1. Doppler Shift (Redshift & Blueshift)

• Redshift/Blueshift = change in light wavelength as celestial objects move away or towards us.
• Edwin Hubble first described galactic redshift → proved that the universe is expanding.
• Hubble’s Law: The farther a galaxy → faster it moves away → shows accelerating expansion of the Universe.

2. Cosmic Microwave Background (CMB)

• With optical telescopes → space between galaxies is dark.
• Radio telescopes detect a faint glow → strongest in the microwave region → called Cosmic Microwave Background (CMB).
• CMB = relic radiation from the Big Bang; the oldest observable light in the Universe.
• Present in all directions; key proof for Big Bang Theory & accelerating expansion./li>

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3. Gravitational Waves

• Predicted by Einstein (1916) in General Relativity.
• “Ripples” in spacetime are produced by massive accelerating bodies (e.g., merging black holes, neutron stars).
• Travel at the speed of light, carrying information about the origin.
• LIGO (USA), 2015: first physical detection of two merging black holes, 1.3 billion light-years away.
• Waves reaching Earth are extremely faint (billions of times smaller).

4. Einstein’s Relativity & Spacetime

• Special Relativity (1905):
  • Laws of physics are identical for all non-accelerating observers.
  • Speed of light constant → space and time merge into spacetime.
• General Relativity (1915):
  • Massive objects distort spacetime → felt as gravity.
  • Evidence: Gravitational lensing, gravitational waves.

5. Gravitational Lensing

• Light bends around massive objects (e.g., black holes).
• Acts like a “lens” magnifying objects behind it.

6. Importance of Gravitational Waves

• “Standard sirens” for measuring the Hubble Constant (rate of universal expansion).
• Velocity estimated from light flash (from merging bodies).
• Distance estimated from gravitational waves.
• Together → more precise, independent measurement of the Universe’s expansion rate.