Open Questions:

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The universe is a big place, perhaps the biggest.

Philip Jose Farmer (writing as Kilgore Trout)

Hubble Telescope Deep Field View of Galaxies
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The standard model of cosmology

Cosmology and high-energy physics

Open questions

In depth

The Big Bang

The cosmic microwave background

Dark matter

Cosmic inflation

Dark energy

Large-scale structure of the universe

Galaxy formation, structure, and evolution

The early universe

Matter/antimatter asymmetry

Cosmic strings

Time asymmetry (time's arrow)

Fundamental constants

Quantum cosmology

Cosmological models

Cosmic magnetic fields

Reference material

Recommended references: Web sites

Recommended references: Magazine/journal articles

Recommended references: Books


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 are:
  • 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 universe.
  • 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 accelerating.
  • 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 previously expected.

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 expanding.

In particular, two of the most pressing questions that now need answers are:

The standard model of cosmology

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

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 universe.

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

Dark matter

"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 part.

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:


Dark energy

Cosmology and high-energy physics

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 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 future.

Open questions

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 long ago.

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:

Recommended references: Web sites

Site indexes

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 found at Netscape.
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 found at Netscape.
Further Reading [Cosmology]
Excellent categorized list by Martin White.
Cosmology Books and Links
Bibliography of books and Web sites, part of a cosmology course, by Joseph Tenn.
Good Cosmology Sites
A short list by Ned Wright, part of his Cosmology Tutorial.
Cosmology Links
By Andrew Hamilton.
Cosmology Pages
Links on general cosmology, dark matter, and large-scale structure, by Douglas Scott.
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 here.
Yahoo Cosmology Links
Annotated list of links.
Yahoo Astrophysics Links
Annotated list of links.
Galaxy: Cosmology
Categorized site directory. Entries usually include descriptive annotations. Has a subcategory for universal origins. (More here.)

Sites with general resources

The New Cosmology: From Quantum Fuzz to the Accelerating Universe
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 here.
Stephen Hawking 60th Birthday Celebration Workshop and Conference
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 and cosmology".
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 tutorial on cosmology.
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 include ACBAR, OVRO/BIMA SZ Imaging, BIMA Fine Anisotropy Survey, APEX, South Pole Telescope, BICEP, DASI, and CBI.
Cosmology Bibliography
Useful bibliography of cosmology books for a general audience, and a few college/university textbooks on astronomy, astrophysics, and cosmology, by Ned Wright.
First Principles of Cosmology
Online supplementary material to the book by Eric V. Linder. Includes astrophysics links and cosmology FAQ.
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, such as 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 energy ("quintessence").
Andrei Linde
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.
Max Tegmark
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 papers describes an apparent anisotropy in the CMB. There's quite a bit of good information on cosmology in these pages.
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 here. He also writes about current topics in the blog Cosmic Variance.
Rocky Kolb
Kolb specializes in, and has made significant contributions to, particle astrophysics. His home page links to a number of informational talks on cosmology.
Neil Cornish
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.
Cosmic Background
A small collection of background material and introductions to some research topics in cosmology, provided by Ben Wandelt.
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 image gallery, and useful external links.
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 stars and galaxies, the large scale strucure of the universe, and the the nature of dark matter. The site contains a lot of information on the background science, and frequently-asked questions.
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 educational resources.
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 cosmological simulation).
Durham Extragalactic & Cosmology Research Group
University research group. Site includes sections on Theoretical Extragalactic Astronomy & Cosmology and Observational Extragalactic Astronomy & Cosmology.
Virtual Universe
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 Wikipedia. See also Universe.
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 as a 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 Ned Wright.
Cosmology Primer
Another very good introduction to cosmology, by Sean Carroll. It's well-organized, covers all the basic topics, and has a FAQ, glossary, and external links.
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 page, here.
What is Theoretical Cosmology?
Excellent set of tutorial material prepared by Joanne Cohn and M. White. 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, Inflation, and 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, dark matter, the search for dark matter, large scale structure, and dark energy.
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 Cosmic Time
Booklet designed as a guide for teachers and students by the American Astronomical Society. The booklet is available as a PDF file
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). See the 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 that provide overviews of dark matter, dark energy, galaxy formation, extraterrestrial life, and multiple universes.
Searching for Answers in a Digital Universe
April 2002 article from Discusses use of computer simulation in cosmology research.
The Universe: Still Boggling the Minds of 'Finite Creatures'
June 2001 article from 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.
Cosmology Teach-ins
A series of lectures given in 2001-02 by leading experts, in PDF format.
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 Physics World, 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.
Cosmology 2001
Slide presentation given by Michael Turner at the 2001: A Spacetime Odyssey conference. Excellent, detailed summary of the state of the universe as of May 2001.
Cosmological Parameters
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 pertinent topics.
Elementary Cosmology
Course material by David Bennett. Also here.
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 Schombert.
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 good links.
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.

Recommended references: Magazine/journal articles

Beyond the Big Bang
Marcia Bartusiak
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.
[Additional resources]
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 resources.]
Braney Universe
Ron Cowen
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
Mark Sincell
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
Robert Zimmerman
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 the details.
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
Martin Rees
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 of many.
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"
Andreas Albrecht
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 cosmology.
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
Robert Irion
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
Rocky Kolb
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
James Glanz
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 astrophysics.
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.

Recommended references: Books

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.
[Book review]
Charles Seife -- Alpha & Omega: The Search for the Beginning and End of the Universe
Viking, 2003
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 accelerating expansion.
[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 supernovae.
Marcus Chown -- The Universe Next Door: The Making of Tomorrow's Science
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 matter.
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 laws.
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
Springer-Verlag, 2001
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
Springer-Verlag, 2000
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 (such as 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 Universe
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 dimensions.
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 Our Universe
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
Addison-Wesley, 1997
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 understood.
Stephen Hawking -- Black Holes and Baby Universes and Other Essays
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 New Cosmology
Cambridge University Press, 1989
Collection of survey chapters by eminent specialists. Topics include dark matter, distribution of galaxies, and cosmic inflation.
Heinz Pagels -- Perfect Symmetry: The Search for the Beginning of Time
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
Springer-Verlag, 1984
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.


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