# Presentation: 201447

## Title

- UC Davis
- APS Meeting
- Philadelphia, April 2003

- Cosmic acceleration and fundamental physics

## I) Challenges for models of cosmic acceleration

- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## As far as we can tell so far, the cosmic acceleration might be the result of  

- I.1 The cosmological constant problem / fundamental gravity

##  = 10120

-   0 ?

**Vacuum Fluctuations**

- I.1 The cosmological constant problem / fundamental gravity

## Challenges:

-    0 dream may not work
-  A more mature fundamental theory of gravity could overturn ideas about the origins of acceleration
-  Huge potential for observations to impact fundamental theories

- I.1 The cosmological constant problem / fundamental gravity

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## (not really a separate “why now problem”, with  )        

- _Why this?_
-  Where does this strange parameter come from?

- (not really a separate “why now problem”, with  )

## Why  now?

-  _If_ the source of acceleration is some _dynamical_ matter (“quintessence”), then it has to acquire the value

- _today_ (*t=14.5* Gyr). (not some other time)

- Today

*Time ***

- I.2T he “why this/why now” problems

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## If the acceleration is produce by a scalar field (quintessence):

- If the acceleration is produce by a scalar field (quintessence):
- i) The quintessence field must have a mass
- How can this value stay safe from quantum corrections?

## If the acceleration is produce by a scalar field (quintessence):

- ii) If such a small mass is preserved, there is a new long range force mediated by the quintessence field.
-   How can 5th force bounds be evaded?
-      (Carroll)

- I.3 Quantum corrections/long range forces

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## ia) The entropy bound:

-  If there really is a , the late-time asymptotic state of the universe will have a horizon with area:

-  This implies a Hawking-Beckenstein type upper bound on the entropy:

-  Are we then compelled to only consider only models of physics with a finite dimensional Hilbert space?  (Excludes field theory, M-theory, SHO, etc.)

- T. Banks & W. Fischler

- II.1 The quantum physics of

## ib) Holography & the Causal Patch

-  Two basic facts about de Sitter space:

- II.1 The quantum physics of

- Observer’s Worldline 

- 1) Objects never appear to leave the horizon

- Const. t

- Const. r

- (Gibbons & Hawking)

- 2) A free field appears at Temp:

- [Black Hole Analogy]

## ib) Holography & the Causal Patch

-  Two basic facts about de Sitter space:

- II.1 The quantum physics of

- Observer’s Worldline 

- 1) Objects never appear to leave the horizon

- Const. t

- Const. r

- See Kaloper (Davis Inflation Meeting ’03) (t’ Hooft: BH case)

- 2) A free field appears at Temp:

## ib) Holography & the Causal Patch

-  Causal Patch “unification” hypothesis:

- II.1 The quantum physics of

- 1) Objects never appear to leave the horizon

- 2) A free field appears at Temp.                    (divergent at horizon)

- Holographic (quantum gravity) “magic” gives
-  Cutoff for “divergent” entropy at *r**H * area entropy law
-  “All Physics” inside causal patch*.  Objects thermalize as they approach the horizon (no “outside” = no “inside” of Black Hole)
-  Consistency of the above across different observers.
- *The end of inflation? (Dyson *et al* ’02)

*O*

- This observer sees divergent entropy here, cut off by quantum gravity.

- Banks Bousso Fischler t’Hooft Susskind

**No physics**
** ****out here**

## ii) Other constraints on fundamental theories with  :

-  Do scattering states exist? (Banks, Fischler)
- Does string theory exist in de Sitter space? (see Kachru *et al*  ’03)
-  Does quantum de Sitter space exist? (Goheer et al ’03)

- II.1 The quantum physics of

## iii) Causal set theory of gravity:

- a prediction of causal set theory?!

- (Ahmed et al. Sep 2002)

- II.1 The quantum physics of

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

##  Inflation has taught us how to accelerate the universe with a scalar field... why not try again?

-  Inflation has taught us how to accelerate the universe with a scalar field... why not try again?

*With    f **** 10**18**GeV,    M **** 10**-3**eV*

- PNGB: Frieman, Hill, Stebbins, & Waga 1995

*M=1 meV - 1GeV*

- Zlatev, Wang & Steinhardt 1998

- Scalar field domination/ acceleration

- II.2 Dark energy/quintessence, Stage I (adventures)

##  Inflation has taught us how to accelerate the universe with a scalar field... why not try again?

-  Inflation has taught us how to accelerate the universe with a scalar field... why not try again?

*With    f **** 10**18**GeV,    M **** 10**-3**eV*

- PNGB: Frieman, Hill, Stebbins, & Waga 1995

*M=1 meV - 1GeV*

- Zlatev, Wang & Steinhardt 1998

- Scalar field domination/ acceleration

- N.B Resurrect the   0 dream

- II.2 Dark energy/quintessence, Stage I (adventures)

##  Some more scalar field models of dark energy:

- Dodelson Kaplinghat & Stewart 2000

- Albrecht & Skordis 1999

- _Oscillating_

- _Shadowing_

- There are *many* other examples

- II.2 Dark energy/quintessence, Stage I (adventures)

##  A note about the “why this/now” problem:

-  Essentially all existing quintessence models “solve” it by relating        to parameters in the quintessence equations
-  Attractor behavior gets rid of initial conditions dependence
-  How compelling this “solution” is depends on how convinced you are that nature has chosen those parameters.

- II.2 Dark energy/quintessence, Stage I (adventures)

##  A note about Quantum corrections/long range forces:

-  Almost all models of dynamical dark energy/quintessence completely ignore these issues.    “adventures”

- II.2 Dark energy/quintessence, Stage I (adventures)

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## Stage II:  Take quantum corrections/long range forces seriously

- *PNGB: Frieman, Hill, Stebbins, & Waga 1995

- * Extra dimensions
- A.A, Burgess, Ravndal 2001

- * Supergravity Kalosh et al. 2002

-  Challenging particle physics issues

## Stage II:  Take quantum corrections/long range forces seriously

- *PNGB: Frieman, Hill, Stebbins, & Waga 1995

-  Challenging particle physics issues
- Also: Risky because more radical developments could invalidate these efforts

- II.3 Dark energy/quintessence, Stage II (getting serious)

- * Extra dimensions
- A.A, Burgess, Ravndal 2001

- * Supergravity Kalosh et al. 2002

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## I) Challenges for models of cosmic acceleration

- _I) __Challenges for models of cosmic acceleration_
- I.1 The cosmological constant problem / links to fundamental gravity.
- I.2The “why this/why now” problems
- I.3 Quantum corrections/long range forces

- _III) __Conclusions_

- Cosmic acceleration and fundamental physics,      APS April 2003

## III) Conclusions

- Modeling cosmic acceleration leads to:
-  Exciting challenges and new directions, many of which are connected with very fundamental questions.
-  Real opportunity for dramatic observational advances

- Weller & AA

##  Parameters and Cosmic Confusion                       IV-<number>

- Degeneracy

* **Parameters and Cosmic Confusion                       IV-*

- NB: Don’t be afraid of degeneracies!! Data can still distinguish among many interesting models

- Maor et al.