chapter 29: cosmology - csufresno
TRANSCRIPT
Chapter 29: Cosmology
Cosmology is the study of the Universe as a whole, including its history and origin.
(It has nothing to do with haircuts or makeup: that’s cosmetology.)
What evidence is there that the Big Bang really happened?
(1) The observed expansion of the Universe, discovered in 1929 by Edwin Hubble,
shows that all but the nearest galaxies have redshifts in their spectra. This is because of
the Doppler effect as the galaxies move away (See Chapter 11.) Hubble’s law is the
farther the galaxy, the greater the redshift (so v = H0 D). In other words, the Universe is
observed to be expanding: the farther away a galaxy is, the faster it is moving away. (See
Figures 29-01 to 29-05.)
(2) Gravity always attracts. Because of this, the Universe can’t be static: it must be either
expanding or contracting. In 1916, Einstein discovered that his general theory of
relativity predicts this. That the Universe could be expanding seemed too fantastic even
for Einstein, so he didn’t call attention to it until 1929, after Edwin Hubble published his
observations of it. (See Figures 29-06 and 29-07.)
(3) The Cosmic Background Radiation was discovered in 1964 by Arno Penzias and
Bob Wilson. It is exactly the thermal radiation predicted for a Universe that was once hot
and dense, and has since expanded and cooled. (See Figures 29-08 to 29-12.)
(4) The Olbers paradox: If the Universe were infinitely large and infinitely old, the
night sky should not be dark. The night sky should be as bright as the Sun, because no
matter in which direction one looked, one would eventually see a star. The solution to the
paradox is that the Universe is not infinitely old. In fact, since galaxies are so smoothly
spread out, the Universe may be infinitely large. Since the Universe has a finite age,
though, astronomers can only see so far. (See Figure 29-13.)
(5) The abundances of helium and other light elements: During the first three minutes
of the existence of the Universe, the whole Universe was as hot and dense as the core of a
star. The same nuclear reactions that occur in stars were happening everywhere.
Scientists understand stars (and nuclear weapons) well enough to make predictions of the
abundances of the light elements cooked up in the Big Bang. Measurements from spectra,
starting in 1972, agree with these predictions, and with each other. (See Figure 29-14.)
(6) Recall look-back time (from Chapter 5): as one looks deep into space, one looks back
in time. This is because light has a finite speed, and it takes time to get here.
Looking deep into the Universe, astronomers see change over time, or in other words,
evolution. Examples include:
(a) In the distant past, galaxies of all kinds were much bluer, with many hot, short-lived
stars (OB types), after accounting for the effect of redshift. (See Figure 29-15.)
(b) Galaxies now are more often observed in clusters than they were in the past. This
clustering has progressed over the age of the Universe because of gravity. (See Figure 29-
15.)
(c) Looking deeply into space, astronomers observe that irregular galaxies were much
more common during the first 1-3 billion years of the age of the Universe. Spiral galaxies
have become more common since then, because of collisions and mergers between
galaxies. (See Figure 29-16.)
(d) Giant elliptical galaxies, sometimes called “cannibal galaxies,” have become more
common over the age of the Universe. They are also more common in the centers of
galaxy clusters. This is because they are observed to be eating other galaxies, mainly
spirals. (See Figure 29-16.)
(e) Quasars are the most luminous (powerful) galaxies. Quasars are agitated galaxies,
which are in the process of forming. Observations show that quasars were much more
common in the distant past than they are now. Normal spiral galaxies (like the Milky
Way) are observed to have become more relaxed, having settled into stable orbits because
of gravity. (See Figure 29-17.)
Figure 29-01: In the 1920s, a debate gripped astronomy. On one side was Harlow
Shapley. He said that the Milky Way was the entire Universe. On the other side was
Edwin Hubble. He said that there were other galaxies, or “island universes,” of billions of
stars. Shapley thought that these were nearby objects in the Milky Way. Edwin Hubble
won: his observations settled the argument in 1925.
Top left: Harlow Shapley (NEED PERMISSION from the American Institute of Physics
Emilo Segrè Visual Archive: This is image “Shapley Harlow A4”.)
Bottom left: Edwin Hubble (NEED PERMISSION from the American Institute of Physics
Emilo Segrè Visual Archive: This is image “Hubble Edwin A6”.)
Top right: The Milky Way, shown stretching from horizon to horizon (Image by the
author at Fresno State’s station at Sierra Remote Observatories.)
Bottom right: NGC 891, a galaxy similar to the Milky Way (Image by the author at
Mount Laguna Observatory) ________________
Figure 29-02: Shapley advocated that the Milky Way is the whole Universe. This is
partly because he had shown that the Universe was 100 times larger than previously
assumed.
