What are dark matter and dark energy, as far as is known?
They are fudge factors introduced to make theories (that are well supported in other respects) fit certain, inconvenient observed facts. That is not to say that they don’t exist. Such “fudges” have turned out to be real in the past, and have led to real discoveries, one famous instance being the neutrino. Others, such as negatively weighted phlogiston, have not turned out so well, and the theories that they were intended to save have been replaced by quite different ones.
As I understand it, there are several possible candidate for what dark matter might be, one possibility being neutrinos with mass (it used to be thought that they have no mass, but maybe they have a little after all, and there are lots of them). Not all dark matter has to be the same thing, by the way, so several of the suggested candidates might be contributors to the “missing mass”. Another possibility is planet-like bodies in deep space, not in orbit around any star. One of these has recently been discovered, but I do not think we have much idea about how common they are. Exoplanets in general might account for a lot of dark matter, and we now know there are lots of those.
On the other hand, the last I heard, no-one really has a clue as to what dark energy might be.
NASA has a really good aimed-at-beginners page on the subject of both dark energy and dark matter. Note for the foregoing: We now think dark energy is approximately 70% of the mass-energy in the Universe, dark matter is 25%, and the stuff we can actually see making up the remaining 5% or so.
In both cases, the fundamental story is the same:
[ul]
[li]We have a very good theory that enables us to make a lot of very accurate predictions. In both cases, the theory is General Relativity, the theory behind how we currently think space-time is structured and, therefore, how gravity works.[/li][li]We observe things that we can’t explain using only the theory and the stuff we know exists.[/li][li]So, instead of completely throwing out the very successful, very well-validated theory, we take the more conservative approach of either postulating new stuff that makes the observations work in the context of the existing theory, or making relatively minor tweaks to the theory. We might need to throw out the theory anyway, but we don’t know enough to do that yet.[/li][/ul]
Dark energy is, at present, an expression of our confusion over why the expansion of the Universe seems to be accelerating. We have four current best guesses:
One, the version of Einstein’s theory of the structure of space-time (General Relativity) that has a nonzero cosmological constant, such that as space itself grows out in the great empty expanses between galaxies, more energy is generated by the growth, fueling the expansion.
Two, it could be that empty space spontaneously generates matter in the form of virtual particles, which gives it the energy to keep expanding at an increasing rate. The problem with this theory is, when we try to compute the amount of energy such particles would create, our answer is too big by a factor of 10[sup]120[/sup]. Being wrong on that scale is impressive in and of itself.
The third idea is that empty space is full of a mysterious force that pushes matter apart at very long distances, which some physicists call ‘quintessence’. The problem with that idea is, essentially, “Fucking quintessence, how does it work?” It doesn’t solve the problem to shove the uncertainty around to a new name.
The last option is that General Relativity is wrong, which, as I alluded to above, we’ll only be able to decide based on more evidence.
Dark matter is an expression of our confusion over, for one thing, how the galaxies we observe are able to rotate as fast as they do with as little mass as we observe in them. The obvious answer to this is “They have mass we can’t see”, which is what we call dark matter, which raises interesting questions. Keep in mind: We can see planets, at least to a certain extent. We can see dust. We can see a lot of different things in a lot of different ways, directly and indirectly. What we can’t see is dark matter, which we now think accounts for 25% of all the mass-energy in the Universe, as I mentioned above.
This is a fascinating time to be alive.
I thought that we had been able to indirectly “see” dakr matter in the form of gravitational lensing, no?
I might be thinking of black holes…
I think this is very misleading. When the idea of dark matter was introduced, we could not see planets outside our solar system at all, and it was quite widely believed that planetary systems were quite rare. Even now, we can’t really see extrasolar planets, just detect them indirectly, but the numbers that have been detected over the past few years lead astronomers to think, now, that most or all stars probably have planetary systems, and there also seem to be some planet-like bodies floating around in space, not associated with any particular star.
No one ever said that dark matter has to be magically invisible in some way. It just meant (originally) “matter we cannot detect now” (apart from its general gravitational effect on the behavior of galaxies). The exoplanets that are now believed to be common are at least part of the dark matter that the original dark-matter theorists were talking about. They may not be common or large enough to account for the whole of it, but even if that is the case, there is no particular reason to believe that whatever other components of dark matter there may be, that it would be undetectable (except gravitationally) or even invisible if you got up close. Some of it might be invisible exotic particles, or massive neutrinos, or something else that you would never be able to actually see, but there is no particular reason to think any of it is like that, and some of it is certainly planets.
Also keep in mind that the Sun makes up 99.86% of the total mass of the solar system, so things like planets etc. are not going to be a huge contribution to a galaxy’s mass, even if they are ubiquitous.
If we assume #1 or #2 to be correct, does that mean that Newton was incorrect?
Incorrect about what?
Um.
About nothing. Scratch that.
I was talking about the laws of conservation of mass and conservation of energy, which is an entirely separate concept from the laws of motion.
It’s not possible to describe what Dark Matter is. The entire reason we call it “Dark” is because we cannot identify or account for it.
To break the theory down to its most basic level, the universe is expanding and accelerating, and scientists cannot account for why. We also cannot account for the absence of certain types of matter (such as antimatter) that our equations indicate must exist somewhere but we haven’t found. We discuss “Dark” Energy or Matter as the unknown force that is causing this.
If we ever do discover the cause, it will no longer be “Dark,” and we will give it a proper name.
It’s important to note that the term “dark matter” can have two different meanings. At its simplest, it just means matter, any kind of matter, that isn’t emitting light. By this definition, we are made of dark matter (along with almost everything other than stars). Dark matter of this sort, that’s made of basically familiar kinds of material which just happens to not be glowing, is sometimes called “baryonic dark matter”, since almost all of the mass of such matter is due to baryons (protons and neutrons).
But not all, or indeed even most, of the matter in the Universe is baryonic. A number of isotopes of elements, such as ordinary hydrogen, deuterium, helium-3, and the isotopes of lithium, are what’s called primordial, meaning they were formed by the processes of the Big Bang (this is called “Big Bang Nucleosynthesis”). The relative amount of each of these isotopes is determined by how densely packed baryons were at the time. So by measuring the amounts of these isotopes found in the Universe currently, we can calculate how dense baryons were during the era of BBN, and hence how dense they are now. But this calculation comes up far short of the total mass we seem to have now, as determined by gravitational effects, which means that most of the mass in the Universe (including most of the dark matter, since all of the glowing matter is baryonic) must be of some non-baryonic form. We don’t know what this non-baryonic matter is, and things which we don’t know are always more interesting than the things we do, so the term “dark matter” is sometimes used just to describe this mysterious stuff.
Right, but there could be a lot of baryonic matter in the galaxy that isn’t orbiting a glowing bit. The density of non-glowing matter in interstellar space could possibly be a lot larger than we imagine. Most of that would probably be balls of ice like comets plus a few balls of rock plus maybe some gasballs.
If we imagine that the mass of an object in a galaxy follows some sort of logarithmic pattern, we’d see a few supermassive objects like bright stars (which we do), a lot more massive objects like regular stars (which we do), a whole bunch of less massive objects like red dwarfs (which we do) and a whole bunch of even smaller objects similar to gas giants, rocky planets, and iceballs (which we can’t see because they don’t glow).
But then even if you added up the mass of the small objects, even if there are 100 times more small objects the big glowing objects might be much more than 100s of times more massive, and so the small objects become a rounding error, like they are in our solar system.
Which leaves us empty-handed, right?
ETA: Derleth, great cite.