add and edited by:
By:
Arip Nurahman
Department of Physics
Faculty of Sciences and Mathematics, Indonesia University of Education
and
Follower Open Course Ware at Massachusetts Institute of Technology
Cambridge, USA
Department of Physics
http://web.mit.edu/physics/
http://ocw.mit.edu/OcwWeb/Physics/index.htm
&
Aeronautics and Astronautics Engineering
http://web.mit.edu/aeroastro/www/
http://ocw.mit.edu/OcwWeb/Aeronautics-and-Astronautics/index.htm
Arip Nurahman
Department of Physics
Faculty of Sciences and Mathematics, Indonesia University of Education
and
Follower Open Course Ware at Massachusetts Institute of Technology
Cambridge, USA
Department of Physics
http://web.mit.edu/physics/
http://ocw.mit.edu/OcwWeb/Physics/index.htm
&
Aeronautics and Astronautics Engineering
http://web.mit.edu/aeroastro/www/
http://ocw.mit.edu/OcwWeb/Aeronautics-and-Astronautics/index.htm
In physical cosmology, dark energy is a hypothetical exotic form of energy that permeates all of space and tends to increase the rate of expansion of the universe. Dark energy is the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for 74% of the total mass-energy of the universe.
Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant is physically equivalent to vacuum energy. Scalar fields which do change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.
High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time. In general relativity, the evolution of the expansion rate is parameterized by the cosmological equation of state. Measuring the equation of state of dark energy is one of the biggest efforts in observational cosmology today.
Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model" of cosmology because of its precise agreement with observations. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a cyclic model for the universe.
| Physical cosmology | ||||||||
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| Universe · Big Bang Age of the Universe Timeline of the Big Bang Ultimate fate of the universe
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Contents
- 1 Evidence for dark energy
- 2 Nature of dark energy
- 3 Implications for the fate of the universe
- 4 History
- 5 See also
- 6 References
- 7 Bibliography
- 8 External links
Evidence for dark energy
Supernovae
In 1998, observations of Type Ia supernovae ("one-A") by the Supernova Cosmology Project at the Lawrence Berkeley National Laboratory and the High-z Supernova Search Team suggested that the expansion of the universe is accelerating.
Since then, these observations have been corroborated by several independent sources. Measurements of the cosmic microwave background, gravitational lensing, and the large scale structure of the cosmos as well as improved measurements of supernovae have been consistent with the Lambda-CDM model.
Supernovae are useful for cosmology because they are excellent standard candles across cosmological distances. They allow the expansion history of the Universe to be measured by looking at the relationship between the distance to an object and its redshift, which gives how fast it is receding from us.
The relationship is roughly linear, according to Hubble's law. It is relatively easy to measure redshift, but finding the distance to an object is more difficult. Usually, astronomers use standard candles: objects for which the intrinsic brightness, the absolute magnitude, is known.
This allows the object's distance to be measured from its actually observed brightness, or apparent magnitude. Type Ia supernovae are the best-known standard candles across cosmological distances because of their extreme, and extremely consistent, brightness.
Cosmic Microwave Background
The existence of dark energy, in whatever form, is needed to reconcile the measured geometry of space with the total amount of matter in the universe. Measurements of cosmic microwave background (CMB) anisotropies, most recently by the WMAP satellite, indicate that the universe is very close to flat.
For the shape of the universe to be flat, the mass/energy density of the universe must be equal to a certain critical density. The total amount of matter in the universe (including baryons and dark matter), as measured by the CMB, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy to account for the remaining 70%.
The most recent WMAP observations are consistent with a universe made up of 74% dark energy, 22% dark matter, and 4% ordinary matter.
Large-Scale Structure
The theory of large scale structure, which governs the formation of structure in the universe (stars, quasars, galaxies and galaxy clusters), also suggests that the density of baryonic matter in the universe is only 30% of the critical density.
Late-time Integrated Sachs-Wolfe Effect
Accelerated cosmic expansion causes gravitational potential wells and hills to flatten as photons pass through them, producing cold spots and hot spots on the CMB aligned with vast supervoids and superclusters. This so-called late-time Integrated Sachs-Wolfe effect (ISW) is a direct signal of dark energy in a flat universe, and has recently been detected at high significance by Ho et al. and Giannantonio et al. In May 2008, Granett, Neyrinck & Szapudi found arguably the clearest evidence yet for the ISW effect, imaging the average imprint of superclusters and supervoids on the CMB.
