Why is the Universe Accelerating

1
Why is the Universe Accelerating?

• Sean Carroll
• astro-ph/0310342

Emily Alicea-Muñoz 16/March/2004
Hubble Ultra Deep Field APOD 2004 March 9
2
A problem, a coincidence, a puzzle

• The cosmological constant problem
• or zero-point vacuum energy
• From QFT rL 10112 erg/cm3
• From observations rL 10-8 erg/cm3
• The dark energy puzzle
• What is the nature of dark energy? Why is the
  Universe accelerating?
• The coincidence scandal
• Why is the dark energy density nearly equal to
  the matter density at the present time?

3
A problem, a coincidence, a puzzle

• The cosmological constant problem
• or zero-point vacuum energy
• From QFT rL 10112 erg/cm3
• From observations rL 10-8 erg/cm3
• The dark energy puzzle
• What is the nature of dark energy? Why is the
  Universe accelerating?
• The coincidence scandal
• Why is the dark energy density nearly equal to
  the matter density at the present time?

A difference of 120 orders of magnitude!!!
4
A problem, a coincidence, a puzzle

• The cosmological constant problem
• or zero-point vacuum energy
• From QFT rL 10112 erg/cm3
• From observations rL 10-8 erg/cm3
• The coincidence scandal
• Why is the dark energy density nearly equal to
  the matter density at the present time?
• The dark energy puzzle
• What is the nature of dark energy? Why is the
  Universe accelerating?

A difference of 120 orders of magnitude!!!
5
A problem, a coincidence, a puzzle

• The cosmological constant problem
• or zero-point vacuum energy
• From QFT rL 10112 erg/cm3
• From observations rL 10-8 erg/cm3
• The coincidence scandal
• Why is the dark energy density nearly equal to
  the matter density at the present time?
• The dark energy puzzle
• What is the nature of dark energy? What is
  causing the acceleration?

A difference of 120 orders of magnitude!!!
6
What might be going on?

• Misinterpretation of the data
• Type Ia SN not standard candles?
• Not likely properties do not seem to
• depend on redshift

Figure 1.2
7
What might be going on?

• Breakdown of GR at large scales
• Self-tuning models with extra dimensions?
• Modifications to the GR equations?
• Must be constrained by observations (BBN, LSS,
  CMB)
• Cant solve cosmological constant problem,
• but might explain current acceleration period

maybe
Figure 1.2
8
What might be going on?

• Dynamical dark energy
• Slowly evolving scalar field (quintessence)
• Possible trigger mechanism for acceleration
• (rolling to the minimum of the potential)
• Related to inflation?
• Problem light scalar fields unnatural in QFT

maybe
Figure 1.2
9
What might be going on?
?
?
Figure 1.2
10
Observational issues

• Equation-of-state parameter
• would result in a Big Rip (but not
  necessarily)

11
Observational issues

• Direct detection of dark energy
• Gravitational interaction of dark energy scalar
  field with ordinary matter would result in
  time-dependence of the constants of nature
• From quasar absorption spectra (Murphy et.al.
  2001. MNRAS 327, 1208), at
  redshifts
• Possible detection of quintessence???
• Non-trivial interactions between dark matter and
  dark energy
• Numerous models proposed (Farrar Peebles,
  astro-ph/0104464 Hoffman, astro-ph/0307350
  Fardon, Nelson Weiner, astro-ph/0309800) How
  to prove/disprove them???

12
Conclusions

• Theoretical and observational values of L differ
  by many orders of magnitude
• We dont know why WL and WM are so close in value
  at the present time
• Possible causes for acceleration
• Modification of GR
• Dynamical dark energy (quintessence)
• Others???
• Theres still many issues with cosmology that
  havent been solved (no kidding)