The 74% Solution (Dark Energy)
To recapitulate; we live in a Universe that "exploded" into existence about 13.7 billion years ago. We don't know why or how this happened. The matter in the early Universe clumped together because of the gravitational influence of invisible stuff called Dark Matter. We don't know why or how this happened or what this Dark Matter is. While we call the explosion the Big Bang, it wasn't an explosion at all in the sense that we use the word. Some force expanded the Universe carrying all matter with it. We don't know why or how this happened or what this expansive force is. Since the Big Bang, we thought the Universe has, under the influence of gravity, been slowing down its rate of expansion. We thought we know why and how this happened. It feels so good to know something.That was the Universe as we understood it about ten years ago. 10% or so of the stuff in the Universe was "baryonic" matter as we know it: protons, electrons, quarks, photons, neutrinos, and all the other quantum particles in what physists call the Standard Model. The other 90% was something else; something we called Dark Matter. We had nifty new telescopes on the ground and in space and we had lots of bright people trying to figure out what this Dark Matter could be. Sometimes, however, when you reach into the cookie jar there's a mousetrap inside.
One of the ideas that astronomers were pretty sure of was that the expanding Universe has been slowing down. This makes sense. We may not understand what caused the Big Bang but whatever it was, the gravity of all the stuff in the Universe, normal matter and Dark Matter, should be slowing things down. The big question was if there was enough matter to finally stop the Universe from expanding and start collapsing everything back to a Gnab Gib (Big Bang backwards). This is a very subtle measurement. The difference between an open Universe that expands forever and a closed Universe that someday reverses direction to collapse, is very, very small. From observations it looked very much like our Universe was flat -- right at the perfect balance between open and closed Universes. The problem with all of this is that, if you add up all the ordinary matter and Dark Matter in the Universe you don't get enough stuff to match what we measured. We end up in a Universe with "missing" matter making up about 75%
Telescopes look into the past. Since light travels at a known speed, we can look at galaxies farther and farther away and look farther and farther back in time. We should be able to graph how fast space is expanding between us and nearby galaxies and then galaxies a bit farther away, and then at galaxies a bit farther away and eventually see exactly how much the Universe is slowing down. This sounds easy but is very, very hard. Astronomers about ten years ago finally had telescopes and instruments that could finally make this measurement. We would finally know the future of the Universe! Just as we reached for this ultimate "cookie," our fingers closed on the mousetrap.
Remember back when we were talking about standard candle stars and we discussed type 1a (one A) nova exploding stars? With our nifty new telescopes we can see a 1a nova if it explodes in the farthest galaxy we can detect. With very careful measurement of many 1a novi, astronomer found that the expansion of the Universe is speeding up!!!!!! SNAP! Goes the mousetrap! It's even crazier than that. Until "recently" (compared to 13.7 billion years), the Universe was slowing down. Now it is accelerating! Why? You may well ask.
Back when we were discussing the Big Bang we made the careful distinction between objects moving and the space between them getting larger. This sounds like just playing with words but the difference is important. Here's why. Let's say you have a box one meter on a side. The box is empty. You have carefully removed every last electron, proton, photon, and bit of Dark Matter (not that we know how to do this, mind you). So what's in the box? Duh! Nothing! Right? Right? Well you'd think so. It turns out that "nothing" is built out of lots of "something". No, really! It seems that most explanations of Quantum Mechanics, that wonderfully bizarre theory that strives to explain the very tiny in the Universe, require something called vacuum energy. Now this isn't the stuff that powers your Hoover or Oreck. No, this is the almost magical idea that particles, like electrons, can spontaneously appear out of absolutely nothing as long as at the exact same moment its anti-particle, like a positron, would also spontaneously appear and the two particles would collide and "poof" they would disappear! Crazy, right? In the instant that these particles exist, they occupy space and time. The whole E=mc2 equation says that if you have mass you've got energy. This vacuum energy, also known at the "cosmological constant," is the price for space to exist.
This vacuum energy acts something like "anti-gravity". Gravity "pulls" things together. Vacuum energy creates space which takes ordinary matter with it. Vacuum doesn't "push" things apart but it does increase the distance between objects. Gravity works best when thing are close together. So in the early years of the Universe when everything was in a much smaller volume of space, gravity had a stronger influence over the expansion rate. Now, since everything is much farther away from everything else, and since vacuum energy creates more space which creates more vacuum energy which creates more space....., the Universe's rate of expanding in increasing.
74% of everything in the Universe seems to be Dark Energy. This dark energy is probably (but not positively) the effect of vacuum energy expanding space and making even more vacuum energy.
22% of everything in the Universe seems to be Dark Matter. We don't what this dark matter is but it seems to only interact with "ordinary" matter gravitationally.
4% of everything in the Universe seems to be Baryonic "ordinary" Matter. This is the stuff we are made of and the only stuff we have ever seen and touched.
It just keeps getting stranger and stranger.