Isotropic and homogeneous of space

[sadegh4137]

we say the universe around us is isotropic and homogeneous.

it means that all direction and points are the same for some special class of reference.

if this is true why we say in large scale universe is isotropic and homogeneous?

it seems that the space, itself, to be isotropy and homogeneous.

 

[Drakkith]

I'm sorry I don't understand what you are asking. Could you try to make your specific question a little clearer?

 

[phinds]

we say the universe around us is isotropic and homogeneous.

yes we do

it means that all direction and points are the same for some special class of reference.

NO ... it means that for ALL frames of reference. There IS no "special class of reference" (which I take to be your way of saying frame of reference)

if this is true why we say in large scale universe is isotropic and homogeneous?

because it is, as far as we can tell. Why do you think otherwise?

it seems that the space, itself, to be isotropy and homogeneous.

I have no idea what you mean by "space itself". Space is just the volume that contains matter. It is the distribution of matter that is isotropic and homogeneous (on large scales)

 

[Chalnoth]

NO ... it means that for ALL frames of reference. There IS no "special class of reference" (which I take to be your way of saying frame of reference)

Well, actually, the universe only appears homogeneous and isotropic for a particular class of observers: comoving observers (that is, observers that are stationary with respect to the overall expansion).

It is still an interesting statement that such a class of observers exists (the universe could in principle be different such that no class of observers saw our universe as homogeneous or isotropic).

And to answer the OP's question, no, this isn't simply about the fundamental symmetry of space-time, because all observers see the same laws of physics: space itself is perfectly symmetric, for all observers, in all directions. But our universe is only homogeneous and isotropic for a particular class of observers. That fundamental symmetry, in other words, is broken by the fact that our universe is expanding. And that expansion makes the past look different from the future while also imposing a sensible meaning of simultaneity. The fundamental physics has neither an arrow of time nor a notion of simultaneity.

To expand a bit on what I mean by simultaneity, there is no way within General (or special) Relativity to say definitively that two events separated by some distance happened at the same time. On observer might observe two events as happening at the same time, but an observer moving relative to them in general will tend to interpret one as happening before the other. But because our universe has a reference frame in which it appears symmetric, we can set up a notion of simultaneity by picking out just those observers for whom this symmetry appears.

 

[sardar]

I can confirm that the concept of isotropy and homogeneity in the universe is well-supported by scientific evidence. Isotropy refers to the idea that the universe looks the same in all directions, while homogeneity refers to the idea that the universe is uniform on a large scale.

One of the key pieces of evidence for this is the cosmic microwave background radiation, which is a remnant of the hot, dense early universe. This radiation is incredibly isotropic, meaning it has the same temperature in all directions, to a precision of about one part in 100,000. This supports the idea that the universe was once in a highly uniform state.

Additionally, observations of the large-scale structure of the universe, such as the distribution of galaxies, also support the idea of homogeneity. On scales larger than a few hundred million light-years, we see that the universe looks roughly the same in all directions, with clusters and voids of matter appearing in a regular, almost uniform pattern.

While it is true that there may be some variations in the universe on smaller scales, such as the clustering of galaxies and the presence of structures like galaxy clusters and superclusters, these do not significantly impact the overall isotropic and homogeneous nature of the universe on a large scale.

In conclusion, the idea of isotropy and homogeneity in the universe is a well-supported concept in the scientific community, and it is based on a wealth of observational evidence. While there may be some variations and structures on smaller scales, the overall nature of the universe is remarkably uniform and consistent, leading us to describe it as isotropic and homogeneous.