--------------------- #1517 - What is the Universe Made Of?
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- What is the Universe made of? We do not know? 95% of our knowledge is just “dark”. We think 75% is energy and 25% is matter. Remember matter and energy are the same thing, simply separated by the speed of light squared, c^2.
-------------------------- E = mc^2
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------------------------- Energy = mass * 90,000,000,000,000,000
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- Matter is just a huge concentration of energy.
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- Matter made of atoms is what we call our “ natural world”. But, this represents less than 5% of the total. The other 95% is what we call Dark Matter and Dark Energy.
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- Dark is the names we have given this “ stuff” because we do not know what it is. We can’t see it. We know its existence due to its influence. We can see the “ influence”, we can’t see the “ stuff”.
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- The most telling influence is the effect of gravity. For galaxies to behave the way they do there must be 10 times as much mass out there than we can detect with light, or any other part of the electromagnetic spectrum.
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- We understand gravity fairly well. We would expect that gravitational influences being only an attractive force would eventually slow down and reverse the expansion of the Universe. However, evidence to the contrary suggests that the Universe is accelerating in its rate of expansion and has no intention of slowing down. We call the mysterious repelling force Dark Energy, for lack of a better name. ( See Review #1518 for more about Dark Energy).
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- What exactly is the evidence for Dark Matter?
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---------------------- (1) Mass and gravity calculations for the Milky Way Galaxy
--------------------- (2) Mass calculations for other spiral galaxies
---------------------- (3) Mass calculations for large galaxy clusters using gravity lenses.
---------------------- (4) Explaining galaxy formations with computer simulations.
--------------------- (5) Explaining the structure of the Universe with computer simulations.
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----------------------- (1) Mass and gravity calculations for the Milky Way Galaxy
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- Astronomers can calculate the amount of gravity and mass by measuring the orbital velocities of stars in the galaxy. The mass inside the orbit of a star is directly proportional to the radius of orbit and the square of the orbiting velocity.
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--------------------------- Mr = r * v^2 / G
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- See footnote (1) to see exactly how this calculation is made.
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- Hydrogen gas atoms emit a spectral line of 21 centimeters in the radio wavelengths. The spectral line is blue-shifted for stars moving towards us and red-shifted for stars moving away from us. This Doppler Shift of wavelengths is used to calculate the orbiting velocity of stars at various radii from the center of the galaxy.
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- The orbital velocities of stars in our Galaxy are so high at the outer edges our Milky Way Galaxy should be flying apart. There is not enough visible mass to hold it together.
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- Our Solar System is orbiting the center of the Galaxy at a radius of 26,000 lightyears. Our orbital velocity is 230 kilometers per second.
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- The orbital speed of stars at the farther edges of the Galaxy, out to 100 lightyears have orbital velocities that are essentially the same, 250 kilometers per second.
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- This “ flat” rotational velocity with respect to radius indicates that there is 10 times more mass at distances tens of thousands of lightyears from the center. We can not see this halo of mass. We call the mystery Dark Matter.
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--------------------- (2) Mass calculations for other spiral galaxies
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- Astronomers measure the galaxy’s mass to its luminosity, called the mass to light ratio. The rotational curves of spiral galaxies are all nearly flat over a wide range of distances from the center. Both these measurements suggest Dark Matter is common in all spiral galaxies. The calculations put mass to be 90% Dark Matter and 10% ordinary visible matter.
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- The same measurements are more difficult for elliptical galaxies but, so far, the conclusions are the same. Galaxies are made of about 90% Dark Matter.
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---------------------- (3) Mass calculations for large galaxy clusters using gravity lenses.
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- When these measurements are made on large clusters of galaxies they conclude there is much greater mass existing within the cluster then the luminosity would indicate. The amount of Dark Matter in clusters is up to 50 times that of the combined mass of the stars in the cluster’s galaxies.
