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This week 50 years ago
This week–
Can’t live without dark energy STUART CLARK
IT’S like the gatecrasher who won’t leave the party. The cosmological constant, the simplest explanation of why the expansion of the universe is accelerating, just won’t go away, much as astronomers would like to see the back of it. The cosmological constant originated as a “fudge factor” that Einstein added to general relativity to square its predictions with a universe that he thought was static. He later disowned it, when it was realised that the universe is expanding, but
with the discovery in the late 1990s that this expansion is accelerating, the cosmological constant came back into favour. In essence, the cosmological constant represents the “dark energy” of space-time that repulses gravity. There is one huge problem with it, though: the value for the cosmological constant predicted by particle physics is about 120 orders of magnitude greater than is consistent with the observed expansion of the universe. So could there be another explanation for the universe’s behaviour? Some have suggested
The Wellcome Trust & New Scientist Essay Competition 2007
HOW WELL CAN YOU WRITE
5
Are you a current postgraduate student in science, engineering or technology? Can you captivate an audience with a lively, fresh 700-word article on your research and its implications for society? Then why not tell the world?
Your chance to win a Wellcome Media Training Placement with New Scientist
Enter this year’s essay competition and show your skills – it could gain you a career training opportunity
PRIZES FIRST a two-week expenses paid media placement with New Scientist, £1000 spending money and your essay published in New Scientist
The prizewinners plus the 10 runners-up get a free one-year subscription to New Scientist. The prizewinners will be invited to give a short talk at an exciting event to be held in London later this year and to share their work with key players from science and the media
SECOND £750 THIRD £500
Closing date Friday 8 June
For more information and a full list of the competition rules go to www.wellcome.ac.uk/scienceessay 18 | NewScientist | 19 May 2007
that modifying general relativity to weaken gravity could explain the accelerated expansion, without the need for dark energy. To check whether modified gravity is at work, Shen Wang of Brookhaven National Laboratory in Upton, New York, and colleagues from Columbia University, New York, looked at the large-scale structure of the universe today. Dark energy influences the growth of clusters of galaxies, so cluster formation can be used to estimate how much dark energy there is. The researchers found that the cluster growth they observed needs about 70 per cent of the universe to be dark energy of the kind represented by the cosmological constant. This agrees very well with estimates of dark energy derived from supernova studies (www.arxiv. org/abs/0705.0165). Their conclusion: no modified gravity is necessary. So the cosmological constant wins out over modified gravity, this time round at least. It’s not yet a knock-out, though. Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena, thinks a definitive
“A definitive statement on whether modified gravity explains the accelerating universe will have to wait” statement on whether modified gravity is needed to explain the accelerating universe will have to wait until cosmologists have collected more accurate data. “This is a good analysis, showing the kind of things that we can do with current data, but the story isn’t over,” he says. “They certainly haven’t ruled out modified gravity as an alternative to dark energy.” The options have, however, become much more limited. Wang and his team say they have put tight constraints on a new theory of gravity: any tweak to the strength of gravity can only deviate from general relativity by less than 1 per cent. ●
THIS WEEK 50 YEARS AGO All jammed up The end of petrol rationing has brought back the traffic jam to our cities. Before long there will be complete congestion in some places. One of the main causes of traffic jams is parked vehicles impeding flow. Another is a consequence of having so many intersecting roads. While science cannot stop people parking their cars, it can do much to improve traffic flow at intersections. In the last 30 years automatic travel signals have revolutionised traffic control. First installed in Leeds, they were originally designed to work on a fixed time cycle. More recently, signals have been developed which are operated by the traffic itself, which indicates its presence by running over a rubber pad. But where a number of intersections occur, the limited intelligence and foresight of the ordinary traffic light becomes problematic. A vehicle passing through the intersections may be stopped at each one. With this in mind, a rather more advanced system has been installed on Oxford Street in London. This groups the lights together so that the traffic flows steadily at a reasonable speed. However, there is still a great deal which could be done to improve traffic control at urban intersections: particularly in the field of detecting approaching traffic. Photo-electric or capacity-operated detectors, which also provide information about the speed of approaching vehicles, would enable traffic lights to be built which could assess traffic conditions more accurately. The development of such devices would do much to reduce the congestion in cities, but even the best system is bound to cause some interruption in traffic flow. With this in mind, we conclude that the only satisfactory solution is to build intersections with bridges on two levels in all our cities so that through traffic is not impeded. From The New Scientist, 23 May 1957
www.newscientist.com
Can’t live without dark energy STUART CLARK
IT’S like the gatecrasher who won’t leave the party. The cosmological constant, the simplest explanation of why the expansion of the universe is accelerating, just won’t go away, much as astronomers would like to see the back of it. The cosmological constant originated as a “fudge factor” that Einstein added to general relativity to square its predictions with a universe that he thought was static. He later disowned it, when it was realised that the universe is expanding, but
with the discovery in the late 1990s that this expansion is accelerating, the cosmological constant came back into favour. In essence, the cosmological constant represents the “dark energy” of space-time that repulses gravity. There is one huge problem with it, though: the value for the cosmological constant predicted by particle physics is about 120 orders of magnitude greater than is consistent with the observed expansion of the universe. So could there be another explanation for the universe’s behaviour? Some have suggested
The Wellcome Trust & New Scientist Essay Competition 2007
HOW WELL CAN YOU WRITE
5
Are you a current postgraduate student in science, engineering or technology? Can you captivate an audience with a lively, fresh 700-word article on your research and its implications for society? Then why not tell the world?
