Comments about "Observable universe" in Wikipedia

This document contains comments about the article Observable universe in Wikipedia
In the last paragraph I explain my own opinion.




The article starts with the following sentence.
The observable universe is a spherical region of the Universe comprising all matter that can be observed from Earth at the present time, because electromagnetic radiation from these objects have had time to reach Earth since the beginning of the cosmological expansion.
At present there exists no spherical region of the Universe which we can observe. What we observe at present is the state of the Universe in the past. In fact what we can observe is all the matter (galaxies) that there are at present, but not in their present state but in their state in the past. The futher away the earlier.
In that sense the word Observable Universe is misleading as if we can observe the observe the total world at present. We cannot. What we see is the past.
There are at least 2 trillion galaxies in the observable universe, containing more stars than all the grains of sand on planet Earth.
A much better sentence is something like:
Current observational technology allows us to destinquish at least 2 trillion galaxies.
The second part of the sentence should be deleted.
That is, the observable universe is a spherical volume (a ball) centered on the observer.
From our point of such a spherical volume does not exist.
The second reason is that before the recombination epoch, about 378,000 years after the Big Bang, the Universe was filled with a plasma that was opaque to light, and photons were quickly re-absorbed by other particles, so we cannot see objects from before that time using light or any other electromagnetic radiation.
Of course you cannot see objects, because if the universe was filled with plasma, there were no objects.
Gravitational waves and neutrino background would have been unaffected by this, and may be detectable from earlier times.
There is a huge difference between gravitational waves and neutrino's. Gravitational waves are the results replacements of massive objects. This leads to the conclusion that the existance of gravitational waves is doubtfull.

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 lacks a definition of what is the Universe and what is the Observable Universe. Reflection 1 - Universe versus Observable Universe.
In the future, light from distant galaxies will have had more time to travel, so additional regions will become observable.
In fact there are two issues related to what we will observe: (1) Improving current observation techniques. (2) Keeping the same observation techniques into the future.
However, due to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light (special relativity prevents nearby objects in the same local region from moving faster than the speed of light with respect to each other,but there is no such constraint for distant objects when the space between them is expanding; see uses of the proper distance for a discussion) and furthermore the expansion rate appears to be accelerating due to dark energy.
Hubble's Law and special relativity have nothing to do with this. Both are not the cause of any physical behaviour; they are descriptions.
Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become extremely redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible.
This is a very tricky sentence because the outer edge of the Universe is already expanding at a speed of roughly 3 times the speed of light.
As already mentioned the overall picture is that we will see less galaxies instead of more (keeping the same technology)

2 Size

So if the matter that originally emitted the oldest CMBR photons has a present distance of 46 billion light years, then at the time of decoupling when the photons were originally emitted, the distance would have been only about 42 million light-years.
That means the size of the Universe at the time of decoupling was 42 million light-years and the size at presnt is 46 billion light years.

2.1 Misconceptions on its size

Many secondary sources have reported a wide variety of incorrect figures for the size of the visible universe. Some of these figures are listed below, with brief descriptions of possible reasons for misconceptions about them.
The most important issue to discuss is the physical size of the Universe, not the Observable size.
This reasoning would only make sense if the flat, static Minkowski spacetime conception under special relativity were correct. In the real universe, spacetime is curved in a way that corresponds to the expansion of space, as evidenced by Hubble's law.
The expansion of space is based on observations i.e. speed and frequency shift are equivalent. Hubble's law captures this behaviour but that does not mean that spacetime is curved. Observations reveal that the path of a lightray is not always straight, but can be bended. However this does not validate that spacetime is bended. The first question is what is spacetime compared to space.

3. Large-scale structure

Sky surveys and mappings of the various wavelength bands of electromagnetic radiation (in particular 21-cm emission) have yielded much information on the content and character of the universe's structure.
Should this not be the structure of the Observable universe?

3.1 Walls, filaments, nodes, and voids

3.2 End of Greatness

At this scale, no pseudo-random fractalness is apparent.
To concept pseudo-random fractalness is misleading. The concept has nothing to do with physics, so why mention?

3.3 Observations

Gravitational lensing (bending of light by gravitation) can make an image appear to originate in a different direction from its real source.
Correct. This is a physical effect.
This is caused when foreground objects (such as galaxies) curve surrounding spacetime (as predicted by general relativity), and deflect passing light rays.
The cause is physical the same as why the Earth moves around the Sun. The path of both are bended. Newton's Law and GR describe this behaviour.
Thus, the environment of the cluster looks a bit squashed if using redshifts to measure distance.
That is correct. The problem is that all intervening masses (galaxies, stars or BH's) can cause errors in observations, resulting in errors in the true distance calculations.

3.4 Cosmography of our cosmic neighborhood

These galaxies are all redshifted, in accordance with Hubble's law.
That is not true. These galaxies are redshifted because they move away, which results in a frequency shift, as capsulated by Hubble's Law.
This indicates that they are receding from us and from each other, but the variations in their redshift are sufficient to reveal the existence of a concentration of mass equivalent to tens of thousands of galaxies.
The individual redshifts of a large cluster of galaxies is not according to Hubble's law. It is easy possible that you have two galaxies, at the same distance, in a cluster who's redshifts are different. This is for example the case when one moves towards the observer and the other one away from the observer.

4 Mass of ordinary matter

The mass of the observable Universe is often quoted as 10^50 tonnes or 10^53 kg
The discussion apperently is here about baryonic matter.
However, it excludes dark matter and dark energy.
Dark energy belongs to the parameter Lambda.

4.1 Estimates based on critical density

5. Matter content number of atoms

6. Most distant objects

7. Horizons

8. See also

Following is a list with "Comments in Wikipedia" about related subjects

Reflection 1 - Universe versus Observable Universe.

The Universe is a physical entity and encompasses the total world in which we live. The current main stream opinion is that the universe started with a Big Bang or a Big Explosion. What caused this explosion or what happened before this explosion has no general accepted explanation. The Obervable Universe defines no physical entity. The Observable Universe defines what we can see from our point in space of the Universe. What we see is the physical state of the universe in the past. The futher away we look the earlier. In principle we could observe the Big Bang, but that depends about the parameters that describe the evolution of the universe.

Reflection 2

Reflection 3


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Created: 13 December 2017

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