what is dark matter and the black hole?

Answer 1

Scientists using different methods to determine the mass of galaxies have found a discrepancy that suggests ninety percent of the universe is matter in a form that cannot be seen. Some scientists think dark matter is in the form of massive objects, such as black holes, that hang out around galaxies unseen. Other scientists believe dark matter to be subatomic particles that rarely interact with ordinary matter. This paper is a review of current literature. I look at how scientists have determined the mass discrepancy, what they think dark matter is and how they are looking for it, and how dark matter fits into current theories about the origin and the fate of the universe.


In 1933, the astronomer Fritz Zwicky was studying the motions of distant galaxies. Zwicky estimated the total mass of a group of galaxies by measuring their brightness. When he used a different method to compute the mass of the same cluster of galaxies, he came up with a number that was 400 times his original estimate (1). This discrepancy in the observed and computed masses is now known as "the missing mass problem." Nobody did much with Zwicky's finding until the 1970's, when scientists began to realize that only large amounts of hidden mass could explain many of their observations (2). Scientists also realize that the existence of some unseen mass would also support theories regarding the structure of the universe (3). Today, scientists are searching for the mysterious dark matter not only to explain the gravitational motions of galaxies, but also to validate current theories about the origin and the fate of the universe.

Mass and Weight. What exactly is mass? Most people would say that mass is what you weigh. But to scientists, mass and weight are different things. Mass is the measure of a quantity of matter--how much stuff there is. Weight, on the other hand, is the effect that gravity has on that stuff. Weight is dependent on mass--the more mass you have, the more gravity pulls you down, and the more you weigh. When an astronaut floats in space, we say that the astronaut is weightless. But the astronaut still has a body, and so has mass.

Hide and Seek. Scientists estimate that 90 to 99 percent of the total mass of the universe is missing matter (4). Actually, "missing matter" may be misleading--it's really the light that is missing (5). Scientists can tell that the dark matter is there, but they cannot see it. Bruce H. Margon, chairman of the astronomy department at the University of Washington, told the New York Times, "It's a fairly embarrassing situation to admit that we can't find 90 percent of the universe" (6). This problem has scientists scrambling to try and find where and what this dark matter is. "What it is, is any body's guess," adds Dr. Margon. "Mother Nature is having a double laugh. She's hidden most of the matter in the universe, and hidden it in a form that can't be seen" (5).



Determining the Mass of Galaxies
How do we measure the mass of the universe? Since the boundaries (if there are any) of the universe are unknown, the actual mass of the universe is also unknown. But scientists talk of the missing mass of the universe in percentages, not real numbers. Since the majority of the matter that we can see is clumped together into galaxies, the total mass of all the galaxies should be a good indication of the mass of the universe. Although it isn't possible to add up an infinite number of galaxies, scientists can infer the percentage of the universe's missing mass from estimates of the missing mass in galaxies and clusters of galaxies (7). And because scientists (like Fritz Zwicky) use different techniques to determine the masses of galaxies, they can perceive mass that they cannot see.

The Doppler Shift. One of the tools that scientists use to detect the motions of galaxies is the Doppler Shift. The Doppler Shift was discovered in the 1800's by Christian Doppler when he noticed that sound travels in waves much like waves on the surface of the ocean (7). Doppler also noticed that when the source of the sound is moving, the pitch of the sound is different, depending on whether the source is moving toward or away from the observer. Take, for example, the horn on a train. As the speeding train passes by you, the sound of the horn changes to a lower pitch. This is the Doppler Shift. When the train approaches, the sound waves get pushed together by the motion of the train. As the train speeds away, the sound waves get stretched out.




The Doppler Shift also works with light. When a light source is moving toward you, the light becomes bluer (called a blue shift). When a light source is moving away from you, the light becomes redder (called a red shift). And the faster something is moving, the farther the light is shifted. But the Doppler shift for light is very subtle and cannot be detected with the naked eye. Scientists use a device called a spectroscope to measure Doppler Shift and determine how fast stars and galaxies are moving (7).

Rotational Velocity. Using the power of the Doppler Shift, scientists can learn much about the motions of galaxies. They know that galaxies rotate because, when viewed edge-on, the light from one side of the galaxy is blue shifted and the light from the other side is red shifted. One side is moving toward the Earth, the other is moving away. They can also determine the speed at which the galaxy is rotating from how far the light is shifted (7). Knowing how fast the galaxy is rotating, they can then figure out the mass of the galaxy mathematically.
As scientists look closer at the speeds of galactic rotation, they find something strange. The individual stars in a galaxy should act like the planets in our solar system--the farther away from the center, the slower they should move. But the Doppler Shift reveals that the stars in many galaxies do not slow down at farther distances. And on top of that, the stars move at speeds that should rip the galaxy apart; there is not enough measured mass to supply the gravity needed to hold the galaxy together (7).
These high rotational speeds suggest that the galaxy contains more mass than was calculated. Scientists theorize that, if the galaxy was surrounded by a halo of unseen matter, the galaxy could remain stable at such high rotational speeds.

