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----------------- 1738 - The Dark Matter in the Universe?
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- Dark Matter is the hypothetical matter of unknown composition that does not emit or reflect electromagnetic radiation ( light ). But, it does interact with gravity. Gravity effects is how it is inferred to exist.
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- Gravity effects include:
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---------------------- Rotational speeds of galaxies
---------------------- Orbital velocities of galaxy clusters
---------------------- Gravitational lensing of background galaxies.
---------------------- Temperature distribution of hot gases in galaxies
---------------------- Computer models of galaxy evolution.
--------------------- Anisotropy of CMB
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- Anisotropy means having different physical properties existing in different directions. That is, it’s lumpy, it has temperature variations along different axes. We will review each of these gravity effects to learn how astronomers infer the existence of Dark Matter.
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- 95% of mass-energy in the Universe is some unknown stuff , or energy. 22% of this is Dark Matter which is 6 times more plentiful than Ordinary Matter, which is 4.5% of the total. That would be like me weighing myself on Earth at 180 pounds. Then, flying off into space and calculating the pull of gravity to give me a mass of 1,080 pounds. I have no idea where the extra weight came from.
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- Where did the extra 900 pounds come from? Either my observations are wrong, my math is wrong, or I have a 900 pound halo that I never knew was there.?
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- This is what we are dealing with in galaxies. The calculations have been made several ways and astronomers always get the same answers to infer Dark Matter’s existence.
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- Gravity, telling us there is missing mass, has happened to us before. Uranus was discovered because the orbits of the other planets needed some unknown planet’s mass to be affecting them for the math to match observations. When astronomers looked to where this missing mass ought to be they discovered the planet Uranus.
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- Mercury’s orbit did not match the Newtonian math calculations until Albert Einstein discovered that the speed of light had to be constant and gravity had to be a distortion of space and time, they were the variables. Space and Time were not constants. When the math from Relativity was added to Newton’s math the calculations exactly matched the observations for Mercury’s orbit.
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- These were slight discrepancies to be corrected. Dark Matter’s discrepancies are huge. We should be able to figure this out.
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---------------------- Rotational speeds of galaxies:
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- The inner part of spiral galaxies is mostly luminous matter according to the rotation curves. Plotting velocity versus distance from the center, the radius, the velocities should trail off. They don’t. The velocities remain the same even though the radius increases in distance. In order to make these velocities match the math 95% of the spiral galaxy has mass that is not represented by the visible mass around the center.
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---------------------- Orbital velocities of galaxy clusters:
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- The same orbital velocities are calculated for clusters of galaxies. The same results occur. There is 6 times as much mass in the cluster that is not visible in order to mathematically account for the orbital velocities observed.
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- What about elliptical galaxies that do not rotate. Therefore, the velocity calculations do not work. Some other means is needed to calculate the total mass of the galaxy. The technique is to measure how the galaxies hold on to X-ray emitting gas. X-ray emissions are a giant halo surrounding these galaxies. The gravity needed to account for holding these halos may be 20 times the mass that can be accounted for in visible matter. The missing matter is simple called, Dark Matter.
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---------------------- Gravitational lensing of background galaxies.
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- See Review # 1534 for how gravitational lensing is used to calculated the total mass in a galaxy cluster. The gravity bends the light from a background galaxy and focuses it on our line of sight. Einstein’s ring was the math needed to calculate the amount of mass in the cluster. Again, astronomers get the same answer, 22% of the Universe is Dark Matter.
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- Relativity equations tell us how much space is curved in the vicinity of strong gravitational fields. The images of background galaxies are distorted by the gravitational lensing effect by an amount that depends on the mass of the intervening galaxy or galaxy cluster. These calculations agree with the X-ray calculations.
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---------------------- Temperature distribution of hot gases in galaxies:
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- X-ray telescopes have discovered vast clouds of multi-million degree gas in clusters of galaxies . But, there is not enough mass to account for the gravity needed to hold on to it. The pressure of the hot gas is balanced with the gravitational pull of all the mass of the galaxy. These galaxies must contain 5 times more Dark Matter as that present is stars and interstellar gas.
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---------------------- Computer models of galaxy evolution:
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-- The Cosmic Microwave Background radiation varies in intensity, fluctuations in temperature, due to clumps of matter that is either hotter or cooler than the average. Calculating how the clumps would grow using different mixtures of photons, protons, neutrinos, and Dark Matter only matches today’s observations if there is 6 times more Dark Matter than Ordinary Matter.
--------------------- Anisotropy of CMB
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- The density of ordinary baryons, ( protons and neutrons ) is 1 hydrogen atom per cubic meter of space. Only 4% of the total mass-energy density of the Universe is these baryons, ( visible matter ).
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---------------------- 4% Ordinary Matter
---------------------- 22% Dark Matter
---------------------- 74% Dark Energy
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- The Dark Ages of the Universe is that time before the first stars were born. The Dark Ages ended 550,000,000 years after the Big Bang. But, photons were released earlier. It was 370,000 years after the Big Bang when photons were no longer impeded by charged particles, ( protons and electrons). This Gamma Ray light was first released and it has been stretched by expanding space by a factor of 1,000 to become the Cosmic Microwave Background Radiation that we see today. Expanding space stretched the wavelengths from Gamma Rays to Microwaves.
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- Solving the Dark matter mystery will require a profound change of our understanding of the Universe:
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- Candidates for explaining Dark Matter:
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-------------------- Jupiter sized planets
-------------------- Black holes
-------------------- White Dwarf stars
-------------------- Unknown subatomic particles.
-------------------- MACHOS, Massive compact halo objects.
-------------------- WIMPS, Weakly interactive massive particles.
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- WIMPS are the leading candidate. A typical WIMP particle could be 100 times more massive that a proton. Examples:
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---------------------- Neutrinos
---------------------- Gravitinos
---------------------- Axions
---------------------- Sterile neutrinos
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- Expect many more reviews on Dark Matter. There is so much more to learn.
- Here are some other reviews completed previously, back to June 25, 2003:
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- #1535 Gravity Lensing
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- #1636 How a galaxy cluster was used to measure the mass of Dark Matter.
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- #1722 Dark Matter gravity should be slowing down the expansion of the Universe , But, instead expansion is accelerating. In order for this to happen 74% of the mass-energy in the Universe must be anti-gravity, or Dark Energy.
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- #1658 Could there be a new state of matter yet to be discovered to account for Dark Matter?
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- #1576 Supersymmetry creates the math and 17 new particles to add to the Standard Model of Particle Physics. The math looks promising because it does account for the Weak Nuclear Force being 10^32 times stronger than the force of Gravity.
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- #1517, #1485 Our Sun contains 99% of the mass in our total Solar System. But, the Dark Matter Halo is 80% of the mass in the galaxy and mass is not the Blackhole at the center of our galaxy that is 4,500,000 times more massive than our Sun.
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- #1427 A colliding cluster of billions of stars is 2.4 light years from us? Astronomers can see Dark Matter and Ordinary separating in the collision.
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- #1341 Measuring the halos of Dark Matter.
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- #1218 Could Dark Matter be another Universe?
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- #1204 Does Dark Matter have anti-Dark Matter? If they collide the two annihilate each other into pure Gamma Rays.
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- #1075 A calendar for 10^100 yeas of the Universe.
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- #837 Kepler’s formula for the period of the planets, and the mass of the Sun.
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- #718 There is no evidence that Neutron Stars and Blackholes would explain the mass of Dark Matter.
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- #692 Dark Galaxies outnumber visible galaxies by 100 to 1.
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- #14 Dark Matter contains most of the energy in the Universe, according to E=mc^2. June 25,2003
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