ASTR 121 (O'Connell) Optional Reading
COSMIC HISTORY: A BRIEF NARRATIVE
A frame from a supercomputer simulation
of a forming cluster of galaxies (B. Moore)
We have been discussing the impact astronomy has had on human society
amd civilization. One of its fundamental contributions has been to
provide our basic perspective of space and time. It is the only
science that attempts to understand the ultimate origin of the
universe around us (in empirical, not religious or mythological,
terms). The study of the origin and evolution of the universe
is called cosmology.
Even though cosmology is not the subject of this course (it is covered
more thoroughly in ASTR 124 and 348), it helps to set the stage to
briefly describe what we have learned so far about the universe and
how its contents have changed through time.
You may want to refer to Supplements 2 and 3
for background on some of the technical topics included in the following
narrative.
A. THE BIG BANG
We think the Universe began in a Big Bang---that is, a
superdense and superhot state from which it has been expanding
ever since. The best evidence for the existence of this hot state is
the very faint radiation called the cosmic microwave
background. This was discovered in 1964 by scientists at Bell
Labs, although it had actually been predicted (on the basis of the Big
Bang concept) 16 years earlier. It was recently studied in great
detail by the NASA "COBE" and "WMAP" satellites, and its properties
are in complete agreement with the Big Bang picture.
WHEN the Big Bang?
A very long time ago. As best we can determine, the Universe is
almost 14 billion years old. This estimate is based on
observations by WMAP coupled with measures of the expansion rate of
the universe by the Hubble Space Telescope and other instruments.
(The expansion rate of the universe implies an age, since by comparing
the current velocity of expansion to the separation between galaxies,
you can determine the time in the past when they were all on top of
one another.)
One of the firm predictions of the Big Bang picture is that there can
be nothing in the universe older than the time of the Bang.
Astronomers can test that by determining the ages of stars, star
clusters, and galaxies. So far, the oldest of these are about 12
billion years old---consistent with the predictions.
Recall the time-scale analogy we discussed in the first lecture.
We compared the number of letters in the textbook to
the age of the Sun and decided that if the whole text represented
the age of the Sun (5 billion years), then a single letter
represented 2500 years.
So: in the textbook analogy, the Universe would be almost 3 textbooks long.
WHERE the Big Bang?
When you think of an ordinary explosion, say of a firecracker, it is
always localized in a small volume, and the effects expand outward.
The Big Bang was not like this: in the case of the BB, the explosion happened
everywhere simultaneously. It filled the whole spatial
volume of the universe. The unimaginable pressure of this event
caused not only the matter of the universe to expand with great
violence, but also caused space to expand as well.
This is because, according to the theory of General Relativity as
formulated by Einstein (1916), space and time are not independent of
mass and energy----space and time are dragged with the expanding
matter. So, if you could run the "movie" of the Big Bang
backwards, you would see material around you getting ever more
compressed and ever hotter, no matter where you are in the universe.
As for the volume of space in the universe at the time of the Bang,
the best evidence is that it was infinite then and has remained
so ever since (after all, space is expanding). The universe has no
center, and we are certainly not at a center. [Yes, this is a difficult
concept---so don't worry if you can't visualize it.]
WHY the Big Bang?
Now you've got us. In order to answer that question, we would have to
be able to assess the state of things before the Big Bang.
But, as just described, space and time are tied to matter and energy,
and, according to our best understanding, space and time did not
exist before the Bang itself. There is no way to probe what
happened earlier because basic conceptual equipment
becomes meaningless at the instant of the Bang.
B. EARLY EVOLUTION OF MATTER
Physical conditions near the Bang were utterly alien to everyday
intuition. Just after the Bang, the universe was filled with an
unimaginably hot, disorganized mixture of subatomic particles.
The temperature was too high for even ordinary protons and neutrons to
exist, and certainly too high for atoms and molecules. But the
expansion caused quick cooling, and more familiar kinds of matter
rapidly began to "freeze out."
@ 3 Minutes After the Big Bang: Atomic Nuclei Form
At this point, the universe is "cool" enough [about 10-20 million
degrees] for ordinary protons and neutrons to form---the
constituents of atomic nuclei. Conditions are similar to those in the
cores of stars like the Sun, and the nuclei interact the same way they
do inside of stars (or a hydrogen bomb)---in nuclear fusion reactions.
These reactions turn hydrogen nuclei (one proton) into heavier nuclei,
like helium (2 protons, 2 neutrons). If the universe lingered at this
density and temperature, all of the heavier elements of the
Periodic Table
could
have been made. But it was still expanding and cooling quickly, so
only helium and few traces of other light elements like lithium were
made this way.
@ About 1 Million Years After the Big Bang: Atoms Form
By this time, the universe has cooled to about 3000 degrees, at
which temperature atoms can form. The nuclei acquire electrons by
electromagnetic attraction from the hot surrounding gas, forming
stable atoms. But still, because only a few types of nuclei exist,
the gas is mainly only H and He. This stage is called "recombination"
(electrons "combine" with nuclei)---something of a misnomer since they
had never previously been combined.
It is at this point that the cosmic microwave background
radiation is released. Until this time, electromagnetic
radiation had been strongly trapped by interactions with matter (as
daylight is strongly scattered in fog, for instance). With the
formation of ordinary atoms, most of this radiation is now free to
stream away, even to cross the universe. That is, in fact, how we we
are able to measure it directly today. We see that radiation coming
to us from enormous distances (corresponding to a distance of almost
14 billion light-years). Because it has continued to "cool down" as
the universe expands with time, it is detected today as being at an
equivalent temperature of only about 3 degrees above absolute zero.
