Astro 105: Our Place in the Universe

1
Astro 105 Our Place in the Universe
Lecture 14

• Lecturers
• J.P.Ostriker
• A.E.Shapley
• J.E. Gunn
• P. Steinhardt

2
Logistics

• Assignment 5 due today!
• Solution set 4 is posted on the course website
  and on Blackboard, so no more late assignment
  4s accepted.
• No homework over Thanksgiving (except to think
  about the expansion and geometry of the Universe.
  Yeah, right.)

3
Overview

• Review of CMB highlights, some questions that
  arose last time
• How do ripples in the CMB tell you about the
  geometry of the Universe? (Finish up cosmological
  parameters)
• How are the fluctuations in the CMB radiation
  related to the rich structure (galaxies,
  clusters) we see around us today?
• Attempts to map out the distribution of galaxies
  in the local and distant Universe
  (distantlooking back in time)
• Galaxy formation and evolution

4
Review

• Hot Big Bang Model (LeMaitre, Gamow, Alpher,
  Herman)
• Blackbody radiation (Universe as a blackbody)
• When Universe cools to 3000K, it is no longer
  opaque, photons can get to us. Radiation that is
  coming from so far away that it probes this
  Recombination epoch, said to come from the
  Surface of Last Scattering
• Search for CMB radiation (Penzias, Wilson,
  Dicke, Wilkinson, Roll, Peebles)
• COBE (FIRAS told us it was a blackbody spectrum,
  DMR showed the anisotropies of 1 part in 100,000)
• WMAP, current state of the art CMB mission, maps
  the surface of last scattering with much higher
  spatial resolution, allows for exquisite study of
  the characteristic size of the ripples

5
Side note on the Big Bang
Last time, the balloon analogy was used to show
the expanding Universe. Note this is a 2D
analogy for 3D space. Where is the center of the
explosion on the surface of the balloon? (not the
center of the balloon!!!!!) There is no center of
the Big Bang explosion -- every galaxy sees the
same thing.
At t0, space is infinitely dense
EVERYWHERE!! Not just at one singularity. Dont
think of it like a bomb exploding.
6
The Hot Big Bang Model

• George Gamow, Ralph Alpher, and Robert Herman
• Predicted elemental abundances formed in Hot Big
  Bang, and also that the Universe should be filled
  with relic radiation corresponding to a blackbody
  at 5K

7
Review

• Hot Big Bang Model (LeMaitre, Gamow, Alpher,
  Herman)
• Blackbody radiation (Universe as a blackbody)
• When Universe cools to 3000K, it is no longer
  opaque, photons can get to us. Radiation that is
  coming from so far away that it probes this
  Recombination epoch, said to come from the
  Surface of Last Scattering
• Search for CMB radiation (Penzias, Wilson,
  Dicke, Wilkinson, Roll, Peebles)
• COBE (FIRAS told us it was a blackbody spectrum,
  DMR showed the anisotropies of 1 part in 100,000)
• WMAP, current state of the art CMB mission, maps
  the surface of last scattering with much higher
  spatial resolution, allows for exquisite study of
  the characteristic size of the ripples

8
Blackbody Radiation

• Radiation emitted by opaque surface that absorbs
  all the light that falls on it, and is
  characterized by a certain temperature, T.
• Temperature measures the amount of microscopic
  motion within a system.
• Object at temperature, T, emits a very specific
  spectrum, that is a function of the temperature,
  T, and the wavelength, l.
• Spectrum has a peak at a characteristic
  wavelength, which is bluer for higher
  temperatures and redder for lower temperatures

9
Review

• Hot Big Bang Model (LeMaitre, Gamow, Alpher,
  Herman)
• Blackbody radiation (Universe as a blackbody)
• When Universe cools to 3000K, it is no longer
  opaque, photons can get to us. Radiation that is
  coming from so far away that it probes this
  Recombination epoch, said to come from the
  Surface of Last Scattering
• Search for CMB radiation (Penzias, Wilson,
  Dicke, Wilkinson, Roll, Peebles)
• COBE (FIRAS told us it was a blackbody spectrum,
  DMR showed the anisotropies of 1 part in 100,000)
• WMAP, current state of the art CMB mission, maps
  the surface of last scattering with much higher
  spatial resolution, allows for exquisite study of
  the characteristic size of the ripples

10
Surface of Last Scattering

• The key is that something important happens when
  the Universe cools to 3000K, which is about
  300,000 years after the Big Bang.
• Protons and electrons combine to form atoms, and
  photons no longer scatter off of electrons. The
  primeval fireball cools to the point that it is
  no longer opaque.
• Photons can free stream to us, from z1100.
  Along the way they redshift, and reflect the
  overall expansion and cooling of the universe.
• These photons make up the microwave background
  radiation.
• We cant see farther back than z1100, because
  the Universe was opaque.

