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# 5.8: Dark Matter and Dark Energy

Difficulty Level: At Grade Created by: CK-12

## Lesson Objectives

• Describe what led to the theory of dark matter.
• Describe what dark matter may be made from.
• Describe what led to the concept of dark energy.

## Overview

There is much that scientists don’t know. When astronomers peer into space, they take pictures and make observations about the change in locations of the stars and galaxies above. From this data they propose theories and make sense of motions. One of the most exciting events is when the galaxies and stars don’t behave as predicted. Scientists then begin to think how and why they are getting unusual results. Eventually a theory will arise that is supported more than others. It does not mean that it is correct, it may just be the most heavily tested at the time. Now is one of those times and dark matter and dark energy is one of those theories.

When astronomers look at the speed of each planet in our solar system, they see that the farther away the planets are from the Sun the smaller the planet’s velocity. This can be calculated according to Newton’s law of universal gravity and the concepts of circular motion. This concept extrapolates to the motion of galaxies as well as our solar system. But when astronomers look at the motion of other galaxies to examine the velocities of the stars in the systems, the results do not match the expectations. Instead, after a certain distance the speeds remain relatively constant.

Astronomers measure the mass of a galaxy by looking at the average luminosity of the galaxy and the star density. This luminosity is then proportioned to our Sun’s luminosity to mass ratio. If Newton’s law of universal gravity is used to verify the the motion of the galaxies, then it turns out that more mass must be in the galaxy than can be accounted for. About $50$ percent or more of the needed mass is unaccounted for (Imamura, 2008). This is too much to be accounted for by the unseen planets in the galaxy’s solar systems. Not enough additional objects can be seen using frequencies above or below the visible light spectrum to account for the $50$% missing mass. Because this mass is not giving off any form of energy in the electromagnetic spectrum, it is given the name "dark matter.”

Dark matter is not detectable by looking in the electromagnetic spectrum. The collisions at the LHC may discover evidence of dark matter. It could find a connection between the lightest super partner and dark matter, or it may find evidence of multi-dimensions supporting string theory. A lot is to be determined (Green, 2008).

## More Evidence of Dark Matter: Einstein’s Rings

Before Einstein it was thought that all light traveled in a straight line between the galaxies and the observers on Earth. Einstein proposed as part of his theory of general relativity that gravity not only curves the trajectory of objects and particles like baseballs and electrons, but it also bends light by bending the trajectory of photons.

Light from a distant galaxy travels in all directions. Some of the light travels straight to the observers on the Earth. Many light rays would pass the Earth.

If there is a massive galaxy between the Earth observer and the distant galaxy, the light could be bent toward the Earth as pictured above.

The Earth observer will see the galaxy as if the galaxy cluster in the middle were not there. The observer will see the galaxy at the end of the dotted line.

The distant galaxy will also emit other rays that will bend around the galaxy to reach the Earth.

This means that the Earth observer will see the distant galaxy in another position.

Because Earth exists in a three–dimensional space, the Earth observer will see more than these galaxies. He will see an infinite number of galaxies. All these galaxies will form a distorted ring in space. This distorted ring is called an Einstein ring.

Using the Hubble telescope, astronomers have discovered many visual examples of an Einstein ring. This is an image of Galaxy Cluster Abell 2218. In this image you can see white circular streaks. These streaks form the image of Einstein’s rings.

For these rings to appear in images, there must be something in between the Earth and the observer. It is theorized that that something is dark matter—A substance that does not reflect or emit any energy in the electromagnetic spectrum but does exert the forces of gravity on photons.

## Vocabulary

B-field
The abbreviation for magnetic field. The use of the letter “$b$” is rumored to have come for the variable “$b$” that was used in a published paper by Michael Faraday.
boson
A subatomic particle, such as proton, that has no quantum spin. They follow the description given by Bose and Einstein. These particles are responsible for forces in the universe.
bunch
A collection of electrons or nucleons. For the LHC a bunch equals $2808$ charges.
CERN
European Organization for Nuclear Research: The Abbreviation originates from the original title, Conseil Europeén pour la Recherche Nucléaire.
collider
A machine in which two particles are guided into a head–on collision.
Coulomb
The Systems International’s standard unit of charge. Abbreviated with a capital “$C$.” Named after Charles Coulomb.
dark matter
A substance with mass that does not emit, absorb, or reflect any type of electromagnetic energy.
E-field
The abbreviation for electric field.
electric field
A force field that moves objects with a charge that is positive or negative. Measured with the standard Systems International units of a Newton/Coulomb or the non-standard unit of a volt/meter.
electron volt
A small unit of energy directly proportional to the charge of an electron. $eV$: Abbreviated $eV$.
fermion
A subatomic particle, such as electrons, a quantum spin of a half. They follow the description given by Fermi and Dirac. These particles are responsible for mass.
giga
Prefix standing for billions. Example: A $4$ gigabyte hard drive stores four billion bytes of information.
A subatomic particle including baryons and mesons.
Higgs
A subatomic particle believed to be responsible for mass. Direct evidence of its existence has not been found as of February 2009.
Joules
The Systems International’s standard unit of energy. Abbreviated with a capital "$J$." Named after James Joules.
kinetic energy
The energy associated with moving objects.
LHC
lineac
Linear accelerator used to accelerate subatomic particles to high velocities.
magnetic field
A force field that affects moving charges. Natural sources are iron, nickel, cobalt, etc. The standard Systems International unit is the tesla.
mega
Prefix standing for millions. Example: Six megavolts is six million volts.
tera
Prefix standing for trillions.

Feb 23, 2012

Nov 25, 2014