We have now detected the CMB coming to us from all directions in the
universe.
C. FORMATION OF GALAXIES, STARS, AND THE CHEMICAL ELEMENTS
Gravity Takes Over
What we've described so far is based on the fundamental physics of
elementary particles, which can mostly be very well tested in the
laboratory. Conditions like those in the Big Bang are simulated in
the microscopic fireballs produced when protons or electrons collide
with one another at near the speed of light in particle accelerators.
New experiments like the Large Hadron Collider (in Europe) will
further test the kinds of physics prevailing only microseconds after
the Big Bang. The interactions we've described are based on the
so-called nuclear short-range forces and the longer-range
electromagnetic forces.
What happens over the subsequent 12 billion years depends on how the
basic constituents of the Universe interact with each other
gravitationally. Even though gravity is the weakest of the
basic forces, it dominates the growth of
organization in the universe from now on.
Physical structure in the present-day universe originated in tiny
irregularities in the distribution of matter during the Big Bang which
have been "amplified" over the intervening time by the weak,
but inexorable, force of
gravity and blown up to cosmic size by the expansion.
The next phase of evolution depends very strongly on how clumpy the
matter in the universe is, how fast the clumps move, and so forth. We
are just now starting to understand this, and the details are not well
in hand. We do know that following recombination the universe will be
"dark" for a period of some hundreds of millions of years, in that the
blaze of the Bang will have faded but stars will not yet have
formed.
Star Formation and Nucleosynthesis
Gas begins to concentrate under gravity slowly at first but then
faster. Eventually, the density of the largest clumps become high
enough for gas clouds within them to cool, compress, and begin to
collapse under their own self-gravity and hence to form the first
stars. This could have happened anywhere from 50 to 500 million
years after the Bang.
The earliest generations of stars will consist only of H and He. But
because they burn these nuclei in their centers through nuclear
fusion reactions, they create heavier elements, like C, N, O.
These in turn are the basis of the organic molecules we find in our
bodies and other life on Earth.
But the elements wouldn't do us much good if they stayed trapped in
stars. Fortunately, they don't. Stars like the Sun, in their old
age, tend to shed their outer envelopes, and this carries some of the
heavier elements back into space (into the so-called "interstellar
gas"). Even more importantly, some kinds of stars can explode
violently in their old age. These supernovae not only spew
most of the star's processed material back into space, but the
explosions themselves generate the heavier elements like iron through
uranium.
So the stars create the chemical elements---a process called
nucleosynthesis. These get incorporated into later
generations of stars, which can enrich the mixture further.
Galaxy Formation
The earliest stars were probably unusually massive, but as they seed
their surroundings with heavy elements, ordinary star formation
proliferates, and billions of stars form rapidly in starbursts
within the still-collapsing big protogalaxies.
Galaxy build-up continues for the next 5 billion years or so, often
involving turbulent and violet gravitational interactions between
galaxies that result in mergers between them. Star formation and
galaxy build-up is still continuing today, though at a much reduced
pace.
Supercomputers allow astronomers to make fairly realistic numerical
simulations of this phase of evolution. Click on the links
below for some examples:
D. FORMATION OF PLANETS AND LIFE
The Sun formed in the outer parts of an ordinary galaxy about 7
billion years after the oldest stars in the galaxy. Several
generations of chemical enrichment had occurred, so about 2%
of its mass is in the form of heavy elements.
The planets formed from a debris layer which accompanied the
formation of the Sun. The terrestrial planets like the Earth
accumulated from solid grains and chunks of materials in
the inner part of the debris disk. These were predominantly rocky
because the temperature of the inner disk was high. The outer
planets accumulated from more icy materials in the cooler part
of the disk.
By several 100 million years after the Sun settled down to burning
hydrogen steadily in its center, the planets had assumed their present
structures. The Earth's atmosphere and surface were very different
from now, and both were subject to violent change by occasional
collisions with asteroids and comets.
Within another billion years or so, there was sufficient stability and
favorable conditions (water content, temperature) that primitive
living organisms had begun to thrive. Whether these formed on
Earth or arrived from elsewhere we do not yet know. But the process
of biological evolution through natural selection, which has continued
to the point of producing us, had begun. Lifeforms on the planet
underwent a series of dramatic transformations until, about 50 million
years ago, mammals and flowering plants began to spread rapidly across
Earth's surface. And that brings us (on a cosmic scale) to the present
time.
One of the most important lessons of astronomy to date is that there
is nothing special about the Sun or the solar system. In fact, we
already know of over 200 planets in orbit around other stars.
Planet-based lifeforms are therefore probably widespread in our galaxy
and the universe, and life could even exist in interstellar
environments. The universe probably teems with life.
Web Links:
Supplements 2 & 3 (Study Guide 10)
provide background information on elementary particles, interparticle
forces, and atomic structure. [This won't be formally assigned
reading in the course until after the first midterm exam.]
To keep this narrative focussed on things we understand fairly well,
I've deliberately omitted discussion of topics like cosmic
inflation, dark matter, and dark energy. These
ideas have helped explain some mysterious aspects of the universe but
have added new puzzles of their own. For more information, look them
up in Wikipedia
and follow the links there. If you're very curious about such topics,
you might also consider taking ASTR 348,
which is taught every spring semester.
Last modified
November 2007 by rwo
Text copyright © 1998-2007 Robert W. O'Connell. All
rights reserved. These notes are intended for the private,
noncommercial use of students enrolled in Astronomy 121 at the
University of Virginia.
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