11
Review

• Hot Big Bang Model (LeMaitre, Gamow, Alpher,
  Herman)
• Blackbody radiation (Universe as a blackbody)
• When Universe cools to 3000K, it is no longer
  opaque, photons can get to us. Radiation that is
  coming from so far away that it probes this
  Recombination epoch, said to come from the
  Surface of Last Scattering
• Search for CMB radiation (Penzias, Wilson,
  Dicke, Wilkinson, Roll, Peebles)
• COBE (FIRAS told us it was a blackbody spectrum,
  DMR showed the anisotropies of 1 part in 100,000)
• WMAP, current state of the art CMB mission, maps
  the surface of last scattering with much higher
  spatial resolution, allows for exquisite study of
  the characteristic size of the ripples

12
Detection of CMB

• Dicke, Wilkinson, Roll, Peebles at Princeton,
  both experimentalists and theorists
• Penzias and Wilson at Bell Labs
• 1964-1965, Detection of excess noise, turned out
  to be CMB radiation, fit with predictions
• What exactly did they detect (what does 3K noise
  mean)??

13
Detection of CMB

• Radio antenna collects radio waves (which are
  light waves with long wavelengths, longer than 1
  mm)
• Examples of antennae on TVs, radio dishes,
  horns
• Radio waves sent to a receiver which records
  the strength of the radio signal
• CMB detection was a certain flux of radio waves.
  If you have a blackbody at 3K, you know exactly
  what flux to detect at each wavelength, according
  to the BB spectrum.

14
Big Bang Nucleosynthesis

• One of the predictions of the Big Bang model is
  that, for a given baryon density, Wb, the
  primordial abundances of Helium, Deuterium, and
  Lithium are all fixed (reaction rates depend on
  Wb).
• The predictions of the Big Bang model for the
  abundances of these light elements are consistent
  with the observed values, for a single Wb (value
  of Wb is consistent with other estimates)

Major success of Big Bang Model
15
Big Bang Nucleosynthesis

• What is deuterium?
• Heavy hydrogen, nucleus with one proton and
  one neutron
• Produced in Big Bang along with helium and
  lithium, and not made anywhere else.
• In fact it is destroyed in stars
• The amount of deuterium produced in the Big bang
  is very sensitive to the density of baryons
• Higher baryon density -- less deuterium, which
  gets converted to helium
• Measurements made in intergalactic gas at high
  redshift

Major success of Big Bang Model
16
Review

• Hot Big Bang Model (LeMaitre, Gamow, Alpher,
  Herman)
• Blackbody radiation (Universe as a blackbody)
• When Universe cools to 3000K, it is no longer
  opaque, photons can get to us. Radiation that is
  coming from so far away that it probes this
  Recombination epoch, said to come from the
  Surface of Last Scattering
• Search for CMB radiation (Penzias, Wilson,
  Dicke, Wilkinson, Roll, Peebles)
• COBE (FIRAS told us it was a blackbody spectrum,
  DMR showed the anisotropies of 1 part in 100,000)
• WMAP, current state of the art CMB mission, maps
  the surface of last scattering with much higher
  spatial resolution, allows for exquisite study of
  the characteristic size of the ripples

17
2 Big Results From COBE

• FIRAS showed that the spectrum of the CMB
  radiation was that of a blackbody with T2.725K
  (consistent with prediction that Universe cooled
  from initial hot big bang).
• DMR showed fluctuations of 1 part in 100,000
  (once the motion of the sun, and the emission
  from the galaxy are subtracted off) The
  fluctuations are regions slightly hotter or
  colder than the average temperature of 2.725.

18
What are we looking at?

• This is an all-sky map in Galactic coordinates
  from COBE.
• The plane of our Milky Way runs horizontally
  across the middle of the map.
• Top map shows the solar system motion in the
  Universe (we are not at rest wrt the CMB in every
  direction) middle map shows our Galaxy bottom
  map shows density fluctuations in the early
  Universe (denser-- intrinsically hotter, less
  dense-- intrinsically colder), but gravitational
  redshift reverses it
• This is a map of the surface of last scattering.
  These photons have traveled to us since the
  Universe was a few 100,000 yrs old. Remember

