Neutrino Model of Dark Energy

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

2. 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)

3. Experimental Observations(1)

4. 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.

5. 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 energy density which appears to determine the universe. • Coincidence Scandal: Why is the dark energy density approximately equal to the matter density today

6. Candidates of Dark Energy • Cosmological Constant (consntant) • Dynamical Cosmological Constant (Time-dependent; Quintessence field) • Modified Gravity (Modified Friedmann Eq.)

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

9. 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 !!!

10. ( 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:

11. 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.

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

13. 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 !!!

14. 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.

15. Interacting dark energy model

16. 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:

17. 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