March 16, 2005
STEP ON IT, WOUDJA:
Was Einstein right when he said he was wrong?: Why is the universe expanding at an accelerating rate, spreading its contents over ever greater dimensions of space? An original solution to this puzzle, certainly the most fascinating question in modern cosmology, was put forward by four theoretical physicists, Edward W. Kolb of the U.S. Department of Energy's Fermi National Accelerator Laboratory, Chicago (USA): Sabino Matarrese of the University of Padova; Alessio Notari from the University of Montreal (Canada); and Antonio Riotto of INFN (Istituto Nazionale di Fisica Nucleare) of Padova (Italy). Their study was submitted yesterday to the journal Physical Review Letters. (PhysOrg.com, 3/16/05)
Over the last hundred years, the expansion of the universe has been a subject of passionate discussion, engaging the most brilliant minds of the century. Like his contemporaries, Albert Einstein initially thought that the universe was static: that it neither expanded nor shrank. When his own Theory of General Relativity clearly showed that the universe should expand or contract, Einstein chose to introduce a new ingredient into his theory. His "cosmological constant" represented a mass density of empty space that drove the universe to expand at an ever-increasing rate.When in 1929 Edwin Hubble proved that the universe is in fact expanding, Einstein repudiated his cosmological constant, calling it "the greatest blunder of my life." Then, almost a century later, physicists resurrected the cosmological constant in a variant called dark energy. In 1998, observations of very distant supernovae demonstrated that the universe is expanding at an accelerating rate. This accelerating expansion seemed to be explicable only by the presence of a new component of the universe, a "dark energy," representing some 70 percent of the total mass of the universe. Of the rest, about 25 percent appears to be in the form of another mysterious component, dark matter; while only about 5 percent comprises ordinary matter, those quarks, protons, neutrons and electrons that we and the galaxies are made of.
"The hypothesis of dark energy is extremely fascinating," explains Padova's Antonio Riotto, "but on the other hand it represents a serious problem. No theoretical model, not even the most modern, such as supersymmetry or string theory, is able to explain the presence of this mysterious dark energy in the amount that our observations require. If dark energy were the size that theories predict, the universe would have expanded with such a fantastic velocity that it would have prevented the existence of everything we know in our cosmos."
The requisite amount of dark energy is so difficult to reconcile with the known laws of nature that physicists have proposed all manner of exotic explanations, including new forces, new dimensions of spacetime, and new ultralight elementary particles. However, the new report proposes no new ingredient for the universe, only a realization that the present acceleration of the universe is a consequence of the standard cosmological model for the early universe: inflation.
"Our solution to the paradox posed by the accelerating universe," Riotto says, "relies on the so-called inflationary theory, born in 1981. According to this theory, within a tiny fraction of a second after the Big Bang, the universe experienced an incredibly rapid expansion. This explains why our universe seems to be very homogeneous.
It's all so easy once you incorporate fine-tuning. Posted by Orrin Judd at March 16, 2005 3:17 PM
This isn't fine-tuning, but reasoning backward from the (known) conclusion, which is a time-honored method of science.
Posted by: pj at March 16, 2005 4:59 PMA note on the physics:
1) All quantum theories have a ground state (lowest energy state), and there is no reason why the energy of the ground state needs to be zero (it's not for the harmonic oscillator, for instance). So, from the perspective of quantum mechanics, there's no reason why the ground state of empty space needs to be one of zero energy density. Whatever the energy density of the vacuum is in a unified quantum field theory, implies a certain cosmological constant. That's why Einstein was actually right, from a quantum mechanics perspective, to put in a cosmological constant.
(2) A puzzle is that the magnitude of the energy density is very very close to zero. It's easy to come up with quantum models that have energy densities that are zero or very large, but not so easy to come up with ones that match what is observed (if dark energy or dark matter is a quantum ground state energy, which is by no means assured).
(3) The inflationary cosmology is the only known explanation for another puzzle -- the universe's age and mass. For the universe to be long-lived, the mass of the universe has to be very closely related to the velocity of expansion shortly after the Big Bang: too low a velocity compared to escape velocity, and everything falls back very soon; too high a velocity, and everything spreads apart and the universe becomes large, cold, and lonely very fast. In order to support life, the early expansion velocity has to be almost exactly equal to escape velocity (within a part in 10^15 a few seconds after the Big Bang). Inflation explains the connection, albeit with an uncheckable assumption.
So what these guys have done is connect the inflationary physics to the quantum energy density physics.
Posted by: pj at March 16, 2005 5:12 PMDon't know much about quantum physics ... don't know it from Shinola, really ... but that sounds suspiciously like an INSTANT universe to moi. Hmm...
Posted by: ghostcat at March 16, 2005 5:58 PM"I do know the difference. Shinola is a shoe polish."
"Inflation" sounds like the cosmolgists' equivalent of the "old universe" which some Creationists maintain was created 6 thousand years ago with bilion year old fossils and distant galaxies.
As for "Dark Matter", major constituents are phlogiston and left-over epicycles.
"inflationary cosmology is the only known explanation for another puzzle -- the universe's age and mass."
Necessity is the mother of invention.
Posted by: oj at March 16, 2005 8:04 PMPJ:
Perhaps you can answer some questions which confuse me. It seems that the vacuum-energy/dark-energy is constant per unit volume. Since the universe is expanding and its volume increasing:
1) Doesn't this mean that the total mass-energy of the universe is likewise increasing, violating the conservation of mass-energy? and
2) Wouldn't this increase in the mass-energy of the universe be reflected in the mass-energy tensor of the field equation, resulting in increased gravitation and slowing, instead of an acceleration?
"When in 1929 Edwin Hubble proved that the universe is in fact expanding, Einstein repudiated his cosmological constant, calling it 'the greatest blunder of my life.'"
Einstein also apologized to Georges Lemaitre--the Catholic priest who, before Hubble worked out the implications of Red Shift, developed the Big Bang theory.
Einstein had told Lemitre that while his math was correct his grasp of physics was abysmal. After Hubbell's discovery supported Lemaitre's work, Einstein called Lemaitre's theory "beautiful."
For some reason, Lemaitre is rarely mentioned in the popular media when the Big Bang theory he fathered is discussed.
Posted by: David at March 16, 2005 11:44 PMNo OJ, General Relativity is not evolution. Its a solid theory that is well supported. Applying it to understand the history of the universe has been difficult, but it not impossible.
What the article says is that these scientists realized that they had a way to account for some observed effects of cosmic expansion, itself an observed phenomenon, within GR, by using GR and without hypothesizing any unobserved entiity, such as "dark matter" or "dark energy."
Nice work. Sounds very elegant. Are they correct? More data will tell.
Posted by: Robert Schwartz at March 17, 2005 2:42 AMI am also confused.
Albert Einstein initially thought that the universe was static: that it neither expanded nor shrank.
O.K.
When his own Theory of General Relativity clearly showed that the universe should expand or contract, Einstein chose to introduce a new ingredient into his theory. His "cosmological constant" represented a mass density of empty space that drove the universe to expand at an ever-increasing rate.
If Einstein's theory said that the universe should expand or contract, why had he need to introduce an "expansion factor" if General Relativity already provided for that?
When in 1929 Edwin Hubble proved that the universe is in fact expanding, Einstein repudiated his cosmological constant, calling it "the greatest blunder of my life."
Hubble showed the universe is expanding, and Einstein repudiated the "cosmological constant" that enabled the universe to expand? Huh?