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Search for the Missing Universe
Chris Gondek writes "The Sydney Morning Herald has reported that one of the greatest discoveries of our time could be made under the Yorkshire moors. Deep in a Yorkshire mine, scientists are toiling to solve a cosmic puzzle that has baffled astronomers for 70 years: about 90 per cent of the universe is missing. Analyse the movements of stars and you can work out how much matter is making them swirl round in galactic islands and how much makes galaxies cluster together as they do - in other words, you can work out how much mass makes the universe look the way it does. But measurements suggest that the universe is not what it appears."
Yes, modifications to Gravity is one of the way to explain away dark matter/dark energy problems. It is an active field of research, but it is a hard one.
The problem is that while there is no direct tests of gravity at very large scales, there are a lot of "consistency" checks of the various cosmological observations (say of the cosmic microwave background anisotropies) that you have to satisfy.
In other words, there is no proof that such theories of modified gravity do not exist. But to find one is really hard.
Mode (3) smart-aleck mode. Press * to return to main menu.
Dark Matter isn't the only explanation for Fritz Zwicky's 1993 observation.
MOND or Modified Newtonian Dynamics proposed by Moti Milgrom is I think better. If I were to bet on someone winning a future Nobel, Milgrom would be the person.
I'm driving the VLT as I type this...sentence was interrupted for a preset...I'm back now.
Anyway, I know a number of scientists that seriously consider the Newton's may not work at large scales. Nature recently rejected a paper from some rather prominent that seemed to confirm that gravity behaves differently at large scales. But, science is very reluctant to change its equations and publication will have to await more data.
Just remember - Dark matter may not exist. Be skeptical of those who treat it as fact.
MOND FAQ
Dark-Matter Heretic [This is a wonderful article]
This argument is actually flawed.
The answer is a bit involved.
But basically, the weighing of the matter (as quoted in the article) does not depend on just mass, but a quantity called "mass-energy". It is true that a particle moving at very high speeds seemed to gain "mass". But depending on observers travelling at different velocities relative to this particle, each will see a different mass. However this particle, irregardless of its velocity, will have a consistent "mass-energy" to all observers. In other words, everybody in the unvierse can agree on the amount of "mass-energy" each particle have. So there is a consistent picture of weighing the amount of mass of the universe.
That is the beauty of Einstein's Special Relativity, which is to unify mass and energy into a (jargon warning) relativistically consistent picture of mass-energy.
Mode (3) smart-aleck mode. Press * to return to main menu.
..IANAP (i'm not a physicist), though.
There are two strage things happening in the universe on the large scale. The first one is the "dark matter". Basically, if we apply Newton's equations for gravity to various galaxies, we find out that they are spinning too fast. If the force holding them together is what we think it is, most of the stars in a galaxy should have been slingshoted away and left the galaxy. So there must be something making the attraction stronger than we think.
The second strangeness - the "dark energy" - concerns the expansion of the universe. Different pieces of matter in the universe attract to each other by gravity. This slows down the expansion of the universe. As far as we know, gravity is the only thing that can affect the universe on a large scale. So, the expansion of the universe should be slowing down. However, as WMAP showed, the rate of expansion of the universe is actually speeding up. So, there must be something that makes the universe speed up faster than we think.
In both cases, there are two possibilities. The first one is that the anomality is equally distributed through space. This would mean that our equations are a little bit off. For instance, we can account for the "dark energy" by adding an extra term to Einstein's equation for the expansion of the universe. If we change Newton's equation to make gravity stronger over large distances, we can eliminate dark matter.
Yet, there is a possibility that there can be more of the "strangeness" in one point in the universe than in another. For example, one galaxy may be held together tighter than another one of the same size. That would mean that there is another strange beast in the universe apart from the types of matter and energy we know. A whole new branch of physics will be needed to deal with the beast and ask questions like "Why is there more dark matter here than there is there?" and "Does dark matter interact with ordinary matter in any other way than gravity?". Dark matter will compress things on a smaller scale; dark energy will expand things on a larger scale. Obviosly, the statement that "Universe is 75% (or whatever) dark matter" will only be meaningful in this case. As far as I know, we need more precise observations to choose between the two possibilities.
I hope that someone who actually is a physicist, is not asleep, or can reach the "Reply" button will explain all the points I'm wrong on...
I recall an earlier article about the universe being topologically equivolent to a torus. Could this topology account for some of the inconsistancies in these "mass of the universe" calculations?
Consider any two stars of mass m and M. With distance r between them:
The Gravitational force of attraction is G*M*m/r^2.
But you'd also have a gravitional force wrapped once around the torus of G*M*m/(r+L)^2.
Then you could wrap around again and again and again....
Of course, generally the distance would be too huge to make difference, but when you consider how many stars there are and the infinite number of loops around the torus you could make, it would add up eventually.
Any thoughts on this?
Fight or flight its all the same
Live to die another day
--Ryan
I can't believe no one brought this up yet. Recently some astronomers have been using hubble to look at the middle of galaxies and have discovered Supermassive Black Holes there. In fact, they've found a bunch of 'em, and there's a relationship between the size of the galaxy and the size of the singularity, and every galaxy seems to have one, even our own! And IIRC they figured this would account for the missing stuff.
-Peverbian