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Spacetime –
where all of the activity in the universe takes place, in a four
dimensional coordinate system (three dimensions plus time).
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In order to record events in spacetime, we use a graph called a spacetime diagram
that usually plots time on the yaxis vs. spatial direction (space) on the xaxis.
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Cosmological Horizon –
the boundary between what we can and cannot see in
terms of a light source.
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Observable Universe –
the distance from Earth to our cosmological horizon.
This distance is approximately 46 billion light years away
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Due to cosmic expansion, particles continually get farther and farther away from
each other even though they themselves are “sitting still”.
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Cosmic expansion however, can only be applied on a large scale. On a small
scale, such as the distance between two trees on Earth, the trees do not move apart
due to intermolecular forces and gravitational forces acting on them. Both of these
forces are greater than that of cosmic expansion.
•
The scale of the universe changes as a function of time and is denoted by the
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This note was uploaded on 12/28/2010 for the course ASTRONOMY 142 taught by Professor Bregman during the Winter '10 term at University of Michigan.
 Winter '10
 BREGMAN
 Space

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