See also
References
- ^ P. J. E. Peebles and Bharat Ratra (2003). "The cosmological constant and dark energy" (subscription required). Reviews of Modern Physics 75: 559–606. doi:. http://www.arxiv.org/abs/astro-ph/0207347.
- ^ Sean Carroll (2001). "The cosmological constant". Living Reviews in Relativity 4: 1. doi:10.1038/nphys815-. http://relativity.livingreviews.org/Articles/lrr-2001-1/index.html. Retrieved on 28 September 2006.
- ^ L.Baum and P.H. Frampton (2007). "Turnaround in Cyclic Cosmology" (subscription required). Physical Review Letters 98: 071301. doi:. http://www.arxiv.org/abs/hep-th/0610213.
- ^ a b S. Perlmutter et al. (The Supernova Cosmology Project) (1999). "Measurements of Omega and Lambda from 42 high redshift supernovae" (subscription required). Astrophysical J. 517: 565–86. doi:. http://www.arxiv.org/abs/astro-ph/9812133.
- ^ a b Adam G. Riess et al. (Supernova Search Team) (1998). "Observational evidence from supernovae for an accelerating universe and a cosmological constant" (subscription required). Astronomical J. 116: 1009–38. doi:. http://www.arxiv.org/abs/astro-ph/9805201.
- ^ a b D. N. Spergel et al. (WMAP collaboration) (March 2006). Wilkinson Microwave Anisotropy Probe (WMAP) three year results: implications for cosmology. http://lambda.gsfc.nasa.gov/product/map/current/map_bibliography.cfm.
- ^ "Looking for Lambda with the Rees-Sciama Effect", Crittenden R.G., & Turok N., 1996, Phys. Rev. Lett., 76, 575
- ^ "Correlation of CMB with large-scale structure: I. ISW Tomography and Cosmological Implications", Ho et al., 2008, Phys Rev. D, submitted
- ^ "Combined analysis of the integrated Sachs-Wolfe effect and cosmological implications", Giannantonio et al., 2008, Phys. Rev. D, in press
- ^ "An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect", Granett, Neyrinck & Szapudi, 2008, ApJL, submitted
- ^ a b c Hogan, Jenny (2007). "Unseen Universe: Welcome to the dark side". Nature 448 (7151): 240–245. doi:.
- ^ Weinberg, Steven (1987). "Anthropic bound on the cosmological constant". Physical Review Letters 59 (22): 2607–2610. doi:.
- ^ A.M. Öztas and M.L. Smith (2006). "Elliptical Solutions to the Standard Cosmology Model with Realistic Values of Matter Density". International Journal of Theoretical Physics 45: 925–936.
- ^ D.J. Schwarz and B. Weinhorst (2007). "(An)isotropy of the Hubble diagram: comparing hemispheres". Astronomy & Astrophysics 474: 717–729. doi:.
- ^ Stephon Alexander, Tirthabir Biswas, Alessio Notari, and Deepak Vaid (2008). "Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae". ArΧiv ePrint. arΧiv:astro-ph/0712.0370v2.
- ^ Gough, Paul (2008). "Information Equation of State" (PDF). Entropy: 150–159. doi:. http://www.mdpi.org/entropy/papers/e10030150.pdf.
- ^ Wiltshire, David L. (2007). "Exact Solution to the Averaging Problem in Cosmology". Phys. Rev. Lett. 99: 251101. doi:.
- ^ [1] arXiv:0708.2943v1 Dark energy as a mirage HIP-2007-64/TH
- ^ The first appearance of the term "dark energy" is in the article with another cosmologist and Turner's student at the time, Dragan Huterer, "Prospects for Probing the Dark Energy via Supernova Distance Measurements", which was posted to the ArXiv.org e-print archive in August 1998 and published in Physical Review D in 1999 (Huterer and Turner, Phys. Rev. D 60, 081301 (1999)), although the manner in which the term is treated there suggests it was already in general use. Cosmologist Saul Perlmutter has credited Turner with coining the term in an article they wrote together with Martin White of the University of Illinois for Physical Review Letters, where it is introduced in quotation marks as if it were a neologism.
