Saturday, 14 July 2007

Dark Energy and Dark Matter

Dark Energy and Dark Matter:

The Results of Flawed Physics?



Dark Energy, Dark Matter 



In the early 1990's, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the Universe had to slow. The Universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the Universe was actually expanding more slowly than it is today. So the expansion of the Universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.

Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.

Author by: 

Edited By:

Arip Nurahman

Dark matter and dark energy are two of the most vexing problems in science today. Together they dominate the universe, comprising some 96 percent of all mass and energy.

But nobody knows what either is. It's tempting to consider them products of the same unknown phenomenon, something theorist Robert Scherrer suggests. The professor of physics at Vanderbilt University says "k-essence" is behind it all.

Dark matter was invoked decades ago to explain why galaxies hold together. Given regular matter alone, galaxies might never have formed, and today they would fly apart. So there must be some unknown stuff that forms invisible clumps to act as gravitational glue.

Dark energy hit the scene in the late 1990s when astronomers discovered the universe is not just expanding, but racing out at an ever-faster pace. Some hidden force, a sort of anti-gravity, must be pushing galaxies apart from one another in this accelerated expansion.
Separate theories have been devised to try and solve each mystery.

To explain dark energy, for example, theorists have re-employed a "cosmological constant" that Einstein first introduced as a fudge factor to balance the force of gravity. Einstein called the cosmological constant a great blunder and retracted it. Yet many theorists now are comfortable re-employing it to account for the effects of dark energy. But it does not reveal what the force is.
Scherrer agrees two explanations might be necessary, but he's also bothered by that complexity.

Best fit theoretical rotation curves superimposed on data (dotted lines) from galaxy “NGC 4455” (left) and galaxy “NGC 5023” (right). The solid line is the curve predicted by the new gravity model. Also shown are the Newtonian curve (short dashes) and the Newtonian curve corrected for dark matter (long dashes). There are few scientific concepts as intriguing and mysterious as dark energy and dark matter, said to make up as much as 95 percent of all the energy and matter in the universe. And even though scientists don't know what either is and have little evidence to prove they exist, dark energy and dark matter are two of the biggest research problems in physics.

This is what three Italian physicists have recently asked. In a paper in the August 3 online edition of the Institute of Physics' peer-reviewed Journal of Cosmology and Astroparticle Physics, they put forth the idea that scientists were forced to propose the existence of dark energy and dark matter because they were, and still are, working with incorrect gravitational theory.

The group suggests an alternative theory of gravity in which dark energy and dark matter are effects – illusions, in a sense – created by the curvature of spacetime (the bending of space and time caused by extremely massive objects, like galaxies). Their theory does not require the existence of dark energy and dark matter.

“Our proposal implies that the 'correct' theory of gravity may be one based solely on directly observed astronomical data,” said lead author Salvatore Capozziello, a theoretical physicist at the University of Naples, to

Dark energy and dark matter were originally conceived to explain, respectively, the accelerating expansion of the universe (despite the tendency of gravity to push matter together) and the discrepancy between the amount of matter scientists expect to observe in the universe but have not yet found. Astronomers suggested the existence of dark matter when they noticed something odd about spiral galaxies: Stars at the middle and edge of a spiral galaxy rotate just as fast as stars near the very center. But according to Newtonian mechanics (the physics of bodies in motion), stars further away from the galactic center should rotate more slowly. Scientists thus assumed that some sort of “dark” matter, not observable by emitted light, must be boosting the total gravity of the galaxy, giving the stars extra rotational speed.

“We can show that no 'exotic' ingredients have to be added to fill the gap between theory and observations,” said Capozziello.

In their paper, he and his co-authors demonstrate this using data from 15 well-studied galaxies. Among this data was each galaxy's “rotation curve,” a graph that plots the rotational speed of the stars in the galaxy as a function of their distance from the galaxy's center. These curves were successfully fit to curves produced using the new theory. Since these 15 galaxies are believed to be dominated by dark matter, fitting their rotation curves using this new gravity model is strong evidence to support an alternative theory of gravity.

Despite this, the notion that dark matter and dark energy are “wrong” is potentially very unpopular. Capozziello and his colleagues are aware that a new theory of gravity impacts the dynamics of the universe as scientists now understand them.

“Any extended theories of gravity must be tested on all the astrophysical scales, ranging from the Solar System to galaxies to galaxy clusters, and all of cosmology,” said Capozziello. “Performing these tests is the cornerstone of our research program.”

See also



  1. ^ P. J. E. Peebles and Bharat Ratra (2003). "The cosmological constant and dark energy". Reviews of Modern Physics 75 (2): 559–606. arXiv:astro-ph/0207347. Bibcode 2003RvMP...75..559P. doi:10.1103/RevModPhys.75.559. 
  2. ^ a b "Seven-Year Wilson Microwave Anisotropy Probe (WMAP) Observations: Sky Maps, Systematic Errors, and Basic Results" (PDF). Retrieved 2010-12-02.  (see p. 39 for a table of best estimates for various cosmological parameters)
  3. ^ a b Sean Carroll (2001). "The cosmological constant". Living Reviews in Relativity 4. Retrieved 2006-09-28. 
  4. ^ L.Baum and P.H. Frampton (2007). "Turnaround in Cyclic Cosmology". Physical Review Letters 98 (7): 071301. arXiv:hep-th/0610213. Bibcode 2007PhRvL..98g1301B. doi:10.1103/PhysRevLett.98.071301. PMID 17359014. 
  5. ^ a b Adam G. Riess et al. (Supernova Search Team) (1998). "Observational evidence from supernovae for an accelerating universe and a cosmological constant". Astronomical J. 116 (3): 1009–38. arXiv:astro-ph/9805201. Bibcode 1998AJ....116.1009R. doi:10.1086/300499. 
  6. ^ a b S. Perlmutter et al. (The Supernova Cosmology Project) (1999). "Measurements of Omega and Lambda from 42 high redshift supernovae". Astrophysical J. 517 (2): 565–86. arXiv:astro-ph/9812133. Bibcode 1999ApJ...517..565P. doi:10.1086/307221. 
  7. ^ The first paper, using observed data, which claimed a positive Lambda term was G. Paal et al. (1992). "Inflation and compactification from galaxy redshifts?". ApSS 191: 107–24. Bibcode 1992Ap&SS.191..107P. doi:10.1007/BF00644200. 
  8. ^ "The Nobel Prize in Physics 2011". Nobel Foundation. Retrieved 2011-10-04. 
  9. ^ The Nobel Prize in Physics 2011. Perlmutter got half the prize, and the other half was shared between Schmidt and Riess.
  10. ^ a b D. N. Spergel et al. (WMAP collaboration) (March 2006). Wilkinson Microwave Anisotropy Probe (WMAP) three year results: implications for cosmology.

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