The universe is a big place, perhaps the biggest.
Philip Jose Farmer (writing as Kilgore Trout)
Hubble Telescope Deep Field View of Galaxies
Cosmology is the study of the orgins, history, and evolution of the
universe. Philosophical speculation on such questions has
been active since long before recorded history. But it is only since the early
1920s that we have even had observational evidence that gave a somewhat
accurate picture of the actual shape and dynamics of the universe as
we now understand it. It was at that time that Edwin Hubble
found evidence that there are astronomical objects
outside our own galaxy -- namely other similar galaxies -- and that
all but the closest of them appear to be receding from us at a velocity
proportional to their distance.
Both observational and theoretical developments in cosmology proceded
during the 20th century at an accelerating pace. Some of the milestones
- Einstein's general theory of relativity, which provides the
mathematical basis for describing the universe.
- Hubble's discovery of the distance to galaxies outside our own and
the fact of the expansion of the universe.
- Formulation of the Big Bang theory of the early history of the
- Discovery of the Cosmic Microwave Background radiation (CMB), which
provided a major piece of evidence for the Big Bang theory.
- Recognition that a substantial amount of "dark matter" (matter not
visible by its own or reflected radiation) must exist in order to
account for the observed dynamics of galaxies and clusters of galaxies.
- Theoretical calculations which demonstrated that the relative abundance of
certain light elementts (deuterium, helium, lithium) is exactly what
the Big Bang threory predicts.
- Addition of the idea of "Inflation" to the Big Bang theory in order
to solve several important problems in the theory.
- Many developments in the theory of high-energy particle physics
which inform and support our cosmological theories of the earliest
events in the origins of the universe.
- Discoveries made by the Cosmic Background Explorer satellite which
confirmed the existence of expected temperature variations in the CMB.
- Measurements, by means of observation of supernovae,
of accurate distances to high-redshift galaxies that
indicate that the rate of expansion of the universe is actually
- More accurate measurements of temperature fluctuations in the CMB
that indicate the large-scale geometry of the universe is flat, as
predicted by inflationary theory.
The last two of these milestones are especially recent, having been
completed only since 1998. Yet their importance can hardly be
exaggerated. We now, finally, have good evidence that answers the
old question about the geometry of the universe -- that is is flat.
And we have the very surprising conclusion that the rate of the
expansion of the universe is increasing, rather than decreasing as
Thus we have two major open questions of cosmology that appear to
have received answers from observation. And yet, as is almost always
the case, each new answer makes it possible to ask entirely new
questions about phenomena which before had not even been suspected.
Just as occurred with the original discovery that the universe was
In particular, two of the most pressing questions that now need answers
- Exactly what is it that is responsible for the observed
acceleration of expansion?
- Will even more detailed measurements of
fluctuations in the temperature of the CMB provide positive confirmation
of the predictions of the inflationary hypothesis?
Cosmology today has a "standard model" just as elementary particle
physics does, and it is called the Big Bang. Ever since the discovery
of cosmic microwave background radiation (CMB) in 1965, the evidence has been
strongly in favor of the Big Bang scenario. As a result of much
more detailed information about the CMB obtained by a number of
experiments beginning with the Cosmic Microwave Background Explorer (COBE)
satellite mission in the early 90s, the evidence in favor of the Big
Bang model has become overwhelming.
We can list the main elements of this standard model as follows:
- Expansion of the universe, starting about 13.7 billion years ago
- Nucleosynthesis: the formation of hydrogen, helium, and other
low-mass chemical elements
- Cosmic microwave background radiation
- Dark matter
- Dark energy
Expansion of the universe
Around 1920 it was still an open question as to whether the whole universe
consisted of the stars which were easily identifiable by telescopes of
the time and other objects at roughly the same distance.
Most stars seemed to be grouped into the band
known as the Milky Way, which stretches across the whole sky. Astronomers
had also observed objects which did not seem to be stars, because they
appeared fuzzy in telescopes, rather than pointlike as stars did. These
were called nebulae (singular: nebula, from the Latin for "cloud".)
There was an active debate as to whether these objects were relatively
nearby, like observable stars, or whether they were much more distant
and separate collections of stars like our own galaxy. ("Galaxy" is
from the Greek word for milk, and related to the Latin lac.)
The basic problem was that it was very difficult to estimate
distances to astronomical objects. The first tool that astronomers
had for doing this was the method of parallax, which depends on
slight differences in relative position of a distant object
when the object is viewed from two different locations. If a difference
of position can be observed, then it is simply a matter of trigonometry
to figure out the distance. In this case, since the Earth is about 93
million miles from the Sun, the difference in Earth's location relative
to the Sun is about 186 million miles at two times that are six months
apart. Knowing this, it was possible to calculate the distance to many
stars in the Milky Way. But since no difference of relative position
could be detected in many nebulae, it was impossible to calculate
their distance -- though they must have been farther than stars whose
distance could be calculated.
Fortunately, among the stars whose distance could be calculated, there
is a special type known as Cepheid variables. The brightness of
such stars varies in a predictable way, going from a minimum to a
maximum and back again to a minimum in a length of time that does not
change, the period of the variation. Furthermore, the length
of this period was found to be proportional to the absolute brightness
of the star at its brightest. Of course, the absolute brightness could
be known only if the distance to the star was known, which limits
when this technique can be used. When the distance was known,
the absolute brightness could be inferred from the observed brightness,
and the uniformity of the period-luminosity relationship for
Cepheid variables could be proven. But once this relationship was
established, astronomers could then infer the absolute brightness of a star
by measuring its period, and then its distance
could be inferred from the known observed brightness, since brightness
of any object is inversely proportional to the square of its distance.
A breakthrough occurred in 1923 when Edwin Hubble (for whom the Hubble
Space Telescope was named) was able to observe a Cepheid variable in the
Andromeda galaxy (then usually known as the Andromeda nebula).
Assuming his observation wasn't erroneous, the Andromeda galaxy had to
be much farther away than any known star in the Milky Way, so almost
certainly the Andromeda galaxy was not part of the Milky Way galaxy, and
the same was probably true of many other similar objects which were
known as "spiral nebulae" at the time. Hubble's initial distance
estimate was not quite correct, and it's now known that the Andromeda
galaxy is about 2 million light-years away, whereas the total diameter
of the Milky Way is about 100,000 light-years. Nevertheless, it suddenly
appeared that the Milky Way was merely a small part of a much larger
Within just a few more years Hubble made an even more astonishing
discovery, which not only solidified his estimate of the distance to
the Andromeda galaxy, but also revealed a property of the universe
at least as surprising as its size. Namely, the universe apparently
was not static in size, but actually expanding.
