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- May 2015 astronomers observing the Abell 3827 cluster of galaxies find some more clues about the existence of Dark Matter. The cluster is four colliding galaxies. Outside the cluster is light from background galaxies in long blue arcs evidence of gravitational lensing.
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- The gravity of Dark Matter separated in the galaxy collisions is bending the background light, focused in our line of sight and acting like a magnifying glass.
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- The image suggests that Dark Matter “particles” are interacting with one another and slowing themselves down during the collision. These interactions did not affect normal matter during the collision. There are many different theories as to what Dark Matter is. Self-interaction is one more clue in the search for the answer.
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- Astronomers believe that galaxies are mostly Dark Matter. “ Dark” because this matter does not emit or reflect photons of light like Ordinary Matter. “ Matter” because it still behaves the same with gravity.
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----------------------------------- F = G * M * m / r^2
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- The Force of gravity is equal to a Constant of Coefficients times the product of mass
(or, matter) divided by the square of the distance of separation.
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- Johannes Kepler ( 1571 - 1630) discovered that in our Solar System the outer planets orbital speed is slower and have further to go than the inner planets.
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- For example: Jupiter’s distance from the Sun is 5.2 times larger that Earth’s distance from the Sun. That Earth - Sun distance is called an “ Astronomical Unit”. If Jupiter was orbiting at the same speed as Earth traveling further to complete one orbit would take 5.2 years. But, Jupiter takes almost 12 years to complete one orbit.
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- This relationship is true for all the planets and other orbiting bodies. The further from the Sun, the larger the circumference of orbit and the slower the orbiting speed.
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- Kepler discovered the mathematical relationship between the time period of orbit and the distance from the Sun. The period squared is equal to the distance cubed:
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----------------------------- ( P years)^2 = ( Astronomical Units)^3
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- In metric units this formula becomes:
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----------------------- (P seconds)^2 = 2.9*10^-19 ( A meters)^3
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- But, it is easier to work in Astronomical Units. For example: Neptune’s orbital period is 165 years.
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------------------ (165)^2 = A ^3
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----------------- 27,225 = A ^3
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-------------- A = 30.1 Astronomical Units.
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- Earth’s orbital period is 1 year with a radius of orbit of 1 Astronomical Unit.
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-------------------- (1 year)^2 = ( 1 AU)^3
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------------------------ 1 = 1
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- 30.1 AU is the average distance between Neptune and the Sun. This relationship works the same for all the planets.
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- Isaac Newton related Kepler’s law to the total mass of the orbiting system with this ratio:
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--------------------- Solar Mass = A^3 / P^2
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- For example: The mass of the Milky Way Galaxy interior to the Sun’s orbit is:
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------------------- Mass = (1.6*10^9 Astronomical Units)^3 / ( 250*10^6 years )^2
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------------------ Mass = 6.6*10^10 Solar Mass.
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- The mass of the Milky Way interior to the Sun is 66 billion times the mass of the Sun.
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- If the mass distribution of the Galaxy is like that of the Solar System then we would see mass concentration at the center and fast orbital velocities near the center with slower orbital velocities farther out.
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- To the total surprise for astronomers they discovered that the orbital velocities did not slow down farther out ,but, remained constant out to the edge of the spiral galaxy. For this to be true the mass distribution in galaxies must be different than that of solar systems.
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- For the observed orbital velocities to hold true there must be a greater total mass halo extending outside the visible galaxy that we can see. This mass we cannot see but can be measured from the effects of gravity.
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- After studying many galaxies in this way astronomers conclude that about 90% of the total mass of a typical galaxy is in the form of Dark Matter that we can not see.
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- Here are some more Reviews that try to explain this situation:
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- #1636 - Dark Matter accounts for 5.4 times as much mass as Ordinary Matter in the Universe.
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- # 1722 - If you include Dark Energy than 95% of the Universe is made of this stuff. Remember matter and energy are the same thing, E = mc^2.
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- #1658 - Gravity grows weaker with distance, 1 / r^2, therefore distant objects must orbit more slowly or they would fly off into space.
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- #1594 - 4% Ordinary Matter and the rest is Dark Matter and Dark Energy.
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- #1585 - Today the space is expanding due to Dark Energy at 47,000 miles per hour per million lightyears, but, it is also accelerating faster and faster.
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- #1596 - Using Redshift to measure the expansion of space.
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- #1597 - Using Brightness to measure the expansion of space.
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- #1598 - Stars and galaxies would be flying our of their orbits if it were not for unseen mass existing around the galaxies.
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- #1599 - We need a breakthrough in physics to explain Dark Matter.
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- #1517 - 5 calculations used to measure Dark Matter.
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- #1485 - #594 - 10 more reviews about the subject. Stay tuned we have much more to learn.
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