The “perfect cosmological principle” isn’t true for our Universe

Back in 1923, an unprecedented scientific revolution occurred.

Distances to individual stars were measured beyond the Milky Way.

By combining measured extragalactic distances with observed recession speeds, we discovered the expanding Universe.

But was the Universe evolving temporally, as the Big Bang predicts?

Or was it dynamic but unchanging: cosmically indistinguishable at different moments?

The Universe could obey the perfect cosmological principle: identical in all locations and throughout time.

Merely a small, constant rate of spontaneous matter creation would be required.

Galaxies and stars of all ages should be found everywhere, universally.

The expansion rate and density shouldn’t evolve over cosmic time.

And the only cosmic backgrounds would arise from reflected starlight and radiating matter.

Observational evidence, beginning in the 1950s and 60s, overwhelmingly refuted the idea.

Distant galaxies appear:

• younger,
• bluer,
• lower in mass,
• and less morphologically (shape) evolved.

The total density, measured by mass and galaxy number counts, increases with distance.

The expansion rate changes with time: the “Hubble constant” is an evolving parameter.

And a cosmic radiation background exists, whose spectrum doesn’t match reflected starlight’s or heated dust’s.

The Universe really does change with time, supporting the Big Bang.

The “perfect cosmological principle” doesn’t hold true.

Mostly Mute Monday tells an astronomical story in visuals, images, and no more than 200 words.

This article was first published in February of 2023. It was updated in September of 2026.

This article is featured on Big Think.