special lectures on experimental gravity gravitational lensing · experimental gravity --...

Special Lectures onExperimental Gravity

GRAVITATIONAL LENSING

Paul Lasky

Experimental Gravity -- Gravitational Lensing

Contents★ A history of Gravitational Lensing

★ On the contents of the Universe

★ Strong Lensing

★ Weak Lensing

★ Microlensing


A (very short) history ofGravitational Lensing


★ 1704 - Newton, Optiks: “Do not Bodies act upon Light at a distance, and by their action bend its rays; and is not this action strongest at the least distance?”

★ 1784 - Henry Cavendish calculates deflection angle of corpuscular light ray in Newtonian gravity

★ 1804 - von Soldner calculates deflection of 0.84 arcsecs for stars close to the limb of the sun. Value considered too small to be observationally relevant.

★ 1800’s - Corpuscular theory of light is disfavoured for wave theory. General confusion over whether deflection is expected for a light wave.

Lensing prior to Einstein

! = 2GM/Rc2Newton

Cavendish


★As early as 1912, Albert Einstein predicted that a geometric theory of gravity would exhibit this phenomena

★He begins to search for observers to verify his prediction:★ Erwin Findlay-Freundlich - a German national went to

Crimea in August 1914 to observe eclipse. War is declared and he is arrested!

★ William Wallace Campbell - a US citizen is free to leave Russia on an expedition in 1914, but his equipment is impounded! Inferior equipment at an eclipse in 1918 yields a null result. He does not publish.

★ 1915 - Einstein publishes theory of GR and predicts new value for deflection angleSolar deflection angle is calculated to be 1.75 arcsecs - double the Newtonian (corpuscle) value.

Lensing as a purely geometric effect

! = 4GM/Rc2

Einstein

Findlay-Freundlich

William Wallace


1919 Eclipse★ In 1919, (Sir) Arthur Eddington went to Island

of Principe, off west Africa to observe a total eclipse. Another team went to Sobral, Brazil.

★ 2 plates in Principe successful; measured

★One telescope in Sobral gave★Another gave

★Eddington only published the two values in agreeance with GR prediction

★First experimental verification of GR!!★Einstein becomes famous!!!!★ (The 2nd telescope from Sobral has since been heavily debated.

Possibly out of focus, possibly also gives good agreement with other plates!)

Principe

! = 0.93 arcsec! = 1.61± 0.30 arcsec

! = 1.90± 0.11 arcsecEinstein & Eddington


Pospieszalska-Surdej, A., Surdej, J., and Veron,

P., ``The `Gravitational Lensing'

Bibliography'', in Astrophysical

Applications of Gravitational Lensing: IAU

Symposium 173, 415-418, (Kluwer,

Dordrecht, 1996).!


Pospieszalska-Surdej, A., Surdej, J., and Veron,

P., ``The `Gravitational Lensing'

Bibliography'', in Astrophysical

Applications of Gravitational Lensing: IAU

Symposium 173, 415-418, (Kluwer,

Dordrecht, 1996).!

Press (1996): “This is easily done by the rigorous mathematical technique of linear extrapolation... One sees that by the year 2008, all astronomical work will be devoted to gravitational lenses; by the year 2046, gravitational lens work will take over all scientific research; and by the year 2100, all humans will be devoted to our field.”


★Coles 1999, Einstein and the total eclipse

★Crelinsten 2006, Einstein’s jury: The race to test relativity

★Stanley 2007, Practical mystic: Religion, Science and A. S. Eddingtion

★Gates 2009, Einstein’s telescope: The hunt for dark matter and dark energy in the Universe

Much Longer Histories of Lensing


A (very short) Cosmology Primer


Standard Cosmology Ingredients★General Relativity★Cosmological Constant★Homogeneity & isotropy assumptions (FRW

Universe)

“Precision” cosmological observations★Supernova (SN Ia) (Riess et al. 1998, Perlmutter et al.

1999 - both over 5000 citations - cosmological revolution!)

★Cosmic microwave background (WMAP1 - 2003, WMAP3 - 2007, WMAP5 - 2009, Plank)

★Baryon acoustic oscillations (SDSS)

★Gravitational lensing surveys (CASTLES, CLASS, etc.)

★Integrated Sachs-Wolfe effect (Cross-correlations)

★Rotation curves of galaxies


Combining the assumptions with the observations leads to...

The currently accepted “Standard model of Cosmology”

ΛCDM



ΛCDMDark Energy Cold Dark Matter

★ Cosmological Constant?★ Dark fluid?★ ??????

