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Astronomical time-of-flight photon speedometer.

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Scientists measured the speed of light from distant galaxies using a new instrument. The photon group velocity is constant across vast cosmic distances, supporting general relativity.

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Area of Science:

  • Astrophysics and Cosmology
  • Experimental Physics

Background:

  • Measuring the velocity of photons from faint extragalactic sources is challenging.
  • Previous studies have relied on indirect methods or less precise measurements.

Purpose of the Study:

  • To develop and utilize a novel dual-band, fiber-optic, photon time-of-flight instrument.
  • To perform the first direct photon group velocity measurements for extragalactic objects.
  • To test the invariance of photon group velocity across cosmic distances and validate cosmological models.

Main Methods:

  • Development of a specialized fiber-optic instrument for photon time-of-flight measurements.
  • Optimization for detecting visible photons from dim astronomical sources (apparent magnitude m>12).
  • Measurement of photon group velocity for objects with redshifts ranging from 0 to 1.33.

Main Results:

  • The photon group velocity was measured to be 3.00±0.03×10^8 m/s.
  • The measured velocity was found to be invariant within experimental error across the observed redshift range (0≤z≤1.33).
  • The results provide direct experimental evidence for the constancy of light speed in extragalactic space.

Conclusions:

  • The findings validate Einstein's theory of general relativity.
  • The results are consistent with the Friedmann-Lemaître-Robertson-Walker and hyperbolic anti-de Sitter cosmological models.
  • The data exclude the elliptical de Sitter metric, providing constraints on cosmological parameters.