The Big Bang Theory (CIE IGCSE Physics)

• 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

• Redshift in the light from distant galaxies is evidence that the Universe is expanding and supports the Big Bang Theory
  • 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

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

  

Evidence for the Big Bang

• The Big Bang theory is very well supported by evidence from a range of sources
• The main pieces of evidence are
  • Galactic red-shift
  • Cosmic Microwave Background Radiation (CMBR)

Evidence from Galactic Red-Shift

• 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

Comparing the light spectrum produced from the Sun and a distant galaxy

• 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

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

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Evidence from CMB Radiation

• The discovery of the CMB (Cosmic Microwave Background) radiation 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 radiation

• 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 observable 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 radiation 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

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Using Redshift Observations to Measure the Universe

• The change in wavelength of the galaxy’s starlight due to redshift can be used to find the velocity, v, with which a galaxy (or any distant object) is moving away from Earth
  • Using an equation to compare the ratio of the expected wavelength with the observed wavelength, the velocity can be found;

   

   

This equation will not be directly examined but the idea that the velocity of distant objects can be found from the redshift seen in easily observed wavelengths is an important one   

Measuring Distance Using Supernovae

• Redshift and CMB radiation allow various measurements of the Universe to be accurately made 
  • Measuring distance is done using different methods
  • A key method is the use of standard candles, including supernovae
• Supernovae are exploding stars
  • Certain types have the same peak level of brightness (absolute magnitude), making them extremely useful in measuring the distance to remote stars and galaxies
  • Type 1a supernovae are so bright that they can be seen clearly even though they may be deep inside their parent galaxy
  • This allows the distance to the galaxy to be calculated