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What is a universe, a universe in its general definition?

First, a stock universe is comprised of things like galaxies that undergo formation and changes across vast spans of time, so an observer (similar to us) basically cannot directly determine the formation, the evolution, the changes that occur in a universe. Direct knowledge that we are so accustomed to in our daily existence cannot be obtained about a universe.  We cannot reach out and measure it, take a picture that captures its wholeness, put it on a scale, or gather a sampling of all its facets.   Everything a sentient observer may know is based on estimation and extrapolation of facts gathered during a small portion of a fraction of a blink in the cosmic timeline.

A universe is fundamentally characterized by time and matter and the closely related energy.  Einstein's relativity theories bound these two basic elements of a universe together as the conceptual spacetime and so they remain today in our present science understanding.

Remember, this is being written in the autumn of 2016 and, as science progresses. New revolutions, new paradigm shifts will bring us a different picture than we have today and, at the same time, that picture will be based on what we know in the winter of 2016, just as our knowledge today is based on the gathered knowledge of Copernicus and Galileo and Newton and Einstein and Faraday and Planck and Boltzmann and Euler and Hertz and Michelson and Hawking and ...

Dimensions of a Universe

One way to describe a universe is to describes it's basic metrics.  We measure it in estimations and extrapolations.

A universe has dimension, although most theories hold that many of a universe's dimensions are infinite.  Nonetheless, we estimate.  We estimate the dimensions of the "observable universe."  We estimate its mass, we estimate its widths.  We estimate its age.

Note that I said "observable universe" because many theories characterize the extent of the universe as infinite and always expanding.  And if the universe has infinite size, within that infinite space is infinite mass and infinite energy and infinite beauty.

There are many characteristic attributes of a universe but we'll start with the obvious ones that help us first visualize a universe:

  • Age of the Universe << Click here
  • Diameter - 8.8E26 m
  • Mass of the universe - 6.0E51 kg
  • Density - 3.0E-28 kg/m³
  • Temperature
  • Rate of expansion

How We See the Universe

Since in many theories things, like the universe itself, exist because we perceive them. I find it interesting to ponder just exactly what we are seeing.  

We perceive things with our senses, and beyond our senses we are able to perceive many spectrums within the electromagnetic spectrum using sensor devices tuned to specific ranges of that specrum. In the figure below, and thanks to NASA, we see our own Milky Way illuminated in 10 different parts of the electromagnetic spectrum.

Our human senses cannot see the infrared range but seeing the Milky Way in infrared emanations paints the background "in a different light."

Multi-wave Milky way_8x10.jpg

The image above shows several ways we can view the universe -- literally -- as NASA has so kindly provided a set of comparative images of our own Milky Way galaxy.

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  • Mass of the Universe by MichaelBartmess
  • Density of the Universe by MichaelBartmess