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

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The angular diameter distance decreases after a redshift of z = 1.5 because the universe was significantly smaller at that time. The angular diameter of an object is determined by its size when the light was emitted, not by the expansion of the universe, which does not alter angles. As light travels, it stretches, but the angular size distance reflects the distance of the object at the time the light was emitted, which is smaller than its current distance. For example, matter that emitted the Cosmic Microwave Background (CMB) light was only about 41 million light-years away when that light was emitted, despite being approximately 45 billion light-years away today. Understanding these concepts clarifies why angular diameter distance behaves this way at higher redshifts.
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Hi,
Can anyone explain (physically) why the angular diameter distance starts to decrease after a certain redshift (around z = 1.5)? Thanks
 
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semiserious said:
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.
 
semiserious said:
Hi,
Can anyone explain (physically) why the angular diameter distance starts to decrease after a certain redshift (around z = 1.5)? Thanks

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

Chalnoth said:
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
 

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