Lois & Clark Forums Forums General Discussion Off Topic The Big Bang and the universe, part 10: Enter the dark forces of the universe!

In this post I'm going to talk about the dark forces of the universe. I don't mean the Dark Knight:



Or the Dark Vader:



Or the Dark Penguin:



Or the Dark Horse:



No, I mean the dark forces of the universe! (With apologies to Florida for stealing their thunder!)



Meet the really important dark forces of the universe, Dark Matter and Dark Energy!



I'll return to this post later, but for now... consider that most astronomers and physicists are convinced that the world we can see around us, including ourselves, our world, the solar system, all other stars, planets, gas clouds, galaxies, E.T. guys and girls etcetera... make up only 4% of everything that exists in the universe! So in this thread I thought I'd introduce you to the other 96% of reality!

But you'll have to wait for my next post to find out...

Ann

In 1985, Very Rubin presented evidence that there is something weird about how galaxies rotate.



Vera Rubin in 1970, studying galactic spectra.

Galaxies were supposed to rotate in much the same manner that our own solar system is rotating. After all, a galaxy can be regarded as a monstrously blown-up version of a solar system. In the middle of both these systems you have a large concentrations of mass - in our solar system it is the Sun, and in a galaxy it is the supermassive black hole that is found in the center of most galaxies. Further out you have all the matter orbiting the center of mass. In our solar system it is all the planets, asteroids and comets that orbit the Sun. In a galaxy it is all the stars, planets and gas clouds that orbit the central black hole.



The planets orbit the Sun.



In a galaxy, all stars, planets, gas clouds and other matter orbit the central black hole.

In our solar system, planets orbit faster the closer they are to the Sun. Mercury, the innermost planet, orbit the fastest. Neptune, the outermost planet, ponderously moves around the Sun at the slowest speed. (Pluto doesn't count here since it is no longer considered a planet, just a dwarf planet.)



Pluto, a planet no more.

Anyway. In our solar system, objects move faster the closer they are to the Sun. They move more slowly the farther they are from the Sun. There is a reason for that. Neptune (and Pluto!) feels the Sun's gravity much less than Mercury does, and if Neptune (and Pluto) moved as fast as Mercury does, then their own speed "along the tangent of their orbits" would easily overcome the Sun's attempts to "rein them in" with its own gravity. Conclusion? If Neptune and Pluto moved as fast as Mercury does, then their own velocity along the tangents of their orbits would flung them clear out of the solar system until they were lost in space!

Lost in space, but for real!

It was expected to be the same way with galaxies as it is with our solar system, namely, that objects orbiting far from the center of the galaxy would have to orbit much more slowly than objects orbiting deeper inside. The farther away you got from the center, the more slowly the stars, gas and other matter was expected to rotate.

It turned out that it was not so, as Very Rubin found out.



This is a "rotation curve" of stars at different distances from the center of our smallish neighbouring galaxy, M33.

It is not too hard to measure how stars rotate in galaxies, as long as the galaxies are somewhat tilted to our line of sight, so that the stars move either toward us or away from us during some parts of their orbits. When they move toward us or away from us their light will be either blueshifted or redshifted, and by measuring how much the spectral lines have moved from their "proper place" you can rather easily deduce how fast they move. (See part 4 of my series for an explanation of redshift and blueshift.

It was, as I said, Very Rubin who was the one who started systematically measuring the rotation curve of galaxies. What she found was that stars farther out from the galaxies' centers don't move more slowly than stars closer to the galactic center. This was completely unexpected.



A typical galactic rotation curve. But how can it be explained?

In 1983, Mordehai Milgrom proposed a new theory of gravity called MOND, Modified Newtonian Dynamics.



According to MOND, Newtonian gravity as we know it doesn't work for objects where the gravitational field is low. What MOND means for galactic rotational curves is that stars close to the center of a galaxy move according to classical Newtonian laws of gravity, but stars further out from the center move according Modified Newtonian Dynamics. Interestingly, MOND has proved itself quite good at predicting how how stars in galaxies and galaxies in galactic clusters will move. Even so, MOND isn't very popular with astronomers, at least not so far. So we need to take a look at the only other explanation that astronomers have come up with to explain the strange rotation curves of galaxies, and that is dark matter.

Ann

I have been interested in astronomy since 1969. In the mid-seventies I started reading an astronoomy magazine on an on-and-off basis (mostly off). It wasn't until the late 1980s that I really started reading astronomy magazines very regularly.

Well, I can definitely remember that as far back as in the 1980s, my astronomy magazines said that there is mass in the universe that has gone "missing". This mass must be there, my magazines told me, and astronomers can calculate that it is there, because the gravity of it is needed to "balance" the universe and make it look and work the way it does today. Indeed, astronomers in my old astronomy magazines argued that the universe must contain at least ten times as much matter as we have detected in stars, dust, gas and galaxies, or else the universe would look completely different than it does. So the "missing mass" had to be there there.



But the astronomers in my old astronomy magazine reported that they couldn't see it.

My astronomy magazines regularly told me about new efforts to find the missing mass. Speculation also ran rife about what the missing mass could be. Could the mass be there in the shape of an incredible profusion of planets, which would be hard to detect with the technology of the late 1900s? But seriously, was it realistic to assume that planets could make up 90% of the matter in the universe, when we know that our own Sun makes up 99% of the mass in our own solar system? Could the missing mass be some sort of huge clouds of cold gas in various parts of the cosmos? But if that was the case, then why weren't the radio telescopes of that time able to detect the clouds, even though such gas clouds would invaribly emit radio waves that the radio telescopes of that time would definitely be able to pick up?



Nobody knew what the missing mass was.

Let's return to Very Rubin. I don't remember reading about her or about galactic rotational curves in the eighties and nineties. But I think that her rotational curves slowly insinuated themselves on the astronomical community. Probably someone made the connection between the missing mass that everyone was wondering about and the weird rotational curves of normal galaxies.

What if the missing mass was right there, in the normal galaxies? Or better yet, what if the missing mass was there as huge haloes surrounding normal galaxies?

Astronomers started calculationg. And, yes indeed, if there was a lot of that strange matter around normal galaxies, then it would actually make sense that the outer parts of galaxies rotated as fast as they did. If they were immersed in that huge halo of matter, they would feel the gravity of that matter and rotate faster because of it.



What if normal galaxies were immersed in huge haloes of unknown matter, like the huge blue halo surrounding the small yellowish galaxy in this picture? If such massive but unseen haloes of matter were there, the galaxy rotational curves would make sense.

Astronomers soon realized that clusters of galaxies suffer from the same problem as galaxies - their individual members seem to move too fast in relation to how much matter there seems to be in the clusters.



A galaxy cluster. There seems to be more matter in it than we can detect.

Take a look at the picture again. Can you see that the yellow galaxy cluster is surrounded by drawn-out, thin bluish arcs and lines? These arcs and lines are background galaxies. Their apparent shapes have been "drawn out" and the light from them has been magnified by the huge concentration of mass in the foreground cluster. This huge concentration of mass literally "bends" space and causes it to work as a gravitational lens. But this lens wouldn't be so "strong" and powerful unless the yellow galaxy cluster contained more mass than we can see.

Gradually, more and more astronomers became convinced that we have found the missing mass of the universe, and we can detect its gravitational effects on ordinary matter. We can also say where this unknown matter is to be found: it is particularly plentiful around galaxies and in galaxy clusters. But that's all we can say about it. Because apart from its gravitational effects, we can't detect this kind of matter at all. This matter is completely "dark".

And that is why it is called "dark matter".

Ann