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Cosmological perturbations

- Karim A. Malik, D. Wands
- Physics
- 29 September 2008

We review the study of inhomogeneous perturbations about a homogeneous and isotropic background cosmology. We adopt a coordinate based approach, but give geometrical interpretations of metric… Expand

A general proof of the conservation of the curvature perturbation

- D. Lyth, Karim A. Malik, M. Sasaki
- Physics
- 9 November 2004

Without invoking a perturbative expansion, we define the cosmological curvature perturbation, and consider its behaviour assuming that the universe is smooth over a sufficiently large comoving scale.… Expand

A New approach to the evolution of cosmological perturbations on large scales

- D. Wands, Karim A. Malik, D. Lyth, A. Liddle
- Physics
- 20 March 2000

We discuss the evolution of linear perturbations about a Friedmann-Robertson-Walker background metric, using only the local conservation of energy-momentum. We show that on sufficiently large scales… Expand

Evolution of second-order cosmological perturbations.

- Karim A. Malik, D. Wands
- Physics
- 2 July 2003

We present a method for constructing gauge-invariant cosmological perturbations which are gauge-invariant up to second order. As an example, we give the gauge-invariant definition of the second-order… Expand

Non-Gaussianity of inflationary field perturbations from the field equation

- D. Seery, Karim A. Malik, D. Lyth
- Physics
- 5 February 2008

We calculate the tree-level bispectrum of the inflaton field perturbation directly from the field equations, and construct the corresponding fNL parameter. Our results agree with previous ones… Expand

A not so short note on the Klein–Gordon equation at second order

- Karim A. Malik
- Physics
- 29 October 2006

We give the governing equations for multiple scalar fields in a flat Friedmann–Robertson–Walker (FRW) background spacetime on all scales, allowing for metric and field perturbations up to second… Expand

Cosmological Perturbations in an Inflationary Universe

- Karim A. Malik
- Physics
- 31 January 2001

After introducing gauge-invariant cosmological perturbation theory we give an improved set of governing equations for multiple fluids including energy transfer. Having defined adiabatic and entropic… Expand

Generalised constraints on the curvature perturbation from primordial black holes

- A. S. Josan, A. Green, Karim A. Malik
- Physics
- 18 March 2009

Primordial black holes (PBHs) can form in the early Universe via the collapse of large density perturbations. There are tight constraints on the abundance of PBHs formed due to their gravitational… Expand

New calculation of the mass fraction of primordial black holes

- A. Green, A. Liddle, Karim A. Malik, M. Sasaki
- Physics
- 8 March 2004

We revisit the calculation of the abundance of primordial black holes (PBHs) formed from primordial density perturbations, using a formation criterion derived by Shibata and Sasaki which refers to a… Expand

Large scale curvature and entropy perturbations for multiple interacting fluids

- Karim A. Malik, D. Wands, Carlo Ungarelli Institut d'Astrophysique de Paris, Portsmouth, Birmingham
- Physics
- 27 November 2002

We present a gauge-invariant formalism to study the evolution of curvature perturbations in a Friedmann-Robertson-Walker universe filled by multiple interacting fluids. We resolve arbitrary… Expand

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