This year’s Nobel Prize for Physics has today been awarded to Saul Perlmutter, Brian Schmidt and Adam Reiss for the discovery in the 1990s that the expansion of the universe is accelerating. In turn, this implies the existence of “dark energy” driving the acceleration. We don’t know what dark energy is, but it appears to constitute about 3/4 of mass of the Universe. The discovery is obviously incredibly important to our understanding of the nature of our Universe, and the Nobel Prize is well deserved.
The accelerating expansion was detected from observations of Type Ia Supernovae (SN Ia for short) in distant galaxies. A supernova is the catastrophic explosion of a star. When observed, SN Ia characteristically have no hydrogen lines in their spectra, and so they are thought to result from the total destruction of a white dwarf star. White dwarfs are the remnants of the cores of stars that were once like the Sun. Over 90% of stars end their lives as white dwarfs. Since stars burn hydrogen (and subsequently helium) in nuclear fusion reactions, white dwarfs are largely composed of the products: mainly carbon and oxygen. An earlier Nobel prize winner, Subrahmanyan Chandrasekhar, described how white dwarf stars prevented themselves from collapsing under their own gravity (since they don’t undergo nuclear fusion in their cores), and calculated the maximum mass for a white dwarf: about 1.4 times the mass of the Sun. Beyond this, they will collapse, presumably catastrophically. Hence, it is generally believed white dwarfs are behind SN Ia.
Not only that, but since all collapsing white dwarfs will have the same mass, then all SN Ia should have the same intrinsic luminosity and can be used as “standard candles” to measure distance and the speed with which distant galaxies are receding from us. By this method, Perlmutter, Schmidt and Reiss determined that the expansion of the Universe is accelerating and hence showed the existence of dark energy.
And this is the story you will read in the papers and online today and tomorrow.
The problem is: we don’t really know what causes SN Ia. We believe a white dwarf is involved based on the evidence we have. But we don’t know for sure how the white dwarf gains enough mass to exceed the “Chandrasekhar limit”, nor how the explosion proceeds, and we’ve never, despite huge effort, discovered a bona fide progenitor of an SN Ia.
There are two main candidates.
The first is a short-period (say few hours) binary system containing a white dwarf and a Sun-like star that has probably started evolving away from the “main sequence” (it’s become a red giant). The white dwarf, having a large gravitational field, pulls matter off the red giant and accretes this onto it’s surface. If the white dwarf had sufficiently large mass to start with, it may accrete enough to push it over the Chandrasekhar limit and explode as an SN Ia.
The second candidate is also a binary system, this time containing two white dwarf stars. As time goes on, the orbit of these two stars shrinks through the emission of gravitational wave radiation. Eventually, they will merge, and if their combined mass is greater than the Chandrasekhar limit, the merged object will explode. Maybe.
As a white dwarf specialist I’m pretty familiar with the searches for both these types of progenitors. Unfortunately, despite huge effort, *no convincing progenitor of either type* has ever been found. Sure, there are a small number of extremely interesting objects that could be argued may represent a progenitor (eg the subdwarf+white dwarf binary KPD 1930+2752, Maxted et al., 2000). But they are not universally accepted. Indeed, it may be the case that both the scenarios described above can
lead to SN Ia.
My white dwarf colleagues and I were a little perturbed by the use of SN Ia as standard candles when Perlmutter and Schmidt’s results were announced in the late 1990s, and remain so to this day. And while I understand cosmologists are happy that other methods appear to support the SN Ia results and that the existence of dark energy is widely accepted, it is worth re-iterating we still don’t know what causes SN Ia. Personally, I think that’s a little bit worrying, especially when Nobel prizes are being handed out.
Throughout my career, the white dwarf field has remained pretty small compared to
the main extragalactic and cosmology communities. There’s maybe 150 people,
including students and theoreticians, seriously investigating white dwarf stars worldwide. As a result, it’s hard to get grants and telescope time. And those important citations are few and far between. Yet, understanding white dwarfs and their evolution is clearly of utmost importance to Nobel Prize-winning science.
As my colleague Stu Littlefair (@slittlefair) tweeted: “Today just provided me with the first sentence for many grant and telescope applications to come!”