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- W. H. Hu, Ignacy Sawicki
- 2007

We study a class of metric-variation f R models that accelerates the expansion without a cosmological constant and satisfies both cosmological and solar-system tests in the small-field limit of theâ€¦ (More)

- W. H. Hu, Ignacy Sawicki
- 2007

We develop a parameterized post-Friedmann (PPF) framework which describes three regimes of modified gravity models that accelerate the expansion without dark energy. On large scales, the evolution ofâ€¦ (More)

We study the evolution of linear cosmological perturbations in f(R) models of accelerated expansion in the physical frame where the gravitational dynamics are fourth order and the matter is minimallyâ€¦ (More)

- Ignacy Sawicki, W. H. Hu
- 2007

Ignacy Sawicki and Wayne Hu* Kavli Institute for Cosmological Physics, University of Chicago, Chicago, Illinois 60637, USA Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USAâ€¦ (More)

Ignacy Sawicki, Yong-Seon Song, and Wayne Hu* Kavli Institute for Cosmological Physics, Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637, USA Department of Physics, Universityâ€¦ (More)

- Ippocratis D. Saltas, Ignacy Sawicki, Luca Amendola, Martin Kunz
- Physical review letters
- 2014

We make precise the heretofore ambiguous statement that anisotropic stress is a sign of a modification of gravity. We show that in cosmological solutions of very general classes of models extendingâ€¦ (More)

where u is the velocity of an observer while "u is the energy density and P is the pressure measured by the observer and we use the (Ã¾ ) convention for the metric. The equation-of-state parameter isâ€¦ (More)

- Valerio Marra, Luca Amendola, Ignacy Sawicki, Wessel Valkenburg
- Physical review letters
- 2013

There is an approximately 9% discrepancy, corresponding to 2.4 Ïƒ, between two independent constraints on the expansion rate of the Universe: one indirectly arising from the cosmic microwaveâ€¦ (More)

We study the evolution of linear cosmological perturbations in f(R) models of accelerated expansion in the physical frame where the gravitational dynamics are fourth order and the matter is minimallyâ€¦ (More)

- Ignacy Sawicki
- 2004

We introduce the motivation for in ation as an additional phase of the early universe and then discuss some of the ways in which its theoretical grounding is being approached from the point of viewâ€¦ (More)