the dark universe
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THE DARK UNIVERSE. The Copernican Revolution continues… Caty Pilachowski, Mini-University 2010. DARK MATTERS. “Extraordinary claims require extraordinary evidence.” (Carl Sagan) “Extraordinary claims require extraordinary proof.” ( Marcello Truzzi ) - PowerPoint PPT PresentationTRANSCRIPT
THE DARK UNIVERSE
The Copernican Revolution continues…
Caty Pilachowski, Mini-University 2010
DARK MATTERS• “Extraordinary claims require
extraordinary evidence.” (Carl Sagan) • “Extraordinary claims require
extraordinary proof.” (Marcello Truzzi) • “The weight of evidence for an
extraordinary claim must be proportioned to its strangeness” (Laplace)
• “A wise man, therefore, proportions his belief to the evidence” (David Hume)
Journey into the
Dark Universe Foundation Concepts
Key Observations Explaining Our Universe Concordance Model History of the Universe
Physical SizesEarth’s Diameter: 13000 kmMoon’s Diameter: ¼ Earth’s DiameterMoon’s Distance: 400,000 kmDistance from Sun: 150,000,000 km
8 light minutes
The Universe We See:
Earth and Moon
Physical Size Diameter: 1,400,000 km
(about 100 x the diameter of Earth)Distance: about 100 x the Sun’s diameter
The Universe We See: The Sun
The Univers
e We See:The
Nearest Stars
The closest star to our Sun is Proxima Centauri, about 4 light years distant(that’s about 40 trillion km)
The Universe We See: The Milky Way Galaxy
Our Milky Way galaxy contains two hundred billion stars
The Sun is about 26,000 light
years from the center
100,000 LY
Our Milky Way Galaxy is part of a cluster of about 3 dozen galaxies
The Universe We See: The Local Galaxy Group
Andromeda is 3 million LY distant
Virgo Supercluster
Our Local Group of galaxies is part of a larger Supercluster of galaxy groups
The Universe We See: The Local Supercluster
Virgo is 60M LY distant
Galaxies and clusters of galaxies collect into vast streams, sheets, and walls of galaxies.
The Univers
e We See
The Visible Universe On the largest scales, the universe seems to be more or less uniform
The Universe we see is made of hydrogen and helium
everything
else90% hydrogen atoms10% helium atoms
Less than 1% everything else(and everythingelse is made in stars!)
Astronomers can see into the past
The Universe we see far away appears younger than the Universe
nearby
“Lookback time”
Foundation ConceptsKey Observations Explaining our Universe Concordance Model History of the Universe
Missing MassExpansionAcceleration
THE DARK UNIVERSE
The Case of the Missing Mass
• In the 1930s, astronomer Fritz Zwicky noticed that galaxies in clusters were moving at speeds to fast to hold the cluster together
• In the 1950s, astronomer Vera Rubin found that galaxies were spinning too fast to hold together
Galaxies and galaxy clusters both behave as if more mass is present than we can see…
Mass within Sun’s orbit:
~1011 MSun
Total mass:
~1012 MSun
Galaxy Rotation
What’s the PROBLEM???The orbits of stars suggest that galaxies contain
several times more mass that we can find in stars, gas and dust
MISSING MASS!
Dark matter is the material believed to account for the discrepancy between the mass of a galaxy as found from the orbits of stars and the mass observed in the form of gas and dust
The visible portion of a galaxy lies deep in the heart of a large halo of dark matter
Evidence for Dark MatterRotation
of galaxies
Velocities of stars in dwarf
galaxies
Velocities of galaxies in clusters
Gravitational lensing
Collisions of galaxy clusters
Hot gas in galaxy
clusters
Galaxy interactions
Velocity Dispersions in
Dwarf Galaxies
Count the stars Add up the light Look for any gas Add up the mass
Velocity Dispersions in Dwarf Galaxies
From spectra and the Doppler shift Measure the velocity dispersion Determine the total mass
Calculated for a sample of 194 stars with 32-33 stars per bin
astro-ph/0704126
M/L Ratios for MW DwarfsGalaxy MV L Radiu
sTotal mass M/L Gas
Fraction
(mag) (106 LSun) (pc) (106
MSun) (Sun=1)
Sculptor -11.1 2.15 110 6.4 3.0 0.004
Phoenix -10.1 0.90 310 33 37 0.006
Fornax -13.2 15.5 460 68 4.4 <0.001
Carina -9.3 0.43 210 13 31 <0.001
Leo I -11.9 4.79 215 22 4.6 <0.001
Sextans -9.5 0.50 335 19 39 <0.001
Leo II -9.6 0.58 160 9.7 17 <0.001 Ursa Minor -8.9 0.29 200 23 79 <0.002
Draco -8.8 0.26 180 22 84 <0.001
Galaxy interactions require more mass than we can see
Antennae Galaxy (HST)Computersimulation
The real thing
Evidence for dark matter in clusters of galaxies
We can measure the velocities of galaxies in a cluster from their Doppler shiftsThe mass we find from galaxy motions in a cluster is about 50 times larger than the mass in stars!