19
Review

• Hot Big Bang Model (LeMaitre, Gamow, Alpher,
  Herman)
• Blackbody radiation (Universe as a blackbody)
• When Universe cools to 3000K, it is no longer
  opaque, photons can get to us. Radiation that is
  coming from so far away that it probes this
  Recombination epoch, said to come from the
  Surface of Last Scattering
• Search for CMB radiation (Penzias, Wilson,
  Dicke, Wilkinson, Roll, Peebles)
• COBE (FIRAS told us it was a blackbody spectrum,
  DMR showed the anisotropies of 1 part in 100,000)
• WMAP, current state of the art CMB mission, maps
  the surface of last scattering with much higher
  spatial resolution, allows for exquisite study of
  the characteristic size of the ripples

20
Current State of the Art WMAP

• Wilkinson Microwave Anisotropy Probe
  (NASA/Goddard/Princeton), launched in 2001, still
  collecting data, named for Princeton physicist
  David Wilkinson
• All-sky map, but at much higher angular
  resolution than COBE (sharper image)
• Allows us to answer questions about the geometry
  of the Universe (as well as a bunch of other
  parameters). Is it closed, flat, or open?
• COBEs angular resolution was about 7 degrees
  (14 times the diameter of the full moon), while
  WMAP has resolution of 0.3 degrees (a bit less
  than the diameter of the full moon)

21
Current State of the Art WMAP
Resolution more than 20 times better with WMAP
22
A Characteristic Scale

• It turns out that there is a specific angular
  scale (hot/cold patch size), on which we expect
  to see the most contrast. This scale corresponds
  to the angular size that the observable Universe
  at z1100 subtends on the sky today (about 1
  degree)
• Figuring out the exact angular size where we see
  the most variation tells us about the global
  geometry of the Universe (closed/flat/open)

23
A Characteristic Scale
24
A Characteristic Scale Geometry

• Hopefully, you can see how the characteristic
  size of the pattern depends on the overall
  geometry of space
• When the Universe is closed, the pattern subtends
  a larger angular size (like the angles of the
  triangle adding up to more than 180 degrees)
  when the Universe is open, the pattern subtends a
  smaller angular size for a flat universe, the
  pattern will subtend a size that is intermediate.
• The verdict from BOOmerang and WMAP The Universe
  is spatially flat!!
• This means WmWL 1
• Next big mission Planck, to be launched in 2007.

25
A Characteristic Scale Geometry

• Weve already made a pretty good case for Wm0.3.
• The CMB therefore offers independent evidence
  that WL0, and yields a value (0.7) consistent
  with the results from Type Ia Supernovae and the
  acceleration of the Universe.
• This is amazing the CMB anisotropies and the
  measurements of distances and redshifts of
  supernovae are two completely indpendent methods
  that provide evidence for a spatially flat
  Universe with non-zero cosmological constant and
  less than critical matter density.

26
Working Timeline since BB
27
Evidence for BB Model

• Galaxies are receding from each other, the
  expansion of the universe
• Success of the Hot Big Bang model for producing
  the relative abundances of light elements like
  Helium, Deuterium, and Lithium
• Detection of 3K blackbody CMB radiation

28
The Ripples (Anisotropies)

• Weve discussed how the temperature anisotropies
  correspond to density fluctuations in the
  Universe at z1100.
• Two important questions about the anisotropies
• 1. What caused the initial fluctuations in
  density?
• How do the fluctuations evolve?
• Or, more generally how do we go from a
  Universe that appears so simple and smooth (the
  maps enhance the fluctuations) to one that
  contains so much structure?
• Mapping the distribution of galaxies and
  comparing with simulations helps us understand
  2., at least.

29
The Ripples (Anisotropies)

• Temperature fluctuations in the early universe,
  from the slight clumping of matter (map on
  surface of last scattering)
• Structure growing as denser regions attract more
  matter, seeded from the early fluctuations
• The first stars form in the regions of highest
  density
• More stars turning on, galaxies forming
• The modern era

30
Ideas of structure formation
31
Structure Formation

• While we believe that on the largest scales is
  homogeneous and isotropic, the distribution
  of matter in the Universe is not completely
  uniform
• Our galaxy lies in what is called the Local
  Group, with M31 (falling towards us), the Large
  and Small Magellanic Clouds, and lots of other
  small galaxies
• Already in the time of Hubble and Zwicky, we
  knew that galaxies were also found in groups and
  clusters. Nearby clusters include the Virgo
  Cluster (hundreds, maybe thousands of galaxies),
  and the Coma cluster. Clusters are grouped in
  even larger structures.
• Hierarchical clustering.
• Deep Thoughts Questions What came first? The
  galaxies or the clusters? When did the galaxies
  start to form?

