Dark Energy

I gave a pedagogic review of dark energy in this talk.

The first observations which suggested that the present cosmic
expansion is accelerating were of Type 1A supernovae in 1998.
The WMAP data on the CMB released in Feb 2003 led to the same conclusion
independently and provided a breakdown of the cosmic energy components as
4% baryons, 23% nonbaryonic dark energy and 73% dark energy.
The dark energy could be a cosmological constant but there are two
difficulties: the value is 122 orders of magnitude less than the natural
gravity value and 54 orders below that from the electroweak vacuum density.
Second, there is the coincidence that the dark energy and dark matter have
comparable densities within a factor 3.2 at the present time.

One approach which accommodates the coincidence is transPlanckian tail modes which,
on a visit to Scuola Normale Superiore in Pisa during 2001, were accommodated
into a string theory picture as stringy dark energy in the paper

[308] "Modified Dispersion Relations in Toroidal Cosmology" by M. Bastero-Gil,
P.H. Frampton and L. Mersini, Phys. Rev. D65, 106002 (2002).

where we used T-duality for closed strings. Laura Mersini is, since January 1, 2004, a
faculty member in Chapel Hill. A subsequent analysis of the string approach was in

[318] " How to Test Stringy Dark Energy" by P.H. Frampton,
Phys. Lett. B555, 139 (2003).

in which it was suggested that an equation of state for dark energy satisfying w
less than -1 is a test for string theory. In the presence of dark energy the
simple connection between geometry and destiny is lost and in the paper

[321] "The Fate of Dark Energy" by P.H. Frampton and T. Takahashi
Phys. Lett. B557, 135 (2003).

it was shown how the future of the Universe could involve quite different
scenarios depending on the nature of dark matter. A philosophically attractive
possibility is a Big Rip at a finite future time which restores more symmetry
between past and future. The issues of stability for dark energy equation of
state below -1 was discussed in

[322]"Stability Issues for Dark Energy" by P.H. Frampton
Mod. Phys. Lett. A19, 801 (2004).

where it is argued that no terrestrial experiment (indeed any experiment smaller
than a galaxy!) can be sensitive to dark energy. One alternative to dark energy is
to modify general relativity in the infra-red or equivalently at cosmological
distances. In the paper

[337]"Bigger Rip with No Dark Energy" by P.H. Frampton and T. Takahashi
Astropart. Phys. 22, 307-312 (2004). astro-ph/0405333.

it is shown how the Big Rip can become more singular without dark energy in a
modified gravity theory.

[361] "Turnaround in Cyclic Cosmology" by L. Baum and P.H. Frampton.
Phys. Rev. Lett. 98, 071301 (2007). hep-th/0610213 (2006).

In [361] phantom dark energy is used in a Cyclic Universe model which solves
the entropy problem.



home   <-- Research homepage