The Distance Scale
The Distance Scale
The Universe has been expanding continuously since the Big Bang, about 13.7 billion years ago. Its expansion gradually slowed because of the total gravity of matter in the Universe. However, it was recently discovered that during the past few billion years the expansion has increased. This acceleration is attributed to something astronomers call “dark energy.” Dark energy is the mysterious force that is causing the expansion of the Universe to speed up, overcoming the force of gravity.
In 1998, astronomers looked at Type Ia supernovae, exploding stars with predictable brightness. They found that supernovae observed in distant galaxies were dimmer than they should have been, meaning they are farther than predicted and the Universe must be expanding faster than previously thought. Astronomers don't know what dark energy is, but they do know some force is overcoming the gravitational pull within the Universe, speeding up its rate of expansion.
For the Digital Universe, we use this information to compute distances to the galaxies and quasars in the Extragalactic Atlas. This provides you with the size and scale of the Universe at the present epoch. This distance scheme sets the scale of the Universe so that the quasars range past 20 billion light-years, while the CMB is fixed to a sphere with a radius of 42 billion light-years. Of course, these objects were much closer when their light left. If we plotted the Universe according to where an object's light left from, then the CMB would be about 13.7 billion light-years away. However, we have opted to plot the Universe at its current state, lending reality to the Atlas and one's impression of the Universe.
© 2002-2005 American Museum of Natural History
Last Modified: 2007-12-19 by Brian Abbott