In 1915, Shapley showed that the Solar System is not at the Milky Way’s center. He did
this by noticing that globular clusters (left), now known to be ancient clusters of stars, are
not evenly spread out over the sky: nearly all of them are observable during boreal
summer.
Shapley measured the distances to many globular clusters, using a method discovered by
the Harvard women. He found that the Milky Way’s globular clusters are centered on a
point in space about 30,000 light-years from Earth, in the constellation Sagittarius. The
Milky Way passes through Sagittarius, and is brightest and thickest there. Shapley
realized that he had discovered the center of the Milky Way (right), and that the Solar
System is not near it.
(Left: NASA/STScI; Right: Image by the author) ________________
Figure 29-03: Again, don’t confuse galaxies with planetary systems, or systems of
planets.
Left: The Solar System is our planetary system. It has one star (the Sun) and its system of
planets. (NASA/LPI)
Right: This is a whole galaxy, with hundreds of billions of stars. Nearly every one of
these stars probably has planets. (Image by the author at Mount Laguna Observatory)
________________
Figure 29-04: Left: In 1925, Edwin Hubble showed that galaxies are islands of billions
of stars, outside the Milky Way. (NEED PERMISSION from Jean-Leon Huens/Getty
image 80151535)
Right: This is the Great Galaxy in Andromeda, also known as M31. It is our nearest
neighbor galaxy. Edwin Hubble (actually, Milton Humason) obtained the first
observations of individual stars in this galaxy that were detailed enough to show that they
were similar to stars in the Milky Way—only much fainter, because they are much
farther away. (Image courtesy of Frank S. Barnes III)
________________
Figure 29-05: In 1929, Edwin Hubble discovered that the Universe is expanding.
Left: The spectra of nearly all galaxies are redshifted. This is because of the Doppler
effect: light from a galaxy moving away from the observer looks redder than it would if
the galaxy were stationary, because the light waves get stretched. (See Chapter 11.)
(Image by the author)
Right: Hubble’s law is: v = H0 D. The farther a galaxy is from us (D), the greater its
redshift, and the faster it moves away from us (v). (Image by the author)
(NOTE TO EDITOR: THIS FIGURE NEEDS TO BE IN COLOR.)
________________
Figure 29-06: Top left: If the Universe is expanding, this implies that sometime in the
past, everything was much closer together. The Universe is made largely of gas, and gas
follows the ideal gas law, so the denser a gas, the hotter it gets.
This implies that the Universe began in a hot, dense state, widely referred to today as “the
Big Bang.” Recent measurements with Hubble Space Telescope and other instruments of
the rate of expansion imply that the Big Bang happened 13.80 ± 0.02 billion years ago.
(Image by the author)
Bottom left: One way to understand the expansion of the Universe is to make an analogy
to an expanding balloon, with galaxies painted on it. As the balloon expands, the galaxies
move apart. This illustrates Hubble’s law, v = Ho D, since the farther apart the galaxies
are (D), the faster they move away from each other (v).
The expanding balloon analogy also illustrates that there is no center of the Universe.
From any galaxy, all the other galaxies appear to be moving away from it. This is because
the expansion of the Universe isn’t an explosion of particles into space, like a firecracker:
it’s an expansion of space itself. (Image by the author)
Right: Einstein visited Edwin Hubble at Mount Wilson in 1931. Einstein predicted the
expansion of space over time, in his general theory of relativity in 1916. Einstein did not
call attention to this, though, since it seemed too fantastic. He did come around to the
idea when Edwin Hubble was able to show him the expansion. (NEED PERMISSION
from the Archives of the California Institute of Technology Photo ID 1.6-16)
________________
Figure 29-07: “Extraordinary claims require extraordinary evidence.” It is a lot to expect
anyone to accept that the Universe, and time itself, had a beginning, isn’t it? What other
evidence supports this?
Left: Fred Hoyle was an author of the Steady State theory of cosmology, an early
competitor to the Big Bang. In 1949, he made up the name “Big Bang,” in an off-the-cuff
remark on a BBC radio show. He pointed out that there was no other evidence known—
in 1949—that the Universe had a hot, dense origin, aside from the observed expansion.
(NEED PERMISSION from the American Institute of Physics Emilo Segrè Visual
Archive: This is image “hoyle_fred_a1”.)
Right: The Steady State theory assumed that the Universe is infinitely old, and expands
because matter is created continuously. Before dismissing this as a crazy idea, remember
that it’s no crazier than supposing that all matter came into existence all at once in the
Big Bang. And so cossmology sat, until 1964. (Image by the author)
________________
Figure 29-08: In 1964, Arno Penzias (right) and Bob Wilson (left) were two engineers,
not astronomers. They were building the first satellite communications system for long-
distance telephone.
Satellites were crude and not-powerful in 1964. To send phone signals up and down to a
satellite, Penzias and Wilson used the antenna shown here. This was because this antenna
is larger and was much more sensitive than the satellite TV dishes on many houses today.
