1. Cosmic Odyssey: Past, Present and Future
T. Padmanabhan
IUCAA, Pune

2. THE COSMIC QUESTIONS
What is the cosmos made of ? How does the universe evolve ? Where did it all come from ?

3. Cosmology is the youngest of all sciences !
Born around about 25 years ago !

4. Cosmology today is driven by direct observational results – not by theoretical speculations !

5. LOOKING FARTHER IN THE UNIVERSE IS THE SAME AS LOOKING BACK IN TIME!
WE CAN SEE THE PAST !
LOOKING FARTHER IN THE UNIVERSE IS THE SAME AS LOOKING BACK IN TIME!

6. No longer an arm-chair subject !
Astrosat
AAT (2dfGRS)
GMRT

7. What do cosmologists do ?

8. Structure of Physical Laws Structure of the Universe
Explain cosmos from known physical laws
From a billionth of a second to 14 billion years
Before one billionth
of a second
Test physical theories using cosmology
Structure of the Universe

9. Structure of
Physical Laws

10. Newtonian
Gravity
SPECIAL
RELATIVITY
qUANTUM
THEORY

11. Newtonian
Gravity
SPECIAL
RELATIVITY
qUANTUM
THEORY

12. Rules over the
macroworld

13. Newtonian
Gravity
SPECIAL
RELATIVITY
qUANTUM
THEORY

14. Space + Time = Spacetime
E = mc2

15. Newtonian
Gravity
SPECIAL
RELATIVITY
qUANTUM
THEORY

16. Rules over microworld
Uncertainty Principle

17. Newtonian
Gravity
SPECIAL
RELATIVITY
Newtonian
Gravity
SPECIAL
RELATIVITY
qUANTUM
THEORY
GENERAL
RELATIVITY

18. Gravity = Curvature of Spacetime

19. Matter drives the expansion in accordance
with Einstein’s General Relativity !
Courtesy: Roy Martens

21. Newtonian
Gravity
SPECIAL
RELATIVITY
qUANTUM
THEORY
SPECIAL
RELATIVITY
qUANTUM
THEORY
GENERAL
RELATIVITY
QUANTUM FIELD
THEORY

22. Vacuum fluctuations
Antiparticles
Standard Model

23. Newtonian
Gravity
SPECIAL
RELATIVITY
qUANTUM
THEORY
GENERAL
RELATIVITY
QUANTUM FIELD
THEORY
GENERAL
RELATIVITY
QUANTUM FIELD
THEORY
QUANTUM
Gravity

24. Unknown, final frontier! Can be tested by cosmology?
QG!
Unknown, final frontier!
Needed in cosmology
Can be tested by cosmology

25. Structure of
the universe

31. IF YOU HAVE THE EYES FOR THE GALAXIES ……
Night sky with over a million galaxies!

32. 2dF galaxy survey, there are galaxies forming the cosmic web with voids (gaps) in between filaments of galaxies. This survey was performed at Siding Springs in Australia!
2dF GRS team 32

33. COMPOSITION OF THE COSMOS
What is it made of ?

34.  A KEY PARAMETER OF THE UNIVERSE
Measures the relative strength of expansion rate
compared to the gravity of matter

35. Density of visible matter
Count all that you can see!
Then you get
 ≈ 0.046

36. But, does the universe have more matter than meets the eye ?
YES! A LOT MORE !!

37. VISIBLE EFFECTS OF DARK MATTER

38. Dark matter contributes about WDM  0.25
VISIBLE EFFECTS OF DARK MATTER
BOTTOM LINE:
Dark matter contributes about WDM  0.25

41. 2.73 K

45. HOW DID THESE
STRUCTURES FORM?

46. LET US MAKE A UNIVERSE!

47. HOW DO WE TEST THIS
IDEA OBSERVATIONALLY?

48. THE TINY WIGGLES ARE A GOLD MINE OF INFORMATION !
2.73 K

50. WHERE DID THESE DENSITY FLUCTUATIONS COME FROM?
CAN THESE LEAD TO THE
STRUCTURES WE SEE?

51. IN THE VERY EARLY UNIVERSE
VACUUM FLUCTUATIONS
IN THE VERY EARLY UNIVERSE
COSMIC STRUCTURES
SEEN TODAY

52. Requires an inflationary phase in the very early universe
VACUUM FLUCTUATIONS
IN THE VERY EARLY UNIVERSE
Requires an inflationary phase in the very early universe

