Question

1. What do we mean when we say the Universe is expanding? Is the space between planets in our solar system, or the space between stars in our galaxy expanding, or is something else meant entirely? How does the concept of the expansion of the Universe give us its current age?
2. Carl Sagan famously said that we are "star stuff." In what sense is this true?

3. What is meant by observable Universe? What is the difference between the observable Universe and the entire Universe?

Answer 1

1. We mean a homogenous, isotropic expanding Universe whose expansion carries Heavenly bodies, constantly between planets, stars.

By calculating since this movement is constant and there's a relation V/d gives us the Δt. (Explanation below)

2. In the sense of how tiny we are compared to the Universe.

3. Observable Universe, the part of the Universe bounded by the horizon of the viewer as the observer was the center of a sphere whose radius was the size of 4.6508 x $10^{10}$ light-years

Explanation:

1) Because according to researches conducted by Slipher, and others Hubble come up to the conclusion that the universe moves away gradually, with a speed proportionally to the distance. It's like the space among planets and other Heavenly bodies are getting larger.

Moreover to that, Hubble developed an equation called Hubble's Law.

V =H.D where V=Velocity, H= Hubble's constant and D= Distance

With this equation, we can calculate the age of the universe.

Calculation the age of the Universe

The Hubble law, affirms that galaxies sited at a distance d and they are receding from us at a given speed v

Universe Age = Distance to given galaxy/Its own velocity of recession

Universe Age =1/$H_{0}$

Universe age = $\frac{72km}{sMpc}$=13.6 billion years

PS.

Despite the fact, that the Hubble Constant changes it's considered to be constant because its variations on H, are not relevant in terms of our human parameters.

There were some calculations over the Units Megaparsec, omitted for the sake of explanation, but necessary due to the nature of the kilometer unit and the nature of the Universe as nonlinear.

2. In the sense that the Universe is gigantic and complex enough and the planet Earth, of the amount of the information, we know about it. And how tiny we are. Like some say we're just a small ball or rock in one galaxy.

3.  The observable Universe is finite and bounded by the event horizon. The observable Universe is huge with a radius of 4.6508 x $10^{10}$ light-years and according to 2016 update there was also a finite total rounded numbers: $10^{11}$ galaxies, $10^{21} stars$ and so on.

The Entire Universe is much bigger, and observation spots are different.

The entire Universe is composed of the non-observable universe there are many unknown things to discover about the Non-observable Universe, much inference using our pure mathematics and theoretical physics has been doing. But still, there are more questions than answers about the whole Universe.