TWO mysteriously off-course spacecraft and the biggest puzzle in physics today 鈥� the nature of dark energy 鈥� could have a common explanation. If so, be prepared to accept that our sun is putting on some weight.
Hans Fahr at the University of Bonn in Germany and Michael Heyl at the German Aerospace Centre, also in Bonn, set out to solve a puzzle that has been vexing cosmologists ever since it was first noticed that the expansion of the universe is accelerating. This acceleration has been attributed to dark energy, which many think is the inherent energy of space-time, described by the so-called cosmological constant (CC). But there鈥檚 a problem. Particle physics predicts that the CC should be more than 10120 times larger than observed 鈥� a value so large it would have blown the universe apart before stars or galaxies formed.
Fahr and Heyl take the view that this constant can change over time. They suggest that it did indeed start out at its huge predicted value after the big bang, but has been decaying ever since, reaching its relatively small value today. 鈥淭he natural question is, what happens to this energy during decay?鈥� Fahr says.
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In their model, the missing energy is converted to mass in the form of new particles. Most of this new matter would have appeared soon after the birth of the universe, as the cosmological constant decayed rapidly, Fahr says, but small amounts of matter will be spontaneously popping up even today (Naturwissenschaften, DOI: 10.1007/s00114-007-0235-1).
Astrophysicist James Overduin at Stanford University, California, likes the idea. 鈥淚t鈥檚 not as crazy as it sounds,鈥� he says. 鈥淎fter all, the big bang is just matter creation all at once. Fahr just wants to spread this out over time.鈥�
鈥淚t鈥檚 not as crazy as it sounds. After all, the big bang is just matter creation all at once. Fahr just wants to spread this out鈥�
Recent observations of the galaxy cluster A586 suggest that dark energy does interact with matter 鈥� in this case dark matter (New 杏吧原创, 7 April, p 10). This lends credibility to the notion that dark energy could decay into dark matter, says Orfeu Bertolami at the Instituto Superior T茅cnico in Lisbon, Portugal, who has studied A586. He cautions that cosmologists will need much more evidence before seriously considering Fahr and Heyl鈥檚 model, though.
That鈥檚 where the Pioneer anomaly comes in. Launched 35 years ago, NASA鈥檚 Pioneer 10 was the first spacecraft to reach the outer solar system. Its sister ship, Pioneer 11, set out a year later to visit Saturn. Before losing contact with Earth, both spacecraft seemed to be drifting off course, as if they were being tugged towards the sun slightly more than expected 鈥� enough to shift Pioneer 10 400,000 kilometres from its expected position.
According to Fahr and Heyl, this mysterious drift can be explained if the sun is gradually increasing in mass from matter being created by a decaying cosmological constant. They show that this increase would shift the wavelength of the radio signals exchanged with the Pioneers as the signals pass by the sun. 鈥淭he spacecraft are actually on the correct course, but this distortion of radio signals gives the illusion that they are off track,鈥� says Fahr.
However, there is a wealth of alternative theories to explain the Pioneer anomaly. Bertolami鈥檚 team have developed their own, to appear in the journal Physical Review D: they calculate that an extra gravitational force, which takes different values at different points in space-time, could be shifting the spacecraft.
Fahr agrees that Bertolami鈥檚 explanation is just as valid as his own. We need a test, he says, to differentiate between theories such as Bertolami鈥檚, in which the Pioneers are really veering off course, and theories such as his own, where just the tracking signals are being distorted.
Bertolami and his colleague Jorge P谩ramos are putting together a proposal to the International Space Science Institute in Bern, Switzerland, for just such a test. They want to launch two spheres from a mother ship and track them with lasers. The spheres would have different mass-to-surface-area ratios such that any mysterious extra force would cause them to accelerate relative to each other. 鈥淭his would verify that the Pioneer anomaly is real and not just an error with spacecraft instrumentation,鈥� Bertolami says.
The mother ship would also be subject to any unknown force. It could be tracked using radio signals and also by an on-board accelerometer: if anomalous motion was picked up in the radio signals, but didn鈥檛 show up on the accelerometer, that would favour Fahr鈥檚 model, Bertolami says. 鈥淲e hope to piggy-back on another mission to keep down the cost,鈥� he says. 鈥淚t would be a cheap way to test one of the biggest puzzles in physics.鈥�
