COSMIC CONUNDRUM: What causes white dwarfs to explode as type Ia supernovae?
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When stellar cataclysms known as type Ia supernovae flare up far across the universe, their brightness and consistency allow astronomers to use them as so-called standard candles to measure cosmological distances. Just over a decade ago, two teams used the supernovae to show that the universe is accelerating in its expansion due to the influence of dark energy, a shocking discovery that thrust type Ia supernovae into the astrophysical limelight. But how exactly did these cosmic mileposts come to be?
A type Ia supernova arises from the explosion of an ultradense stellar remnant known as a white dwarf, but it is less than clear how the white dwarf comes to ignite in a thermonuclear blast. The traditional view held that a white dwarf, locked in a binary pairing with another star, sucked matter from its companion, growing ever larger in size until it could no longer support its own weight. Once a white dwarf reaches the Chandrasekhar limit, roughly 1.4 times the mass of the sun, it contracts and explodes in a massive blast.
But a new study presents evidence that, for at least one kind of galaxy, the binary-accretion model should not be more than a minor contributor to the observed type Ia supernovae population. Marat Gilfanov, an astrophysicist at the Max Planck Institute for Astrophysics (M.P.A.) in Garching, Germany, and the Space Research Institute in Moscow, along with M.P.A. graduate student Ákos Bogdán looked at elliptical galaxies for x-rays expected to arise during the accretion process. In a study published in the February 18 issue of Nature, Gilfanov and Bogdán report that they found just a fraction of the x-rays expected from white dwarfs accreting matter from their neighbors. (Scientific American is part of Nature Publishing Group.) The standard path to type Ia supernovae, the study's authors wrote, should have produced 30 to 50 times the x-rays observed, indicating that accreting white dwarfs account for less than 5 percent of the explosions.
As a white dwarf draws off hydrogen-rich material from a binary companion over millions of years, Gilfanov says, it experiences a steady process of nuclear fusion on its surface that gives off tremendous amounts of radiation. That radiation should be detectable in the x-ray band, although interstellar gas and dust would absorb some of it. That is why the researchers focused on elliptical galaxies, which have less obscuring material than spiral and irregular ones. Gilfanov says they are now working on characterizing the type Ia progenitors in other galactic types, such as the spiral cousins of our own Milky Way.
Andrew Howell, a staff scientist with the Las Cumbres Observatory Global Telescope Network in Santa Barbara, Calif., says that alternative origins for type Ia supernovae are becoming more compelling. "The evidence has been building for years that the classical paradigm, the single-degenerate scenario, is not enough to explain every type Ia that we see," Howell says. The favored alternative at present is the so-called double-degenerate scenario, in which two white dwarfs locked in a binary pairing spiral inward and merge, triggering an explosion. Such an explosion, which would have more fuel to burn than a single detonated white dwarf, might explain certain bright supernovae that appear to be powered by an object above the Chandrasekhar mass.
Howell says that these mergers have been less favored because it is difficult to make them work in a three-dimensional computer model, although recent work has offered promise. "Nature is telling us that these mergers happen, but we're not smart enough yet to figure out how this happens," he says.
The use of type Ia supernovae for cosmic distance measurements does not depend heavily on knowing the mechanism by which they detonate, so the new work will not unseat dark energy as a widely accepted component of the universe. But Howell notes that the mix of supernova brightnesses changes as astronomers look farther across the universe and, by extension, further back in time. Understanding the progenitors of the explosions might help unravel their evolution through cosmic time.
And Gilfanov says that resolving the underlying astrophysics of type Ia supernovae would help make standard-candle measurements more precise. "If we want to go from 10 percent to 1 percent [uncertainty] in measuring cosmological parameters," he says, astronomers need a better understanding of why white dwarfs explode in supernovae. "Dark energy will not go away, and the concept of standard candles will not go away," Gilfanov says. "It just gives us a better understanding and a better set of tools."
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13 Comments
Add CommentI have to keep this simple, but as I understand the reported revolutionary evidence for the accelerating expansion of the universe, which extraordinarily requires the proposed unidentified Dark Energy, is based entirely on the assumption that all type Ia SN (or at least those included in the reports) have the identical peak emission luminosity. In other words, if the emission luminosity of the SN included in the studies reporting definitive determination that universal expansion was not that presumed, those studies are invalidated.
Reply | Report Abuse | Link to thisThe crucial finding of those studies was that the ‘actual’ distance to the more distant SN observed, estimated using the diminishment of their peak observed luminosity from the presumedly constant peak emission luminosity, exceeded the distance estimated by standard cosmological models, which rely on the observed redshift of light from their emission galaxies. From this finding it was extraordinarily concluded that the universe is accelerating.
Peter Nugent, an astrophysicist with Berkeley Lab's Computational Research Division and one of the principal authors of one of the reports cited, is quoted in that report: "Type Ia supernovae are thought to be reliable distance indicators because they have a standard amount of fuel — the carbon and oxygen in a white dwarf star — and they have a uniform trigger."
Meanwhile, this SciAm article states: “The use of type Ia supernovae for cosmic distance measurements does not depend heavily on knowing the mechanism by which they detonate, so the new work will not unseat dark energy as a widely accepted component of the universe.”
These statements are contradictory [additional remarks omitted here.]
Well stated, and I have to say that I would agree. From the first time I was introduced to the theory of dark matter / energy driven acceleration - and more imporantly the manner in which such a conclusion was reached - I found myself doubting the validity of such a proposition. Occam's Razor, when applied in this instance, compels me to suspect that there is some fault with our measurement process.