- ^ Pierre Astier et al. (Supernova Legacy Survey) (2006). "The Supernova legacy survey: Measurement of omega(m), omega(lambda) and W from the first year data set" (subscription required). Astronomy and Astrophysics 447: 31–48. doi:. http://www.arxiv.org/abs/astro-ph/0510447.
Author By:
Robert Roy Britt
Senior Science Writer
Robert Roy Britt
Senior Science Writer
PHILADELPHIA -- At first there was disbelief. Then widespread befuddlement. Then a period of quantification. Now, five years after discovering that the universe is expanding at an ever-faster pace, scientists know exactly how much mysterious "dark energy" is behind the acceleration and have turned to figuring out what it is.
The task may eat up a lifetime, researchers admit. Or perhaps some new Einstein will figure it out next year in a light-bulb moment. The reward will be a far more complete understanding of the history and fate of the cosmos.
At a meeting of the American Physical Society (APS) here this week, front-line observational astronomers and big-thinking theorists said important clues to one of the most vexing mysteries of modern science are already rolling in. Cautious optimism infused the gathering.
Much important but unpublished data are already collected, SPACE.com has learned, and by the end of this year the history of the universe's expansion could become a bit clearer, further illuminating the path toward understanding dark energy.
Change of pace
Our universe has always been expanding, with practically all galaxies receding from each other, except for those bound in clusters.
The expansion was decelerating until about 6.3 billion years ago, however. Then an important switch to acceleration occurred. Something caused the universe to step on the gas, driving a growth that now speeds up each day. It is like a rocket whose speed increases 100 mph in the first mile, then by the same amount the next half-mile, then in a quarter-mile, and so on.
Scientists admit they've made almost zero progress in understanding dark energy. They have no idea what it is or how it works.
Various researchers describe the phenomenon as a repulsive force, as vacuum energy, as anti-gravity, and as possibly no more than a different manifestation of gravity over large distances. Some say the repulsion could be a response to dark matter, unseen stuff that is known to make up nearly a quarter of the universe, but such a link has never been established.
All that's clear is that dark energy comprises 73 percent of the mass-energy budget of the universe, and that it is no longer an arguable point for theorists but instead is a viable quarry for astronomers.
Surprise finding
While not even trying, the Hubble Space Telescope just spotted two very distant exploding stars that represent baby steps ahead of a footrace of expected observations.
The so-called supernovae, announced this morning, are 5 billion and 8 billion light-years away and were found serendipitously by Hubble's new Advance Camera for Surveys, installed a year ago, while it was making a calibration run. Hubble officials, who have been saying the new camera would turn Hubble into a supernova hunting machine, offered the discoveries as proof of that claim.
The supernovae bracket the presumed time of the switch from deceleration to acceleration, and examination of them has helped build the observational case that the shift occurred. Two other groups have used Hubble to purposely collect data on several more supernovae and results will likely be reported by the end of the year, researchers told SPACE.com.
Hubble is but one tool being used to probe dark energy.
Scientists are also exploring so-called cosmic microwave background radiation that carries an imprint of the baby universe's structure. Just two months ago, that effort led to the first firm determination of the age of the universe, a solid estimate for when the first stars were born, plus undisputed confirmation of dark energy's expansive role.
Other teams are examining the structure of the space through time by noting how interstellar hydrogen absorbs light. Still other researchers are pushing back the frontier of time, finding galaxies that formed when the universe was less than 10 percent its present age.
For the first time in history, experts suggest, cosmological data is accumulating faster than the wild theories that try to describe it all.
"By far the best is yet to come," said Max Tegmark, a cosmologist at the University of Pennsylvania. "We're not even halfway through this avalanche of data in cosmology."
Going back in time
Theorists have known since the 1920s that the universe was expanding. They wondered if that expansion would go on forever or if common gravity might eventually win out and pull everything back together in a sort of Big Crunch.
Then in 1998 two separate groups hunting faraway supernovae found several that were dimmer than they should have been, indicating that the universe is not just expanding, but accelerating.
The supernovae are of a particular variety, known as Type IA, that all shine with the same intrinsic brightness. Astronomers use them as "standard candles," their observed brightness revealing their distance. Light from the objects is analyzed to determine how much the waves have stretched, which bears an exact relationship to how much the universe has expanded since the light left its source -- the exploded star.
The 1998 finding of an accelerating universe was initially met with disbelief by its discoverers. Once digested -- in some cases only in the last couple of years by skeptics -- its profound implications for the composition and fate of the cosmos brought the term dark energy into common use.
