we’re going to do our best, guys. I’m going to try to get better pens, and find some better paper as we go. So without much more rambling:
DARK MATTER: It’s not hocus pocus.
So. Dark matter…
Hey ben! Civilization has Collapsed! And we need to know how much the galaxy weighs! What should we do? (the answer is “stop using ballpoint pens they’re awful”)
Two ways to figure it out: directly (though counting) and indirectly (deduce it from evidence)
Due diligence requires that I outline the special trick* which lies at the heart of the physics of the next reasoning bit. THE NEXT PANEL IS OPTIONAL FOR PEOPLE WHO WANT TO UNDERSTAND WHAT IS GOING ON.
The moral of the story is that the gravity of a big system of orbiting planets is WAY SIMPLER than you would guess from the outset. all you care about is the TOTAL mass of all the stars lying inside your orbit. everything OUTSIDE your orbit doesn’t have an effect.
BUT HOW CAN WE TELL HOW FAST STARS ARE MOVING? the answer comes from The special theory of relativity: Redshift and blueshifting of the colour spectra of stars!
ok. well, that answers that question. but there’s a problem with what we’ve observed so far:
I should probably do my due diligence and qualify the things i just said.
MaCHOs are probably not really candidates anymore due to observational evidence. but they’re important to add because, back in the 90’s dark matter was all about MaCHOs vs. WIMPs. which is hilarious.
Modified Non-newtonian theories of gravity are popular; but Einstein’s General Relativity (the modern theory of gravity) is very successful when it comes to satisfying experimental predictions, and relativity seems to agree with Newtonian gravity at the length scales we’re talking about. So the non-newtonian gravity game includes more than just changing the (1/r^2) force law; the theorist must also kind of address how GR will change. there are a lot of candidates, but none have really caught a foothold.
WIMPs are the most probable candidate theoretically, since their gravitational signature has been seen astronomically (see the Bullet Cluster), and they’re an important component in modeling the evolution of the universe. We’ve also measured the effect of their gravity in cosmological evolution models. In other words, since they were proposed, we’ve found that their inclusion is integral to modeling the cosmos. There are a variety of physicists who are currently concerned with detecting them, and there are a lot of different guesses as to what these shy particles might be.