32
Structure Formation
Northern galactic hemisphere

• Lick Observatory Counts of galaxies in 10
  arcminute by 10 arcminute cells (1967)
• No redshifts, but still the clustering is clear,
  started the theorists (i.e. Peebles) thinking

33
Structure Formation

• 1960-1980s
• Bottom-up structure formation (Peebles) begin
  with small objects that form first, which then
  merge to form larger objects
• Top-down structure formation (Zeldovich, Silk)
  structure forms first on the largest scales
  pancakes (supercluster and cluster), which then
  fragment into smaller objects
• It turns out that the nature of dark matter
  affects what type of structure formation occurs
• If we have hot dark matter, i.e. matter that
  moves at close to the speed of light at z1000,
  the idea is that all the small-scale fluctuations
  get washed out early in the Universe by the
  relativistic motions of the particles, and you
  only get large-scale clumps. One such example is
  the neutrino.
• If we have cold dark matter, matter that moves
  non-relativistically in the early universe,
  small-scale fluctuations dont get destroyed

34
Structure Formation
z0, i.e. today

• CDM simulation, more clustering of objects on
  smaller scales (small clumps)
• 300 Mpc on a side, slice of simulation with dark
  matter

• HDM simulation, less clustering of objects on
  smaller scales (small clumps)
• 300 Mpc on a side, slice of simulation with dark
  matter

(courtesy, M. White)
35
Structure Formation

• We now know that Hot Dark Matter doesnt work.
  Galaxies form too late (we have now observed
  galaxies, which correspond to small-scale
  fluctuations) out to high redshift. In HDM model,
  these dont form until zcorrelations observed today among the positions
  of galaxies mapped out in redshift surveys dont
  match the predictions for HDM
• Cold Dark Matter with bottom-up structure
  formation is the favored model.
• BUT in the 1960s and 1970s, we didnt have a
  detailed map of the galaxies and (by inference)
  the underlying dark matter distribution, to test
  the models.
• Therefore, an important goal was to map out the
  3D large-scale structure in the Universe. This
  large-scale structure is the descendant of the
  density ripples revealed in the CMB.

36
Galaxy Redshift Surveys Mapping the Universe
37
Galaxy Redshift Surveys

• As of 1970s, there were only 250 published
  redshifts of galaxies, mainly from Humason,
  Mayall, and Sandage
• (Side note remember from Jim Gunns lecture and
  lecture on the expansion of the Universe, Sandage
  was Hubbles student (heir), has spent his lifes
  work trying to determine Hubble Constant and
  deceleration of the Universe.)
• Why so few redshifts?
• Redshifts were HARD!! As opposed to images,
  where all the wavelengths of light that emanate
  from a given galaxy location expose the same
  corresponding location on the photographic plate.
  With spectra (required for redshifts), the light
  gets spread out into all of its component
  wavelengths. So, any specific place on the
  photographic plate receives less light.
• Goal of redshift surveys use Hubbles law to
  approximate distances with redshifts. Make a 3D
  map of the galaxies.

38
CfA Redshift Surveys

• Mid-Late 1970s Marc Davis, Princeton graduate
  student of Wilkinson and Peebles, tries to
  calculate how galaxies should be clustered in
  space, goes to Harvard Center for Astrophysics
  (CfA) as an assistant professor in the late
  1970s, embarks on a survey of galaxies, along
  with Dave Latham, John Huchra, and grad student
  John Tonry
• Use a new detector for measuring spectra (not
  photographic plates), but not CCDs yet

Davis
Latham
Huchra
Tonry
Geller
39
CfA Redshift Surveys

• Conduct the survey on Mt. Hopkins near Tucson,
  on a 60 telescope, 1977-1982
• Obtain redshifts for 2400 galaxies drawn from
  catalog of Zwicky, brighter than a certain limit
  (mag14.5), 3000 times fainter than what the
  naked eye can see. The average redshift of the
  galaxies they target is 0.015, or 5000
  km/s. The furthest is about z0.03.
• Covered a large area on the sky 2.7 steradians
  (how big is that?)
• First quantitative estimates of the clustering
  of galaxies

Davis
Latham
Huchra
Tonry
Geller
40
CfA Redshift Surveys

• Second CfA Survey, started by John Huchra and
  Margaret Geller in 1984, completed 1995
• Measured redshifts for 18,000 galaxies
• What did they find?
• Frothy structure, large filamentary
  superclusters up to 60 Mpc in extent, large voids
  20-30 Mpc in diameter
• Is this a fair sample? Complementary to pencil
  beam surveys that go to higher redshift, but
  cover smaller area on the sky

Davis
Latham
Huchra
Tonry
Geller
41
CfA Redshift Surveys

• Slice of the Universe , 1100 galaxies in a
  strip 6 degrees by 130 degrees, part of CfA2
  redshift survey (credit SAO Huchra)
• Stick man!!!

