AY 101 Chapter Notes - Chapter 18: Cosmic Microwave Background, Dark Matter, Dim Stars
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The Big Picture
We have found that there may be much more to the universe than meets the eye. Dark matter too dim for
us to see seems to far outweigh the stars, and a mysterious dark energy may be even more prevalent.
Together, dark matter and dark energy have probably been the dominant agents of change in the overall
history of the universe. Here are some key “big picture” points to remember about this chapter:
•Dark matter and dark energy sound very similar, but they are hypothesized to explain different
observations. Dark matter is thought to exist because we detect its gravitational inﬂuence. Dark energy
is a term given to the source of the force that is accelerating the expansion of the universe.
•Either dark matter exists or we do not understand how gravity operates across galaxy-size distances.
There are many reasons to be conﬁdent about our understanding of gravity, leading most astronomers to
conclude that dark matter is real.
•Dark matter seems to be by far the most abundant form of mass in the universe, and therefore the
primary source of the gravity that formed galaxies and larger-scale structures from tiny density
enhancements that existed in the early universe. We still do not know what dark matter is, but we
suspect it is largely made up of as-yet-undiscovered subatomic particles.
•The existence of dark energy is supported by evidence from observations both of the expansion rate
through time and of temperature variations in the cosmic microwave background. Together, these
observations have led to a model of the universe that gives us increasingly precise values for the
inventory of its contents and its age.
•The fate of the universe seems to depend on whether its expansion continues forever, and the
acceleration of the expansion suggests that it will. Nevertheless, forever is a long time, and only time
will tell whether new discoveries will alter our speculations about the distant future.
Summary of Key Concepts
What do we mean by dark matter and dark energy?
•Dark matter and dark energy have never been directly observed, but each has been proposed to exist
because it seems the simplest way to explain a set of observed motions in the universe. Dark matter is
the name given to the unseen mass whose gravity governs the observed motions of stars and gas clouds.
Dark energy is the name given to the form of energy though to be causing the expansion of the
universe to accelerate,
What is the evidence for dark matter in galaxies?
•The orbital velocities of stars and gas clouds in galaxies do not change much with distance from the
center of the galaxy. Applying Newton’s laws of gravitation and motion to these orbits leads to the
conclusion that the total mass of a galaxy is far larger than the mass of its stars. Because no detectable
visible light is coming from this additional mass, we call it dark matter.
What is the evidence for dark matter in clusters of galaxies?
•We have three different ways of measuring the amount of dark matter in clusters of galaxies: from
galaxy orbits, from the temperature of the hot gas in clusters, and from the gravitational lensing
predicted by Einstein. All these methods are in agreement, indicating that the total mass of dark matter
in a galaxy cluster is at least 40 times the mass of its stars.
Does dark matter really exist?
•We infer that dark matter exists from its gravitational inﬂuence on the matter we can see, leaving two
possibilities: either dark matter exists or there is something wrong with our understanding of the galaxy.
We cannot rule out the latter possibility, but we have good reason to be conﬁdent about our current
understanding of gravity and the idea that dark matter is real.
What might dark matter be made of?
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