How can we say that the Universe is accelerating outwards when the only observations of this are billions of years out of date, and the true situation now may be that is slowing down or even starting to retract?
This is a good question!
The universe is currently expanding and it will not start retracting. Of course, observations of nearby objects cannot provide information about the future expansion of the universe. It is the current cosmological model that enables a prediction of the fate of the universe.
However, even the most elegant and complete theory can describe something that does not exist. It is for this reason that observations become crucial, even if they refer to events that happened millions or billions of years ago. In fact, a complete cosmological model should be able to make predictions for different epochs of the history of the Universe, both in the past and in the future. When observations of past events agree with the model, the theory becomes successful and its predictions are considered reliable. This is generally true in science, but for the cosmological model there is even stronger evidence that the Universe is expanding and accelerating.
The fate of the Universe is completely dominated by its composition, which can be measured with great accuracy using at least three independent methods. The first method is by measuring the CMB, the cosmic microwave background radiation detected by the satellite WMAP (http://map.gsfc.nasa.gov/universe/bb_tests_cmb.html). This is isotropic light (it comes from every direction) that originated when the universe was hot in its early stages. The second method is the distribution of the galaxies in the universe which shows “crowded” regions surrounded by voids with no galaxies (http://www.sdss.org/includes/sideimages/sdss_pie2.html). The third method is the study of the supernovae Ia, stars that at the ends of their lives explode and, though distant, become easily visible. These methods are independent and probe the universe at three different times in the past. All together, they show that the mass of the universe is not enough to slow the expansion of the universe by gravity. Moreover, these observations show that the major component of the universe is the “dark energy”, whose nature is still unclear, but that causes an acceleration in the expansion of the universe.