astronomer Hermann Bondi and others, concerns the role of the expansion of the universe in diluting light. Discovered by Edwin Hubble in the 1920s, this expansion reveals itself in the recession (outward movement) of remote galaxies, relative to our galaxy, the Milky Way. Hubble found that the farther away a galaxy, the faster it appeared to be moving away from us. Noting this universal relationship, known as Hubble’s law, Bondi argued that the light from distant bodies would become weaker and less energetic the farther it has to travel toward Earth. This effect would greatly dilute the amount of radiation that reaches us, thereby enabling darkness at night.

Considering its critical scientific importance, it was natural for scientists to bring Hubble’s law into the discussion. Hubble’s revelation of the universal expansion of space is one of the greatest astronomical discoveries of all time. He discovered this effect through a careful study of the atomic spectral lines found in galactic light.

Spectral lines are the fingerprints of atoms, uniquely characterizing their internal structures. As quantum physics tells us, each type of atom has a particular arrangement of energy levels that its electrons can occupy. Like workers ascending or descending a ladder, stepping up and down on certain rungs, electrons are restricted to specific energy states. Each time an electron drops from a higher to a lower level it emits a photon that carries away that energy difference. On the other hand, whenever an electron rises from a lower to a higher level, it must absorb a photon that infuses the required amount of energy. Quantum physics further informs us that photons have wavelike characteristics, vibrating at various rates depending on their energies. The more energetic a photon, the greater its frequency (rate of vibration). Therefore, the energy profile of an atom translates into a unique arrangement of frequencies of the light emitted or absorbed. Physicists refer to these, respectively, as the emission and absorption spectra.

A well-known property of waves, known as the Doppler effect, is that their observed frequencies shift with the speed of the source. A



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