Astronomy 305 frontiers in astronomy
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Astronomy 305/Frontiers in Astronomy. Class web site: Office: Darwin 329A and NASA EPO (707) 664-2655 Best way to reach me: [email protected] Group 14. Great job, Group 14!. What is the Universe made of?. Regular matter

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Astronomy 305/Frontiers in Astronomy

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Astronomy 305/Frontiers in Astronomy

Class web site:

Office: Darwin 329A and NASA EPO

(707) 664-2655

Best way to reach me: [email protected]

Prof. Lynn Cominsky

Group 14

Great job, Group 14!

Prof. Lynn Cominsky

Prof. Lynn Cominsky

What is the Universe made of?

  • Regular matter

    • Heavy elements 0.03%

    • Stars 0.5%

    • Free Hydrogen and Helium 4% (Lecture 11)

  • Neutrinos 0.3% (Lecture 10)

  • Dark Energy 60% (Lecture 13)

  • Dark Matter 30% (this lecture)

You can't see it but you can feel it!

Prof. Lynn Cominsky

Kepler’s Third Law movie

  • P2 is proportional to a3

Prof. Lynn Cominsky

Dark Matter Evidence

  • In 1930, Fritz Zwicky discovered that the galaxies in the Coma cluster were moving too fast to remain bound in the cluster according to the Virial Theorem

KPNO image of the Coma cluster of galaxies - almost every object in this picture is a galaxy! Coma is 300 million light years away.

Prof. Lynn Cominsky

Virial Theorem

  • Stable galaxies should obey this law: 2K = -U

  • where K=½mv2 is the Kinetic Energy

  • U = -aGMm/r is the Potential Energy (a is usually 0.5 - 2, and depends on the mass distribution)

  • Putting these together, we have M=v2r/aG.

  • Measure M, r and v2 from observations of the galaxies; then use M and r to calculate vvirial

  • Compare vmeasured to vvirial

  • vmeasured > vvirial which implies M was too small

Prof. Lynn Cominsky

NGC 3198

Dark Matter Evidence

  • Measure the velocity of stars and gas clouds from their Doppler shifts at various distances

  • Velocity curve flattens out!

  • Halo seems to cut off after r= 50 kpc

  • Galaxy Rotation Curves

v2=GM/r where M is mass within a radius r

Since v flattens out, M must increase with increasing r!

Prof. Lynn Cominsky

Dark Matter Evidence

  • Cluster Mass Java simulation

  • Rotation Curve Java simulation

Prof. Lynn Cominsky

Dark Matter Evidence

  • Measure the mass of light emitting matter in galaxies in the cluster (stars)

  • Measure mass of hot gas - it is 3-5 times greater than the mass in stars

  • Calculate the mass the cluster needs to hold in the hot gas - it is 5 - 10 times more than the mass of the gas plus the mass of the stars!

  • Hot gas in Galaxy Clusters

Prof. Lynn Cominsky

Dark Matter Halo

  • The rotating disks of the spiral galaxies that we see are not stable

  • Dark matter halos provide enough gravitational force to hold the galaxies together

  • The halos also maintain the rapid velocities of the outermost stars in the galaxies

Prof. Lynn Cominsky

Types of Dark Matter

  • Baryonic - ordinary matter: MACHOs, white, red or brown dwarfs, planets, black holes, neutron stars, gas, and dust

  • Non-baryonic - neutrinos, WIMPs or other Supersymmetric particles and axions

  • Cold(CDM) - a form of non-baryonic dark matter with typical mass around 1 GeV/c2 (e.g., WIMPs)

  • Hot (HDM) - a form of non-baryonic dark matter with individual particle masses not more than 10-100 eV/c2 (e.g., neutrinos)

Prof. Lynn Cominsky

Hydrogen = 1p + 1e

Deuterium = 1p + 1e + 1n

Helium = 2p + 2e + 2n

Primordial Matter

  • Normal matter is 3/4 Hydrogen (and about 1/4 Helium) because as the Universe cooled from the Big Bang, there were 7 times as many protons as neutrons

  • Almost all of the Deuterium made Helium

Prof. Lynn Cominsky

Primordial Matter

  • The relative amounts of H, D and He depend on h = (protons + neutrons) / photons

  • h is very small - We measure about 1 or 2 atoms per 10 cubic meters of space vs. 411 photons in each cubic centimeter

  • The measured value for h is the same or a little bit smaller than that derived from comparing relative amounts of H, D and He

  • Conclusion:we may be missing some of baryonic matter, but not enough to account for the observed effects from dark matter!

Prof. Lynn Cominsky

Baryonic Dark Matter

  • Baryons are ordinary matter particles

  • Protons, neutrons and electrons and atoms that we cannot detect through visible radiation

  • Primordial Helium (and Hydrogen) – recently measured – increased total baryonic content significantly

  • Brown dwarfs, red dwarfs, planets

  • Possible primordial black holes?

  • Baryonic content limited by primordial Deuterium abundance measurements

Prof. Lynn Cominsky

Baryonic - Brown Dwarfs

  • Mass around 0.08 Mo

  • Do not undergo nuclear burning in cores

  • First brown dwarf star Gliese 229B

Prof. Lynn Cominsky

Expected 38 red dwarfs: Seen 0!

Baryonic - Red Dwarf Stars

  • HST searched for red dwarf stars in the halo of the Galaxy

  • Surprisingly few red dwarf stars were found, < 6% of mass of galaxy halo

Prof. Lynn Cominsky

Ghost Galaxies

  • Also known as low surface brightness galaxies

  • Studies have shown that fainter, elliptical galaxies have a larger percentage of dark matter (up to 99%)

  • This leads to the surprising conclusion that there may be many more ghostly galaxies than those we can see!

  • Each ghost galaxy has a mass around 10 million Mo

Prof. Lynn Cominsky

Baryonic –MACHOs

  • Massive Compact Halo Objects

  • Many have been discovered through gravitational micro-lensing

  • Not enough to account for Dark Matter

  • And few in the halo!

Mt. Stromlo Observatory in Australia (in better days)

Prof. Lynn Cominsky

Baryonic – MACHOs

  • 4 events towards the LMC

  • 45 events towards the Galactic Bulge

  • 8 million stars observed in LMC

  • 10 million stars observed in Galactic Bulge

  • 27,000 images since 6/92

Prof. Lynn Cominsky

Gravitational Microlensing

  • Scale not large enough to form two separate images


Prof. Lynn Cominsky

Baryonic – black holes

  • Primordial black holes would form at 10-5 s after the Big Bang from regions of high energy density

  • Sizes and numbers of primordial black holes are unknown

  • If too large, you would be able to see their effects on stars circulating in the outer Galaxy

  • Black holes also exist at the centers of most galaxies – but are accounted for by the luminosity of the galaxy’s central region

Prof. Lynn Cominsky

So, it must be a black hole!

Black Hole MACHO

  • Isolated black hole seen in Galactic Bulge

  • Distorts gravitational lensing light curve

  • Mass of distorting object can be measured

  • No star is seen that is bright enough…..

Prof. Lynn Cominsky

Strong Gravitational Lensing

Prof. Lynn Cominsky

Strong Gravitational Lensing

  • HST image of background blue galaxies lensed by orange galaxies in a cluster

  • “Einstein’s rings” can be formed for the correct alignment

Prof. Lynn Cominsky

Large Survey Synoptic Telescope

  • At least 8 meter telescope

  • About 3 degree field of view with high angular resolution

  • Resolve all background galaxies and find redshifts

  • Goal is 3D maps of universe back to half its current age

Prof. Lynn Cominsky

Gravitational Lens Movie #1

  • Movie shows evolution of distortion as cluster moves past background during 500 million years

  • Dark matter is clumped around orange cluster galaxies

  • Background galaxies are white and blue

Prof. Lynn Cominsky

Gravitational Lens Movie #2

  • Movie shows evolution of distortion as cluster moves past background during 500 million years

  • Dark matter is distributed more smoothly around the cluster galaxies

  • Background galaxies are white and blue

Prof. Lynn Cominsky


Strong Gravitational Lensing

  • Spherical lens

  • Perfect alignment

  • Note formation of Einstein’s rings

Prof. Lynn Cominsky


Strong Gravitational Lensing

  • Elliptical lens

  • Einstein’s rings break up into arcs if you can only see the brightest parts

Prof. Lynn Cominsky

Gas clouds in Lagoon nebula

Baryonic – cold gas

  • We can see almost all the cold gas due to absorption of light from background objects

  • Gas clouds range in size from 100 pc (Giant Molecular Clouds) to Bok globules (0.1 pc)

  • Mass of gas is about the same as mass of stars, and is part of total baryon inventory

Prof. Lynn Cominsky

Dust clouds of the dark Pipe nebula

Baryonic –dust

  • Dust is made of elements heavier than Helium, which were previously produced by stars (<2% of total)

  • Dust absorbs and reradiates background light

Prof. Lynn Cominsky

Non-baryonic: Neutrinos

  • There are about 100 million neutrinos per m3

  • More (or less) types of neutrinos would lead to more (or less) primordial Helium than we see

  • Neutrinos with mass affect the formation of structure in the Universe

    • Much less small scale structure would be present

    • Observed structure sets limits on how much mass neutrinos may have, and on their contribution to dark matter.

