The only thing we can ever hope to do in an expanding universe is to determine our velocity (speed and direction) with respect to ever larger collections of matter, and larger scales. The Earth moves at about 30 km/sec in an orbit centered on the Sun. The sun looks like a stationary object locally, but we know it is moving around the center of the galaxy with a speed of 220 km/sec. For many kinds of astronomical study, the center of the Milky Way is 'at rest' and stars and gas clouds circulate around it. For other studies, we have to consider the motion of our galaxy relative to the center of mass of our local cluster of galaxies, or the motion of our local cluster relative to more distant clusters. The speeds and directions of all these motions, at any given time, have to be vectorially added.
The largest reference frame we know about is that of the cosmic background radiation itself...the fireball light left over from the Big Bang. It samples a scale of the universe over 15 billion light years in radius. If you have no 'peculiar' speed relative to this frame, your galaxy is exactly partaking of the expansion of the universe with no additional local speed added to it. The NASA Cosmic Background Explorer confirmed many previous measurements of the motion of our earth's speed and found that it has a peculiar speed of 360 +/- 20 kilometers/sec in the direction of the constellations Leo and Crater. This means that when you add up all of the speeds and directions of the different local reference frames, the net vector sum is the amount seen by COBE. Astronomers, knowing how fast the sun travels around the milky way, the milky way through the local group, the local group towards the Virgo supercluster etc, have been able to account for much of the COBE result, but it seems that the entire collection of galaxies and clusters of galaxies out to 100 million light years, may have its own residual velocity compared to the cosmic background 'rest frame'. If this tendency persists as we explore the motions of larger and larger systems of galaxies, this MIGHT mean that the Big Bang was not as smooth as we have been assuming.
All answers are provided by Dr. Sten Odenwald (Raytheon STX)
NASA IMAGE/POETRY Education and Public Outreach program.