What are the Cosmological and the Perfect Cosmological Principles?
This essay was written as part of a philosophy course at the Australian National University and my first time writing such essays.
Cosmological Principle
In a nutshell, the Cosmological Principle (CP) is that the universe no matter who you are or where you are in space, looks exactly the same regardless of position or direction, as Liddel A. (2003) puts it, the place that we occupy is nothing special.
The Cosmological Principle is based upon two key points:
- Homogeneity
Every region in the universe is the same, the patterns of galaxies, distribution of superclusters; there is no unique spot in the universe when observing at a sufficiently large enough scale.
- Isotropy
The same observation evidence is available to any observer in the universe by looking in any direction; No location has a special or unique view than any other.
Perfect Cosmological Principle
The Perfect Cosmological Principle (PCP) extends the Cosmological Principles into time; stating that the universe is homogeneous and isotropic in space AND time.
It states that the density of matter in the expanding universe is the same by the continuous creation of matter, in other words, the universe is the same and unchanging, homogeneous and isotropic throughout time and space.
(Viewing at large scales)
What are their roles in the Big Bang and Steady State models of the Cosmos?
Big Bang and the Cosmological Principle
The Big Bang Model is dependant on the assumptions that the Cosmological Principles are true Using the Cosmological Principle a number of inferences can be made about the evolution and behaviour of the universe.
Applying the cosmological principle (CP), distant galaxies are clustered together and contain less heavy elements than current time galaxies, this suggests that heavier elements were not created in the big bang but produced in stars and expelled across a series of supernovas, meaning that heavier elements will accumulate over time and the universe will change as more nucleosynthesis occurs, using the cosmological principles we can make similar deductions about how the universe evolves over time.
Steady-State and the Perfect Cosmological Principle
The Steady-State Theory (SST) is an alternative to the big bang model of universe evolution and in order to adhere to the PCP the SST asserts that the expanding universe maintains its density by the continuous creation of matter and is unchanged across time, resulting in a universe without beginning or end.
How do they relate to the idea that space and time have boundaries?
CP in Space and Time
The cosmological principle means that every region in the universe must look the same no matter the position and direction we are looking at, This means that there can be no edges, boundaries or centre to the universe as this would than disrupt both the principles of isotropy and homogeneous by having points that were more unique and different from other points when comparing regions near a “edge”.
PSP Space-Time
PCP and the Steady State model of the universe assert that even though the universe is expanding it remains un-changed with no beginning or end. This means a lacking of a boundary of time resulting in an infinite universe without a beginning or end.
IF the universe had boundaries than not only would regions at those boundaries differ in structure and distribution as areas within the boundaries but an observer near the edge would view different amounts of galaxies in different directions.
Does any account of the scientific method make sense of the roles these principles play?
There so far are a number of scientific observations that reinforce the predictions of the Big Bang and the Cosmological Principles whilst rejecting the Steady State Model and the Perfect Cosmological Principle.
The observation evidence from the big bang that reinforces CP homogeneity and isotropy, comes mainly from the observations of cosmic background radiation an electromagnetic radiation leftover as a remnant from the early stages of the universe and the evolution of the distribution of galaxies over time. Early galaxies were more fragmented, interacting and unusually shaped then the current time local galaxies which suggest an evolution of galaxy structures over time.
One of the most damning evidence against the steady-state model, the existence of uniform background radiation in the universe known as the cosmic microwave background radiation (CMBR).
According to the big bang theory, the universe was a nearly perfect black body of energy a second after the big bang, the CMBR is remnants of the energy unleashed at that early stage of the universe and is close to that of an ideal black body when accounting for redshift, currently, the big bang model is the only theory that can explain Cosmic Background Radiation.
The Steady-State Model states that the microwave background radiation was from ancient stars scattered by space dust. However, the cosmic background radiation is spread uniformly in all directions, showing no evidence of scattering characteristics like polarization.
Are the roles these Principles play philosophically justified?
Our observations and measurements of the universe tell us that the premise of a homogeneous and isotropic universe in space is true, whilst a homogeneous and isotropic universe in time is false.
The main premises behind PCP that the universe is homogeneous and isotropic in space and time leads to the conclusion that the universe does not change. However our observations tell us that the universe does in fact change, therefore whilst the premise of homogeneous and isotropic is true in regards to space and time the conclusion is invalid resulting in an unsound deductive argument.
Space and Time Boundaries
In space a homogeneous and isotropic universe has no unique regions, a universe with an edge has different regions at the edge than it does away from the edge both inhomogeneity and isotropic, therefore a homogeneous and isotropic universe can have no edge and no boundary.
References
Liddle A. 2003, and Introduction to Modern Cosmology
