Edexcel iGCSE Physics -8.18P Expansion of the Universe- Study Notes- New Syllabus

Edexcel iGCSE Physics -8.18P Expansion of the Universe- Study Notes- New syllabus

Edexcel iGCSE Physics -8.18P Expansion of the Universe- Study Notes -Edexcel iGCSE Physics – per latest Syllabus.

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Edexcel iGCSE Physics -Concise Summary Notes- All Topics

Why Red-Shift of Galaxies Is Evidence for the Expansion of the Universe

The light received from distant galaxies is observed to be red-shifted. This observation provides strong evidence that the universe is expanding.

Key Statement

Statement: The red-shift of light from galaxies shows that they are moving away from Earth, which provides evidence that the universe is expanding.

Key idea: If almost all galaxies are moving away from us, space itself must be expanding.

What Red-Shift Shows

  • Red-shift is an increase in the wavelength of light.
  • It occurs when a light source moves away from an observer.
  • The greater the red-shift, the faster the source is moving away.

Observations of Galaxies

  • Light from almost all distant galaxies is red-shifted.
  • Very few galaxies show blue-shift.
  • More distant galaxies show larger red-shifts.

Conclusion: Most galaxies are moving away from Earth.

Link Between Red-Shift and Expansion

  • If galaxies were moving randomly, some would move towards Earth.
  • This would produce blue-shift.
  • Instead, red-shift is observed in nearly all directions.

Key idea: This pattern shows that space itself is expanding uniformly.

Distance and Red-Shift

  • Nearby galaxies show small red-shifts.
  • Distant galaxies show larger red-shifts.
  • This means distant galaxies are moving away faster.

Important: This relationship supports a steadily expanding universe.

Mathematical Support

\( \mathrm{\dfrac{\Delta \lambda}{\lambda_0} = \dfrac{v}{c}} \)

  • Larger \( \mathrm{\Delta \lambda} \) means larger velocity \( \mathrm{v} \).
  • Larger velocity corresponds to greater distance.

Why This Means the Universe Is Expanding

  • Galaxies are moving away in all directions.
  • No central point of expansion is observed.
  • This is explained by the expansion of space itself.

Key idea: Every observer sees galaxies moving away.

Exam-Focused Explanation

  • Red-shift → galaxies moving away.
  • More distant → greater red-shift.
  • All directions → uniform recession.
  • Therefore → universe is expanding.

Example

Explain why the observation that almost all galaxies show red-shift provides evidence that the universe is expanding.

▶️ Answer / Explanation
  • Red-shift shows galaxies are moving away.
  • Almost all galaxies show red-shift.
  • This means galaxies are moving away in all directions.
  • This cannot be explained by random motion.
  • It shows that space itself is expanding.

Example

Two galaxies are observed, one near Earth and one very distant. The distant galaxy shows a much larger red-shift. Explain how this supports the expansion of the universe.

▶️ Answer / Explanation
  • Larger red-shift means higher recessional speed.
  • The distant galaxy is moving away faster.
  • This shows recession speed increases with distance.
  • This pattern is expected in an expanding universe.
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