Misinterpreting Modified Gravity as Dark Energy: a Quantitative Study
  Standard cosmological data analyses typically constrain simple
phenomenological dark-energy parameters, for example the present-day value of
the equation of state parameter, $w_0$, and its variation with scale factor,
$w_a$. However, results from such an analysis cannot easily indicate the
presence of modified gravity. Even if general relativity does not hold,
experimental data could still be fit sufficiently well by a phenomenological
$w_0w_a$CDM, unmodified-gravity model. Hence, it would be very useful to know
if there are generic signatures of modified gravity in standard analyses. Here
we present, for the first time to our knowledge, a quantitative mapping showing
how modified gravity models look when (mis)interpreted within the standard
unmodified-gravity analysis. Scanning through a broad space of modified-gravity
(Horndeski) models, and assuming a near-future survey consisting of CMB, BAO,
and SNIa observations, we report values of the best-fit set of cosmological
parameters including $(w_0, w_a)$ that would be inferred if modified gravity
were at work. We find that modified gravity models that can masquerade as
standard gravity lead to very specific biases in standard-parameter spaces. We
also comment on implications for measurements of the amplitude of mass
fluctuations described by the parameter $S_8$.
