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The Big Bang: Dark Energy

A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the metric seen on the left. Image from WMAP press release, 2006. (Detail) A graphical representation of the expansion of the universe with the inflationary epoch represented as the dramatic expansion of the metric seen on the left. Image from WMAP press release, 2006.

Continuation From Big Bang

Measurements of the redshift-magnitude relation for type Ia supernovae have revealed that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed "dark energy". Dark energy is indicated by several other lines of evidence. Measurements of the cosmic microwave background indicate that the universe is very nearly spatially flat, and therefore according to general relativity the universe must have almost exactly the critical density of mass/energy. But the mass density of the universe can be measured from its gravitational clustering, and is found to have only about 30% of the critical density. Since dark energy does not cluster in the usual way it is the best explanation for the "missing" energy density. Dark energy is also required by two geometrical measures of the overall curvature of the universe, one using the frequency of gravitational lenses, and the other using the characteristic pattern of the large-scale structure as a cosmic ruler.

Negative pressure is a property of vacuum energy, but the exact nature of dark energy remains one of the great mysteries of the Big Bang. Possible candidates include a cosmological constant and quintessence. Results from the WMAP team in 2006, which combined data from the CMB and other sources, indicate that the universe today is 74% dark energy, 22% dark matter, and 4% regular matter. The energy density in matter decreases with the expansion of the universe, but the dark energy density remains constant (or nearly so) as the universe expands. Therefore matter made up a larger fraction of the total energy of the universe in the past than it does today, but its fractional contribution will fall in the far future as dark energy becomes even more dominant.

In the Lambda-CDM model, the best current model of the Big Bang, dark energy is explained by the presence of a cosmological constant in the theory of General Relativity. However, the size of the constant that properly explains dark energy is surprisingly small relative to naive estimates based on ideas about quantum gravity. Distinguishing between the cosmological constant and other explanations of dark energy is an active area of current research.

Before observations of dark energy, cosmologists considered two scenarios for the future of the universe. If the mass density of the universe were greater than the critical density, then the universe would reach a maximum size and then begin to collapse. It would become denser and hotter again, ending with a state that was similar to that in which it started-a Big Crunch. Alternatively, if the density in the universe were equal to or below the critical density, the expansion would slow down, but never stop. Star formation would cease as all the interstellar gas in each galaxy is consumed; stars would burn out leaving white dwarfs, neutron stars, and black holes. Very gradually, collisions between these would result in mass accumulating into larger and larger black holes. The average temperature of the universe would asymptotically approach absolute zero-a Big Freeze. Moreover, if the proton were unstable, then baryonic matter would disappear, leaving only radiation and black holes. Eventually, black holes would evaporate. The entropy of the universe would increase to the point where no organized form of energy could be extracted from it, a scenario known as heat death.

Modern observations of accelerated expansion imply that more and more of the currently visible universe will pass beyond our event horizon and out of contact with us. The eventual result is not known. The ΛCDM model of the universe contains dark energy in the form of a cosmological constant. This theory suggests that only gravitationally bound systems, such as galaxies, would remain together, and they too would be subject to heat death, as the universe expands and cools. Other explanations of dark energy-so-called phantom energy theories-suggest that ultimately galaxy clusters, stars, planets, atoms, nuclei and matter itself will be torn apart by the ever-increasing expansion in a so-called Big Rip.

While the Big Bang model is well established in cosmology, it is likely to be refined in the future. Little is known about the earliest moments of the universe's history. The Penrose-Hawking singularity theorems require the existence of a singularity at the beginning of cosmic time. However, these theorems assume that general relativity is correct, but general relativity must break down before the universe reaches the Planck temperature, and a correct treatment of quantum gravity may avoid the singularity.

There may also be parts of the universe well beyond what can be observed in principle. If inflation occurred this is likely, for exponential expansion would push large regions of space beyond our observable horizon.

Some proposals, each of which entails untested hypotheses, are:

  • models including the Hartle-Hawking boundary condition in which the whole of space-time is finite; the Big Bang does represent the limit of time, but without the need for a singularity.
  • brane cosmology models in which inflation is due to the movement of branes in string theory; the pre-big bang model; the ekpyrotic model, in which the Big Bang is the result of a collision between branes; and the cyclic model, a variant of the ekpyrotic model in which collisions occur periodically.
  • chaotic inflation, in which inflation events start here and there in a random quantum-gravity foam, each leading to a bubble universe expanding from its own big bang.

Proposals in the last two categories see the Big Bang as an event in a much larger and older universe, or multiverse, and not the literal beginning.

The Big Bang is a scientific theory, and as such stands or falls by its agreement with observations. But as a theory which addresses, or at least seems to address, the origins of reality, it has always been entangled with theological and philosophical implications. In the 1920s and '30s almost every major cosmologist preferred an eternal universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of the steady state theory. This perception was enhanced by the fact that Georges Lemaître, who put the theory forth, was a Roman Catholic priest.

