This animation shows how galaxies separate with the passage of time in three different universe models. Galaxies separate horizontally as time progresses vertically. The three models show the same separation rate today, making the point that we cannot distinguish between these models if our only information is the present expansion rate. However, the models have distinct pasts and futures.
The yellow rings show the simplest expansion. This case corresponds to a very low-density universe, whose expansion rate neither slows down nor speeds up.
The blue rings illustrate motion in a universe with high density. In this case, the expansion began later but was initially much more rapid. Over time, the gravity from the high density has been slowing the expansion and in the distant future this universe would collapse back in on itself.
The real universe is now thought to behave in the manner of the red rings. There has been a competition between gravity, which initially slowed down the expansion, and an energy, which is now causing an acceleration of the expansion. The flash at a certain point along each trajectory signals a supernova event at the same "redshift,' or spectral Doppler displacement, for each model. The flash occurs closer in time and distance for the blue model, the dense universe. The flash occurs farthest in distance and time for the red model, the one that corresponds to observations.
Image: Visualization by Brent Tully and Karen Teramura.