There are two ways to slide easily through life; to believe everything or to doubt everything. Both ways save us from thinking. -Alfred Korzybski
I’ve recently received a number of messages from readers of this site expressing doubts about the existence of dark matter. As someone who’s researched dark matter extensively, it is my determination that dark matter most likely exists, although explaining exactly what it is is a challenge. Over this next series, I would like to lead you through the evidence, observations and discoveries that have led me to this conclusion, and I hope that I explain this well enough that it leads you to draw the same ones for yourself.
Stars, mass, and light: One of the wonderful things that astronomers have learned about stars is the relationship between the light they emit and how massive they are. Not surprisingly, the more massive a star is, the more light it typically emits. We’ve figured out what the relationship is, and we have not only uncovered the relationship between the mass of a star and how much light it gives off, we’ve also taken a census of how many stars are out there.
What is surprising in this regard is that — on average — our Sun is not typical. More than 90% of the stars in our galaxy (and in all galaxies) are less massive than our Sun, meaning they give off much less light than our Sun does. But this relationship is severe: a star half as massive as our Sun gives off only 10% of the light that our Sun does. All-in-all, it means that when we look at anything in the Universe that emits light — stars, galaxies, clusters of galaxies, etc. — we can figure out how much mass is there in the form of stars. (Please keep in mind that over 99% of the mass of our solar system is in our star.)
Gravity and mass: But measuring starlight isn’t the only way we know of to find out how much mass is in something. We have another tool: gravity! Gravitationally, we have a number of different tools to measure how much mass is in something. For individual galaxies, we can look at how they rotate (or, for close enough galaxies, how individual stars move) and figure out how much mass there must be to hold them together. For clusters of galaxies, we can look at how the galaxies on the outskirts move about the center, like Fritz Zwicky did.
More Gravity: But there are more ways than just looking at velocities to measure the amount of mass in a galaxy or cluster with gravity. We can look at — over the history of the Universe — how quickly (and on what scales) objects collapse gravitationally, to form the structures we currently see in our Universe.
Gravitational Lenses: The Universe is also full of matter. Some of it’s closer to us, and some of it’s farther away. Every once in awhile, we’ll find a cluster of galaxies in front of another galaxy. When this happens, the background galaxy can be lensed by the mass of the foreground cluster, the same way an optical lens can bend light. The distorted arcs in the image below are not only evidence of lensing, they are — in many instances — multiple images of the same background galaxy! They also allow us to measure how much mass is in the foreground cluster.
We can also look at the rate of expansion of the Universe. It’s highly dependent on all of the energy in it, including normal matter, radiation, neutrinos, and any “dark” things that may exist.
Although there are other ways to measure mass gravitationally, these are the main ones. They all give the same answer, too: about 25% to 30% of the total energy in the Universe is some form of gravitational mass. But what of the stars? It turns out that stars are only about 0.5% of the mass in the Universe.
This is a difference of a factor of FIFTY!
So what we learn from this is that all the different ways we can measure gravitational mass give the same result. That result tells us that when we look at an image like this, and see all the light we see:
We’re only seeing light from about 2% of the total mass. This leads us to one of only two conclusions: either there is a bunch of mass that is invisible to us or there is a different law of gravity for galaxies, clusters, and the Universe than we’ve currently considered. But right off the bat, we find that starlight is woefully inadequate to explain our observations. And we need a little more information to decide which of these two possibilities is correct, which we’ll work towards in a few days in part II!