Skip to content
Starts With A Bang

The Universe violates the perfect cosmological principle

Generations ago, cosmologists asserted that the Universe might not just be the same in all directions, but at all times. But is that true?
halo evolve cosmic time millennium II
This set of images shows four different "slices" of the Universe's most massive cosmic halo on three different distance scales. If the Universe obeyed the perfect cosmological principle, these structures would not be seen to evolve over time. Observationally, the Universe itself must decide.
(Credit: Millennium-II simulation/M. Boylan-Kolchin et al., MNRAS, 2009)
Key Takeaways
  • One of the great scientific revolutions began with Copernicus, who opined that we occupied no special place in the Universe.
  • As our view of the Universe expanded, we developed the cosmological principle: that the Earth, Sun, and even Milky Way occupied no special place in the cosmos.
  • Maybe, as Fred Hoyle, Hermann Bondi, and Tommy Gold suggested, we also occupy no special “time” in the Universe, and that the Universe truly never changes. But can that be true?

100 years ago, an unprecedented scientific revolution occurred.

Hubble discovery cepheid andromeda
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.
Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA); Illustration via NASA, ESA, and Z. Levay (STScI)

Individual stars were measured in galaxies outside the Milky Way.

hubble plot expanding universe
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.
Credit: E. Hubble; R. Kirshner, PNAS, 2004

By combining measured distances with observed recession speed, we determined the Universe was expanding.

big bang
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. This balance was essential if complex structures, like stars and galaxies, were to arise within the Universe.
Credit: NASA / GSFC

But was the Universe truly evolving, like the Big Bang predicts?

fractal universe
This image shows a slice of the matter distribution in the Universe as simulated by the GiggleZ complement to the WiggleZ survey. There are many cosmic structures that seem to repeat on progressively smaller scales, but does that imply that the Universe is truly a fractal? And does that mean it’s truly unchanging with time?
(Credit: Greg Poole, Centre for Astrophysics and Supercomputing, Swinburne)

Perhaps it was dynamic but unchanging: cosmically indistinguishable at different times.

big bang vs steady state
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.
(Credit: E. Siegel)

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

every square degree
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.
(Credit: NASA/ESA/A. Feild (STScI))

Only a small, constant amount of spontaneous matter creation would be required.

hubble tension
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. Within these bound structures, however, no expansion occurs.
Credit: designua / Adobe Stock

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

galaxy cluster distance
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.
(Credit: CTIO/NOIRLab/DOE/NSF/AURA; Image processing: Travis Rector (University of Alaska, Anchorage/NSF’s NOIRLab), Jen Miller (Gemini Observatory/NSF’s NOIRLab), Mahdi Zamani & Davide de Martin (NSF’s NOIRLab))

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

lookback time galaxies
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. It’s important to remember that the light we see is only the light arriving right now, after journeying through the vast expanse of space.
Credit: NASA, ESA and Z. Levay, F. Summers (STScI)

And the only cosmic backgrounds would arise from reflected starlight and heated dust.

NGC 2014
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.
(Credit: NASA, ESA and STScI)

The evidence, beginning in the 1950s and 60s, quickly demolished the idea.

milky way galaxies cosmic time
Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, more chaotic, and richer in gas in general than the galaxies we see today. For the first galaxies of all, this effect goes to the extreme. As far back as we’ve ever seen, galaxies obey these rules.
Credit: NASA, ESA, P. van Dokkum (Yale U.), S. Patel (Leiden U.), and the 3-D-HST Team

Distant galaxies are younger, bluer, lower in mass, and less morphologically (shape) evolved.

map of universe
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.
Credit: SDSS and the Planck Collaboration

The density of objects, measured by mass and galaxy counts, increases with distance.

Friedmann equation
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.
Credit: Ned Wright/Betoule et al. (2014)

The expansion rate evolves with time: the “Hubble constant” isn’t really a constant.

universe temperature
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.
Credit: Sch/Wikimedia Commons (L); COBE/FIRAS, NASA/JPL-Caltech (R)

And a cosmic background exists, with a spectrum incompatible with reflected starlight or heated dust.

temperature universe time
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.
(Credit: P. Noterdaeme et al., Astronomy & Astrophysics, 2011)

The Universe really does change with time, supporting the Big Bang and ruling out the Steady State model.

big crunch
The far distant fates of the Universe offer a number of possibilities, but if dark energy is truly a constant, as the data indicates, it will continue to follow the red curve, leading to the long-term scenario frequently described on Starts With A Bang: of the eventual heat death of the Universe. If dark energy can strengthen, weaken, or reverse sign over time, however, all bets are off.
Credit: NASA/CXC/M. Weiss

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


Related

Up Next