Luminosity-Distance versus red-shift for Type Ia Supernovae

In 1998 two independent teams of astronomers and physicists were able to measure the relation between distance and redshift for samples of Type Ia Supernovae, for redshifts extending to values above 1.

The redshift is related to the speed at which the galaxy that contains the supernovae is receding from us, while the distance is related to the time the light has been traveling to us. The data indicate that in the past the universe was expanding at a lower rate than at present, contrary to what one would expect if the universe only contained matter and ordinary energy.

The Figure on the right (Perlmutter, Physics Today) shows magnitude (related to luminosity) versus red-shift for two samples of Type Ia Supernovae obtained by two different teams. The lines show the expectations for several models of the universe, with and without dark energy

Temperature fluctuations in the CMB

Independent evidence for dark energy comes from the study of the total amount of mass-energy in the universe.

From the detailed study of temperature fluctuations in the CMB it is possible to obtain the overall density of matter and energy in the universe and thus its overall geometry. If the density is above a certain value, named critical density, the universe will be curved in itself. The critical density corresponds to a flat universe and a density below critical corresponds to an open universe.

The precise measurements of the cosmic microwave background temperatures obtained by the WMAP satellite in 2003, together with earlier measurements, give a strong indication that the universe is flat. On the other hand, the mass density, as inferred from several determinations, is only about 25% of the critical density. The rest should be other type, namely dark energy.