New Fuel for Type Ia Supernova Debate

Type Ia supernovae leaped into astronomical fame in 1998 when they helped astronomers discover the accelerating universe and suggested the presence of a mysterious anti-gravity force known as dark energy.

Yet even though Type Ia's continue to be astronomy's best cosmological rulers, astronomers are still hotly debating the cause of these powerful explosions, a question that becomes ever more important in this age of precision cosmology.

The more familiar kind of supernova (either Type Ib, Type Ic, or Type II) is pretty well understood. When a massive star runs out of fuel, its core collapses to form a neutron star or black hole. The outer layers ricochet off the collapsing core in a spectacular explosion. But a Type Ia supernova, astronomers agree, is an altogether different beast — a white dwarf that becomes a thermonuclear bomb, leaving nothing behind but wisps of gas.

An artist's illustration of two merging white dwarfs. As they spiral inward, the white dwarfs emit gravitational waves, bending the fabric of space-time into outward ripples as if they were two oars paddling through water. This lost energy saps the orbital energy of the pair, causing the orbit to shrink. After millions of years the white dwarfs will merge, perhaps in a cataclysmic explosion seen by astronomers as a Type Ia supernova.
So what ignites the bomb? Astronomers have argued over the answer for some 40-odd years. One camp holds that white dwarfs explode when they collect too much gas from a "normal" companion star. The extra mass tips the white dwarf past the Chandrasekhar limit, the point at which the pressure between electrons can no longer support the dwarf's weight. The dwarf starts to collapse in on itself, setting off the runaway nuclear fusion reactions that make it explode.

A single exploding white dwarf used to be the textbook answer to Type Ia supernovae, and evidence suggests that, at least in some cases, it's true. But another contentious study, which looked for the X-ray emission expected from white dwarfs accreting matter, found much less than expected.

Meanwhile, another theory is gaining traction. This theory is based on binary systems, where two white dwarfs revolve around each other in an ever-shrinking orbit. When they merge, kaboom.

To see if this theory could be a valid contender in the Type Ia debate, Carlos Badenes (University of Pittsburgh) and Dan Maoz (Tel Aviv University) rifled through the Sloan Digital Sky Survey, a massive archive of images and spectra, to find 4,000 white dwarfs floating in the Milky Way Galaxy. They measured each white dwarf's motion by measuring the Doppler effect in the archived spectra. Fifteen white dwarfs whirled around an unseen companion faster than 250 km/second (560,000 mph), implying a tight orbit around another white dwarf. Such a fast, close pair is destined to spiral together.

Centered in these frames are 72 of the 4,000 white dwarfs that Badenes and Maoz observed in the Sloan Digital Sky Survey.
Carles Badenes and the SDSS-III team
Using their observed numbers, Badenes and Maoz compared the rate of such white dwarf mergers expected in our galaxy to the rate of Type Ia supernovae in Milky Way-type galaxies, only to find that the rates are almost exactly the same: on average, one white-dwarf pair merges, and one Type Ia supernova explodes, in each galaxy every century. This doesn't prove that white dwarf mergers produce Type Ia supernovae, but it proves that they exist in sufficient numbers.

"We don't theorize about the existence of white-dwarf pairs, we observe them," Badenes says. "We have to apply models to interpret their velocities in terms of a merger rate, but the 15 short-period binaries in our sample are real. Those 15 systems unavoidably lead to a high merger rate."

The models used by Badenes and Maoz contain significant uncertainties, but the results still represent progress in the field, says Andrew Howell (University of California, Santa Barbara), a supernova expert not involved in the study.

If the study holds up, however, a question remains. Most of the white-dwarf pairs that Badenes and Maoz found add up to less than the Chandrasekhar limit: less than about 1.4 times the mass of our Sun. Astronomers have a handle on what happens in a Type Ia supernova if a white dwarf exceeds the Chandrasekhar limit. But if less massive white-dwarf mergers cause Type Ia supernovae, then "what happens during the merger is very much an open question," says Badenes. "From a modeling point of view, it’s a very messy problem." But that's something they'll leave to the theorists.

