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

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

Cosmology is the youngest of all sciences ! Born around 1990 - about 25 years ago !

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

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!

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

What do cosmologists do ?

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

Structure of Physical Laws

Newtonian Gravity SPECIAL RELATIVITY qUANTUM THEORY

Newtonian Gravity SPECIAL RELATIVITY qUANTUM THEORY

Rules over the macroworld

Newtonian Gravity SPECIAL RELATIVITY qUANTUM THEORY

Space + Time = Spacetime E = mc2

Newtonian Gravity SPECIAL RELATIVITY qUANTUM THEORY

Rules over microworld Uncertainty Principle

Newtonian Gravity SPECIAL RELATIVITY Newtonian Gravity SPECIAL RELATIVITY qUANTUM THEORY GENERAL RELATIVITY

Gravity = Curvature of Spacetime

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

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

Vacuum fluctuations Antiparticles Standard Model

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

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

Structure of the universe

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

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

COMPOSITION OF THE COSMOS What is it made of ?

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

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

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

VISIBLE EFFECTS OF DARK MATTER

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

2.73 K

 

HOW DID THESE STRUCTURES FORM?

LET US MAKE A UNIVERSE!

HOW DO WE TEST THIS IDEA OBSERVATIONALLY?

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

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

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

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

   

     

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

Cosmological constant … Courtesy: A. Paranjape

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

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!

TRC, TP (2003, 2013)

 

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!

 

   

     

     

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 !

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

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

“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)

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

   

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 ?

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, 2002 - 2014 )

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.

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’’!

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

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

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 )

     

     

     

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

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

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

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