(NEED PERMISSION from the American Institute of Physics Emilo Segrè Visual
Archive, image c4)
________________
Figure 29-09: Because their antenna was so sensitive, Penzias and Wilson discovered
this faint glow of microwaves, coming from everywhere in the sky. Today, this is called
the Cosmic Microwave Background, also known as the Cosmic Background Radiation.
At first, Penzias and Wilson didn’t know what it was. They hypothesized that it was from
pigeons making nests in their antenna, and covering the inside with “a dielectric
substance.” They got rid of the dielectric substance, and the pigeons, but the microwave
background persisted.
They then went to a science talk by Robert Dicke. In this talk, Dicke predicted the
existence of the Cosmic Background Radiation. Penzias and Wilson realized what their
mysterious microwave signal was. They asked Dicke if he’d collaborate with them, and
published side-by-side papers. For reasons that are still obscure, they won the Nobel
Prize and Dicke didn’t.
As Dicke explained, the Cosmic Background Radiation is thermal radiation (see Chapter
11). The Cosmic Background Radiation is the light that was given off by the primordial
fireball, which was the hot, dense gas that filled the Universe at its origin. Since the
Universe has expanded so much since this early time, the Cosmic Background Radiation
is now a thin soup of microwaves that fills the Universe.
As the Universe expanded, it also cooled. The Cosmic Background Radiation now has a
temperature of 3 Kelvins, which is why it is sometimes called the 3 K background.
(Image courtesy of Professor John Ruhl/BOOMERANG collaboration)
________________
Figure 29-10: Recall look-back time. (See Chapter 5.) Because it takes time for light to
travel, when one looks deeply into the Universe, it’s like looking into a time machine.
What does one see, when one looks back all the way? One sees the origin of the
Universe. The Cosmic Background Radiation is the first light in the Universe. In other
words, it is the light given off when the cosmic fireball first cooled enough to become
transparent to light. In other words, the Cosmic Background Radiation is an actual
picture of the Big Bang!
(Image by the author, using NASA images)
________________
Figure 29-11: Top: Measuring the variation in the temperature of the Cosmic
Background Radiation can show the conditions of the gas in the cosmic fireball. This is
much like how geologists use sound waves to tell what’s inside Earth. (NASA)
Bottom: The Universe is observed to be within 0.4% of the density it needs to keep
expanding forever. (Image by the author) ________________
Figure 29-12: The discovery of the Cosmic Background Radiation supports the Big Bang
theory, which predicted it. It disproved the Steady State theory, which did not predict it.
The Oscillating Universe is the idea that the expansion of the Universe will eventually
stop, because of the gravity of all the mass in the Universe. The Universe will then
collapse billions of years in the future, in a “Big Crunch” (or a “Gnab Gib,” which is
“Big Bang” spelled backwards). The Oscillating Universe has also been discredited, since
precise observations of the Cosmic Background Radiation show it is within 0.4% of the
density the Universe needs to keep expanding forever. (Image by the author)
________________
Figure 29-13: The Olbers paradox is that if the Universe were infinitely large and
infinitely old, the sky at night shouldn’t be dark. It should be as bright as the Sun, since
no matter in which direction one looked, sooner or later there should be a star. One can’t
see the forest, because there are too many trees in the way, as in this Louisiana swamp.
The solution to the paradox is that the Universe isn’t infinitely old. The Big Bang theory
predicts the Universe to have a finite age. (Image by the author)
________________
Figure 29-14: What other evidence did the Big Bang leave? During the first three
minutes of the existence of the Universe, the whole Universe was as hot and dense as the
interior of a star. The same nuclear reactions that occur in stars were happening
everywhere. Today, we observe remnants of this: the abundances of helium and other
light elements.
Top: Big Bang nucleosynthesis predictions use well-known fusion reactions to predict the
abundances of elements and isotopes left over from the Big Bang. (Image courtesy of Dr.
Scott Burles, Dr. Kenneth Nollett, and Prof. Michael Turner and redrawn by the author)
Bottom: Observations of the abundances agree with theoretical predictions and with each
other. (NASA/WMAP Science Team)
(NOTE TO EDITOR: THIS FIGURE NEEDS TO BE IN COLOR.)
________________
Figure 29-15: These examples of galaxy evolution further discredit the Steady State
theory. This is because the Steady State theory predicts that there should be no overall
evolution of galaxies, if the Universe were in a steady state.
Top left: Because of look-back time, the little faint red galaxies shown here are in their
infancy, as they were during the first billion years of the history of the Universe.
(NASA/STScI)
Bottom left: Galaxies in the early Universe had more hot, short-lived blue stars (OB
types), after counting for redshift. (NASA/STScI)
Right: Galaxies now are more highly clustered than in the past, because of gravity.
