Looking into the past, and mapping the...
Transcript of Looking into the past, and mapping the...
Looking into the past, and mapping the structure.
Based on slides by Joe Mohr
Part 1: Timeline into the PastTen Epochs
–1. The Nearby Universe–2. The Universe at One Half Its Present Age–3. The Universe at One Quarter Its Present Age–4. The Universe at One Sixteenth Its Present Age–5. The Dark Ages–6. The Epoch of Recombination –7. Entering the Radiation Dominated Epoch–8. Primordial Nucleosynthesis–9. The Inflationary Epoch–10. The Current Limits of Physical Extrapolation
A Hubble Space Telescope image of a star forming region roughly 7,000 light years away
The Milky Way contains gas, and new stars- and planetary systems-are continuously forming where conditions are favorable
New Stars Form Continuously in Our Galaxy
aAn HST Image ofthe Eagle Nebula
Hubble Space Telescope (HST) Image
Our Galaxy Also Contains Ancient StarsGlobular clusters are stellar condensations that orbit through the Milky Way and other galaxiesContain millions of starsIntriguing because they contain no young, hot blue stars
Stellar evolution calculations indicate that the stars in the oldest globular clusters have been burning Hydrogen for ~12 billion years.
Globular Cluster Messier 13
Globular Cluster NGC 5904
The Milky Way is a Spiral GalaxyAnatomy of a spiral galaxy
» a central bulge filled with older, cooler stars that appear yellow
» a gaseous disk filled with old and new stars, and so it appears bluer
» a dark, massive halo
Scale and Nature of spiral galaxy» gas becomes new stars in the disk» it takes light 100,000 years to cross the
disk of our galaxy» spirals contain hundreds of billions of
stars
Spiral galaxies are most common
M31, our nearest (big) neighbor is 2 million light years away
Messier 31, our galactic neighbor
Structure in the Nearby UniverseThere exists a hierarchy of evolving structures in the nearby universe» Galaxies come in three main types
– spirals are gas rich and the most common– ellipticals are gas poor and are only common in clusters of galaxies– irregulars are gas rich but are not as structured as spirals
» Groups of galaxies– tens of galaxies– the Milky Way is part of the Local Group
» Clusters of galaxies– hundreds to thousands of galaxies– hot, 108 Kelvin gas which emits X-rays
» Superclusters– tens of galaxy clusters
» Large scale structures (LSS)– filaments and sheets with scales of hundreds of millions of light years
if the Chicago Loop were a galaxy then LSS would be the size of North America
The microwave background
Dark matter constitutes a majorityof the mass in all these structures.This dark matter is detected onlythrough its gravitational effects.Determining what this dark matterconsists of is a major research focus.
M83, a Nearby Spiral Galaxy
M51, The Whirlpool GalaxyHubble Space Telescope (HST) Image
With the superb imagequality available with HSTone can see that the spiralarms of M51 are filled withbright blue knots of newlyformed stars, very similar to those we see in the Milky Way.
M87, a Giant Elliptical Galaxy in the Virgo Cluster
Note the lack of hot, youngblue stars. There is littlegas in this giant. The tiny, bright knots are globular clusters.
Photo by David Malin
M104, the Sombrero Galaxy
The dust in the disk of this galaxy is seenin silhouette and in re-flection against the huge bulge.
X-ray Emission from Galaxy ClustersGalaxy clusters contain thousands of galaxies and a hot gas
» Galaxies orbit through the cluster like stars in an elliptical galaxy» Hot, 100 million Kelvin gas emits X-rays» Entire cluster lights up like an X-ray lantern!
Galaxy clusters are young because they are still accreting mass
Typical size of cluster galaxy
X-ray Images from the Roentgen Satellite
In these false color images, yellow denotes the brightest regions, dark green the faintest.
Two clustersabout to merge
The Universe at Half Its Present AgeYoung galaxy clusters present
» galaxies within clusters appear bluer and brighter than in nearby clusters» clusters tend to be surrounded by galaxies with lots of gas and young stars
Objects in Universe look different…. younger.Radiation background: microwaves at a temperature of 4 Kelvin
Hubble Image of Distant Galaxy Cluster Abell 2218
Merger of two galaxy clusters
Arcs are gravita-tionally lensed back-
ground galaxies.
