Back in 1923, an unprecedented scientific revolution occurred.
Hubble’s discovery of a Cepheid variable in the Andromeda galaxy, M31, opened up the Universe to us, giving us the observational evidence we needed for galaxies beyond the Milky Way and leading us, in short order, to the discovery of the expanding Universe.
Distances to individual stars were measured beyond the Milky Way.
Edwin Hubble’s original plot of galaxy distances, from 1929, versus redshift (left), establishing the expanding Universe, versus a more modern counterpart from approximately 70 years later (right). Many different classes of objects and measurements are used to determine the relationship between distance to an object and its apparent speed of recession that we infer from its light’s relative redshift with respect to us. As you can see, from the very nearby Universe (lower left) to distant locations over a billion light-years away (upper right), this very consistent redshift-distance relation continues to hold. Earlier versions of Hubble’s graph were composed by Georges Lemaître (1927) and Howard Robertson (1928), using Hubble’s preliminary data.
By combining measured extragalactic distances with observed recession speeds, we discovered the expanding Universe.
There is a large suite of scientific evidence that supports the expanding Universe and the Big Bang. At every moment throughout our cosmic history for the first several billion years, the expansion rate and the total energy density balanced precisely, enabling our Universe to persist and form complex structures. Today, dark energy dominates the Universe, while early on, prior to the onset of the hot Big Bang, a phase of cosmological inflation occurred, preceding it and setting it up.
But was the Universe evolving temporally, as the Big Bang predicts?
This image shows a slice of the matter distribution in the Universe as simulated by the GiggleZ complement to the WiggleZ survey. The large-scale structure of the Universe grew from a more uniform, hotter, denser state, and only grew into its present web-like state as the Universe gravitated, expanded, and cooled.
Or was it dynamic but unchanging: cosmically indistinguishable at different moments?
In the Big Bang, the expanding Universe causes matter to dilute over time, while in the steady-state theory, continued matter creation ensures that the density remains constant over time.
The Universe could obey the perfect cosmological principle: identical in all locations and throughout time.
If you look farther and farther away, you also look farther and farther into the past. If the number of galaxies, the densities and properties of those galaxies, and other cosmic properties like the temperature and expansion rate of the Universe didn’t appear to change, you’d have evidence of a Universe that was constant in time.
Merely a small, constant rate of spontaneous matter creation would be required.
Just like a ball of dough leavens, causing the various parts of the dough to “expand” away from one another, so too does the fabric of spacetime expand on cosmological scales, driving structures like galaxies, galaxy groups, and galaxy clusters apart from one another, where the effects of relativity must be taken fully into account to understand separation distances and light-travel-times. Within each of these bound structures individually, however, no expansion occurs.
Galaxies and stars of all ages should be found everywhere, universally.
Most of the largest known galaxies in the Universe are found at the hearts of massive galaxy clusters, like the nearby galaxy cluster shown here. If galaxies assemble and grow over cosmic time, it’s the closest galaxies, on average, that will be most likely to be the largest ones we observe today, whereas if the steady-state model and the perfect cosmological principle are correct, galaxies like these will be found at all distances, including in the more distant background objects visible here.
The expansion rate and density shouldn’t evolve over cosmic time.
Galaxies identified in the eXtreme Deep Field image can be broken up into nearby, distant, and ultra-distant components, with Hubble only revealing the galaxies it’s capable of seeing in its wavelength ranges and at its optical limits. The changing populations and densities of galaxies reveals a Universe that does, in fact, evolve with time.
And the only cosmic backgrounds would arise from reflected starlight and radiating matter.
This one small region near the heart of NGC 2014 showcases a combination of evaporating gaseous globules and free-floating Bok globules, as the dust goes from hot, tenuous filaments at top to denser, cooler clouds where new stars form inside below. The mix of colors reflects a difference in temperatures and emission lines from various atomic signatures. This neutral matter reflects starlight, where this reflected light is known to be distinct from the cosmic microwave background.
Observational evidence, beginning in the 1950s and 60s, overwhelmingly refuted the idea.
Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, and richer in gas in general than the galaxies we see today. Fewer galaxies have disks and spiral shapes as we look farther back in time. Over time, many smaller galaxies become gravitationally bound together, resulting in mergers, but also in groups and clusters containing large numbers of galaxies overall.
Distant galaxies appear:
- younger,
- bluer,
- lower in mass,
- and less morphologically (shape) evolved.
Looking out at any “slice” of the Universe allows us to see stars, galaxies, and the leftover glow from the Big Bang going all the way back a full 13.8 billion years to the start of the hot Big Bang. As the data clearly indicates, galaxies farther away have younger stars, are less massive and less evolved, and appear with greater number densities in space than they do today.
The total density, measured by mass and galaxy number counts, increases with distance.
A plot of the apparent expansion rate (y-axis) vs. distance (x-axis) is consistent with a Universe that expanded faster in the past, but where distant galaxies are accelerating in their recession today. This is a modern version of, extending thousands of times farther than, Hubble’s original work. Note the fact that the points do not form a straight line, indicating the expansion rate’s change over time. The fact that the Universe follows the curve it does is indicative of the presence, and late-time dominance, of dark energy.
The expansion rate changes with time: the “Hubble constant” is an evolving parameter.
The Sun’s actual light (yellow curve, left) versus a perfect blackbody (in gray), showing that the Sun is more of a series of blackbodies due to the thickness of its photosphere; at right is the actual perfect blackbody of the CMB as measured by the COBE satellite. Note that the “error bars” on the right are an astounding 400 sigma. The agreement between theory and observation here is historic, and the peak of the observed spectrum determines the leftover temperature of the cosmic microwave background: 2.73 K.
And a cosmic radiation background exists, whose spectrum doesn’t match reflected starlight’s or heated dust’s.
The observational evidence that probes the temperature of the cosmic microwave background at different epochs in the Universe, including at present (red star), in the relatively nearby Universe (blue points), and in the distant Universe (red points) all shows that the Universe was hotter in the past, and has cooled as it’s expanded exactly as predicted by the Big Bang theory.
The Universe really does change with time, supporting the Big Bang.
Our Universe, from the hot Big Bang until the present day, underwent a huge amount of growth and evolution, and continues to do so. Our entire observable Universe was approximately the size of a modest boulder some 13.8 billion years ago, but it has expanded to be ~46 billion light-years in radius today. The complex structure that has arisen must have grown from seed imperfections of at least ~0.003% of the average density early on, and has gone through phases where atomic nuclei, neutral atoms, and stars first formed, eventually giving rise to our Solar System, planet, life, and humans.
The “perfect cosmological principle” doesn’t hold true.
From objects within our Solar System to stars within our galaxy to distant galaxies as far as our telescopes can see, space is populated with objects at a specific location in space that emit light. We can only observe the light that is arriving right now: after journeying through the expanding Universe. However, the leftover light from the Big Bang, the CMB, was emitted from all locations at a specific moment in time. With each passing moment, light from a slightly more distant location than the previous moment arrives.
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 The “perfect cosmological principle” isn’t true for our Universe is featured on Big Think.