Dipanjan Dey

The idea that coherent oscillations of a scalar field, oscillating over a time period that is much shorter than the cosmological timescale, can exhibit cold dark matter (CDM) like behavior was previously established. In our work we first show that this equivalence between the oscillating scalar field model and the CDM sector is exact only in a flat Friedmann–Lemaitre-Robertson–Walker (FLRW) spacetime in the absence of cosmological constant and any other possible matter components in the universe when the mass of the scalar field is very large compared to the Hubble parameter. Then we show how to generalize the equivalence between the coherently oscillating scalar field model and the CDM sector in a spatially curved universe with multiple matter components. Using our general method, we will show how a coherently oscillating scalar field model can represent the CDM sector where it decays into the radiation. Our method is powerful enough to work out the dynamics of gravitational collapse in a closed FLRW spacetime where the coherently oscillating scalar field model represents the CDM sector. We have, for the first time, presented a consistent method which specifies how a coherently oscillating scalar field model, where the scalar field is ultralight, acts like the CDM sector in a multicomponent universe.

This study examines the gravitational collapse of an overdense dark matter region in a coupled scalar field dark energy scenario within a flat FLRW background. It finds that, depending on the initial conditions, some overdense regions avoid collapse and expand eternally with the background. The interior overdense region follows a closed FLRW metric, while its boundary is described by generalized Vaidya spacetime, which allows flux across the boundary while preserving the homogeneity of dark energy inside. Dark matter evolves as cold dark matter, but in non-minimal coupling, the modified Klein-Gordon equation alters dark energy evolution. The results highlight the impact of coupled dark energy on dark matter virialization and cosmic structure formation.

This paper explores the cosmological implications of a scalar field with a specific potential, crucial for achieving the final equilibrium state of gravitational collapse. We consider a system with two fluids: minimally coupled matter representing dust-like dark matter and a scalar field acting as dark energy. Our model, akin to the top-hat collapse model, focuses on isolated over-dense regions within a closed FLRW metric, while the background follows a flat FLRW metric. We analyze spacetime configurations where these regions undergo initial expansion followed by contraction, deriving the scalar field potential responsible for their equilibrium state. Our fully relativistic approach offers a comprehensive understanding of stable cosmic over-dense regions, without the need for ad-hoc Newtonian virialization.

In this article we propose a new way to handle interactions between two scalar fields, in the cosmological backdrop, where one scalar field oscillates rapidly in the cosmological time scale while the other one evolves without showing any periodic behavior. We have interpreted the rapidly oscillating scalar field as the dark matter candidate while the other scalar field is the canonical quintessence field or the non-canonical phantom field. A model of a generalized top-hat-like collapse is developed where the dark sector is composed of the aforementioned scalar fields. We show how the non-minimal coupling in the dark sector affects the gravitational collapse of a slightly overdense spherical patch of the universe. The results show that one can have both unclustered and clustered dark energy in such collapses, the result depends upon the magnitude of the non-minimal coupling strength.

This paper explores the evolution of the overdense region of dark matter in the presence of a nonminimally coupled scalar field which is used to model quintessence and phantomlike dark energy. We focus on algebraic coupling, where the interaction Lagrangian is independent of the derivatives of the scalar field. To make our model more relativistic, like the minimal coupling scenario we studied earlier, we consider a spacetime structure that is internally closed Friedmann-Lemaître-Robertson-Walker (FLRW) spacetime and externally the generalized Vaidya spacetime. This structure allows nonzero matter flux at the boundary of the overdense region. Our investigation reveals that an increment of the coupling strength causes dark energy to cluster with dark matter at a certain cosmological scale where the influence of dark energy cannot be ignored. This phenomenon arises from the specific nature of the nonminimal coupling considered in this paper. While the evolution of matter’s energy density remains unchanged, the scalar field’s Klein-Gordon equation is modified, causing dark energy to deviate from its homogeneous state and cluster with dark matter. Similar to minimal coupling scenarios, closed spherical regions do not collapse within certain parameter ranges, exhibiting eternal expansion within the spatially flat FLRW spacetime and acting as voids with decreasing matter density. The study extends our understanding of the cosmological scenarios where the virialization of the overdense regions of dark matter is influenced by the nonminimally coupled dark energy.

This paper explores the cosmological implications of a scalar field with a specific potential, crucial for achieving the final equilibrium state of gravitational collapse. We consider a system with two fluids: minimally coupled matter representing dust-like dark matter and a scalar field acting as dark energy. Our model, akin to the top-hat collapse model, focuses on isolated over-dense regions within a closed FLRW metric, while the background follows a flat FLRW metric. We analyze spacetime configurations where these regions undergo initial expansion followed by contraction, deriving the scalar field potential responsible for their equilibrium state. Our fully relativistic approach offers a comprehensive understanding of stable cosmic over-dense regions, without the need for ad-hoc Newtonian virialization.