Notes and references

  1. ^ "Even though the Universe has been expanding and cooling ever since, the sound waves have left their imprint as temperature variations on the afterglow of the big bang fireball..." Chown, Marcus (30 October 2003). "Big Bang sounded like a deep hum". New Scientist
  2. ^ Slipher, V. M.. "The radial velocity of the Andromeda nebula". Lowell Observatory Bulletin 1: 56-57. 
    Slipher, V. M.. "Spectrographic observations of nebulae". Popular Astronomy 23: 21-24. 
  3. ^ Friedman, A (1922). "Über die Krümmung des Raumes". Z. Phys. 10: 377-386.  (German) (English translation in: Friedman, A (1999). "On the Curvature of Space". General Relativity and Gravitation 31: 1991-2000. doi:10.1023/A:1026751225741. )
  4. ^ Lemaître, G. (1927). "Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extragalactiques". Annals of the Scientific Society of Brussels 47A: 41.  (French) Translated in: (1931) "Expansion of the universe, A homogeneous universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae". Monthly Notices of the Royal Astronomical Society 91: 483-490. 
  5. ^ Lemaître, G. (1931). "The evolution of the universe: discussion". Nature 128: suppl.: 704. 
  6. ^ a b c Edwin Hubble (1929). "A relation between distance and radial velocity among extra-galactic nebulae". Proceedings of the National Academy of Sciences 15: 168-173. 
  7. ^ E. Christianson (1995). Edwin Hubble: Mariner of the Nebulae. Farrar Straus & Giroux. ISBN 0374146608
  8. ^ a b c P. J. E. Peebles and Bharat Ratra (2003). "The cosmological constant and dark energy". Reviews of Modern Physics 75: 559-606. doi:10.1103/RevModPhys.75.559. arXiv:astro-ph/0207347
  9. ^ E. A. Milne (1935). Relativity, Gravitation and World Structure. Oxford University Press. 
  10. ^ R. C. Tolman (1934). Relativity, Thermodynamics, and Cosmology. Oxford: Clarendon Press. LCCN 340-32023.  Reissued (1987) New York: Dover ISBN 0-486-65383-8.
  11. ^ Zwicky, F (1929). "On the Red Shift of Spectral Lines through Interstellar Space". Proceedings of the National Academy of Sciences 15: 773-779.  Full articlePDF (672 KiB).
  12. ^ Hoyle, Fred (1948). "A New Model for the Expanding universe". Monthly Notices of the Royal Astronomical Society 108: 372. 
  13. ^ R. A. Alpher, H. Bethe, G. Gamow (1948). "The Origin of Chemical Elements". Physical Review 73: 803. 
  14. ^ R. A. Alpher and R. Herman (1948). "Evolution of the Universe". Nature 162: 774. 
  15. ^ Simon Singh. Big Bang. Retrieved on 2007-05-28.
  16. ^ It is popularly reported that Hoyle intended this to be pejorative. However, Hoyle denied that and said it was just a striking image meant to emphasize the difference between the two theories for radio listeners. See chapter 9 of The Alchemy of the Heavens by Ken Croswell, Anchor Books, 1995.
  17. ^ a b A. A. Penzias and R. W. Wilson (1965). "A Measurement of Excess Antenna Temperature at 4080 Mc/s". Astrophysical Journal 142: 419. 
  18. ^ a b Boggess, N.W., et al. (COBE collaboration) (1992). "The COBE Mission: Its Design and Performance Two Years after the launch". Astrophysical Journal 397: 420, Preprint No. 92-02. doi:10.1086/171797
  19. ^ a b c D. N. Spergel et al. (WMAP collaboration) (2006). "Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology". Retrieved on 2007-05-27
  20. ^ S. W. Hawking and G. F. R. Ellis (1973). The large-scale structure of space-time. Cambridge: Cambridge University Press. ISBN 0-521-20016-4
  21. ^ There is no consensus about how long the Big Bang phase lasted: for some writers this denotes only the initial singularity, for others the whole history of the universe. Usually at least the first few minutes, during which helium is synthesised, are said to occur "during the Big Bang".
  22. ^ a b c Spergel, D. N.; et al. (2003). "First-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Determination of Cosmological Parameters". The Astrophysical Journal Supplement Series 148: 175-194. doi:10.1086/377226
  23. ^ Guth, Alan H. (1998). The Inflationary Universe: Quest for a New Theory of Cosmic Origins. Vintage. ISBN 978-0099959502
  24. ^ Schewe, Phil, and Ben Stein (2005). "An Ocean of Quarks". Physics News Update, American Institute of Physics 728 (#1). Retrieved on 2007-05-27
