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Super-supernovae spell trouble for dark energy
46 | NewScientist | 27 July 2013
f r a w d The w e r g that g i b o o t
ernovae p u s w o h w we kne lew We thought along and b e m a c e n o til worked – un k, says Colin Stuart boo up the rule
W
HEN Andy Howell announced what he had found, a gasp filled the lecture hall. “Nobody saw it coming,” he says. That was in Prague in 2006. Howell, now at the University of California, Santa Barbara, was at a meeting of the International Astronomical Union presenting his team’s latest observations of the exploding stars known as type-1a supernovae. We think we know what makes these stellar bombs tick – and a lot rides on us knowing it. Above all, they detonate with a similar brightness, a fact that allows us to calibrate distance in the universe. Observations of type-1a supernovae led 15 years ago to one of the landmark discoveries of modern cosmology: that the expansion of the universe is accelerating, fuelled by a
simon danaher
”We think we know what makes these stellar bombs tick – and a lot rides on us knowing it”
shadowy agent since dubbed dark energy. Except if what Howell was saying was right, things weren’t that simple. He and his colleagues had seen a monstrously bright supernova that exploded all the rules. It did not mean the accelerating universe was wrong – but unmasking whatever was responsible was going to be even harder than we thought. To understand why this was so we need to wind back to the summer of 1930, and a boat afloat on the Indian ocean. On board was the 19-year-old prodigy Subrahmanyan Chandrasekhar, en route from Madras to take up a doctoral position as an astrophysicist at the University of Cambridge. To while away the time, he did a few calculations to resolve a mystery surrounding the existence of a type of star called a white dwarf. White dwarfs are a cosmic curiosity. The stability of any star relies on a delicate balance between the inward pull of gravity and some form of internal resistance. In an ordinary star, this is supplied by the thermal pressure of nuclear fusion reactions at its core. When a star begins to run out of fuel, gravity wins out and the star starts collapsing. This collapse does not continue forever. When the diminishing star reaches about the size of Earth, electrons within it offer up their own form of resistance, called degeneracy pressure. This comes about because if the electrons were squeezed any further, they would be forced into the same energy state as each other, something a quantum rule called the Pauli exclusion principle forbids. Unable to
be corralled any further, the electrons prop up a semi-collapsed stellar remnant: a white dwarf. That explained how these mysterious objects came to be, but Chandrasekhar’s calculations also revealed a limit to their size. If the collapsing object’s mass tipped the scales at more than 1.4 times that of the sun, degeneracy pressure would not be sufficient to counteract its gravity. It would evolve into a much denser object: a neutron star or even a black hole. At the time, to mention black holes was astronomical blasphemy, and the opposition of luminaries of the age, such as the director of the Cambridge Observatory where Chandrasekhar was bound, Arthur Eddington, meant Chandrasekhar’s work was initially ignored. It took more than 50 years for full recognition to come his way, when he was awarded a share of the 1983 Nobel prize in physics for his work. The ceiling of 1.4 solar masses, known as the Chandrasekhar limit, is now universally accepted, and has been borne out time and again by observations. What bearing does this have on supernovae, dark energy and the rest? That lies in the fact that it is rare for stars to live out a solitary existence; they are more often found orbiting each other in a binary system. If one of this gravitationally conjoined pair collapses to form a dense white dwarf, it can begin to devour material from its companion star, bulking itself up. In this way a white dwarf can approach the Chandrasekhar limit even after it has formed. As it does so, it can further > 27 July 2013 | NewScientist | 47
One dwarf or two?