57. DEPENDS ON THE NATURE AND AMOUNT OF MATTER IN THE UNIVERSE
Gravitational
instability
DEPENDS ON THE NATURE AND
AMOUNT OF MATTER IN THE UNIVERSE

58. Cosmological constant…
Courtesy: A. Paranjape

59. Cosmological constant will accelerate the expansion of the universe
CRUCIAL TEST
Cosmological constant will accelerate the expansion of the universe

60. LOOKING FARTHER IN THE UNIVERSE IS THE SAME AS LOOKING BACK IN TIME!
HOW DO WE TEST THIS?
LOOKING FARTHER IN THE UNIVERSE IS THE SAME AS LOOKING BACK IN TIME!

64. TRC, TP (2003, 2013)

66. COSMOLOGICAL CONSTANT
ILLOGICAL
COSMOLOGICAL CONSTANT
A term in Einstein’s equation which acts like matter with negative pressure
Drives the accelerated expansion of the universe at the present epoch
THE MOST IMPORTANT PUZZLE IN THEORETICAL PHYSICS TODAY!

72. High energy physics can, in principle, determine rinf , req
High energy physics can, in principle, determine rinf , req
But we have no clue
about the value of rL !

73. Cosmological constant
Gravity
Quantum Theory
G h c3 L 
10-123
Relativity
OBSERVATIONS
Cosmological constant
[ dimension: cm-2 ]

74. Cosmological constant
Gravity
Quantum Theory
G h c3 L  ?
Relativity
THEORY !!
Cosmological constant
[ dimension: cm-2 ]

75. “It’s not that they can’t see the solution,
it’s that they can’t see the problem”
G.K. Chesterton,
Scandal of Father Brown (1935)

76. Is a problem with gravity!
The real problem with 
Is a problem with gravity!
Why is gravity immune to energy shifts ?

78. Is a problem with gravity!
The real problem with 
Is a problem with gravity!
Why is gravity immune to energy shifts ?
What fixes the zero level of energy ?

79. Recent work suggests that we need another paradigm shift!
A NEW PERSPECTIVE
Recent work suggests that we need another paradigm shift!
Gravity could be an
emergent phenomenon
( TP, )

80. What is an emergent phenomenon?
Examples: Elasticity, gas dynamics, ….
Dynamics can be described in terms of macroscopic variables; e.g. PV = RT
Dynamics is reasonably independent of the microstructure, atoms etc.

81. What is an emergent phenomenon?
Examples: Elasticity, gas dynamics, ….
Dynamics can be described in terms of macroscopic variables; e.g. PV = RT
Dynamics is reasonably independent of the microstructure, atoms etc.
Gravity is the fluid mechanics of the “atoms of spacetime’’!

82. Study gravity the way physicists studied gas dynamics before knowing the atomic structure of matter

83. 1030 Planck length 1020 GUTs 1010 Electroweak 10-10 10-20 10-30 cm
Quantum Gravity!
Planck length
1020
GUTs
1010
Electroweak cm
10-10
10-20
10-30

84. Emergent gravity paradigm:
Key results
Field equations are like laws of gas dynamics
Gravity IS immune to energy shifts
Can determine the “Avogadro number of spacetime” : Nsur = (Area / LP2 )
Dynamics arises from (Nsur – Nbulk )

88. TAKE-HOME MESSAGE
TP, HP (2013,2014)

90. To Sum Up……..
We understand the evolution of the universe from a fraction of
a second to 14 billion years !
Vacuum fluctuations generated in the early universe can act as seeds for the cosmic structures we see today
The cosmological constant, contributing about 71% to the energy density today, is accelerating the universe

91. To Sum Up……..
We understand the evolution of the universe from a fraction of
a second to 14 billion years !
Vacuum fluctuations generated during inflation can act as seeds for the cosmic structures we see today.
The cosmological constant, contributing about 71% to the energy density today, is accelerating the universe
The explanation for the value of the cosmological constant might require another paradigm shift about gravity

92. Thank you for your time Acknowledgements
Vasanthi Padmanabhan for help in producing the presentation
Collaborators: Tirthankar Roy Choudhury, Hamsa Padmanabhan, Aseem Paranjape, …
Discussions: James Bjorken, Sunu Engineer, D. Lynden-Bell, …. and several others
Thank you for your time