Dark Energy & Dark Matter (OCR A Level Physics): Revision Note

Dark Energy & Dark Matter

Dark Energy

• Scientists know that the universe is accelerating as it expands

• Dark Energy is a hypothetical form of energy which is used to try and explain the accelerating expansion

• Dark energy is defined as:

  A type of energy that permeates the whole universe and opposes the attractive gravitation force between galaxies via the exertion of a negative pressure

• Dark energy cannot be detected directly

  • It should make up 68% of the total energy in the universe

  • So far experiments have not been able to find the form of the energy

Dark Matter

• Astronomers expect to observe the velocity of an object within a galaxy decrease as it moves away from the galaxy’s centre

  • This is thought to be the case because of weakening gravitational field strength further from the centre

• This is observed in smaller mass systems, such as the in the solar system, where planets orbiting furthest from the Sun have the slowest orbital velocity

  • This is not the case in larger mass systems, such as entire galaxies

• In fact, mass is not concentrated in the centre of galaxies - it is spread out

• However, all the observable mass of a galaxy is observed to concentrate in the centre of galaxies

  • Therefore, there must be another type of matter that can't be observed

  • This is known as Dark Matter

• Dark matter is defined as:

  Matter which cannot be seen and that does not emit or absorb electromagnetic radiation

• Dark matter cannot be detected directly through telescopes

  • It is estimated to make up 27% of the mass in the universe

  • It is detected based on its gravitational effects relating to either the rotation of galaxies or by the gravitational lensing of starlight