Comments about "Big Bang nucleosynthesis" in Wikipedia

Introduction

• The text in italics is copied from that url
  
• Immediate followed by some comments
  

Contents

• 1. Characteristics
• 2. Important Parameters
• 2.1 Neutron-Proton Ratio
• 2.2 Baryon-Photon Ratio
• 3. Sequence
• 3.1 History of theory
• 3.3 Helium
• 3.4 Deuterium
• 4. Measurements and status of theory
• 5. Non-standard scenarios

Introduction

Primordial nucleosynthesis is believed by many scientists to have taken place just a few moments after the Big Bang and is believed to be responsible for the formation of a heavier isotope of hydrogen known as deuterium

This sentence is not very professional. A better description is:

Nucleosynthesis is the name of the process which started a few moments after the Big Bang and which describes the forming of a heavier isotope of hydrogen known as deuterium etc and continued until ?

1. Characteristics

The era began at temperatures of around 10 MeV (116 gigakelvin) and ended at temperatures below 100 keV (1.16 gigakelvin). The corresponding time interval was from a few tenths of a second to up to 10^3 seconds.

It is very important to explain the physical meaning of the word temperature. Is this a form of radiation ? Also is important: 1. A temperature of what and 2. how do we know those two spcific moments.

The rate of cooling in this era can be given by the equation:
t*T^2=0.74(10.75/g_* )^{1/2} Note #3
Note #3: 21. BIG-BANG COSMOLOGY

The reference Note #3 is wrong, because the document does not discuss this specific equation. The document is interesting becauses it discusses the issues involved.
Next we read in the Wikipedia document:

It was widespread, encompassing the entire observable universe.

This sentence requires modification. The concept of observable universe is based on human qualities and should not be used. To delete the word observable creates a sentence with no meaning.
It is better to delete this sentence all together. An alternative is: Cooling was uniform (But how do we know this ?)
Alan Guth in his book "The inflationary Universe" also uses the concept of Observable Universe. See For example: "The Inflationary Universe book review" page 75.

The key parameter which allows one to calculate the effects of BBN is the number of photons per baryon

Of course such a sentence requires a clear explanation. How do you measure or calculate this value and specific what are the effects.

Document Note #3 just below equation 21.38 writes:

In the Standard Model, a chemical potential is often associated with baryon number, and since the net baryon density relative to the photon density is known to be very small (of order 10^-10),

This means that the photon mass is a factor 10^10 higher then baryon mass. Strange ?. How do they know this.
Next we read in the Wikipedia document:

This parameter corresponds to the temperature and density of the early universe and allows one to determine the conditions under which nuclear fusion occurs.

The question how do we know all of this. Specific which conditions are meant.
IMO the whole paragraph is not very clear.

2. Important Parameters

The creation of light elements during BBN was dependent on a number of parameters; among those was the neutron-proton ratio and the baryon-photon ratio.

A very important aspect is the creation of protons and neutrons, which consists of quarks
If you want to understand the Big Bang nucleosymthesis you have to take quarks into consideration
The Quark epoch is not mentioned in this document. To read more see: Wikipedia Quarck epoch

2.1 Neutron-Proton Ratio

Neutrons can react with positrons or electron neutrinos to create protons and other products in one of the following reactions:

n + e⁺ --> anti-NU_e + p

n + NU_e --> p + e^-

A neutron consists of two down and one up quark. mass = 939,56 MeV
A proton consists of two up and one down quark. mass = 938,27 MeV and stable
What each reaction describes is transformation of a neutron into a proton (specific from a down quark into an an up quark) and a transformation between two leptons. Between those two transformation an exchange of charged immediate vector boson takes place. Either W⁺ or W^-
The question is how important are these reactions just after the Big Bang. To answer this question you must understand the major processes that took place to create neutrons and protons in the first place. Maybe it is much more practical to think about a quark soup which created all types of elementary particles "slowly" when this soup cooled down. Or better: when the universe expanded and the density decreased.
This type of thoughts should be explained in the text. The problem is there is almost no way to prove this by experiment.