This conclusion was
based on the observed red shift of distant galaxies. Another
astronomer, Vesto Slipher, had been analyzing the spectra of stars
and nebulae since about 1912. Stellar spectra have a distinct structure
which includes bright bands at specific wavelengths of light. These
bands are generated by individual chemical elements such as sodium in
stellar atmospheres. All spectra are more or less the same in terms
of the relative position of these bands, but the spectrum as a whole
can be shifted to either higher (red) or lower (blue) wavelengths.
Although other explanations for such a shift are conceivable, the most
natural explanation is as a Doppler shift that occurs in the
light emitted by an object that is moving towards or away from the Earth.
If a light-emitting object is moving away from the Earth, the light's
spectrum will be shifted towards red, and if in Earth's direction it
will be shifted towards blue.
By 1925 Slipher had measured the spectral shifts of about 40 galaxies.
In parallel, Hubble was locating Cepheid variables in other galaxies and
thereby inferring their distance. In combining these two sorts of
observation, a curious fact emerged: the spectra of all galaxies except
for the nearest ones (not farther, roughly, than Andromeda) were shifted
to the red. Surely that was no coincidence. By 1929, Hubble had discovered
the astonishing fact that the amount of spectral (red) shift was
proportional to the distance of the galaxy.
Hence, on the assumption that spectral shift is caused by relative
motion, it seemed that the speed of recession of all but the closest
galaxies was proportional to their distance from Earth and from each
other. If this interpretation is correct, then apparently the whole
universe was expanding. Galaxies were spreading apart from
each other like dots on the surface of an inflating balloon or raisins
in a swelling loaf of raisin bread.
Since this relationship appeared to be a linear one, there is a
constant of proportionality between the amount of red shift and the
distance. If the shift is due to relative motion, velocity
is proportional to the amount of shift, and so there must
be a constant of proportionality between velocity and distance.
This constant is now known as the Hubble constant, denoted
by H. The best current estimate of H is about 70 kilometers per second
per megaparsec. (A megaparsec is a million parsecs.
A parsec is a distance unit of about 3.26 light-years used by
astronomers since it's convenient for measurement of distances by the
parallax method. So a megaparsec is about 3.26 million light-years,
or 3.0857 × 1019 kilometers.)
Suppose we now make one more assumption, that the universe has been
expanding at roughly the same rate indefinitely. Then two galaxies
which are now far apart must have been much closer in the distant
past. And at some point in the past, they must have occupied the
same location. How long ago would that be? Note that the reciprocal
of H has units of time. 1/H is in fact the length of time since
everything would have been at the "same" place. The value is about
14 billion years (4.4 × 1017 seconds).
Thus if all the assumptions are approximately correct, about 14 billion
years ago the universe began to expand from a singular state. This
initial state or singularity is what is called the Big Bang.
Of course, we've made a number of assumptions along the way. If any
of them are wrong, this conclusion would not be valid. Indeed, many
people over the years have challenged various of the assumptions.
However, for the sake of argument, say we accept the assumptions and
the conclusion that the universe started to expand from a singularity about
14 billion years ago. Then a number of other conclusions would follow.
These are predictions that this Big Bang model makes. Two of
the main predictions are with regard to nucleosynthesis and
cosmic microwave background radiation. We'll explain those
concepts soon. But the key point now is that these predictions have
been verified by observation. It seems very hard to account
for these observations if something much like the Big Bang did not
occur. And this is the fundamental reason that the Big Bang model is
so widely accepted today by cosmologists.
Let's deal with one assumption here. There isn't any good reason to
suppose that the Hubble constant actually is constant. It may have
been different in the past. Indeed, if the Big Bang were really
like an explosion, with all matter in the universe flung out from
where it began long ago, we would expect the expansion to slow down
over time, due to the gravitational attraction of all matter in the
universe. This would be just like the way that an object thrown upward
from the Earth's surface slows down (and eventually reverses direction).
This means that H may have been greater in the distant past from what
we observe now when measuring galaxies that are relatively nearby. If
this is the case, then it would have reached its present size more
quickly than if H were what we see now, and so the age of the universe
would be less than 14 billion years. In other words, if the average
value of H were more, the age of the universe, which is (roughly)
1/H, would be smaller.
That could be a problem, a big problem -- because it is possible
to estimate the age of certain stars (in globlular clusters of
our galaxy, for example), and there seem to be stars that are 11 or 12
billion years old. Any estimate for the age of the universe that was
less than that would obviously be wrong. In fact, this was a big
worry in the early 1990s.
Fortunately, observations made in 1998 have shown that the rate of
expansion of the universe now is actually more than it was long
ago. So the rate is increasing and thus the expansion is accelerating.
That's very good for yielding an age of the universe around 14 billion
years. But it raises a different problem: there must be some
force other than simple gravitation as it was long understood, a
repulsive rather than an attractive force, to account for this
acceleration. Cosmologists believe there is a way to model such a force,
by adding a term called the cosmological constant to Einstein's
fundamental equation of general relativity. It turns out that this
equation in its simplest form predicts that the universe cannot be
static, and that it must expand or contract. Since, before 1920, it was
believed that the universe is static, Einstein himself proposed
adding the cosmological constant term to the equation to eliminate this
"instability". It would be a sort of anti-gravity that
opposes gravity's effect of diminishing the rate of expansion.
The observational evidence is now very good that the expansion of the
universe is accelerating. A cosmological constant that is nonzero would
explain that. It can be thought of as a sort of dark energy which
is causing this acceleration. Of course how to understand where this dark
energy comes from is a major open question, and we'll go into much more
detail on it.
Although we'll discuss the Big Bang more
elsewhere, there are a few final remarks we can
make here. First, it should not be thought that the Big Bang occurred
at a particular point location in space. Instead, the idea is that
there was no space before the Big Bang, nothing inside which
there was a huge explosion at one particular point. Rather, it is just
something which was "everywhere" at that initial point in time.
