Evidence for the Big Bang:

Alexia Noirot UVI-5

Introduction:

The Big Bang Theory is the theory used to explain the creation of the universe It states that the universe started in a hot, dense state and has been expanding ever since Some suggest that it could have been infinitely hot and infinitely dense. Up until the 1930s, astronomers thought that the universe was infinitely large and static but since, new theories have been produced to suggest otherwise This essay will discuss the relevant theories that support the Big Bang Theory: Galactic Redshift, Hubble’s Law, Cosmic Microwave Background Radiation, Dark Energy and Hydrogen to Helium Abundances

Observing Galactic Doppler Shift can be done through analysing absorption/emission spectra from distant galaxies A reference spectrum is used against the observed one to calculate the difference in wavelength This can be used to calculate how much redshift is present from the galaxy.

This supports the Big Bang Theory because if the universe was not expanding, no redshift would be present The fact that redshift is observable in these spectra shows that the galaxy or star being observed is moving away from us on Earth It also shows that the galaxy is moving away from us at an increasing rate If we on earth and the galaxy far away were moving at the same velocity, there would be no relative speed between us and so no redshift would be observed. Since we do observe redshift, the galaxy must be moving at a higher velocity than us so it must be accelerating at an increasing rate.

Galactic Redshift: d to This is absolute be treated or a source he avelength light that e observe creases e to the ppler effect The waves are pushed towards the longer wavelength side of the Electromagnetic spectrum and we say that they have been redshifted The faster the relative motion is between the source of the light and the observer results in a higher value for redshift

Hubble’s Law:

Edwin Hubble was the first scientist to suggest that the universe was expanding. He did so by using Type 1a Supernovae to calculate distances to galaxies. Using the redshift calculated from spectra, recessional velocity could be calculated

This can be used to calculate

��/�� = ��/�� redshift and the galaxy's velocity (v) The velocity calculated can then be used to calculate the distance to the galaxy:

�� = ���� and d is the distance to the galaxy

Where H is Hubble’s constant

On a graph showing recessional velocity against distance, a directly proportional relationship was found. The gradient of the plotted graph is Hubble’s constant (65 kms^-1MPc^-1)

Analysing this graph can show us that as the distance to a galaxy or star increases, its recessional velocity increases This supports the Big Bang Theory.

Cosmic Microwave Background Radiation:

The Big Bang model predicts that at the moment the universe was created, a large burst of short wavelength electromagnetic radiation (probably Infra-Red) was produced in all directions As the universe has expanded (and subsequently cooled) the wavelength of this radiation has been increased out to the microwave region These waves only started propagating once the universe turned transparent They can be detected by a microwave specialised telescope and can be seen to be the same in whichever direction the telescope is pointed at.

CMBR has a perfect blackbody spectrum

This means a continuous spectrum of all wavelengths The peak in this type of curve can be used to calculate the temperature of the universe though Wien's Law. This peak falls into the microwave region of the Electromagnetic Spectrum. The Big Bang Theory suggests the temperature is around 2.7 Kelvin, and CMBR’s Black Body curve supports this estimate There are small fluctuations in calculated temperatures due to tiny energy density variations which have been present since the early universe.

CMBR also contributes to the cosmological principle, which is the second assumption of the Big Bang Model. This states that the observer’s view of the universe should not depend on the direction of observation or location CMBR can be seen to be homogeneous (every part is the same) and isotropic (everything looks the same in every direction - there is no centre)

Dark

Energy

and Matter:

All mass in the universe feels the gravitational force, one of the four fundamental forces in the Universe. This force is always attractive and is only zero at infinity. In theory, this attractive force felt by all objects with mass should slow down the rate of expansion but that is not the case with the universe. Scientists believe that this is due to something they have called Dark Matter which is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark Matter is composed of particles that do not emit, absorb or reflect light so cannot be detected by observing electromagnetic radiation. This means that it cannot be directly seen, which makes its existence hard to prove. We can however measure the effect it would have on the universe, its acceleration. Since that is something we deem to be true, dark matter, in principle, should exist

Dark Energy is a form of energy that fills the whole space of the universe, it is thought to be the only energy source