2.2 Baryon-Photon Ratio

The baryon-photon ratio Eta, is a strong indicator of the abundance of light elements present in the early universe. Baryons can react with light elements in the following reactions:

(p,n) + 2H <--> (3He, 3H)

(3He, 3H) + (n,p) <--> 4He

The first sentence is not clear. Abundance of light elements versus heavy elements ?
IMO all the elements mentioned are Baryons.
Photons belong to what is called electromagnetic radiation. Photons are typical emitted when electrons switch from one higher state into a lower state. Wikipedia X rays (medium frequency) and Wikipedia Gamma rays belong here. When you discuss Wikipedia radiation in general than also alha and Beta particles are considered.
There is one issue: How are the two quantities i.e. photon energy mass and baryon energy mass, measured in the early Universe?

See also: http://arxiv.org/pdf/0803.3465v3.pdfCONSTRAINING THE EARLY-UNIVERSE BARYON DENSITY AND EXPANSION RATE

3. Sequence

This time is essentially independent of Dark matter content, since the universe was highly radiation dominated until much later, and this controls the temperature/time relation

Of course you can write this, but is it correct ?

The relative abundances of protons and neutrons follow from simple thermodynamical arguments, combined with the way that the mean temperature of the universe changes over time.

The realtive abundance of p and n versus what ?. Same comment as above.

If the reactions needed to reach the thermodynamically favoured equilibrium values are too slow compared to the temperature change brought about by the expansion, abundances would have remained at some specific non-equilibrium value.

During the whole period of Big Bang nucleosynthesis, which is a very dynamic period, a concept like equilibrium should not be used because this implies static behaviour.
Anyway how do you know all of this which partly did not happen. It is better to remove the sentence all together.

Combining thermodynamics and the changes brought about by cosmic expansion, one can calculate the fraction of protons and neutrons based on the temperature at this point.

Remember what we are discussing here: The time interval from a few tenths of a second to up to 10^3 seconds Think about this same but slightly modified sentence:

Combining thermodynamics and the changes brought about by cosmic expansion, one can calculate the fraction of protons versus neutrons during the whole period of Big Bang nucleosynthesis.

Is that true ? I doubt this. The problem is you must understand the processes involved when the universe was very dense.

The answer is that there are about seven protons for every neutron at the beginning of nucleosynthesis. This fraction is in favour of protons initially, primarily because their lower mass with respect to the neutron favors their production.

That means we can calculate this 1 to 7 ratio for the moment a few "tenths of a second" after the Big Bang.
What is the reasoning behind that the tiny lower mass of a proton explains that protons are stable and neutrons not ?

Part of the problem is that at present we have 1 Helium atom compared with 12 protons. This number is observed in all nearby galaxies. This same relation is supposed to have existed at the end of nucleosynthesis i.e. 10^3 seconds after the Big Bang. During nucleosynthesis He was formed. That means at the beginning of nucleosynthesis there were 2 neutrons for every 14 protons because 1 He atom contains 2 protons and 2 neutrons. The problem is: is this physical true.

3.1 History of theory

During the 1970s, there was a major puzzle in that the density of baryons as calculated by Big Bang nucleosynthesis was much less than the observed mass of the universe based on calculations of the expansion rate.

Both calculations are very problematic. Issues to consider are:

• The initial conditions. Protrons and neutrons are created as a result of quarks of which there are many different types after the quark epoch.
• The reaction rates of the different reactions which are also a function of the density at each moment.
• The expansion rate calculation and Friedmann's equation.

