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Recent Developments in Cosmology. Josh Frieman. Quarknet, Argonne National Laboratory, July 2002. Cosmology: an ancient endeavor. How did the world around us come into being? Has it always been like this or has it evolved? If the Universe is changing, how did it begin and what will

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recent developments in cosmology
Recent Developments in Cosmology

Josh Frieman

Quarknet, Argonne National Laboratory, July 2002

cosmology an ancient endeavor
Cosmology: an ancient endeavor
  • How did the world around us come into being?
  • Has it always been like this or has it evolved?
  • If the Universe is changing, how did it begin and what will
  • it be like in the future? And (how) will it end?
  • Early Cosmology: the Universe evolved from a beginning

Babylonian cosmology: Enuma elish

  • Judeo-Christian cosmology: Genesis
  • Greek and Roman myths and philosophers
  • Modern cosmology:expanding Universe established 1929,
  • evolving Universe established in 1965 (discovery of Cosmic
  • Microwave Background Radiation by Penzias & Wilson,
  • Nobel Prize in 1978)
slide3

Modern Science:

--The Universe is knowable through repeatable observations

--The Universe can be described in terms of universal physical laws

Modern Cosmology:Archaeology on the Grand Scale

--We cannot (yet) create universes in the laboratory and study them

--We must observe stars, galaxies, cosmic radiations, etc, and use

them as `pottery shards’ to reconstruct what the Universe was

like at much earlier times, to weave a coherent story of

cosmic evolution based on our understanding of physical laws.

Fortunately, there are surprisingly few ways (given the laws

of physics) to make a Universe that looks like ours today.

slide5

Human scale:

Size ~ 100 cm

Mass ~ 100 kg ~ 1029 atoms

Density ~ 0.6 gm/cm3

Structures organized

by atomic interactions

Sarah Frieman

b. March 26, 2001

slide6

Planets: Size ~ 1010 cm~1010 cm

Mass ~ 1026 kg ~ 1054 atoms

Density ~ 0.6 gm/cm3

Structures determined by atomic interactions & gravity

slide7

Brown Dwarf Star(Planet/star transition)

Ordinary Stars: Size ~ 1011 cm Mass ~ 1030 kg ~ 1057 atoms

Density ~ 0.5 gm/cm3 Hot gas bound by gravity

slide8

M87 Nebula in Orion (star forming region in our galaxy)

Interstellar gas

clouds & star clusters:

Size ~ 1 parsec ~ 3 light-yr

~ 3 x 1018 cm

Mass ~ 105 Msun

slide9

An Infrared view of the Milky Way (our galaxy)

Galaxies: Size ~ 1022 cm ~ 10 kiloparsec (kpc) Mass ~ 1011 Msun

Self-gravitating systems of stars, gas, and dark matter

a brief tour of galaxies
A Brief Tour of Galaxies

Images from the Sloan Digital Sky Survey (SDSS):

An on-going project to map the Universe, the SDSS

will catalog roughly 70 million galaxy images and

measure 3D positions for ~700,000 of them by the

time it is completed in 2005

slide17

Clusters of Galaxies: Size ~ 1025 cm ~ Megaparsec (Mpc)

Mass ~ 1015 Msun

Largest gravitationally bound objects: galaxies, gas, dark matter

slide18

Cluster of Galaxies

`giant arcs’ are galaxies behind the cluster, gravitationally lensed by it

slide19

Apparent position 2

True position 2

Apparent Position 1

True Position 1

Observer

Gravitational “lens”

“Looking into” the lens:

extended objects are

tangentially distorted...

Gravitational Lensing

Basically, the same effects that occur in more familiar optical

circumstances: magnification and distortion

Objects farther from

the line of sight are

distorted less.

slide20

Helen Frieman

b. 9/20/99

slide21

Helen behind

a Black

Hole

Gravitational

Lens

slide22

Mapping

the Mass

in a Cluster

of Galaxies

via

Gravitational

Lensing:

Most of the

Mass in the

Universe is

Dark

(it doesn’t

shine)

Dark Matter

slide23

Superclusters and Large-scale Structure:

Filaments, Walls, and Voids of Galaxies

100 Million parsecs (Mpc)

You Are Here

`Pizza Slice’ 6 degrees thick containing 1060 galaxies:

position of each galaxy represented by a single dot

slide24

Superclusters and Large-scale Structure:

Filaments, Walls, and Voids of Galaxies

100 Million parsecs (Mpc)

You Are Here

slide25

Superclusters and Large-scale Structure:

