OCR A-Level Physics/Fields, Particles and Frontiers of Physics/Evolution of the Universe

The Big Bang Theory
Before the Big Bang the universe was confined to one point in space and time. This point was infinitely dense, infinitely small and infinitely hot. Energy was said to be in the form of pure energy. No kinetic or potential or thermal, just energy. All four fundamental forces are also as one. In a split time frame, something triggered the expansion of the infinitely small point and this is what we call the Big Bang.

Stages of the Big Bang

 * 1) The universe is infinitely dense, infinitely small and infinitely hot.
 * 2) The universe is now a Quark and Lepton 'Soup' where there are only quarks and leptons present. The temperature is approximately 1014K
 * 3) The Quarks combine to form hadrons such as neutrons. The temperature is about 1012K
 * 4) Protons have not been created yet. Neutrons later decay into protons.
 * 5) The protons and neutrons come together to create helium nuclei from fusion reactions. The temperature now is about 107K
 * 6) Electrons join with nuclei to form atoms. Temperature about 104K
 * 7) Matter clumps together to form gas clouds and dust which go on to form stars and galaxies. The temperature now is 2.7K which gives rise to the cosmic microwave background radiation.

Evidence for The Big Bang Theory

 * The temperature of the universe is 2.7K which corresponds to cosmic microwave background radiation. The radiation can be detected at every angle in space and is almost uniform.
 * We can observe red-shift. This shows that galaxies are receding from a central point. This shows that the universe is expanding and not uniform and infinite.
 * Most of the chemical compositions of the universe is hydrogen and helium which is what we can observe from early galaxies.

Fate of the Universe
There are three states that the universe could undergo; Open, Closed, and Flat. What decides what type of universe we live in depends on the critical density.

Critical Density
A critical density is a density that determines the fate of the universe. This is because the three types of ending all depend on the mass of a universe which depends on how dense it is. The critical density acts like a threshold that determines between the three.
 * $$\rho _0 = \frac{3H_0^2}{8\pi G}$$

Open
An open universe is one that will continue to expand forever. The rate of expansion will decrease but it will never stop expanding. This is because the gravitational force cannot decelerate the expansion enough to make it stop. If the density of the universe is less than the critical density, then the universe will be open. This is because there is insufficient gravitational force to slow down the expansion.
 * $$\text{density of universe}<\rho_0$$

Closed
A closed universe is one that will eventually collapse upon itself and end in what is described as The Big Crunch where the universe will return to a state that it was before the Big Bang. The reason for this is that the gravitational forces slow down the expansion and reverse the process because the redshift of the universe is greater than the critical density.
 * $$\text{density of universe}>\rho_0$$

Flat
A flat universe is one where the gravitational forces are just sufficient enough to slow down the process of expansion but no more. So the universe will remain that size forever. For this to happen the density of the universe must be  equal to or close to the critical density.
 * $$\text{density of universe} = \rho_0$$

Fate of our Universe
Calculating the density of our own universe shows the density is close to, if not equal to, the critical density. This shows that our universe may be a flat universe. However, cosmologists have recently believed the existence of Dark Matter. While examining the rate at which our universe is expanding, astronomers have seen that rather than slowing down, the universe has been expanding faster. It has been accelerating and this is due to what cosmologists are calling Dark Energy. We cannot see dark matter because it reflect and emits very little EM radiation but we know it's there because we can see it having a gravitational pull on galaxies.

Dark matter and dark energy
It isn't possible to determine the fate of the universe unless we can determine an accurate value for its density. Scientists believe that dark matter could account for 27% of the matter present in the universe

Evidence of dark matter
By considering the centripetal acceleration of star and interstellar matter as it orbits around the galaxy centre, we can estimate the mass of the galaxy and how this mass is distributed throughout the galaxy. Since the centripetal acceleration of a star orbiting at a distance $$r$$, from the centre of a galaxy of mass M is given by $$v^2 \over r$$, and using Newton's law of gravitation, $$GM \over r^2$$ we equate these and rearrange to get; $$M = {v^2r\over G}$$, assuming M and G are constants we get that $$M = {v^2r\over G}$$ = constant. Hence, as the radius increase the tangential velocity of the star should decrease. However this is not what was observed. This was evidence for a massive amount of unseen matter in the galaxy, which is today called a dark halo. It is now believed that the visible mass only makes up around 5% of the total mass of the universe.

What is dark energy
The four main contenders for the type of particle most likely to make up dark matter are called WIMPs, MACHOs, axions and neutrinos.

Dark energy
Astronomers were observing the expansion of the universe in the hope of determining the rate at which its acceleration is decreasing - however they found that the acceleration of the universe was increasing.

In order for this to happen, material with a gravitationally repulsive effect would be need to be present and occupy around 68% of the universe. It would also need to exert a negative pressure to account for the acceleration of the expansion of the universe. At present, astronomers do not know what it is.