Agreed terms help sensible discussion: Universe


The devil is in the detail
May I please kick off with the word Universe. I will plead my case, but please chip in if you want a plurality of universes, or alternate realities.

First I would like to quote some dictionaries and books on cosmology.

Oxford Dictionary of Astronomy Ian Ridpath OUP 2011.
"Universe Everything that exists, including space, time and matter. The study of the Universe is known as cosmology. Cosmologists distinguish between the Universe, with a capital U, meaning the cosmos and all its contents, and universe with a small u, which is usually a mathematical model derived from some physical theory. The real Universe consists mostly of apparently empty space, with matter concentrated into galaxies consisting of stars and gas. The Universe is expanding, so the space between galaxies is gradually stretching, causing a cosmological red shift in the light from distant objects. There is now strong evidence that space is filled with unseen dark matter that may have many times the mass of the visible galaxies; and even more mass may be accounted for by a still-mysterious dark energy. The most favoured concept of the origin of the Universe is the "Big Bang Theory [BBT], according to which the Universe came into being in a hot, dense fireball 13.7 billion years ago."

The Icon Critical Dictionary of The New Cosmology Ed Peter Coles Icon Books 1998.
Universe The entirety of all that exists. The Greek word cosmos, the root of cosmology, means the same; cosmology is the study of the Universe. This definition seems relatively straightforward, but there are some confusing subtleties, and linguistic confusion. For example, what do we mean by exist?
There are over two ages elaborating. I would summarise, that there is the view of science that only that which can be observed qualifies as Universe. "For some scientists what really exists is the laws of physics; our Universe is merely a consequence, or an outcome of these laws. . . . . . . . . . But do these laws exist, or did we invent them? Is mathematics an intrinsic property of the world, or is it simply a human invention that helps us to describe that world, in much the same way as a language? . . . . . . . . . If the Universe is the entirety of all that exists, then our model universe cannot be embedded in anything. What is outside the Universe must be something that does not exist. It does not therefore make any sense to think of there being anything outside the Universe."

Universe: The Definitive Visual Guide Ed. Martin Rees DK 2012
"The Universe is all of existence - all of space and time and all the matter and energy within it. . . . . . . . . . The Universe encompasses everything from the smallest atom to the largest galaxy cluster, and yet it seems that all are governed by the same basic laws.

Note: In the more specialised texts on cosmology, it is perhaps understandably more difficult to find definitions of Universe. It is taken for granted, unless stated to the contrary, that the accepted definitions, such as the above, apply. Any mentions of "other universes" will be documented.

Bang! The Complete History of the Universe. Brian May Patrick Moore Chris Lintott
Carlton Books 2006.
"Everything, space, time and matter, came into existence with a 'Big Bang' around 13.7 billion years
ago. The Universe then was a strange place - as alien as it could possibly be. . . . . . . . . . how big is the Universe? Either the Universe is of finite size or it isn't. If finite, what lies outside it? The question is meaningless - space itself exists only within the Universe, and literally there is therefore no 'outside'. On the other hand, to say the Universe is infinite is really to say that its size is not definable.

Cosmology A Very Short Introduction Peter Coles OUP 2001
"The word cosmology itself is derived from the Greek cosmos meaning the world as an ordered system or whole. The emphasis is just as much on order as on wholeness, for in Greek the opposite of cosmos is chaos. . . . . . . . . . The advent of mathematical reasoning, and the idea that one can learn about the physical world using logic and reason marked the beginning of the scientific era."
"In the modern era of cosmology . . . . . . began with a complete rewrite of the laws of Nature. (Einstein) demolished Newton's conception of space and time . . . . . . great works by Friedman, Lemaitre, and de Sitter formulated a new and complex language for the mathematical description of the Universe." Einstein's theory plays a fundamental conceptual role in modern cosmology. Hubble's observation of galaxies led to the observation that the Universe is expanding, and Penzias and Wilson discovered the cosmic microwave background (CMB) considered by many as proof that the Universe began with the Big Bang. Whilst accepted by most cosmologists "as being essentially correct, as far as it goes . . . . . . it is important to realise that the Big Bang is not complete. "For one thing, Einstein's theory itself breaks down at the very beginning of the Universe. The Big Bang is an example of what relativity theorists call a singularity, a point where the mathematics fall to pieces and measurable quantities become infinite. While we know how the Universe is expected to evolve from a given stage, the singularity makes it impossible to know from first principles what the Universe should look like in the beginning. . . . . . . . . . Most cosmologists interpret the Big Bang singularity in much the same way as the Black Hole singularity . . . i.e., as meaning that Einstein's equations break down at some point in the early Universe due to the extreme physical conditions present there. . . . . . . . . . This shortcoming is the reason why the word 'model' is probably more appropriate than 'theory' for the Big Bang."

The Theory of (nearly) Everything Ed Daniel Bennett BBC 2016
A lengthy section entitled The Story of the Stuart Clark and Elizabeth Pearson.
Chapter headings are:
1. The Big Bang
2. Inflation 19-35 seconds post-Big Bang
3. Particle Creation 1 minute post-Big Bang
4. The Decoupling of Matter and Energy 380,000 years post-Big Bang
5. The Cosmic Dark Ages 1 million years post-Big Bang
6. The Formation of the Solar System 8.8 billion years post-Big Bang.
As such reference is made to the Big Bang, that Section of Agreed Terms is suggested.

Astronomy Special Issue The Beginning and End of the Universe January 2021
Relevant contents:
It began with a Bang by Dan Hooper
Inflating the universe by Brian Keating
The Emergence of Matter by Christopher Conselice
The Cosmic Dark Ages by Dana Najjar
The First Stars are Born by Michael E Bakich

Astronomy Now Universe's expansion rate still doesn't match up News May 2021
"The controversy over the value of the Hubble constant, which describes the rate of expansion of the Universe, has deepened, thanks to a new method of measuring distances across the cosmos."

Astronomy Now Billions of dwarf galaxies caught in the cosmic web News May 2021
"The light of billions of previously unknown dwarf galaxies has been found illuminating the cosmic web of matter during the first two billion years of the Universe."

Astronomy Now A lens on the Universe by Keith Cooper June 2021
"Space is sculpted by gravity, and gravity is able to distort the path of light and magnify it like an optical lens. These gravitational lenses are our window onto some of the Universe's greatest mysteries."

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