Gravitational lensGravitational lens
b=impact parameterb=impact parameter
θ =2brg
,.....rg =2GM
c 2
Lensing CartoonMassive galaxy cluster acts like a lense, focusing light from distant galaxyPerfect alignment between observer, a spherical cluster and a distant galaxy can result in an Einstein Ring
» typically alignment isn’t perfect and galaxy clusters aren’t perfectly spherical
» observe arcs rather than complete rings
Massive Galaxy Cluster
Distant GalaxyObserver
Direction of light changes.Light bent toward center
of galaxy cluster.
Light from distant galaxy
Cartoon of Einstein Ring
Galaxy Cluster
Background galaxydistorted into ring.
Apparent position of Distant Galaxy
Apparent position of Distant Galaxy
Changes in the Hubble ParameterWhen we study the distant Universe, we look back to a time when the Universe was expanding at a different rate
» simple Hubble law relationship v=H0d only holds if Universe expands homogeneously» gravity has measurably changed the expansion rate over the last 7 Gyr (billion years)
Precision observations of the breakdown of the Hubble law tell us what kind of matter- and how much- there is in the Universe
Distance [Billion Light years]
Vel
ocity
[km
/s]
Hubble Law
Faster expansionin the past.
v = Hod
Hubble Law onlyvalid in nearby uni-verse. Matter in theUniverse changes theexpansion parameterH over time, introduc-ing curvature into the relationship between object distance and re-cession velocity.
The Universe at One Quarter Its Present AgeGalaxies very different
» irregulars very common» extremely blue
No known galaxy clusters» What does this mean?
– really no clusters?– just too faint to detect X-rays?
» New methods of detection
Radiation background » microwaves at temperature of 7 Kelvin
Hubble Image of Faint Blue Galaxies
The Universe at One Sixteenth Its Present AgeA Sampling of Structures
» protogalaxies– faint knots of hot, young stars
» a few quasars– extremely luminous QUAsi-StellAR
objects- powered by black holes
Hubble Image of Protogalaxies
» a single observed galaxy
Radiation background » microwaves at temperature of 16 Kelvin
Limit of current optical observations
The Dark AgesFrom t=500,000 years to t~100-1000million years
Rapid evolution of physical conditions» matter density drops by factor of 10 million» structure formation begins in earnest
– transition from highly homogeneous to collapsed structures» first generation of stars emerge» first elements heavier than He and Li are synthesized» radiation produced by collapsed structures ionizes matter» larger structures begin to be assembled
Radiation background» temperature ranges from 3,000 Kelvin to 16 Kelvin» begins dark ages as optical background» cools to an infrared background» cools to microwave background by end of dark ages
RecombinationUniverse is ~ 500,000 years old
» source of the observed cosmic microwave background radiation– pillar of the Big Bang- implies universe hot and dense in the past
Radiation background» is 3,000 K and an optical background» hot enough that electrons are stripped from atoms
Changes in normal matter» over ~100,000 year period matter goes from neutral to fully ionized
Radiation and matter coupled in ionized Universe» collisions of photons and electrons are common» matter and radiation temperatures equalize» Universe a soup of photons, electrons, nuclei and neutrinos
Epoch of Radiation Domination BeginsUniverse is ~1,500 years old
Radiation background » temperature is ~65,000 Kelvin» ultraviolet radiation
Energy in the radiation background equals energy in the matter» matter energy density um:
» radiation energy density uγ :
At all earlier times radiation energy density larger than matter energy density» matter like flotsam in a stormy sea of energetic photons
um = nm m c2
uγ = nγ Eγ
Primordial NucleosynthesisUniverse is ~1 minute old
Radiation background » temperature of ~1 billion Kelvin» gamma ray radiation
– typical photon many times more energetic than X-rays used for health care imaging
Interactions between matter and radiation» before this time the radiation is energetic enough to blow nuclei apart» around this time nuclei have enough energy to fuse when they collide with one another
Creation of the light elements» Hydrogen fused into deuterium, helium and lithium during this epoch» Pillar of the big bang model
– primordial nucleosynthesis required to explain abundances of the light elements helium, deuterium and lithium
Inflationary EpochUniverse ~10-36 seconds old
Transitions in nature of nuclear and electromagnetic forces» leads to large vacuum energy» vacuum energy is like Einstein’s Λ» acts as a cosmic repulsion
Period of rapid, accelerating expansion» scale of Universe increases by factor of 1020 to 1030
– observable universe goes from atom sized to cherry pit sized» provides solution to the horizon problem
– why is CMB temperature uniform to 1 part in 100,000 even on opposite sides of the sky which could never have been in causal contact in a simple universe?
