Announcements Monday April 24

1
AnnouncementsMonday April 24

• Exam 4
• Wednesday, April 26
• 25 questions, 3 equations
• Hubble law, redshift, age from Hubble constant
• Covers Chapters 19 (Milky Way Galaxy), 20
  (Galaxies), 21 (Galaxy Evolution), 22 (Cosmology
  I)
• Practice exam will be posted by Sunday evening
• Final exam
• Comprehensive, 150 pts
• Wednesday May 10, 230pm - 430pm
• Review sessions Wed (May 3), Fri (May 5)
• Who wants to be a Millionaire astronomer?
• Bring PRS! Extra credit for correct answers

2
The critical density of the Universe

• (blackboard derivation on Friday)
• rcrit 3H02 /8pG
• H0 is the Hubble constant and G is the universal
  constant of gravitation.
• For H0 70 km/s/Mpc, rcrit 9.2 x 10-27 kg/m3

• Density parameter W
• W r/rc
• r is the total average mass density of Universe
  (including dark matt er, dark energy, ordinary
  matter)

3
The shape of the universe indicates its matter
and energy content.
4
Our three cosmological futures
5
Four possible fates of the Universe

• Which one do observations support?

6
Ordinary (Baryonic) Matter

• Baryonic matter is everything we can detect
  (stars, planets, gas ,dust)
• The density of baryonic matter (dividing mass by
  volume) results in a mass density ? only 4 of
  critical density
• Hence Obaryon 0.04

7
Dark Matter
8
Determining Mass Distribution of a galaxy
Evidence for unseen (dark) matter

• In Spiral Galaxies
• measure the Doppler shift of the 21-cm radio line
  at various radial distances
• construct a rotation curve of the atomic Hydrogen
  gas (beyond visible disk)
• calculate the enclosed mass using Keplers Law

9
Galaxy Rotation Curves are flat at large
distances Evidence of Dark Matter!
NGC 3199 Observed
10
Spiral Galaxies Rotation Curves

• Rotation curves of spirals
• are flat at large distances from their centers
• indicates that (dark) matter is distributed far
  beyond disk

11
Dark Matter Distribution in a Galaxy (?)
12

• Vera Rubin Video

13
Dark matter in elliptical galaxies No gas, use
stellar speeds to estimate mass

• In Elliptical Galaxies
• there is no gas
• measure the average orbital speeds of stars at
  various distances
• use broadened absorption lines
• Results indicate that dark matter lies beyond the
  visible galaxy.
• we can not measure the total amount of dark
  matter, since we can see only the motions of
  stars

14
Mass-to-Light Ratio

• is the mass of a galaxy divided by its
  luminosity.
• we measure both mass M? and luminosity L? in
  Solar units
• Within the orbit of the Sun, M/L 6 M?/ L? for
  the Milky Way
• this is typical for the inner regions of most
  spiral galaxies
• for inner regions of elliptical galaxies, M/L
  10 M?/ L?
• not surprising since ellipticals contain dimmer
  stars
• However, when we include the outer regions of
  galaxies
• M/L increases dramatically
• for entire spirals, M/L can be as high as 50 M?/
  L?
• dwarf galaxies can have even higher M/L
• Thus we conclude that most matter in galaxies are
  not stars.
• the amount of M/L over 6 M?/ L? is the amount of
  dark matter

15
Measuring the mass of clusters of galaxies

• There are three independent ways to measure
  galaxy cluster mass
• measure the speeds and positions of the galaxies
  within the cluster (Zwicky 1930s)
• measure the temperature and distribution of the
  hot gas between the galaxies
• observe how clusters bend light as gravitational
  lenses

All three methods give same answer There is
100x as much mass as can be seen in luminous
matter (stars, gas) gt Dark matter dominates
total mass
16
Method 1 Orbital velocities of galaxies in
cluster

• Velocities of Orbiting Galaxies in clusters
• This method was pioneered by Fritz Zwicky.
• assume the galaxies orbit about the cluster
  center
• measure the orbital velocities of the galaxies
• measure each galaxys distance from the center
• apply Keplers Law to calculate mass of cluster
• Zwicky found huge M/L ratios for clusters.
• his proposals of dark matter were met with
  skepticism in the 1930s

17
Method 2 Use intra-cluster hot gas (emits X-rays)

• Intra-cluster Medium is the hot (107108 K) gas
  between the cluster galaxies
• this gas emits X-rays
• from the X-ray spectrum, we can calculate the
  temperature
• this tells us the average speed of the gas
  particles
• again, we can estimate mass required to keep
  gass confined
• Coma Cluster

Coma cluster of galaxies (optical)
(X-ray)
Mcluster 1015 solar masses (100x mass of stars!)
18
Method 3 Gravitational Lensing

• This is a gravitational lens.
• Einsteins Theory of Relativity states that
  massive objects distort spacetime.
• a massive cluster will bend the path of light
  which approaches it (like a lens)
• the blue arcs are the lensed images of a galaxy
  which is behind the cluster

19
Gravitational lensing

• The angle at which the light is bent depends on
  the mass of the cluster.
• by analyzing lensed images, we can calculate
  cluster mass
• Analysis of gravitational lenses shows dark
  matter dominates cluster mass (100x luminous
  matter)

20

• The cluster masses which are measured by all
  three of these independent methods agree
• M/L for most galaxy clusters is greater than 100
  M?/ L?
• galaxy clusters contain far more mass (10x-100x)
  in dark matter than in stars

21
Dark Matter What is it?
22
What is Dark Matter Made Of?

• Dark matter could be made out of protons,
  neutrons, electrons.
• so-called ordinary matter, the same matter we
  are made up of
• if this is so, then the only thing unusual about
  dark matter is that it is dim
• However, some or all of dark matter could be made
  of particles which we have yet to discover.
• this would find this to be extraordinary matter
• Physicists like to call ordinary matter baryonic
  matter.
• protons neutrons are called baryons
• They call extraordinary matter nonbaryonic matter.

23
An Ordinary Matter Candidate MACHOs

• Our Galactic halo should contain baryonic matter
  which is dark
• low-mass M dwarfs, brown dwarfs, and Jovian-sized
  planets
• they are too faint to be seen at large distances
• they have been called MAssive Compact Halo
  Objects or MACHOs

• We detect them if they pass in front of a star
  where they
• gravitationally lens the stars light
• the star gets much brighter for a few days to
  weeks
• we can measure the MACHOs mass
• These events occur to only one in a million stars
  per year.
• must monitor huge numbers of stars
• Number of MACHOs detected so far does not account
  for the Milky Ways dark matter

24
Dark Matter Non-baryon Candidates

• We have already studied a nonbaryonic form of
  matter
• the neutrinodetected coming from the Sun
• neutrinos interact with other particles through
  only two of the natural forces
• gravity
• weak force (hence we say they are weakly
  interacting)
• their masses are so low speeds so high, they
  will escape the gravitational pull of a
  galaxythey can not account for the dark matter
  observed
• But what if there existed a massive weakly
  interacting particle?
• physicists call them Weakly Interacting Massive
  Particles or WIMPs
• these particles are theoretical they have not
  yet been discovered
• they would be massive enough to exert
  gravitational influence
• they would emit no electromagnetic radiation
  (light) or be bound to any charged matter which
  could emit light
• as weakly interacting particles, they would not
  collapse with a galaxys disk
• yet they would remain gravitationally bound in
  the galaxys halo

25
Dark Matter Summary

• Dark matter total is 10x mass of known
  luminous matter
• But total of DM and ordinary matter is only 30
  of matter needed to stop Hubble expansion
• Evidence for dark matter
• Spiral galaxies flat rotation curves
• Elliptical galaxies Stellar velocities (no gas)
• Galaxy Clusters
• Galaxy speeds
• X-ray halo mass needed for confinement
• Gravitational lensing
• Dark matter what is it?
• Baryonic (ordinary) matter too low MACHO
  searches
• Non-baryonic matter
• Neutrinos? Probably not (mass too low, speed too
  high)
• WIMPs (weakly interacting massive particles) -
  maybe

26
Dark Energy The other 70 of the mass-energy
of the Universe?
27
Observations of distant supernovae indicate that
we live in an accelerating universe.
28
Supernovae of type Ia (white dwarf in binary
system) all have (nearly) the same peak
luminosity Can be used to measure distance
29
Evidence for Dark Energy (negative pressure of
the vacuum) comes from observations of distant
exploding stars (supernovae)
30
Evidence of acceleration from supernova
observations
Universes Fate Accelerating!
Current Observations
31
What causes this cosmic acceleration?
Key idea The vacuum isnt empty It has (a lot
of) enrgy and negative pressure (causes
acceleration)
32
(No Transcript)
33
An accelerating Universe
34
Future SNAP telescope (can detect much fainter
Supernovae). Possible launch 2012
35
Dark Energy Summary

• Dark energy Total is 70 of mass-energy of
  Universe
• By including dark energy, O 1 (Universe is flat)
• The long-term future of Universe is acceleration
• Evidence for dark energy
• Observation of distant Type Ia supernova bursts
• Measure brightness, redshift, get distance
• Plot on Hubble plot no longer linear
  acceleration
• Observation of CMB
• Fluctuations indicate Ototal 1
• Dark energy what is it?
• Most likely negative pressure of vacuum an
  intrinsic property
• This was already predicted in Einsteins General
  theory of relativity (cosmological constant ?)!

36
(No Transcript)
37
Another cosmological standard model.