One subject that seems to keep coming up in class is dark energy or dark matter. I had only briefly heard about these concepts before recently. So what exactly are dark matter and dark energy? According to the Cornell astronomy department here is the break down of our universe; 0.03% heavy elements, 0.3% neutrinos, 0.5% stars, 4% free hydrogen and helium, 25% dark matter, and 70% dark energy. So the two “darks” definitely make up a large majority, but what exactly do the two effect? Before I continue I want to clarify that both dark energy and dark matter are hypotheses unconfirmed from direct observation, but thought to exist due to inference from other observable aspects of our universe.
Visualization of the Breakdown
For the past eighty years we have known the universe is expanding. According to our understanding of gravity, the universe’s expansion should be slowing down due to objects pulling on each other. After scientists Perlmutter, Schmidt, and Riess independently discovered the universe’s expansion was accelerating in 1998 they were quite surprised. There need be something out there causing this acceleration. Some type anti-gravity effect, which is now commonly referred to as dark energy. So dark energy is basically just a name given to explain the acceleration of the universe expanding, but that doesn’t tell us exactly what it is. The two main ideas as to what dark energy really is are the cosmological constant and quintessence. The concept of a cosmological constant is concept that Einstein originally came up with, which in simple terms means that “a volume of space has some intrinsic, fundamental energy” or vacuum energy. The actual cold hard number is though to be around 10^−29 g/cm^3. There are some serious discrepancies with this explanation. The largest being that quantum theory predicts a cosmological constant that is 100 times larger. Quintessence explains dark energy by saying there is a field all through out space and that the potential energy generated by this field is what causes the acceleration of the universe. The only issue? Currently there is no solid evidence to support this hypothesis. In the end we still know very little about dark energy, but research on the topic is on going.
Now how about that dark matter. The galaxy we live in along with all others would rip apart and fly out into space if it wasn’t for the gravity holding them together. This gravity is caused by the mass of things such as stars, planets, and of course black holes, but according to calculations there is no where near enough mass in galaxy’s to keep them together. So there must be some unobservable other mass accounting for the extra gravity hold galaxies together. This unseen mass is called dark matter and it accounts for 84.5% of the total matter in our universe.
Something else to consider is that our understanding of gravity and maybe completely wrong and that we are like the fish looking out through their bowl, with an incomplete view of their world. In this case dark energy and dark matter could just be a part of gravity we haven’t worked into our model yet. We’ll leave that to the scientists to figure out. More realistically (or not) we will find a way to directly detect the two or more accurately study them so their existence can be proven once and for all.
Sources:
http://www.space.com/20929-dark-energy.html
http://curious.astro.cornell.edu/question.php?number=634
http://www.lsst.org/lsst/public/dark_energy
