SPACE IN EXPANDING UNIVERSE:
With General Relativity, Einstein showed that space (and time) is flexible and relative. It again came to our rescue by providing a perfect reason for space to be expanding.
Alexander Friedmann and Georger Lemaitre, after solving Einstein’s equation said that if if throw a ball in the sky, it will be either moving up or moving down (stopping once at the peak height). Similarly space is either contracting or expanding.
TIME IN EXPANDING UNIVERSE:
In my previous post, I have written about bread and resin model of universe. (click this link to read part 1:EXPANSION OF THE UNIVERSE: PART 1 ). Now we will take out each of the resins and replace them with clock which are perfectly synchronized. When the bread expands the each clocks moves away from one another. Now according to special relativity clocks moving at different speed through the universe goes out of synchronization. But the clocks in bread does not move out of synchronization because these clocks are not moving through the space . They are rather moving with the space and hence remain synchronized. So if you are in Milkyway and your friend is in Andromeda galaxy and both of you posses perfectly synchronized clocks, then they will remain synchronized even if both of you are moving away from each other along with your galaxy (provided none of you move at all).
BOUNDARY OF UNIVERSE:
Titus Lucretius Carus (ca. 99 BC – ca. 55 BC) said that space is, by his own words, ‘infinite’. His reasoning is: if there is a purported boundary to the universe, we can toss a spear at it. If the spear flies through, it isn’t a boundary after all; if the spear bounces back, then there must be something beyond the supposed edge of space. Either way, there is no edge to the universe; space is boundless. This means that space cannot be contained like an object can be contained in a box. Space is indeed not finite, not physical; space is nothing.
Lucretius also reasoned that nothing comes from ‘nothing’, and nothing can be destroyed. Matter exists in imperceptible objects (atoms) separated from one another by space. The atoms are solid, indivisible, and eternal.
IF THERE IS NO BOUNDARY HOW IS UNIVERSE EXPANDING?:
expansion of universe does not mean that boundary (if present) are expanding. It means that each and every thing are moving away from each other.
If each and everything is moving away, then why doesn’t the spaces between each atom, molecules, matter increase? In bread and resin model, the distances between each resins increases but the resins itself does not expand. The molecular forces and electromagnetic forces keep the resins intact.