14 research outputs found
Tachyons, Scalar Fields and Cosmology
We study the role that tachyon fields may play in cosmology as compared to
the well-established use of minimally coupled scalar fields. We first elaborate
on a kind of correspondence existing between tachyons and minimally coupled
scalar fields; corresponding theories give rise to the same cosmological
evolution for a particular choice of the initial conditions but not for any
other. This leads us to study a specific one-parameter family of tachyonic
models based on a perfect fluid mixed with a positive cosmological constant.
For positive values of the parameter one needs to modify Sen's action and use
the sigma process of resolution of singularities. The physics described by this
model is dramatically different and much richer than that of the corresponding
scalar field. For particular choices of the initial conditions the universe,
that does mimick for a long time a de Sitter-like expansion, ends up in a
finite time in a special type of singularity that we call a "big brake". This
singularity is characterized by an infinite deceleration.Comment: 7 figures. Enlarged discussion of the big brake cosmology.
Continuation of the model clarified. References adde
Tolman-Oppenheimer-Volkoff equations in presence of the Chaplygin gas: stars and wormhole-like solutions
We study static solutions of the Tolman--Oppenheimer--Volkoff equations for
spherically symmetric objects (stars) living in a space filled with the
Chaplygin gas. Two cases are considered. In the normal case all solutions
(excluding the de Sitter one) realize a three-dimensional spheroidal geometry
because the radial coordinate achieves a maximal value (the "equator"). After
crossing the equator, three scenarios are possible: a closed spheroid having a
Schwarzschild-type singularity with infinite blue-shift at the "south pole", a
regular spheroid, and a truncated spheroid having a scalar curvature
singularity at a finite value of the radial coordinate. The second case arises
when the modulus of the pressure exceeds the energy density (the phantom
Chaplygin gas). There is no more equator and all solutions have the geometry of
a truncated spheroid with the same type of singularity. We consider also static
spherically symmetric configurations existing in a universe filled with the
phantom Chaplygin gas only. In this case two classes of solutions exist:
truncated spheroids and solutions of the wormhole type with a throat. However,
the latter are not asymptotically flat and possess curvature singularities at
finite values of the radial coordinate. Thus, they may not be used as models of
observable compact astrophysical objects.Comment: A reference added, matches the version published in Physical Review
Gravity of a static massless scalar field and a limiting Schwarzschild-like geometry
We study a set of static solutions of the Einstein equations in presence of a
massless scalar field and establish their connection to the Kantowski-Sachs
cosmological solutions based on some kind of duality transformations. The
physical properties of the limiting case of an empty hyperbolic spacetime
(pseudo-Schwarzschild geometry) are analyzed in some detail.Comment: 13 pages, 4 figure
Cosmological zoo -- accelerating models with dark energy
ecent observations of type Ia supernovae indicate that the Universe is in an
accelerating phase of expansion. The fundamental quest in theoretical cosmology
is to identify the origin of this phenomenon. In principle there are two
possibilities: 1) the presence of matter which violates the strong energy
condition (a substantial form of dark energy), 2) modified Friedmann equations
(Cardassian models -- a non-substantial form of dark matter). We classify all
these models in terms of 2-dimensional dynamical systems of the Newtonian type.
We search for generic properties of the models. It is achieved with the help of
Peixoto's theorem for dynamical system on the Poincar{\'e} sphere. We find that
the notion of structural stability can be useful to distinguish the generic
cases of evolutional paths with acceleration. We find that, while the
CDM models and phantom models are typical accelerating models, the
cosmological models with bouncing phase are non-generic in the space of all
planar dynamical systems. We derive the universal shape of potential function
which gives rise to presently accelerating models. Our results show explicitly
the advantages of using a potential function (instead of the equation of state)
to probe the origin of the present acceleration. We argue that simplicity and
genericity are the best guide in understanding our Universe and its
acceleration.Comment: RevTeX4, 23 pages, 10 figure
Nonlinear evolution of dark matter and dark energy in the Chaplygin-gas cosmology
The hypothesis that dark matter and dark energy are unified through the
Chaplygin gas is reexamined. Using generalizations of the spherical model which
incorporate effects of the acoustic horizon we show that an initially
perturbative Chaplygin gas evolves into a mixed system containing cold dark
matter-like gravitational condensate.Comment: 11 pages, 3 figures, substantial revision, title changed, content
changed, added references, to appear in JCA
Hessence: A New View of Quintom Dark Energy
Recently a lot of attention has been drawn to build dark energy model in
which the equation-of-state parameter can cross the phantom divide .
One of models to realize crossing the phantom divide is called quintom model,
in which two real scalar fields appears, one is a normal scalar field and the
other is a phantom-type scalar field. In this paper we propose a non-canonical
complex scalar field as the dark energy, which we dub ``hessence'', to
implement crossing the phantom divide, in a similar sense as the quintom dark
energy model. In the hessence model, the dark energy is described by a single
field with an internal degree of freedom rather than two independent real
scalar fields. However, the hessence is different from an ordinary complex
scalar field, we show that the hessence can avoid the difficulty of the Q-balls
formation which gives trouble to the spintessence model (An ordinary complex
scalar field acts as the dark energy). Furthermore, we find that, by choosing a
proper potential, the hessence could correspond to a Chaplygin gas at late
times.Comment: Latex2e, 12 pages, no figure; v2: discussions and references added,
14 pages, 3 eps figures; v3: published versio
The Chaplygin gas, a model for dark energy in cosmology.
We review the essential features of the Chaplygin gas cosmological models and provide some examples of appearance of the Chaplygin gas equation of state in modern physics. A possible theoretical basis for the Chaplygin gas in cosmology is discussed. The relation with scalar field and tachyon cosmological models is also considered
The Chaplygin gas as a model for dark energy
We review the essential features of the Chaplygin gas cosmological models and provide some examples of appearance of the Chaplygin gas equation of state in modern physics. A possible theoretical basis for the Chaplygin gas in cosmology is discussed. The relation with scalar field and tachyon cosmological models is also considered.