Why Does Angular Diameter Distance Decrease After Redshift z=1.5?

[semiserious]

Hi,
Can anyone explain (physically) why the angular diameter distance starts to decrease after a certain redshift (around z = 1.5)? Thanks

 

[Chalnoth]

Hi,
Can anyone explain (physically) why the angular diameter distance starts to decrease after a certain redshift (around z = 1.5)? Thanks

Simply put: our universe was much smaller back then.

 

[marcus]

Hi,
Can anyone explain (physically) why the angular diameter distance starts to decrease after a certain redshift (around z = 1.5)? Thanks

Simply put: our universe was much smaller back then.

Semi, you got your answer! Chalnoth put it concisely.

One thing you could concentrate on understanding is this: the angular diameter of something is the angular diameter it had when the light was emitted and started on its way to us.

Because pure expansion does not change angles. If you think diagrammatically, the lightrays are not spread apart or squinched together by expansion. They are just stretched out longer.

But the angular-size distance is just based on the angular diameter of some standard ruler (like a 100 thousand lightyear galaxy), and since the angle spread of the incoming light does not change
the angular size distance equals the distance of the object when the light was emitted.

And that is smaller than the presentday distance by a factor of z+1.

 

[marcus]

Hi,
Can anyone explain (physically) why the angular diameter distance starts to decrease after a certain redshift (around z = 1.5)? Thanks

Simply put: our universe was much smaller back then.

Semi, you got your answer! Chalnoth put it concisely.

One thing you could concentrate on understanding is this: the angular diameter of something is the angular diameter it had when the light was emitted and started on its way to us.

Because pure expansion does not change angles. If you think diagrammatically, the lightrays are not spread apart or squinched together by expansion. They are just stretched out longer.

But the angular-size distance is just based on the angular diameter of some standard ruler (like a 100 thousand lightyear galaxy), and since the angle spread of the incoming light does not change
the angular size distance equals the instantaneous proper distance to the object measured on the day when the light was emitted.

And that is smaller than the presentday distance by a factor of z+1.

So think about this example: the matter that emitted the CMB which we are now detecting is now about 45 billion LY from us. But the redshift of that ancient light is z = 1090. So the distances have increased by a factor of z+1 = 1091. Not to be too fussy, distances have increased by a factor of 1100.

So when the light was emitted, that matter was only about 41 million LY from our matter! When the ancient light was emitted, the matter was much much closer