Astroparticle Physics

186 8 Cosmology8.8 Experimental Evidencefor the Vacuum Energy“It happened five billion years ago. Thatwas when the Universe stopped slowingdown and began to accelerate, experiencinga cosmic jerk.”Adam Riessdeceleration parameteracceleration parameterFor a given set of contributions to the energy density of theuniverse, the Friedmann equation predicts the scale factor,R(t), as a function of time. From an observational standpoint,one would like to turn this around: from measurementsof R(t) one can make inferences about the contents ofthe universe. Naïvely one would expect the attractive forceof gravity to slow the Hubble expansion, leading to a deceleration,i.e., ¨R < 0. One of the most surprising devel-opments of recent years has been the discovery that the expansionis, in fact, accelerating, and apparently has been forseveral billion years. This can be predicted by the Friedmannequation if one assumes a contribution to the energy densitywith negative pressure, such as the vacuum energy pre-viously mentioned. Such a contribution to ϱ is sometimescalled dark energy.Because of the finite speed of light, observations ofgalaxies far away provide information about the conditionsof the universe long ago. By accurately observing the motionof very distant galaxies and comparing to that of those closerto us, one can try to discern whether the universe’s expansionis slowing down or speeding up. From each observedgalaxy, two pieces of information are required: its speed ofrecession and its distance. As type-Ia supernovae are extremelybright and therefore can be found over cosmologicaldistances, they are well suited for this type of study. Toobtain the speed, the redshift z of spectral lines can be used.As type-Ia supernovae have to good approximation a con-stant absolute luminosity, the apparent brightness providesinformation on their distance. This is essentially the sametype of analysis that was carried out to determine the Hubbleparameter H 0 as described in Sect. 8.1. Here, however,one is interested in pushing the measurement to distances sofar that the rate of expansion itself may have changed in thetime that it took for the light to arrive at Earth.The relation between the apparent brightness of a supernovaand its redshift can be obtained once one knowsthe scale factor as a function of time. This in turn is deter-negative pressuredark energytype-Ia supernovaerelation between brightnessand redshift