Showing posts with label Cosmology. Show all posts
Showing posts with label Cosmology. Show all posts

Monday, April 30, 2012

A Superset Universe?

How would you draw a Universe with all theories as being part of,  as a subset?

Pictorial representations can be very useful in presenting information or assisting reasoning. Venn diagram is an example. Venn diagrams are used to represent classes of objects, and they can also assist us in reasoning about the relations between these classes. They are named after the English mathematician John Venn (1834 - 1923), who was a fellow at Cambridge University.

A few may have taken in the link supplied to a lecture given by Thomas Campbell with regard to his MBT book he had written. Now, I was drawn to the idea of a Venn diagram presented in his lecture and the idea of how one might have use this diagram as a question about the universe and it's subsets? How would you draw it?

I give a current posting by Sean Carroll with regards to his opinion on a book written by Lawrence Krauss. So there all these theories about the nature of the universe and some scientists of course have their opinions.

............Or not, of course. We should be good empiricists and be open to the possibility that what we think of as the universe really does exist within some larger context. But then we could presumably re-define that as the universe, and be stuck with the same questions. As long as you admit that there is more than one conceivable way for the universe to be (and I don’t see how one could not), there will always be some end of the line for explanations. I could be wrong about that, but an insistence that “the universe must explain itself” or some such thing seems like a completely unsupportable a priori assumption. (Not that anyone in this particular brouhaha seems to be taking such a stance.) SEE:A Universe from Nothing?

Physicists have proposed several theories to explain why Λ is so small. One of the most popular -- the "anthropic principle" -- states that Λ is randomly set and has very different values in different parts of the universe (figure 1). We happen to live in a rare region, or "bubble", where Λ has the value we observe. This value has allowed stars, planets and therefore life to develop. However, this theory is also unsatisfactory for many scientists because it would be better to be able to calculate Λ from first principles.

See also:

Wednesday, December 08, 2010

Conformal Cyclic Cosmology....

Penrose's Conformal Cyclic Cosmology, from one of his Pittsburgh lecture slides in June, 2009. Photo by Bryan W. Roberts

....... (CCC) is a cosmological model in the framework of general relativity, advanced by the theoretical physicist Sir Roger Penrose.[1][2] In CCC, the universe undergoes a repeated cycle of death and rebirth, with the future timelike infinity of each previous universe being identified with the Big Bang singularity of the next.[3] Penrose outlines this theory in his book Cycles of Time: An Extraordinary New View of the Universe.


Basic Construction

Penrose's basic construction[4] is to paste together a countable sequence of open FLRW spacetimes, each representing a big bang followed by an infinite future expansion. Penrose noticed that the past conformal boundary of one copy of FLRW spacetime can be "attached" to the future conformal boundary of another, after an appropriate conformal rescaling. In particular, each individual FLRW metric gab is multiplied by the square of a conformal factor Ω that approaches zero at timelike infinity, effectively "squashing down" the future conformal boundary to a conformally regular hypersurface (which is spacelike if there is a positive cosmological constant, as we currently believe). The result is a new solution to Einstein's equations, which Penrose takes to represent the entire Universe, and which is composed of a sequence of sectors that Penrose calls "aeons."

Physical Implications

The significant feature of this construction for particle physics is that, since baryons are obey the laws of conformally invariant quantum theory, they will behave in the same way in the rescaled aeons as in the original FLRW counterparts. (Classically, this corresponds to the fact that light cone structure is preserved under conformal rescalings.) For such particles, the boundary between aeons is not a boundary at all, but just a spacelike surface that can be passed across like any other. Fermions, on the other hand, remain confined to a given aeon. This provides a convenient solution to the black hole information paradox; according to Penrose, fermions must be irreversibly converted into radiation during black hole evaporation, to preserve the smoothness of the boundary between aeons.

The curvature properties of Penrose's cosmology are also highly desirable. First, the boundary between aeons satisfies the Weyl curvature hypothesis, thus providing a certain kind of low-entropy past as required by statistical mechanics and by observation. Second, Penrose has calculated that a certain amount of gravitational radiation should be preserved across the boundary between aeons. Penrose suggests this extra gravitational radiation may be enough to explain the observed cosmic acceleration without appeal to a dark energy matter field.

