Lecture 10 Flashcards
two methods for weighting galaxies
1: measure brightness, assume galaxies are mostly sun-like stars, convert brightness to equivalent mass
2: measure gravity, measure orbital velocity of material, use newtons law of gravity and centripetal force
how to measure orbital velocity of a galaxy
measure spectrum at each point, find redshift, convert to velocities
compare spectrum at 3 points
get redshifts
issue with the 2 methods to weigh a galaxy?
every known galaxy the measurements disagree,
approx grav mass ~ 10 x stellar mass
more careful measurements including a range of star masses, gas and dust mass comes to the same conclusion
what are the 3 rotation curve types
solid - constant orbital period
keplerian - orbiting enclosed mass
differential - orbiting inside constant density field
what are rotation curves always
flat - implies mass inside radius keeps increasing well beyond visible radius of galaxies
rotation curve conclusions
dark matter dominates the gravitation mass in outer regions, ~90% of total mass is dark
galaxies embedded into much larger invisible halo of matter
what would happen to galaxies if dark matter didn’t exist
galaxies spin so fast they would fly apart if only gravity from stars and gas held them together
how can we estimate galaxy clusters mass
random motions from the random orbits of galaxies
what are the most dark matter dominated parts of the universe
galaxy clusters - 95% whereas galaxies are 90%
what are the normal and exotic candidates for dark matter
exotic - Weakly Interacting Massive Particles (WIMPs)
normal - MAssive Compact Halo Objects (MACHOs)
what are MACHOS
large numbers of massive but small objects surrounding galaxies
too faint to see individually but collectively proposed to form dark matter
what would MACHOS cause e.g. issues with the theory
would cause frequent microlensing when it moves infront of a star or galaxy
these have been detected but not large enough scale to be dark matter
so DM is not primarily MACHOs
what are wimps
some wimps exist - neutrinos, but too small and fast to be dark matter
particle accelerator experiments have created particles so creating another isn’t surprising
but needs to be a different mass regime
current explanation for DM
how is direct detection looking for DM
particle accelerators and direct detectors looking for rare interactions with ordinary matter causing flashes or tiny heat increases
can have solid detectors made of germanium or liquid Nobel gas ones
how is astrophysical detection looking for DM
looked for effects of possible DM particle types on light from distant quasars
or glow from self interacting dm in core of galaxy on cmb maps
some have been ruled out, some not compelling as too hard to separate from the known phemonema
issues with direct detection?
no detections after decades
dark matter alternative for modified newton laws
- lots of things in physics seems to go wrong when accelerations are small.
- a0 = 10^-10 m/s^2 is the scale of the gravitational force due to stars at the edges of galaxies
- dark energy has effect
- milgrom proposed a0 was a fundamental physical constant and developed modified newtonian dynamics
what is MOND (modified newtons dynamics)
rather than changing universe, change 2nd law to include specifics for small accelerations
since we know relativity is needed, when you combine MOND and relativity weird things happen and need new energy fields doing strange things
MOND fails in galaxy clusters as gets known motions very wrong
dark matter alternative: primordial BH
very small window possible for black holes formed in early universe to be dark matter, need to find some exotic mechanism to form them as cant be from regular stars