Filaments, Walls, and Voids of Galaxies

Coma cluster

of galaxies

100 Million parsecs (Mpc)

You Are Here

slide26

Early

SDSS

Data

~200,000

Galaxies

Mapped in

3D so far

slide27
The Big Bang Theory:a well-tested framework for understanding the observationsand for asking new questions

The Universe has been expanding isotropically from a hot, dense `beginning’ (aka the Big Bang) for about 14 billion years

The only successful framework we have for

explaining several key facts about the Universe:

Hubble’s law of galaxy recession:expansion

Uniformity (isotropy) of Microwave background

Cosmic abundances of the light elements:

Hydrogen, Helium, Deuterium, Lithium, cooked in the first 3 minutes

the big bang theory
The Big Bang Theory

Not `just a theory’, but one of the most firmly established

paradigms in science:

The Standard Cosmological Model

the big bang theory1
The Big Bang Theory

The Big Bang is an idealization, a simplified

description (analogous to the approximation of the Earth as a

perfect sphere), and cosmologists are now occupied with

mapping out/filling in the details.

Even so, certain basic elements of the model remain to be

understood: e.g., the natures of the Dark Matter & Dark Energy

which together make up 95% of the mass-energy of the Universe

These puzzles do NOT mean that the Big Bang Theory is

wrong—rather, it provides the framework for investigating

them.

slide36

Spectrum of

Light from

Galaxies

Redshift

of Galaxy

Emission &

Absorption

Lines:

recession

velocity

v/c ≈ z = /0

(approximation

for objects

moving with

v/c << 1)

receding

slowly

receding

quickly

slide37

Hubble

Space

Telescope

in Orbit

Measured

distances to

galaxies

using Cepheid

Variable stars

slide38

Hubble

(1929)

Hubble

Space

Telescope

(2000)

slide39

Modern

`Hubble

Diagram’

Extend to

larger

distances

using

objects

brighter

than

Cepheids

slide40

The Microwave

Sky:

The Universe is

filled with

thermal radiation:

Cosmic Microwave

Background (CMB)

COBE Map of the Temperature

of the Universe

On large scales, the Universe is (nearly) isotropic around us (the same in all directions): CMB radiation probes as deeply as we can, far beyond optical light from galaxies: snapshot of the young Universe (at 400,000 years old)

T = 2.7 degrees

above

absolute zero

Scale of the Observable

Universe:

Size ~ 1028 cm

Mass ~ 1023 Msun

slide41

CMB

(nearly)

isotropic

Earth

not

the cosmological principle
The Cosmological Principle

A working assumption (hypothesis) aka the Copernican Principle:

We are not priviledged observers at a special place in the

Universe:

At any instant of time, the Universe should appear

ISOTROPIC

(over large scales) to All observers.

A Universe that appears isotropic to all observers is

HOMOGENEOUS

i.e., the same at every location (averaged over large scales).

slide43

The Microwave

Sky:

COBE Map

of the

Temperature

of the Universe

Dipole

anisotropy

due to our

Galaxy’s

motion through

the Universe

T = 2.728 deg

above

absolute zero

Red: 2.7+0.001

Blue:2.7-0.001

Red:

2.7+0.00001 deg

Blue:

2.7-0.00001 deg

slide44

The Microwave

Sky:

COBE

Map of the

Temperature

of the Universe

Map with

Dipole anisotropy

removed:

fluctuations of

the density of

the Universe (plus

Galactic emission)

T = 2.7 degrees

above

absolute zero

Red: 2.7+0.001

Blue:2.7-0.001

Red:

2.7+0.00001 deg

Blue:

2.7-0.00001 deg

slide45

Cosmology as Metaphor:

From The New Yorker, March 5, 2001:

`A hiss of chronic corruption suffuses the capital

like background radiation from the big bang.’

--Hendrik Hertzberg

`The Talk of the Town’

slide46

Physical Implications of Expanding Universe

An expanding gas cools and becomes less denseas

it expands. Run the expansion backward: going back into

the past, the Universe heats up and becomes denser.

Expanding Universe plus known laws of physics

imply the Universe has finite age and a `singular’

(nearly infinite density and Temperature) beginning

about 14 Billion years ago:

THE BIG BANG

big bang nucleosynthesis
Big Bang Nucleosynthesis

Origin of the Light Elements: Helium, Deuterium, Lithium,…

When the Universe was younger than about 1 minute old,

with a Temperature above ~ 1 billion degees,

atomic nuclei (e.g., He4 nucleus = 2 neutrons + 2 protons

bound together) could not survive: instead the baryons

formed a soup of protons & neutrons.

