DARK MATTER IN GALAXIES. Deqian Yuan

Transcription

1 DARK MATTER IN GALAXIES Deqian Yuan

2 INTRODUCTION Dark Matter(From Merriam Webster): Nonluminous matter not yet directly detected by astronomers that is hypothesized to exist to account for various observed gravitational effects. Dark : Dark matter cannot be seen directly with telescopes; because it cannot emit or absorb any light or other form of electromagnetic radiation. Simply not reactant to light. The only way to confirm its existence is its gravitational effects.

3 DARK MATTER

4 HISTORY OF DISCOVERY 1. In 1932, Jan Oort postulated that there should be some missing mass in the universe because the orbital velocities of the stars in the Milky Way would be way different from the measured value if there is no Dark Mass in universe. 2. In 1933, Fritz Zwicky posted a similar viewpoint giving the orbital speed of galaxies in clusters behave the same way which also provides the evidence of the existence of dark matter.

5 TRADITIONAL NEWTONIAN PREDICTION From Traditional Newtonian Dynamics: F=G(M*m)/r 2 =mv 2 /r V 2 =GM/r Traditional Newtonian Theories actually predict that the velocity will decrease as the object is further away from the center of the orbit.

6 OORT S OBSERVATION Blue line: Predicted movements of stars versus the distance from galaxy center Red line: Measured value of the speed.

7 DARK MATTER HYPOTHESIS Astrophysicists make the hypothesis that dark matter does exist due to discrepancies between the mass of large astronomical objects determined from their gravitational effects. According to consensus among cosmologists, dark matter is composed primarily of a not yet characterized type of subatomic particle which interact only through gravity and weak force. (WIMPs) Again, this is yet a hypothesis. Alternative explanations have been proposed, we still cannot decide which is right.

8 ALTERNATIVE THEORIES Modified Newtonian Dynamics(MOND): Proposed by Mordehai Milgrom in 1983 Newton's law for gravitational force has been verified only where gravitational acceleration is large For extremely small accelerations the theory may not hold

9 ALTERNATIVE THEORIES Tensor vector scalar gravity (TeVeS): Developed by Jacob Bekenstein. Adjustments made to MoND theories. TeVeS reproduces the MoND acceleration formula. Accommodate gravitational lensing. Respects conservation laws.

10 OORT S OBSERVATION In 1932, Jan Oort was studying stellar motions in the local galactic neighbourhood and measuring their velocities by examining the Doppler Shifts of the spectrum of stars. The result showed that the actual velocities were way more than expected. Then he proposed that there should be other sources that provide gravitational force to affect the movements of the stars. later. Swiss astrophysicist Zwicky made a similar interference Jan Oort( )

11 OORT S OBSERVATION A typical star's tangential velocity as a function of its distance from the galactic center.

12 GRAVITY LENSING

13 UNIVERSES INGREDIENTS Dark Energy: A special form of potential energy that fills everywhere in universe. It will accelerate the expansion velocity of the universe.

14 OBSERVATION Being dark, Dark Matter can only be detected by gravity methods. Assume it is made up with WIMPs, which we are able to detect. The majority of present experiments use one of two detector technologies: Cryogenic Detectors, operating at temperatures below 100mK, detect the heat produced when a particle hits an atom in a crystal absorber. Noble Liquid Detectors detect the scintillation light produced by particle collisions of noble liquid.

15 FORMS OF DARK MATTER There are 3 forms of dark matter. 1. Cold: Objects with a free-streaming length much smaller than a protogalaxy; 2. Warm: Particles with a free-streaming length similar to a protogalaxy; 3. Hot: particles with a free-streaming length much larger than a protogalaxy.

16 REFERENCES 1. First observational evidence of dark matter The Distribution and kinematics of neutral hydrogen in spiral galaxies of various morphological types. Rijksuniversiteit Groningen.

17 THANK YOU Good luck with the midterms and enjoy the spring break!