Further, the initial singularity need not have been a mathematical
point at all. It could have been a 3-dimensional (or some other
dimensional) volume which, at the time of the singularity, began to
expand. No one really has a good idea of conditions at the singularity.
But there are various theoretical ideas about the singularity, such
as Hawking's "no boundary" proposal. Quantum mechanics, in some form,
must play a part in describing the singularity. But such issues are
very speculative, and not currently part of cosmology's "standard
model". As to conditions immediately after the Big Bang, that is
addressed in the theory of "cosmic inflation", about which we'll
have a lot more to say later.
Cosmic microwave background radiation
"Matter", in the sense used here, is everything in the universe that has
mass. Some of that matter is stuff that glows -- basically, stars and
the galaxies they comprise. Roughly, then, "dark matter" is all the
remaining matter. And, the evidence shows, that is by far the larger
The dark matter can be subdivided into two types: baryonic and non-baryonic.
In the standard model of particle physics, any particle composed of quarks
is called a "baryon", and so any matter composed of particles
containing quarks is called baryonic. (The standard model also contains
particles called "leptons", mainly electrons, and they are counted as
"baryonic" as well even though they are not composed of quarks,
but their contribution to the total is very small.)
Baryonic dark matter could be in the form of large clumps of ordinary
matter which are too small to produce light by the process of nuclear
fusion, so they do not glow. Objects of this sort are called "massive
compact halo objects", or "MACHOs". The baryonic dark matter could
also be in the form of diffuse hydrogen and helium gas. But in any
case, it is known that the total of all baryonic matter (dark and otherwise)
is still just a relatively small percentage of all matter.
The remaining, large majority portion of matter must be in some
exotic form that is dark and not accounted for by the standard model of
particle physics. There are many possibilities for what this could be,
but very little evidence that even gives us a good clue about what
it actually is. It may well be a mixture of some of the following:
- Massive neutrinos -- neutrinos are now known to have very small
mass, though the standard model of particle physics doesn't account for
it. Though the mass of individual neutrinos is quite small, they
could exist in sufficient abundance to make up some, but not all,
of the dark matter.
- Hypothetical particles called "axions", which have been conjectured
as an explanation for the slight breaking of "CP symmetry" in particle
- Particles predicted to exist on the basis of "supersymmetry",
as supersymmetric counterparts to standard model particles.
- Any number of other exotic sorts of particles which have been
conjectured to exist from other considerations outside the standard
model of particle physics.
- Small or microscopic black holes.
- Primordial magnetic monopoles.
Although we now have farily good answers to the "what", "when", and "where"
questions of cosmology, we are still a long way from answering many of
the "how" and "why" questions. Interestingly enough, the answers are
most likely to be found from an entirely different field: very high
energy particle physics. The reason for this is simple enough: given
the Big Bang model, we know that at a very early stage in the development
of the universe, there was an enormous amount of energy confined to a
very small space. This is precisely the domain that particle physics
is concerned with.
Even so, the number of ways in which the studies of cosmology and particle
physics interact is surprising:
- The "inflation" hypothesis is not merely a single theory, but actually
a whole class of theories that may be able to explain what sort of force
could drive the whole universe to double in size every
seconds. The answer is likely to be found when we understand theoretically
how to unify the electromagnetic, weak, and strong forces.
- At some even earlier time (around the "Planck time" of
seconds) in the history of the universe, the force of gravity must have
been unified with the other three forces. In this epoch, spacetime itself
would have been quantized in some sort of "quantum foam", but
we lack a theory of quantum gravity to address this issue.
- The most likely candidates for (non-baryonic) dark matter include
neutrinos with mass and as yet undetected particles such as axions, and
supersymmetric partners of the known elementary particles.
- The asymmetry between matter and anti-matter (the fact that almost
all of the universe appears to consist of the former rather than the latter)
seems to be a consequence of symmetry breaking in certain particle
- Measurements of the relative abundances of light elements (deuterium,
helium, lithium) have not only been used to confirm details of the Big
Bang model, but the calculations can also be used to show that there
are no more than three families of leptons and quarks, in accord with
the standard model of particle physics.
The fact that cosmology and particle physics are so thoroughly entangled
has profound implications for both fields. The consequence for cosmology
is that we may expect to develop theories which will genuinely answer
"how" and "why" questions. The consequence for particle physics is that
its theories can be tested by deriving predictions about events in the
very early, very high energy epoch in the history of the universe --
predictions which we have no way to test using the very low-energy
devices our technology limits us to construct for the foreseeable
Many of the still open questions about cosmology will be answered
by advances in high-energy physics. But not all. In fact, we are reaching
the practical technological limits of studying particle physics by means
of massive accelerators. In the future, our best sources of observational
information for both high-energy physics and cosmology will be telescopes
and other sorts of energy detectors that are tuned to observe processes
in the universe which happened -- literally -- very far away and very
The topics covered by other pages under this one (listed and described
in the box above) indicate most of the major open questions. But we can
make a brief summary.
Here are the big questions which we can realistically hope to address
specifically in the area of cosmology:
- Is the inflationary hypothesis -- which determines the "initial conditions"
that control practically everything we can now observe in the universe --
- If the inflationary hypothesis is generally correct, did inflation occur in
such a way that the universe we now observe is only one of countless
"bubble universes" that could have arisen out of the same process?
- How many forms of dark matter are really present in the universe, what
is the relative percentage of each, and how has the dark matter affected
the observable structure of the universe?
- What is the true cause of the accelerating expansion we seem to observe
now, and is it likely to continue indefinitely into the future?
- What is the true dimensionality of spacetime, and how did the apparent
three dimensions of space and one of time (along with any other "hidden"
dimensions) come to be as we see them?
- Are the laws of physics, in particular the "fundamental constants",
truly the same everywhere and at every time in the observable universe,
or do they vary in some slight but predictable way?
- Is the topology of the observable universe truly infinite, or is it
finite in such a way that we could in principle observe our own section
of the universe if we could only see "far" enough?