Meanwhile, theorists had already figured out that an accelerating universe would necessarily be preceded by a period of deceleration, which would have followed an initial phase of rapid inflation associated with the Big Bang.
Here is why things must have slowed down:
"Early on the universe had lots of mass in a small volume," explains John Blakeslee of Johns Hopkins University. "The pull from gravity must have been enormous." As the universe expanded, gravity would have become less effective over the larger distances, and dark energy would have taken over.
One previously detected supernova, at about 10 billion light years away, supported this idea when reported in 2001, but the object proved difficult to study. The Hubble observations presented today also support the switch, said Blakeslee, lead author of a paper on the findings that will be published in the June Astrophysical Journal.
Had the universe always been accelerating, the supernova that's 8 billion light-years distant would have been dimmer, Blakeslee said in a telephone interview.
"It's not conclusive at all," Blakeslee said of his work. Another 20 or so very distant supernova are needed to make a strong case, he added.
Those 20, and then some, will not take long.
Hubble's new eyesight "should allow astronomers to discover roughly 10 times as many of these cosmic beacons as was possible with Hubble's previous main imaging camera," Blakeslee said.
Already in the bag
A separate Hubble project, led by Adam Riess of the Space Telescope Science Institute, has already bagged several distant supernovae. Riess, who worked on one team that made the 1998 acceleration breakthrough, reported preliminary results of his latest work at the APS meeting and is expected to publish a paper soon, possibly later this year.
Eleven other distant supernovae have been examined by Hubble in another study headed up by Saul Perlmutter of the Lawrence Berkeley National Laboratory. Perlmutter led the other team involved in the 1998 discovery of acceleration.
Perlmutter was not involved in the two new Hubble discoveries, but he told SPACE.com they are among many important steps that could lead to a firm determination of when acceleration began. Many of the discoveries are coming from ground-based telescopes, he noted, but Hubble "is really becoming a key for everybody in terms of follow-up" to glean the necessary detail.
By building a strong historical timeline of the universe, astronomers and cosmologists hope to answer a pressing question: Do the properties of dark energy change over time?
The answer would help them determine what dark energy is and would allow refined predictions about the origin and fate of the universe. No one expects a quick resolution, however.
Michael Turner, one of the world's foremost cosmologists from the University of Chicago, said solving the dark energy problem "is going to require a crazy idea." While most leading theories project the universe will accelerate forever, perhaps even to the wild point that it rips all matter apart, the notion that it might eventually collapse has not been ruled out, Turner said at the APS meeting.
"The destiny question is wide open," Turner said.
Dark energy more evidence for the accelerating universe
Ever since two teams of astronomers announced in 1998 that the expansion of the universe appeared to accelerating with time, other teams of astronomers have been double-checking their conclusions with data from other sources. Although some astronomers still have doubts about the accelerating universe, evidence in its favor continues to grow. The latest comes from studies of the deviation of galaxy velocities from a smooth universal expansion (I Zehavi and A Dekel 1999 Nature 401 252).
The first evidence for the accelerating universe came from observations of distant supernovae. However, the data were also consistent with an open universe - a universe that would expand forever because the total energy density was less than the so-called critical density - with a low mass density and no cosmological constant. The energy density of the universe is composed of matter (both ordinary visible matter and invisible or "dark" matter) and the energy density of the vacuum. The size of the latter, which is sometimes called quintessence or "dark energy", defines the cosmological constant. This constant was first introduced by Einstein to explain why the universe did not appear to be expanding. Hubble later showed that the universe was expanding, causing Einstein to call it his "biggest blunder".
Now Idit Zehavi of the Hebrew University in Jerusalem and Avishai Dekel of Fermilab in the US have studied the "peculiar" velocities of over 4000 galaxies on scales of about 300 million light years. These peculiar velocities are due to variations in the mass distribution of the universe which either slow down or increase the expansion due to the big bang. Zehavi and Dekel find that their data and the supernova data favour an almost flat universe in which the mass density and the cosmological constant are comparable. However, they add that "this type of universe seems to require a degree of fine tuning of the initial conditions that is in apparent conflict with 'common wisdom'". All of which means that the accelerating universe still presents lots of challenges to astronomers.
