The Big Bang - by Joseph Silk 1989 - Book review

This document contains comments about chapter HORIZONS of the book "The Big Bang " by Joseph Silk written 1989
For more reading see:

Contents

Reflection

Chapter 5.

Cosmological Models page 90

page 89

In this chapter we shall describe some simple analogies that help to clarify these questions, and we shall then examine the models that remain viable alternative descriptions of the real universe.
This imposes a problem. The problem is if you use a model of something, meaning you try to describe the universe or a process simpler as the reality, this model should always be physical possible.
The common understanding is that the Universe consists of 3D objects. In a mathematics this is also the common understanding. However in mathematics also you can think about 1D, 2D and 4D objects.

page 90

5.1 The curvature of space page 90

One of the fundamental questions of cosmology concerns the nature of space.
What is the definition of space?
The common understanding is, that this is the 3D area between the stars we see at night.
The universe in that respect is even larger, because it is the same as this 3D area with all the stars included.
Two of the standard big bang models assume that space is curved. What does that mean?
One way to visualize the concept of curved space is two use a two-dimensional analoque
Remember a 2D object does not physical exist. 
                                                            
              B            
          3   .                9
      A       .      .     12     .      .     D
              5                         .
          4   .               ?
              .       .
              C

Figure 5.1 A Map of Lilliput

  • The length between A and B is 3 units. The length A and C is 4 units. The length between C and B is 5 units.
    That means the triangle A,B,C can be drawn in a flat plane. The angle BAC is 90 degrees.
  • The length between A and B is 3 units. The length B and D is 9 units. The length between A and D is 12 units.
    That means the triangle A,B,D can not be drawn in a flat plane, because AB + BD = AD.
    To be physical realistic (on a flat surface) the straight length AD should be smaller than 12 units.
    If the measured length of AD is 12 units (or more) than the surface of the map is not flat.
    • This can be a hill. It also be a valley, on the surface of the earth.
    Any way this map has nothing to do with the curvature of space, but more with curvature of the hilly surface of a 3D object.
The next map shows the same information, but in a more convincing matter. 
                 B                                          
             .   .   .          
          5      3       5
      .          .           .                 
   A  .   4  .   C   .   6   .   D
      .          .           .      
          5      3       ?
             .   .   .
                 E

Figure 5.1.1 A Map of a kite

  • The length between A and B is 5 units. The length A and C is 4 units. The length between C and B is 3 units.
    That means the triangle A,B,C can be drawn in a flat plane. The angle BAC is 90 degrees.
  • The same for the triangle A,E,C.
  • The length between A and B is 5 units. The length B and D is 5 units. The length between A and D is 10 units.
    That means the triangle A,B,D can not be drawn in a flat plane, because AB + BD = AD.
    The same is true for the triangle CBD with the angle BCD being 90 degrees. To be physical realistic (on a flat surface) the straight length CD should be smaller than 6 units, i.e. 4 units
    If the measured length of CD is 6 units (or more) than the surface of the map is not flat.
    • This can be a hill or a valley, on the surface of the earth.
    Any way this map has nothing to do with the curvature of space, but more with curvature of the hilly surface of a 3D object.
The real lesson of this discussion is that you can only discuss actual performed experiments or actual performed observations which can be repeated.
The lesson of Figure 5.1 is that the flat map, based on the shown information, can not be a projection of a flat physical surface.

page 93

5.2 Horizons page 93

Imagine a rubber balloon etc.
Suppose the balloon is gradually inflated. If we imagine that the universe is confined to the surface of the balloon we now have a two-dimensional model of a closed expanding universe.
This picture is wrong we are living inside such a balloon.
This picture is correct if it shows the evolution of "our" night sky.
Now imagine the earth as a point on the surface of the balloon.
That is physical wrong.
An observer on the surface could only survey a fraction of the area of the balloon.
That is correct.
Similarly we on earth will never be able to see very much more of the universe than we see at present because we are limited by the observable horizon (Figure 5.4)
Figure 5.4 shows a complete different situation which does not include a balloon.

Page 94

Consider a galaxy separated from us by a distance of D light-years.
At present
Light takes D years to travel to us.
That depends if space expansion is taken into account.
Without space expansion this is D years.
With space expansion this is longer.
Only after the universe has been expanding for a time in excess of D years will the galaxy become visible to us.
That is not true. The galaxy is already visible to us right now at an earlier age when the galaxy was younger and its distance was less than D light-years.
We say that a galaxy first comes within our horizon after the universe has been expanding for a period equal to the time it takes for light to travel to us from the galaxy.
The first part is wrong. You have to take space expansion into account. As a result we can observe the CMB radiation emitted 300000 years after the Big Bang and almost all galaxies, the further away the younger.

Figure 5.4 Horizons

At the initial of the Big Bang two hypothetical observers A and B could not communicate with each other.
In order to understand the evolution of the universe after the Big Bang communication between two observers is not an issue.
The biggest issue is that starting from the Big Bang throughout the entire universe grandi moso the same chemical reactions took place, (as indicated in Table 4.1 The Cosmic Time Scale at page 72/73) which resulted in a local homogeous universe consisting of galaxies inter connected by filaments.
This has nothing to do with the concept of horizons nor with communication. 2024: The most important physical issue is not communication, but how two regions A and B can influence each other.