As the Temperature dropped below this value (set by the

binding energy of light nuclei), protons and neutrons

began to fuse together to form bound nuclei:the light

elements were synthesized as the Universe expanded and

cooled.

slide48

BBN predicted abundances

h = H0/(100 km/sec/Mpc)

Fraction of

baryonic

mass in He4

Light

Element

abundances

depend

mainly on

the density

of baryons

in the

Universe

Deuterium to

Hydrogen

ratio

Lithium to

Hydrogen

ratio

baryon/photon ratio

slide49

BBN Theory vs. Observations:

Observational constraints

shown as boxes

Remarkable agreement

over 10 orders of magnitude

in abundance variation

Concordance region:

b = 0.04

Strongest constraint comes from

Deuterium.

Excellent agreement w/ more

recent CMB measurements

b

4He

slide50

Recent CMB experiments:

Going to smaller angular scales  higher resolution

cmb angular power spectrum
CMB Angular Power Spectrum

Angular power spectrum is a statistical way to characterize

the spatial structure in a 2-dimensional image or map

slide55

Oscillation of the Photon-

  • Baryon fluid when the
  • Universe was 400,000 yrs old
  • Imprint on the Microwave

sky

slide56

Theoretical dependence of CMB

anisotropy on the baryon density

Angular frequency

Angular separation on the sky

microwave background anisotropy probes w b baryon density
Microwave Background AnisotropyProbesWb(Baryon Density)

Boomerang experiment (2001)

Wb= 0.04

DASI experiment (2001)

slide58

Einstein’s General Relativity

Matter and Energy curve

Space-Time

All bodies move in this

curved Space-time

A massive star

attracts nearby objects

by distorting spacetime

gravity newton vs einstein
Gravity: Newton vs. Einstein

Newton: 1) gravitation is a force exerted by one massive

body on another.

2) a body acted on by a force accelerates

Einstein: 1) gravitation is the curvature of spacetime due to a

nearby massive body (or any form of energy)

2) a body follows the `straightest possible path’

(aka geodesic) in curved spacetime

slide60

Einstein: space can also be globally curved

What is the geometry of three-dimensional space?

slide61

Microwave photons

traverse a significant

fraction of the

Universe,

so they can probe

its spatial curvature

Sizes of hot and cold

spots in the CMB

give information

on curvature of space:

In curved space, light bends as it travels: fixed object has larger

angular size in a positively curved space: CMB spots appear larger.

Opposite occurs for negatively curved space.

slide63

Position

of first

Peak

probes

the

spatial

Curvature

of the

Universe

microwave background anisotropy probes spatial curvature
Microwave Background AnisotropyProbes Spatial Curvature

Boomerang experiment (2001)

W0 = 1.03 0.06

W0 = 1.04 0.06

DASI experiment (2001)

slide65

Einstein: space can also be globally curved

What is the geometry of space? Recent observations of the

Microwave background anisotropy indicate it is flat

probes of the matter density w m
Probes of the Matter Density:Wm

From galaxy clusters

and other probes:

Wm~ 0.3

Galaxy kinematics

Current evidence:

Lensing

X-ray gas

slide67

rotation velocity

Observed: flat, M ~ d

Keplerian: v ~ d-1/2

blueshift

redshift

Typical rotation speed ~200 km/sec and visible disk size ~ 10 kpc

slide68

Clusters of Galaxies: Size ~ 1025 cm ~ Megaparsec (Mpc)

Mass ~ 1015 Msun

Largest gravitationally bound objects: galaxies, gas, dark matter

the 2 dark matter problems
The 2 Dark Matter Problems

Observations indicate:

visible matter ~ 0.01 baryons ~ 0.04 dark matter ~ 0.3

BBN+CMB

Dark Baryonic matter

composed of protons, neutrons,

(more fundamentally of quarks)

Dominant component

of Dark Matter is

Non-baryonic

requires a new component

beyond quarks,...

basic dark matter questions
Basic Dark Matter Questions

How much is there?

What is the value of ? Current evidence suggests ~0.3.

Where is it?

Is it just clustered with the luminous material? Not precisely,

since Dark halos extend beyond luminous galaxies. Are

there completely dark galaxies or clusters?

What is it?

BBN+CMB  mostly not made of baryons (i.e., protons,

neutrons, quarks, etc). It could be a new Weakly Interacting

Massive Particle (WIMP). Supersymmetry models predict these.

