Download Recent Developments in Cosmology

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
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)
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.
The macroscopic
Universe observed:
a hierarchy of Structure...
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
Planets:
Size ~ 1010 cm~1010 cm
Mass ~ 1026 kg ~ 1054 atoms
Density ~ 0.6 gm/cm3
Structures determined by atomic interactions & gravity
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
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
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
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
UGC 03214: edge-on spiral galaxy in Orion
NGC 1087 spiral galaxy in Aries
Clusters of Galaxies: Size ~ 1025 cm ~ Megaparsec (Mpc)
Mass ~ 1015 Msun
Largest gravitationally bound objects: galaxies, gas, dark matter
Cluster of Galaxies
`giant arcs’ are galaxies behind the cluster, gravitationally lensed by it
Gravitational Lensing
Basically, the same effects that occur in more familiar optical
circumstances: magnification and distortion
Apparent position 2
True position 2
Apparent Position 1
True Position 1
Objects farther from
the line of sight are
distorted less.
Observer
Gravitational “lens”
“Looking into” the lens:
extended objects are
tangentially distorted...
Helen Frieman
b. 9/20/99
Helen behind
a Black
Hole
Gravitational
Lens
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
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
Superclusters and Large-scale Structure:
Filaments, Walls, and Voids of Galaxies
100 Million parsecs (Mpc)
You Are Here
Superclusters and Large-scale Structure:
Filaments, Walls, and Voids of Galaxies
Coma cluster
of galaxies
100 Million parsecs (Mpc)
You Are Here
Early
SDSS
Data
~200,000
Galaxies
Mapped in
3D so far
The Big Bang Theory:
a well-tested framework for understanding the observations
and 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
Not `just a theory’, but one of the most firmly established
paradigms in science:
The Standard Cosmological Model
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.
The Big Bang Theory:
Are there human implications?
(as seen on public buses and roadside billboards)
Spectrum of
Light from
Galaxies
receding
slowly
Redshift
of Galaxy
Emission &
Absorption
Lines:

recession
velocity
v/c ≈ z = /0
(approximation
for objects
moving with
v/c << 1)
receding
quickly
Hubble
Space
Telescope
in Orbit
Measured
distances to
galaxies
using Cepheid
Variable stars
Hubble
(1929)
Hubble
Space
Telescope
(2000)
Modern
`Hubble
Diagram’
Extend to
larger
distances
using
objects
brighter
than
Cepheids
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
CMB
Earth
(nearly)
isotropic
not
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).
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
The Microwave
Sky:
COBE
Map of the
Temperature
of the Universe
T = 2.7 degrees
above
absolute zero
Red: 2.7+0.001
Blue:2.7-0.001
Map with
Dipole anisotropy
removed:
fluctuations of
the density of
the Universe (plus
Galactic emission)
Red:
2.7+0.00001 deg
Blue:
2.7-0.00001 deg
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’
Physical Implications of Expanding Universe
An expanding gas cools and becomes less dense as
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
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.
BBN predicted abundances
Fraction of
baryonic
mass in He4
Deuterium to
Hydrogen
ratio
h = H0/(100 km/sec/Mpc)
Light
Element
abundances
depend
mainly on
the density
of baryons
in the
Universe
Lithium to
Hydrogen
ratio
baryon/photon ratio
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
Recent CMB experiments:
Going to smaller angular scales  higher resolution
Boomerang
Recent CMB Anisotropy Experiments: South Pole
DASI
CMB Angular Power Spectrum
Angular power spectrum is a statistical way to characterize
the spatial structure in a 2-dimensional image or map
Oscillation of the PhotonBaryon fluid when the
Universe was 400,000 yrs old
 Imprint on the Microwave
sky
Theoretical dependence of CMB
anisotropy on the baryon density
Angular frequency
Angular separation on the sky
Microwave Background Anisotropy
Probes b (Baryon Density)
b = 0.04
Boomerang experiment (2001)
DASI experiment (2001)
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
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
Einstein: space can also be globally curved
What is the geometry of three-dimensional space?
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.
Position
of first
Peak
probes
the
spatial
Curvature
of the
Universe
Microwave Background Anisotropy
Probes Spatial Curvature
0 = 1.03
0.06
Boomerang experiment (2001)
0 = 1.04
0.06
DASI experiment (2001)
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: m
Current evidence:
Galaxy kinematics
X-ray gas
From galaxy clusters
and other probes:
m ~ 0.3
Lensing
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
Clusters of Galaxies: Size ~ 1025 cm ~ Megaparsec (Mpc)
Mass ~ 1015 Msun
Largest gravitationally bound objects: galaxies, gas, dark matter
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
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
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*:
Dubya
DARK ENERGY
Characterize by its equation of state:
*more specifically, p < 3 (w < 1/3)
pressure
w = p/
density
p =  (w = 1)
Size of the
Universe
Accelerating
SNe Ia + CMB
indicate
m  0.3
DE  0.7
Empty
Open
Closed
Today
Cosmic Time
Evidence for Dark Energy
I.
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
Discovery
of SNe Ia
at `high’
redshift
z ~ 0.5 – 1
Intrinsic Brightness
vs. Time
Physical model:
White dwarf star,
accreting mass from a
companion star,
explodes when it
exceeds a critical
mass (Chandrasekhar)
Luminosity
Type Ia
Supernovae
Peak Brightness
as a calibrated
`Standard’
Candle
Time
Apparent Brightness
42 SNe Ia
distance
m(z) = M+5log(H0dL)=(1+z)  dz’/H(z’)
CMB and Supernovae
• CMB + SNIa
• orthogonal constraints
m = 0.31
L = 0.71
Dark matter density
0.13
0.11
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
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
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.
Inflation Models: Scalar Field slowly rolls down a hill
Potential energy
density
High
Temp.
High Temperature:
Symmetry is restored,
 = 0.
Low
Temperature
Low Temperature:
Symmetry is broken
 = + or -
field
Potential energy function must be fairly
`flat’ so the field rolls slowly: probably not a
Higgs field, must be something else
After rolling down, scalar field oscillates around
the bottom REHEATING
Potential energy
density
High Temperature:
Symmetry is restored,
 = 0.
High
Temp.
Low
Temperature
Low Temperature:
Symmetry is broken
 = + or -
At the end of inflation,
the Universe is very cold.
field

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?
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
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.
1-dimensional
cross-section
field space
field
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
The Structure Formation Cookbook
1. Initial Conditions: Start with a Theory for the Origin of
Density Perturbations in the Early Universe
Your Favorite Inflation 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.
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
Evolution of
Structure in the
Universe
SDSS 2.5 meter Telescope
Galaxy
Clustering
in the
SDSS
Redshift
Survey
~100,000
galaxies
Voids, sheets,
filaments
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 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?
CMB Sky:
1992
circa Jan.
2003
MAP
Satellite
launched
June
2001
Planck
Satellite
planned
for
~2008
Proposed
satellite
mission to
observe
several
thousand
SNe Ia out to
z ~ 1.7
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
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