Energy and Human Ambitions on a Finite Planet, 2021a

D.2 Cosmic <strong>Energy</strong> Conservation 393
that existed before this time is opaque 1 to light travel. The universe only
became “clear” after this time, when the plasma cooled into neutral
(mostly) hydrogen atoms. So we can see almost back to the Big Bang, at
least 99.997% of the way before the scene becomes opaque.
1: . . . much as the sun, another plasma is
opaque: we only see light from its outer
surface
So that’s the limit to our vision, based on the idea that light has not had
time to travel farther since the universe began. This is what we mean by
theedgeofthevisible universe.
But is it a real edge? All indications are that it is certainly not. When we
look 13.8 billion light years away, we just see this glowing plasma (the
CMB). But in the intervening years, galaxies <strong>and</strong> stars <strong>and</strong> planets have
formed in that region of space, <strong>and</strong> would appear “normal,” or mature
today. So imagine a being on such a planet looking at us today, 13.8
billion light years distant. But they see us 13.8 billion years ago, when
our neighborhood was still a glowing plasma well before the formation
of galaxies, stars, <strong>and</strong> planets.
Let’s say the distant being is directly behind you <strong>and</strong> you are both
looking off in the same direction—the alien essentially looking over your
shoulder as you look directly opposite the direction to the alien. You
(or the primordial gas that will someday become you) are at the limit
of their vision, <strong>and</strong> they can’t see anything beyond you. You sit at their
edge. But to you it’s no edge. You have no trouble seeing more “normal”
universe stretching another 13.8 billion light years beyond what our
distant friend can see. It’s only a perceived edge, based on the limit of
light travel time.
A nice way to think of it is familiar scenes of limited vision, like in a fog
or in the ocean, or even on the curved surface of Earth. All cases have a
horizon: a limit to the distance visible. Yet moving to the edge of vision
reveals a whole new region that was before invisible. Keep going <strong>and</strong>
your starting region will no longer be visible, or within your horizon.
But it has not ceased to exist.
Similarly, the universe would seem to be much larger than our visible
horizon. Measurements of the “flatness” of the geometry of space suggest
a universe that is at least 100 times larger than our horizon, <strong>and</strong> may in
fact be unfathomably larger. We may never know for sure, as limits to
light travel seal us off from direct observation of most of the universe.
D.2 Cosmic <strong>Energy</strong> Conservation
Sec. 5.2 (p. 69) discussed the foundational principle of the conservation
of energy, claiming that the principle is never violated except on cosmic
scales. Besides elaborating on that point, this section follows the story
of energy across vast spans of time as our sun forms <strong>and</strong> ultimately
delivers energy to propel a car. We also clarify what it means for energy
to be “lost” to heat.
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.