Empirical Tests

In 2010, Penrose and V. G. Gurzadyan published a preprint of a paper claiming that observations of the cosmic microwave background made by the Wilkinson Microwave Anisotropy Probe and the BOOMERanG experiment showed concentric anomalies which were consistent with the CCC hypothesis, with a low probability of the null hypothesis that the observations in question were caused by chance.[5] However, the statistical significance of the claimed detection has since been questioned. Three groups have independently attempted to reproduce these results, but found that the detection of the concentric anomalies was not statistically significant.[6][7][8]

See also


  1. ^ Palmer, Jason (2010-11-27). "Cosmos may show echoes of events before Big Bang". BBC News. Retrieved 2010-11-27. 
  2. ^ Penrose, Roger (June 2006). "Before the big bang: An outrageous new perspective and its implications for particle physics". Edinburgh, Scotland: Proceedings of EPAC 2006. p. 2759-2767. Retrieved 2010-11-27. 
  3. ^ Cartlidge, Edwin (2010-11-19). "Penrose claims to have glimpsed universe before Big Bang". Retrieved 2010-11-27. 
  4. ^ Roger Penrose (2006). "Before the Big Bang: An Outrageous New Perspective and its Implications for Particle Physics". Proceedings of the EPAC 2006, Edinburgh, Scotland: 2759-2762. 
  5. ^ Gurzadyan VG; Penrose R (2010-11-16). "Concentric circles in WMAP data may provide evidence of violent pre-Big-Bang activity". arΧiv:1011.3706 [astro-ph.CO]. 
  6. ^ Wehus IK; Eriksen HK (2010-12-07). "A search for concentric circles in the 7-year WMAP temperature sky maps". arΧiv:1012.1268 [astro-ph.CO]. 
  7. ^ Moss A; Scott D; Zibin JP (2010-12-07). "No evidence for anomalously low variance circles on the sky". arΧiv:1012.1305 [astro-ph.CO]. 
  8. ^ Hajian A (2010-12-8). "Are There Echoes From The Pre-Big Bang Universe? A Search for Low Variance Circles in the CMB Sky". arΧiv:1012.1656 [astro-ph.CO].

See Also: Penrose's CCC cosmology is either inflation or gibberish

Tuesday, December 07, 2010

Big Bounce

Physical cosmology
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Universe · Big Bang
Age of the universe
Timeline of the Big Bang
Ultimate fate of the universe
The Big Bounce is a theorized scientific model related to the formation of the known Universe. It derives from the cyclic model or oscillatory universe interpretation of the Big Bang where the first cosmological event was the result of the collapse of a previous universe.[1]


Expansion and contraction

According to some oscillatory universe theorists, the Big Bang was simply the beginning of a period of expansion that followed a period of contraction. In this view, one could talk of a Big Crunch followed by a Big Bang, or more simply, a Big Bounce. This suggests that we might be living in the first of all universes, but are equally likely to be living in the 2 billionth universe (or any of an infinite other sequential universes).
The main idea behind the quantum theory of a Big Bounce is that, as density approaches infinity, the behavior of the quantum foam changes. All the so-called fundamental physical constants, including the speed of light in a vacuum, were not so constant during the Big Crunch, especially in the interval stretching 10−43 seconds before and after the point of inflection. (One unit of Planck time is about 10−43 seconds.)

If the fundamental physical constants were determined in a quantum-mechanical manner during the Big Crunch, then their apparently inexplicable values in this universe would not be so surprising, it being understood here that a universe is that which exists between a Big Bang and its Big Crunch.

Recent developments in the theory

Martin Bojowald, an assistant professor of physics at Pennsylvania State University, published a study in July 2007 detailing work somewhat related to loop quantum gravity that claimed to mathematically solve the time before the Big Bang, which would give new weight to the oscillatory universe and Big Bounce theories.[2]

One of the main problems with the Big Bang theory is that at the moment of the Big Bang, there is a singularity of zero volume and infinite energy. This is normally interpreted as the end of the physics as we know it; in this case, of the theory of general relativity. This is why one expects quantum effects to become important and avoid the singularity.