48 | NewScientist | 27 July 2013
contract, heat up and reignite nuclear fusion. The white dwarf is ripped apart in the blink of an eye: it explodes as a type-1a supernova. Other types of supernova exist, with chemical signatures that indicate origins in different types of stars. But because type-1a supernovae are always filled with the same amount of fuel, they always explode with the same intrinsic brightness. Their brightness as observed by us is then just a measure of how far away the explosion occurred. This apparent brightness can be used as a “standard candle” to determine the distance to the supernova’s host galaxy. This chain of reasoning led to the shock discovery, made independently by two teams in 1998, that certain supernovae are further away than they should be. Another measure of the amount of space between us and a cosmic object is its redshift: as light travels towards us ”With no idea what causes from distant parts of the universe, the the oddball explosions, we expansion of the intervening space stretches it to longer, redder wavelengths. The redshift cannot tell where they are of type-1a supernovae showed consistently leading us astray” that space had expanded more than we would expect given the supernovae’s distance as type-1a explosions, fewer than 1 per cent are measured by their brightness (see diagram, below right). The effect was so convincing and confirmed rule-breakers. Not many, but so consistent that there seemed to be only one enough to raise a pressing question: where possible conclusion: the space between us and does it leave ideas of an accelerating universe ? First up, it almost undoubtedly does not them had started to expand faster, pushing change the bald fact. Since the supernova the supernovae to greater distances. This was measurements, dark energy’s presence has a turn-up for the books. If anything, we had thought that gravity’s merciless pull would be been gleaned from other independent sources such as the patterns of fluctuations in the gradually slowing the universe’s growth. Not so: the universe was accelerating away from us. cosmic microwave background, the radiation left over from the big bang. What it does is threaten to foil attempts to Sacred violation unmask dark energy’s identity. One leading If that was a shock, what Howell presented in contender is the cosmological constant. Prague was a counter-shock. He was describing Einstein infamously introduced this quantity a type-1a supernova that had exploded three into his equations of general relativity in 1917 years earlier, known by the trip-off-the-tongue to act as a buffer against gravity and maintain name of SNLS-03D3bb. The sting in his tale the static universe that he and most others at was that the supernova’s extreme brightness the time believed to exist. He realised he had indicated a fuel stock close to two solar masses erred in the 1920s when the universe was (Nature, vol 443, p 308). Chandrasekhar’s found to be expanding from an origin in a big unbreakable limit had seemingly been broken. bang, something perfectly explicable without “I was saying that we’ve found something that a hand-spun patch on the equations. But after appears to violate this practically sacred the supernova revelations, a gravity-defying Nobel-worthy calculation that every constant with the same density in all of space astrophysicist knows,” says Howell. provided a neat way to explain why the While theorists debate how that might be universe’s expansion has been picking up possible (see “One dwarf or two?”, left), more pace: as the amount of space increases, there of these “super-Chandra” supernovae have is more of the constant, so its effects increase. been popping up. Over the past five years, the If dark energy really does take the guise of a intermediate Palomar Transient Factory (iPTF) cosmological constant, its value must never has been scanning the skies above California change over space and time. Other suggested and classifying supernovae. Of over 1000 forms for dark energy, such as an energy field Ray Bertram / Steward Observatory
Upasana Das thinks she knows how the white dwarf stars that cause type-1a supernovae can get so much more massive than the 80-year-old Chandrasekhar limit allows (see main story). “Chandrasekhar didn’t consider highly magnetised white dwarfs,” says Das, of the Indian Institute of Science in Bangalore. Das has shown, together with her colleague Banibrata Mukhopadhyay, that a concentrated magnetic field provides an extra quantum-mechanical effect known as Landau quantisation within a white dwarf. This gives electrons additional resistance against gravity, staving off collapse up to a mass of 2.6 suns, rather than the 1.4 suns suggested by Chandrasekhar (Physical Review Letters, vol 110, p 071102). The Sloan Digital Sky Survey, a cosmic trawl that has already scanned a quarter of the sky, has uncovered several white dwarfs thought to have strong enough magnetic fields for the mechanism to work. An earlier suggestion was that a rapidly rotating white dwarf might be better able to withstand gravitational pressures and accumulate more mass before exploding. But models of rotating stars suggest they would have a different chemical composition from that seen in massive supernovae, says Stefan Taubenberger of the Max Planck Institute for Astrophysics in Garching, Germany. His own idea for overbright type-1a supernovae is more radical. The standard picture has a single white dwarf devouring a companion until it reaches the Chandrasekhar limit and explodes. Taubenberger’s scenario has two white dwarfs each of about 1 solar mass that initially coexist peacefully. But one is slightly larger than the other and starts to tear material from its partner. On reaching the Chandrasekhar limit it explodes, with its ejected material ploughing into the remains of the diminished partner and making it glow brightly (arxiv.org/abs/1304.4952). “You can produce a lot of additional luminosity because some of the explosion’s kinetic energy is converted into light,” says Taubenberger. As yet these are just ideas. “There is a whole mess of data out there and observation needs to catch up with theory,” says Richard Scalzo of the Australian National University in Canberra. In April this year, Xiaofeng Wang of Tsinghua University in Beijing, China, showed that the light spectra from over 100 well-defined type-1a supernovae suggest their progenitors have two distinct chemical compositions (Science, vol 340, p 170). If that is confirmed, it might be that type-1a supernovae are not even one type at all.
When completed, the LSST will use one of the world’s largest mirrors to hunt for supernovae
called “quintessence”, would have different characteristics, for example becoming more diluted as space stretches. This suggests a way to unmask the agent of cosmic acceleration. “We need to look at dark energy’s evolution over its entire history,” says Richard Scalzo of the Australian National University in Canberra – from the close-by recent universe to the far-flung regions that reveal the universe in its infancy. Supernovae, which have been around for almost as long as stars, are the most direct way to do that. The superChandra supernovae urgently need to be
eliminated from the data sets, so as not to skew the distance measurements. That’s where things get tricky. With no idea what causes the oddball explosions, we cannot tell where they might be leading us astray. There are a few indications that the superChandra explosions tend to happen in places with low levels of elements heavier than hydrogen and helium, says Stefan Taubenberger of the Max Planck Institute for Astrophysics in Garching, Germany. That suggests the top-heavy interlopers might be more abundant in the early universe, before
Surprising supernovae In a universe expanding at a fixed rate, type-1a supernovae would be expected to look dimmer as redshift, a measure of their distance from us, increases. But observations have thrown up some surprises
Apparent brightness
DECELERATING EXPANSION
Far-off supernovae measured in the 1990s were fainter than expected, suggesting that the expansion of the universe is being accelerated by something – DARK ENERGY
More recently, some anomalously bright supernovae have broken the pattern, suggesting a different physical origin for these events
UN IFO RM
ACCELERATING EXPANSION Redshift
EX PA NS ION
stars had had much opportunity to fuse many heavy elements. “My guess is their relative rate would be higher, but we don’t have an idea of how much higher at the moment,” says Mark Sullivan of the University of Southampton, UK. The further away a supernova is, the harder it is to work out whether it is an anomaly: existing telescopes cannot provide an accurate enough spectrum of the light from the exploding star. Distant super-Chandras could already be masquerading as run-of-the-mill type-1a supernovae. “There is a real danger that these objects might creep in to the data and not be recognised,” says Taubenberger. The present clutch of super-Chandras probably represent extreme cases: there is likely to be a sliding scale of rule-breakers between the standard supernovae and the known band of rebels. These will be even harder to weed out. Sky surveys such as the iPTF and the Nearby Supernova Factory, which uses a telescope atop Mauna Kea in Hawaii, currently log thousands of type-1as, but classifying them more effectively requires knowledge of the subtle differences between millions of exploding stars. That is why a plan is afoot to kick supernova surveys into overdrive. Construction has just started on the Large Synoptic Survey Telescope, a massive instrument high in the Chilean Andes. Its primary mirror, 8.2 metres across, has already been cast and ground, and will eventually be attached to a 3.2-gigapixel digital camera, one of the largest ever made. The LSST will be able to examine 10 square degrees of the sky at a time, equivalent to the space nearly 50 full moons would take up in the sky. At 800 snaps per evening, it will cover the entire sky over just three nights, allowing it to create a database of up to a million supernova explosions over 10 years, as well as do other things such as keep a lookout for potentially hazardous near-Earth asteroids. But it will be at least a decade before we have access to that database. In the meantime, without certain knowledge of what triggers the anomalous supernovae it is hard for astronomers to know whether they are doing the right things for the right reasons. “We’re making blind corrections at the moment based on empirical observation, not a deep astrophysical understanding,” says Howell. Seven years on from that moment in Prague, the shock waves from his explosive announcement continue to reverberate. n Colin Stuart is a freelance science writer based in London. His first book, Big Questions in Science, is published on 12 September 27 July 2013 | NewScientist | 49