(Image by the author) ________________
Figure 29-16: Top left: Edwin Hubble’s “tuning fork” classification of galaxies has been
explained by observations by Hubble Space Telescope. It turns out to be an evolutionary
sequence: the earliest galaxies are irregulars, which merge together into spirals like the
Milky Way, which merge into giant elliptical galaxies. (Image by the author)
Bottom left: Primordial irregular galaxies (NASA/STScI)
Top right: A spiral galaxy, similar to the Milky Way Galaxy (Image courtesy of Dr. Greg
Morgan)
Bottom right: A giant elliptical galaxy that is eating spiral galaxies (NASA/STScI) ________________
Figure 29-17: Spiral galaxies themselves show evolution, throughout the age of the
Universe. Early in the Universe, spiral galaxies were much more active, with bright
nuclei from stars falling into their centers. Now, they are observed to be much more
relaxed, with more stars having settled into stable orbits. (NASA/STScI/Frank Summers)
________________
Some questions about the cosmos
(Q) Where did the energy of the Big Bang come from? Doesn’t the Big Bang violate the
first law of thermodynamics, which says that energy can’t just appear from nowhere?
(A) Not necessarily: the energy of the observed expansion of the Universe just balances
the energy of the gravity holding the Universe together, within 1%. The net energy of the
Universe may therefore be zero. As an accountant might say, “You just broke even.”
(Q) Where is the center of the Universe?
(A) There is no center. The whole Universe is expanding. This means that from any
galaxy, it looks like you are in the center, because the other galaxies are moving away
from you. They’re all moving away from each other, too.
(Q) What came before the Big Bang?
(A) We don’t know: astronomers have no information from this early time.
(Q) What is outside the Observable Universe?
(A) We don’t know: again, astronomers have no information, since we can’t see “outside”
the Universe. Since the speed of light is finite, it limits how far away we can see.
Remember look-back time: the speed of light limits how far back in time we can see.
(Q) Can the Universe be infinite in size?
(A) Maybe: the mass of the Universe is spread very evenly, on its largest scale.
(Q) If the Universe is expanding, what is it expanding into?
(A) The short answer is that we don’t know.
A longer answer may be that, in a way similar to how a two-dimensional surface of a
balloon can expand into a three-dimensional room, our three-dimensional Universe (plus
time) may be expanding into a higher-dimensional space. Higher dimensions do get a lot
of attention from theorists these days. They may or may not exist: scientists currently
have no observations or experimental evidence that they do.
(Q) Why don’t scientists know everything about the Universe?
(A) Scientists can’t currently answer everything. This does not mean that we don’t know
anything. Astronomers see that the Universe is expanding. Astronomers also observe the
Cosmic Background Radiation, which is the light left over from the hot, dense origin of
the Universe. Astronomers can also see other traces of its origin, and can see the galaxies
evolving (so the Universe can’t be in a steady state since it’s changing).
Why are we here?… from: Carroll, B.W., and Ostlie, D.A., 1996, An Introduction to Modern Astrophysics,
Addison-Wesley Pub. Comp. Inc (A.K.A "BOB": the Big Orange Book)
In our astrophysics class, a student once asked, “Why are we here?”
The answer was as amazing to us as it was to the class:
We are here because, more than ten billion years ago, the Universe borrowed energy from
the vacuum to create vast amounts of matter and antimatter in nearly equal numbers.
Most of it annihilated and filled the Universe with photons. Less than one part per billion
survived to form protons and neutrons, and then the hydrogen and helium that makes up
most everything there is. Some of this hydrogen and helium collapsed to make the first
generation of massive stars, which produced the first batch of heavy elements in their
central nuclear fires. These stars exploded and enriched the interstellar clouds that would
form the next generation of stars. Finally, about five billion years ago, one particular
cloud in one particular galaxy collapsed to form our Sun and its planetary system. Life
arose on the third planet, based on the hydrogen, carbon, nitrogen, oxygen, and other
elements found in the protostellar cloud. The development of life transformed Earth's
atmosphere and allowed small furry mammals to take center stage. Humans evolved and
moved out of Africa to conquer the world with their new knowledge of tools, language,
and agriculture. After raising food on the land, your ancestors, your parents, and then
you consumed this food and breathed the air. Your own body is a collection of the atoms
that were created billions of years earlier in the interior of stars, the fraction of a fraction
of a percent of normal matter that escaped annihilation in the first microsecond of the
Universe. Your life and everything in the world around you is intimately tied to countless
aspects of modern astrophysics.
(If you think this should be “How did we come to be here,” rather than “Why are we
here,” consider that at least we got this answer by observing the Universe around us, and
by thinking about the observations rationally. It wasn’t done entirely by making up
stories.)