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- Galaxy Clusters have enormous mass that can bend light coming from more distant galaxies in the same line of sight. This gravitational lensing can indicate the light-bending angle caused by the mass. From this astronomers can calculate the amount of mass that is distorting the path of light. Equations from General Relativity are used. The same conclusions ,Clusters contain substantial amounts of Dark Matter.
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---------------------- (4) Explaining galaxy formations with computer simulations.
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- If Dark Matter is the most common form of mass in the galaxies it must have provided most of the gravitational attraction responsible for creating he proto-galactic clouds starting a few million years after the Big Bang
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--------------------- (5) Explaining the structure of the Universe with computer simulations.
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- Super computer models of the structure formation of galaxies and the galaxies expanding in the Universe can simulate densities needed to evolve to what we see today. To get the results expected the models must assume there is at least 6 times more Dark Matter as ordinary matter in the Universe.
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(1) Calculate the mass inside the orbital velocity of the outer galaxy in a galaxy cluster. The radius is the distance to the center of the cluster.
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------------------------ radius = 6,200,000 lightyears.
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----------------------- orbital velocity = 3,020,000 miles per hour.
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---------------------------- Mr = r * v^2 / G
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- “G” = the Gravitational Constant = 6.67*10^-11 meters^3 / ( kilogram * seconds^2)
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------------------------ r = 5.9*10^22 meters
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------------------------- v = 1.35 *10^6 meters / second
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------------------------- Mr = 5.9*10^22 * (1.35 *10^6 )^2 / 6.67*10^-11
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-------------------------- Mr = 1.6*10^45 kilograms
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- One Solar Mass = 2*10^30 kilograms. The cluster’s gravity inside the radius is 8*10^14 Solar Mass. The cluster’s mass is equivalent to 800 trillion Suns. In comparison the Milky Way Galaxy is 1 trillion Solar Mass. The galaxy cluster is the mass of 800 Milky Way-size galaxies.
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(2) It would be interesting to know if the Dark Matter was spread uniformly through the cluster, or if it was concentrated in the center, or maybe in an outside halo. We can use these calculations for 2 different radii and calculate the mass density in the center compared to an outer ring. The galaxy cluster chosen is 4.5 billion lightyears away. It contains 100 individual galaxies. The immense gravity of the cluster creates a gravitational lens which in turn creates 47 ghost images of 12 background galaxies that lie directly behind the large cluster of galaxies.
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- Measurements made for the mass inside a radius of 515,000 lightyears yielded a mass of 1.3*10^14 Solar Mass.
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- The measurements were repeated for a radius of 312,000 lightyears yielding 8*10^13 Solar Mass.
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- We first calculate the density of mass in the center of the cluster out to the 312,000 lightyear radius.
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----------------- Volume of a sphere = 4/3 * pi * r^3
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------------------ Volume of a sphere = 4/3 * pi * (312,000)^3
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------------------ Volume of a sphere = 1.3 * 10^17 lightyears^3
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------------------ Density = Mass / Volume
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--------------- Density = 8*10^13 Solar Mass / 1.3 * 10^17 lightyears^3
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---------------- Density = 0.00062 Solar Mass / cubic lightyear
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- Next we make the same calculation for the outer shell to determine the mass density there. The outer shell is between the radii 312,000 lightyears and 515,000 lightyears.
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- ------------------ Volume = 4/3 * pi * (515,000)^3 - 4/3 * pi * (312,000)^3
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---------------------- Volume = 4.4*10^17 lightyears ^3
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--------------------Mass of outer shell = 1.3*10^13 - 8 * 10^13 = 5 * 10^13 Solar Mass
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------------------ Density = 0.00011 Solar Mass / LY^3
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- The Density comparison between 0.00062 and 0.00011 tells us that Dark Energy is 6 times more concentrated in the center of the cluster than in the outer shell. Dark Energy is not uniformly distributed throughout the cluster. But, we still don’t know what it is? An announcement will be made shortly, stay tuned.
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- Other Reviews available about Dark Matter available upon request:
1485, 1427, 1341, 1218, 1204, 1075, 837, 718, 692, 594.
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707-536-3272, Tuesday, November 20, 2012
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