The most basic, nitty-gritty theory physicists have for explaining the Universe is called Quantum Field Theory - it's the deep-down set of laws we assume everything else is based on. Like learning the basic moves of a game of chess, if we understand the laws of QFT we can explain any phenomenon you care to mention.
There are just two problems. QFT doesn't explain the existence of gravity and QFT leaves an enormous mystery open: why is nature so messy? What I mean is that the raw ingredients of our Universe don't follow any logic, they're a higgeldy piggeldy mess of particles whose properties seem to be random.
It's like discovering that the ingredients of the Universe are a bag of revels: some large, some small, some with caramel centres, some Malteser etc. There's no reason to it but it definitely seems to be true. But since QFT can't explain this wierdness, QFT must be incomplete and we're on the lookout for a new theory to explain reality. One of the possible avenues is an idea called string theory.
In the 1970s a group of theoretical physicsts began tackling the QFT problems in a casual way - mainly out of curiosity and for fun, they began imagining that the deep structure of the world wasn't made of fields and their particles, but made of strings. Everything in the Universe was really made of a tiny string-object and all the particles we can see are just these strings coiled up and vibrating in different ways.
The first string theories were only used to deal with a small part of QFT however, what's called the bosonic aspect, so the theory's full name was Bosonic String Theory. It was never intended to be rigorous though and was mainly developped as a way to keep the equations simple. In fact, Leonard Susskind (the theory's main inventor) has even described it as something "we were just playing around with", but then something rather weird happened - bosonic string theory turned out to accidentally explain the existence of gravity.
Suddenly, people got interested and wanted to see if they could use this strange new idea to explain all of QFT, not just the bosonic part of it. Could we use string theory to maybe explain all the particles in the Universe? One of the tricks used was to assume that strings could act in symmetrical ways - vibrating left, but also vibrating right - spinning clockwise, but also spinning anticlockwise (or something similar to that, the reality of what the strings are doing is too hard for humans to visualise).
By imagining that all these string-like objects had a property called "supersymmetry" Physicists discovered they could explain quantum field theory and include gravity, as well as giving us an idea of where the randomness of particles comes from. So the strings were renamed "Superstrings" and the idea is still being worked on today!
Let's assume we really could dig a tunnel all the way through the Earth's core, right to the other side. If we dropped a rock at one end, the first thing that would happen would be an acceleration toward the centre. It would get pulled by the gravity of an entire planet and hurtle down at 9.81 ms-2. But here's where it gets interesting.
The deeper you got into the Earth, the more of the planet would be above you. As the rock falls, it gradually finds more and more of the Earth is now above it (rather than below it), and all of this plant-stuff has a gravitational attraction of its own. This means as it started to approach the centre of the Earth it would begin to slow down, it starts to get pulled "up" as well as "down".
It's going to have a lot of momentum however, which will carry it through the centre of the tunnel, but it's already begun slowing down, so it won't go shooting to the other side of the planet. What will happen is that it will begin to yo-yo around the centre of the Earth, being pulled back and forth for several minutes until eventually it reaches an equilibrium and will then sit in the centre, hovering perfectly. At this point, it is equally attracted outwards (because all the gravity is surrounding it) so it will become weightless and will sit there, floating, until we drop another rock down in an attempt to knock it out.
The Universe is mostly empty space, with a few stars floating around. Steafan's suggested we take all the heat energy from all the suns, plus all the emptiness, and average it out. How cold would it end up being? The question sounds a bit unusual but there's a good reason to ask it; this is one of the ways Scientists think the Universe might end. It's called the "Heat death of the Universe" hypothesis and the idea is that the Universe is gradually stretching out, so everything will eventually become so far apart that nothing will heat anything up and the Universe will become cold. So, how cold will the Universe be at this point?
The answer is - pretty much the same temperature as it is now! Thing is, the suns and planets account for very little of the Universe's total volume and mass. Most of space is pretty empty, hovering at -270 degrees Celsius (about 3 Kelvin). If we averaged out all of the suns, it would barely make any difference. It would be like taking a handful of matches to Antarctica, lighting them all and working out what the average temperature of Antarctica would now be. While technically we have increased the average heat of Antarctica, it's by such a small amount as to be barely noticeable. Same principle with the Suns and the Universe. In other words, the thermal energy of the Universe already is pretty well distributed.
There are pockets of heat (galaxies) but once they've cooled down, space's temperature won't have changed much. There will still be lone particles, plus empty space has an energy value associated with it, so the temperature will never drop to absolute zero, but it will probably hover around -270 degrees.
The Cassini-Huygens spacecraft is an unmaned vehicle gathering data about Saturn and its various moons. A few years ago, it sent back the following transmission:
What's going on there??? The answer is the Northen Lights, except the north of Saturn, rather than Earth.
The Sun spits out a huge number of electrically-charged, radioactive particles in all directions with a significant amount of force. It's called solar wind and you can picture it like a river of energy flowing from the surface of the Sun.
When electrically charged particles (like those in the solar wind) interact with magnetic fields they get deflected. On Earth, most of these particles are specifically directed to the north and south poles, where the magnetic field points. When these particles start flying into the Earth, they get mixed up in the ionosphere, Earth's very own electrically-charged bubble. The resulting mixture of electric and magnetic interactions causes the Northern Lights (Aurora Borealis) or the Southern Lights (Aurora Australis). But Earth is not unique.
Jupiter and Saturn have their own magnetic fields, and their own ionospheres (Uranus and Neptune probably have them too) which means solar wind does the same thing to them, creating the same beautiful light displays. But not all of light is visible. Some of it falls in the very low-energy part of the light spectrum: invisible radio-waves. These radio waves aren't picked up by our eyes, but to a probe like Cassini, they're a beautiful symphony.
What's happening in that video is that Cassini is picking up on radio waves from the North (or possibly South) pole of Saturn. Those radio waves can then be converted into audio sounds for the human ear. In other words, Saturn is acting like a radio station, pumping out its own surreal music. The resulting radio-to-audio waves are what you're hearing in the youtube clip!