In the meantime, find more details in the press release or in their paper, to be published in the Astrophysical Journal Letters.

13 thoughts on “New Fuel for Type Ia Supernova Debate

  1. Rod

    Indeed there is debate about these supernovae and if they support the accelerated expansion of 3D space. Astronomers report the necessary time dilation effect in their light curves as John Hartnett documented in JOC 25(3):109-114, 2011, Does observational evidence indicate the universe is expanding?-part I: the case for time dilation. What I rarely see is what JOC published. Recent work by Mike Hawkins of the Royal Observatory in Edinburgh, UK examined nearly 900 QSOs imaged and documented over 28 years and did not find the predicted time dilation. That means the expanding universe has real contradictions between Type Ia and QSOs.

  2. Edward Schaefer

    I took a look at the Mike Hawkins paper and quickly realized the flaw: These results can be explained by quasar brightnesses changing chaotically. With chaos, you will see the same (or at least a very similar) frequency/power spectrum at different time scales. So these results are evidence for chaos and not evidence against time dilation.

  3. Bruce Mayfield

    Rod, I’m puzzled by your posts. Several of your comments have shown that you at least lean toward Creation as an expaination for existance. As a student of the Bible, I certainly believe that "God created the heavens and the earth." My question is, what’s the issue with universal expansion? Expansion actually fits very nicely with numerous biblical texts referring to God’s "stretching out the heavens."

  4. Peter WilsonPeter Wilson

    Have not read Mike Hawkins’ paper, but the accelerated expansion of 3D space is real. The Earth has been pulling on the Moon gravitationally for the last 4.5 billion years, yet it has been getting further away the whole time. Like space, its orbit is expanding. And like the cosmos, its expansion is accelerating! Once one understands why the expasnsion of the Moon’s orbit is accelerating, the accelerating expansion of the cosmos makes sense, as well.

  5. Allan Holmgren

    As to the type 1a supernovae that don’t seem to exceed the Chandrasekhar limit, as the two White Dwarfs orbit each other at a high rate of speed and spiral toward a merger there must be a lot of kinetic energy between them. Relativitys’ equivalence could account for enough mass from this energy to exeed the Chandrasekhar limit and produce the type 1a explosion.

  6. Rod

    Bruce Mayfield, glad to see you read my post. My concern is that there is a growing volume of data that conflicts with or appears not to support 3D space expanding. Some in this thread claim Mike Hawkins paper is flawed due to chaos in QSO spectrums and the transform analysis used. That is a subject for a peer reviewed journal like ApJ or MNRAS or even JOC or perhaps S&T could publish a rebuttal. John Hartnett in JOC reported on Type Ia spectrums and the selection process used to determine which to keep and which to toss so selection bias plays a role too. Hartnett published a table listing 19 items (ranging from Hubble Law to HI cloud spacing vs. z) that either support expanding universe or do not. As he concluded – "Taking together all the evidences presented here in parts 1 and 2 (see table 1), in my opinion, it is impossible to conclude either way whether the universe is expanding or static. The evidence is equivocal. It would seem that cosmology is far from a precision science, and there is still a lot more work that needs to be done to resolve the apparently contradictory evidence." For those who read the Bible I caution that we do not tie expansion into Genesis and be dogmatic on the topic.

  7. Peter WilsonPeter Wilson

    Rod: What about the the Moon’s orbit? Is there any data that conflicts with its measured rates of expansion and acceleration? Can we agree that in a 2-body problem, such as the Earth-Moon system, accelerating expansion has been both predicted and measured?

  8. Rod

    Peter, you bring up an interesting point concerning the lunar orbit expanding. It is based upon Earth’s tidal dissipation rate, conservation of angular momentum and the Moon’s orbital angular momentum changing, thus the Moon is slowly receding from Earth while the Earth’s length of day is slowly increasing, i.e. the Earth spin is slowing down and the Moon is moving away. This is not the same as 3D space expanding which is based upon equations derived from Einstein’s GR. The CMBR redshift of z > 1100 is an example of 3D space expanding and the redshift of galaxies in the big bang model. Edwin Hubble thought the z values were related to Doppler shifts but later this was changed to 3D space expansion where photons redshift as they travel through expanding 3D space. This is an important area of theoretical physics that as far as I know, has not been confirmed experimentally in the labs. Hope this helps.