★ Non-baryonic★ Weakly

Interacting Massive Particles (WIMPS)????

★ ?????

★ Stars, galaxies, clusters, you, me, ... ALL baryons!



ΛCDMGravitational lensing is one of the KEY tools to

discovering more about cosmology:★dark matter★dark energy★the accelerating Universe★the law of gravity

PLUS other exciting astrophysical objects including:★Extra-solar planets★black holes★wormholes (?) ...


Simply... the deflection of light by mass

What is gravitational lensing?

Gravitational lensing allows us to:★ Study the object emitting the light that gets deflected -- the “source”

★ The source can be amplified allowing us to see objects we would not otherwise have been able to.

★ Study the object causing the deflection -- the “lens”★ Lensing traces mass, even if we can’t see the mass!

Gravitational lensing comes in 3.5 flavours:

Strong, Weak and Microlensing(flexion)


Strong Gravitational Lensing


Strong lensing - Multiple Images

Quasar (source)at z ~ 1.69

lens at z ~ 0.04

Quasar (source)at z ~ 1.41

lens at z ~ 0.36


Strong lensing - Arcs and Arclets


Strong lensing

Observer

Source


Strong lensingSource

Observer



Observer

Image

Image

Multiple images with high magnification


Perfect alignment of lens and source ==> Einstein ring

Hubble has even detected adouble Einstein ring!

what could this be?


Dark Matter Maps

Position and sizes of arcs allows for the creation of dark matter maps of clusters


Testing theory of gravity?Abell 1689 - a straight lens

But MANY people don’t believe dark matter exists

Require a modification of gravity to achieve this!

Many such theories exist that modify gravity on various scales


Testing theory of gravity?Abell 1689 - a straight lens

Then how do we explain “straight” arcs??


Weak Gravitational Lensing


Weak Lensing - Small shape distortions


Observer

Source

Weak lensing


Weak lensing

Observer

Source

Shape changes with low magnification


Weak lensing Surveys (cosmic shear)★ Simulate entire Universe in a computer (Nbody simulations)★ “ray-trace” path of photons through simulation★ Collect statistics on change of shape of images (shear)★ Compare with large

scale surveys which observe large numbers of distant galaxies


Back to ΛCDMMany free parameters, requires fitting by various observations

rms l

inea

r de

nsity

con

tras

t(C

lum

pine

ss o

f Uni

vers

e)

total matter density(dark matter + baryons)

Density perturbation spectral index(Measure of flatness at end of inflation)


Determining the nature and quantity of dark matter in the Galaxy and in Clusters of galaxies

Elliptical galaxies

Globular Clusters

UCDs

Forbes et al. 2008

(a small lie: virial mass here NOT measured using grav. lensing. But it could have been, and the principal is the same!)


Determining the nature and quantity of dark matter in the Galaxy and in Clusters of galaxies

Elliptical galaxies

Globular Clusters

UCDs

Forbes et al. 2008

(a small lie: virial mass here NOT measured using grav. lensing. But it could have been, and the principal is the same!)

Extend this plot much higher in masses by using weak lensing maps of clusters!


Testing theory of gravity?The Bullet Cluster

Contours - Mass inferred from weak lensing measurementsPretty colours - X-ray emission

Can this be explained without dark matter??


Microlensing



Observer

Image

Image


Strong lensing+ Low mass lens

Observer


Strong lensing+ Low mass lens= Microlensing

Observer

★Can not resolve images★Magnification effects★Lens mass: 10-6 < M/M_sun < 106


Microlensing:Requires relative

motions

Observer

Magnification varies on scales of days to weeks

Source

Lens

Velocity orthogonal to line of sight


Microlensing - Light curves and magnification


Extrasolar planets from microlensing

Large microlensing event changes magnification of source objectTiny perturbation reveals a low-mass planet!

time


Extrasolar planets from microlensing


Lots of things not covered!!!

★ Schneider, Ehlers & Falco (1992) Gravitational lenses, Springer ★ does not include recent developments, but very thorough introduction to theory of

lensing)

★ Bartelmann & Schneider (2001) Weak gravitational lensing, Physics Reports 340, 291

★ Wambsganss (1998 - updated in 2002) Gravitational lensing in astronomy, Living Reviews in Relativity

★www.livingreviews.org; emphasis on strong lensing

★ Ellis (2010) Gravitational lensing: A unique probe of dark matter and dark energy, Philosophical Transactions of the Royal Society A 368, 967

★Very nice review of recent developments associated with cosmological applications

http://www.living

http://www.living

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