» implies that the geometry of the Universe is flat– a balance between expansion and gravitational attraction– flat refers to nature of geometry- Euclidean rather than more complicated- (and not to the
shape of the universe)
Limits of Physical Extrapolation Universe is 10-48 seconds old
The Planck epoch» enough energy available that nature of gravity expected to change» quantum corrections to general relativity expected to render GR invalid» wavelength of typical object is comparable to size of observable universe
Further extrapolation requires new physical theories
Sketch of Inflation TheoryVacuum: ground state, lowest energy stateAt t=10-34 s 1027 K: universe enters supercooled state of FALSE VACUUMTemperature falls below GUT symmetry breaking temperature.Supercooled, energy density of FV=1094 Jm-3
UUFVFV, P, PFVFV
False vacuumFalse vacuumUUTVTV=0, P=0, PTVTV=0=0True VacuumTrue Vacuum
Change volume Change volume dVdV
dE=dE=UUFVFVdVdV, but dE=, but dE=--PPFVFVdVdVby first law of TDby first law of TD
Hence PHence PFVFV= = --UUFVFV
False vacuum has negative False vacuum has negative presurepresure
Apply Friedmann Equation to False Vacuum
Ý Ý a = −4πG3
U + 3Pc 2
⎛ ⎝ ⎜
⎞ ⎠ ⎟ a
Ý Ý a = 8πGUFV
3c 2 a
a(t) = aie−
tτ i
τ i = 3c 2
8πGUFV
≈ 10−34 s
Ω(t) =1+kc 2
Ý a 2=1+
kc 2τ i2
a2(t)
FE with pressure
Normally U= ρc and P=0For inflation P=PFVand this dominates over U
Solution
where
Inflation increases scale factor by e100 or 1043
Latent heat released at GUT transition reheats to 1027 KTodays observable universe originated within a small bubble of true vacuum: initial size 10-26 m, final size 1024mTodays observable universe was cm in size at end of inflationFraction of original bubble in obs. universe today: 10-52
Inflation forces Ω=1,k=0
A TimelineThe Present
The Beginning
Time
Time
The Nearby Universe
Galaxies with younger starsYoung galaxy clusters
Faint Blue GalaxiesNo galaxy clusters?
ProtogalaxiesQuasars
One observed galaxy
The Dark Ages
Recombination, Radiation domination, Nucleosynthesis and Before
Dir
ect O
bser
vatio
n
CM
B R
adia
tion
Lig
ht E
lem
ent A
bund
ance
s
t02
t04
t 016
Spirals and EllipticalsGroups and Clusters
SuperclustersLarge Scale Sheets and Filaments
Part 2Mapping the Structures of the Nearby Universe
A Perspective on Mapping» The view outward from a spiral galaxy» Stars, the scourge of extragalactic astronomy» The distribution of galaxies on the sky» Accessing the third dimension
The First Map of the Nearby Universe» Mapping strategy and techniques» The structure of the universe revealed!» Observed structures within the context of the Big Bang model
Seeking Maps of Ever Larger Pieces of the Observable Universe» The Las Campanas Redshift Survey
– in search of the largest structures in the universe» The Sloan Digital Sky Survey
– accessing the details of structure formation with a million galaxy redshifts
The Complications of Life in a Spiral Galaxy
The Milky Way is a spiral galaxy» solar system is located in the dust and gas
rich disk about 25,000 light years from the galactic center
Structure of the galaxy affects our extragalactic view
Consider view outwardfrom within the disk
The Zone of AvoidanceThe gas, dust and stars in the disk of the Milky Way form a circular strip, the so-called zone of avoidance, where it is extremely difficult to study extragalactic objects
Many extragalactic studies are focused near the north and south galactic poles, where interference from objects in the Milky Way is minimized
An optical view of the sky
An infrared view of the sky
A radio view of the sky
Three Views of the Disk of the Milky WayThree Views of the Disk of the Milky Way
Galaxies Around the South Galactic PoleAPM galaxy survey- 2 million galaxies over 10% of the sky (50x100 degrees)
» scans of photographic plates followed by automatic star/galaxy classification– only 1 out of every six objects is a galaxy down to classification magnitude limit of B<20.5
APM survey by Maddox, Sutherland, Efstathiou, Loveday and Dalton, Oxford University Astronomy
Blue - brightGreen - medium
Red - faint
Projected Distribution of GalaxiesNonuniformities clearly apparent- overdensities, underdensities, bridging structuresHow large are these structures?
