Lecture 7: Threshold 2—The First Stars and Galaxies

Threshold 2—The First Stars and Galaxies ....................................

LECTURE

So, here’s the problem. How can you move from something this simple

to the vastly more varied and beautiful world that we inhabit today?

How can you generate complex things with such simple components?

T

his lecture leads us across the second major threshold of this course

by describing how the early Universe created the ¿ rst complex

objects. How do you make a star? The early Universe contained vast

clouds of hydrogen and helium atoms with energy pouring through them.

Enter gravity. Gravity is a force, so it can rearrange things in interesting

ways. Gravity began the process of sculpting more complex things. Newton

described gravity as a universal force of attraction acting between all forms

of matter.

Einstein showed that it also acted on energy, because energy and matter are

different forms of the same underlying “stuff.” He also showed that gravity

is caused by the changing geometry of “space-time.” Here, we stick with

Newton’s simpler idea of gravity as a “force,” because it is easier to grasp

and can explain all the phenomena we need to understand in this course.

Newton showed that the strength of gravity (1) increases with the mass

of the objects involved and (2) declines as the distance between objects

increases. This means that if the early clouds of matter had been distributed

perfectly equally, gravity would have acted equally on everything, leading to

a sort of cosmic traf¿ c jam in which nothing could move and nothing could

happen. But gravity can magnify even the tiniest unevennesses, because

its power increases where matter is more concentrated. So, where matter

and energy are not distributed evenly, gravity can rearrange them in more

interesting ways.

In 1992, George Smoot used the COBE (Cosmic Background Explorer)

satellite to detect tiny variations in the temperature of the cosmic background

radiation. He showed that there were tiny differences in density and energy

within the clouds of hydrogen and helium that ¿ lled the early Universe.