  • The sum of all the mn~ 5 h502 eV (due to models of Hot and Cold DM)

Prof. Lynn Cominsky

Non-baryonic - axions

  • Extremely light particles, with typical mass of 10-6 eV/c2

  • Interactions are 1012 weaker than ordinary weak interaction

  • Density would be 108 per cubic centimeter

  • Velocities are low

  • Axions may be detected when they convert to low energy photons after passing through a strong magnetic field

Prof. Lynn Cominsky

Searching for axions

  • Superconducting magnet to convert axions into microwave photons

  • Cryogenically cooled microwave resonance chamber

  • Cavity can be tuned to different frequencies

  • Microwave signal amplified if seen

Prof. Lynn Cominsky

Non-baryonic - WIMPs

  • Weakly Interacting Massive Particles

  • Predicted by Supersymmetry (SUSY) theories of particle physics

  • Supersymmetry tries to unify the four forces of physics by adding extra dimensions

  • WIMPs would have been easily detected in acclerators if M < 15 GeV/c2

  • The lightest WIMPs would be stable, and could still exist in the Universe, contributing most if not all of the Dark Matter

Prof. Lynn Cominsky

CDMS Lab 35 feet under Stanford

Cryostat holds T= 0.01 K


  • Cryogenic Dark Matter Search

  • 6.4 million events studied - 13 possible candidates for WIMPs

  • All are consistent with expected neutronflux

Prof. Lynn Cominsky

Detecting WIMPs?

  • Laboratory experiments - DAMA experiment 1400 m underground at Gran Sasso Laboratory in Italy announced the discovery of seasonal modulation evidence for 52 GeV WIMPs

  • 100 kg of Sodium Iodide, operated for 4 years

  • CDMS has 0.5 kg of Germanium, operated for 1 year, but claims better

    background rejection techniques


Prof. Lynn Cominsky



HDM vs. CDM models

  • Supercomputer models of the evolution of the Universe show distinct differences

  • Rapid motion of HDM particles washes out small scale structure – the Universe would form from the “top down”

  • CDM particles don’t move very fast and clump to form small structures first – “bottom up”

Prof. Lynn Cominsky

Largest structures are now just forming




Critical density

Low density

CDM models vs. density

  • CDM models as a function of z (look-back time)

Prof. Lynn Cominsky

Dark Matter Activity

  • You will search a paper plate “galaxy” for some hidden mass by observing its effect on how the “galaxy” “rotates”

In order to balance, the torques on both sides must be equal:

T1 = F1X1 = F2X2 =T2


F1 = m1g and

F2 = m2g

Prof. Lynn Cominsky


  • Strings are little closed loops that are 1020 times smaller than a proton

  • Strings vibrate at different frequencies

  • Each resonant vibration frequency creates a different particle

  • Matter is composed of harmonies from vibrating strings – the Universe is a string symphony

“String theory is twenty-first century physics that fell accidentally into the twentieth century” - Edward Witten

Prof. Lynn Cominsky


  • Strings can execute many different motions through spacetime

  • But, there are only certain sets of motions that are self-consistent

  • Gravity is a natural consequence of a self-consistent string theory – it is not something that is added on later

Self-consistent string theories only exist in 10 or 26 dimensions – enough mathematical space to create all the particles and interactions that we have observed

Prof. Lynn Cominsky

Superstring Dimensions

  • Since we can observe only 3 spatial and 1 time dimensions, the extra 6 dimensions (in a 10D string theory) are curled up to a very small size

  • The shape of the curled up dimensions is known mathematically as a Calabi-Yau space

Prof. Lynn Cominsky

Superstring Universe

  • At each point in 3D space, the extra dimensions exist in unobservably small Calabi-Yau shapes

Prof. Lynn Cominsky

Superstring Theories

  • There are at least five different versions of string theory, which seem to have different properties

  • As physicists began to understand the mathematics, the different versions of the theories began to resemble each other (“duality”)

  • In 1995, Edward Witten showed how all five versions were really different mathematical representations of the same underlying theory

  • This new theory is known as M-theory (for Mother or Membrane)

Prof. Lynn Cominsky


  • Unification of five different types of superstring theory into one theory called M-theory

  • M-theory has 11 dimensions

Prof. Lynn Cominsky

Some questions

  • Can we find the underlying physical principles which have led to us to string theory?

  • Does the correct string (or membrane) theory have 10 or 11 dimensions?

  • Will we ever be able to find evidence for the curled up dimensions?

  • Is string theory really the long-sought “Theory of Everything”?

  • Will any non-physicists ever be able to understand string theory?

  • Hear and see Brian Greene in NOVA’s “Elegant Universe”

Prof. Lynn Cominsky

Web Resources

  • VROOM visualization of 4 dimensions

  • Ned Wright’s Cosmology Tutorial

  • Fourth dimension web site


Prof. Lynn Cominsky

Web Resources

  • Michio Kaku’s web site

  • E. Lowry’s EM Field in Spacetime

  • Visualizing tensor fields

  • Exploring the Shape of Space

Prof. Lynn Cominsky

Web Resources

  • Astronomy picture of the Day

  • Imagine the Universe

  • Center for Particle Astrophysics

  • Dark Matter telescope

  • Dark Matter Activity #2

Prof. Lynn Cominsky

Web Resources

  • Jonathan Dursi’s Dark Matter Tutorials & Java applets

  • MACHO project

  • National Center for Supercomputing Applications

  • Pete Newbury’s Gravitational Lens movies

Prof. Lynn Cominsky

Web Resources

  • Alex Gary Markowitz’ Dark Matter Tutorial

  • Martin White’s Dark Matter Models

  • Livermore Laboratory axion search

  • Dark Matter Activity #1

Prof. Lynn Cominsky

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