Open Directory Project: Astronomy: Cosmology
- Categorized and annotated links. A version of this
list is at
Google, with entries sorted in "page rank" order. May also be
Open Directory Project: Physics: Cosmology
- Categorized and annotated links. A version of this
list is at
Google, with entries sorted in "page rank" order. May also be
Further Reading [Cosmology]
- Excellent categorized list by
Cosmology Books and Links
- Bibliography of books and Web sites, part of a
cosmology course, by
Good Cosmology Sites
- A short list by
Ned Wright, part of his
- Links on general cosmology, dark matter, and large-scale
The Net Advance of Physics: Cosmology
- An index of tutorial and research articles
located at the
physics preprint archive. Topics include the big bang, cosmic strings,
inflation, large-scale structure.
A different form of this list, including other Web sources, is
Yahoo Cosmology Links
- Annotated list of links.
Yahoo Astrophysics Links
- Annotated list of links.
- Categorized site directory. Entries usually include
Has a subcategory for
universal origins. (More
Sites with general resources
The New Cosmology: From Quantum Fuzz to the Accelerating
- A "Chautauqua Short Course for College Teachers" held in
October 2001. The site contains
online versions of the talks,
suggested readings, and
external links. A similar conference called
Cosmology at the Millennium was held in June 1999.
Happy Birthday Stephen Hawking!
- Special issue of Plus
magazine honoring Stephen Hawking's 60th birthday. Contents include
interviews with Martin Rees, Gerardus 't Hooft, Roger Penrose,
and Kip Thorne, reviews of related books,
as well as the text of Hawking's own address:
Stephen Hawking's 60 years in a nutshell.
Tales of Singularities
- February 22, 2002 article from Science magazine summarizing
presentations at the workshop and symposium in honor of Stephen
Hawking's 60th birthday. Provides a brief glimpse at many of the
cutting edge ideas in theoretical physics and cosmology.
Stephen Hawking: The Birthday Lecture
- BBC-produced multimedia (sound and video) presentation of
Hawking's "60 years in a nutshell" lecture. Includes a text
transcript of the lecture.
Stephen Hawking 60th Birthday Symposium
- General Web site for the public symposium held in honor of
Stephen Hawking's 60th birthday, January 11, 2002. The theme of
the symposium was "the future of theoretical physics and cosmology".
A summary of public reports on the meeting is
Stephen Hawking 60th Birthday Celebration Workshop and
- General Web site for the workshop and conference held in
honor of Stephen Hawking's 60th birthday, January 7-11, 2002.
The theme of the meetings was "the future of theoretical physics
Best of Physics Web: Astronomy and Astrophysics
- Directory of best feature articles, news stories, and
external links on astronomy, astrophysics, and cosmology at the
Physics Web site.
Microwave Anisotropy Probe
- NASA project launched in June 2001
to probe conditions in the early universe by measuring the
properties of the cosmic microwave background radiation over
the full sky. Site contains
mission information, an
image gallery, and an extensive
Beyond Einstein: Structure and Evolution of the Universe
- NASA site that offers information and images related to the
agency's programs in this area.
Holzapfel Laboratory for Experimental Cosmology
- Provides desriptions of research projects managed by this
laboratory of the University of California at Berkeley. Projects
OVRO/BIMA SZ Imaging,
BIMA Fine Anisotropy Survey,
South Pole Telescope,
- Useful bibliography of cosmology books for a general audience,
and a few college/university textbooks on astronomy, astrophysics,
and cosmology, by
First Principles of Cosmology
- Online supplementary material to the book by
Eric V. Linder. Includes
astrophysics links and
Paul J. Steinhardt
- Steinhardt is a leading cosmologist who recently announced a
new cosmological theory known as the "cyclical model". His home
page contains various popular and technical papers on the subject,
A Brief Introduction to the Cyclic Model and
A Brief Introduction to the Ekpyrotic Universe.
Other topics covered at the site include dark matter and dark
- Linde is one of the authors of the inflationary cosmology and of
the theory of the cosmological phase transitions. Other research
interests include investigation of the global structure of the
universe, cosmological constraints on the properties of
elementary particles, and quantum cosmology.
- Tegmark is an expert in several important areas of cosmology,
such as the cosmic microwave background and large scale structure.
He describes his work as "precision cosmology". One of his recent
describes an apparent anisotropy in the CMB.
There's quite a bit of good information on cosmology in these
Sean M. Carroll
- Sean M. Carroll (not to be confused with the biologist
Sean B. Carroll) is a physicist who has written many articles
and a couple of textbooks on topics in cosmology and relativity.
A long list of reviews and talks is
He also writes about current topics in the blog
- Kolb specializes in, and has made significant contributions to,
particle astrophysics. His home page links to a number of
informational talks on cosmology.
- Cornish's "research focuses on the inteface between general
relativity, astrophysics and early universe cosmology." Specific
topics include using the
cosmic microwave background to
study the size and shape of the universe, black holes, quantum
gravity, and gravitational wave astronomy.
- A small collection of background material and introductions to
some research topics in cosmology, provided by
Sloan Digital Sky Survey
- "The Sloan Digital Sky Survey is the most ambitious astronomical
survey project ever undertaken. The survey will map in detail
one-quarter of the entire sky, determining the positions and
absolute brightnesses of more than 100 million celestial objects."
The Web site includes information about the project, a
Q & A page, an
Sloan Digital Sky Survey SkyServer
- The Sloan Digital Sky Survey is an ambitious project to map
the "entire" universe. The
Web site presents data from the SDSS. It
contains information about the SDSS project, many astronomical
images, and useful overviews of
galaxies and quasars,
cosmic structures, the
expanding universe, and
modern cosmology in general.
James Webb Space Telescope
- "JWST is a large, infrared-optimized space telescope.
It will have an 18-segment, 6.5-meter primary mirror and will
reside in an L2 Lissajous orbit. JWST is scheduled for launch in 2011."
The mission was formerly known as SIRTF, the Space InfraRed Telescope
Facility. It is designed for research into the early history of
the universe and such questions as the origin of
large scale strucure of the universe, and the
the nature of dark matter.
The site contains a lot of information on the
Kavli Institute for Cosmological Physics
- A major center for research in cosmology, at the University
of Chicago. Formerly known as the
Center for Cosmological Physics.
Cosmic Background Explorer Home Page
- Includes a good page of
Berkeley Cosmology Group
- Consists of various research groups that do research on
dark matter, the cosmic microwave background, supernova
cosmology, and neutrino physics. Site has
external links to some good FAQs and tutorials.
Hubblesite News & Views: Cosmology
- Press releases concerning Hubble Space Telescope findings.
