Size of the Universe
the post:
' Let me propose a logical argument regarding the size of the universe, and get your thoughts and feedback.
It's a popular notion that at the Big Bang, or right afterward, the entire universe was a very small object. One popular saying is that it was smaller than a proton.
While I think that last sentence is debatable, let me offer it as a given that the universe was small.
The next thought to offer is that it was also finite in size. It seems to me that small implies finite. Let me know if you agree or disagree.
The next thought to offer is that something finite in size cannot grow to be infinite in size. At least not in a finite amount of time.
The universe is on the order of 13.8 billion years old  it's a finite amount of time.
Putting it all together: The universe was small and finite. It has existed only a finite amount of time. It must still be finite  since the finite cannot become infinite within a finite amount of time.
Agree? Disagree? Other thoughts? ' my first comment:
The real universe is infinite. The definition of universe is everything. The expanding universe initiated by the big bang is in that. This expanding universe with its expanding fabric has an identified edge when you describe it with a geometry that does have a boundary. A geometry describes the observer's space; it does not define the space.
Therefore the size of this expanding universe is described by the observer's geometry. If all observers use the same geometry then they observe the universe the same.
My second comment:
I should have added: Spacetime defined by relativity is the observer's geometry. It is not real space. In the special theory of relativity it is the accelerating observer's geometry, that can be curved by gravity.
There is no universal geometry or universal coordinate system because there is no fixed point to serve as a zero reference for any defined planes in a geometry to cover the entire universe, including beyond what we can see from Earth. Our celestial coordinate system is defined from Earth with rules and zero reference points for the two planes (plus distance to fully describe celestial positions).
The spacetime defined by relativity can never describe something in the universe other than by the observer's geometry.
Therefore our view of the universe is always described by our (the observer) geometry. Its size is determined by our geometry.
My third comment is next:.
Spacetime defined by relativity is the observer's geometry. It is not real space. On Earth we agreed on the GPS geometry with its rules for latitude, longitude, altitude, and their zero references. We can share our described locations using this common geometry. On Earth we agreed on the celestial coordinate system, with its rules for right ascension, declination, distance, and their zero references. We can share our described locations for everything in our observable universe, from here on Earth, using this common geometry. I cannot share observations with those beyond Earth unless we share a geometry.
Satellites have a known position relative to Earth so the celestial coordinates can be adjusted accordingly. Someone on Mars could share observations with someone on Earth only with a common geometry. This is difficult when using Earth's celestial coordinate system without the common zero reference for the two celestial planes from that location; the distance conversion is possible.
There is no universal coordinate system because there is no fixed point to serve as a zero reference for any defined planes in a geometry to cover the entire universe, including beyond what we can see from Earth.
The spacetime defined by relativity can never describe something in the universe other than by the observer.
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