L11 dark stuff

dark matter

an undetected form of mass that does not interact with light, but whose existence we infer from its gravitational influence on observed motions of stars and gas clouds

dark energy

an unknown form of energy that seems to cause a repulsive force accelerating the Universe’s expansion

current best model of BB and cosmology is called

lambda CDM

Lambda CDM meaning

Cold - moving at nonrelativistic speeds

Dark - does not emit electromagnetic radiation (light)

Matter - we see it through its gravitational pull, like normal masses

Lambda - cosmological constant associated with dark energy

overall geometry of universe is

flat (critical density possessed by total mass + energy)

composition of universe percentages

5% baryons, 27% dark matter, 68% dark energy

baryons

“normal” matter e.g. protons and neutrons

vera rubin’s findings

rotation curves of spiral galaxies are flat; outermost components move as quickly as those close to the center, indicating large amounts of dark matter

how to measure galaxy rotation

doppler shift of atomic hydrogen’s 21cm line

elliptical galaxy

ellipsoidal or spherical shape with no well organized disk, composed of older stars with little new star formation; believed to form from merger of smaller galaxies

orbital speeds in elliptical galaxies

while randomized, they still depend on the mass within their orbit, and these galaxies also have dark matter

we can tell how fast stars are orbiting through

broadening of spectral lines

dark matter halo

dark matter appears to be contained in a sphere around a galaxy

dark matter is collisionless, meaning it

doesn’t lose energy through friction or gas shocks

galaxy clusters

massive and bound by gravity, contain galaxies + gas + dark matter

% composition of galaxy clusters

85% dark matter, 13% hot gas, 2% stars

fritz zwicky found that

orbits of galaxies around a gravitational center disagreed with their apparent baryonic mass content

galaxy cluster motion

can be measured from doppler shifts, and the mass we find from these motions is about 40 times larger than the mass in stars

hot gas in galaxy clusters

10x star mass in hot, x-ray emitting gas in an equilibrium where gas pressure balances gravity

temperature is the KE of particles, which is determined by

their gravitational potential (and therefore the mass of the cluster)

gravitational lensing

cluster’s gravity bends light from a single galaxy so that it reaches Earth from multiple directions; direct measure of mass

einstein ring

gravitational lensing with the exact alignment of the source, lens, and observer

bullet cluster

two colliding galaxy clusters, separating the x-ray emitting hot gas from the bulk of stars and galaxies

options for existence of dark matter

either it really exists and we are observing its effects, or something is wrong with our understanding of gravity causing a mistaken inference

properties of dark matter that theory must fit

interacts strongly via gravity and maybe weak force, does not emit or interact with detectable light, cold (non-relativistic), stable (on the timescales of the universe)

ordinary dark matter

made of protons, neutrons, and electrons, but too dark to detect with current instruments

extraordinary dark matter

new types of particles (weakly interacting massive particles, axions or axion-like, sterile neutrino) and primordial black holes

ordinary matter possibility

unlikely, but would be if ejected into the outer reaches of a galaxy and are dim e.g. planets, dwarfs, black holes - a massive compact halo object

most dark matter is unlikely to be ordinary matter because

large, dark, compact objects seem to be rare and % of the observed elements do not match this theory

big bang nucleosynthesis predicts

observed abundance of elements

based on density of matter we can predict

how much deuterium should be left over from when protons and neutrons formed it in the early universe

if we were 100% density of ordinary matter, abundance of deuterium would be

very low, mostly fusing into helium

deuterium we have measured indicates

we have around 5% density of ordinary matter, so dark matter is unlikely to be made of baryons

wimps could come from

big bang; if produced back then at theorized mass range, they would naturally exist in the abundance to explain dark matter, but we haven’t detected them yet

axions

hypothetical elementary particles, become more popular due to non-detection of WIMPs, theory that explains strong force violation

weakly interacting massive particle (wimp)

e.g. a neutrino; dark by nature, no electrical charge or radiation, only interact via gravity and weak force - neutrinos move too fast but similar, heavier particles could exist

sterile neutrino

hypothetical particle that interacts only via gravity, theorized to explain neutrino masses in the standard model

primordial black holes

hypothetical black holes that formed soon after BB; collisionless, stable (as long as they are large enough to avoid evaporation on universe’s timescale), and nonrelativistic

how to detect dark matter

indirect detection, direct detection, observe effects through large scale structure surveys

indirect detection

measuring the expected annihilation or decay products expected from dark matter candidates

direct detection

trying to detect the constituent components of dark matter themselves

dark matter detectors are located

underground to minimize background signals

axion dark matter experiment

axions should weakly convert to photons in presence of a large magnetic field, producing a very small signal from the interaction between possible axions in the milky way dark matter halo and the magnetic field in the cavity

protogalactic clouds in early times contracted because of

dark matter’s gravity

universe grows lumpier with time because

gravity of dark matter pulls mass into denser regions

large scale structure of universe is made up of

superclusters and voids

dark matter is cold because

hot dark matter would smear out large scale structure