The evolution of the Universe

In a nutshell

Our universe is thought to be about $13.7$ billion years old and it is suggested that it started with the Big Bang. However we still do not fully understand what $95\%$ of it is made of. About $27\%$ of the universe is dark matter, which is composed of particles which do not emit or absorb light, so cannot be detected.

Equations

DESCRIPTION	EQUATION
Age of the universe	$t=\dfrac{1}{H_0}$

Constants

CONSTANT	SYMBOL	VALUE
$speed \space of \space light \space in \space a \space vacuum$	$c$	$3 × 10^{8}\, ms^{-1}$
$Hubble \, constant$	$H_0$	$65 km s^{–1} Mpc^{–1} = 2.2 × 10^{−18} s^{−1}$
$age\, of\, the\, universe$	$t$	$13.7\, billion \,years$

The Big Bang Theory

The Big Bang model is the most widely believed theory explaining how the universe began. It states that in the past, all the matter in the universe was contained in one single point, the singularity. Initially the universe was hot and dense. When the Big Bang occurred, it began to cool rapidly, allowing elements to form, and was the beginning of life today.

Evidence for the Big Bang

From Hubble's law, it was concluded that the universe is expanding, which leads us to believe that all matter was concentrated at a single point in the past. It was from this point, it began to expand. However, more evidence is needed, as the expanding universe can also be explained by other competing theories.

Cosmic Microwave Background Radiation (CMBR)

The Big Bang theory states the universe was initially very hot and dense, then rapidly expanded and cooled, at this time the universe was populated with high energy gamma photons. The expansion of the universe stretched out the wavelength of these photons, which would now be observed as microwaves.

In 1964, Arno Penzias and Robert Wilson were trying to detect signals from space and instead observed a microwave signal they could not account for, which is now known as CMBR. This could only be described as remnants of the Big Bang.

The age of the Universe

Hubble's law allowed astronomers to calculate how long ago galaxies were compressed in a singularity. From this it can be estimated when the Big Bang occurred, and when our universe began. This estimate, however, assumes the universe expanded uniformly over time, which is now known not to be true, but can still give us a good enough estimate:

$v = H_0d \newline \\[0.1in]t = \dfrac{d}{v} \to v = \dfrac{d}{t} \newline \\[0.1in]\dfrac{d}{t} = H_0d \\[0.1in]t = \dfrac{1}{H_0}$

where $d$ is the distance the galaxy is moving away and $v$ is the recessional velocity.

Example:

Calculate the age of the universe.

First state variables:

$H_0 = 2.2 × 10^{−18} s^{-1}$

State the equation:

$t = \dfrac{1}{H_0}$

Substitute in:

$t = \dfrac{1}{2.2 × 10^{−18}\, s^{−1}} \newline \\[0.1in]t = 4.5 × 10^{17}\, s \ (\approx14 \,billion\, years).$

The age of the universe is about $\underline{4.5 \times 10^{17} \, s}$ .

Evolution of the Universe

As shown above, the universe is thought to be about $13.7$ billion years old, over this time the state of our universe has changed dramatically through expansion, cooling and formation of matter. The early universe started as a hot dense singularity, the Big Bang then occurred which was followed by a period of expansion and cooling called 'inflation'.

At this time there is no matter in the universe, only high energy gamma photons, however over the next second the first fundamental particles form and combine to make protons and neutrons. In the next $100$ seconds, protons and neutrons fuse to form nuclei.

It would then be $380,000$ years before the Earth was cool enough for the first atoms to form, and at this stage electromagnetic radiation, now detected as CMBR, was emitted. About $8$ million years after the Big Bang, the expansion of the universe began to accelerate, cosmologists believe this is down to the mysterious 'dark energy', which causes matter to repel other matter.

Physics; Cosmology; KS5 Year 12; The evolution of the Universe

1.	Quantum fluctuations
2.	Inflation
3.	Afterglow
4.	Dark ages
5.	First stars
6.	Development of galaxies
7.	Dark energy accelerating expansion
8.	Big Bang expansion

Dark matter

Dark matter doesn't absorb or emit light, making it difficult to detect, it has only been observed through the gravitational effect it has on visible matter. It makes up about $27\%$ of the universe, however not much is known about it.