Special relativity says that time is a separate ________ in the ...

[jerry coleman]

Special relativity says that time is a separate ________ in the workings of the universe.
 
  Fill in the blank(s) with correct word

Question 2

What do we mean by the critical density of the universe?
 
  What will be an ideal response?

[atrochim]

Answer to Question 1

dimension

Answer to Question 2

In the absence of any mysterious repulsive force (or dark energy), a universe with the average mass density exactly equal to the critical density is the densest universe that will expand forever. A denser universe will eventually reverse its expansion and contract; a less dense universe will expand forever.