly stayed in place from the beginning of the Universe.

IOW it is believed the pennies were created at the relative place they are today upon the rubber band.

It is believed that a long time ago the rubber band had zero length.

It is believed that in the future the rubber band will have infinite length.

 

Tell me I have understood nothing.

[StringJunky]

Right.

Except that the dots are not dots anymore, they expanded together with the rubber band.

What you should have done in order to represent the standard model is to stick pennies on the rubber band. Better, it should have been pennies with rubber band collated between them (no rubber between the pennies, or rubber not expanding beneath the penny).

 

Here below what I understand:

 

In this analogy, the penny is a galaxy cluster.

What maintains the penny from expanding is what we call gravity.

The material the rubber band is made of is unknown (as far as I know)

The force that makes the rubber band expand is unknown (as far as I know)

What glues the pennies to the rubber is unknown (as far as I know)

and BTW what gravity is made of is also unknown (as far as I know)

 

It is believed that the pennies don't move very much along the rubber band, they roughly stayed in place from the beginning of the Universe.

IOW it is believed the pennies were created at the relative place they are today upon the rubber band.

It is believed that a long time ago the rubber band had zero length.

It is believed that in the future the rubber band will have infinite length.

 

Tell me I have understood nothing.

 

Imatfaal's demo was fine for me...I was only interested in the increasing rate-of-recession velocity aspect which he clearly demonstrated his suggestion that it is an artifact of expansion; it didn't matter about the marks expanding as that was irrelevant in this instance. You might take some time out studying what analogies are and their limitations and also you can't expect someone to give a whole treatise covering every aspect of cosmological expansion in one post...that's what books are for.

[michel123456]

Imatfaal's demo was fine for me...I was only interested in the increasing rate-of-recession velocity aspect which he clearly demonstrated his suggestion that it is an artifact of expansion; it didn't matter about the marks expanding as that was irrelevant in this instance. You might take some time out studying what analogies are and their limitations and also you can't expect someone to give a whole treatise covering every aspect of cosmological expansion in one post...that's what books are for.

(bolded mine)

In this case you should be interested that any scaling up has this effect. It is caused by standard Euclidian geometry.

Scale down has the contrary effect : the farther you see, the less it contracts.

 

When you drive on the highway and look in the mirror, you see a car far away behind you that drives faster than you. You may wait a minute or 2 before the car comes close. Then suddenly the car passes by very quickly, zzzzouf. After that you see the car in front of you vanishing slowly and slowlier to the horizon. The object far away looks like moving slowly, the object close looks like moving fast.

It is an effect of perspective which is nothing else than an everyday geometric contraction.

[StringJunky]

(bolded mine)

In this case you should be interested that any scaling up has this effect. It is caused by standard Euclidian geometry.

Scale down has the contrary effect : the farther you see, the less it contracts.

 

When you drive on the highway and look in the mirror, you see a car far away behind you that drives faster than you. You may wait a minute or 2 before the car comes close. Then suddenly the car passes by very quickly, zzzzouf. After that you see the car in front of you vanishing slowly and slowlier to the horizon. The object far away looks like moving slowly, the object close looks like moving fast.

It is an effect of perspective which is nothing else than an everyday geometric contraction.

 

That's what was puzzling me: the further away something is the the more the perspective compresses and so should appear to move slower with increasing distance. I need to get down to first principles on this subject which I lack.

Edited October 23, 2012 by StringJunky

[michel123456]

That's what was puzzling me: the further away something is the the more the perspective compresses and so should appear to move slower with increasing distance. I need to get down to first principles on this subject which I lack.

 

Maybe you are confusing what appears to be and what is measured.

In my example if you measure the car speed with a laser, you will get a correct standard speed.

[StringJunky]

Maybe you are confusing what appears to be and what is measured.

In my example if you measure the car speed with a laser, you will get a correct standard speed.

 

Exactly...measurement is king.

[michel123456]

Exactly...measurement is king.

Hmm I am not sure. You have to interpret what you are measuring.

 

In Imatfaal's analogy, everything happens in real time, one second.

 

When observing the universe, nothing happens in real time.

What we are observing is a pack of parallel rubber bands, each of one being extended at 1 sec time interval.

 

The first band below is extended 1 sec late, the 2nd band is extended 2 sec late, the 3rd band 3 sec, etc.

 

And not exactly that: we are observing the first dot on the first parallel band, the second dot on the 2nd parallel band, the 3rd dot on the 3rd band etc.

 

That makes interpretation of measurement a little bit complicated.

Edited October 24, 2012 by michel123456

[StringJunky]

Hmm I am not sure. You have to interpret what you are measuring.

 

In Imatfaal's analogy, everything happens in real time, one second.

 

When observing the universe, nothing happens in real time.

What we are observing is a pack of parallel rubber bands, each of one being extended at 1 sec time interval.

 

The first band below is extended 1 sec late, the 2nd band is extended 2 sec late, the 3rd band 3 sec, etc.

 

And not exactly that: we are observing the first dot on the first parallel band, the second dot on the 2nd parallel band, the 3rd dot on the 3rd band etc.

 

That makes interpretation of measurement a little bit complicated.

 

What I meant was when I looked at imatfaal's example initially I couldn't see the furthest dot accelerating away in my mind but when I actually replicated it I did see it. Interpretation is important as you say as well.