The Big Bang Theory (Edexcel 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

• 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

• 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 main pieces of evidence for 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