» additional information required
APM survey by Maddox, Sutherland, Efstathiou, Loveday and Dalton, Oxford University Astronomy
Projected DistributionA projection provides limited information
» angular or apparent sizes of structures» degeneracy- difficult to differentiate between small and large structures
With measure of distances» one can determine true sizes of structures
θdistance
Two structures with the same angular or apparent sizeTwo structures with the same angular or apparent size
Big structureat large distance?
Small structurenearby?
Observer
Accessing the Third DimensionMeasuring galaxy distances directly is difficult and time consuming
Recall the Hubble Law» in nearby universe galaxy recession
velocities are proportional to distances
Galaxy recession velocity provides good estimate of galaxy distance
v = Hod Blue points: 19 SNeRed line: Hubble Law
Ho=19.6 km/s/MLy
Type Ia SNe MeasurementsRiess, Press & Kirshner
A lazy man’s distance requires a galaxy spectrum to measure the galaxy recession velocity.
Galaxy Spectroscopy
Spectra of a nearby star and a distant galaxy» Star is nearby, approximately at rest» Galaxy is distant, traveling away from us at
12,000 km/s
Calcium
Magnesium
Sodium
Galaxy Spectrum
Stellar Spectrum
Emission and absorption of light occurs at specific energies for each element
» creates an elemental fingerprint, recognizable even in light from extremely distant objects
Expansion of the Universe stretches light wavelengths
» detected spectrum is shifted to longer wavelengths with respect to emitted spectrum
Effective recession velocity of object is determined by carefully determining the amount of redshifting
Large Scale Structure Revealedde
Lap
pare
nt, G
elle
r &
Huc
hra,
ApJ
1986
A map of 1,061 galaxies with B<15.5 in one sliceof the nearby Universe.
Galaxy distribution on the sky.
Slice comes from this regionand is 6o thick, ~100o longand ~1 billion light years deep.
PiePie--slice gives slice gives good good representation representation of various of various featuresfeatures
Structures Extend over 500 million light years
slice from 8.5o<δ<14.5o slice from 26.5o<δ<44.5o
The Great Wall is roughly500 million light years long
and stands at least300 million light years tall!
Perhaps more importantly,structures extend to the
edges of the survey and soone wonders if there areeven larger structures.
Gel
ler
& H
uchr
a, S
cien
ce 1
989
Underdense regions orvoids with scales of a few
hundred million light years.
Evolution of Density
Perturbations
Increasing Time
Simulation results from Evrard & Crone, ApJ 1992
Simulations of three different models, each described by a different spectrum of initial density fluctuations.
tage=1.6 Gyr tage=4.6 Gyr tage=13 Gyr
Least SmallScale Power
Most SmallScale Power
Incr
easi
ng S
mal
l Sca
le P
ower
Simulation ResultsSimulation Results
What Have We Learned About the Universe?The nearby Universe is lumpy
» Galaxies are not distributed homogeneously » Heirarchy of structures with very large range of scales
– galaxies with scales of 100,000 light years– galaxy clusters with scales of tens of millions of light years– large scale structures with scales approaching a billion light years
» CMB radiation tells us that universe was homogeneous to a part in 100,000 when it was ~500,000 years old
– gravity has been very busy over the last 13 billion years!– gravity works as an amplifier, taking very tiny inhomogeneities and amplifying them over time
Challenges to the Big Bang model?» Largest structures not much smaller than the observable universe
– are these structures expected?» Enough time available for these structures to form?» Favored structure formation models (at the time) in serious trouble
Are there even larger structures in the Universe or is the Great Wall unique?» The race was on!
