Cosmos
🪐 An orderly harmonious systematic universe
Cosmos implies viewing the universe as a complex and orderly system or entity
🌌 Cosmos vs Universe
Cosmos often simply means "universe". But the word is generally used to suggest an orderly or harmonious universe, as it was originally used by Pythagoras in the 6th century B.C.
Thus, a religious mystic may help put us in touch with the cosmos, and so may a physicist.
The same is often true of the adjective cosmic: Cosmic rays (really particles rather than rays) bombard us from outer space, but cosmic questions come from human attempts to find order in the universe.
☄️ Cosmology
A field of study that brings together the natural sciences, particularly astronomy and physics, in a joint effort to understand the physical universe as a unified whole.
The “observable universe” is the region of space that humans can actually or theoretically observe with the aid of technology.
It can be thought of as a bubble with Earth at its centre. It is differentiated from the entirety of the universe, which is the whole cosmic system of matter and energy, including the human race.
💫 The cosmological expansion
When the universe is viewed in the large, a dramatic new feature, not present on small scales, emerges—namely, the cosmological expansion.
On cosmological scales, galaxies (or, at least, clusters of galaxies) appear to be racing away from one another with the apparent velocity of recession being linearly proportional to the distance of the object. This relation is known as the Hubble law.
3.8 billion years ago all of the matter in the universe was closely packed together in an incredibly dense state and that everything then exploded in a “big bang,” the signature of the explosion being written eventually in the galaxies of stars that formed out of the expanding debris of matter.
If one looks up on a clear night, one will see that the sky is full of stars.
🌠 The nature of space and time
Finite or infinite?
An issue that arises when one contemplates the universe at large is whether space and time are infinite or finite. After many centuries of thought by some of the best minds, humanity has still not arrived at conclusive answers to these questions.
Gravitation and The Geometry of Space-time
The physical foundation of Einstein's view of gravitation, general relativity, lies on two empirical findings that he elevated to the status of basic postulates.
The first postulate is the relativity principle: local physics is governed by the theory of special relativity. The second postulate is the equivalence principle: there is no way for an observer to distinguish locally between gravity and acceleration.
🌟 The Hot Big Bang
Given the measured radiation temperature of 2.735 kelvins (K), the energy density of the cosmic microwave background can be shown to be about 1,000 times smaller than the average rest-energy density of ordinary matter in the universe.
Thus, the current universe is matter-dominated.
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