HOT GAS IN GALAXY CLUSTERSClusters contain X-ray emitting hot gasTemperature of hot gas tells us cluster mass 7 x more gas than stars, but not enough!
Visible Light X-Ray Light
Coma Cluster of Galaxies
Gravitational Lensing
Light from a distant galaxy bends around a massive object (such as cluster of galaxies) between the distant galaxy and the observer
Gravitational lensing is predicted by Einstein's theory of general relativity
General Relativity
Gravity bends the paths of light rays
Gravity curves space, so mass acts as a lens
The paths of all objects, whether or not they have mass, are curved if they pass near a massive body
Prediction confirmed in the 1919 solar eclipse
Discovering Gravitationa
l Lenses
Mysterious arcs discovered in 1986 Cluster Abell 370 (left) Cluster C12244 (right)
The clusters and the arcs are at very different distances
The arcs are highly distorted, very distant galaxies
Gravitational Lenses Produce Multiple Images
source andlens aligned
source andlens not aligned
If the source, the lens, and the observer lie in a straight line, the source will appear as a ring around the lens
If the lens is off-center, multiple, distorted images will be seen
Cluster of Galaxies Cl0024+16The reddish
objects are galaxies in the lensing cluster
The bluish objects are multiple images of a much more distant galaxy
Reconstruct the distant galaxy from individual pieces of the arc
Arcs let us map the distribution of dark matter in clusters of galaxies
The Bottom Line…The visible matter
does not provide enough gravity to produce the gravitational lenses we see from galaxies and galaxy clusters
Dark matter must be present to account for what we observe
cluster center
1. 2% Stars2. 13% Hot Gas3. 85% Dark Matter
The galaxies we see are only 2% of the mass
A cluster of galaxies consists of three components
1E 0657-56 – The Bullet Cluster
Direct observation of Dark Matter
What’s going on with Cluster 1E 0657-56?
• TWO clusters of galaxies collide
The gas interacts, the dark matter and galaxies don’t
The galaxies and dark matter pass through unimpeded, but the hot gas is separated from the clusters
The Bullet Cluster
Direct observation of Dark Matter
False Color:Blue = DMRed= Hot GasWhite = Galaxies
All methods of measuring cluster mass indicate similar amounts of dark matter
Clusters of Galaxies
Hercules
CentaurusComa
Perseus
Gravity holds
clusters together
Gotta have Dark Matter!
Small galaxies form first, grow, and merge to form larger galaxies
The two objects approaching at the end will merge in about a billion years
Many of the small galaxies become satellites orbiting larger galaxies
4.3 Mpc or 14 million LY
Galaxy formation is dominated by the gravitational pull of dark matter
Dark Matter makes galaxies grow
The cosmic web of dark matter, gas, and galaxies in a young universe
Intergalacticgas
Clumpsconcentratedby darkmatter lead to galaxies
Galaxies Grow through MergersGalaxy
building blocksobserved with
Hubble
SimulationThe real thing
Dark Matter The universe contains matter we cannot
see Dark matter interacts with normal
matter through gravity Dark matter does NOT interact with light
the way the normal matter does The Universe contains 5 or 6 times
MORE dark matter than normal matterAll galaxies are embedded in clouds of
dark matter
What is DARK MATTER?Can’t see it, taste it, touch it, smell
it…We can only detect it by gravityWe don’t know!Detecting Dark Matter is one of the most active areas of high energy physics, and a reason to build large accelerators.
