Lecture 17: The Expanding Universe

Question 1

Redshift of Galaxies and Distances

Answer 1

• form of doppler shift for galaxies at large distances
• Large redshift seen at large distances
• Hubble Diagram is often plot of redshift vs distance
• Hubble diagram is plot of redshift vs distance

Question 2

Galaxy Masses

Answer 2

• Rotation curve works well for nearby edge-on galaxies
• best measured form gas
• mass of luminous matter is not enough to match rotation curve
• not applicable to faint or elliptical galaxies
• Dark matter is in a large volume around galaxy: A dark matter halo
• velocity dispersion measures the spread in velocities
• velocity dispersion depends on mass
• we can measure the rate at which galaxies rotate from doppler shift, but rotation rate is determined by mass of galaxy
• Flat rotation curve means mass increase with distance
• mass of luminous matter is not enough to match rotation curve (dark matter in large volume)

Question 3

Physical properties of galaxies

Answer 3

• Mass to light ratio has small range, also related to type of galaxy
• Elliptical galaxies have the highest mass to light ratio, also most dark matter
• Irregulars have smallest mass to light ratio
• Black hole masses related to mass of galaxy

Question 4

Galaxy Clusters

Answer 4

• Found in groups, clusters, and superclusters
• Milky way and potentially 100 other galaxies located in the local group

Question 5

Measuring the Mass of a cluster

Answer 5

• measure the speeds and positions of the galaxies within the cluster
• measure the temperature and distribution of the hot gas between the galaxies
• observe how clusters bend light as gravitational lenses
• Pairs of galaxies in orbit around each other have no transverse motion seen, only radial velocity

Question 6

Measuring the Mass of a Cluster: Galaxy Orbits

Answer 6

• Assume the galaxies orbit about the cluster center
• Observe Orbital velocities and distance from centre to use Kepler’s third law to find mass

Question 7

Measuring the Mass of a Cluster: Hot gas between galaxies, and intracluster medium

Answer 7

• hot gas between galaxies emits x-rays
• from the x-ray spectrum, we can calculate the temperature
• temp tells us average speed of the gas particles
• we can estimate the mass required to retain the hot gas

Question 8

Measuring the Mass of a Cluster: Gravitational Lensing

Answer 8

• The angle at which the light is bent depends on the mass of the cluster
• by analyzing lensed images, we can calculate cluster mass
• All previous methods for finding mass depended on newton’s law of gravity
• this method uses a different model of gravity
• every one galaxy in the foreground can act as a lens for galaxies in the background

Question 9

Galaxy and Galaxy Cluster Masses: Dark Matter

Answer 9

• M/L for for galaxy clusters is greater than 100 M/L for the sun
• galaxy clusters contain far more mass in dark matter than in stars
• this is even greater than the factor of 10 found for individual galaxies
• could be because we don’t understand gravity on galaxy scales
• could be because the observed velocities are caused by the gravitational attraction of unseen matter: dark matter

Question 10

Hubble Relation and the Expanding Universe

Answer 10

• Hubble observed that all galaxies are moving away fro milky way
• distance between galaxies is increasing
• Hubble Constant is different depending on measurements
• Depends on distance from us, closer to us, larger the Hubble constant, called the Hubble tension
• Scale of universe is growing, not galaxies expanding

Question 11

The expansion of the universe

Answer 11

• Expansion should get slower over time because of gravity
• Expansion must have been faster in the past

Question 12

The Big Bang

Answer 12

• At some time the universe started out very small and expanding at a high rate
• Similar to an explosion
• Would have been very hot at the beginning and has cooled as it expanded

Question 13

Critical Density

Answer 13

• Gravitational attraction between galaxies can overcome the expansion of the universe in localized regions
• For there to be enough gravity to stop the expansion of the entire universe there would need to be a certain mass density of the universe
• if this critical density is hit, the universe will begin to contract
• all of the luminous matter we observe only accounts for less than 4% of critical density
• Research suggests the universe will expand forever

Question 14

Does gravity alone influence expansion?

Answer 14

• the further the galaxy, the faster it should be receding if the universe is slowing down
• However, high redshift supernova are moving slower than models for an expanding constant velocity empty universe predict
• must be dark energy, a force that repels the galaxies

Question 15

Models of the future universe

Answer 15

• Recollapsing universe where the expansion will someday halt and reverse
• Critical Universe where the universe will expand slower and slower with time, but not collapse
• Coasting universe where the universe will expand forever with little slowdown
• Accelerating universe where the expansion will accelerate with time