Institute for Computational Cosmology
- Home of the "Cosmology Machine" (a supercomputer for
Durham Extragalactic & Cosmology Research Group
- University research group. Site includes sections on
Theoretical Extragalactic Astronomy & Cosmology and
Observational Extragalactic Astronomy & Cosmology.
- Site about extensive computer simulations of the early
universe to model the evolution of structure. A section for
novices explains the general ideas of the research, while
the section for
Hubble volume simulations provides much more detail.
Surveys, overviews, tutorials
- Article from
Cosmology 101: The Study of the Universe
- A good, fairly complete overview of cosmology,
Part of NASA's
Microwave Anisotropy Probe (MAP) site.
Has many surveys and tutorials on topics related to the big bang
and cosmology in general. It includes discussions of
observational tests of the big bang theory,
limitations and extensions of the big bang theory, our
present knowledge about the universe, and
galaxies and stars.
Ned Wright's Cosmology Tutorial
- Good site with detailed technical information. Also available
single page. There are recommended links to other
good cosmology sites. Ned Wright's
home page contains links to some of his publications
and related information.
Frequently Asked Questions in Cosmology
- Excellent collection of questions and answers by
- Another very good introduction to cosmology, by
It's well-organized, covers all the basic topics, and has a
Martin White's Pages
- Pages deal with a variety of topics in cosmology, including
the cosmic microwave background, structure formation, dark
matter, and nucleosynthesis.
Fundamental Issues in Cosmology
- By Joseph Silk - a single page overview, based on an earlier
What is Theoretical Cosmology?
- Excellent set of tutorial material prepared by
Joanne Cohn and
Topics include the cosmic microwave background,
large scale structure, Lyman alpha systems, galactic clusters,
gravitational lensing, and early universe field theory.
A brief guide to cosmology
- Nice May 2009 overview by John and Mary Gribbin.
From Cosmos Magazine.
KIAS Winter School for Particle Physics: Cosmology
- Provides lectures slides from a 2004 course
on introductory topics, in PDF form:
Basics of cosmology,
Large scale structure: CMB observations
The New Cosmology: From Quantum Fuzz to the Accelerating
Universe -- Online Lectures
- Contains lectures from an October 2001
conference, mostly in PDF format. Topics include
the big picture,
the cosmic microwave background,
the search for dark matter,
large scale structure, and
Cosmology: The Structure & Future of the Universe
- Good explanations and external links, part of
Gene Smith's Astronomy Tutorial.
An Ancient Universe: How Astronomers Know the Vast Scale of
- Booklet designed as a guide for teachers and students by the
American Astronomical Society.
The booklet is available as a
Cosmology Essays from the Center for Particle Astrophysics
- About 10 good survey articles by various authors, mainly
concerning the big bang and dark matter.
Cosmology and the Structure of the Universe
- A ScienceWeek
"symposium" consisting of excerpts and summaries of
articles from various sources.
Cambridge Cosmology Public Home Page
- The hot big bang model, galaxies and clusters, relic
radiation, cosmic strings, inflation.
Level 5: Cosmology
- Collection of online articles and papers on many topics in
cosmology. (From the
Level 5 project.)
Ask a High-Energy Astronomer: Cosmology
- Common questions, with answers, provided by NASA's
Ask a High-Energy Astronomer service.
Imagine the Universe!
- A service of NASA's High-Energy Astrophysics Learning Center.
Presents news, features, and tutorial information on astronomy,
astrophysics, and cosmology.
Includes a good list of resources (Web, magazines, and books).
science page and the
advanced science page for lists of the main topics.
For frequently asked questions, see
Ask a High Energy Astronomer.
Frequently Asked Questions in Astronomy: Cosmology
- Questions and answers from the Usenet sci.astro newsgroup.
11 key questions about the universe
- January 2001 news article listing 11 key questions developed by
a panel of U. S. physicists and astronomers.
Contains links to articles at the
Physics Web site related
to some of the questions.
5 Great Cosmic Mysteries
- Good series of 5 early 2002 articles from
Space.com that provide overviews of
extraterrestrial life, and
Searching for Answers in a Digital Universe
- April 2002
article from Space.com.
Discusses use of computer simulation in cosmology research.
The Universe: Still Boggling the Minds of 'Finite Creatures'
- June 2001
article from Space.com.
Discusses various speculative ideas in cosmology, such as the
topology of the universe, the "ekpyrotic" theory, and
ideas of multiple universes.
The Big Questions
- Material derived from a 1995 Australian TV series featuring
Paul Davies, who tends to mix religion with science. Contains a few
external links. There was a sequel entitled
More Big Questions.
- A series of lectures given in 2001-02 by leading experts, in
Astronomy 123: Cosmology and the Origin of Life
- An online "distance education" course, by the University of
Oregon. Lectures on the principal topics of cosmology. Includes
good external links.
Galaxies and the Universe
- Course notes by
William C. Keel
that cover a wide variety of topics in cosmology, such as the formation
of stars and galaxies, dark matter, and active galactic nuclei.
Astrophysics and cosmology: the golden age
- December 1999 article from
Physics World, by
Michael Rowan-Robinson. "Our understanding of the universe has advanced
immeasurably in the last century, and there seems no end to the wealth
of new discoveries as we approach the next millennium."
New age of precision cosmology
- July 1999 article from
by Richard Ellis. Measurements of fundamental parameters of the
universe, such as its age and the value of the Hubble constant,
are finally being determined with "good" precision.
- Slide presentation given by Michael Turner at the
2001: A Spacetime Odyssey conference.
Excellent, detailed summary of the state of the universe as of
- Slide presentation given by Wendy Freedman at the
2001: A Spacetime Odyssey conference.
Contains a nice summary of
the important parameters as of May 2001.
The Nature of the Universe
- Selected articles on cosmology, astronomy, and space science from the
New York Times. Articles are from April-May 2001.
The Big Bang
- Part of Mike Guidry's
Violence in the Cosmos. Material on
expansion of the universe, the cosmic microwave background,
large-scale structure, and matter in the universe.
Cosmology and What Happened Before the Big Bang
- Collection of goog tutorial papers by Sten Odenwald. Includes:
Galaxy Redshifts Reconsidered and
Einstein's Cosmic Fudge Factor.