So, What Could It Be?Dark Matter Candidates:Black holes
Low-mass objects like loose planets
Elementary particles
What about WIMPS??
• “Weakly Interacting Massive Particles”– As yet undiscovered elementary
particles• High energy particle theories suggest
such elementary particles exist WIMPS are a plausible, but not firm,
consequence of several theories in particle physics
What about Dark Energy?First, the expansion of the Universe!
Slipher*, Hubble, and Einstein
*V. M. Slipher is an IU alumnus!
Hubble found that the spectra of more distant galaxies are shifted toward the red – the further the galaxy, the larger the shift
Hubble’s Law
Distance - Velocity Relation
0
200
400
600
800
1000
0 10000 20000 30000 40000 50000 60000 70000
Velocity (km/sec)
Dis
tanc
e (M
pc)
More distant galaxies are moving away from us at greater speedTHE UNIVERSE IS EXPANDING
It depends on the density of the UniverseThe critical density in the current epoch is 10-29
g/cm3, about one hydrogen atom per cubic meter
About 25 times more than the observed mass of stars and gas
Will dark matter recollapse the Universe?There isn’t nearly enough to re-collapse the Universe
Will the Universe Keep Expanding Or Re-
Collapse????
But the universe isn’t even slowing down….
The more we learn, the stranger it gets…
The speed of a ball tossed up in the air slows down because of gravity
Observations at the end of the 20th century established that the Universe is not just coasting, or slowing down because of its own gravitational pull, but actually speeding up.
We observe Type Ia supernovae (exploding dead stars) to measure the distances of extremely distant galaxies
This tells us how fast the Universe expanded when it was youngerThe speeds of very distant galaxies tell us the
Universe is expanding faster today than in the past
Supernova 1998ba
WhyType Ia Supernovae
?All Type Ia supernovae are due to implosion
of dead stars
They all have the same amount of fuel to burn and produce the same luminosity
They therefore act like “standard candles”
Distant Type Ia Supernovaeare too faint…Distant
supernovae are further away than predicted by Hubble’s Law
The expansion of the Universe was slower in the past
The universe is expanding faster today than it did in early times
This expansion cannot be caused by ordinary matter or dark matter
The acceleration suggests the possibility of a new type of repulsive force (anti-gravity) that acts on very large scales
The Universe is speeding up!
DARK ENERGY
The universe is NOT what it seems…
When Einstein developed the theory of general relativity, astronomers thought the Universe was “static” – neither expanding nor contracting
Einstein included a repulsive force called the cosmological constant to balance gravity (anti-gravity!)
The cosmological constant was rejected when we learned the Universe was expanding
The cosmological constant is now needed to explain why the Universe is speeding up
A little history….
A “fifth force?” …in addition to strong force (holds nuclei together)weak force (interactions of electrons)electrical/magnetic force (holds atoms
together)gravity (works over large distances)
Something else?
Another big question!
What is Dark Energy?
Foundation ConceptsKey ObservationsExplaining our
UniverseConcordance ModelHistory of the Universe
The Big Bang & The Dark Universe
THE DARK UNIVERSE
Origin of Structure
Inflation
• The Universe began in a very hot, very dense, very compact state
• All of our visible universe was contained in a very small volume
The Big Bang!
• Space expanded to its present volume
• The Universe continues to expand today – and dark energy is increasing the rate of expansion
Evidence for the Big
Bang
The expansion of space• The abundance of helium (and other
light isotopes)• Cosmic microwave background radiation
Structure in Our Universe
We see galaxies organized in a fluffy, sponge-like web in space
Galaxies are grouped in clusters, sheets, and filaments that surround great voids
Seeing Structure in the Hubble Deep Field
Studying the locations of galaxies in the HDF shows the structure of the Universe
Why is DARK MATTER important?
