In the case of the Sun's orbit around the Milky Way, we only say that the vacuum energy density is less than half of the average matter density in a sphere centered at the Galactic Center that extends out to the Sun's distance from the center. If the vacuum energy density were more than this, there would be no centripetal acceleration of the Sun toward the Galactic Center.
The best limit on the vacuum energy density comes from the largest possible system: the Universe as a whole. The vacuum energy density leads to an accelerating expansion of the Universe.
If the vacuum energy density is greater than the critical density, then the Universe will not have gone through a very hot dense phase when the scale factor was zero the Big Bang. We know the Universe went through a hot dense phase because of the light element abundances and the properties of the cosmic microwave background.
The green region in the upper left is ruled out because there would not be a Big Bang in this region, leaving the CMB spectrum unexplained. The blue wedge shows the parameter space region that gives the observed Doppler peak position in the angular power spectrum of the CMB.
The purple region is consistent with the CMB Doppler peak position and the supernova data. The big pink ellipse shows the possible systematic errors in the supernova data. The figure above shows the scale factor as a function of time for several different models. Since both the CMB and the supernova data have improved. The figure below repeats the diagram above with new error ellipses for the supernova data and a new CMB allowed region shown. The allowed region consistent with both the CMB and the supernova data has shrunk dramatically toward a flat but vacuum energy dominated model.
The CMB models also give a Hubble constant, which is shown by the color coding of the dots. Conclusion In the past, we have had only upper limits on the vacuum density and philosophical arguments based on the Dicke coincidence problem and Bayesian statistics that suggested that the most likely value of the vacuum density was zero.
Now we have the supernova data that suggests that the vacuum energy density is greater than zero. This result is very important if true. We need to confirm it using other techniques, such as the WMAP satellite which has observed the anisotropy of the cosmic microwave background with angular resolution and sensitivity that are sufficient to measure the vacuum energy density. CMB data combined with the measured Hubble constant do confirm the supernova data: there is a positive but small vacuum energy density.
This would mean that the universe is open, contrary to the prediction of the inflationary theory of the origin of the universe. The unexpected discovery of vacuum energy added the required 70 percent to the sum, thus confirming one of the most important predictions of inflationary cosmology. The tiny vacuum energy is large enough to make our universe slowly accelerate. It will take more about 10 billion years for the universe to double in size, but if this expansion continues, in about billion years all distant galaxies will run away from our galaxy so far that they will forever disappear from our view.
That was quite a change from our previous expectations that in the future we are going to see more and more…. The possibility that vacuum may have energy was discussed almost a century ago by Einstein, but then he discarded the idea.
Particle physicists re-introduced it again, but their best estimates of the vacuum energy density were way too large to be true. For a long time they were trying to find a theory explaining why vacuum energy must be zero, but all such attempts failed.
Explaining why it is not zero, but incredibly, excruciatingly small, is a much greater challenge. And there is an additional problem: At present, vacuum energy is comparable with the average energy density of matter in the universe.
In the past, the universe was small, and vacuum energy was negligibly small compared to the energy density of normal matter. In the future, the universe will grow and density of normal matter will become exponentially small. Why do we live exactly at the time when the energy of empty space is comparable to the energy of normal matter? Thirty years ago, well before the discovery of the energy of nothing, Steven Weinberg and several other scientists started to argue that observing a small value of the vacuum energy would not be too surprising: A universe with a large negative vacuum energy would collapse before life could have any chance to emerge, whereas a large positive vacuum energy would not allow galaxies to form.
Thus we can only live in a universe with a sufficiently small absolute value of vacuum energy. To investigate the mysteries of the void, some physicists are using the biggest scientific instrument ever built—the just-completed Large Hadron Collider, a huge particle accelerator straddling the French-Swiss border.
Others are designing tabletop experiments to see if they can plumb the vacuum for ways to power strange new nanotech devices. Until the s physicists viewed the vacuum much as the rest of us still do: as a featureless nothingness, a true void. That all changed with the birth of quantum mechanics. Those virtual quantum particles are more than a theoretical abstraction.
Sixty years ago a Dutch physicist named Hendrik Casimir suggested a simple experiment to show that virtual particles can move objects in the real world. What would happen, he asked, to two metal plates placed very close together in a complete vacuum?
In the days before quantum mechanics, physicists would have said that the plates would just sit there. But Casimir realized that the net pressure of all the virtual particles—the stuff of empty space—outside the plates should exert a minuscule force, a nudge from nothing that would push the plates together.
In that year, physicist Steve Lamoreaux, now at Yale , managed to detect the feeble Casimir force on two small surfaces separated by a few thousandths of a millimeter. At first most physicists regarded the Casimir force as a quantum oddity, something of no practical value. Now that has changed: Forward thinkers see it as an important energizer for the tiniest of machines, devices on the nano scale, and a few labs are working on ways to use the force to defy the conventional limitations of mechanical design.
Federico Capasso, a physicist at Harvard, leads a small team that is trying to create a repulsive Casimir force by tinkering with the shapes of plates or with the coatings used to cover them. His entire set of experiments fits on a desktop, and the objects he works with are so small that most of them cannot be seen without a microscope. That could lead to a host of useful applications, including tiny frictionless bearings or nanogears that spin without touching.
It is garden-variety quantum mechanics, weird but not unexpected. The explosions revealed a universe expanding at an ever-faster rate , a finding at odds with previous expectations that the expansion of the cosmos should be slowing down, braked by the collective gravitational pull of all the matter out there. Some unknown form of energy—physicists call it dark energy simply for lack of a more descriptive term—appears to be built into the very fabric of space, countering the gravitational pull of matter and pushing everything in the universe apart.
Some theorists speculate that dark energy might cause a runaway expansion of the universe, resulting in a so-called Big Rip some 50 billion years from now that would tear the cosmos to pieces, shredding even atoms. The observations have allowed physicists to estimate the quantity of dark energy by deducing the force needed to produce the accelerating effect.
The result is a minuscule amount of energy for every cubic meter of vacuum. Since most of the cosmos consists of empty space, though, that little bit adds up, and the total amount of dark energy completely dominates the dynamics of the universe. With the discovery of dark energy came difficult questions: What is this energy, and where does it come from? Physicists simply do not know.
According to quantum mechanics, the energy of empty space comes from the virtual particles that dwell there. But when physicists use the equations of quantum theory to calculate the amount of that virtual energy, they get a ridiculously huge number—about orders of magnitude too large.
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