Page 95

The galaxies we observe just coming over the horizon are highly redshifted.
The oldest galaxies we observe are highly redshifted.
Their recession velocities relative to us are close to the speed of light (otherwise they would have been observable long ago)
The importance is the distance and the speed of the object at the moment in the past when light was emitted. The distance is much closer than at present. The speed could be larger than c.
The distance to the observable horizon can be most simply expressed as the distance that a light signal can travel in the available time since the Big Bang.The distance to the horizon increases directly with the age of the universe.
This concept is of no relevance.
For a IMO more realistic idea what is happening study this: Friedmann's equation - 13 Questions
This distance (15 billion light-years) thus constitutes the extent of the observable universe.
The concept observable universe is not very helpfull. The oldest "objects" we can observe is the CMB radiation when the age of 300000 years after the Big Bang. The size of the universe was then roughly 3 * 300000 light-years.
To get an idea about the size at present select this: Friedmann Lambda = 0.01155 The document shows that the size of the universe is roughly 3 times its age in light-years or at present roughly 30 billion light-years.

In a more realistic model of the universe, the galaxies are slowed by the effects of gravity

2024: This model may not apply to all galaxies in the universe. The current opinion is that the universe is expanding with a speed 3*c.
Your writer does not know how this physical can be explained.
Our horizon actually expands at a greater rate than the rate at which the galaxies are receding from one another.
2024: The concept our horizon is not very 'practical' to understand the evolution of the universe. If you want to understand the evolution of the universe, starting from the Big Bang you should use a global view and not a local view, centered around use, now.

Reflection 1

See also below Figure 5.4 above
What we observe of the universe has nothing to do with the concept of a horizon which is primarily important to describe here on earth when you are on the top of a mountain or in an airplane.
To use that to describe what we observe in the nightsky could lead to the wrong conclusions.
What we observe is the state in the past. When we observe a flash as a Super Novae this flash happened not at present but in the past as a function of the distance.

Reflection 2 - Curvature of the universe - 2024

  1. Is the universe curved?
    The problem with this question is, like most questions, that the question only makes sense if there are more universes. That means there are some universes with are curved and others not. In that case you can study the differences.
  2. Can a 2D object being curved?
    With the help of: Figure 5.1 A Map of Lilliput the conclusion is that this 2D map (Showing straight lines) can not be the projection of a flat 3D object, but of a curved 3D object.

  3. Can space around a mass being being curved?
    The answer is discussed below with the aid of a star cluster. 
                          B

                  .

              .                                                 

          .           b          
              .               . 
      A       .       M              C
                                       
          .         
                     
              .

                  .
                     
                      D

    Figure 5.9 A 3D Map of cluster of stars

    		 Figure 5.9 shows 4 stars idicated as A,B,C, and D and a central mass indicated as M. M can also be a BH. The importance is that M is invisible.
    There are also 4 lines (visible lines) drawn starting from point A.
    • The line AB shows a straight light signal going to star B. A similar light signal goes from star B to star A
    • The line AD shows a straight light signal going to star D. A similar light signal goes from star D to star A
    • What are not show are 2 stars E and F. Star E is situated straight above mass M, i.e. the plane ABCD. Star F is situated straight below the mass.
      Not shown are light signals going from A to E and from A to F, and in opposite directions.
    • The line AM shows a straight light signal going to mass M. There is no light signal going from M to star A.
    • The line AC shows a bended line going via point b to star C. A similar light signal goes from star C to star A.
      The bended line AC can also be called: AbC
    • Not shown are the bended lines AdC (going in between mass M and star D), AeC, AfC, and many others.
      The result is that observed from point C there is a ring of light around mass M orinating from star A . Such a ring is called an Einstein ring.
    In the above the lightpath in 4 directions is discussed. In reality the stars emits light in all directions. In reality the area between the two points b and M is the most interesting. 
    
                                                                .      b      .
                                                 .          o     o                          .
                                     .     o        o       o         o o                                .
                            .   o      o       o        o          o       o                                      .
                    .  o    o    o        o         o            o           o                                            .
           . o  o  o         o                                  o             o                                                    . 
    Aoooo     .         .         .         .         .         o      M      o         .         .         .         .         .         C 
           .                                                    o             o
                    .                                            o           o                                            .
                            .                                      o       o                                       .
                                     .                                o o                                .
                                                 .                                           .
                                                                .      d      .

    Figure 5.10 Lightrays around mass M or a 'BH'
    Figure 5.10 shows at the center the mass M, surrounded by a ring of 20 o's indicating the size of the mass M. Not on scale.
    At the left is star A and towards star C. The point b is a virtual point towards star B and the point d towards star D. Figure 5.10 shows the light rays originating from star A
    1. The line AbC shows the path of a light ray emitted at A, bended by b and and that goes in the direction of C. That means an observer at C observes the star A in the direction Cb. This line goes towards star B
    2. The line AdC shows the path of a light ray emitted at A, bended by d and and that goes in the direction of C. That means an observer at C observes the star A in the direction Cd. This line goes towards star D
    3. Not showing is, that using the axis AM, and under the same angle bAM and dAM, a 3D cone of light is emitted towards point C. From the point C this is observed as a ring.(An Einstein ring.
    4. However the same is observed at the stars A (Emitted from star C),B (Emitted from star D),D,E and F. See Figure 5.9
    5. However Fig 5.10 shows more light rays. 4 colored lines in total. The outer line has the color orange, the next light green, next blue and the inner line red. Each of these light signals are emitted from star A, under different angles and collide with the mass M.
      All these signals more or less go into the direction of the virtual point b.
      The same type of signals also go to the virtual point d.

    Fig 5.10 shows (some of) the light signals emanating from star A. The same set of signals also are emitted from the stars B,C,D,E and F.
    What this implies that all the stars A,B,C,D,E and F emit lightsignals in all directions. The mass M influences some of these lightsignals
    Does that means that the area or space around the mass M is curved? IMO it does not. It is the mass M that causes the light rays to curve. IMO that is the most simple physical description or explanation.

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Created: 22 September 2014

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