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AiSC STI 04 Team Number: 3.0 Team Name: Heavy Thinkers Area of Science: Astronomy, Mathematical Modeling Project Title: Variable Gravity or How Much Do You Want to Weigh?

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Team Members Jeffrey K Raloff Dale Henderson Sponsoring Teacher(s) Challenge Betsy Nick Bennett Project Mentor(s) Isaac Newton Albert Einstein David Kratzer

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Abstract: The current theory of the universe calls for a strange substance called "dark matter" that provides the mass to hold together the universe and the rate which the galaxies spin. This stuff has many strange characteristics much like the mythical ether that was proposed to carry light one hundred years ago. No experiments to date have confirmed the existence of this dark matter”. Instead what we observe in the universe can be demonstrated if gravity were not held constant. We will take current mathematical models of gravity and modify them to see if the variability of gravity would hold together galaxies as they really are.

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Background In 1983 Moti Milgrom proposed a different solution to the “dark matter” problem.Moti Milgrom Estimates of at least 90% of the matter of the whole universe had to be this “dark matter”. Without this matter the galaxies and the universe could not stay together.

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MOND The alternative solution is Modified Newtonian Dynamics [MOND ]. Traditional Newtonian would have a solar system with the velocities of the planets decreasing away from the sun. However, on the macro-galaxy scale, the MOND solution proposes the acceleration of gravity changes at “very large” distances. This is much like the fact that forces on an atomic scale are very different than our scale, requiring a whole new set of laws – quantum dynamics.

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When it applies When the accelerations of a star in orbit in a galaxy are much above a value of 1 angstrom per sec per sec,{a[0]}, which reaches about to the outer edge of our solar system, regular Newtonian dynamics apply. For the larger sizes like the size of a galaxy, the acceleration of gravity g[N] would be: a[0]*mu(x) = g[N], where x = radius. Two commonly assumed forms which are acceptable to galaxy data are MOND has been shown to follow the actual shape of velocity versus radius determinations in more than one hundred galaxies so far. In the following slide the blue line represents a typical Newtonian prediction and the pink line the MOND prediction – which follows a real distribution.

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Simulation of is called V vs R in excel

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N-Body “Galaxy” programs We would find a program in C++ [StarX] that we could run and modify to check out MOND. A main part of a completed project would be Java programs to model the variation in orbital velocity first, and second a small number of stars orbiting a central [black hole] with a Java applet. In these programs we could put the actual parameters from our research to get more realistic models. Original approach

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Excel Formulas SQRT(3/A5)*100 SQRT(SQRT(A5*10))*100 – are the excel formulas for Newtonian Velocity and MOND velocity respectively. A5 if the radius variable and the constants were chosen to have the graphs start near each other and show the exponential parts. Started off with…

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Then …. Starlogo Model Starlogo is used to model a number of galaxy systems with different parameters: 1.Basic solar system orbit purely with Newtonian physics [01] 2.Solar system evolution with constant center of mass [xx] 3.Solar system evolution with variable center of mass [02] [03] 4.“big-bang” with constant gravity [04] 5.“big-bang” with variable gravity [05vg]

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Main hurdles Only a crude model of a “galaxy” with a few stars rotating about a center* The N-body problem has made any precise modeling of a real galaxy impossible to date, even with super- computers, more extensive models may well be difficult to run in Starlogo or Java. However, we still have some success. lack of “space” lack of “bodies”

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Sources – http:// Cosmological models in the relativistic theory of gravitation Physics Demonstrations on (1L) - Gravity : Class Model UA Astronomy - Normal Galaxy Images Surface of Section Cosmology JavaLab [XSTAR] The XStar N-body Solver Galaxies The MOND pages NASA ADS: ADS Home Page [astro-ph/0107284] How Cold Dark Matter Theory Explains Milgrom's Law Search results_MOND 0207469.pdf (application/pdf Object)_equation! MOND_discussion forum http://www2.iap.fr/users/alard/mond/ Modified Newtonian Dynamics and the physics aesthetic PhysicsWeb - homepage PhysicsWeb - Shadow cast on dark matter

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More MOND page background We usually think first in terms of a modification at some length scale: galaxies are big, so maybe gravity is different on large scales. This does not work. But there are other scales which are different about galaxies. One of them is the very low centripetal acceleration experienced by stars orbiting within galaxies. This is just as far removed from our laboratories as is the size scale of galaxies. MOND was motivated by two observations: 1) the asymptotic flatness of rotation curves and 2) the slope of the Tully-Fisher relation (M ~ Vflat4). These two things lead to an acceleration scale: Newton: GM/R = V2 Observed: M = AV4 where A is a constant which holds irrespective of differences in R. Squaring Newton,A is a constantirrespective V4 = (GM/R)2 = M/A A-1 = G2M/R2 = G*(G*surface density) and G*surface density has units of acceleration: A-1 = Ga0

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Newtonian Orbital Theory Equating the force from Newton’s 2 nd law of motion and his Law of Gravitation we get the first equation, then deriving v.

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MOND [material from MOND Pages FAQ] A[0] = 1.2 x 10-10 m s-2, i.e., about one Angstrom per second per second. This is one part in 1011 of what we feel on the surface of the earth. The precise value depends on the distance scale to galaxies, so perhaps it would be better to say a[0] = 1.2 x 10-10 m s-2 h752, where h = H0/75 is the Hubble Constant (the expansion rate of the universe) in units of H0 = 75 km s-1 Mpc-1. (Currently, most measurements report values in the neighborhood of H0 = 72 km s-1 Mpc-1.)

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