Lecture on Redshift and Expanding Universe

Introduction to Redshift

• Evidence of universe expansion seen in light from distant galaxies.
• Spectrum of the sun has dark lines; similar lines in galaxies' spectra are shifted to longer wavelengths (redshifted).
• Cosmological redshift explanation: light traveling through expanding space stretches photons, increasing wavelength.

Types of Redshift

1. Doppler Redshift

• Due to relative motion between observers and the source.

2. Gravitational Redshift

• Different wavelengths measured at varying gravitational potentials.
• Experiment by Pound and Rebka (1959) showed redshift in photons traveling in a gravitational field.

3. Cosmological Redshift

• Observing photons over vast distances in an expanding universe results in wavelength changes.

Thought Experiments

Gravitational Field and Accelerating Rockets

• Equivalence principle suggests identical results in gravitational fields and accelerating spaceships.
• Photon color perceived differently by stationary observers and those accelerating.

Observers and Photon Systems

• Wavelength and energy are properties of the photon-observer system, not intrinsic to the photon itself.
• Different observers perceive different wavelengths due to their relative motion or gravitational field.

Redshift Unification

• Redshift is not three distinct phenomena; it’s reframed by the observer’s perspective.
• Gravitational and Doppler redshifts can be viewed as one, depending on observer motion or field strength.

Cosmological Redshift in an Expanding Universe

• Universe can be seen as a cosmic fluid (homogeneous and isotropic).
• Observers in this fluid exchange photons and perceive redshift due to relative motion (Doppler effect).

Implications and Misconceptions

• Redshift does not mean space is physically stretching objects like photons, stars, or galaxies.
• Local conditions (e.g., electromagnetic forces) prevent expansion at molecular or galactic scales.

Conclusion

• Personal expansion with the universe does not occur under normal conditions.
• Expansion only noticeable if forces binding bodies are removed and under dominance of dark energy.

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