REPORT OF DISCUSSION CLUB

SESSION NO.7(A)

Topic:Dark Matter

Presenter:Vidhi Bafna

Date:25th February,2020.

Dark Matter





WHY ARE ASTROPHYSICISTS SO CONVINCED OF THE EXISTENCE OF THIS MYSTERIOUS, INVISIBLE MATTER??





Without dark matter, the universe wouldn’t be the way it is. There isn't enough gravity for the stars and galaxies to rotate at the speed we observe. Since the number of starts are not enough to hold the Milky Way together , it is fortunate for us that there exist extra dark matter that provides an extra force of gravity and keeps the whole Milky Way together. If it weren’t the case there won't be enough force to hold Milky Way together and the stars would fly off into the Cosmos.

The idea behind dark matter is this, there are anomalies in the behaviour and dynamics of galaxies and to understand these anomalies, we suppose that there exists an invisible dark matter which affects the movements of the galaxies, but the origin and nature remain unknown.









THEN WHAT IS DARK MATTER??? We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the universe to make up the 27% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons.Baryons are basically subatomic particles such as protons and neutrons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.





THEORIES BEHIND ITS EXISTENCE (OBSERVATIONAL EVIDENCES): Galaxy rotation curves:

The arms of spiral galaxies rote around the galactic centre. The luminous mass (normal matter) density of a spiral galaxy decreases as one goes from the centre to the outskirts. If luminous mass were all the matter, then we can model the galaxy as a point mass in the centre and test masses orbiting around it. The galaxy rotation curve remains flat as distance from the center increases(indicated by the graph). There is a lot of non-luminous matter (dark matter) in the outskirts of the galaxy.

