The Inflationary Universe in Scientific American May 1984

This document contains comments about the article The Inflationary Universe by Alan H. Guth and Paul J. Steinhardt in Scientific American of May 1984.
For the text of this article (1984) select: This copy has no page numbers and pictures. Only text.

The article starts with the following sentence.
A new theory of cosmology suggests that the observable universe is embedded in a much larger region of space that had an extraordinary growth spurt a fraction of a second after the primordial big bang
It is very important to study all what happened after the Big Bang independent from the human point of view. Concepts like the observable universe should be very critically investigated.
The word suggest implies that all what is written is not sure. It is a proposal, a hint.
The article starts with the following sentence:
In the past few years certain flaws in the standard big-bang theory of cosmology have led to the development of a new model of the very early history of the universe.
The whole article is a clear and easy to understand document, that does not mean that there are no comments to make.
If you think that a theory has "flaws" you clearly have to identify what these errors are. In general this means that an old theory makes certain predictions which are in error in the sense that your new theory makes more accurate predictions which are demonstrated by observations. Immediate next:
The model known as the inflationary universe agrees precisely with the generally accepted description of the observed universe for all times after the first 10^-30 second.
The reader should remind that the earliest we can observe is the CMB radiation which reflects the state of the universe (a sphere) 300000 years after the Big Bang.
At page 90 left column:
The inflationary process also has important implications for the present universe. If the new model is correct the observed universe is only a very small fraction of the entire universe.
The article should clearly describe how this is demonstrated.
When you study Friedmann's equation The path of a light ray and specific Figure 3, you should understand that the black line defines the size of the universe. The blue line the path of a light ray originating very close to the Big Bang. What this means that the radius of the observed universe at any moment is smaller than the size of the universe at each of these moments. This discrepancy is the largest at present and the smallest near the Big Bang.
However this has nothing to do with the inflation theory.
At page 90 left column
In both models the universe began between 10 and 15 billion years ago as a primeval fireball of extreme density and temperature, and it has been expanding and cooling ever since.
This picture has been succesfull in explaining many aspects of the observed universe, including the red-shifting of the light from distant galaxies the cosmic microwave background radiation and the primordial abundances of the light elements.
It is important to make a distinction between observations, a model and predictions. The red shift of light and the CMB radiation can be both observations and predictions of the Big Bang model dependent what came first. The primordial abundances of the light elements is a prediction.
At page 90 middle column:
The equations that describe the period of inflation have a very attractive feature: from almost any initial conditions the universe evolves to precisely the state that had to be assumed as the initial one in the standard model.
The Friedmann equations describe the standard evolution of the universe starting from the Big Bang until present. What the above text indicates is that only for the first second you have to use the inflation equations after that the Friedmann equations apply.
When you study Friedmann's equation - Question 14 you will see that the parameter v0 which defines the intial speed or distance after the Big Bang has (almost) no influence on the present size of the universe.
Page 93 left column:
The Big Bang model leads to three experimentally testable predictions. First etc Second etc Third etc.
The question is if this are predictions or observations.
page 93 middle column:
The first problem is the difficulty of explaining the large-scale uniformity of the observed universe.
What we observe are galaxies everywhere almost randomly distributed.
The large-scale uniformity is most evident in the CMB radiation
The emphasis is on radiation. If similar processes emmit similar radiation than it is not so strange that the radiation (photon) density is uniform.
Page 93 right column:
In the standard model the source of the microwave background radiation observed from opposite directions in the sky were separated from each other by more than 90 times the horizon distance when the radiation was emitted.
Specific study Reflection 1
Since the regions could not have communicated it is difficult to see how they could have evolved conditions so nearly identical.
Physics has nothing to do with communication. Specific study Reflection 1
The problem is that one of the most salient features of the standard universe - its large scale uniformity - cannot be explained by the standard model; it must be assumed as an initial condition.
One of the initial states or era of the universe is what is called the quarck soup. During that period (almost) all the elementary particles were quarcks. Was this quarck soup homogeneous? most probably not but close. There will be density perturbations but limited.
Page 94 bottom:
Horizon Problem
The gray horizontal plane shows the time at which the microwave background radiation was released
The horizon problem is the difficulty of explaining how the radiation received from the two opposite directions came to be at the same temperature.
This is not a temperature problem but a frequency problem. Temperature is a human based concept.
To rephrase the horizon problem you get:
The horizon problem is the difficulty of explaining how the frequency of the radiation received from the two opposite directions came to be the same
The first step is to recognize that the problem is not on the receiving side but at the moment when the radiation was emitted i.e. during the decoupling era.

The following two drawings show what happened thera after

 ^                   A
 |                 .   .
 |               .       .
 |            _.           ._
 |           . |           | .
 |         .                   .
 |       .                       .
 |     B                           C  
 |   .   .                       .   .
 | .       .                   .       .
            Horizon problem 1

 ^ <--BB             A              CC-->           
 |                 .   .
 |               .       .
 |             .           .
 |            .             .
 |           .               .         
 |          /.\             /.\
 |           .               .
 |           .               .
 |       b    .             .   c
 |              .         .
 |                  B C       
             Horizon problem 2
The left drawing shows the same picture as in the Scientific American article. The right drawing shows a modification.

The most important difference between the two drawings are two issues: The lesson to be learned is that there is no horizon problem. Than you also do not need the inflation theory to solve the problem
See also: Reflection 2 - The Inflation theory

Reflection 1 - Horizon problem

The CMB radiation was emitted 300000 years after the Big Bang. The size of the universe was than already larger than 300000 light-years. The horizon problem starts from the idea that the state of the entire universe at that moment is not uniform because in order to reach equilibrium communication with all parts in the system are required. In a sense that is true but this is not a horizon problem but a space expansion problem and is valid for the whole evolution of the universe.
When you study the book "The Big Bang" by Joseph Silk at page 72 he considers 6 era: Inflation era, Hadronic era, Leptonic Era, Radiation Era, Matter era and Decoupling era
All these era are supposed to happen synchroneous throughout the entire universe which was expanding. The whole issue is how many individual processes are there which generate the CMB radiation and how different is the radiation (energy density) of each.
When there is only one process involved than there is no communication involved to make the radiation uniform throughout the entire universe.

Reflection 2 - The inflation Theory

The Big Bang theory supports the concept that the evolution of the universe went through different phases or more practial called era. During each era the whole of the universe changed drastically.
The most important parameter that drives these changes is the radius of the universe. As a consequence the average density changes but that does not mean that there are no 3D density perturbations or that all the phases hapenned everywhere synchroneous.
The horizon problem implies lack of communication but IMO there is no global communication involved which reduces these inhomogenieties.
The inflation theory consists of two types of events or era: An acceleration event where the excessif expansion starts and a deacceleration event where the excessif expansion stops. The issue that both should start and stop throughout the whole universe simultaneous. If they don't and if the period of inflation is not everywhere the same than the resulting density will not be uniform. Specific to stop inflation is the largest and physical the most difficult problem.
What that means is that the inflation theory more problems causes than it solves.

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Created: 1 August 2014

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