Seeing the Light. Another method astronomers use to determine the mass of a galaxy (or cluster of galaxies) is simply to look at how much light there is. By measuring the amount of light reaching the earth, the scientists can estimate the number of stars in the galaxy. Knowing the number of stars in the galaxy, the scientists can then mathematically determine the mass of the galaxy(1).
Fritz Zwicky used both methods described here to determine the mass of the Coma cluster of galaxies over half a century ago. When he compared his data, he brought to light the missing mass problem. The high rotational speeds that suggest a halo reinforce Zwicky's findings. The data suggest that less than 10% of what we call the universe is in a form that we can see (8). Now scientists are diligently searching for the elusive dark matter--the other 90% of the universe.



Dark Matter
What do scientists look for when they search for dark matter? We cannot see or touch it: its existence is implied. Possibilities for dark matter range from tiny subatomic particles weighing 100,000 times less than an electron to black holes with masses millions of times that of the sun (9). The two main categories that scientists consider as possible candidates for dark matter have been dubbed MACHOs (Massive Astrophysical Compact Halo Objects), and WIMPs (Weakly Interacting Massive Particles). Although these acronyms are amusing, they can help you remember which is which. MACHOs are the big, strong dark matter objects ranging in size from small stars to super massive black holes (1). MACHOs are made of 'ordinary' matter, which is called baryonic matter. WIMPs, on the other hand, are the little weak subatomic dark matter candidates, which are thought to be made of stuff other than ordinary matter, called non-baryonic matter. Astronomers search for MACHOs and particle physicists look for WIMPs.
Astronomers and particle physicists disagree about what they think dark matter is. Walter Stockwell, of the dark matter team at the Center for Particle Astrophysics at U.C. Berkeley, describes this difference. "The nature of what we find to be the dark matter will have a great effect on particle physics and astronomy. The controversy starts when people made theories of what this matter could be--and the first split is between ordinary baryonic matter and non-baryonic matter" (10). Since MACHOs are too far away and WIMPs are too small to be seen, astronomers and particle physicists have devised ways of trying to infer their existence.

Answer 2

The expansion of the universe is not constant as thought infact it is slow when compared to the ratio of matter around the universe . according to the present matter which is visible the calculations show that none of the galaxy has enough gravity to hold another one..so that means all the galaxies should go away with huge velocities than the present movement.this means that there is some matter in the universe which is holding the visible matter with gravity and it is not showing any radiation.so scientists name it dark matter ,which cant be seen but its effect is felt..infact scientists estimate the dark matter should be covering 97% of the matter of the universe this makes accurate prediction why the galaxies are not traveling at huge speeds.


When a star which is 10-100 times bigger than the sun completes its fuel ie, all the hydrogen in it is converted to helium
there is no more gas to burn..As the stars gravity pulls the matter into the center the gas which is burning will eject the matter out ward so the gravity is canceled by outward thrust ,this resultant force makes star stable. After the gas is burned out there is nothing to make the matter throw away from the center of the star ,so the star starts to collapse into its center.this eventually turns into a black hole whose gravity is so high that nothing .not even light can escape its force...


This is the difference between two

Answer 3

A black hole is an object (usually a collapsed star) with such an enormous gravitational pull that not even light can escape from it (which is why it's black)

No one actually knows what dark matter is - it would appear to be a mysterious substance which makes up a very large proportion of the universe. It does not reflect light, so we can't see it, and it does not seem to be made of atoms like everything else. So it's very mysterious stuff.

Answer 4

Dark matter and black holes are not related to each other. Astronomers realized that there was more matter in galaxies than could be accounted for by the mass of the stars and gasses it it. The rate of spin meant there was more matter in the galaxy than we could measure. The extra mass is called dark matter and no one know what it is.

http://en.wikipedia.org/wiki/Dark_matter

A black hole is a large star that has collapsed on itself. It is so dense that even light cannot escape from it.

http://en.wikipedia.org/wiki/Black_hole

Answer 5

we have recently "mapped" the "structure" of dark matter, and it appears to act as some sort of "skeleton" to the universe.
matter we cannot perceive, which existence was only guessed when we tried to work out the mass of the entire universe.
a black hole is a singularity.
a star collapsed on itself, reaching such degrees of density that nothing, not even light can escape its gravitational field.
our most recent system still predict that black holes shouldn't exist.
beautiful!!!

Answer 6

If only the great minds of science and astronomy had thought to ask Yahoo answers about this one, the entire mystery of 96% of the universe that is unaccounted for could have been solved in a trice by Jo Public. Well done Einstein nice move, maybe we could ask the meaning of life while we're about it.

Answer 7

Since most scientists/astronermers only know 97% of the universe a black hole gathers speed as it moves, dark matter is entirely different. Part from that it could be considered my purse.!!

Answer 8

have you tried running a search for them? it's much better learning about things that way. (too much typing to do AND too much insufficiency of data here!)

try wikipedia.com. it's the best! i always use it for everything i want to know about!