But what does that have to do with the flatness of the universe and the hard to reconcile curved-space phenomenon resulting from Einstein's general theory of relativity? More from The Economist:
Many of those 5,000 papers deal with something that has come to be known as dark energy. One reason for its popularity is that, at one fell swoop, it explains another big cosmological find of recent years. In the early 1990s studies of the cosmic microwave background (CMB), an all-pervading sea of microwaves which reveals what the universe looked like when it was just 380,000 years old, showed that the universe, then and now, was "flat". However big a triangle you draw on it—the corners could be billions of light years apart—the angles in it would add up to 180°, just as they do in a school exercise book.
That might not surprise people whose geometrical endeavours have never gone beyond such books. But it surprised many physicists. At some scales space is not at all flat: the power of Albert Einstein's theory of general relativity lies in its interpretation of gravity in terms of curved space. Cosmologists were quite prepared for it to be curved at the grandest of scales, and intrigued to discover that it was not.
So far so good, but where does the amount and nature of dark energy come into play, and why does the relationship of its postive and negative qualities have to prove a cosmological or universal constant?
Relativity says that for the universe to be flat, it has to have a very particular density—which in relativity is a measure not just of the mass contained in a certain volume, but also of the energy. The puzzle was that various lines of evidence showed that the universe's endowment of ordinary matter (the stuff that people, planets and stars are made of) would give it just 4% of that density. Adding in extraordinary matter—"dark matter", not made of atoms, that interacts with the rest of the universe almost only by means of gravity—gets at most an extra 22%. That left almost three-quarters of the critical density unaccounted for. Theorists such as Michael Turner, of the University of Chicago, became convinced that there was something big missing from their picture of the universe.
Whatever it is that is driving the universe's accelerating expansion fits the bill rather well. Add the amount of energy needed to keep cosmic acceleration going to the amount of matter and energy in the universe already accounted for and you have more or less exactly the density of matter and energy needed to make the universe flat. But there is a catch; for the sums to tally, that "dark energy"—Dr Turner is thought to have coined the term— must be very strange stuff indeed. According to Einstein's theory of relativity, energy in the form of radiation has the same sort of gravitational effect as matter does—the photons of which light is made exert a pressure, and this in turn gives rise to a gravitational attraction. In order to drive its acceleration, then, dark energy must instead have a repulsive effect. It must, in other words, exert a negative pressure.
Divide dark energy's pressure (negative) by its energy density (positive) and you get something cosmologists label "w". It is easy to see that w must be negative. Observations made since 1998 suggest that w is pretty close to -1. If it were found to be exactly -1, that would make dark energy something physicists call a cosmological constant. A cosmological constant is the same no matter where in the universe you look—an inherent, unchanging feature of the fabric of creation, however much it expands, twists or ties itself in knots.And the cosmological constant also has it's history inextricably bound up in the history of Einstein's general theory of relativity.
The cosmological constant is another thing first dreamed up by Einstein. On realising that the equations of general relativity allowed for the universe's expansion (or, indeed, contraction), he added a parameter describing just such a constant in order to keep it from doing either. For all his notoriously counterintuitive predictions, an expanding universe was one he was not prepared to countenance, at least not in 1917, when he published his theory. After Edwin Hubble's discovery 12 years later that other galaxies were indeed streaming away from Earth's Milky Way backyard, Einstein dropped the tweak. No doubt miffed that he had not trusted his maths in the first place, he later called the cosmological constant his "biggest blunder".
By then, though, the cosmological constant had been seized upon by quantum theorists [the physicists who focus on the physics of the smallest things, the sub-atomic particles], themselves in the midst of turning physics on its head. Quantum theory says that the seemingly empty vacuum of space is, in fact, not empty at all. Instead it is constantly abuzz with "virtual" particles flitting in and out of existence. The energy resulting from all this buzzing—vacuum energy—should be a fixed feature of space—in other words, a cosmological constant.