  9. Bruce Mayfield

    Rod, thanks for your explaination of your veiwpoint. Even the Bible advises us not to put faith in every word. (Proverbs 14:15) Actual, ground truth fact is not determined merely by consensus or majority opinion. That being said, the expansion of the universe does seem to be a solidly proven fact, given the redshifted galaxies you just mentioned. Even the recent discovery of accelerated expansion seems to be on solid ground. Are the researchers you site trying to resurrect the old "Steady State" theory?
    And to Peter Wilson, Rod’s explanation for the moon’s receeding orbit is correct. For more, see par. 2 under the "Tidal effects" heading in the "Moon" article in Wikipedia. It lists the moon’s receedence as 38mm/year. If universal expansion were the cause of the moon’s receeding then all planets everywhere would be pulling away from the stars they orbit. The earth’s orbit is over 389 times as large as the moon’s orbit, so earth would be receeding from the sun at about 14.8 meters/year. I agree with you that the universe is expanding, but my quess is that the place where it will eventually be proven to be happening is in the voids between galactic clusters.

  10. Rod

    Bruce a few quick answers here. Concerning expansion of the universe as "solidly proven fact" I would agree if the physics were based upon the well tested Doppler effect but it is not. Perhaps a best fit interpretation based upon theoretical physics using Occam’s razor. Concerning the researchers resurrecting the Steady State model, I don’ know. However, I think it is good science to test other parameters to see if the expanding universe is consistent with other evidence. The big bang model has no center and is based upon the Copernican principle. I suspect you could construct a cosmology with Earth at or near the center of an expanding universe and astronomers could never disprove that model. It is rejected in favor of the Copernican principle mostly I suspect because of philosophical grounds vs. empirical science.

  11. Peter WilsonPeter Wilson

    Rod, in terms of energy, expansion is expansion. The Earth’s orbit is either expanding or contracting, I would have to look it up, depending on whether the other planets are adding or taking away net energy. Expansion of the cosmos involves energy. The math Newton used "blows up" when an infinite number of bodies is involved. Einstein to the rescue, but his equations mostly keep track of energy. Whether it is expansion of a two-body system or an infinite one, adding energy to a gravitational system increases the distance between objects. The way it increases depends on the way energy is moving in the system and the geometry. When comet-dust is blown out of the solar system, both the radiation-pressure driving it and the sun’s gravity opposing it decrease with the inverse-square of distance, so the dust’s orbit expands at a constant rate. You can use GR to calculate the effect of radiation pressure on comet-dust, and you also find the dust neither speeds up nor slows down. In other situations, such as the Moon’s orbit and the 3D expansion of the cosmos, the energy input is nearly constant, but the gravity field decreases with distance, so accelerating expansion is what we should expect. Accelerating expansion is what we see; there is no reason to doubt it.

  12. jorge Llanos

    I tried to switch to adifferent location and I was unable to switch from you interactive screen. Please explain what should be done to activate location change.

    Thank you.

  13. Rod

    To clear up some items here, expansion of 3D space in cosmology is based upon the Friedmann-Robertson-Walker (FRW) metric derived from GR and the metric uses the cosmological constant. Type Ia SNe and accelerated expansion is based upon dark energy, not the cosmological constant in the FRW metric. The FRW metric has a definition for redshift where 3D space is stretched, so are all the electromagnetic waves passing through the space, thus the photons are redshifted. My comments about Doppler effect well tested is based upon this difference in explaining redshifts. The FRW metric for photons redshifting still has not been confirmed in experimental tests.

All comments must follow the Sky & Telescope Terms of Use and will be moderated prior to posting. Please be civil in your comments. Sky & Telescope reserves the right to use the comments we receive, in whole or in part, and to use the commenter’s username, in any medium. See also the Terms of Use and Privacy Policy.


This site uses Akismet to reduce spam. Learn how your comment data is processed.