The Cosmological Principle is a fundamental idea in cosmology that states the universe is uniform (homogeneous) and looks the same in all directions (isotropic) when viewed on large scales, despite local irregularities.[1]
Homogeneity is a property that describes something with similar elements throughout, like milk, which has a uniform consistency even though it contains various ingredients. A salad on the other hand, is not homogeneous because it’s a mix of veggies and fruits.
Isotropic describes a property that’s the same in all directions. Milk is again a good example, a log of wood however is not isotropic. You can break it easily along the grain, but not across.
Imagine cutting a cake to observe its insides. Up close, you might see crumbs, tunnels, and cavities – irregularities on a small scale.
But zoom out, and the cake appears smooth and deliciously uniform on your plate.
Similarly, while the universe appears clumpy and irregular when we zoom in, on the grandest scales of billions of light-years, it exhibits a remarkable smoothness and symmetry described by the Cosmological Principle.
It’s also important to note the scale where this definition applies, it’s usually at scales larger than about 250-500 million light-years (a more precise scale is still a matter of debate).
On smaller scales, you won’t find perfect homogeneity and isotropy. You’ll find galaxies clustered together in some areas and vast emptiness in others.
Origin of the Cosmological Principle
The concept of the cosmological principle, which states that the universe is homogeneous (uniform) and isotropic (the same in all directions) on large scales, has its roots in the work of Isaac Newton.
While the term itself wasn’t coined until later, Newton’s ideas in his groundbreaking 17th-century work, Philosophiæ Naturalis Principia Mathematica, laid the foundation for this principle.
Before Newton, prevailing cosmological models placed Earth at the center of the universe. Newton, through his laws of motion and universal gravitation, challenged this geocentric view.
His work implied a universe that functioned according to the same physical laws everywhere, not just around Earth. This universality hinted at a large-scale uniformity in the cosmos.
Newton’s law of gravitation suggests a universe that tends to clump together due to gravity. However, his observations of the night sky didn’t reveal a collapsing universe.
To reconcile this, Newton proposed the idea of an infinite universe. In an infinitely large universe, the attractive force of gravity would be balanced out on a large scale, leading to a uniform distribution of matter.
The cosmological principle gained further momentum in the early 20th century when observational evidence from Edwin Hubble‘s work on galaxies and their redshifts supported the idea of an expanding universe.
This led to the development of the Big Bang theory, which provided a theoretical framework for the cosmological principle.
In 1935, the cosmologist Edward Arthur Milne formally introduced the term “cosmological principle” and proposed that the universe should appear essentially the same everywhere and in all directions when viewed on sufficiently large scales.
What Does the Cosmological Principle Tell Us?
Imagine looking up at the night sky and wondering if it’s the same everywhere you go. In the 1920s, scientists started getting hints that it might be!
They observed galaxies scattered fairly evenly across the sky.
The cosmological principle imposes homogeneity and isotropy on the Universe and it tells us there are no “special places” in the Universe.
No matter in what direction you point your telescope, the picture of the Universe appears remarkably uniform on large scales.[2].
This large-scale uniformity implies that the laws of physics are the same everywhere which helps us make sense of several critical observations about the universe.
Expanding Universe and the Big Bang
In 1929, Edwin Hubble discovered that galaxies outside our Milky Way were moving away from us, with their velocities proportional to their distances from Earth.
These observations provided evidence that the Universe was expanding, and to explain these observations, the Belgian physicist Georges Lemaître first proposed in 1927 what became known as the Big Bang theory.
This theory describes the Universe starting in an incredibly hot and dense state around 13.8 billion years ago and expanding and cooling to its present state over time.
Then, in 1964, came a major discovery, the Cosmic Microwave Background (CMB) radiation. It is a faint afterglow or an echo from the Big Bang, filling the Universe, which also represents the temperature of the overall Universe.
The CMB emerged about 380,000 years after the Big Bang. Before that, the universe was incredibly hot and dense, filled with a soup of elementary particles.
This dense state prevented light from traveling freely, making the universe essentially opaque to radiation.
As the universe expanded and cooled, it eventually reached a point where these particles could combine and form neutral atoms.
This transition allowed light to travel freely for the first time, and the faint afterglow of that hot, early universe is what we detect today as the Cosmic Microwave Background Radiation (CMB).
Interestingly, In 1992, a NASA satellite called COBE (Cosmic Background Explorer) made super precise measurements of the density and temperature variations of the CMB.
COBE found tiny temperature variations, but these variations were strangely similar across the entire observable universe, still, supporting the idea of a uniform universe and cosmological principle.
Inflation Theory
Now, we have a problem here, as the observations from COBE explain, the CMB radiation we observe today is remarkably uniform across the universe.
This means that temperatures in distant regions of the universe must have been nearly identical at the time when the CMB emerged.
Using the standard Big Bang model, we can estimate the size of the observable universe at the point when it first became transparent enough to emit the CMB, which is about 1.6 billion light-years in diameter.
With objects in the universe already so far apart at that point, they wouldn’t have had enough time to interact and establish a uniform temperature before emitting the CMB, so how do we explain the uniformity now?
To explain this, cosmologists think of a brief period of extremely rapid inflation, even faster than the speed of light, just after the Big Bang. The inflation stretched away the space between regions of the universe incredibly far apart.
This expansion was of the order 1050, in a fraction of time, about 10-30 seconds, so inflation perhaps was the real bang after the big bang.
Even though these distant regions never interacted “directly” before inflation, they originated from the same, very small, and uniform patch of the universe.
Inflation’s rapid expansion “stretched” this uniformity across a much larger scale.
The inflationary process itself left a uniform imprint on these regions, explaining their similar properties despite the lack of recent interaction, and during inflation, tiny quantum fluctuations are thought to have occurred.