Your chance to win a Wellcome Media Training Placement with New Scientist
Enter this year’s essay competition and show your skills – it could gain you a career training opportunity
PRIZES FIRST a two-week expenses paid media placement with New Scientist, £1000 spending money and your essay published in New Scientist
The prizewinners plus the 10 runners-up get a free one-year subscription to New Scientist. The prizewinners will be invited to give a short talk at an exciting event to be held in London later this year and to share their work with key players from science and the media
SECOND £750 THIRD £500
Closing date Friday 8 June
For more information and a full list of the competition rules go to www.wellcome.ac.uk/scienceessay 18 | NewScientist | 19 May 2007
that modifying general relativity to weaken gravity could explain the accelerated expansion, without the need for dark energy. To check whether modified gravity is at work, Shen Wang of Brookhaven National Laboratory in Upton, New York, and colleagues from Columbia University, New York, looked at the large-scale structure of the universe today. Dark energy influences the growth of clusters of galaxies, so cluster formation can be used to estimate how much dark energy there is. The researchers found that the cluster growth they observed needs about 70 per cent of the universe to be dark energy of the kind represented by the cosmological constant. This agrees very well with estimates of dark energy derived from supernova studies (www.arxiv. org/abs/0705.0165). Their conclusion: no modified gravity is necessary. So the cosmological constant wins out over modified gravity, this time round at least. It’s not yet a knock-out, though. Sean Carroll, a cosmologist at the California Institute of Technology in Pasadena, thinks a definitive
“A definitive statement on whether modified gravity explains the accelerating universe will have to wait” statement on whether modified gravity is needed to explain the accelerating universe will have to wait until cosmologists have collected more accurate data. “This is a good analysis, showing the kind of things that we can do with current data, but the story isn’t over,” he says. “They certainly haven’t ruled out modified gravity as an alternative to dark energy.” The options have, however, become much more limited. Wang and his team say they have put tight constraints on a new theory of gravity: any tweak to the strength of gravity can only deviate from general relativity by less than 1 per cent. ●
THIS WEEK 50 YEARS AGO All jammed up The end of petrol rationing has brought back the traffic jam to our cities. Before long there will be complete congestion in some places. One of the main causes of traffic jams is parked vehicles impeding flow. Another is a consequence of having so many intersecting roads. While science cannot stop people parking their cars, it can do much to improve traffic flow at intersections. In the last 30 years automatic travel signals have revolutionised traffic control. First installed in Leeds, they were originally designed to work on a fixed time cycle. More recently, signals have been developed which are operated by the traffic itself, which indicates its presence by running over a rubber pad. But where a number of intersections occur, the limited intelligence and foresight of the ordinary traffic light becomes problematic. A vehicle passing through the intersections may be stopped at each one. With this in mind, a rather more advanced system has been installed on Oxford Street in London. This groups the lights together so that the traffic flows steadily at a reasonable speed. However, there is still a great deal which could be done to improve traffic control at urban intersections: particularly in the field of detecting approaching traffic. Photo-electric or capacity-operated detectors, which also provide information about the speed of approaching vehicles, would enable traffic lights to be built which could assess traffic conditions more accurately. The development of such devices would do much to reduce the congestion in cities, but even the best system is bound to cause some interruption in traffic flow. With this in mind, we conclude that the only satisfactory solution is to build intersections with bridges on two levels in all our cities so that through traffic is not impeded. From The New Scientist, 23 May 1957
www.newscientist.com