Comments about "Observable Universe" in Wikipedia


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The observable universe consists of the galaxies and other matter that can, in principle, be observed from Earth in the present day because light and other signals from these objects has had time to reach the Earth since the beginning of the cosmological expansion.
The problem with the concept observable universe is that you define a space which state at present cannot be observed at present. The important point to consider is that all the galaxies observed at present define conditions (events, positions) in the past. That means in order to calculate the present state (positions) you need Friedmann's equation.

1. The universe versus the observable universe

Some parts of the universe are too far away for the light emitted since the Big Bang to have had enough time to reach Earth, so these portions of the universe lie outside the observable universe.
This sentence reflects the wrong physical interpretation of what we observe. The most important issue is, that what we observe is not the state of the universe at present but of the past. The second issue is that the further away we look the universe was much smaller implying that the concept of observable universe does not define a physical entity.
In the future, light from distant galaxies will have had more time to travel, so additional regions will become observable.
Completely wrong physical interpretation. In the future we will not see additional regions. In general we will always observe the same regions and we will always observe the same galaxies but they will have aged.
The only exception is that some times we will not see certain galaxies any more because they will have merged.
However, due to Hubble's law regions sufficiently distant from us are expanding away from us faster than the speed of light and furthermore the expansion rate appears to be accelerating due to dark energy.
Sufficient distant regions are not expanding away from us (faster than the speed of light) because of Hubble's Law. The physical evolution of the universe has nothing to do with any law. Laws are descriptions of physical phenomena but they are not the reason why something happens the way it happens.
Assuming dark energy remains constant, so that the expansion rate of the universe continues to accelerate, there is a "future visibility limit" beyond which objects will never enter our observable universe at any time in the infinite future, because light emitted by objects outside that limit would never reach us.
Wrong physical interpretation. See below Reflection 2 - Size of the Universe .

2. Size

The figures quoted above are distances now (in cosmological time), not distances at the time the light was emitted.
Correct. You can add the sentence: "Which happened 13 billion years ago"
This CMB radiation was emitted by matter that has, in the intervening time, mostly condensed into galaxies, and those galaxies are now calculated to be about 46 billion light-years from us.
To write this slightly different: "The CMB radiation, this region, is now 46 billion light-years away from us." Which is correct.
That means we can observe the origin of these far away regions at present as CMB radiation. That means no regions disappeared outside our vision, nor new regions became vissible.
This picture is in conflict with the one "painted" in paragraph 1.

A different question to answer if these far away regions look the same at present as the galaxies surrounding us.

2.1 Misconceptions

In the paragraph "13.8 billion light-years" we read:
it is a common misconception that the radius of the observable universe must therefore amount to only 13.8 billion light-years.
The problem is specific of the concept "observable universe", which is a misnomer. See also: Reflection 1 - General

Reflection 1 - General

The problem with the concept "Observable Universe" that at present nor at any moment there exists something what you can call an "Observable Universe".
The problem with the concept "Observable Universe" is, that it is not a physical concept, because it is based what we humans observe. These observations are important because based on these we try envision how the entire universe looks like.
What is a physical concept is the size of the entire Universe. What we mean by that is the size of all of space influenced as a result of the Big Bang.

What we humans observe is the situation of the Universe in the past at many different moments. The further away the younger. An important issue is to what extend the state of the entire universe at present is isotropic and homogeneous. Your author has serious doubts.

Reflection 2 - Size of the Universe

In order to study the size of the entire universe using Friedmann's equation and based on the mainstream accepted cosmological parameters please study this: Friedmann Lambda=0.01155 What this document shows:
  1. First (The blue line is) is the path of a light ray emitted very close to the Big Bang.
  2. The size of the entire universe as a function of the age of the universe. (black line)
When you compare the blue line with the black line you can see that of the entire universe we can only observe a tiny bit at present and that the size of the entire Universe is approximate 35 billion light-years

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Created: 20 October 2014

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