These fluctuations served as the seeds for the large-scale structures we see today, like galaxies and galaxy clusters or superclusters—they were once part of an incredibly tiny, causally connected patch in the very early universe.
We observe remarkable uniformity in the large-scale structures throughout the observable universe, in the patterns and distributions of cosmic structures across vast distances and regions.
These large-scale structures include galaxy groups, galaxy clusters, superclusters, the intergalactic medium, galactic walls, and the cosmic web.
Even though distant regions are causally disconnected, the leftover imprint from inflation and these quantum fluctuations ensured a surprising degree of uniformity across the universe.
The observed homogeneity and isotropy of the universe strongly support the predictions of inflationary cosmology.
Accelerating Universe and the Λ-CDM Model
71 years later in 1998 two teams of Astronomers led by Saul Perlmutter and Brian Schmidt, discovered that the cosmic expansion was not just continuing, but accelerating.
Their observations of distant supernovae provided evidence for a mysterious dark energy counteracting gravity and causing the expansion rate to increase over time.
It is now thought that after the inflation the expansion decelerated for another 7-8 billion years, after which the acceleration again began eventually due to the dark energy.
All of these theories are subsumed in the (Λ) Lambda-CDM model which describes the universe’s composition and evolution, using the cosmological principle of homogeneity and isotropy of the universe, as a fundamental assumption.
In Λ-CDM, the Lambda (Λ) represents dark energy and CDM stands for Cold Dark Matter, an invisible form of matter that makes up most of the universe’s mass which helps explain why the universe’s expansion is accelerating.
By assuming the universe is essentially the same everywhere, the Λ-CDM model can use simpler geometric descriptions and equations of state, instead of having to account for complex variations.
The isotropic nature implied by the cosmological principle allows us to project our local observations of a smaller region of the cosmos to a representative picture of the entire universe.
Violating the cosmological principle of uniformity would undermine many of our established theories about how the cosmos began and evolved to its present state.
Observational Evidence for the Cosmological Principle
Earlier, we discussed the CMBR’s near-uniformity in all directions supports the isotropy aspect of the Cosmological Principle.
Edwin Hubble’s discovery of the expanding universe in 1920 is also key evidence, implying that galaxies are moving away from each other, indicating that the universe is not static but dynamic.
Besides these, there is also a Cosmic X-ray background, which is highly isotropic, the resolved sources that contribute to the X-ray background are primarily distant and powerful active galaxies.
These galaxies host supermassive black holes at their centers, which produce intense X-ray emissions as surrounding material falls into the black hole’s accretion disk.
Isotropy of cosmic X-ray background suggests the isotropic distribution of these sources across the universe.[3]
Extensive galaxy surveys reveal that on scales larger than about 600 million light-years, galaxies are distributed uniformly with no preferred directions or locations. This uniformity extends even beyond our local supercluster of galaxies.
Astronomers have mapped the expansion history of the universe by studying the brightnesses of extremely distant Type Ia supernovae. These observations indicate an extremely uniform expansion rate in all directions on cosmic scales.
Measurements of the peculiar motions and velocities of galaxies relative to the cosmic expansion reveal no significant bulk flows or directional preferences on large scales beyond about 300 million light-years.
The polarization patterns imprinted on the CMB by early quantum fluctuations exhibit the same striking isotropy as the CMB temperature map, reinforcing the universe’s smoothness.
These periodic fluctuations in the matter distribution, originating from sound waves in the early universe, provide a robust standard ruler. Baryon Acoustic Oscillations (BAO) measurements confirm the predicted large-scale uniformity.
Discoveries Posing Challenges to the Cosmological Principle
The cosmological principle has been a strong assumption in cosmology for a long time, but in recent years, cosmologists have made discoveries that challenge this long-standing concept.
In 2021, Subir Sarkar, a professor at the University of Oxford, found that the large-scale anisotropy of the universe, as measured by the dipole in the angular distribution of a flux-limited sample of quasars.[4]
This observation directly conflicts with the concordance Lambda-CDM model by challenging the cosmological principle.
Some large-scale complex structures with sizes beyond a billion light-years have also been observed, such as galaxies forming a giant arc of 2.3 billion light years across, along with an enormous ring of galaxies named Big Ring.[5]
For the cosmological principle to be true, there should not exist structures larger than 1.2 billion light years across.
Still, since we’re already observing far larger structures, we may need an alternate model of cosmology.
But so far, there hasn’t been any cosmological model as good as the standard cosmological model (Λ-CDM) that follows the cosmological principle, as it’s still able to explain most of the observations made so far.
References
- Richard Gelderman, ‘Cosmological Principle,’ Wku.edu, 2014, astro.wku.edu/astr106/structure/cosmologicalprinciple.html Accessed 4 April 2024[↩]
- Barbara Ryden, ‘Introduction to Cosmology,’ Addison Wesley, p9, 2006[↩]
- University of Groningen, ‘Cosmological Principle: the Evidence’, Cosmology lectures, https://www.astro.rug.nl/~weygaert/tim1publication/cosmo2019/cosmology2019.lect3a.cosmological_principle.pdf, Accessed on 4 April 2024[↩]
- Secrest, Nathan J., et al. ‘A Test of the Cosmological Principle with Quasars,’ The Astrophysical Journal. Letters (Print), vol. 908, no. 2, IOP Publishing, Feb. 2021, pp. L51–51, https://doi.org/10.3847/2041-8213/abdd40, Accessed 4 April 2024[↩]
- Lopez, Alexia M., et al. ‘A Giant Arc on the Sky’ Monthly Notices of the Royal Astronomical Society, vol. 516, no. 2, Oxford University Press, Aug. 2022, pp. 1557–72, https://doi.org/10.1093/mnras/stac2204, Accessed 4 April 2024[↩]
