## Comments about "Friedmann equations" in Wikipedia

This document contains comments about the article Friedmann equations in Wikipedia
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• Immediate followed by some comments
In the last paragraph I explain my own opinion.

### Introduction

The article starts with the following sentence.
The Friedmann equations are a set of equations in physical cosmology that govern the expansion of space in homogeneous and isotropic models of the universe within the context of general relativity.
The word govern is wrong. A law or equation describe a physical phenomena.

### 1. Assumptions

The Friedmann equations start with the simplifying assumption that the universe is spatially homogeneous and isotropic, i.e. the cosmological principle; empirically, this is justified on scales larger than ~100 Mpc.
The issue is that this assumption should be valid, at all scales, for the (entire) Universe at present. What this means that the density in the entire Universe at any moment should be the same.
To get a better idea what is involved study: Friedmann's Equation & The path of a light ray

### 3 Density parameter

The density parameter, Omega, is defined as the ratio of the actual (or observed) density rho to the critical density rho_c of the Friedmann universe.
It is important to note that the critical density rho_c can not be measured directly.
The value can only be calculated indirectly by setting the value equal to the observed density rho (Which makes Omega=1) and other parameters equal to zero.
To date, the critical density is estimated to be approximately five atoms (of monatomic hydrogen) per cubic metre, whereas the average density of ordinary matter in the Universe is believed to be 0.2–0.25 atoms per cubic metre.
As stated above the critical density can not directly be measured (observed)
Although the total density is equal to the critical density (exactly, up to measurement error), the dark energy does not lead to contraction of the universe but rather may accelerate its expansion.
This is the parameter Lambda.

### 5. Rescaled Friedmann equation

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

### Reflection 1 - Is the Universe homogeneous and isotropic?

Homogeneous and isotropic means that the Universe in all directions (at present) is the same. At present should be considered broader. It means during the whole evolution of the Universe starting direct after the Big Bang untill at present.

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Created: 17 December 2017

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