186 research outputs found
The Statefinder hierarchy: An extended null diagnostic for concordance cosmology
We show how higher derivatives of the expansion factor can be developed into
a null diagnostic for concordance cosmology (LCDM). It is well known that the
Statefinder -- the third derivative of the expansion factor written in
dimensionless form, a^{(3)}/aH^3, equals unity for LCDM. We generalize this
result to higher derivatives of the expansion factor and demonstrate that the
hierarchy, a^{(n)}/aH^n, can be converted to a form that stays pegged at unity
in concordance cosmology. This remarkable property of the Statefinder hierarchy
enables it to be used as an extended null diagnostic for the cosmological
constant. The Statefinder hierarchy combined with the growth rate of matter
perturbations defines a composite null diagnostic which can distinguish
evolving dark energy from LCDM.Comment: 6 pages, 6 figures; to appear in Phys. Rev.
Generalizing the Cosmic Energy Equation
We generalize the cosmic energy equation to the case when massive particles
interact via a modified gravitational potential of the form phi(a, r), which is
allowed to explicitly depend upon the cosmological time through the expansion
factor a(t). Using the nonrelativistic approximation for particle dynamics, we
derive the equation for the cosmological expansion which has the form of the
Friedmann equation with a renormalized gravitational constant. The generalized
Layzer-Irvine cosmic energy equation and the associated cosmic virial theorem
are applied to some recently proposed modifications of the Newtonian
gravitational interaction between dark-matter particles. We also draw attention
to the possibility that the cosmic energy equation may be used to probe the
expansion history of the universe thereby throwing light on the nature of dark
matter and dark energy.Comment: 11 pages; technical issue in the previous submission corrected;
version to match publication in Phys. Rev. D [Rapid Communication
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