One of the most enduring principles in the entire Universe is the cosmological principle: an outgrowth of the Copernican revolution. Copernicus, nearly 500 years ago now, hypothesized that the Earth occupied no special place in the Universe when he put forth the heliocentric picture, instead recognizing that Earth might be just an ordinary planet like all the others: revolving around the Sun. We’ve since expanded our view of the Universe substantially, and with it, our view of the cosmological principle.
In every way fathomable, there’s nothing special or preferred about our location in space and time — in the here and now — relative to the vast, expanding Universe that we ourselves are a part of. But any number of possible signals, coming from the Universe itself, could call that very principle into question. Such signals could include: While extremely large studies show excellent agreement with the longstanding assumptions of isotropy and homogeneity for the Universe, recent studies of the CMB — first from WMAP, then from Planck, and now from ACT as well — indicate a small but positive, real, and nonzero effect for cosmic birefringence of that light.
Behind the Hubble tension, it’s probably the second most exciting tension we’re seeing in observational cosmology today, and the three leading explanations all give us something to look forward to with new, superior observations on their way. Light is nothing more than an electromagnetic wave, with in-phase oscillating electric and magnetic fields perpendicular to the direction of light’s propagation. The shorter the wavelength, the more energetic the photon, but the more susceptible it is to changes in the speed of light through a medium.
One of the most foundational aspects of our Universe is the principle of relativity itself: that uniform motion itself is indistinguishable from being at rest. Dating all the way back to the time of Galileo, it’s the oldest physical principle that’s still thought to be 100% correct. Einstein built upon this to conclude that the speed of light in a vacuum was indeed invariant for all observers, and that was the key insight that led to special relativity.
All observers, everywhere, no matter where or when they were or how they were moving, measured the same speed for all forms of light. That principle was later extended by Einstein to include acceleration and gravitation as well, leading to general relativity. However, since light itself is an electromagnetic wave in nature — with oscillating, in-phase, mutually perpendicular electric and magnetic fields — different observers in relative motion to one another will observe the same light wave to have different wavelengths: the light will be redshifted or blueshifted depending on the relative motion of the observer to the direction of propagation of the light wave itself.
The light’s speed is invariant (the speed of light in a vacuum), but the observed wavelength changes. This is thought to be absolutely true in a vacuum. However, it isn’t always true when light passes through a material, as materials can slow down light in a wavelength-dependent fashion.
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