However, research in loop quantum cosmology purported to show that a previously existing universe collapsed, not to the point of singularity, but to a point before that where the quantum effects of gravity become so strongly repulsive that the universe rebounds back out, forming a new branch. Throughout this collapse and bounce, the evolution is unitary.

Bojowald also claims that some properties of the universe that collapsed to form ours can also be determined. Some properties of the prior universe are not determinable however due to some kind of uncertainty principle.

This work is still in its early stages and very speculative. Some extensions by further scientists have been published in Physical Review Letters.[3]

Peter Lynds has recently put forward a new cosmology model in which time is cyclic. In his theory our Universe will eventually stop expanding and then contract. Before becoming a singularity, as one would expect from Hawking's black hole theory, the Universe would bounce. Lynds claims that a singularity would violate the second law of thermodynamics and this stops the Universe from being bounded by singularities. The Big Crunch would be avoided with a new Big Bang. Lynds suggests the exact history of the Universe would be repeated in each cycle. Some critics argue that while the Universe may be cyclic, the histories would all be variants.

See also


  1. ^ "Penn State Researchers Look Beyond The Birth Of The Universe". Science Daily. May 17, 2006.  Referring to Ashtekar, Abhay; Pawlowski, Tomasz; Singh, Parmpreet (2006). "Quantum Nature of the Big Bang". Physical Review Letters 96 (14): 141301. doi:10.1103/PhysRevLett.96.141301. PMID 16712061. 
  2. ^ Bojowald, Martin (2007). "What happened before the Big Bang?". Nature Physics 3 (8): 523–525. doi:10.1038/nphys654. 
  3. ^ Ashtekar, Abhay; Corichi, Alejandro; Singh, Parampreet (2008). "Robustness of key features of loop quantum cosmology". Physical Review D 77: 024046. doi:10.1103/PhysRevD.77.024046. 

Further reading

  • Magueijo, João (2003). Faster than the Speed of Light: the Story of a Scientific Speculation. Cambridge, MA: Perseus Publishing. ISBN 0738205257. 
  • Bojowald, Martin. "Follow the Bouncing Universe". Scientific American (October 2008): 44–51. 

External links

Cyclic model

Physical cosmology
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Universe · Big Bang
Age of the universe
Timeline of the Big Bang
Ultimate fate of the universe
A cyclic model is any of several cosmological models in which the universe follows infinite, self-sustaining cycles. For example, the oscillating universe theory briefly considered by Albert Einstein in 1930 theorized a universe following an eternal series of oscillations, each beginning with a big bang and ending with a big crunch; in the interim, the universe would expand for a period of time before the gravitational attraction of matter causes it to collapse back in and undergo a bounce.



In the 1930s, theoretical physicists, most notably Albert Einstein, considered the possibility of a cyclic model for the universe as an (everlasting) alternative to the model of an expanding universe. However, work by Richard C. Tolman in 1934 showed that these early attempts failed because of the entropy problem that, in statistical mechanics, entropy only increases because of the Second law of thermodynamics.[1] This implies that successive cycles grow longer and larger. Extrapolating back in time, cycles before the present one become shorter and smaller culminating again in a Big Bang and thus not replacing it. This puzzling situation remained for many decades until the early 21st century when the recently discovered dark energy component provided new hope for a consistent cyclic cosmology.[2]

One new cyclic model is a brane cosmology model of the creation of the universe, derived from the earlier ekpyrotic model. It was proposed in 2001 by Paul Steinhardt of Princeton University and Neil Turok of Cambridge University. The theory describes a universe exploding into existence not just once, but repeatedly over time.[3][4] The theory could potentially explain why a mysterious repulsive form of energy known as the "cosmological constant", and which is accelerating the expansion of the universe, is several orders of magnitude smaller than predicted by the standard Big Bang model.