f r a w d The w e r g that g i b o o t
ernovae p u s w o h w we kne lew We thought along and b e m a c e n o til worked – un k, says Colin Stuart boo up the rule
W
HEN Andy Howell announced what he had found, a gasp filled the lecture hall. “Nobody saw it coming,” he says. That was in Prague in 2006. Howell, now at the University of California, Santa Barbara, was at a meeting of the International Astronomical Union presenting his team’s latest observations of the exploding stars known as type-1a supernovae. We think we know what makes these stellar bombs tick – and a lot rides on us knowing it. Above all, they detonate with a similar brightness, a fact that allows us to calibrate distance in the universe. Observations of type-1a supernovae led 15 years ago to one of the landmark discoveries of modern cosmology: that the expansion of the universe is accelerating, fuelled by a
simon danaher
”We think we know what makes these stellar bombs tick – and a lot rides on us knowing it”
shadowy agent since dubbed dark energy. Except if what Howell was saying was right, things weren’t that simple. He and his colleagues had seen a monstrously bright supernova that exploded all the rules. It did not mean the accelerating universe was wrong – but unmasking whatever was responsible was going to be even harder than we thought. To understand why this was so we need to wind back to the summer of 1930, and a boat afloat on the Indian ocean. On board was the 19-year-old prodigy Subrahmanyan Chandrasekhar, en route from Madras to take up a doctoral position as an astrophysicist at the University of Cambridge. To while away the time, he did a few calculations to resolve a mystery surrounding the existence of a type of star called a white dwarf. White dwarfs are a cosmic curiosity. The stability of any star relies on a delicate balance between the inward pull of gravity and some form of internal resistance. In an ordinary star, this is supplied by the thermal pressure of nuclear fusion reactions at its core. When a star begins to run out of fuel, gravity wins out and the star starts collapsing. This collapse does not continue forever. When the diminishing star reaches about the size of Earth, electrons within it offer up their own form of resistance, called degeneracy pressure. This comes about because if the electrons were squeezed any further, they would be forced into the same energy state as each other, something a quantum rule called the Pauli exclusion principle forbids. Unable to
be corralled any further, the electrons prop up a semi-collapsed stellar remnant: a white dwarf. That explained how these mysterious objects came to be, but Chandrasekhar’s calculations also revealed a limit to their size. If the collapsing object’s mass tipped the scales at more than 1.4 times that of the sun, degeneracy pressure would not be sufficient to counteract its gravity. It would evolve into a much denser object: a neutron star or even a black hole. At the time, to mention black holes was astronomical blasphemy, and the opposition of luminaries of the age, such as the director of the Cambridge Observatory where Chandrasekhar was bound, Arthur Eddington, meant Chandrasekhar’s work was initially ignored. It took more than 50 years for full recognition to come his way, when he was awarded a share of the 1983 Nobel prize in physics for his work. The ceiling of 1.4 solar masses, known as the Chandrasekhar limit, is now universally accepted, and has been borne out time and again by observations. What bearing does this have on supernovae, dark energy and the rest? That lies in the fact that it is rare for stars to live out a solitary existence; they are more often found orbiting each other in a binary system. If one of this gravitationally conjoined pair collapses to form a dense white dwarf, it can begin to devour material from its companion star, bulking itself up. In this way a white dwarf can approach the Chandrasekhar limit even after it has formed. As it does so, it can further > 27 July 2013 | NewScientist | 47
One dwarf or two?
