Gravitational lensing occurs when the light from distant galaxies and quasars is bent and distorted by the gravitational field of intervening massive objects like galaxies or galaxy clusters. This causes several observable phenomena including Einstein rings, giant luminous arcs, and multiply imaged quasars. Gravitational lensing has many applications such as determining the mass distribution of galaxy clusters and measuring the Hubble constant, and will continue to be an important tool in astronomy.

1. Gravitational Lensing

2. INTRODUCTION
Einstein’s General
theory of relativity
Gravitation is only
a superficial force
Method of
measuring space
and time using
properties of
Euclidian geometry
Reality
Space time
continuum in
gravitational field of
body
Non Euclidian law of
geometry or curved
geometry

4. GRAVITATIONAL LENSING
 An astrophysical phenomena in which the distribution of mass in the
universe affects the propagation of light.
 Works in an similar way to the description of the propagation of light through
normal lenses
Photon
travels across
the universe
Gravitational
effect of
mass
concentration
Trajectories
arepurturbed
Refocuses
somewhere
else

6. GRAVITATIONAL LENSING
Opticallensing
• light bends due to refraction at
spherical end and focuses into
the eyes.
Gravitationallensing
• light bends around a
concentration of mass (usually
galaxies, stars, black holes) and
refocuses somewhere else.

7.  When contemplating near incredibly dense
masses, the impacts can be exceedingly odd
and very intense.
 As the light from lensed galaxies and clusters
approaches the foreground cluster, it is
stretched into arcs.

9. HISTORY OF GRAVITATIONAL
LENSING

10. 1804
• “On The Deflection Of Light Ray From Its Straight MotionDueToThe
Attraction Of A World Body Which It Passes Closely,"
• byJohannSoldner
• Predictedthat a light ray passing close to the solar limb would be deflected by
an angleα= 0:84arcsec.
1911
• Albert Einsteindirectly addressedthe influence of gravity on light
1913
• Einsteincontacted the director of Mount Wilson observatory, George Hale,
and asked if it will be possible to measure the positions of stars near the sun
during the day in order to establish the angular deflection effect of the sun

11. 1914
1916
• the completion of the General Theory of Relativity byEinstein
• derivethe first correct formula for the deflection angleαof a
light.
• obtained deflection angle for sun asα= 1.74arcsec
1919
• .
• Einstein foundα= 0.83arcsecfor the deflection angle of a light
ray grazing the sun
•The first observational attempt to test Einstein's prediction for
the deflection angle
•never accomplished
•predicted value for the angular deflection was actually wrong.

12. 1924
• Chwolsonmentioned the idea of a“factious double star."
• also mentioned the symmetric case of star exactly behind star,
resulting in a circular image.
1936
• Einsteinalso reported about the appearance of a “luminous"
circle of perfect alignment between the source and the lens, such
a configuration is called“Einstein Ring".
1937
• Fritz Zwickypointed out that galaxies are more likely to be
gravitationally lensed than a star and that one can use the
gravitational lensas natural telescope.

13. GRAVITATIONAL LENSING
PHENOMENA

14. PHENOMENA
 When a light ray follows the curvature, it
bends towards the mass, causing space-time
to curve.
This bending gives rise to several important
phenomena
1. Einstein rings
2. Giant luminous arcs
3. Multiply imaged quasars

15. EINSTEIN RINGS
Special case of Gravitational lensing in which the source
lies exactly behind the lens, a ring like image will be
produced, called as Einstein rings.

17. Einstein Rings
conditions to be
satisfied in order to be
able to observe Einstein
rings
the source must lie
exactly on top of the
resulting degenerate
point-like caustic.
the source must lie
exactly on top of
theresulting degenerate
point-like caustic.

18. EINSTEIN RINGS
 Their diameters vary between 0.33 to 2
arcseconds
 They're all found in the radio frequency range,
with several having vissible or infrared
counterparts as well
 Some of them are "broken" rings having one or
two interruptions along the circle
 The compact component is changeable and
usually viewed as a double image divided by the
it's diameter.
 The changeability of conpact source helps in
measuring time delay and hubble constant in
these systems.

19. EINSTEIN RING
preliminary pictures of
the gravitational lensing
effect seen with a wine
glass! Below is an image
of a classic ‘Einstein ring’
that occurs due to
extreme strong lensing
effects. This is rarely
seen in nature.

20. THE FIRST EINSTEIN RING
 The extended radio source
MG1131+0456
 Observed in 1988
 discovered by Hewitt
 Diameter of about 1.75 arcsecond

21. GIANT LUMINOUS ARC
magnified, distorted and strongly elongated images of background
galaxies which happened to lie behind foreground clusters of galaxies.

22. GIANT LUMINOUS ARC
• Clusters of galaxies with masses of order
10^14Mo are very effective lenses
• TheirEinstein radii are of the order of 20
arcseconds
• Giant arcs can be exploited in two ways :
1. they provide us with strongly magnified galaxies
at (very) high redshifts.
2. as tools to study the potential and mass
distribution of the lensing galaxy cluster

23. GIANT LUMINOUS ARC
• An interesting result from the analysis of giant
arcs in galaxy clusters is that Clusters of galaxies
are dominated by dark matter
• The fact that we see such arcs shows that the
central surface mass density in clusters must be
high.
• core radii of clusters - the radii at which the
mass profile of the cluster flattens towards the
center-must be of order of this distance or
smaller.

