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Dark Matter & Dark Energy
Transcript of Dark Matter & Dark Energy
What is Dark Energy?
In 1933, Swiss astronomer, Fritz Zwicky first proposed
Dark Matter's existence. He was studying the Coma Galaxy Cluster when he found out the galaxy's gravity alone was much too small to hold the cluster together.
In the 1970's the next evidence of Dark Matter was discovered. This was when astronomers measured the velocity (speed) of stars at different distances from the centre of a galaxy. Then, they plotted the velocity versus the distance. It was expected to reach its maximum velocity then decrease farther from the centre of the galaxy. However, the data showed it reached its maximum velocity and then it plateaued. With velocities of this magnitude the outer edge of the galaxy's stars should fling out of their orbits but they do not. Therefore, some mass that we cannot detect must hold these outer stars in orbit.
The mass of galaxy clusters also show evidence of Dark Matter. A galaxy cluster can bend and magnify light from galaxies behind it. The galaxy cluster acts as a gravitational lens.
Dark Energy is the name given to an unexplained force that is drawing galaxies away from each other. Also, its density remains constant as the universe expands. According to current theories Dark Energy has always been there and has always exerted the same level of force. However, its existence isn't proven, but Dark Energy is scientists' theory to explain why the universe's expansion is speeding up.
What is Dark Matter?
Using various research methods, astronomers have discovered that the elements the human eye can see in space (such as stars, gas and dust) only make up 4.6% of the universe. This is classified as ordinary matter, therefore made up of protons, neutrons and electrons. Another 24% of the universe is something called Dark Matter and the other 71.4% is Dark Energy. I will be talking about both Dark Matter and Dark Energy with you today.
Dark matter is a very mysterious mass that does not emit, absorb, or reflect any type of light. Therefore, Dark Matter is invisible. Dark Matter is not like anything scientists have ever encountered on Earth. Each second, billions of these strange particles pass through everything they encounter due to their extremely small size. However, they are so massive in weight that they have the power to influence the galaxies.
How do astronomers know it is there?
Dark Matter interacts with ordinary matter through gravity. Saying this, astronomers first found out it was there from its gravitational interaction. Its gravitational influence can be seen in the motion of gasses and stars in galaxies. However, astronomers are not 100% positive these particles even exist.
In this image of the galaxy cluster, Abell 2218, you can see numerous blue arcs. These are background galaxies that are distorted and magnified by the cluster's gravity.
Astronomers study the sizes and shapes of these arcs to determine the cluster's mass. Scientists then compare this calculated mass to the mass that comes from only luminous objects (stars, gas and dust). The remaining number determines how much dark matter is in the cluster.
I now have a video that summarizes Dark Matter and how the space station is trying to solve what Dark Matter is.
The concept of dark energy was first proposed, and then discarded, by Albert Einstein early in the 20th century. His general theory implied that the pull of gravity would make the universe collapse, but, like many scientists of his time, he assumed the universe was going to be changeless. To make his equations fit these assumptions; Einstein added a "cosmological constant" whose effect was uncertain. When American astronomer, Edwin Hubble, discovered that the universe was expanding, it led Einstein to remove the cosmological constant from his equations. Einstein then said that it had been the biggest careless mistake of his career.
In the 1990's, astronomers set out to see how fast the universe was expanding. They expected that the universe's expansion rate was constant or slowing down. However, from the Hubble Space Telescope it appeared that the expansion was doing neither: it was speeding up. The evidence was based on measurements of bright exploding stars called supernovas. Astronomers were using supernovas as lampposts to track distances. The scientists observed many supernovas at different distances to determine how fast they are speeding away from Earth. These measurements gave astronomers a picture of how fast the universe was expanding at different points in its history.
At this point, Scientists knew the earth was expanding, but didn't know what was causing it.
Eventually, theorists came up with three sets of explanations:
1. It was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." 2. There was some strange kind of energy-fluid that filled space.
3. There is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration.
Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called Dark Energy.
To conclude, scientists hope to find more information and proof about the existence of both Dark Matter and Dark Energy. These discoveries will hopefully help reveal key facts about the universe that are still unknown.
Thank-you for listening to my presentation!
This diagram reveals changes in the rate of expansion since the universe's birth, 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart at a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart.
This pie chart shows the components that make up the universe.
Hot Dark Matter is composed of particles that have zero or a mass that is near zero. The Special Theory of Relativity requires that particles without a mass move at the speed of light and particles that barely have a mass move at nearly the speed of light. Therefore, very low mass particles must move at very high velocities and form very hot gases.
On the other hand, cold Dark Matter is composed of objects sufficiently massive that move at sub-relativistic velocities. They form much colder gases. The difference between cold Dark Matter and hot Dark Matter is significant in the formation of structures, because the high velocities of hot Dark Matter cause it to wipe out structures.
Difference between cold and hot Dark Matter: