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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

Gravitational Lensing: How to See the Dark

J. E. BjorkmanUniversity of Toledo

Department of Physics & Astronomy



Dark matter
Dark Matter

  • How do we know its there?

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

    • Cluster Simulation

    • Rotation Velocities




What is the dark
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.






Eclipse astrometry
Eclipse Astrometry

  • How do we know the stars moved?








Measuring the dark
Measuring the Dark

  • 0.5% of Universe is luminous

  • 99.5% of Universe is dark matter


Stellar lenses orion behind a black hole
“Stellar Lenses”Orion behind a Black Hole


Gravitational microlenses
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
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









How big is the lens how close did it get
How Big is the Lens? How Close did it get?



Follow up monitoring
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 bifocals binary stars as lenses
Looking Through BifocalsBinary Stars as Lenses





Micolensing results
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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