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Explanation of the Effect of Dark Matter and Dark Energy on Gravitation - Succeed in Understanding Physics. Also refer to invisible, force, astronomy, galaxies, stars, big bang, Modified Newtonian Dynamics theory, physical science, Ron Kurtus, School for Champions. Copyright © Restrictions
Effect of Dark Matter and Dark Energy on Gravitation
by Ron Kurtus (22 September 2009)
Dark matter is invisible material that seems to add to the gravitation caused by visible matter in galaxies. On the other hand, dark energy seems to be a force that acts in the opposite of gravitation, pushing stars and galaxies apart instead of toward each other. It seems to be a form of anti-gravitation.
Neither dark matter nor dark energy has been directly observed, so each is really a way to explain anomalies in gravitation for objects at the galaxy scale of measurement.
Questions you may have include:
- What is dark matter?
- What is dark energy?
- What are problems and alternative to these theories?
This lesson will answer those questions. There is a mini-quiz near the end of the lesson.
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Dark matter
The motion of planets orbiting the Sun is readily calculated using the Universal Gravitation Equation or the General Relativity gravitation equations. The same holds true for most stars rotating around a center of mass in a galaxy.
However, astronomical observations of stars at the outer edges of galaxies show that they are traveling faster than what the available visible matter in the galaxy would allow to keep them in orbit. This pointed to the possibility of some sort of invisible matter within the galaxies that is adding to the gravitational effect.
Ordinary matter reacts with light or electromagnetic waves by reflecting, absorbing or transmitting the energy. Dark matter seems not to do any of those interactions and thus cannot be seen. However, the gravitational effect of the dark matter can be observed.
One observation that seems to point to the existence of dark matter is what is called gravitational lensing, which is when a light from a distant galaxy is bent by the mass of a closer galaxy. The more distant galaxy seems magnified, allowing astronomers to calculate the apparent amount of dark matter present.
Dark Energy
While dark matter seems to be adding to the gravitation in the Universe, dark energy seems to be a force pushing stars and galaxies apart.
The Universe appears to be expanding away from a center point, presumably the source of the Big Bang. Scientists and astronomers have calculated that the expansion would slow down with time, due to the mutual gravitational attraction between the galaxies. However, this does not seem to be the case, as the Universe is actually expanding at a faster rate than before.
A possible explanation for the increased rate of expansion is a form of anti-gravitational force called dark energy. Unfortunately, dark energy has never been observed or measured. Instead it is an effort to explain the increased rate of expansion of the Universe.
Problems and alternatives
The idea of invisible matter and energy is troubling to some people, as well as scientists.
Alternative to dark matter
An alternative to dark matter is the Modified Newtonian Dynamics theory, which proposes that at higher speeds or accelerations, seen in stars at the outer edges of galaxies, gravitational attraction would fall off as a simple inverse of the distance instead of the inverse square of the distance in the Universal Gravitation Equation. This would allow stars on the outer edge of a galaxy to be held by a stronger gravitational pull.
Problem with dark energy
Likewise, the existence of dark energy begs the question of why it only affects galaxies and not smaller objects of matter. Instead of being anti-gravitation, it could be a characteristic of space or perhaps some other unknown force that is applicable for only extremely large masses.
Summary
Dark matter and dark energy affect gravitation in opposite ways. Dark matter is invisible material that seems to add to the gravitation in galaxies. Dark energy seems to accelerate the expansion of the Universe with an anti-gravitation force. Both dark matter and dark energy are theories to explain anomalies in gravitation for objects at the galaxy scale of measurement.
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Resources
The following resources provide information on this subject:
Websites
Dark Matter - Wikipedia
Dark Energy - Wikipedia
Gravitation and Gravity Resources
Books
Top-rated
books on Simple Gravity Science
Top-rated
books on Advanced Gravity Physics
Mini-quiz to check your understanding
If you got all three correct, you are on your way to becoming a Champion in Physics. If you had problems, you had better look over the material again.
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Effect of Dark Matter and Dark Energy on Gravitation