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Abstract Non-canonical scalar fields with the Lagrangian $${{\mathcal {L}}} = X^\alpha - V(\phi )$$ L = X α - V ( ϕ ) , possess the attractive property that the speed of sound, $$c_s^{2} = (2\,\alpha - 1)^{-1}$$ c s 2 = ( 2 α - 1 ) - 1 , can be exceedingly small for large values of $$\alpha $$ α . This allows a non-canonical field to cluster and behave like warm/cold dark matter on small scales. We derive a general condition on the potential in order to facilitate the kinetic term $$X^\alpha $$ X α to play the role of dark matter, while the potential term $$V(\phi )$$ V ( ϕ ) playing the role of dark energy at late times. We demonstrate that simple potentials including $$V= V_0\coth ^2{\phi }$$ V = V 0 coth 2 ϕ and a Starobinsky-type potential can unify dark matter and dark energy. Cascading dark energy, in which the potential cascades to lower values in a series of discrete steps, can also work as a unified model.

Abstract It is well known that a canonical scalar field is able to describe either dark matter or dark energy but not both. We demonstrate that a non-canonical scalar field can describe both dark matter and dark energy within a unified setting. We consider the simplest extension of the canonical Lagrangian $${\mathcal {L}} \propto X^\alpha - V(\phi )$$ L ∝ X α - V ( ϕ ) where $$\alpha \ge 1$$ α ≥ 1 and V is a sufficiently flat potential. In this case the kinetic term in the Lagrangian behaves just like a perfect fluid, whereas the potential term mimicks dark energy. For very large values, $$\alpha \gg 1$$ α ≫ 1 , the equation of state of the kinetic term drops to zero and the universe expands as if filled with a mixture of dark matter and dark energy. The velocity of sound in this model and the associated gravitational clustering are sensitive to the value of $$\alpha $$ α . For very large values of $$\alpha $$ α the clustering properties of our model resemble those of cold dark matter (CDM). But for smaller values of $$\alpha $$ α , gravitational clustering on small scales is suppressed, and our model has properties resembling those of warm dark matter (WDM). Therefore our non-canonical model has an interesting new property: its expansion history resembles $$\Lambda $$ Λ CDM, while its clustering properties are akin to those of either cold or warm dark matter.

Abstract  The seminal paper by Ya. B. Zeldovich (Soviet Physics Uspekhi 11, 381–393, 1968) is reprinted here, together with an editorial comment on its lasting scientific relevance, and a biography of the author. [ABSTRACT FROM AUTHOR]

An outstanding issue in braneworld theory concerns the setting up of proper boundary conditions for the brane-bulk system. Boundary conditions (BC's) employing regulatory branes or demanding that the bulk metric be nonsingular have yet to be implemented in full generality. In this paper, we take a different route and specify boundary conditions directly on the brane thereby arriving at a local and closed system of equations (on the brane). We consider a one-parameter family of boundary conditions involving the anisotropic stress of the projection of the bulk Weyl tensor on the brane and derive an exact system of equations describing scalar cosmological perturbations on a generic braneworld with induced gravity. Depending upon our choice of boundary conditions, perturbations on the brane either grow moderately (region of stability) or rapidly (instability). In the instability region, the evolution of perturbations usually depends upon the scale: small-scale perturbations grow much more rapidly than those on larger scales. This instability is caused by a peculiar gravitational interaction between dark radiation and matter on the brane. Generalizing the boundary conditions obtained by Koyama and Maartens, we find for the Dvali-Gabadadze-Porrati model an instability, which leads to a dramatic scale-dependence of the evolution of density perturbations in matter and dark radiation. A different set of BC's, however, leads to a more moderate and scale-independent growth of perturbations. For the mimicry braneworld, which expands like LCDM, this class of BC's can lead to an earlier epoch of structure formation. [ABSTRACT FROM AUTHOR]

We investigate constraints on some key cosmological parameters by confronting metastable dark energy (DE) models with different combinations of the most recent cosmological observations. Along with the standard ΛCDM model, two phenomenological metastable DE models are considered: (i) DE decays exponentially, (ii) DE decays into dark matter. We find that: (1) when considering the most recent supernovae and BAO data, and assuming a fiducial ΛCDM model, the inconsistency in the estimated value of the parameter obtained by either including or excluding Planck cosmic microwave background (CMB) data becomes very much substantial and points to a clear tension; (2) although the two metastable DE models that we study provide greater flexibility in fitting the data, and they indeed fit the supernovae (SNe) Ia+BAO data substantially better than ΛCDM, they are not able to alleviate this tension significantly when CMB data are included; (3) while local measurements of the Hubble constant are significantly higher relative to the estimated value of H0 in our models (obtained by fitting to SNe Ia and BAO data), the situation seems to be rather complicated with hints of inconsistency among different observational data sets (CMB, SNe Ia+BAO, and local H0 measurements). Our results indicate that we might not be able to remove the current tensions among different cosmological observations by considering simple modifications of the standard model or by introducing minimal DE models. A complicated form of expansion history, different systematics in different data and/or a nonconventional model of the early universe might be responsible for these tensions. [ABSTRACT FROM AUTHOR]

It is generally believed that a cosmological arrow of time must be associated with entropy production. Indeed, in his seminal work on cyclic cosmology, Tolman introduced a viscous fluid in order to make successive expansion/contraction cycles larger than previous ones, thereby generating an arrow of time. However, as we demonstrate in this letter, the production of entropy is not the only means by which a cosmological arrow of time may emerge. Remarkably, systems which are dissipationless may nevertheless demonstrate a preferred direction of time provided they possess attractors. An example of a system with well defined attractors is scalar-field driven cosmology. In this case, for a wide class of potentials (especially those responsible for inflation), the attractor equation of state during expansion can have the form , and during contraction . If the resulting cosmology is cyclic, then the presence of cosmological hysteresis, during successive cycles, causes an arrow of time to emerge in a system which is formally dissipationless. An important analogy is drawn between the arrow of time in cyclic cosmology and an arrow of time in an N-body system of gravitationally interacting particles. We find that, like the N-body system, a cyclic Universe can evolve from a single past into two futures with oppositely directed arrows of time. [ABSTRACT FROM AUTHOR]