New_Scientist_May_27_2017

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COVER STORY Our leaking universe The steady drip of energy through the fabric of space-time could be ripping the universe apart, findsJoshuaSokol IF PHYSICISTS went in for commandments, the first would surely be: thou shalt not get something from nothing. Also known as the principle of energy conservation, this universal accounting law makes it impossible for energy to be magicked either into or out of existence. Whenever a suspicious transaction seems to take place in physics, a careful audit with the principle of energy conservation usually reveals the source of the error – some overlooked entry in the ledger that, once taken into account, helps balance the books. This time-honoured technique has allowed us to predict planets and discover particles. But now it appears to be under attack. Look out into the depths of the universe today, and you see a vast quantity of energy. It is so vast, in fact, that it accounts for over two-thirds of all the energy there is. And this mysterious stash is growing continuously – energy laundering on the grandest of cosmic scales. Working out where this so-called dark energy comes from is probably the biggest problem in physics. We have long been frustrated in finding a solution, but now two groups of physicists think they have it. If they are right, we may have found dark energy’s source in the imperfect joins of the universe where different theories of reality meet. Follow the trail back, and we could even arrive at a better theory of reality. We’ve known about the invisible elephant in the universe for some time. In 1998, astronomers observing distant supernovae noticed that they were even dimmer than expected. We expected their light to fade as it travelled towards us across an expanding universe, but these new results suggested there was a foot on the accelerator. Dark energy is the mysterious substance conjured up to explain what is pushing the universe apart ever faster. And as the latest “Dark energy may be a cosmic accounting error, rather than an unknown substance” data from sources like the Planck space telescope reveal, it is spread evenly throughout the universe at a density equivalent to around half a dozen protons in every cubic metre of space. The simplest way to explain this allpervasive energy is to think of empty space as not being empty after all. On very small scales, quantum mechanics says that any vacuum is filled with the wriggling of quantum fields. But calculations following that approach give us a dark energy density that is 120 orders of magnitude larger than the one astronomers measure from the accelerating expansion of the universe. Almost laughably wrong. Some researchers, however, haven’t given up on making these two numbers square. According to a new paper by Qingdi Wang, a student of theoretical physicist Bill Unruh at the University of British Columbia in Canada, these jiggling fields would tend to cancel each other out on larger scales, drastically deflating the prediction. But frustration at the inability to make progress has now led some to suggest that it’s all down to a cosmic accounting error. The idea is that dark energy is not actually a substance held in the universe’s vaults – it’s something that appears on the books purely because there’s something else we’ve overlooked. Energy conservation is such a basic principle that any apparent violation should give pause for thought. The seminal work of mathematician Emmy Noether in the early 20th century showed that energy conservation was an expression of something even more fundamental: the idea that the laws of physics are immutable over time. And indeed, it is a principle that has paved the way for centuries of discovery (“What has conservation of energy ever done for us?”, see page 31). When it comes to dark energy, cosmologists > 28 | NewScientist | 27 May 2017
ANDREW ARCHER 27 May 2017 | NewScientist | 29
Page 1 and 2: WEEKLY May 27 - June 2, 2017 PROOF
Page 3 and 4: CONTENTS Volume 234 No 3127 This is
Page 5 and 6: LEADERS LOCATIONS USA 45 Prospect S
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Page 9 and 10: In this section ■ Why lack of sle
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