Closing
Kenyataan dalam observasi memang menuntut adanya materi gelap dan energy gelap dalam upaya penyokongan jagat raya yang seimbang, tapi harus diingat bahwa keduanya ini merupakan sebuah hipotesis yang belum di buktikan secara experimental.
Sementara itu, penelitian terus berlanjut mengenai apa itu dark energy (energy gelap) dengan para astronom muda lainnya
The task may eat up a lifetime, researchers admit. Or perhaps some new Einstein will figure it out next year in a light-bulb moment. The reward will be a far more complete understanding of the history and fate of the cosmos.
At a meeting of the American Physical Society (APS) here this week, front-line observational astronomers and big-thinking theorists said important clues to one of the most vexing mysteries of modern science are already rolling in. Cautious optimism infused the gathering.
Much important but unpublished data are already collected, SPACE.com has learned, and by the end of this year the history of the universe's expansion could become a bit clearer, further illuminating the path toward understanding dark energy.
Change of pace
Our universe has always been expanding, with practically all galaxies receding from each other, except for those bound in clusters.
The expansion was decelerating until about 6.3 billion years ago, however. Then an important switch to acceleration occurred. Something caused the universe to step on the gas, driving a growth that now speeds up each day. It is like a rocket whose speed increases 100 mph in the first mile, then by the same amount the next half-mile, then in a quarter-mile, and so on.
Scientists admit they've made almost zero progress in understanding dark energy. They have no idea what it is or how it works.
Various researchers describe the phenomenon as a repulsive force, as vacuum energy, as anti-gravity, and as possibly no more than a different manifestation of gravity over large distances. Some say the repulsion could be a response to dark matter, unseen stuff that is known to make up nearly a quarter of the universe, but such a link has never been established.
All that's clear is that dark energy comprises 73 percent of the mass-energy budget of the universe, and that it is no longer an arguable point for theorists but instead is a viable quarry for astronomers.
Surprise finding
While not even trying, the Hubble Space Telescope just spotted two very distant exploding stars that represent baby steps ahead of a footrace of expected observations.
The so-called supernovae, announced this morning, are 5 billion and 8 billion light-years away and were found serendipitously by Hubble's new Advance Camera for Surveys, installed a year ago, while it was making a calibration run. Hubble officials, who have been saying the new camera would turn Hubble into a supernova hunting machine, offered the discoveries as proof of that claim.
The supernovae bracket the presumed time of the switch from deceleration to acceleration, and examination of them has helped build the observational case that the shift occurred. Two other groups have used Hubble to purposely collect data on several more supernovae and results will likely be reported by the end of the year, researchers told SPACE.com.
Hubble is but one tool being used to probe dark energy.
Scientists are also exploring so-called cosmic microwave background radiation that carries an imprint of the baby universe's structure. Just two months ago, that effort led to the first firm determination of the age of the universe, a solid estimate for when the first stars were born, plus undisputed confirmation of dark energy's expansive role.
Other teams are examining the structure of the space through time by noting how interstellar hydrogen absorbs light. Still other researchers are pushing back the frontier of time, finding galaxies that formed when the universe was less than 10 percent its present age.
For the first time in history, experts suggest, cosmological data is accumulating faster than the wild theories that try to describe it all.
"By far the best is yet to come," said Max Tegmark, a cosmologist at the University of Pennsylvania. "We're not even halfway through this avalanche of data in cosmology."
Going back in time
Theorists have known since the 1920s that the universe was expanding. They wondered if that expansion would go on forever or if common gravity might eventually win out and pull everything back together in a sort of Big Crunch.
Then in 1998 two separate groups hunting faraway supernovae found several that were dimmer than they should have been, indicating that the universe is not just expanding, but accelerating.
The supernovae are of a particular variety, known as Type IA, that all shine with the same intrinsic brightness. Astronomers use them as "standard candles," their observed brightness revealing their distance. Light from the objects is analyzed to determine how much the waves have stretched, which bears an exact relationship to how much the universe has expanded since the light left its source -- the exploded star.
The 1998 finding of an accelerating universe was initially met with disbelief by its discoverers. Once digested -- in some cases only in the last couple of years by skeptics -- its profound implications for the composition and fate of the cosmos brought the term dark energy into common use.
Meanwhile, theorists had already figured out that an accelerating universe would necessarily be preceded by a period of deceleration, which would have followed an initial phase of rapid inflation associated with the Big Bang.
