What is the "universe"?

The universe is not just all the matter there is; but, following Einstein theory of general relativity , also all the energy and spatial-temporal dimensions. Recall that space is warped by a strong gravitational force. Thus, the universe has no real "edge", or an inside or an outside. Perhaps it is worth consider a well often quoted analogy. We said that, because of the gravitational pull, space might be warped. Consider the lives of individuals living in a fictitious two-dimensional space; assume, for simplicity, that this two-dimensional space is closed, such as the surface of a sphere. For those two-dimensional individuals the surface of the sphere is the universe, and there is clearly no inside or outside. If you extrapolate that argument to our three dimensional lives, and consider that we really live in a four dimensional space (3 space dimension plus time), then you see that the universe is really what there is, and no more (not even "nothing").

What do we know about the universe?

A simple observation of the night sky would suggest that not much is changing in the sky; there are a few rare occasions in which things happen in an unpredictable way, such as the appearance of shooting stars and comets; once in a great while, big explosions (supernovae) are observed. And this has been the belief for centuries and centuries. But in the last century, we have assembled enough evidence to obtain a rather good picture of what the universe looks like and how it formed.

The first important discovery is of Edwin Hubble (1889-1953), who found out that galaxies move away from us (on Earth) at a great speed. Using large telescopes, he found that the light emitted by distant galaxies was shifted toward the red, due to the Doppler effect. For very distant galaxies, this speed approaches the speed of light (300,000 Km/sec) which is the ultimate speed according to Einstein theory of special relativity. Specifically, Hubble found that the further a galaxy is from us, the faster it moves. By plotting the estimated distance versus speed, he found that the slope of the line going through his observation had a value of 50 Km/sec/Mpc.

Note:
1Mpc=10⁶pc(parsec)
1pc=3.34 light-years
(1 light year equals the distance traveled by light in vacuum in a year).

Schematic plot illustrating the relationship between distance and speed of galaxies.

Hubble found out that galaxies recede from the Earth in all directions. Are we allowed to conclude that we are at the center of the Universe? NO !

The answer is subtler. Consider an inflatable balloon (not necessarily spherical). Paint some dots on its outer surface and inflate it. You will see that the distance between any two dots increases, although none can be considered at the center of the expansion.

A similar argument can be used for the universe. Thus, from Hubble discovery we now know that the universe is not static, but rather is expanding.