An Upper Limit On Not Knowing What the F*** They're Doing

First, I should say that the Supernova Cosmology Group and others using Type Ia supernova as standard candles are very precise in their work and I don't seriously doubt their results as they have been very consistent with other observations. There is though the one dark shadow looming over all their results and that is systematic error. Cosmologists use Type Ia supernova as a lighthouse in the dark because we can assume that all lighthouses have the same intrinsic luminosity and therefore any difference in observed luminosity is due solely to the distance from us. Thus by observing distant supernovae and recording their various properties such as luminosity and recession velocity from us we can plot their velocity versus distance and we can learn about the expansion of our universe and the cosmological constant. However, we assumed that we knew their intrinsic luminosity, but of course there are always unknown unknowns:

As we know,
There are known knowns.
There are things we know we know.
We also know
There are known unknowns.
That is to say
We know there are some things
We do not know.
But there are also unknown unknowns,
The ones we don’t know
We don’t know.

—Donald Rumsfeld, Feb. 12, 2002, Department of Defense news briefing

Today I read two things online that I really enjoyed and I realized that they are actually very connected. On The Blog of Steve Shwartz I read that No One Knows What the F*** They're Doing (or "The 3 Types of Knowledge") and couldn't agree more (for example, I certainly don't know what I am doing). And in Nature I read about An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate (and the arXiv preprint here) which raised questions about possible systematics in the use of supernovae in cosmology. The abstract from the nature article:

There is wide agreement that type Ia supernovae (used as standard candles for cosmology) are associated with the thermonuclear explosions of white dwarf stars. The nuclear runaway that leads to the explosion could start in a white dwarf gradually accumulating matter from a companion star until it reaches the Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in a compact binary system. The X-ray signatures of these two possible paths are very different. Whereas no strong electromagnetic emission is expected in the merger scenario until shortly before the supernova, the white dwarf accreting material from the normal star becomes a source of copious X-rays for about 10⁷ years before the explosion. This offers a means of determining which path dominates. Here we report that the observed X-ray flux from six nearby elliptical galaxies and galaxy bulges is a factor of ~30–50 less than predicted in the accretion scenario, based upon an estimate of the supernova rate from their K-band luminosities. We conclude that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs make a significant contribution to the supernova rate.

So, what the researchers found using Chandra data is observational evidence that type Ia supernovae are not simply explosions of Chandrasekhar mass white dwarfs, which would have been the simple case. The 'classic' picture is that when the amount of material accreted onto a white dwarf exceeds the Chandrasekhar mass the dwarf explodes:

The new Chandra results indicate that some Type Ia supernovae probably originate from the collision of white dwarf binaries. The collision occurs because the stars radiate away gravitational waves and move inevitably closer. The result is an explosion of two stars that are near the Chandrasekhar mass so the observed luminosity may not be so standard:

There is at least one caveat to the results and the explanation given above. The Chandra observations were focused on elliptical galaxies and on the the center of one spiral galaxy because these areas had minimal amounts of gas and dust which block X-rays from reaching detectors. To summarize the results, the dominant mechanism for Type Ia supernovae in the elliptical early type galaxies Chandra observed is white dwarf mergers and not mass accretion. The take away point is that cosmologists need to take into account the galaxy type when using supernovae as standard candles because elliptical and spiral galaxies have different supernova progenitors; the supernova cosmology surveys have only used a small fraction of supernova from elliptical galaxies though, so it wont really change current results! So all that worry to discover nothing so troubling, but perhaps we gain assurance that soon even more distant standard candles can be trusted (like the GRB as a standard candle) despite that we can never really place anything more than an upper limit on unknown unknowns.


References:


Marat Gilfanov, & Akos Bogdan (2010). An upper limit on the contribution of accreting white dwarfs to the type
Ia supernova rate Nature, 18 February 2010, Vol.463, p.924 arXiv: 1002.3359v1