What is the mysterious dark energy that's causing the expansion of theuniverse to accelerate? Is it some form of Einstein's famouscosmological constant, or is it an exotic repulsive force, dubbed"quintessence," that could make up as much as three-quarters of thecosmos? Scientists from Lawrence Berkeley National Laboratory (BerkeleyLab) and Dartmouth College believe there is a way to find out.
Ina paper to be published in Physical Review Letters,physicists Eric Linder of Berkeley Lab and Robert Caldwell of Dartmouthshow that physics models of dark energy can be separated into distinctscenarios, which could be used to rule out Einstein's cosmologicalconstant and explain the nature of dark energy. What's more, scientistsshould be able to determine which of these scenarios is correct withthe experiments being planned for the Joint Dark Energy Mission (JDEM)that has been proposed by NASA and the U.S. Department of Energy.
"Scientists have been arguing the question 'how precisely do we need tomeasure dark energy in order to know what it is?'" says Linder. "Whatwe have done in our paper is suggest precision limits for themeasurements. Fortunately, these limits should be within the range ofthe JDEM experiments."
Linder and Caldwell areboth members of the DOE-NASA science definition team for JDEM, whichhas the responsibility for drawing up the mission's scientificrequirements. Linder is the leader of the theory group for SNAP — theSuperNova/Acceleration Probe, one of the proposed vehicles for carryingout the JDEM mission. Caldwell, a professor of physics and astronomy atDartmouth, is one of the originators of the quintessence concept.
In their paper in Physical Review LettersLinder and Caldwell describe two scenarios, one they call "thawing" andone they call "freezing," which point toward distinctly different fatesfor our permanently expanding universe. Under the thawing scenario, theacceleration of the expansion will gradually decrease and eventuallycome to a stop, like a car when the driver eases up on the gas pedal.Expansion may continue more slowly, or the universe may evenrecollapse. Under the freezing scenario, acceleration continuesindefinitely, like a car with the gas pedal pushed to the floor. Theuniverse would become increasingly diffuse, until eventually our galaxywould find itself alone in space.
Either ofthese two scenarios rules out Einstein's cosmological constant. Intheir paper Linder and Caldwell show, for the first time, how tocleanly separate Einstein's idea from other possibilities. Under anyscenario, however, dark energy is a force that must be reckoned with.
SaysLinder, "Because dark energy makes up about 70 percent of the contentof the universe, it dominates over the matter content. That means darkenergy will govern expansion and, ultimately, determine the fate of theuniverse."
In 1998, two research groups rocked the field of cosmology withtheir independent announcements that the expansion of the universe isaccelerating. By measuring the redshift of light from Type Iasupernovae, deep-space stars that explode with a characteristic energy,teams from the Supernova Cosmology Project headquartered at BerkeleyLab and the High-Z Supernova Search Team centered in Australiadetermined that the expansion of the universe is actually accelerating,not decelerating. The unknown force behind this accelerated expansionwas given the name "dark energy."
Prior to thediscovery of dark energy, conventional scientific wisdom held that theBig Bang had resulted in an expansion of the universe that wouldgradually be slowed down by gravity. If the matter content in theuniverse provided enough gravity, one day the expansion would stopaltogether and the universe would fall back on itself in a Big Crunch.If the gravity from matter was insufficient to completely stop theexpansion, the universe would continue floating apart forever.
"Fromthe announcements in 1998 and subsequent measurements, we now know thatthe accelerated expansion of the universe did not start until sometimein the last 10 billion years," Caldwell says.
Cosmologists are now scrambling to determine what exactly dark energyis. In 1917 Einstein amended his General Theory of Relativity with acosmological constant, which, if the value was right, would allow theuniverse to exist in a perfectly balanced, static state. Althoughhistory's most famous physicist would later call the addition of thisconstant his "greatest blunder," the discovery of dark energy hasrevived the idea.
"The cosmological constantwas a vacuum energy (the energy of empty space) that kept gravity frompulling the universe in on itself," says Linder. "A problem with thecosmological constant is that it is constant, with the sameenergy density, pressure, and equation of state over time. Dark energy,however, had to be negligible in the universe's earliest stages;otherwise the galaxies and all their stars would never have formed."
For Einstein's cosmological constant to result in the universe wesee today, the energy scale would have to be many orders of magnitudesmaller than anything else in the universe. While this may be possible,Linder says, it does not seem likely. Enter the concept of"quintessence," named after the fifth element of the ancient Greeks, inaddition to air, earth, fire, and water; they believed it to be theforce that held the moon and stars in place.
"Quintessenceis a dynamic, time-evolving, and spatially dependent form of energywith negative pressure sufficient to drive the accelerating expansion,"says Caldwell. "Whereas the cosmological constant is a very specificform of energy — vacuum energy — quintessence encompasses a wide classof possibilities."
To limit the possibilitiesfor quintessence and provide firm targets for basic tests that wouldalso confirm its candidacy as the source of dark energy, Linder andCaldwell used a scalar field as their model. A scalar field possesses ameasure of value but not direction for all points in space. With thisapproach, the authors were able to show quintessence as a scalar fieldrelaxing its potential energy down to a minimum value. Think of a setof springs under tension and exerting a negative pressure thatcounteracts the positive pressure of gravity.
"Aquintessence scalar field is like a field of springs covering everypoint in space, with each spring stretched to a different length,"Linder said. "For Einstein's cosmological constant, each spring wouldbe the same length and motionless."
Under theirthawing scenario, the potential energy of the quintessence field was"frozen" in place until the decreasing material density of an expandinguniverse gradually released it. In the freezing scenario, thequintessence field has been rolling towards its minimum potential sincethe universe underwent inflation, but as it comes to dominate theuniverse it gradually becomes a constant value.
TheSNAP proposal is in research and development by physicists,astronomers, and engineers at Berkeley Lab, in collaboration withcolleagues from the University of California at Berkeley and many otherinstitutions; it calls for a three-mirror, 2-meter reflecting telescopein deep-space orbit that would be used to find and measure thousands ofType Ia supernovae each year. These measurements should provide enoughinformation to clearly point towards either the thawing or freezingscenario — or to something else entirely new and unknown.
SaysLinder, "If the results from measurements such as those that could bemade with SNAP lie outside the thawing or freezing scenarios, then wemay have to look beyond quintessence, perhaps to even more exoticphysics, such as a modification of Einstein's General Theory ofRelativity to explain dark energy."
"The limits of quintessence," by R.R. Caldwell and Eric V. Linder, is now online at http://arxiv.org/abs/astro-ph/0505494 and will appear in a forthcoming edition of Physical Review Letters.
BerkeleyLab is a U.S. Department of Energy national laboratory located inBerkeley, California. It conducts unclassified scientific research andis managed by the University of California. Visit our website at http://www.lbl.gov/.
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