And, in principle, it could also propel the universe's expansion. Thus vacuum energy and dark energy might be the same thing. But this theoretical neatness runs into a practical problem. A naive approach to quantum theory says that vacuum energy should be a whopping 1060 to 10120 times bigger than dark energy's estimated energy density. Some physicists call this "the worst prediction ever". Working out why vacuum energy is not so vast has been a problem for physics ever since.And that brings us around to those who theorize the answer is connected to "string theory."
Cliff Burgess, from Perimeter Institute for Theoretical Physics in Waterloo, Ontario, and the author of a handful of the 5,000 papers Dr Perlmutter has dug up, thinks he has a solution; the vacuum energy is vast, but it is almost all hidden away in extra spatial dimensions. Unlike the familiar three of length, breadth and height, these extra dimensions are curled up so tightly that they elude detection (though scientists are trying to prise them open in particle accelerators like the Large Hadron Collider near Geneva). Extra dimensions are of interest because string theory, a class of mathematical models based on quantum theory that seeks to describe reality in the most fundamental way, requires that there be at least six of them, maybe more.
What makes Dr Burgess's proposal unusual is that he went out on a limb and suggested that these energy-sapping, curled-up extra dimensions should be as big as a few microns across, gargantuan by string-theory standards. The reason they have not been noticed by chipmakers, virologists and others who pay attention to things on the micron scale, he contends, is that, like dark matter, they are sensitive only to gravity, and relatively oblivious to the other three of nature's fundamental interactions: electromagnetism and the weak and strong nuclear forces. This may sound like a cheap excuse but it makes robust mathematical sense. And it makes predictions; at micron scales the attraction between two masses will no longer depend on the square of the distance between them in the way that physicists since Newton have required it to.
[...] If Dr Burgess is right, vacuum energy and dark energy are the same thing, a cosmological constant, and w is exactly equal to -1. What, though, if it is not? Then dark energy would have to be something that varies in space, time, or both, and is close to -1 today just by coincidence. Names applied to this something else include quintessence, k-essence, phantom energy and a bunch more, depending on which theorist you ask and what properties you think likely. It would be a new fundamental force, one that rears its head only at vast cosmic distances.
[...]The more precisely w comes to look like -1, the more enthusiasm there will be for cosmological constant theories, which require that value, and the less enthusiasm there will be for fifth forces and modified gravity, part of the charm of which is that they can work with other values. This is where telescopes like Cerro Tololo come in. Existing data from ground-based and space telescopes put w at between -1.1 and -0.9. DES will aim to narrow the margin of uncertainty down to just 0.01. To do so, it will take 400 one-gigabyte snaps a night for 525 nights over five years (the remaining telescope time will be split between other science projects). And it will use an array of clever techniques to analyse the data.But there's an overarching problem, possibly an insuperable one, in proving the cosmological constant inherent in the positive and negative qualities of dark energy--at least at this time. The article concludes:
The rub is that no amount of observations can ever pin down the figure for w with perfect accuracy. That would require infinite precision, something impossible to achieve even in an ever-expanding universe. And the whole constant idea falls to pieces if w is even a smidgen off -1.
More than any other scientific problem the cosmic-expansion conundrum presents scientists with an existential quandary. "It could be a 22nd-century problem we stumbled upon in the 20th century," says Dr Turner. Some researchers may begin to feel time would be better spent on other scientific pursuits.
Many astronomers, including Dr Perlmutter, are quietly hoping that as DES and the host of other acronyms come online, they will spring another surprise, like the one that first propelled cosmic acceleration into the limelight in 1998. Whether they do or not, though, dark energy—or whatever else is causing the universe to speed up—is probably too big a conundrum for one generation to crack. It will cause boffins to rack their brains for years to come.
There is much more shared in this article: the details of the more recent research, formulation of next questions and next steps in the research, and the projects and research methodologies organized to address them. It is fully worth your time to read it all.
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