Davis
Latham
Huchra
Tonry
Geller
42
CfA Redshift Surveys

• 6 Slices of the Universe (thicker than the
  last image, credit SAO, John Huchra)
• Do you see the Great Wall, largest structure
  detected in any redshift survey when it was
  found? Hundreds of Mpc in extent in longest
  dimension.

Davis
Latham
Huchra
Tonry
Geller
43
CfA Redshift Surveys

• The CfA redshift surveys allowed for comparison
  with theoretical predictions.
• In subsequent years, other surveys pushed out to
  higher redshifts. Most recently, two large
  surveys are the 2 Degree Field (2dF) Galaxy
  Redshift survey, and the Sloan Digital Sky Survey
  (SDSS).
• These (especially SDSS) represent the state of
  the art in redshift surveys.

Davis
Latham
Huchra
Tonry
Geller
44
Sloan Digital Sky Survey

• Started in the late 1980s, by Jim Gunn
  (Princeton), Richard Kron, and Donald York (U.
  Chicago).
• Goal Surveying 25 of the whole sky. How many
  square degrees are there in the sky (lets
  calculate that in terms of full moons)?
• Observations started in 1998-1999, conducted on
  a special-purpose 2.5 meter telescope in Apache
  Point, New Mexico. Use of CCDs for both imaging
  and spectroscopy much more sensitive!
• First part, imaging in 5 color filters, 100
  million celestial objects
• Second part, spectroscopy of more than 1 million
  galaxies and quasars, 608 objects at a time (vs.
  CfA redshift survey, which was 1 object at a
  time), using a fiber spectrograph

(credit M. Strauss Sci. Am. Article)
45
Sloan Digital Sky Survey
(credit M. Strauss Sci. Am. Article)

• Comparison of SDSS slice with CfA2 slice (find
  stick man!)
• SDSS Great Wall 300 Mpc long
• This SDSS slice represents 1 of SDSS volume

46
Sloan Digital Sky Survey

• SDSS provides the best statistical map of galaxy
  positions and luminosities to date, for
  comparison with numerical simulations, which have
  also advanced a lot in the last twenty years.
• One important question is, are galaxies
  unbiased tracers of the underlying dark matter
  distribution.

(credit M. Strauss Sci. Am. Article)
47
Sloan Digital Sky Survey

• Not just redshift survey (though it is the best
  one at z0!!)
• So much more, in countless areas of astronomy
• Galaxy luminosity function what is the relative
  number of galaxies as a function of intrinsic
  luminosity (there are a lot more faint galaxies
  than bright ones)
• The structure of our Milky Way and Local Group
• Exotic stellar and moving objects
• Galaxy and Black Hole evolution
• Quasars in the early universe, understand how
  the intergalactic gas returned from neutral to
  ionized at z6.
• More, more, more. This is a true goldmine of
  data, and its right here!

48
Higher zDEEP2 Redshift Survey

• University of California project to survey
  40,000 galaxies at z1 (8 billion light years
  away) using the Keck 10-meter telescope in
  Hawaii, 4 square degrees on the sky.
• Galaxies about 10,000 times fainter than faint
  limit of original CfA survey
• Goal is to look at large scale structure at z1
  (how has the large-scale structure evolved over 8
  billion years), as well as galaxy evolution.
• This project is led by Marc Davis and Sandy
  Faber.

Map of galaxies in DEEP2 field (courtesy A. Coil)
49
Even Higher z

• We have statistical samples of galaxies at z2,
  z3, z4, back to when the Universe was only 1-2
  billion years old. Interesting selection
  techniques. You have to be trickier than just
  trying to get redshifts for all galaxies brighter
  than a certain flux value.
• We can even study the large-scale distribution
  of these distant galaxies.
• We also want to understand how the galaxy
  internal properties evolve, and how the galaxies
  actually form. We can tell from just looking at
  them that these galaxies in the early Universe do
  not look like nearby spirals and ellipticals, but
  much more irregular.
• But, galaxy formation is a topic for another
  day

50
What caused the initial ripples?

• One of the questions at the beginning was what
  caused the initial fluctuations in density that
  we observe in the CMB radiation?
• One theory is something called inflation. This
  theory is described in Chapter 9, pp. 137-148, in
  the book by Martin Rees. We may discuss this
  later on.