Reply | Report Abuse | Link to thisThere was ten times more hydrogen gas in the early universe, and far less old white dwarf collisions taking place despite more stars forming then today. The dark energy measurements are corrupted and the search will continue for the same reasons.
Reply | Report Abuse | Link to thisgalaxy_man - Thanks: I did the best I could at the time. Actually, the last sentence of the first paragraph should have read:
Reply | Report Abuse | Link to this"In other words, if the actual emission luminosity (of the SN included in the studies that reported definitively determining that universal expansion was accelerating) was not the presumed luminosity, the conclusions of those studies are invalid."
It seems that once any interpretation of observations 'passes the test of time' (even if otherwise untested), extraordinary proof is demanded of any contradictory interpretation. This alone accounts for the establishment and perpetuation of such highly questionable and weakly founded conclusions.
Binary white dwarfs orbit each-other. How can any of them colide with the other? One of them would have to increase its gravitational force to pull the other .
Reply | Report Abuse | Link to thisI can't answer your question, but alternatively they could somehow lose orbital velocity...
Reply | Report Abuse | Link to thiswhy would a variance in peak luminosity affect the REDSHIFT that determines an expanding cosmos. their not measuring fading light. are you quite sure there is a discrepency?
Reply | Report Abuse | Link to thisthe idea that the cosmos has an "end' or "margin" is tremendous fallacy. how simply one can conceive of an infinite continuous cosmos much the same throughout as in our vicinity.
Reply | Report Abuse | Link to thisthen what about the BigBang. it must have no specific center but occur uniformly throughout infinite borderless space. it is not like a ripple in a pond but really a continuous cyclical transformation. as there is no margin to the universe . space. there is no standing outside of space. therefore no standing outside of time. maybe time is unlike what we measure with clocks -the efects of time. time must be a constant momentum that carries this cyclical transformation. surely when the transformation from a dead universe to a new one the conversion is 1% . 1% Matter and Energy -leaving 99% DARK MATTER AND DARK ENERGY. Dark Matter then may be the matrix of the cosmos -and Dark Energy, the momentum of time.
see if it doesn't make more sense that the cosmos is infinite and continuous and that the entirety transforms simultatneously as it is continuous. in all ten quarters with chilicosms -and an eternal but mundane cycle. maybe the proportion of the infinite 1% and 99% determines the periodic table and the qoutient the natural laws.
peak luminosity might necessarilly be discarded -but REDSHIFT does not depend on luminosity but on the shifting of identifiable spectrums -spectrums that can be determined because the relationships of dark lines doesn't change for these elemental fingerprints. only their placement in natural light.
Reply | Report Abuse | Link to thisBinary white dwarfs are believed to be uncommon, and shown not to supernova when reaching a certain mass increase ratio by accretion. This was the basis for a standard candle for dark energy, where the absolute magnitude would not vary between explosions. Type 1a SN collisions are between two widely varying masses, I believe this would invalidate the use of them for standard candles. Also, white dwarfs are old stars and were less in numbers in the early univese, so they should be more difficult to detect at greater distances in the past, being collisions and not accretion are the cause. this would make an accelerated expansion more then it should be.
Reply | Report Abuse | Link to thisIt is difficult for me to explain clearly, but I’ll do my best. The luminosity and redshift mentioned represent two separate measurements.
Reply | Report Abuse | Link to thisGenerally, the method of these studies was to more accurately determine distance from the presumedly constant peak emission luminosity of the type Ia SN and the peak observed luminosity. Luminosity decreases at a fixed rate over distance: presuming the emission luminosity allowed them to calculate distance from the observed luminosity. Since the emission luminosity of other stars varies, this method is only useful if the luminosity at the emission source is a known quantity.
The broader spectrum light from the galaxy in which the type Ia SN was found is input into cosmological models that also estimate distance based on redshift. It was also presumed that the redshift would also indicate the amount of spacetime expansion that had occurred since emission.
The more distant SN observed produced a discrepancy in the estimated distance of these two methods: the SN luminosity method indicated the SN was further away than the distance cosmological models estimated for the galaxy in which the SN resided. The nearer group of SN observed did not produce this discrepancy in estimated distances. Since the discrepancy occurred only for more distant SN, it was concluded that the rate of expansion was increasing. I cannot explain how they came to that conclusion.
It seems to me that since the more distant observations, representing the earlier state of the universe, were interpreted to indicate increased expansion relative to the nearer, more recent emissions, it should have been concluded the expansion of spacetime has decelerated, as had been previously expected. However, that’s a separate issue from the reliability of the type Ia SN peak emission luminosity estimate.
jimhenson – As I understand, it was expected that type Ia SN occurred when a white dwarf star in orbit with another main-course (hydrogen) star had accreted a sufficient mass from its partner, estimated to be about 1.4 Solar masses, it would supernova. Since this SN event was thought to always occur at a fixed mass quantity, the peak luminosity at the emission source would always be constant. In theory, this would allow accurate estimation of distance using only its observed peak luminosity.
Reply | Report Abuse | Link to thisThe principal issue is that if other conditions may produce SN indistinguishable from type Ia that occur at a mass other than the expected roughly 1.4 Solar masses, or that are composed of different materials, their peak emission luminosity can vary, making them an unreliable standard candle. While some may minimize the potential for this variability to have affected distance estimations for the SN observations used in studies determining that the universe is accelerating (as did SciAm in this article), it certainly requires reexamination of that conclusion.
Science News says the new findings "WILL SHIFT DARK ENERGY MEASUREMENTS." They'd better start believing that knowing the mechanism matters. the false belief in accretion rather then widely varying masses colliding, will not produce a constant peak luminosity for a standard candle. I knew this from elementary astronomy and posted it on my website when the story hit at quantauniverse.com.
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