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Lecture notes of 2 pages for the course 1740 at Ebor (homework answers)
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NATS1740 HW 5The expansion of space is a challenge to picture. In the first half of the 20 th
century astronomers studied galaxies near our Milky Way for the very first time.
In every direction they looked they found that the most distant galaxies had
velocities carrying them away from our own galaxy. Was everything running away
from us?
In this homework you will try to picture the expansion of 3-dimensional space on
a 2-dimensional surface (this page). To do this, we will imagine a universe of 2
dimensions like what might sit on the surface of a balloon. On a surface like that,
items can only move in 2 directions (left-right or up-down), they can never leave
the surface. If the balloon inflates, any galaxies on the surface will see every other
galaxy appear to be moving away, but actually it is the balloon itself that
stretches.
Shown in the figure on the next page are 3 instances in time for a balloon
universe from 2 different perspectives. Perspective A (or galaxy a) and Perspective
B (for galaxy b). The axis on the right give each image scale in millions of light
years (Mly).
The time elapsed between each time period of the universe is in gigayears where
1Gyr = 1 billion years. Note that the 3 time periods are given in the rows of the
figure (i.e. the bottom row is 7.5 Gyr).
You’ll need to use the figure to answer the questions on the homework.
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