A different cyclic model relying on the notion of phantom energy was proposed in 2007 by Lauris Baum and Paul Frampton of the University of North Carolina at Chapel Hill.[5]

The Steinhardt–Turok model

In this cyclic model, two parallel orbifold planes or M-branes collide periodically in a higher dimensional space.[6] The visible four-dimensional universe lies on one of these branes. The collisions correspond to a reversal from contraction to expansion, or a big crunch followed immediately by a big bang. The matter and radiation we see today were generated during the most recent collision in a pattern dictated by quantum fluctuations created before the branes. Eventually, the universe reached the state we observe today, before beginning to contract again many billions of years in the future. Dark energy corresponds to a force between the branes, and serves the crucial role of solving the monopole, horizon, and flatness problems. Moreover the cycles can continue indefinitely into the past and the future, and the solution is an attractor, so it can provide a complete history of the universe.
As Richard C. Tolman showed, the earlier cyclic model failed because the universe would undergo inevitable thermodynamic heat death.[1] However, the newer cyclic model evades this by having a net expansion each cycle, preventing entropy from building up. However, there are major problems with the model. Foremost among them is that colliding branes are not understood by string theorists, and nobody knows if the scale invariant spectrum will be destroyed by the big crunch. Moreover, like cosmic inflation, while the general character of the forces (in the ekpyrotic scenario, a force between branes) required to create the vacuum fluctuations is known, there is no candidate from particle physics. [7]

The Baum–Frampton model

This more recent cyclic model of 2007 makes a different technical assumption concerning the equation of state of the dark energy which relates pressure and density through a parameter w.[5][8] It assumes w < -1 (a condition called phantom energy) throughout a cycle, including at present. (By contrast, Steinhardt-Turok assume w is never less than -1.) In the Baum-Frampton model, a septillionth (or less) of a second before the would-be Big Rip, a turnaround occurs and only one causal patch is retained as our universe. The generic patch contains no quark, lepton or force carrier; only dark energy - and its entropy thereby vanishes. The adiabatic process of contraction of this much smaller universe takes place with constant vanishing entropy and with no matter including no black holes which disintegrated before turnaround. The idea that the universe "comes back empty" is a central new idea of this cyclic model, and avoids many difficulties confronting matter in a contracting phase such as excessive structure formation, proliferation and expansion of black holes, as well as going through phase transitions such as those of QCD and electroweak symmetry restoration. Any of these would tend strongly to produce an unwanted premature bounce, simply to avoid violation of the second law of thermodynamics. The surprising w < -1 condition may be logically inevitable in a truly infinitely cyclic cosmology because of the entropy problem. Nevertheless, many technical back up calculations are necessary to confirm consistency of the approach. Although the model borrows ideas from string theory, it is not necessarily committed to strings, or to higher dimensions, yet such speculative devices may provide the most expeditious methods to investigate the internal consistency. The value of w in the Baum-Frampton model can be made arbitrarily close to, but must be less than, -1.


  1. ^ a b R.C. Tolman (1987) [1934]. Relativity, Thermodynamics, and Cosmology. New York: Dover. LCCN 34032023-{{{3}}}. ISBN 0486653838. 
  2. ^ P.H. Frampton (2006). "On Cyclic Universes". arΧiv:astro-ph/0612243 [astro-ph]. 
  3. ^ P.J. Steinhardt, N. Turok (2001). "Cosmic Evolution in a Cyclic Universe". arΧiv:hep-th/0111098 [hep-th]. 
  4. ^ P.J. Steinhardt, N. Turok (2001). "A Cyclic Model of the Universe". arΧiv:hep-th/0111030 [hep-th]. 
  5. ^ a b L. Baum, P.H. Frampton (2007). "Entropy of Contracting Universe in Cyclic Cosmology". arΧiv:hep-th/0703162 [hep-th]. 
  6. ^ P.J. Steinhardt, N. Turok (2004). "The Cyclic Model Simplified". arΧiv:astro-ph/0404480 [astro-ph]. 
  7. ^ P. Woit (2006). Not Even Wrong. London: Random House. ISBN 97800994488644. 
  8. ^ L. Baum and P.H. Frampton (2007). "Turnaround in Cyclic Cosmology". Physical Review Letters 98 (7): 071301. doi:10.1103/PhysRevLett.98.071301. arXiv:hep-th/0610213. PMID 17359014. 