Here is why things must have slowed down:
"Early on the universe had lots of mass in a small volume," explains John Blakeslee of Johns Hopkins University. "The pull from gravity must have been enormous." As the universe expanded, gravity would have become less effective over the larger distances, and dark energy would have taken over.
One previously detected supernova, at about 10 billion light years away, supported this idea when reported in 2001, but the object proved difficult to study. The Hubble observations presented today also support the switch, said Blakeslee, lead author of a paper on the findings that will be published in the June Astrophysical Journal.
Had the universe always been accelerating, the supernova that's 8 billion light-years distant would have been dimmer, Blakeslee said in a telephone interview.
"It's not conclusive at all," Blakeslee said of his work. Another 20 or so very distant supernova are needed to make a strong case, he added.
Those 20, and then some, will not take long.
Hubble's new eyesight "should allow astronomers to discover roughly 10 times as many of these cosmic beacons as was possible with Hubble's previous main imaging camera," Blakeslee said.
Already in the bag
A separate Hubble project, led by Adam Riess of the Space Telescope Science Institute, has already bagged several distant supernovae. Riess, who worked on one team that made the 1998 acceleration breakthrough, reported preliminary results of his latest work at the APS meeting and is expected to publish a paper soon, possibly later this year.
Eleven other distant supernovae have been examined by Hubble in another study headed up by Saul Perlmutter of the Lawrence Berkeley National Laboratory. Perlmutter led the other team involved in the 1998 discovery of acceleration.
Perlmutter was not involved in the two new Hubble discoveries, but he told SPACE.com they are among many important steps that could lead to a firm determination of when acceleration began. Many of the discoveries are coming from ground-based telescopes, he noted, but Hubble "is really becoming a key for everybody in terms of follow-up" to glean the necessary detail.
By building a strong historical timeline of the universe, astronomers and cosmologists hope to answer a pressing question: Do the properties of dark energy change over time?
The answer would help them determine what dark energy is and would allow refined predictions about the origin and fate of the universe. No one expects a quick resolution, however.
Michael Turner, one of the world's foremost cosmologists from the University of Chicago, said solving the dark energy problem "is going to require a crazy idea." While most leading theories project the universe will accelerate forever, perhaps even to the wild point that it rips all matter apart, the notion that it might eventually collapse has not been ruled out, Turner said at the APS meeting.
"The destiny question is wide open," Turner said.
Dark energy more evidence for the accelerating universe
Ever since two teams of astronomers announced in 1998 that the expansion of the universe appeared to accelerating with time, other teams of astronomers have been double-checking their conclusions with data from other sources. Although some astronomers still have doubts about the accelerating universe, evidence in its favor continues to grow. The latest comes from studies of the deviation of galaxy velocities from a smooth universal expansion (I Zehavi and A Dekel 1999 Nature 401 252).
The first evidence for the accelerating universe came from observations of distant supernovae. However, the data were also consistent with an open universe - a universe that would expand forever because the total energy density was less than the so-called critical density - with a low mass density and no cosmological constant. The energy density of the universe is composed of matter (both ordinary visible matter and invisible or "dark" matter) and the energy density of the vacuum. The size of the latter, which is sometimes called quintessence or "dark energy", defines the cosmological constant. This constant was first introduced by Einstein to explain why the universe did not appear to be expanding. Hubble later showed that the universe was expanding, causing Einstein to call it his "biggest blunder".
Now Idit Zehavi of the Hebrew University in Jerusalem and Avishai Dekel of Fermilab in the US have studied the "peculiar" velocities of over 4000 galaxies on scales of about 300 million light years. These peculiar velocities are due to variations in the mass distribution of the universe which either slow down or increase the expansion due to the big bang. Zehavi and Dekel find that their data and the supernova data favour an almost flat universe in which the mass density and the cosmological constant are comparable. However, they add that "this type of universe seems to require a degree of fine tuning of the initial conditions that is in apparent conflict with 'common wisdom'". All of which means that the accelerating universe still presents lots of challenges to astronomers.
Closing
Kenyataan dalam observasi memang menuntut adanya materi gelap dan energy gelap dalam upaya penyokongan jagat raya yang seimbang, tapi harus diingat bahwa keduanya ini merupakan sebuah hipotesis yang belum di buktikan secara experimental.
Sementara itu, penelitian terus berlanjut mengenai apa itu dark energy (energy gelap) dengan para astronom muda lainnya
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