What do we mean by "The Universe is expanding"



The Universe is very wierd but this is what that makes us want to know more about it!
The Universe is an empty space containing billions of galaxies that are moving away from one another. The space which appears empty to us is not actually empty but contains dark energy throughout. This dark energy is a theoretical predicted energy that is thought to exist in the empty space to explain the Expansion of Universe as a consequence of the Big Bang! With the recent discovery of Gravitational waves , we hope that the existence of Dark energy will also be proved in the future which supposedly occupies more than 60% of the Universe which is greater than that occupied by all the matter and dark matter in the Universe!
We all must have heard at some point that the Universe is expanding continuously and that the rate at which it expands is increasing with time. That is, in the past it expanded at slower rate than it is doing today. This observation was discovered by observing various white dwarfs at different distances from the planet and measuring their red shift. By comparing their distances and red shifts, we came to a conclusion that the universe is expanding and farther the galaxies were, the more faster were they moving from us.  That is, the rate at which Universe expands is accelerating with time!
You can consider this as analogous to a tooty-fruity bread loaf. The bread dough initially has raisins embedded in it. As it is heated in an Oven, the dough expands. In this process, the raisins do not move relative to one another but the dough between them is expanding, thus , overall increasing the space between these fruits or raisins. These raisins are the galaxies and the dough is the Universe which is expanding. The space between these galaxies is expanding which is supposed to be caused by the dark energy which by it's nature has a negative or repulsive Pressure which is proportional to the square of speed of light and Energy density. The constant of proportionality is a negative number.
Hence, the galaxies move apart from one another!
The speed of expansion of Universe is faster than speed of light but that doesn't violate the rule that no matter can attain the light speed as we saw how the space between the galaxies is expanding and that galaxies are not moving!
We hope that the existence of dark energy is confirmed soon thus providing a more stronger confirmation to the Theory of relativity!

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