See also

Further reading

  • P.J. Steinhardt, N. Turok (2007). Endless Universe. New York: Doubleday. ISBN 9780385509640. 
  • R.C. Tolman (1987) [1934]. Relativity, Thermodynamics, and Cosmology. New York: Dover. LCCN 34032023-{{{3}}}. ISBN 0486653838. 
  • L. Baum and P.H. Frampton (2007). "Turnaround in Cyclic Cosmology". Physical Review Letters 98 (7): 071301. doi:10.1103/PhysRevLett.98.071301. arXiv:hep-th/0610213. PMID 17359014. 
  • R. H. Dicke, P. J. E. Peebles, P. G. Roll and D. T. Wilkinson, "Cosmic Black-Body Radiation," Astrophysical Journal 142 (1965), 414. This paper discussed the oscillatory universe as one of the main cosmological possibilities of the time.
  • S. W. Hawking and G. F. R. Ellis, The large-scale structure of space-time (Cambridge, 1973).

External links

Physical Cosmology

Physical cosmology

Physical cosmology
WMAP 2010.png
Universe · Big Bang
Age of the universe
Timeline of the Big Bang
Ultimate fate of the universe
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution.[1] For most of human history, it was a branch of metaphysics and religion. Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics, which first allowed us to understand those laws.

Physical cosmology, as it is now understood, began with the twentieth century development of Albert Einstein's general theory of relativity and better astronomical observations of extremely distant objects. These advances made it possible to speculate about the origin of the universe, and allowed scientists to establish the Big Bang Theory as the leading cosmological model. Some researchers still advocate a handful of alternative cosmologies; however, cosmologists generally agree that the Big Bang theory best explains observations.

Cosmology draws heavily on the work of many disparate areas of research in physics. Areas relevant to cosmology include particle physics experiments and theory, including string theory, astrophysics, general relativity, and plasma physics. Thus, cosmology unites the physics of the largest structures in the universe with the physics of the smallest structures in the universe.


History of physical cosmology

Modern cosmology developed along tandem observational and theoretical tracks. In 1915, Albert Einstein developed his theory of general relativity. At the time, physicists believed in a perfectly static universe without beginning or end. Einstein added a cosmological constant to his theory to try to force it to allow for a static universe with matter in it. The so-called Einstein universe is, however, unstable. It is bound to eventually start expanding or contracting. The cosmological solutions of general relativity were found by Alexander Friedmann, whose equations describe the Friedmann-Lemaître-Robertson-Walker universe, which may expand or contract.

In the 1910s, Vesto Slipher (and later Carl Wilhelm Wirtz) interpreted the red shift of spiral nebulae as a Doppler shift that indicated they were receding from Earth. However, it is difficult to determine the distance to astronomical objects. One way is to compare the physical size of an object to its angular size, but a physical size must be assumed to do this. Another method is to measure the brightness of an object and assume an intrinsic luminosity, from which the distance may be determined using the inverse square law. Due to the difficulty of using these methods, they did not realize that the nebulae were actually galaxies outside our own Milky Way, nor did they speculate about the cosmological implications. In 1927, the Belgian Roman Catholic priest Georges Lemaître independently derived the Friedmann-Lemaître-Robertson-Walker equations and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"—which was later called the Big Bang. In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of the brightness of Cepheid variable stars. He discovered a relationship between the redshift of a galaxy and its distance. He interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds directly proportional to their distance. This fact is now known as Hubble's law, though the numerical factor Hubble found relating recessional velocity and distance was off by a factor of ten, due to not knowing at the time about different types of Cepheid variables.
Given the cosmological principle, Hubble's law suggested that the universe was expanding. There were two primary explanations put forth for the expansion of the universe. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other possibility was Fred Hoyle's steady state model in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly the same at any point in time.

For a number of years the support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. The discovery of the cosmic microwave background in 1965 lent strong support to the Big Bang model, and since the precise measurements of the cosmic microwave background by the Cosmic Background Explorer in the early 1990s, few cosmologists have seriously proposed other theories of the origin and evolution of the cosmos. One consequence of this is that in standard general relativity, the universe began with a singularity, as demonstrated by Stephen Hawking and Roger Penrose in the 1960s.

History of the Universe

The history of the universe is a central issue in cosmology. The history of the universe is divided into different periods called epochs, according to the dominant forces and processes in each period. The standard cosmological model is known as the ΛCDM model.

Equations of motion

The equations of motion governing the universe as a whole are derived from general relativity with a small, positive cosmological constant. The solution is an expanding universe; due to this expansion the radiation and matter in the universe are cooled down and become diluted. At first, the expansion is slowed down by gravitation due to the radiation and matter content of the universe. However, as these become diluted, the cosmological constant becomes more dominant and the expansion of the universe starts to accelerate rather than decelerate. In our universe this has already happened, billions of years ago.

