How big do we need for comsological principle?

[skydivephil]

When we say the universe is homogenous and isotropic . How many Mega PArsecs are we talking about? Preusmably we all agree its not homogenous or isotropic on small scales. So where is the border and how when was it established? Is the current estimate, whatever it is, as recent as Sloan Digital Sky survey or much older than that?

 

[Ich]

How many Mega PArsecs are we talking about?

There's a diagram on Ned Wright's site. It's a gradual transition - the larger the scale, the more homogeneous.

 

[Chalnoth]

When we say the universe is homogenous and isotropic . How many Mega PArsecs are we talking about? Preusmably we all agree its not homogenous or isotropic on small scales. So where is the border and how when was it established? Is the current estimate, whatever it is, as recent as Sloan Digital Sky survey or much older than that?

Roughly 80Mpc or so.

 

[skydivephil]

Agh thanks for that, wanted it for my cosmology exams which was yesterday ,think I aced it and you guys have been enormous help in general, Physics forums rock

 

[sardar]

The size of the observable universe is approximately 93 billion light years in diameter, which is equivalent to about 880 Mega Parsecs. This is the scale at which we can observe the universe, and it is considered to be homogeneous and isotropic on this large scale.

However, on smaller scales, such as within galaxies and galaxy clusters, the universe is not homogeneous and isotropic. This is due to the presence of structures such as galaxies, stars, and planets, which create variations in density and distribution.

The border between the homogenous and isotropic universe and the non-homogenous and non-isotropic universe is not clearly defined. It is a gradual transition as we move from larger to smaller scales. The exact scale at which this transition occurs is still a topic of debate among scientists.

The current estimate of the size of the observable universe is based on data from the Sloan Digital Sky Survey, as well as other cosmological observations and models. However, the concept of the homogenous and isotropic universe has been around for much longer, dating back to the early 20th century when scientists first began to study the structure of the universe.

In summary, the size of the universe that we consider to be homogenous and isotropic is approximately 93 billion light years, or 880 Mega Parsecs. This estimate is based on current observations and models, but the concept of a homogenous and isotropic universe has been studied for much longer. The border between the homogenous and isotropic universe and the non-homogenous and non-isotropic universe is a gradual transition and is still being studied by scientists.