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