Ultimate Copernican principle:

We’re not even made of the central stuff of the Universe!

dark energy and the accelerating universe
Dark Energy and the Accelerating Universe

Brightness of distant Type Ia supernovae indicates the expansion

of the Universe is accelerating, not decelerating.

If General Relativity is valid, this requires a new form of

stuff with negative effective pressure*:

DARK ENERGY

Characterize by its equation of state:w = p/

*more specifically, p <  (w < 1/3)

Dubya

pressure

density

slide72

p =  (w = 1)

Accelerating

SNe Ia + CMB

indicate

m  0.3

DE  0.7

Empty

Size of the

Universe

Open

Closed

Today

Cosmic Time

evidence for dark energy
Evidence for Dark Energy
  • Direct Evidence for Acceleration
  • Brightness of distant Type Ia supernovae:
  • Standard candles  measure luminosity distance dL(z):
  • sensitive to the expansion history H(z)
  • Supernova Cosmology Project
  • High-Z Supernova Team
  • II.Evidence for `Missing Energy’
  • CMB Flat Universe: 0 = 1
  • Clusters, LSS  Low matter density m  0.3
  • missing = 1 – 0.3 = 0.7 and missing stuff can only
  • dominate recently for structure to form: w < – 0.5
slide74

Discovery

of SNe Ia

at `high’

redshift

z ~ 0.5 – 1

slide75

Type Ia

Supernovae

Peak Brightness

as a calibrated

`Standard’

Candle

Intrinsic Brightness

vs. Time

Physical model:

White dwarf star,

accreting mass from a companion star,

explodes when it

exceeds a critical

mass (Chandrasekhar)

Luminosity

Time

slide76

42 SNe Ia

Fainter

Apparent Brightness

distance

m(z) = M+5log(H0dL)=(1+z)  dz’/H(z’)

cmb and supernovae
CMB and Supernovae
  • CMB + SNIa
  • orthogonal constraints

Dark Energy density

Wm= 0.31 0.13

WL = 0.71 0.11

Dark matter density

the early universe the key to large scale structure
The Early Universe:the key to Large-scale Structure

From our vantage point 13 billion years after the Big Bang,

we are now trying to unravel what happened in the earliest tiny fraction of a second, when the Universe was

0.000000000000000000000000000000000001 seconds old!

We can test our ideas about the Very Early Universe by

observing the distributions of galaxies and of cosmic

radiations in space.This has been a major breakthrough in cosmology over the last decade.

inflation
Inflation

An epoch of very rapid expansion, during which the

size of the Universe grows faster than time

This means that comoving observers appear to be

accelerating away from each other.

As we saw, there is mounting evidence (from Type Ia

Supernovae) that the Universe recently (~10 billion years ago)

entered such an accelerating phase of expansion.

The Universe may now be in the early stages of Inflation.

inflation in the early universe
Inflation in the Early Universe

A hypothetical epoch of very rapid (`accelerated’)

expansion very early in cosmic history (perhaps around

t ~ 10-33 seconds), during which the size of the Universe

grew faster than time.

If this period of `Superluminal’ expansion lasts long enough,

then it effectively stretches any inhomogeneity & space

curvature, `explaining’ why the Universe today appears

homogeneous and flat.

Theory arose in 1980 (A. Guth) from considerations of

symmetry-breaking phase transitions in Grand Unified

Theories.

slide82

Inflation Models: Scalar Field slowly rolls down a hill

Potential energy

density

High

Temp.

High Temperature:

Symmetry is restored,

 = 0.

Low Temperature:

Symmetry is broken

 = + or -

Low

Temperature

field

Potential energy function must be fairly

`flat’ so the field rolls slowly: probably not a

Higgs field, must be something else

slide83

After rolling down, scalar field oscillates around

the bottom REHEATING

Potential energy

density

High

Temp.

High Temperature:

Symmetry is restored,

 = 0.

Low Temperature:

Symmetry is broken

 = + or -

Low

Temperature

field

At the end of inflation,

the Universe is very cold.

Reheating: Oscillating field energy transformed to other particles

as it decays: Universe heats back up to high Temperature: `another’ bang

that creates all the matter and energy in the Universe.

who is the inflaton field
Who is the `inflaton’ field?

Originally it was thought a GUT Higgs field would do the

trick. With the death of `old’ inflation, this hope dimmed.

Inflation requires a new scalar field with a very flat

potential energy function. Currently, there is

no consensus among particle physics theorists as to

the identity of this hypothesized inflaton field.

