Lecture 17 Flashcards
(14 cards)
how can you measure recession velocity
using redshifts - higher recession velocity shifts spectra towards red
what do galaxy spectras show
emission lines (spikes), and absorption lines(dips)
these correspond to specific atomic transitions of hot gas (emission) or cool outer parts of stars (absorption)
can be used as markers to measure redshift
How to do a redshift survey
- map a region of the sky
- for each galaxy, take a spectrum
- identify known features, measure wavelength shift relative to a nearby reference galaxy
- use doppler formula and Hubbles law
- make a 3-d catalogue of right ascension(cosmic longitude), declination(cosmic latitude) and distance(approximate)
why is the distance approximate in redshift surveys
recession velocity includes Hubble flow plus any peculiar velocity along that direction, we have no way to correct for peculiar velocities
this causes an effect called fingers of god
what is the fingers of god effect
dense regions have more gravity causing galaxies to orbit faster causing a higher peculiar velocity
dense groups get broadened in redshifts
what did the first redshift survey find
galaxies are not randomly distributed
evolution of redshift surveys
1st gen - thousands of galaxies. 15th-17th magnitude
2nd - millions, 19th-20th
current - 10’s millions, 22nd magnitude depth
what surveys looked remarkably similar to real redshift surveys
mock surveys based on gravitational collapse
what is the trade off in redshift surveying
can map either large areas to shallow depths or small area to get larger, but fainter, depths
what happens if the universe has a higher density of matter
more matter, more gravity, faster growth, more structure
what happens if the universe had no dark energy
structure can grow early on before dark energy dominated but amount of dark matter and normal matter would have to increase so densities still add to 1
how are large scale structures growth rates measured since the CMB (galaxy clusters, redshift space distortions, weak lensing, voids)
galaxy clustering - the more galaxies clump together, the more structure has grown
redshift space distortions - strength of fingers of god measures growth of the structure
weak lensing - clumped mass distorts light so more distortion means more structure
voids - bigger voids mean more rapid growth
what does the correlation function do
quantifies galaxy clustering, measures the excess probability of finding a galaxy near another
shows different strengths for different cosmologies