48 | NewScientist | 27 July 2013
contract, heat up and reignite nuclear fusion. The white dwarf is ripped apart in the blink of an eye: it explodes as a type-1a supernova. Other types of supernova exist, with chemical signatures that indicate origins in different types of stars. But because type-1a supernovae are always filled with the same amount of fuel, they always explode with the same intrinsic brightness. Their brightness as observed by us is then just a measure of how far away the explosion occurred. This apparent brightness can be used as a “standard candle” to determine the distance to the supernova’s host galaxy. This chain of reasoning led to the shock discovery, made independently by two teams in 1998, that certain supernovae are further away than they should be. Another measure of the amount of space between us and a cosmic object is its redshift: as light travels towards us ”With no idea what causes from distant parts of the universe, the the oddball explosions, we expansion of the intervening space stretches it to longer, redder wavelengths. The redshift cannot tell where they are of type-1a supernovae showed consistently leading us astray” that space had expanded more than we would expect given the supernovae’s distance as type-1a explosions, fewer than 1 per cent are measured by their brightness (see diagram, below right). The effect was so convincing and confirmed rule-breakers. Not many, but so consistent that there seemed to be only one enough to raise a pressing question: where possible conclusion: the space between us and does it leave ideas of an accelerating universe ? First up, it almost undoubtedly does not them had started to expand faster, pushing change the bald fact. Since the supernova the supernovae to greater distances. This was measurements, dark energy’s presence has a turn-up for the books. If anything, we had thought that gravity’s merciless pull would be been gleaned from other independent sources such as the patterns of fluctuations in the gradually slowing the universe’s growth. Not so: the universe was accelerating away from us. cosmic microwave background, the radiation left over from the big bang. What it does is threaten to foil attempts to Sacred violation unmask dark energy’s identity. One leading If that was a shock, what Howell presented in contender is the cosmological constant. Prague was a counter-shock. He was describing Einstein infamously introduced this quantity a type-1a supernova that had exploded three into his equations of general relativity in 1917 years earlier, known by the trip-off-the-tongue to act as a buffer against gravity and maintain name of SNLS-03D3bb. The sting in his tale the static universe that he and most others at was that the supernova’s extreme brightness the time believed to exist. He realised he had indicated a fuel stock close to two solar masses erred in the 1920s when the universe was (Nature, vol 443, p 308). Chandrasekhar’s found to be expanding from an origin in a big unbreakable limit had seemingly been broken. bang, something perfectly explicable without “I was saying that we’ve found something that a hand-spun patch on the equations. But after appears to violate this practically sacred the supernova revelations, a gravity-defying Nobel-worthy calculation that every constant with the same density in all of space astrophysicist knows,” says Howell. provided a neat way to explain why the While theorists debate how that might be universe’s expansion has been picking up possible (see “One dwarf or two?”, left), more pace: as the amount of space increases, there of these “super-Chandra” supernovae have is more of the constant, so its effects increase. been popping up. Over the past five years, the If dark energy really does take the guise of a intermediate Palomar Transient Factory (iPTF) cosmological constant, its value must never has been scanning the skies above California change over space and time. Other suggested and classifying supernovae. Of over 1000 forms for dark energy, such as an energy field Ray Bertram / Steward Observatory
Upasana Das thinks she knows how the white dwarf stars that cause type-1a supernovae can get so much more massive than the 80-year-old Chandrasekhar limit allows (see main story). “Chandrasekhar didn’t consider highly magnetised white dwarfs,” says Das, of the Indian Institute of Science in Bangalore. Das has shown, together with her colleague Banibrata Mukhopadhyay, that a concentrated magnetic field provides an extra quantum-mechanical effect known as Landau quantisation within a white dwarf. This gives electrons additional resistance against gravity, staving off collapse up to a mass of 2.6 suns, rather than the 1.4 suns suggested by Chandrasekhar (Physical Review Letters, vol 110, p 071102). The Sloan Digital Sky Survey, a cosmic trawl that has already scanned a quarter of the sky, has uncovered several white dwarfs thought to have strong enough magnetic fields for the mechanism to work. An earlier suggestion was that a rapidly rotating white dwarf might be better able to withstand gravitational pressures and accumulate more mass before exploding. But models of rotating stars suggest they would have a different chemical composition from that seen in massive supernovae, says Stefan Taubenberger of the Max Planck Institute for Astrophysics in Garching, Germany. His own idea for overbright type-1a supernovae is more radical. The standard picture has a single white dwarf devouring a companion until it reaches the Chandrasekhar limit and explodes. Taubenberger’s scenario has two white dwarfs each of about 1 solar mass that initially coexist peacefully. But one is slightly larger than the other and starts to tear material from its partner. On reaching the Chandrasekhar limit it explodes, with its ejected material ploughing into the remains of the diminished partner and making it glow brightly (arxiv.org/abs/1304.4952). “You can produce a lot of additional luminosity because some of the explosion’s kinetic energy is converted into light,” says Taubenberger. As yet these are just ideas. “There is a whole mess of data out there and observation needs to catch up with theory,” says Richard Scalzo of the Australian National University in Canberra. In April this year, Xiaofeng Wang of Tsinghua University in Beijing, China, showed that the light spectra from over 100 well-defined type-1a supernovae suggest their progenitors have two distinct chemical compositions (Science, vol 340, p 170). If that is confirmed, it might be that type-1a supernovae are not even one type at all.