Inflation has thus been described as a theory in search of a model.

density perturbations structure
Density Perturbations & Structure

Inflation provides a physical mechanism for producing the

initial `seed’ perturbations which grew into Large-scale Structure

Density Perturbations from Quantum Mechanics:

Classically, the scalar field rolls down its potential at the same

speed everywhere in the Universe:  = (t). According to

Quantum Mechanics, the amplitude (or rolling speed) of the field

fluctuates: it differs from place to place by a small amount,

 = (x,t). These field fluctuations imply spatial fluctuations in

the energy density of the Universe. During, reheating, these

become fluctuations in the density of all matter & radiation

particles. This is a crucial but originally unforeseen consequence

of the theory, now seen to be in excellent agreement with

CMB observations.

slide86

1-dimensional

cross-section

space

field

field

evidence for inflation
Evidence for Inflation
  • Large-scale homogeneity and isotropy (by design)
  • Spatial flatness (Euclidean): total = 1
  • (Power) Spectrum of density perturbations inferred
  • from CMB experiments agrees to high precision
  • with spectrum of quantum fluctuations predicted by inflation
  • Future:
  • -more precise measurements by satellites (MAP, Planck)
  • -measurement of CMB polarization possibly test inflationary
  • prediction for gravity wave spectrum and distinguish
  • between different inflation models
slide88

The Structure Formation Cookbook

  • Initial Conditions: Start with a Theory for the Origin of
  • Density Perturbations in the Early Universe
  • Your FavoriteInflation model
  • 2. Cooking with Gravity: Growing Perturbations to Form Structure
  • Set the Oven to Cold, Hot, or Warm Dark Matter
  • Season with a few Baryons and add Dark Energy
  • 3. Let Cool for 14 Billion years (or buy a Really Big Computer)
  • 4. If it looks, smells, and tastes like the real thing, then publish the recipe. If not, publish anyway, and then start over with different ingredients or change the oven settings.
slide89

Early

Evolution of

Structure in a

Simulated

Big Bang

Universe Filled

with Dark Matter

`The Cosmic Web’

Galaxies and

Clusters form at the

intersections of

sheets and filaments,

very similar to the Structure seen in

galaxy surveys

Today

slide90

Evolution of

Structure in the

Universe

slide92

Galaxy

Clustering

in the

SDSS

Redshift

Survey

~100,000

galaxies

Voids, sheets,

filaments

slide93

Probing Neutrino Mass and Baryon Density

Wiggles

Due to

Non-zero

Baryon

Density

SDSS + MAP: will constrain sum of stable neutrino masses as low as ~ 0.5 eV

some key questions for 21 st century cosmology
Some Key Questions for 21st Century Cosmology

How did the hierarchy of large-scale structure, from stars to galaxies to clusters and beyond, originate?

Did this structure arise from the expansion stretching of microscopic quantum ripples in the fabric of spacetime during the earliest moments of the Big Bang, a theory known as Cosmic Inflation?

What is the nature of the Dark Matter that makes up most of the mass of the Universe?Is it in the form of exotic elementary particles as yet undiscovered? (The Ultimate Copernican Principle)

What is the nature of the Dark Energy that is causing the expansion of the Universe to Accelerate?

Will the Universe continue to accelerate forever?

What happened `before’ the Big Bang?Is this question meaningful?

Are there more than 3 spatial dimensions?Can we ever detect them?

slide95

CMB Sky:

1992

circa Jan.

2003

slide96

MAP

Satellite

launched

June

2001

Planck

Satellite

planned

for

~2008

slide97

Proposed

satellite

mission to

observe

several

thousand

SNe Ia out to

z ~ 1.7

despite major recent advances in cosmology fundamental mysteries remain
Despite major recent advances in cosmology, fundamental mysteries remain

Unlike the ancient mystics, however, we hope these unexplained phenomena can in principle be understood, by a combination of new theoretical insight and experimental advances: scientists are perpetual optimists.

So far, this optimism has been justified by the continued

progress of science.

What are the ultimate limits to our understanding of the Universe?

references
References

T. Ferris, The Whole Shebang (Touchstone Books 1997)

B. Greene, The Elegant Universe (Vintage, 1999)

A. Guth and A. Lightman, The Inflationary Universe

J. Silk, A Short History of the Universe

C. Hogan, The Little Book of the Big Bang

More advanced:

A. Liddle, An Introduction to Modern Cosmology

E. Linder, First Principles of Cosmology

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