Big Bang Science
- From the Particle Physics and Astronomy Research Council (UK).
Focus is on particle physics and its role in the big bang model.
Creation of a Cosmology: Big Bang Theory
- By Scott J. Siegel.
The Cosmological Models
- In-depth mathematical exposition by Jeff Bondono.
- Part of the
Stephen Hawking's Universe site which discusses various
various cosmological models.
- Part of an introduction to astronomy by Nick Strobel. Covers many
- Course material by David Bennett. Also
Galaxies, Quasars, and Cosmology
- Course material by Mike Merrifield.
Four Keys to Cosmology
- February 2004 Scientfic American Special Report, subtitled
"The big bang theory works better than ever. If only
cosmologists could figure out that mysterious acceleration."
Cosmos in a Computer
- Excellent multimedia site created by the
National Center for Supercomputer Applications (NCSA). Provides
overview of cosmology and how it is studied by computer modeling.
Relativity and Cosmology
- Outline and overview from a college course, by Jose Wudka.
Notes on Cosmology
- College-level course material by Andreas Albrecht.
String Theory and Cosmology
- From the String Theory Web site.
Foundations of Modern Cosmology
- This is a summary/outline of the book of the same title by
John Hawley and Katherine Holcomb. There is an overview and
questions/answers for each of the 16 chapters. Also a good
list of links to
other astronomy and cosmology sites.
Early Universe Primer
- A lecure on the big bang and related topics, by James
Galaxies and the Expanding Universe
- A lecture course by James Schombert.
- A lecture course by James Schombert.
- A lecture course by James Imamura.
An Introduction to Cosmology
- A lecture course by James Imamura.
The Universe According to Hubble
- 1995 article from the Philadelphia Inquirer. Seems to have
The Greatest Story Ever Told
- December 1998 news article from Science News, about
a debate as to whether cosmologists had finally assembled
a consistent framework that integrates the origin, evolution,
and current appearance of the universe.
- Beyond the Big Bang
National Geographic, May 2005, pp. 110-121
- A relatively brief article considers several speculative
ideas in cosmology that extend the big bang theory, including
multiple inflationary universes, accelerating expansion, and
the cosmological constant.
The Beginning of Time
Craig J. Hogan
Science, March 22, 2002, pp. 2223-2225
- Study of the Cosmic Microwave Background yields a wealth of
information on the state of the universe during the period of
inflation. It may even be possible to deduce information about
other universes in the "multiverse". [A subscription to
Science is required to access this article. It contains
"enhanced" content, including many links to additional related
Science News, September 22, 2001, pp. 184-186
- A new theory -- known as the ekpyrotic theory -- of the origin
of the universe, first made public in April of this year, postulates
that the universe as we know it developed out of a collision between
three-dimensional membranes embedded in a five-dimensional space.
- The 8 Greatest Mysteries of Cosmology
Astronomy, June 2001, pp. 46-52
- The author's list is: (1) true dimensionality of spacetime,
(2) origins of the universe, (3) preponderance of matter over
antimatter, (4) galaxy formation, (5) nature of cold dark matter,
(6) baryonic matter outside galaxies, (7) nature of dark energy,
(8) future of the universe.
- Inside-Out Cosmology
Astronomy, June 2001, pp. 38-43
- The answers to the most important open questions in cosmology
will probably be found in the study of high-energy particle physics.
Making Sense of Modern Cosmology
P. James E. Peebles
Scientific American, January 2001, pp. 54-55
- The science of cosmology is advancing rapidly, thanks to a wealth
of recent data. While support for the basic big bang theory is
strengthening, there is now a lot of activity going into clarifying
- Einstein's Static Universe: An Idea Whose Time Has Come Back?
Aubert Daigneault; Arturo Sangalli
Notices of the AMS, January 2001, pp. 9-16
- There are alternatives to the Big Bang cosmology. One little-known
example is the "chronometric cosmology" developed by the mathematician
I. E. Segal.
[Article in PDF format]
- Exploring Our Universe and Others
Scientific American, December 1999, pp. 78-83
- Increasingly powerful instruments to observe the universe at
all electromagnetic wavelengths and by other means as well will
enable us to understand more about the earliest instants of the
Big Bang, and the possibility that our universe is only one
- A Different Approach to Cosmology
Geoffrey Burbidge, Fred Hoyle, Jayant V. Narlikar
Physics Today, April 1999, pp. 38-44
- The authors argue for a strikingly dissident view of
cosmology that features a steady-state universe, some
quasars nearby, and no big bang.
- Reply to "A Different Approach to Cosmology"
Physics Today, April 1999, pp. 44-46
- Increasingly detailed studies of galaxy redshifts and
fluctuations in the background radiation make it continually
more difficult to justify alternatives to the big bang
- Surveying Space-time with Supernovae
Craig J. Hogan, Robert P. Kirshner, Nicholas B. Suntzeff
Scientific American, January 1999, pp. 46-51
- Describes how observation of supernovae enables good estimates
of the distance of galaxies with large red shift. The new estimates
imply cosmic expansion is accelerating.
- Cosmological Antigravity
Lawrence M. Krauss
Scientific American, January 1999, pp. 53-59
- The acceleration of cosmic expansion implies some sort of
"antigravity", represented by a nonzero cosmological constant
in the equations of general relativity. If the universe is really
flat, as implied by inflation, instead of open, there must be
additional, as yet unexplained energy present.
- The Evolution of Galaxy Clusters
J. Patrick Henry, Ulrich G. Briel, Hans Böhringer
Scientific American, December 1998, pp. 52-57
- Observation of galaxy clusters provides information on
the distribution of matter that is significant for cosmology.
- Exploding Stars Tell All
Astronomy, November 1998, pp. 50-55
- Careful measurement of distances using type Ia supernovae
as "standard candles" indicate that the expansion of the
universe is accelerating.
- Planting Primordial Seeds
Astronomy, February 1998, pp. 38-43
- The large-scale structure of the universe probably originated
as quantum density fluctuations at the time of the Big Bang.
- On Becoming the Material World
Astronomy, February 1998, pp. 44-49
- The relative abundances of nuclei of the very lightest
elements provide important clues to the origin of the universe.
