## 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
• The text in italics is copied from the article.
• Immediate followed by some comments

### 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 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.

### 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.