Neutrino Model of Dark Energy Yong-Yeon Keum Academia Sinica/Taiwan Mujuresort, Feb./16-19/2005

Contents  Experimental evidence of accelerating universe  Candidates of Dark Energy  Neutrino Model of Dark Energy  Lesson from FKNW model (An example of interacting dark matter and dark energy model)

Experimental Observations(1)

Experimental Observation(2)  Our universe is flat, accelerating.  The dominance of a dark energy component with negative pressure in the present era is responsible for the universe’s accelerated expansion.

Accelerating Universe  Cosmological Constant Problem: Why is the energy of the vacuum so much small ?  Dark Energy Puzzle: What is the nature of the smoothly-distributed, persistent e nergy density which appears to determine the universe.  Coincidence Scandal: Why is the dark energy density approximately equal to the matter d ensity today

Candidates of Dark Energy  Cosmological Constant (consntant)  Dynamical Cosmological Constant (Time-dependent; Quintessence field)  Modified Gravity (Modified Friedmann Eq.)

(A) Cosmological Constant Typical scale ： Hence a new energy density scale is too low from the particle physics point of view.

Cosmological constant problem  Many different contributions to vacuum energies: (a) QCD ~  Digression to particle physics and the vacuum energy problem (b) EW physics ~ (c) GUT ~ (d) SUSY ~ All these contributions should conspire to cancel down to. Extreme fine tuning !!!

( B ) Quintessence  Quintessence = dark energy as a scalar field = dynamical cosmological constant  No evidence for evolving smooth energy, but attractive reasons for dynamical origins ! (a) why small, why not zero, why now ??? (b) suggest the physical cosmological evolution.  Canonical quintessence:

Quintessence (2)  If potential energy dominates over the kinetic energy This type of potential is not so easy to stabilize w.r.t. quantum loop corrections.

( C ) Modified Gravity  Newtonian Cosmology: Gravitational Force law determines the evolution Combining the above eqs: Moreover, E = constant and decelleration !!

Modified Force For simplicity g=1 (1) Early times t << tc so that matter domination, no acceleration (2) Later time, t > tc when Accelerated expansion !!!

Neutrino Model of Dark Energy  Cosmological constant:  What physics is associated with this small energy scale ??  It is clearly a challenge to understand dynamically how the small energy scale associated with dark-energy(DE) density aries and how it is connected to particle physics.

Interacting dark energy model

FNW scenario  Fardon, Nelson and Weiner suggested that tracks the energy density in neutrinos  The energy density in the dark sector has two-components:  The neutrinos and the dark-energy are coupled because it is assumed that dark energy density is a function of the mass of the neutrinos:

 Since in the present epoch, neutrinos are non-relativistic (NR),  Assuming dark-energy density is stationary w.r.t. variations in the neutrino mass,  Defining