- Probing Cosmic Mysteries by Supercomputer
Michael L. Norman
Physics Today, October 1996, pp. 42-48
- Advances in computer hardware and software are enabling
numerical investigation of problems in cosmology and
- X Rays from the Rest of the Universe
David J. Helfand
Physics Today, November 1995, pp. 58-64
- X ray astronomy now functions as a laboratory for astrophysics,
nuclear physics, relativity, plasma physics, and cosmology. One
significant accomplishment is the experimental confirmation that
black holes exist.
- Leonard Susskind – The Cosmic Landscape: String Theory
and the Illusion of Intelligent Design
Little, Brown and Company, 2006
- Susskind played a major role in the creation of string
theory in the 1970s. Currently he is the chief exponent of a
conception of cosmology based on the latest forms of superstring
theory. The idea is that string theory does not seem to dictate
a unique set of physical laws for the universe, and perhaps
there are an infinite number of distinct universes, each
realizing one of the infinite sets of physical laws permitted
by string theory. This "multiverse" he calls the "cosmic
landscape". Of course, this is very controversial, but in the
process of explaining the idea, Susskind describes many
somewhat less controversial notions of modern cosmology.
- Charles Seife -- Alpha & Omega: The Search for the Beginning
and End of the Universe
- Seife is a journalist with some training in mathematics,
His book provides yet another nontechnical overview of the
standard topics in cosmology for general readers. It has
the virtues of brevity and not wasting too much time on presenting
the historical background that has been covered so often before.
It does delve into important recent topics like dark matter and
dark energy, the cosmological constant, the Ω parameter,
neutrinos, supersymmetry, and the cosmic microwave background.
All in all, it doesn't pretend to offer technical details, but
it provides an easy, quick introduction (or refresher
course) for most of the latest key ideas in cosmology.
- Tom Yulsman -- Origins: The Quest for Our Cosmic Roots
Institute of Physics Publishing, 2003
- Yulsman is a science journalist whose main work has dealt
with (terrestrial) environmental subjects. His book is intended
for general readers who are curious about the cosmological
background of our terrestrial environment. It covers standard
topics in cosmology, such as the big bang, inflation, and
speculative theories of the "ultimate beginnings". The second
half of the book covers the main events after the big bang --
the origins of stars, solar systems, planets, and life.
- Andrew Liddle -- An Introduction to Modern Cosmology
John Wiley & Sons, 2003
- Yet another good, short introductory cosmology textbook, like
those of Jeremy Bernstein and Eric Linder. Good understanding of
calculus is assumed, but there are not a lot of detailed
calculations. Liddle's book treats the basics somewhat more
briefly than the others, and instead offers some more
advanced topics, such as general relativistic cosmology,
neutrino cosmology, and structure of the universe. Because it
is more recent, the book takes account of evidence for
[Book home page]
- Barbara Ryden -- Introduction to Cosmology
Addison Wesley, 2003
- Ryden's book is similar to Liddle's in content and
mathematical requirements, but about 50% longer because it
is more thorough, with
more descriptive detail. Great care is taken to
explain the important points, which makes Ryden's book one
of the clearest textbooks on any technical subject. It and
Liddle's book are short
enough you might want to read both, as they
reinforce each other.
But if you buy only one undergraduate-level text on the
subject, this is one to get – as of 2003.
- Robert P. Kirshner -- The Extravagant Universe: Exploding
Stars, Dark Energy, and the Accelerating Cosmos
Princeton University Press, 2002
- The discovery in 1998 that the universe was not only
expanding but was doing so at an accelerating rate, and
hence that more that 70% of the "stuff" of the universe was
probably not even matter at all, marks an important watershed
in cosmology. Any overview of cosmology today that predates
this discovery is missing an utterly crucial detail. Kirshner's
book is one of the best on cosmology for general readers
that follows this watershed event. It explains the basic details
of how this discovery was made, by observations of very distant
- Marcus Chown -- The Universe Next Door: The Making of
Oxford University Press, 2001
- Chown covers many topics in a relatively brief space and
shows some predilection for the sensationalistic. But the
ideas presented are plausible science, not New Age mysticism, and
include such topics relevant to cosmology as the direction of
time, multiple universes, unification of relativity and quantum
mechanics, higher dimensions of spacetime, black holes, and mirror
- Peter Coles -- Cosmology: A Very Short Introduction
Oxford University Press, 2001
- This slim volume is one of a series from Oxford of very
short introductions to a large number of subjects. It's a
pocket-size overview of the basic prerequisites anyone should
know before digging deeper. The main topics each occupy their
own chapters: relativity, fundamental principles, expansion of
the universe, the big bang, dark matter, cosmic structure, and
connections with particle physics and fundamental physical
- Ken Croswell -- The Universe at Midnight: Observations
Illuminating the Cosmos
The Free Press, 2001
- Here's a very good survey of modern cosmology for general
readers. Most of the topics that have recently been prominent
are here: the big bang, the cosmic microwave background,
dark matter, the cosmological constant,
refined measurements of Hubble's constant, &Omega, and &Lambda,
and speculation on the future of the universe.
There is also a good glossary of terms, and a very extensive
bibliography which includes the whole range from research
articles to popular books.
- S. Alan Stern, ed. -- Our Universe: The Thrill of Extragalactic
Exploration As Told by Leading Experts
Cambridge University Press, 2001
- Want a really quick introduction, with minimal prerequisites,
to topics like massive black holes, gamma-ray bursts, dark matter,
and large-scale structure? This may be your book. It's told
simply, by ten different contributors, and with some emphasis
on describing how cosmologists actually work.
- James Rich -- Fundamentals of Cosmology
- Rich offers a very good introductory textbook on cosmology
that is a little more sophisticated and detailed than entry-level
texts such as those of Eric Linder, Jeremy Bernstein, and
Andrew Liddle. It has a self-contained introduction
to general relativity, so some proficiency in calculus is needed.
The payoff is that the reader gets to see how the structure
and evolution of the universe is actually calculated.
- P. Binétruy; R. Schaeffer; J. Silk; F. David, eds. --
The Primordial Universe
- This volume consists of a dozen short courses for advanced
students presented at a summer school in Les Houches, France in
1999. (All of the text is in English.) Although there is a lot
of material included that deals with rather advanced topics
supersymmetry, string theory, M-theory), much of it is also
comprehensible to anyone who has a sound basic grasp of the issues
of modern cosmology. The excursions into the more esoteric areas,
even though intended for specialists, do serve to demonstrate
just how interdependent the subjects of cosmology and high
energy physics have become.