The formation of structure and of galaxies requires the extra mass provided by dark matter
Without dark matter, the Universe as we know it would not
exist
Dark Matter Dominates the
Structure of the Universe
Center for Cosmological Physics,University of Chicago
http://cosmicweb.uchicago.edu/index.html
The formation of clusters and filaments in a universe filled with cold dark matter
The box is 140 million light years on a side Simulation begins when the Universe is less than 1%
of its current age and matter is uniformly distributed Small fluctuations grow to large structures Structures formed quickly
Explaining the Origin of Structure
• The simple Big Bang model gives a Universe with no structure
• To explain why the Universe has structure, we need “inflation”
What is Inflation?• In the first 10-38 to 10-36
seconds, regions of the Universe expanded from the size of an atomic nucleus to the size of the Solar System
• The rate of expansion then slowed
• Prior to inflation, matter on opposite sides of our Universe was in contact
• Quantum fluctuations prior to inflation became the seeds of structure
Inflation can make all the structure by stretching tiny quantum ripples to enormous size
These ripples in density then become the seeds for all structures in the Universe
Foundation Concepts
Key ObservationsExplaining our
UniverseConcordance ModelHistory of the
Universe The Modern Cosmologists View of the Universe
THE DARK UNIVERSE
Basic Constraint!• The Universe should not be younger
than the objects in it
• The oldest stars are 13 billion years old
• The Universe must be at least 13 billion years old
The Globular Cluster Messier 3
Mass• Mass alters the expansion of the
Universe through its gravitational effect on space
• • With enough mass in the Universe,
gravity can halt the expansion and cause the Universe to recollapse on itself
• Dark matter and regular matter together provide just 1/3 the amount of mass needed to stop the Universe from expanding forever
The Concordance Model
• Three factors control the fate of the Universe: – the current expansion rate– the amount of mass in the universe– the acceleration factor of the
universe• The Universe is 13.7 billion years
old• The Universe will expand forever,
at a faster and faster rate
Foundation ConceptsKey ObservationsExplaining our
UniverseConcordance ModelHistory of the
Universe*
THE DARK UNIVERSE
*Thanks to Fred Adams, U Michigan
The History…The Universe evolved quickly after the
Big BangStars and galaxies formed within 300
Myr
Gravity vs. Dark EnergyDark energy was
insignificant at early times, otherwise stars and galaxies could not have formed
Gravity dominated when galaxies were close together
As the Universe expanded, galaxies grew further apart, dark energy began to dominate gravity
The FutureDEGENERATE ERA – 10 trillion trillion trillion years after the Big Bang
• Planets detach from stars• Stars and planets evaporate from galaxies• Most ordinary matter in the universe is locked up in degenerate stellar remnants• Eventually, even the protons themselves decay
BLACK-HOLE ERA - 10,000 trillion trillion trillion trillion trillion trillion trillion trillion years after the Big Bang
• The only large objects remaining are black holes, and even they evaporate
THE FINAL, DARK ERA – Only photons, neutrinos, electrons and positrons remain, wandering through a universe bigger than the mind can conceive.
Occasionally, electrons and positrons meet and form "atoms" larger than the visible universe is today.
From here into the infinite future, the universe remains cold, dark and empty.
Final ThoughtsTHE DARK UNIVERSEThe possibility exists that our “bubble
universe” is only one of many universes that could have formed, Each with its own, different version of the Concordance Model
Other universes could have very different physical conditions and we will never see them – they may be on different “branes” or in other dimensions that we cannot measure
From the Universe to the Multiverse
Not only do we not occupy a preferred place in our Universe, we may not occupy any preferred universe in the Multiverse!
Origin of the CMB – the thermal radiation of the first atoms
Isotropic microwave radiation
Testing Inflation
Why is the Cosmic Microwave Background Radiation almost perfectly isotropic?
