(1) An introduction to cosmological models

What is cosmology?  

Is the universe contracting, expanding or staying the same size? 

How to explain the red-shift and its significance?

What is the 'Big Bang Theory of the Universe'?

How old is the universe?  What evidence is there for the Big Bang theory of the universe?

In Part 2 to 5 I'll build up the arguments in four sections to explain why the 'Big bang Theory' seems to be the best theory we have at the moment to describe the origin of the Universe

A general size order: asteroid < moon < planet < star < galaxy < expanding universe

 

Cosmology 

Cosmology is a branch of astronomy that involves observing and studying the origin and evolution of the universe, e.g. from the Big Bang at the start, its current state and what is it's future?

According to NASA, the definition of cosmology is "the scientific study of the large scale properties of the universe as a whole."

At the moment the evidence suggests the Universe is getting bigger - some sort of continuous expansion is taking place.

Two principal models have been put forward to explain this expansion and the creation of the Universe itself.

 

The Steady State model of the universe

The Steady State theory states that the Universe has always existed in its current state and will remain so in the future.

The model is based on the idea that the Universe looks roughly the same in which ever direction you look, and has always looked that way.

In order for this to be so, the expansion is explained (but not proved) by new matter being created all the time i.e. more matter is created to cause the expansion.

In the Steady State model of the Universe, there is no beginning and no end - an interesting philosophical concept!

So, in essence, the Steady State theory of the cosmos, an alternative to the Big Bang, proposed that the universe has always existed and is continuously expanding, with new matter being created to maintain a constant density.

It suggests the universe looks the same at any given time and place, a concept called the perfect cosmological principle

 

The Big Bang model of the universe

The Big Bang theory relies on the idea that all the matter of the Universe started out by occupying a very small space (referred to as a singularity).

This tiny space or volume of material would be incredibly dense and hot - awesome concept!

It then 'explodes' (the 'Big Bang') and expands to create, and continues to create all of the space of the universe and distributes all the matter in it from the singularity.

This expansion is considered to be continuous - see evidence for this cosmological expansions in later parts in this section on cosmology.

Measurements of the rate of this expansion can give you an estimate of the finite age of the Universe - now calculated to be about 13.8 billion years (1.38 x 10¹⁰ years).

See Part (5) How does the red-shift & Hubble equation relate to our big bang theory of the universe, its origins and age?

Note that our Sun, and the planets formed from it, are estimated to be 4.5 billion years old (4.5 x 10⁹ years).

Of the two theories, the Big Bang model is the more accepted theory because it fits the ever increasing experimental data better than the Steady State model - the latter just does not fit in with several important observations.

 

The singularity at a 'deeper' level - BUT not for GCSE level!

The initial singularity was of apparently 'infinite' density and considered to have contained all of the mass and space-time of the Universe that exists today (and in the future).

The current theory suggests that prior to the big bang, some sort of quantum fluctuations caused it to rapidly 'explode' in a 'Big Bang' resulting in massive expansion that ultimately created the present-day Universe as we know it - the mind boggles!

(I wish I understood more, but my old grey cells are not up to it these days, and where did the singularity come from in the first place? - mind boggling again!).

The common misconception about the Big Bang theory of the cosmos is that it was an explosion of matter outward into an existing space. Instead, the Big Bang describes the expansion of space itself, causing galaxies to move apart. It's not galaxies moving through space, but rather the fabric of space expanding and carrying galaxies along with it.

INDEX for physics notes on COSMOLOGY

Know that there are several cosmological models of the universe e.g. the steady state model and the big bang model starting with an initial singularity.

Know that most evidence supports the big bang theory - see Parts 2 to 6.

Know what a singularity is in the context of cosmology.

Appreciate the most likely correct theory is the one that is best supported from the evidence obtained by physicists working in the science of cosmology.