- Donald Goldsmith -- The Runaway Universe: The Race to Find the
Future of the Cosmos
Perseus Publishing, 2000
- This book should be a first choice for the scientifically
literate reader who wants a systematic exposition of the latest
findings about the accelerating rate of expansion of the universe,
how it is measured, what the observations mean in terms of the
cosmological constant and "dark matter", the different lines of
converging evidence, and even what possibilities of misinterpretation
may exist in the data.
- Mario Livio -- The Accelerating Universe: Infinite Expansion,
the Cosmological Constant, and the Beauty of the Cosmos
John Wiley & Sons, 2000
- Livio is head of the Science Division of the Space Telescope
Science Institute, but his approach to the subject heavily
emphasizes an explanation of his ideas of beauty in scientific
theory over orderly development of the most recent findings.
Nevertheless, he covers the usual topics: accelerating expansion,
dark matter, the apparent "flatness" of the universe, possible
origins of the universe, and the "anthropic principle".
- Martin Rees -- Just Six Numbers: The Deep Forces That Shape the
Basic Books, 2000
- Rees, who is England's Astronomer Royal, sets out to show how
there are deep links between the microworld of elementary particles
and the cosmos as a whole. The six numbers that he asserts demonstrate
this link are: the relative strengths of electromagnetic and
gravitational forces, the "fine structure constant", Omega -- a
measure of matter in the universe, lambda -- the "cosmological
constant", the ratio of gravitational binding energy in a
galaxy to its rest mass-energy, and the number of visible spatial
- Michael Rowan-Robinson -- The Nine Numbers of the Cosmos
Oxford University Press, 1999
- The author's purpose in this short book is to give an overview
of what we really know about the universe at this point in time.
Eschewing both speculation and long, drawn-out explanation, he
identifies nine observational facts about the universe and a
corresponding, measurable number which encapsulates each.
- T. Padmanabhan -- After the First Three Minutes: The Story of
Cambridge University Press, 1998
- Excellent introduction to cosmology "in the large", after the
very high-energy initial conditions. Though without mathematics,
it deals directly with the technical issues and does not write
down to the audience.
- Jeremy Bernstein -- An Introduction to Cosmology
Prentice Hall, 1998
- Another good, short introductory cosmology textbook, like
Eric Linder's, providing a relatvively gentle entry to the
mathematical aspects of cosmology.
It's main difference from Linder's book is in choice
of topics, with more emphasis on nucleosynthesis, elementary
particle physics, and inflation.
- John F. Hawley, Katherine A. Holcomb
Oxford University Press, 1998
- The niche served by the present book is that of the textbook
for college undergraduates who want a descriptive introduction
to the history and general themes of modern cosmology, as a
sequel to an introductory course in astronomy. The first
fourth of the book is history and generalities, the next fourth
is a nonmathematical overview of special and general relativity,
and the remainder is a nonmathematical presentation of the
standard topics of cosmology.
- G. Münch, A. Mampaso, F. Sánchez, eds. -- The Universe
at Large: Key Issues in Astronomy and Cosmology
Cambridge University Press, 1997
- Consists of invited papers from leading astronomers and
cosmologists which were presented at a conference dedicated to
describing and organizing the main unsolved problems. The
focus is on the astronomical objects themselves -- quasars,
active galactic nuclei, black holes, galaxies. The papers are
concise and polished, but there are also informal Q/A sections
at the end of each.
- Martin Rees -- Before the Beginning: Our Universe and Others
Perseus Books, 1997
- This is a sweeping survey of cosmology which touches on most
of the important questions: the Big Bang, birth and evolution
of galaxies, black holes, dark matter, inflation. It deals
explicitly with the idea of "multiple universes".
- Eric V. Linder -- First Principles of Cosmology
- This is a good, short, undergraduate-level introductory
cosmology textbook. The only mathematics required is basic
calculus. The main topics covered are cosmological models,
Hubble expansion, nucleosynthesis, cosmic microwave
background radiation, and structure formation.
- Malcolm S. Longair -- Our Evolving Universe
Cambridge University Press, 1996
- Attractively presented non-mathematical overview of astrophysics
and cosmology. It covers successively the origin and development
of stars, quasars, galaxies, and the universe.
- Martin Rees -- Perspectives in Astrophysical Cosmology
Cambridge University Press, 1995
- Brief but technically detailed survey of key issues in
contemporary cosmoloty, such as galaxy formation, orgin of
structure, dark mattter, and background radiation.
- Joseph Silk -- Cosmic Enigmas
American Institute of Physics, 1994
- Silk is both a distinguished cosmologist and a talented
expositor of the subject. Unfortunately, his earlier books on
cosmology are now somewhat out of date, due to rapid
advances in the field. The present book consists of many
shorter essays on topics such as the big bang, galaxy formation,
and large-scale structure. The essays remain stimulating reading,
even though some of the questions discussed are now much better
- Stephen Hawking -- Black Holes and Baby Universes and Other
Bantam Books, 1993
- A collection of essays that is partly autobiographical as
well as scientific. In addition to the subjects of the title, it
considers the possibility of a "theory of everything" and the
origin and future of the universe.
- James Cornell (ed.) -- Bubbles, Voids, and Bumps in Time: The
Cambridge University Press, 1989
- Collection of survey chapters by eminent specialists. Topics
include dark matter, distribution of galaxies, and cosmic
- Heinz Pagels -- Perfect Symmetry: The Search for the Beginning
Simon and Schuster, 1985
- Excellent exposition of the contribution of quantum mechanics
and particle physics to the Big Bang model. Although some of the
details are dated, the book is very worthwhile based on the
author's expertise and explanatory skills.
- G. Contopoulos, D. Kotsakis -- Cosmology: The Structure and
Evolution of the Universe
- Concise technical survey of the main topics of cosmology,
using mathematics where appropriate.
- Steven Weinberg -- The First Three Minutes
Basic Books, 1977
- This rather short book, though out of date on many details
(such as inflation), is a classic. It presents, with great clarity,
an explanation in terms of quantum physics
of the evolution of the universe during
the first three minutes after the Big Bang.
Copyright © 2002-04 by Charles Daney, All Rights Reserved