The CMB is isotropic because regions now on opposite sides of the sky were close together before inflation pushed them far apart
Testing Inflation
Patterns of structure observed by WMAP show the “seeds” of universe
Observed patterns of structure in the Universe agree (so far) with the “seeds” that inflation would produce
WMAP’s observation of the “seeds” of structure inferred from the CMB confirm
the existence of dark energy
• Overall geometry is flat – Total mass+energy has critical density
• Ordinary matter ~ 4.4% of total• Total matter is ~ 27% of total– Dark matter is ~ 23% of total– Dark energy is ~ 73% of total
• Age of 13.7 billion years
Recall the Uncertainty Principle
• The uncertainty principle states that you cannot know both the position x and the momentum p of a particle more precisely than Planck’s constant h/2p “h-bar”
• When dimensions are small, particles must therefore move in order to satisfy the uncertainty principle
• This motion creates a “zero point energy” > 0Uncertainty Principle Dx Dp = h/2
The Universe We See in Microwave
LightIn 1965, two engineers at AT&T’s Bell Labs in New Jersey were puzzled by a constant “noise” in a microwave antenna they were calibrating
Cosmic MicrowavesThe noise
could not be explained by terrestrial or extraterrestrial sources. It comes from every direction
1978 Nobel Prize in Physics to Penzias and Wilson
A uniform, faint signal from all over the sky
Cosmic Microwaves
• The temperature of the Universe is 2.726 K
• The Universe was once much hotter• The light stretched from visible light
to microwave light as the Universe expanded
Spectrum of a 2.726 K degree
blackbody
COBE data points
The universe was hot and dense in the distant pastCOBE
WMAP: Wilkinson Microwave
Anisotropy Probe
The microwave light captured in this picture is from 380,000 years after the Big Bang, over 13 billion years ago
WMAP shows the whole sky in microwavesTiny temperature differences from place to place are
the beginning of cosmic structure in the Universe Temperature varies by only millionths of a degree
A baby picture of the
Universe
• Imagine a temperature map of the Earth in June 1992• WMAP makes a similar map of the sky
What are we seeing in the baby picture?
The microwave radiation comes from the time when the temperature of the Universe became low enough for atoms to form (about 3000K)
Matter became transparent, allowing light to travel great distances
It is like seeing the bottom layer of clouds on an overcast day.
Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3,000 K: visible/IR
On what angular scales do we see variations in the
CMB?
A “Power Spectrum”How strong are variations on different angular scales?
Different angular scales probe different aspects the early
Universe
• Parameters include an expansion rate, a composition, age, etc.
• Astronomers compute a complex physical model of conditions and adjust parameters to match the observed curve
A simple “Big Bang” doesn’t explain all the properties of the Universe
1)Where does structure come from?2)Why is the overall distribution of
matter so uniform?3)Why is the density of the universe so
close to the critical density?
Explaining the Origin of Structure
• The simple Big Bang model gives a Universe with no structure
• To explain why the Universe has structure, we need “inflation”
What is Inflation?• A period of
extremely rapid expansion when the Universe was very young.
• 10-38 to 10-36 seconds
• Regions of the Universe expanded from the size of an atomic nucleus to the size of the Solar System
Alternative to Dark Matter: MOND - Modified Newtonian Dynamics
MOND can‘t explain DM in clusters and far out in halos
For accelerations a less than a0, reduce gravity acceleration by the factor a/a0
a(a/a0) = GM/r2
This gives flat rotation curves
A single value of a0 works for galaxy rotation curves
But MOND is untested experimentally
MOND can’t explain it all• While MOND can reproduce galaxy
rotation curves, it is harder to explain– Galaxy cluster velocity dispersions– Observations of gravitational lenses– The Bullet Cluster and the DM ring
• MOND still requires DM to account for all the observations
• Which is a simpler explanation, DM or MOND+DM?
Galaxy FormationGalaxies form when the
Universe was youngGalaxies form where filaments
of dark matter intersectFilaments provide gas and
dark matterThe gas fuels star formation,
while the galaxy cannibalizes dark matter and smaller galaxies
36 kpc
288 kpc
72 kpc
144 kpc
Dark matter provides the gravitational mass necessary
to form galaxies
Summary: Dark Matter EvidenceMany dynamical phenomena cannot be
explained with the observed mass content of the universe
Problem can be solved with one radical assumption
85% of all matter is dark matter initially distributed as ordinary matterinteracts with normal matter only through
gravityStars, gas are now more concentrated
than dark matter
WMAP’s observation of the “seeds” of structure inferred
from the CMB confirm the existence of dark energy• Overall geometry is flat – Total mass+energy has critical density
• Ordinary matter ~ 4.4% of total• Total matter is ~ 27% of total– Dark matter is ~ 23% of total– Dark energy is ~ 73% of total
• Age of 13.7 billion years