The Cosmic Microwave Background (Edexcel GCSE Physics): Revision Note

The Cosmic Microwave Background

• The discovery of the CMB (Cosmic Microwave Background) led to the Big Bang theory becoming the currently accepted model since it is not supported by the Steady State theory

  • 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 / green 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