Fingers of GodUsing velocities as distances
» Hubble Law describes the effects of the universal expansion on objects» Local mass concentrations can accelerate objects, inducing velocities
– consider Earth’s orbit around the sun– consider the Sun’s orbit through the Milky Way– consider the dipole in the Cosmic Microwave Background
Hubble Law breaks down near galaxy clusters» Galaxy clusters so massive and gravitational attraction so strong» Cluster members move with large velocities approaching 1,000 km/s» This “peculiar” velocity masquerades as a Hubble distance of 60 million light years!» We call this large distortion of Hubble flow a “Finger of God” because they seem to
point towards the observer
Las Campanas Redshift Survey (LCRS)
Designed to search for structures even larger than the Great WallLCRS survey area
» three slices in north galactic cap, and three in the south
» similar to CfA survey in terms of area covered on the sky
» roughly four times deeper– measure redshift for fainter galaxies– probe much larger volumes– 26,000 galaxies surveyed
LCRS Results» many structures like the Great Wall» many voids with scales of several
hundreds of millions of light years» no strong evidence for structures
significantly larger than the Great Wall» more detailed quantitative analysis of
the distribution of galaxes
Shec
tman
, Sch
echt
er, O
emle
r, K
irsh
ner,
Tuc
ker,
Lan
dy, H
ashi
mot
o &
Lin
Ongoing Surveys of the (not so) Nearby UniversePushing to larger areas and greater depths
» higher quality quantitative constraints on the nature of structure formation» probe to significant lookback times
– Universe younger, and so we can observe evolution of large scale structures directly
Use numerical simulations to make quantitative predictions for the large scale distribution of matter
» current models produce structures consistent with observations
» structure depends on cosmological parameters
» comparisons complicated by complex processes like galaxy formation and its sensitivity to environment
Mock Survey from Numerical Simulation by Cole, Weinberg et al
The Sloan Digital Sky SurveyTwo component survey
» CCD imaging of one quarter of the sky» Spectra and redshifts of 1 million galaxies
Industrial strength astronomy» ~ten institutions
– University of Chicago– Fermilab– Princeton– Institute for Advanced Study– University of Washington– Johns Hopkins University– US Naval Observatory– Japanese participation– MPA in Heidelberg
» 168 participants (and counting…)
» large budget
Apache Point Observatory in New Mexico
SDSS Logo
For more info check out http://www.sdss.org
A Dedicated 2.5 m TelescopeImages in 5 bands of 100 million objects
» an unparalleled resource for optical astronomyObject selection and followup spectroscopyYield: 15 terrabytes of data over several years
For more info check out http://www.sdss.org
Simultaneous Spectroscopy of 600 ObjectsOptical fibers collect light from galaxies and transmit that light into spectrograph» measure 600 galaxy spectra- and 600 velocities- with each field
Plug plates must be drilled for each spectroscopy field, and fibers plugged into holes by hand
A Plug Plate from the LCRS
Plugging a fiber bundle, one fiber at a time!
For more info check out http://www.sdss.org
ReviewMapping the Nearby Universe
» Complications:– Zone of avoidance– Star-galaxy differentiation– Accessing the third dimension
A First View of the Distribution of Matter in the Nearby Universe» Geller and Huchra in the 80’s» Large scale structure revealed
– The Great Wall- several hundred million light years across– Voids- hundreds of millions of light years across– Problems for the standard structure formation models
Pushing toward an understanding of structure in the nearby Universe» Las Campanas redshift survey in the 90’s
– Structures apparent in the Geller & Huchra surveys are common– No strong evidence for significantly larger structures
» Family of structure formation models developed– Depend on cosmological parameters– Predict structures which appear to be consistent with those observed
» SDSS over the next several years– High quality images over a quarter of the sky- 100 million objects– Spectra and redshifts of 1 million galaxies