24. two of the most spectacular cluster lenses
producing arcs:
Clusters Abell 2218 CL0024+1654.

25. MULTIPLY-IMAGED QUASAR
Multiply imaged quasar are effect of gravitational lensing. With a
multiple imaged quasar, their light is lensed around an intervening
galaxy before reaching Earth, producing multiple images of the quasar.

26. MULTIPY IMAGED QUASAR
• the first example of a lensed object was a double
quasar Q0957+561
• Quasars are rare and not easy to find-By now
about two dozen multiply-imaged quasar systems
have been found
• The fraction of quasars that are lensed is small
(less than one percent)
• It is not trivial at all to identify the lensed (i.e.
multiply-imaged) quasars among the known
ones.

27. FIRST MULTIPLE IMAGED QUASAR
•Discovered in 1979
•By Dennis Walsh
•found in a radio
survey

28. Splited into
five images

29. STRONG LENSING
• occurs when source, lens and observer are
well positioned and close enough, bending
angle is large enough to resolve various
images.
• a lens with mass density higher than critical
density is necessary
• usually occurs in the central region of galaxies
and clusters

30. STRONG LENSING
• provide cosmological information
1. measurement of hubble constant
2. better understanding of evolution of structures
in the universe
• involves formation of multiple images and
high magnification when source and observer
is in proper alignment - rare event

31. FIRST STRONG LENSING
•first strong lensing
was seen in 1979
•by walsch carswell
and waymann
•doubly imaged
Q0957+561 quasar

32. WEAK LENSING
• small distortion in the patterns of background
galaxies caused by the weak lensing of
foreground galaxies
• by statistical averaging of these small
distortions we could estimate mass of cluster

33. Weak lensing

34. FIRST WEAK LENSING
•first observed in
1990
•around the galaxy
cluster of abell
1689 and
CL409+524

35. MICROLENSING
• type of gravitational lensing in which the
gravitational lensing effect is small-scale
• The lensing is of an image that is so small or faint
that instead of observing multiple images, the
additional light bent towards the observer
making the source appears brighter.
• The surface brightness remains constant but as
numerous images of the object appear, the object
appears brighter and bigger.

37. MACHOS(Massive compact halo object)
Cant see directly but their gravitational lensing effects can be noticed when they pass in
front of a source causes star to brighter for a while

38. FUTURE AND APPLICATIONS

39. • Gravitational lensing is an exceptional field in
astronomy in the sense that its occurence and
many of its features were predicted before they
were actually observed.
example:
a. multiple images
b. quasar microlensing
c. galactic microlensing
d. weak lensing

40. APPLICATIONS
• gravitational lensing is a simple geometrical
concept which easily allows
1. qualitative estimates and quantitative calculations
2. to look forward in time and predict future
applications
• Gravitational lensing is one of three methods used
for the determination of masses of galaxy clusters
Other two are:
I. by X-ray analysis
II. by using the virial theorem and the velocity distribution of
the galaxies

41. MULTIPY-IMAGED QUASAR
• more determinations of accurate time delays
in quasar systems.
• the value of the Hubble constant determined
will be accurate than values obtained with any
other method

42. QUASAR MICROLENSING EVENT
• provide information on the structure of the
quasars and the intervening clumped matter.
• Maybe we can “map” the hot spots of quasars
this way

43. GALACTIC MICROLENSING EVENT
• to study the structure of the Milky Way
• The spectra of highly magnified stars will help
to investigate the metallicity of bulge stars in
detail.
• resolve the stellar surfaces and study their
center-to-limb variations.
• provide unbiased statistics on the fraction of
binary stars (within certain relative distances).

44. GALACTIC MICROLENSING EVENT
• has the advantage compared to all other
Earth-bound planet search techniques that it
is able to detect Earth-mass planets!
• could be detected directly by monitoring
astrometrically the position of the star very
accurately
• provide information on the three-dimensional
mass distribution of the halo

45. WEAK LENSING EVENT
• will be used to map not just the outskirts of
massive galaxy clusters
• to trace the large scale structure by its effect on
the background population of galaxies.
• we can ultimately produce a three-dimensional
map of the matter in the universe (rather than a
light map)!
• useful thing for the understanding of structure
formation and evolution
• we will determine the matter content of the
universe

46. OTHER POSSIBLE APPLICATIONS
• a complementary determination of the black
hole mass.
• help to study the dynamics near the black hole.
• detect the gravitational lens signature of the
matter - the next generation of experiments to
map the cosmic microwave background.
• The redshift of the most distant object will be
magnified.

47. • Ultimately every object in the sky is affected
by (ever so slight) lensing effects
Lensingoutlook at symposium 173
• “gravitational lens effects ... are present along virtually every
line of sight”
• more and more astronomers will (have to) deal withlensingin
the next decade, so thatlensingwill become an “ubiquitous
observational technique” and hence – for better or for worse:
“gravitationallensingmay well disappear as a unique sub-
specialty in astronomy”.
CONCLUSION