When completed, the LSST will use one of the world’s largest mirrors to hunt for supernovae
called “quintessence”, would have different characteristics, for example becoming more diluted as space stretches. This suggests a way to unmask the agent of cosmic acceleration. “We need to look at dark energy’s evolution over its entire history,” says Richard Scalzo of the Australian National University in Canberra – from the close-by recent universe to the far-flung regions that reveal the universe in its infancy. Supernovae, which have been around for almost as long as stars, are the most direct way to do that. The superChandra supernovae urgently need to be
eliminated from the data sets, so as not to skew the distance measurements. That’s where things get tricky. With no idea what causes the oddball explosions, we cannot tell where they might be leading us astray. There are a few indications that the superChandra explosions tend to happen in places with low levels of elements heavier than hydrogen and helium, says Stefan Taubenberger of the Max Planck Institute for Astrophysics in Garching, Germany. That suggests the top-heavy interlopers might be more abundant in the early universe, before
Surprising supernovae In a universe expanding at a fixed rate, type-1a supernovae would be expected to look dimmer as redshift, a measure of their distance from us, increases. But observations have thrown up some surprises
Apparent brightness
DECELERATING EXPANSION
Far-off supernovae measured in the 1990s were fainter than expected, suggesting that the expansion of the universe is being accelerated by something – DARK ENERGY
More recently, some anomalously bright supernovae have broken the pattern, suggesting a different physical origin for these events
UN IFO RM
ACCELERATING EXPANSION Redshift
EX PA NS ION
stars had had much opportunity to fuse many heavy elements. “My guess is their relative rate would be higher, but we don’t have an idea of how much higher at the moment,” says Mark Sullivan of the University of Southampton, UK. The further away a supernova is, the harder it is to work out whether it is an anomaly: existing telescopes cannot provide an accurate enough spectrum of the light from the exploding star. Distant super-Chandras could already be masquerading as run-of-the-mill type-1a supernovae. “There is a real danger that these objects might creep in to the data and not be recognised,” says Taubenberger. The present clutch of super-Chandras probably represent extreme cases: there is likely to be a sliding scale of rule-breakers between the standard supernovae and the known band of rebels. These will be even harder to weed out. Sky surveys such as the iPTF and the Nearby Supernova Factory, which uses a telescope atop Mauna Kea in Hawaii, currently log thousands of type-1as, but classifying them more effectively requires knowledge of the subtle differences between millions of exploding stars. That is why a plan is afoot to kick supernova surveys into overdrive. Construction has just started on the Large Synoptic Survey Telescope, a massive instrument high in the Chilean Andes. Its primary mirror, 8.2 metres across, has already been cast and ground, and will eventually be attached to a 3.2-gigapixel digital camera, one of the largest ever made. The LSST will be able to examine 10 square degrees of the sky at a time, equivalent to the space nearly 50 full moons would take up in the sky. At 800 snaps per evening, it will cover the entire sky over just three nights, allowing it to create a database of up to a million supernova explosions over 10 years, as well as do other things such as keep a lookout for potentially hazardous near-Earth asteroids. But it will be at least a decade before we have access to that database. In the meantime, without certain knowledge of what triggers the anomalous supernovae it is hard for astronomers to know whether they are doing the right things for the right reasons. “We’re making blind corrections at the moment based on empirical observation, not a deep astrophysical understanding,” says Howell. Seven years on from that moment in Prague, the shock waves from his explosive announcement continue to reverberate. n Colin Stuart is a freelance science writer based in London. His first book, Big Questions in Science, is published on 12 September 27 July 2013 | NewScientist | 49