A scanner darkly.
In your March 2007 column, you state: "It is actually two clusters of galaxies, gasses and plasma found about four billion light years from Earth that collided around 100 million years ago."
Isn't there a typing error in this sentence? It seems to not make sense to see an event that happened 100 million years ago if the two clusters in question are four billion light years away from the Earth. I appreciate your kind response.
ANALYTICAL TECHNOLOGIES S.A.
I wrestled with this as well. The original paper, available at chandra.harvard.edu/photo/2006/1e067/medi a, describes the bullet cluster: "... As the line-of-sight velocity difference between the components is only ~600 km/s, the merger must be occurring nearly in the plane of the sky and the cores passed through each other ~100 Myr ago." Various online sources (including en.wikipedia.org/wiki/Bullet_cluster) describe the cluster as being" ... about 4 billion light years from Earth in the constellation of Carina. Observed from Earth, the subcluster passed through the cluster center 150 million years ago...."
I believe the important part is "observed from Earth," so that the temporal reference frame is our own reference frame, i.e., the reference frame of the observatories. So, from our reference frame, the collision happened approximately 100 million years ago, while from the reference frame of the Bullet cluster, it happened around 4 billion + 100 million years ago.