Particle physics in cosmology

Particle physics is important to the behavior of the early universe, since the early universe was so hot that the average energy density was very high. Because of this, scattering processes and decay of unstable particles are important in cosmology.

As a rule of thumb, a scattering or a decay process is cosmologically important in a certain cosmological epoch if the time scale describing that process is smaller or comparable to the time scale of the expansion of the universe, which is 1 / H with H being the Hubble constant at that time. This is roughly equal to the age of the universe at that time.

Timeline of the Big Bang

Observations suggest that the universe began around 13.7 billion years ago. Since then, the evolution of the universe has passed through three phases. The very early universe, which is still poorly understood, was the split second in which the universe was so hot that particles had energies higher than those currently accessible in particle accelerators on Earth. Therefore, while the basic features of this epoch have been worked out in the Big Bang theory, the details are largely based on educated guesses. Following this, in the early universe, the evolution of the universe proceeded according to known high energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the cosmic microwave background was emitted. Finally, the epoch of structure formation began, when matter started to aggregate into the first stars and quasars, and ultimately galaxies, clusters of galaxies and superclusters formed. The future of the universe is not yet firmly known, but according to the ΛCDM model it will continue expanding forever.

Areas of study

Below, some of the most active areas of inquiry in cosmology are described, in roughly chronological order. This does not include all of the Big Bang cosmology, which is presented in Timeline of the Big Bang.

The very early universe

While the early, hot universe appears to be well explained by the Big Bang from roughly 10−33 seconds onwards, there are several problems. One is that there is no compelling reason, using current particle physics, to expect the universe to be flat, homogeneous and isotropic (see the cosmological principle). Moreover, grand unified theories of particle physics suggest that there should be magnetic monopoles in the universe, which have not been found. These problems are resolved by a brief period of cosmic inflation, which drives the universe to flatness, smooths out anisotropies and inhomogeneities to the observed level, and exponentially dilutes the monopoles. The physical model behind cosmic inflation is extremely simple, however it has not yet been confirmed by particle physics, and there are difficult problems reconciling inflation and quantum field theory. Some cosmologists think that string theory and brane cosmology will provide an alternative to inflation.

Another major problem in cosmology is what caused the universe to contain more particles than antiparticles. Cosmologists can observationally deduce that the universe is not split into regions of matter and antimatter. If it were, there would be X-rays and gamma rays produced as a result of annihilation, but this is not observed. This problem is called the baryon asymmetry, and the theory to describe the resolution is called baryogenesis. The theory of baryogenesis was worked out by Andrei Sakharov in 1967, and requires a violation of the particle physics symmetry, called CP-symmetry, between matter and antimatter. Particle accelerators, however, measure too small a violation of CP-symmetry to account for the baryon asymmetry. Cosmologists and particle physicists are trying to find additional violations of the CP-symmetry in the early universe that might account for the baryon asymmetry.

Both the problems of baryogenesis and cosmic inflation are very closely related to particle physics, and their resolution might come from high energy theory and experiment, rather than through observations of the universe.

Big bang nucleosynthesis

Big Bang Nucleosynthesis is the theory of the formation of the elements in the early universe. It finished when the universe was about three minutes old and its temperature dropped below that at which nuclear fusion could occur. Big Bang nucleosynthesis had a brief period during which it could operate, so only the very lightest elements were produced. Starting from hydrogen ions (protons), it principally produced deuterium, helium-4 and lithium. Other elements were produced in only trace abundances. The basic theory of nucleosynthesis was developed in 1948 by George Gamow, Ralph Asher Alpher and Robert Herman. It was used for many years as a probe of physics at the time of the Big Bang, as the theory of Big Bang nucleosynthesis connects the abundances of primordial light elements with the features of the early universe. Specifically, it can be used to test the equivalence principle, to probe dark matter, and test neutrino physics. Some cosmologists have proposed that Big Bang nucleosynthesis suggests there is a fourth "sterile" species of neutrino.

