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Gravitational Lensing: How to See the Dark. J. E. Bjorkman University of Toledo Department of Physics & Astronomy. The Dark Between the Light. Dark Matter. How do we know its there? Answer: It affects the motion of everything we can see. Cluster Simulation Rotation Velocities.

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Gravitational Lensing: How to See the Dark

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Gravitational Lensing: How to See the Dark

J. E. BjorkmanUniversity of Toledo

Department of Physics & Astronomy


The Dark Between the Light


Dark Matter

  • How do we know its there?

  • Answer: It affects the motion of everything we can see.

    • Cluster Simulation

    • Rotation Velocities


Galactic Rotation Curves


Missing Mass in our Galaxy


What is the Dark?

  • MACHOs (Massive Compact Halo Objects)

    • low mass stars - "brown dwarves"

    • "almost" stars (planets, e.g. Jupiters)

    • black holes of less than solar mass

    • The VW graveyard

  • WIMPs (Weakly Interacting Massive Particles)

    • heavy neutrinos (10 to 1000 GeV)

    • new particles predicted by Supersymmetry - 'neutralinos'

    • exotic particles – e.g. axions (particles with mass < 0.1 eV)

  • Modified Gravity - on galactic scales.


Where is the Dark?


Gravity Bends Light (Einstien)


Gravitational Lenses


Einstein tells Eddington gravity bends starlight.


Eclipse Astrometry

  • How do we know the stars moved?


Relativity Verified


Discovery of a Gravitational Lens


Galaxies as Lenses


A Lensing Simulation


A Lens Gallery


Galaxy Clusters as Lenses


Measuring the Dark

  • 0.5% of Universe is luminous

  • 99.5% of Universe is dark matter


“Stellar Lenses”Orion behind a Black Hole


Gravitational Microlenses

  • What are microlenses?

    • Stellar mass (or smaller) lenses

    • Images are unresovled (milliarcsecond separation)

    • Lens focuses light

    • Object appears brighter (several magnitudes!)

  • That’s absurd!

    • You’ll never see one in a million years!

  • Answer – just look at million stars every night!


Microlensing Searches

  • Toward the Magellanic Clouds

    • MACHO (MAssive CompactHalo Objects collaboration)

    • EROS (Experience pour la Recherche d'Objets sombres)

    • DUO (Disk Unseen Objects)

  • Toward the Galactic Bulge

    • OGLE (Optical Gravitational Lensing Experiment)

  • Toward M31:

    • AGAPE (Andromeda Galaxy Amplified Pixel Experiment)

    • MEGA


Ogling the Stars


AGAPE at M31


Looking Through a Lens


A Lens in Motion


What You Really See


Looking for Lenses in Haystacks


Frequency of Events


How Big is the Lens? How Close did it get?


What are They?


Follow-Up Monitoring

  • PLANET (Probing Lensing Anomalies NETwork)

  • Garching Spectroscopic Monitoring Group

  • GMAN (Global Microlensing Alert Network)

  • MPS (Microlensing Planet Search Project)

  • MOA (Microlensing Observations in Astrophysics)


Looking Through BifocalsBinary Stars as Lenses


Binary Stars as Lenses


Looking for Planets


The Planet Search


Micolensing Results

  • They Exist! Future surveys will detect 1/day

  • Fewer than expected toward LMC/SMC

    • 50% of halo may be Machos (M = 0.5Msun)

  • More than expected toward Galactic center

    • Masses are few 0.1 Msun

    • May indicate presence of bar (i.e., Milky Way is a barred spiral)

  • About 10% are binary events

  • Planets

    • No definite detections, yet

    • Fewer that 1/3 of lenses have Jupiter-mass planets at 1-4 AU


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