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AQA GCSE Physics
Revision NotesDark Energy & Dark Matter
Dark Energy & Dark Matter
Dark Matter
Dark matter is a form of matter that does not emit, absorb, or reflect light, making it invisible to telescopes. This is because it does not interact with electromagnetic forces, only through gravity. Despite being unseen, it has mass and exerts a gravitational pull.
Scientists discovered dark matter because of the way galaxies rotate. The stars at the edges of galaxies move faster than expected if only visible matter were present. This suggests there is extra invisible mass holding the galaxy together.
Dark matter acts like a cosmic glue, preventing galaxies from flying apart due to their rotation speeds. It makes up about 27% of the universe's total mass and energy.
For instance, if a galaxy’s visible mass cannot explain the speed of stars orbiting its centre, the extra gravitational pull is attributed to dark matter.
Worked Example
Example: A galaxy’s visible mass suggests stars should orbit at 150 km/s, but observations show stars orbit at 250 km/s. Explain why dark matter is needed.
Dark Energy
Dark energy is a mysterious form of energy that causes the expansion of the universe to speed up. It acts in opposition to gravity, pushing galaxies apart.
Unlike dark matter, dark energy does not clump together and its nature is still unknown. It is often modeled as a cosmological constant or vacuum energy. It makes up about 68% of the universe’s total mass and energy.
The discovery of dark energy came from observing distant supernovae and noticing the universe’s expansion is accelerating, not slowing down as expected.
- Think of dark energy as a force that stretches space itself, making the universe grow faster over time.
- Dark energy is not the same as dark matter; one pulls galaxies together, the other pushes them apart.
Evidence from Red-shift
Red-shift is the increase in the wavelength of light from distant galaxies, making it appear more red. This happens because the galaxies are moving away from us.
When light waves stretch, their wavelength increases, shifting towards the red end of the spectrum. This is similar to the Doppler effect with sound.
The greater the red-shift, the faster the galaxy is moving away. This supports the idea that the universe is expanding.
For example, if a galaxy’s light wavelength is normally 500 nm but is observed at 550 nm, the red-shift shows it is moving away from Earth.
Worked Example
Example: A galaxy emits light with a wavelength of 400 nm, but we observe it at 440 nm. Calculate the red-shift and explain what it tells us.
Worked Example
Example: If a galaxy has a red-shift of 0.2, what fraction of the speed of light is it moving away at? (Use , where )
Role in Cosmology
Dark matter shapes the large-scale structure of the universe by pulling matter together through gravity, helping galaxies and clusters form.
Dark energy drives the accelerated expansion of the universe, stretching space and increasing the distance between galaxies over time.
Both dark matter and dark energy are essential in understanding the Big Bang and the evolution of the universe.
- Remember: dark matter holds the universe’s structure together, dark energy pushes it apart.
- The Big Bang started the universe’s expansion, and dark energy controls how fast it expands now.
Worked Example
Example: Explain why the discovery of red-shift supports the Big Bang theory.
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