Astronomy Without A Telescope – Inconstant Supernovae?


Given the importance of Type 1a supernovae as the standard candles which demonstrate that the universe’s expansion is actually accelerating – we require a high degree of confidence that those candles really are standard.

A paper released on Arxiv, with a list of authors reading like a Who’s Who in cosmology and including all three winners of this year’s Nobel Prize in Physics, details an ultraviolet (UV) analysis of four Type 1a supernovae, three of which represent significant outliers from the standard light curve expected of Type 1a supernovae.

Some diversity in UV output has already been established from observing distant high red-shift Type 1a supernovae, since their UV output is shifted into optical light and can hence be observed through the atmosphere. However, to gain detailed observations in UV, you need to look at closer, less red-shifted Type 1a supernovae and hence you need space telescopes. These researchers used data collected by the ACS (Advanced Camera for Surveys) on the Hubble Space Telescope.

The supernovae studied were SN 2004dt, SN 2004ef, SN 2005M and SN 2005cf. SN 2005cf is considered a ‘gold standard’ Type 1a supernovae – while the other three show considerable diversion from the standard UV light curve, even though their optical light output looks standard.

The left side diagrams show the three anomalous Type 1a supernovae light curves in UV light through three filters. The three outlier supernovae are mapped against the light curve of SN 2005cf (solid line), considered a 'gold standard' light curve. The diversity of the other three supernovae is apparent in UV, but not in optical - as shown in the frames on the right side. Credit: Wang et al.

The researchers also looked at a slightly larger dataset of UV supernovae observations made by the Swift spacecraft – which also showed a similar diversity in UV light, that was not apparent in optical light.

This is a bit of a worry, since the supernovae dataset from which we conclude that the universe is expanding is largely based on observations in optical light which, unlike UV, can make it through the atmosphere and be collected by ground-based telescopes.

Nonetheless, if you are thinking that three outliers isn’t a lot – you’d be right. The paper’s aim is to indicate that there are minor discrepancies in the current data set upon which we have built our current model of the universe. The academic muscle that is focused on this seemingly minor issue is some indication of the importance of isolating and characterising the nature any such discrepancies, so that we can continue to have confidence in the Type 1a supernovae standard candle dataset – or not.

The researchers acknowledge that the UV excess – not seen at all in SN 2005cf, but seen in varying degrees in the other three Type 1a supernovae – with the most pronounced difference seen in SN 2004dt – is a problem, even if it is not a huge problem.

As standard candles, Type 1a supernovae (or SNe1a) are key to determining the distance of their host galaxies. But one key consideration in determining their absolute luminosity is the reddening caused by the dust in the host galaxy. A higher than expected UV flux in some SNe1a could lead to an underestimate of this normal reddening effect, which dims the visible light of the star irrespective of its distance. Such an atypical SNe1a would then be picked up in ground-based SNe1a sky surveys as misleadingly dim – and their host galaxies would be determined as being further away from us than they really are.

The researchers call this another possible systematic error within the current SNe1a-based calculations of the nature of the universe – those other possible systematic errors including the metallicity of the supernovae themselves, as well as the size, density and chemistry of their host galaxy.

The key question to take forward now is what proportion of the total population of SNe1a in the universe might have this high UV flux. To answer that we will need to get more space telescope data.

Further reading:
Wang et al. Evidence for Type Ia Supernova Diversity from Ultraviolet Observations with the Hubble Space Telescope.

11 Replies to “Astronomy Without A Telescope – Inconstant Supernovae?”

  1. I have not read the article yet, but I don’t think this is devastating to cosmology. This appears to be an issue for higher z red shifted SN1As. Since the optical band is not influenced by this UV flux variation, then optical detection can give decent cosmological results for z < 1. The JWST should carry this further with higher z redshift into the IR band. This paper mostly seems to give caveats on certain bands of the EM spectrum being measured.


      1. It is curious that there is such a variation in UV output. The SNIa is due to a white dwarf which reaches the Chandrashenkhar and implodes into a giant fusion bomb. The conditions are fairly generic. Of course the theory is somewhat adiabatic, where the term adiabatic applies to perturbation theory and not thermodynamics. The white dwarf is assumed to reach this limit in a “gentle manner” is a slow accretion process. In some cases though the limit could be reached or even surpassed by a violent process of accretion. There could also be dependencies on the metalicity of the white dwarf. Clearly SNIa events seen further back in time are likely to have low metalicity white dwarfs.


      2. Gamma rays (indeed light in general) are red shifted climbing out of any gravity well.

      3. Well theoretically, yes. You would still be able to determine what the initial ‘rest frame’ wavelength of the light was though – since absorption lines etc get red shifted too.

        So I don’t think this would help explain the phenomenom in question here. If the excess of UV was red-shifted gamma that would be easily determined. I understand that we are dealing with an excess of rest frame UV here.

  2. Likely not much of a problem since the amount of dark energy and its influence are rather finetuned to the flatness of the universe (no apparent dynamics of DE) and its equation of state (cosmological constant).

    A higher than expected UV flux in some SNe1a could enhance their brightness in visible light (in cases where the unexpectedly bright UV is red-shifted into unexpectedly bright blue light). Such SNe1a would then be picked up in ground-based SNe1a sky surveys as misleadingly bright SNe1a – and their host galaxies would be determined as being closer to us than they really are.

    But it seems to me the redshift is accounted for when deriving the visual magnitude.

    From the conclusion of the paper:

    “It provides not only a new clue to the study of SN Ia physics and/or the progenitor environments, but also draws attention to another possible systematic error that might exist in current cosmological studies: the relatively higher UV flux would result in a bluer U ?B color for some SNe Ia, which could lead to an underestimate of the host-galaxy reddening and hence an overestimate of the distance.” [p12]

    The problem is what they mean with “another … error”. It is the only mentioned distance estimate problem.

    1. I read ‘underestimate of the host-galaxy reddening’ as meaning an underestimate of redshift/recession velocity – and was then puzzled as to how that translated to an overestimate of the distance.

      I may have flubbed the redshift issue – but don’t get how a brighter-than-it-should-be SN 1a leads to an overestimate of distance.

      1. Maybe it’s just a typo in the paper… Especially on arxiv such things can happen. 😉

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