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