C onstraints on scalar-tensor m odels ofdark energy from observationaland localgravity tests

f(R)gravity theories corresponding to the coupling Q = 1= p 6. W end that these m odels can bem adecom patible with localgravity constraintseven when jQ j isoftheorderofunity through a cham eleon m echanism ,ifthescalar-� eld potentialischosen to have a suciently large m assin the high-curvatureregions.W eshow thatthesem odelsgenerally lead to thedivergenceoftheequation ofstateofDE,which occuratsm allerredshiftsasthedeviation from theCDM m odelbecom em ore signicant.W e also study theevolution ofm atterdensity perturbationsand em ploy them to place boundson the coupling jQ j aswellasm odelparam etersoftheeld potentialfrom observationsof them atterpowerspectrum and theCM B anisotropies.W end that,aslong as jQ j issm allerthan theorderofunity,thereexistallowed param eterregionsthatareconsistentwith both observational and localgravity constraints. PACS num bers:98.70.V c I. IN T R O D U C T IO N