Cosmic microwave background

The cosmic microwave background is radiation left over from decoupling after the epoch of recombination when neutral atoms first formed. At this point, radiation produced in the Big Bang stopped Thomson scattering from charged ions. The radiation, first observed in 1965 by Arno Penzias and Robert Woodrow Wilson, has a perfect thermal black-body spectrum. It has a temperature of 2.7 kelvins today and is isotropic to one part in 105. Cosmological perturbation theory, which describes the evolution of slight inhomogeneities in the early universe, has allowed cosmologists to precisely calculate the angular power spectrum of the radiation, and it has been measured by the recent satellite experiments (COBE and WMAP) and many ground and balloon-based experiments (such as Degree Angular Scale Interferometer, Cosmic Background Imager, and Boomerang). One of the goals of these efforts is to measure the basic parameters of the Lambda-CDM model with increasing accuracy, as well as to test the predictions of the Big Bang model and look for new physics. The recent measurements made by WMAP, for example, have placed limits on the neutrino masses.

Newer experiments, such as QUIET and the Atacama Cosmology Telescope, are trying to measure the polarization of the cosmic microwave background. These measurements are expected to provide further confirmation of the theory as well as information about cosmic inflation, and the so-called secondary anisotropies, such as the Sunyaev-Zel'dovich effect and Sachs-Wolfe effect, which are caused by interaction between galaxies and clusters with the cosmic microwave background.

Formation and evolution of large-scale structure

Understanding the formation and evolution of the largest and earliest structures (i.e., quasars, galaxies, clusters and superclusters) is one of the largest efforts in cosmology. Cosmologists study a model of hierarchical structure formation in which structures form from the bottom up, with smaller objects forming first, while the largest objects, such as superclusters, are still assembling. One way to study structure in the universe is to survey the visible galaxies, in order to construct a three-dimensional picture of the galaxies in the universe and measure the matter power spectrum. This is the approach of the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey.

Another tool for understanding structure formation is simulations, which cosmologists use to study the gravitational aggregation of matter in the universe, as it clusters into filaments, superclusters and voids. Most simulations contain only non-baryonic cold dark matter, which should suffice to understand the universe on the largest scales, as there is much more dark matter in the universe than visible, baryonic matter. More advanced simulations are starting to include baryons and study the formation of individual galaxies. Cosmologists study these simulations to see if they agree with the galaxy surveys, and to understand any discrepancy.

Other, complementary observations to measure the distribution of matter in the distant universe and to probe reionization include:
  • The Lyman alpha forest, which allows cosmologists to measure the distribution of neutral atomic hydrogen gas in the early universe, by measuring the absorption of light from distant quasars by the gas.
  • The 21 centimeter absorption line of neutral atomic hydrogen also provides a sensitive test of cosmology
  • Weak lensing, the distortion of a distant image by gravitational lensing due to dark matter.
These will help cosmologists settle the question of when and how structure formed in the universe.

Dark matter

Evidence from Big Bang nucleosynthesis, the cosmic microwave background and structure formation suggests that about 23% of the mass of the universe consists of non-baryonic dark matter, whereas only 4% consists of visible, baryonic matter. The gravitational effects of dark matter are well understood, as it behaves like a cold, non-radiative fluid that forms haloes around galaxies. Dark matter has never been detected in the laboratory, and the particle physics nature of dark matter remains completely unknown. Without observational constraints, there are a number of candidates, such as a stable supersymmetric particle, a weakly interacting massive particle, an axion, and a massive compact halo object. Alternatives to the dark matter hypothesis include a modification of gravity at small accelerations (MOND) or an effect from brane cosmology.

Dark energy

If the universe is flat, there must be an additional component making up 73% (in addition to the 23% dark matter and 4% baryons) of the energy density of the universe. This is called dark energy. In order not to interfere with Big Bang nucleosynthesis and the cosmic microwave background, it must not cluster in haloes like baryons and dark matter. There is strong observational evidence for dark energy, as the total energy density of the universe is known through constraints on the flatness of the universe, but the amount of clustering matter is tightly measured, and is much less than this. The case for dark energy was strengthened in 1999, when measurements demonstrated that the expansion of the universe has begun to gradually accelerate.

