PHYS 183 Lecture Notes - Baryon, Gravitational Lens, Bullet Cluster

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Published on 12 Apr 2013
School
McGill University
Department
Physics
Course
PHYS 183
Professor
PHYS 183 The Milky Way Inside and Out Tracy Webb Winter 2013
Lecture 33: April 10th, 2013
general relativity: space is curved by the presence of mass
if you have a lot of mass that is warping space through light, then it changes what we see
a cluster’s gravity bends light from a single galaxy so that it reaches earth from multiple directions
you can therefore see multiple images of what is really a single galaxy
this gives us another way to measure the mass using the velocities of galaxies
the more mass = the more lensing
existence of dark matter we know it exists on large scales
we have 2 options about it
o dark matter exists and we observe the effects of its gravitational attraction
o dark matter doesn’t exist and we don’t understand gravity
we think we understand gravity well, so we choose option 1, which is Occam’s Razor
The Bullet Cluster is 2 different clusters merging together
we still do not know what dark matter is
baryonic matter is a fancy word for regular matter, it is normal stuff that makes everything up
we thought that dark matter was baryonic matter, just very faint
but we found that it cannot be normal matter, because the normal matter would still have to give
gravitational lensing
micro lensing events: a way to detect dark objects in our galaxy
weakly interacting massive particles (WIMPS) is the current best guess for what dark matter is
(non-baryonic matter)
dark matter dominates structure formation
o dark matter & gas collapse together
o dark matter can’t radiate away its energy
o it is left behind in giant halos
o dark matter doesn’t interact with light and doesn’t have electrons to change energy levels,
so it can’t radiate its energy away
o models show that gravity of dark matter pulls mass into denser regions the universe
therefore grows more lumpy with time
the fate of the universe
o does the universe have enough kinetic energy to escape its own gravitational pull?
o you have the combined gravitational pull of everything in the universe that are pulling
back on each other
o it was thought that this should be enough to halt the gravitational pull and reverse it so
that the universe will shrink again
o this is called the big crunch (vs. the big bang)
o so the fate of the universe depends on the amount of dark matter
o the density of matter required to close the universe is known as critical density
o we actually think that we are living in a coasting universe, where it continually expands
o the amount of dark matter is ~25% of the critical density
o this suggests eternal expansion
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Document Summary

Phys 183 the milky way inside and out tracy webb. General relativity: space is curved by the presence of mass. You can therefore see multiple images of what is really a single galaxy. Existence of dark matter we know it exists on large scales. We think we understand gravity well, so we choose option 1, which is occam"s razor. The bullet cluster is 2 different clusters merging together. We still do not know what dark matter is. Baryonic matter is a fancy word for regular matter, it is normal stuff that makes everything up. We thought that dark matter was baryonic matter, just very faint. But we found that it cannot be normal matter, because the normal matter would still have to give gravitational lensing. Micro lensing events: a way to detect dark objects in our galaxy. Weakly interacting massive particles (wimps) is the current best guess for what dark matter is (non-baryonic matter)

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