The field is rich in UDM models [see 128, for a review and for references to the literature]. The models
can grow structure, as well as providing acceleration of the universe at late times. In many cases, these
models have a non-canonical kinetic term in the Lagrangian, e.g., an arbitrary function of the square of the
time derivative of the field in a homogeneous and isotropic background. Early models with acceleration
driven by kinetic energy [-inflation 60, 384, 154] were generalized to more general Lagrangians
[
-essence; e.g., 61, 62, 795]. For UDM, several models have been investigated, such as the generalized
Chaplygin gas [488, 123, 137, 979, 741], although these may be tightly constrained due to the finite sound
speed [e.g. 38, 124, 784, 985]. Vanishing sound speed models however evade these constraints [e.g., the silent
Chaplygin gas of 50]. Other models consider a single fluid with a two-parameter equation of state [e.g
74]), models with canonical Lagrangians but a complex scalar field [55], models with a kinetic
term in the energy-momentum tensor [379, 234], models based on a DBI action [236], models
which violate the weak equivalence principle [375] and models with viscosity [321]. Finally,
there are some models which try to unify inflation as well as dark matter and dark energy
[206, 688, 572, 575, 430].
A requirement for UDM models to be viable is that they must be able to cluster to allow structure to
form. A generic feature of the UDM models is an effective sound speed, which may become significantly
non-zero during the evolution of the universe, and the resulting Jeans length may then be large
enough to inhibit structure formation. The appearance of this sound speed leads to observable
consequences in the CMB as well, and generally speaking the speed needs to be small enough to allow
structure formation and for agreement with CMB measurements. In the limit of zero sound speed,
the standard cosmological model is recovered in many models. Generally the models require
fine-tuning, although some models have a fast transition between a dark matter only behavior and
CDM. Such models [729] can have acceptable Jeans lengths even if the sound speed is not
negligible.
An action which is applicable for most UDM models, with a single scalar field , is
Of interest for Euclid are the weak lensing and BAO signatures of these models, although the supernova
Hubble diagram can also be used [885]. The observable effects come from the power spectrum and
the evolution of the equation-of-state parameter of the unified fluid, which affects distance
measurements. The observational constraints of the generalized Chaplygin gas have been investigated
[706], with the model already constrained to be close to CDM with SDSS data and the
CMB. The effect on BAO measurements for Euclid has yet to be calculated, but the weak
lensing effect has been considered for non-canonical UDM models [202]. The change in shape and
oscillatory features introduced in the power spectrum allow the sound speed parameter to be
constrained very well by Euclid, using 3D weak lensing [427, 506] with errors
[see also
198].
http://www.livingreviews.org/lrr-2013-6 |
Living Rev. Relativity 16, (2013), 6
![]() This work is licensed under a Creative Commons License. E-mail us: |