The expansion rate is calculated based on the velocity of galaxies and using Friedmann's equation. IMO the closer you come to the Big Bang the more difficult it become to use the result of these equations as an actual discription of what happened (the chemical processes involved).
Next:

This puzzle was resolved in large part by postulating the existence of Dark matter

How can you solve a puzzle by introducing a new puzzle? What is Dark matter
See also: Comments: Dark matter.
Assuming that dark matter = non baryonic then sentence becomes:

This puzzle was resolved in large part by postulating the existence of non baryonic matter

But is this correct ? Many people "believe" that dark matter consists of special heavy particles called: Wimps. See: Wikipedia WIMP's
It could also mean that the observations done and or calculations used were wrong.

3.3 Helium

Big Bang nucleosynthesis predicts a primordial abundance of about 25% helium-4 by mass, irrespective of the initial conditions of the universe.

See also the comments at the end of paragraph 3. Sequence
The issue is partly not what happened at the end of nucleosynthesis but at the beginning of nucleosynthesis. How do we explain that there are at that moment 2 neutrons for every 14 protons.
The sentence "irrespective of the initial conditions of the universe." is not true because of the next sentence in the document:

As long as the universe was hot enough for protons and neutrons to transform into each other easily, their ratio, determined solely by their relative masses, was about 1 neutron to 7 protons (allowing for some decay of neutrons into protons).

If protron and neutrons can easily transform into each other than the ratio should be 1, also because there masses are almost the same.
A little further on is written:

One analogy is to think of helium-4 as ash, and the amount of ash that one forms when one completely burns a piece of wood is insensitive to how one burns it.

Let me agree with all what is written about ash, you can also get charcoal, but what has ash (carbon) to do with helium-4 will I expect forever be a mystery for me.

3.4 Deuterium

The problem was that while the concentration of deuterium in the universe is consistent with the Big Bang modle as a whole, it is too high with a model that presumes that most of the universe is composed of protons and neutrons.

Should this "is" not be: "was"

If one assumes that all of the universe consists of protons and neutrons, the density of the universe is such that much of the currently observed deuterium would have been burned into helium-4.

Two reasons could be considered:

• The assumed ratio protons to neutrons is wrong
• The density of the universe is wrong.
• The reaction conditions to convert deuterium are not optimal.

Directly there after we read:

The standard explanation now used for the abundance of deuterium is that the universe does not consist mostly of baryons, but that non-baryonic matter (also known as: Dark matter ) makes up most of the mass of the universe.

This solution is too simple. See also above.
The solution also raises an other problem: Exactly what is non-baryonic matter.
See also: 3.1 History of theory
See also: Comments: Dark matter.

4. Measurements and status of theory

The theory of BBN gives a detailed mathematical description of the production of the light "elements" deuterium, helium-3, helium-4, and lithium-7. Specifically, the theory yields precise quantitative predictions for the mixture of these elements, that is, the primordial abundances at the end of the big-bang.

It is "easy" to calculate precise predictions. It is much more interesting to indicate how accurate these predictions are based on actual observations.
What is written: at the end of the big-bang. What you want is the evolution of these elements.

In order to test these predictions, it is necessary to reconstruct the primordial abundances as faithfully as possible, for instance by observing astronomical objects in which very little stellar nucleosynthesis has taken place (such as certain dwarf galaxies) or by observing objects that are very far away, and thus can be seen in a very early stage of their evolution (such as distant quasars).

As indicated above the initial conditions of all the calculations involved are very important. The question is: if the state of dwarf galaxies is identical as the state when the Big Bang nucleosynthesis processes started.

5. Non-standard scenarios

Reflection

The most important lesson from this document: remove the word dark matter. Only use the word non-baryonic matter or mesons.
A different proposal is to remove the word temperature. This is an human based concept.

A more professional document which describes nucleosynthesis is: http://www.astro.ucla.edu/~wright/BBNS.html by Edward L Wright.

This document discusses Big Bang Nucleosynthesis in Wikipedia. There is also an aditional site which discusses Nucleosynthesis in Wikipedia
To order to read the document select:http://en.wikipedia.org/wiki/Nucleosynthesis

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