Download Chap 18 Cosmology v2

Neil F. Comins • William J. Kaufmann III
Discovering the Universe
Eighth Edition
CHAPTER 18
Cosmology
WHAT DO YOU THINK?
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What is the universe?
Did the universe have a beginning?
Into what is the universe expanding?
How strong is gravity compared to the
other forces in nature?
Will the universe last forever?
What is the Universe?
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The universe consists of all matter,
energy, and space-time that we can ever
detect or that will ever be able to affect us.
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Cosmology is the study of the large –scale
structure and evolution of the universe.
How did the Universe Begin?
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The best theory scientists can come up with is
the Big Bang.
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All matter and energy existed in an infinitely
dense area
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In the first moments of the universe (~13.7 billion
years ago) matter and energy expanded
explosively in an event called the Big Bang
Evidence for the Big Bang
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Cosmic Microwave Background Radiation
-Remnants of the energy from the big bang cause
space to have a temperature of ~2.73 K.
-This background radiation was predicted by
calculations before it was discovered in the 1960’s
Bell Labs Horn Antenna
This Bell Laboratories horn antenna at Holmdel, New Jersey, was
used by Arno Penzias (right) and Robert Wilson in 1965 to detect the
cosmic microwave background.
Evidence for the Big Bang: Cosmological Redshift
Supports that the universe is expanding!
Evidence for the Big Bang

Hubble’s Law allows us to estimate the age
of the universe
(assuming the universe has been expanding at a constant rate)
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1/H0 = 1/71 km/s/Mpc = 13.7 billion years
Unification of the Four Forces
Cosmic Timeline
This figure shows our current thinking about the
evolution of star and galaxy formation in the universe.
Evolutionary Timeline of the Universe
This figure shows that star formation started quickly and has
been tapering off ever since.
Observable Universe
Fate of the Universe
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Models
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1. Closed Universe – the universe will
eventually slow down and collapse again.
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2. Open Universe – the universe will keep
expanding forever
Fate of the Universe
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Scientists believe some kind of dark
energy is causing the universe to
accelerate outward.

This suggests the universe will continue to
expand forever.
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Current evidence suggest we live in an
open universe.
http://hubblesite.org/hubble_discoveries/dark_energy/
Summary of Key Ideas
The Big Bang
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Astronomers believe that the universe began as an
exceedingly dense cosmic singularity that expanded
explosively in an event called the Big Bang. The Hubble
law describes the ongoing expansion of the universe and
the rate at which superclusters of galaxies move apart.

The observable universe extends about 13.7 billion lightyears in every direction from Earth to what is called the
cosmic light horizon. We cannot see any objects that
may exist beyond the cosmic light horizon because light
from these objects has not had enough time to reach us.
The Big Bang
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According to the theory of inflation, early in its existence,
the universe expanded super rapidly for a short period,
spreading matter that was originally near our location
throughout a volume of the universe so large that we
cannot yet observe much of it. The observable universe
today is thus a growing volume of space containing
matter and radiation that was in close contact with our
matter and radiation during the first instant after the Big
Bang. This explains the isotropic and homogeneous
appearance of the universe.
A Brief History of Spacetime, Matter,
Energy, and Everything
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Four basic forces—gravity, electromagnetism, the strong
nuclear force, and the weak nuclear force— explain the
interactions observed in the universe.
According to current theory, all four forces were identical
just after the Big Bang. At the end of the Planck time
(about 10-43 s after the Big Bang), gravity became a
separate force. A short time later, the strong nuclear force
became a distinct force. A final separation created the
electromagnetic force and the weak nuclear force.
Before the Planck time, the universe was so dense that
known laws of physics do not describe the behavior of
spacetime, matter, and energy.
A Brief History of Spacetime, Matter,
Energy, and Everything
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In its first 30,000 years, the universe was radiation
dominated, during which time photons prevented matter
from forming clumps. Then it was matter-dominated, during
which time superclusters and smaller clumps of matter
formed. Today it is dark-energy-dominated. Dark energy of
some sort supplies a repulsive gravitational force that
causes superclusters to accelerate away from each other.
Astronomers think that during the first 379,000 years of the
universe, matter and energy formed an opaque plasma,
called the primordial fireball. Cosmic microwave
background radiation is the greatly redshifted remnant of
the universe as it existed about 379,000 years after the Big
Bang.
A Brief History of Spacetime, Matter,
Energy, and Everything
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By 379,000 years after the Big Bang, spacetime
expansion caused the temperature of the universe to fall
below 3000 K, enabling protons and electrons to
combine to form hydrogen atoms. This event is called
the era of recombination. The universe became
transparent during the era of recombination, meaning
that the microwave background radiation contains the
oldest photons in the universe.
A Brief History of Spacetime, Matter,
Energy, and Everything
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Clusters of galaxies and individual galaxies formed from
pieces of enormous hydrogen and helium clouds, each
of which became a separate supercluster of galaxies.
All of the superclusters and some of the clusters of
galaxies within each supercluster are moving away from
one another.
During the matter-dominated era, structure formed in the
universe. As the universe goes farther into the dark
energy-dominated era, the large-scale structure of
superclusters of galaxies will fade away.
The Fate of the Universe
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The average density of matter and dark energy in the
universe determines the curvature of space and the
ultimate fate of the universe.
Observations show that the universe is flat and that the
cosmic microwave background is almost perfectly isotropic,
resulting from a brief period of very rapid expansion (the
inflationary epoch) in the very early universe.
The universe is accelerating outward and it will expand
forever.