atomic_fungus (atomic_fungus) wrote,

#141: Dark Matter

People persist in believing in the boogy-man. That's all I can figure.

Principle of least action! How hard is this? Occam's Razor

UH? Oh.

There are a few problems with the observable universe. It doesn't seem to behave the way our equations say it should. Galaxies, for example, should not exist. The universe is expanding too quickly. (Or not quickly enough. It depends on who you ask.)

'Way back at the beginning of the 20th Century, Albert Einstein revolutionized physics with his theories of relativity, both the Specific and General. He was working as a patent clerk when he developed these theories, which also proves how productive government workers are. Einstein had time to reinvent physics, for crying out loud. (The work was probably so repetitive and boring that he had to do something with his brain.)

It was a great time for science. Edwin Hubble learned that all the galaxies in the universe are moving away from us at measurable fractions of the speed of light--the universe was expanding! People in Europe learned about radioactivity and began to discover new elements. Really, the early 20th Century was just a continuation of the late 19th Century, when all sorts of new things were discovered.

Anyway, physics and astronomy moved a long way forward. We learned all sorts of interesting things about the properties of matter, and by the 1950s we were well on our way to understanding advanced quantum mechanics.

But there were problems. Einstein's theories fit very well with all observed facts, but little pesky problems began to show up--things like the observed mass of the universe, the existence of galaxies, expansion of space, etc--things which, if true, seemed to contravene Relativity and many other important and otherwise useful theories.

Einstein's General Relativity is a powerful tool. Not only does it describe, fairly accurately, the mechanics of the universe; it also works down to a fairly small level. There is a bit of a fuzzy area from there on down to where quantum mechanics works, but the two disciplines are not mutually exclusive enough to be worrisome. Physicists generally just use relativity where it's needed, and quantum mechanics where it's needed; and this seems fine, most of the time.

Einstein himself had to generate a constant--the cosmological constant--which normalized the equations for the curvature of space. He abandoned the concept as unnecessary, but in light of the debate over dark matter, it's come back to life.

The notion is that the universe seems to be expanding at an accelerating rate. According to our best theories, the universe was created in the Big Bang and expanded from there. At most, the presence of matter in the universe should cause the rate of expansion to slow. At worst, there would not be enough matter in the universe to keep it from expanding, and the rate of expansion would be constant.

But recent findings show that the rate of expansion is increasing.

Absent any other explanation, cosmologists came up with the idea of dark matter and dark energy, things which are undetectable but which comprise up to 95% of the matter and energy in the universe. This would result in a negative cosmological constant, which would explain the accelerating expansion of the universe.

This is where the world of physics, and me, part company. I realize that I am not as highly trained as those who came up with these theories, but it seems to me that if you need to postulate that only five percent of the universe is observable in order to make your theory work, there is something seriously wrong with your theory.

There's no way to generate this cosmological constant from quantum or particle physics.

Doing the essential research for this entry I learned that the problem is even bigger than I thought it was; now some physicists are thinking of doing away with the notion of universal physical law in order to explain all this.

All of this leads to my fundamental point, which is that if the accelerating expansion of the universe requires a mysterious and unobservable phenomenon in order to be explained, then there just might be a problem with the accelerating expansion itself.

Although the mechanisms by which the acceleration of expansion were observed are thought to be well-understood, are they in fact well-understood? The discovery of acceleration was based on the observations of certain types of supernovae, which all behave the same way; but are we sure that is the case?

The constant of gravity is said to change with time. Might this have an effect on the observations? If the gravity constant was higher or lower 8,000,000,000 years ago (or however old the light from those supernovae is--it has taken billions of years to reach us from far distant galaxies) would that change how the supernova took place? Could this variation account for the appearance of accelerating expansion? Can the data be reasonably normalized such that the acceleration disappears? Does this normalization match the data which indicate the change in the constant of gravity?

If we were to see a new supernova within our galaxy tomorrow, it could not have taken place more than about 120,000 years ago. But seeing a new one in a distant galaxy means it could not have taken place less than billions of years ago. Space is time.

Universal physical law demands that the laws of physics are the same throughout the universe--a pint of water weighs one pound when accelerated at 32 feet per second per second, whether you are on the surface of the Earth or on a planet near the Red Shift Limit. If gravity is different out where those supernovas are taking place, then the entire question becomes utterly meaningless anyway. You can't explain accelerating inflation that way; if gravity is different on the edges of the universe than it is here, what is the point of trying to understand anything?

But for me, the biggest issue with dark matter and energy comes from a little-discussed paper which was published some time last year. The writers of the document did something computationally intensive and highly irregular:

they modeled the galaxy's behavior using general relativity rather than classic Newtonian mechanics

...and guess what? Their model fit with the galaxy's observed behavior. No "dark matter" or "dark energy" were required to make it work, although the cosmologists insist that such is needed to explain the workings of galaxies. Our galaxy should not be able to remain in one piece; it should have flung itself apart aeons ago--but it didn't; and many have suggested that dark matter gives our galaxy (and others) enough mass to remain together.

This leads me to another point: WHICH IS IT? Does dark matter repel or attract? Or does it attract on the galactic scale but repel on the universal scale?

And why the hell should its behavior change depending on the scale it's observed at? It makes no sense!

So, to summarize: the universe is acting in ways which don't fit our theories, so rather than fix the theories, we have invented an unobservable phenomenon to explain the discrepancy. This boogy-man has no observable mass at local (planetary) scales; a positive mass, resulting in an attractive force, at galactic scales; and a negative mass, resulting in a repulsive force, at the scale of the entire universe.

Have I summarized it correctly??

The revelation that cosmologists have gone right on using classic Newtonian mechanics, rather than Relativity, should not have been surprising. Einstein's Relativity is not as easy to use. It's not Einstein's fault; Relativity is complex. Newton's mechanics are simple, and when it comes to an object in space you can ignore a lot of factors. You can't do that with Relativity.

But while mechanics are useful for solving problems such as getting a satellite into the right orbit, or moving a box of fruit juice from a conveyor belt into a shipping carton, or designing a braking system for an automobile, they are not useful for making good theories about the working of the universe. They are only good for approximations at a practical level.

That's something which seems to escape most people these days. Newton's mechanics are approximations. Einstein's Relativity is a better approximation. On the scales we're used to working with, Newton's mechanics are just fine. To design a braking system for a car, the engineer need not worry about the speed of light. To reliably transfer a quart of orange juice from a conveyor belt to a box, the robotic technician need only deal with four pounds of mass in a non-varying gravity field, moving at non-relativistic speeds.

When you start dealing with an entire galaxy things change a bit. The gravity field is not uniform. There is an interstellar medium to be considered. Everything is moving at different speeds, some of which may be relativistically significant. These factors, considered only from the viewpoint of Newton, don't add up; but from the viewpoint of Einstein they fit nicely.

I wonder if the "accelerating expansion" theory is Newtonian or Einsteinian?

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