  25. ^ a b Kolb and Turner (1988), chapter 6
  26. ^ Kolb and Turner (1988), chapter 7
  27. ^ a b c Kolb and Turner (1988), chapter 4
  28. ^ Peacock (1999), chapter 9
  29. ^ Ivanchik, A. V.; A. Y. Potekhin and D. A. Varshalovich (1999). "The fine-structure constant: a new observational limit on its cosmological variation and some theoretical consequences". Astronomy and Astrophysics 343: 459. 
  30. ^ Detailed information of and references for tests of general relativity are given at Tests of general relativity.
  31. ^ This ignores the dipole anisotropy at a level of 0.1% due to the peculiar velocity of the solar system through the radiation field.
  32. ^ Goodman, J. (1995). "Geocentrism reexamined". Physical Review D 52: 1821. doi:10.1103/PhysRevD.52.1821
  33. ^ d'Inverno, Ray (1992). Introducing Einstein's Relativity. Oxford: Oxford University Press. ISBN 0-19-859686-3.  Chapter 23
  34. ^ a b Kolb and Turner (1988), chapter 3
  35. ^ Peacock (1999), chapter 3
  36. ^ Astronomers reported their measurement in a paper published in the December 2000 issue of Nature titled The microwave background temperature at the redshift of 2.33771 which can be read here. A press release from the European Southern Observatory explains the findings to the public.
  37. ^ Steigman, Gary. "Primordial Nucleosynthesis: Successes And Challenges". arXiv:astro-ph/0511534
  38. ^ E. Bertschinger (2001). "Cosmological perturbation theory and structure formation". arXiv:astro-ph/0101009
    Edmund Bertschinger (1998). "Simulations of structure formation in the universe". Annual Review of Astronomy and Astrophysics 36: 599-654. 
  39. ^ If inflation is true, baryogenesis must have occurred, but not vice versa.
  40. ^ a b c Kolb and Turner (1988), chapter 8
  41. ^ Strictly, dark energy in the form of a cosmological constant drives the universe towards a flat state; but our universe remained close to flat for several billion years, before the dark energy density became significant.
  42. ^ R. H. Dicke and P. J. E. Peebles. "The big bang cosmology - enigmas and nostrums". S. W. Hawking and W. Israel (eds) General Relativity: an Einstein centenary survey: 504-517, Cambridge University Press. 
  43. ^ A. D., Sakharov (1967). "Violation of CP invariance, C asymmetry and baryon asymmetry of the universe". Pisma Zh. Eksp. Teor. Fiz. 5: 32.  (Russian) Translated in JETP Lett. 5, 24 (1967).
  44. ^ Navabi, A. A.; N. Riazi (2003). "Is the Age Problem Resolved?". Journal of Astrophysics and Astronomy 24: 3. 
  45. ^ Keel, Bill. Galaxies and the Universe lecture notes - Dark Matter. University of Alabama Astronomy. Retrieved on 2007-05-28.
  46. ^ Yao, W. M.; et al. (2006). "Review of Particle Physics". J. Phys. G: Nucl. Part. Phys. 33: 1-1232. doi:10.1088/0954-3899/33/1/001Chapter 22: Dark matterPDF (152 KiB).
  47. ^ (2003) "Phantom Energy and Cosmic Doomsday". Phys. Rev. Lett. 91: 071301. arXiv:astro-ph/0302506
  48. ^ Hawking, Stephen; and Ellis, G. F. R. (1973). The Large Scale Structure of Space-Time. Cambridge: Cambridge University Press. ISBN 0-521-09906-4
  49. ^ J. Hartle and S. W. Hawking (1983). "Wave function of the universe". Phys. Rev. D 28: 2960. 
  50. ^ Langlois, David (2002). "Brane cosmology: an introduction". arXiv:hep-th/0209261
  51. ^ Linde, Andre (2002). "Inflationary Theory versus Ekpyrotic/Cyclic Scenario". arXiv:hep-th/0205259
  52. ^ "Recycled Universe: Theory Could Solve Cosmic Mystery",, 8 May 2006. Retrieved on 2007-07-03
  53. ^ What Happened Before the Big Bang?. Retrieved on 2007-07-03.
  54. ^ A. Linde (1986). "Eternal chaotic inflation". Mod. Phys. Lett. A1
    A. Linde (1986). "Eternally existing self-reproducing chaotic inflationary universe". Phys. Lett. B175
  55. ^ Kragh, Helge (1996). Cosmology and Controversy. Princeton University Press. ISBN 069100546X


  • Kolb, Edward; Michael Turner (1988). The Early Universe. Addison-Wesley. ISBN 0-201-11604-9
  • Peacock, John (1999). Cosmological Physics. Cambridge University Press. ISBN 0521422701


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This article was published on Sunday 11 November, 2007.

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