Cosmology

What is the Big Bang theory?

The Big Bang theory

• Around 14 billion years ago, the Universe began from a very small region that was extremely hot and dense

• Then there was a giant explosion, which is known as the Big Bang

• This caused the universe to expand from a single point, cooling as it does so, to form the universe today

• Each point expands away from the others

  • This is seen from galaxies moving away from each other, and the further away they are the faster they move

• As a result of the initial explosion, the Universe continues to expand

All galaxies are moving away from each other, indicating that the universe is expanding

• An analogy of this is points drawn on a balloon where the balloon represents space and the points as galaxies

• When the balloon is deflated, all the points are close together and an equal distance apart

• As the balloon expands, all the points become further apart by the same amount

• This is because the space itself has expanded between the galaxies

  • Therefore, the density of galaxies falls as the Universe expands

A balloon inflating is similar to the stretching of the space between galaxies

What is the evidence that supports the Big Bang theory?

Evidence for the Big Bang

• Since there is more evidence supporting the Big Bang theory than the Steady State theory, it is the currently accepted model for the origin of the Universe

• The two main pieces of evidence supporting the Big Bang are

  • Galactic red-shift

  • Cosmic Microwave Background (CMB) radiation

Evidence from galactic red-shift

• By observing the light spectrums from supernovae in other galaxies there is evidence to suggest that distant galaxies are receding (moving further apart) even faster than nearby galaxies

  • These observations were first made in 1998

• The light spectrums show that light from distant galaxies is redshifted, which is evidence that the universe is expanding

• As a result, astronomers have concluded that:

  • All galaxies are moving away from the Earth

  • Galaxies are moving away from each other

• This is what is expected after an explosion

  • Matter is first densely packed and as it explodes it, it moves out in all directions getting further and further from the source of the explosion

  • Some matter will be lighter and travel at a greater speed, further from the source of the explosion

  • Some matter will be heavier and travel at a slower speed, closer to the source of the explosion

• If someone were to travel back in time and compare the separation distance of the galaxies:

  • It would be seen that galaxies would become closer and closer together until the entire universe was a single point

• If the galaxies were originally all grouped together at a single point and were then exploded a similar effect would be observed

  • The galaxies that are the furthest are moving the fastest - their distance is proportional to their speed

  • The galaxies that are closer are moving slower

Tracing the expansion of the universe back to the beginning of time leads to the idea the universe began with a “big bang”

Evidence from CMB radiation

• The discovery of the CMB (Cosmic Microwave Background) led to the Big Bang theory becoming the currently accepted model

  • The CMB is a type of electromagnetic radiation which is a remnant from the early stages of the Universe

  • It has a wavelength of around 1 mm making it a microwave, hence the name Cosmic Microwave Background

• In 1964, Astronomers discovered radiation in the microwave region of the electromagnetic spectrum coming from all directions and at a generally uniform temperature of 2.73 K

  • They were unable to do this any earlier since microwaves are absorbed by the atmosphere

  • Around this time, space flight was developed which enabled astronomers to send telescopes into orbit above the atmosphere

• According to the Big Bang theory, the early Universe was an extremely hot and dense environment

  • As a result of this, it must have emitted thermal radiation

• The radiation is in the microwave region

  • This is because over the past 14 billion years or so, the radiation initially from the Big Bang has become redshifted as the Universe has expanded

  • Initially, this would have been high energy radiation, towards the gamma end of the spectrum

  • As the Universe expanded, the wavelength of the radiation increased

  • Over time, it has increased so much that it is now in the microwave region of the spectrum

The CMB is a result of high energy radiation being redshifted over billions of years

• The CMB radiation is very uniform and has the exact profile expected to be emitted from a hot body that has cooled down over a very long time

  • This phenomenon is something that other theories (such as the Steady State Theory) cannot explain

• The CMB is represented by the following map:

The CMB map with areas of higher and lower temperature. Places with higher temperature have a higher concentration of galaxies, Suns and planets

• This is the closest image to a map of the Universe

• The different colours represent different temperatures

  • The red / orange / brown regions represent warmer temperature indicating a higher density of galaxies

  • The blue regions represents cooler temperature indicating a lower density of galaxies

• The temperature of the CMB is mostly uniform, however, there are minuscule temperature fluctuations (on the order of 0.00001 K)

  • This implies that all objects in the Universe are more or less uniformly spread out

What happens when a wave source is moving relative to an observer?

This is called the Doppler Effect. If something that emits a wave moves whilst it is doing so (imagine a noisy motorbike coming towards you then going further away, emitting sound waves the whole time) then the wavelength of the sound will become shorter as it is moving towards you, increasing the frequency, and stretched as it is moving away, decreasing the frequency.   You will hear this as a change in pitch, getting higher as it approaches and lower as it moves away. The same thing happens for a moving object that is emitting light waves –e.g. a galaxy.

How is red-shift in light received from galaxies at different distances away from the Earth?

Galactic redshift

• The Doppler effect affects all types of waves, including light

• Light emitted from stars and galaxies will be at a certain wavelength in the visible part of the electromagnetic spectrum

• If an object moves away from an observer the wavelength of light increases

  • This is known as redshift as the light moves towards the red end of the spectrum

• The redshift definition is therefore:

  The phenomenon of the wavelength of light appearing to increase when the source moves away from an observer

• If an object moves towards an observer the wavelength of light decreases

  • This is known as blueshift as the light moves towards the blue end of the spectrum

Light from a star that is moving towards an observer will show blueshift and light from a star moving away from an observer will show redshift

• An increase in wavelength (redshift) is a decrease in frequency and vice versa

The observer in front observes a blue shift, the observer behind observes a redshift

The expanding Universe

• Galactic redshift provides evidence for the Big Bang Theory and the expansion of the universe

• The diagram below shows the light coming to us from a close object, such as the Sun, and the light coming to the Earth from a distant galaxy

Redshift diagram for a light spectrum

Comparing the light spectrum produced from the Sun and a distant galaxy. The spectral lines from the distant galaxy are redshifted.

Red shift provides evidence that the Universe is expanding because:

• Red shift is observed when the spectral lines from the distant galaxy move closer to the red end of the spectrum

  • This is because light waves are stretched by the expansion of the universe so the wavelength increases (or frequency decreases)

  • This indicates that the galaxies are moving away from us

• Light spectrums produced from distant galaxies are red shifted more than nearby galaxies

  • This shows that the greater the distance to the galaxy, the greater the redshift

  • This means that the further away a galaxy is, the faster it is moving away from the Earth

• These observations imply that the universe is expanding and therefore support the Big Bang Theory

Graph showing the greater the distance to a galaxy, the greater the redshift