Introduction to Cosmology

0 0.5 1 1.5 2

Redshift z

• 0.5

0

0.5

1

Δ (

m

• M

)

always accelerates

accelerates now

decelerates in the past

always decelerates

flat

open

closed

Figure 12.1Evidence for transition from deceleration in the past to acceleration today. The
difference mM is defined in Equation (2.60). The blue line shows a model that fits the data,
where acceleration happens at late epochs in the history of the universe (i.e. starting a few
billion years ago, and billions of years after the Big Bang). The plot uses binned data from the
Union2 compilation [2]. With permission from the author.