Apart from its density and its clustering properties, nothing is known about dark energy. Quantum field theory predicts a cosmological constant much like dark energy, but 120 orders of magnitude larger than that observed. Steven Weinberg and a number of string theorists (see string landscape) have used this as evidence for the anthropic principle, which suggests that the cosmological constant is so small because life (and thus physicists, to make observations) cannot exist in a universe with a large cosmological constant, but many people find this an unsatisfying explanation. Other possible explanations for dark energy include quintessence or a modification of gravity on the largest scales. The effect on cosmology of the dark energy that these models describe is given by the dark energy's equation of state, which varies depending upon the theory. The nature of dark energy is one of the most challenging problems in cosmology.

A better understanding of dark energy is likely to solve the problem of the ultimate fate of the universe. In the current cosmological epoch, the accelerated expansion due to dark energy is preventing structures larger than superclusters from forming. It is not known whether the acceleration will continue indefinitely, perhaps even increasing until a big rip, or whether it will eventually reverse.

Other areas of inquiry

Cosmologists also study:

See also


  1. ^ For an overview, see George FR Ellis (2006). "Issues in the Philosophy of Cosmology". In Jeremy Butterfield & John Earman. Philosophy of Physics (Handbook of the Philosophy of Science) 3 volume set. North Holland. pp. 1183ff. ISBN 0444515607. 

Further reading



  • Cheng, Ta-Pei (2005). Relativity, Gravitation and Cosmology: a Basic Introduction. Oxford and New York: Oxford University Press. ISBN 0-19-852957-0.  Introductory cosmology and general relativity without the full tensor apparatus, deferred until the last part of the book.
  • Dodelson, Scott (2003). Modern Cosmology. Academic Press. ISBN 0-12-219141-2.  An introductory text, released slightly before the WMAP results.
  • Grøn, Øyvind; Hervik, Sigbjørn (2007). Einstein's General Theory of Relativity with Modern Applications in Cosmology. New York: Springer. ISBN 978-0-387-69199-2. 
  • Harrison, Edward (2000). Cosmology: the science of the universe. Cambridge University Press. ISBN 0-521-66148-X.  For undergraduates; mathematically gentle with a strong historical focus.
  • Kutner, Marc (2003). Astronomy: A Physical Perspective. Cambridge University Press. ISBN 0-521-52927-1.  An introductory astronomy text.
  • Kolb, Edward; Michael Turner (1988). The Early Universe. Addison-Wesley. ISBN 0-201-11604-9.  The classic reference for researchers.
  • Liddle, Andrew (2003). An Introduction to Modern Cosmology. John Wiley. ISBN 0-470-84835-9.  Cosmology without general relativity.
  • Liddle, Andrew; David Lyth (2000). Cosmological Inflation and Large-Scale Structure. Cambridge. ISBN 0-521-57598-2.  An introduction to cosmology with a thorough discussion of inflation.
  • Mukhanov, Viatcheslav (2005). Physical Foundations of Cosmology. Cambridge University Press. ISBN 0-521-56398-4. 
  • Padmanabhan, T. (1993). Structure formation in the universe. Cambridge University Press. ISBN 0-521-42486-0.  Discusses the formation of large-scale structures in detail.
  • Peacock, John (1998). Cosmological Physics. Cambridge University Press. ISBN 0-521-42270-1.  An introduction including more on general relativity and quantum field theory than most.
  • Peebles, P. J. E. (1993). Principles of Physical Cosmology. Princeton University Press. ISBN 0-691-01933-9.  Strong historical focus.
  • Peebles, P. J. E. (1980). The Large-Scale Structure of the Universe. Princeton University Press. ISBN 0-691-08240-5.  The classic work on large scale structure and correlation functions.
  • Rees, Martin (2002). New Perspectives in Astrophysical Cosmology. Cambridge University Press. ISBN 0-521-64544-1. 
  • Weinberg, Steven (1971). Gravitation and Cosmology. John Wiley. ISBN 0-471-92567-5.  A standard reference for the mathematical formalism.
  • Weinberg, Steven (2008). Cosmology. Oxford University Press. ISBN 0198526822. 
  • Benjamin Gal-Or, “Cosmology, Physics and Philosophy”, Springer Verlag, 1981, 1983, 1987, ISBN 0-387-90581-2, ISBN 0387965262.

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