Accelerating Universe: Standard ‘light bulbs’ not so standard

I once wrote about one of the problems of determining distance using the so-called standard ‘candle’ of the type Ia supernovae.1  That method is considered to be the gold standard in cosmic distance determination and hence in testing of the expanding universe paradigm. From those measurements, by two independent teams, an accelerating expansion of the Universe was claimed in 1998, for which the Nobel Prize in Physics was awarded in 2011.

That same galaxy in a NASA Swift image is shown, with bars indicating the location of supernova SN 2011fe. The Swift image is a false-color image with UV emission blue and optical emission red. Credit: NASA/Swift

An optical image of the galaxy M101, with bars indicating the location of supernova SN 2011fe. This NASA/Swift image is a false-color image with UV emission shown in blue and optical emission shown in red. Credit: NASA/Swift

Type Ia supernova were (are) believed to be a class of stellar explosions that resulted from progenitor stars with a very small range of masses and chemical properties. It was (is) believed that these could be accurately modelled and therefore they could be relied upon to produce the same intrinsic brightness in their explosions. It was believed therefore that they varied only by a very small degree in a distribution around a well established intrinsic brightness or absolute magnitude near MB ~ -19. That means they were believed to all have the same intrinsic brightness.

Circular reasoning

Previously I pointed out that the supernova chosen as candidates to be used in the distance calculation (in the Hubble diagram of brightness or magnitudeverses redshift) of the source galaxy were biased towards the result they desired. Several researchers had suggested that it was selection bias, a type of  circular reasoning. The astronomers select only the candidates that fit the desired luminosity-distance criteria and use those to determine the luminosity distance.4

And since one cannot determine the absolute magnitudes of the sources without assuming a particular cosmology, the standard concordance criteria for big bang cosmology are used. Those criteria are the total matter density of the Universe at 30% (Ωm ~ 0.3), which includes a normal matter content of only about 4%, a dark energy density of about 70% (ΩΛ ~ 0.7) and a Hubble constant H0 of 70 km/s/Mpc. Using these parameters the absolute magnitudes for the candidate supernovae are calculated and if they fall into a narrow range, near absolute magnitude MB ~ –19, they are accepted, else they are rejected as candidates. Only those that lie within the range are used to test the same model, and therefore determine the ‘correct’ values for the parameters Ωm and ΩΛ,. This is called circular reasoning.

There is a small tweak that is done. A stretch factor is introduced to allow for the small range of absolute brightness that is still acceptable. By applying a small correction factor to the supernova light curves, which describe the change in brightness over real-time as observed on Earth, these can be made to fit a standard brightness template. Hence they then all fall into the category of the standard ‘light bulb’ as desired.

But the problem is that in the big bang paradigm the supernova light curves are also meant to be systematically stretched by a relativistic time dilation factor. In an expanding universe the redshifts measured for the host galaxies of the supernovae are attributed to expansion of  the Universe. The idea is that as the Universe expands the wavelengths of the emitted light from those distant sources is also stretched. Well, relativity theory also requires then that the greater the redshift the slower time passes on those more distant galaxies relative to time on Earth.

Hence there are two mechanisms here affecting all supernova light curves. One is the aforementioned stretch factor, the fudge to get all the type Ia supernova to fit into the same family of standard brightness ‘light bulbs’ and the other is this time-dilation factor, which is found in the luminosity distance used to calculate their distances in the Universe.

But for any individual type Ia supernova the intrinsic width of the light curve (hence knowledge of the stretch factor) is unknown, so without assuming a time-dilation factor, the intrinsic width and time-dilation factor cannot be separated (i.e. disentangled). After assuming the standard time-dilation factor the stretch factor is used as a fudge to standardise the light-curve width, and hence arrive at a standard ‘light bulb’. But is that good science?

A new wrinkle

As reported in the online phys.org news site, an Arizona University-led team of astronomers has now found that these type Ia supernovae, in fact, fall into two distinct populations, not recognized before.  This has the implication that the calculation of the accelerating universe, according to the model, may not be as fast a they thought. The news site writes.5

The team, led by UA astronomer Peter A. Milne, discovered that type Ia supernovae, which have been considered so uniform that cosmologists have used them as cosmic “beacons” to plumb the depths of the universe, actually fall into different populations. The findings are analogous to sampling a selection of 100-watt light bulbs at the hardware store and discovering that they vary in brightness.

“We found that the differences are not random, but lead to separating Ia supernovae into two groups, where the group that is in the minority near us are in the majority at large distances—and thus when the universe was younger,” said Milne, an associate astronomer with the UA’s Department of Astronomy and Steward Observatory. “There are different populations out there, and they have not been recognized. The big assumption has been that as you go from near to far, type Ia supernovae are the same. That doesn’t appear to be the case.” (emphasis added)

The main assumption of the standard brightness used to determine distance is now under question. The standard ‘light bulbs,’ by the astronomers’ own admission, do not neatly fall into the same group. But they are more correctly to be treated as a bifurcated distribution, or actually two completely different distributions or groups.

The discovery casts new light on the currently accepted view of the universe expanding at a faster and faster rate, pulled apart by a poorly understood force called dark energy.5 (emphasis added)

Dark energy, the fictional force that is supposedly pushing the universe apart at an accelerating rate, is admitted to be not so well understood. On top of this, the supposed standard ‘light bulbs,’ which are foundational to the big bang expanding universe paradigm, are not what they thought they were.

Conclusion

What do we draw from this? The science of the expanding universe is not so well understood. The whole concept hinges on circular reasoning and the assumption that the chosen set of standard ‘light bulbs’ is monolithic. But that is not true either. The accelerating expanding universe paradigm is built on assumptions and fudge factors (the light-curve stretch factor, dark matter, dark energy, and even the notion that space is expanding) all of which are unverifiable in the laboratory and in the cosmos they make no sense. And now we have fresh evidence that even the chosen group of supernovae are at least a bifurcated distribution which undermines the very reason they were chosen in the first place.

References and Notes

  1. John G. Hartnett, Does observational evidence indicate the universe is expanding?—part 1: the case for time dilation
  2. The Nobel Prize in Physics 2011, Saul Perlmutter, Brian P. Schmidt, Adam G. Riess, “for the discovery of the accelerating expansion of the Universe through observations of distant supernovae”.
  3. Brightness (or magnitude, the term used by astronomers) is a proxy for distance in the universe. Simply if you have a standard brightness object then it apparent brightness tells you how far away it is based on the inverse square law. The dimmer it is seen from Earth the farther away it must be.
  4. Luminosity distance is the distance as calculated from the brightness of the sources as described in footnote 2. It depends strongly on the cosmology adopted, especially for very distant and therefore high redshift sources, within the standard big bang cosmology.
  5. Accelerating universe? Not so fast, Phys.org, 10 April 2015

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2 thoughts on “Accelerating Universe: Standard ‘light bulbs’ not so standard

  1. “We found that the differences are not random, but lead to separating Ia supernovae into two groups, where the group that is in the minority near us are in the majority at large distances—and thus when the universe was younger,”

    Are they saying that we are in some place in the universe that is different from all other places? How can we be surrounded by predominately one kind of supernovae, when another kind is predominate every where else?

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    • Good point. Not withstanding some selection effect it would appear to be another violation of the cosmological principle. Hence the type Ia progenitors are not homogeneous, at least not temporally if you believe redshifts are indicators of past epochs.

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