Why the Universe Looks the Same from Everywhere?
By Yagyasha
Cosmofluencer (Season 04)
For those wondering, this principle with a very scary name is simply the notion that, from a large enough distance,the universe seems to look the same to every observer. To put it in scientific terms, Cosmological principle says that the spatial distribution of our universe is isotropic and homogeneous when observed from a large distance. This idea is based on the assumption that all the forces in the universe act uniformly and therefore do not produce any irregularities in the large-scale universe.
The Need for this Assumption
The Cosmological Principle is a rather bold assumption that astronomers on Earth have made to study our Universe. This principle stands on the assumption that the laws that we humans have studied on Earth using our tools and techniques govern the whole universe across time and space.
However, the good news is that physicists and astronomers are pretty sure that this assumption is correct. It can be unanimously approved by the abundance of evidence we have collected over the years studying the world outside our home. We see the same types of structures, galaxies and stars everywhere we look. We even get similar spectral data from similar stars irrespective of their distance.
What are the Basic Components of this Principle
The cosmological principle takes into consideration two basic beliefs. The first being Homogeneity, this suggests that the composition of our universe is uniform on a large scale. Now, this does not mean that the concentration of matter is uniform all across space, but that we can see similar types of nebulae, stars and planets everywhere we look.
The second pillar on which this principle stands is that of Isotropy. This very heavy looking term just means that our universe has no preferred direction or orientation. Meaning no matter in which direction we look we should see the same things.
Implications of Cosmological Principle
Most of modern astronomy finds itself depending on this one basic assumption that the laws and forces acting on our observable universe work similarly across the vast expanse of the physical universe.
It helps scientists use the minimal amount of information we have on our observable universe, to determine the properties of our physical reality as a whole and helps us make predictions about its structure, evolution and the ultimate fate.
In addition to this, cosmological principle also sets stage for The Big Bang Theory which states that our universe was born out of an explosion millions of years ago and is expanding in all directions ever since.
Many observations support the cosmological principle. This principle was clearly stated for the first time by Sir Issac Newton in his book Philosophiæ Naturalis Principia Mathematica in 1687. Newton described gravity as a universal force that not only acted on the Sun or the Earth but on every body in the universe far beyond the reaches of our consciousness .Even though this principle has enough evidence to prove itself, it still does not have a solid structure or mathematical derivation. Hence, it still remains very empirical in nature.
The reason for obeying the cosmological principle is not because it is necessarily true but because it allows us to use the extremely limited data provided to cosmology by observational astronomy.
The clearest data that supports this principle is Cosmic Microwave Background or the CMB. The CMB is the image of photons emitted from the early universe. The CMB data reflects isotropy and homogeneity when observed at a large scale.
Interestingly, there was a ‘perfect’ version of this principle which assumed that, in addition to no special orientation or direction, there are no special times in the universe. This idea was however discarded with the discovery of CMB and the realization that the universe indeed had a beginning.
Criticisms and Failures
Like every good theory or law, this principle also has its fair share of criticism and inconsistencies. In adherence to the cosmological principle, studies were done by various teams of researchers and they showed that for the cosmological principle to be true the spatial distribution of galaxies should be averaged over scales 260/h Mpc. This limit is generally referred to as the consistent scale.
However, there are many structures found which exceed this limit:
- The Sloan great wall, found in 2003 has a length of 423 Mpc
- The Huge-large quasar group, discovered in 2012, is another great structure that forces us to rethink our idea of a large scale universe
- In June 2021, the Giant Arc was discovered, a structure spanning approximately 1000 Mpc. It is located 2820 Mpc away and consists of galaxies, galactic clusters, gas, and dust.
These are some of the many structures that challenge the cosmological principle. While the CMB proves to a great extent that the universe is isotropic, the homogeneity of the universe is still up for debate.
References
1. What is the principal evidence for the cosmological principle? | Royal Astronomical Society, Quarterly Journal
2. Cosmological Principle | Wikipedia
3. The Cosmological Principle and the missing rest frame of the Universe by Prof. Mohamed Rameez | SEDS Celestia
4. Scientists Have Discovered The Largest Structure In The Universe | Lightning Round | Joe Scott
5. Cosmological Principle | Modern Physics
6. Cosmological Principle – Definition & Detailed Explanation – Cosmology Glossary | Sentinel Mission
7. Cosmological Principle | Uoregon