Cosmology

Cosmological Principles and Theories

Cosmology is the study of the universe’s structure, origins, evolution, and eventual fate.
The Cosmological Principle posits that the universe is both homogeneous (the same in all locations) and isotropic (the same in all directions).
The Big Bang Theory suggests that the universe originated from a singularity around 14 billion years ago and has been expanding ever since.
The Steady State Theory proposes a self-consistent universe that never began or will never end, continuously creating new matter as it expands.
The Inflationary Universe Theory posits a phase of exponential expansion of the universe a split-second after the Big Bang, explaining ancient cosmic microwave background radiation.

The Observable Universe and Beyond

The cosmic microwave background radiation is radiation left over from the Big Bang, providing evidence of its occurrence.
Cosmic microwave background radiation can be seen in all directions, and its slight fluctuations reveal details about the early universe’s distribution of matter.
The observable universe refers to the portion of the entire universe that we can, in principle, observe.
Beyond the observable universe lies the unobservable universe, which we cannot see due to the finite speed of light and the universe’s on-going expansion.

The Expansion of the Universe

The universe’s expansion was first noted by Edwin Hubble, an observation now known as Hubble’s Law: the velocity of a galaxy is directly proportional to its distance from us.
The redshift of galaxies, which shows motion away from us, provides evidence of this expansion; the more distant a galaxy, the greater its redshift.
The rate of the universe’s expansion, called the Hubble constant, continues to be a point of research and discussion among astronomers.

Structure and Composition of the Universe

The universe is comprised of gravitationally bound systems called galaxies, which can be classified as spiral, elliptical, or irregular.
Dark matter, an unseen form of matter, and dark energy, a mysterious force accelerating the universe’s expansion, make up approximately 95% of the universe’s total energy-density.
The remaining 5% is ‘normal’ matter: stars, galaxies, nebulae, and everything else we can directly detect.

The Fate of the Universe

The universe’s fate depends on its total density. A density less than the critical density would mean an ‘open’ universe, ever expanding. A density greater would yield a ‘closed’ universe, eventually collapsing in a ‘Big Crunch’.
Current observations suggest our universe’s density approximately matches the critical density, implying a ‘flat’ universe that will expand indefinitely, albeit at a decreasing rate.