Dark Energy ... seemed like a good idea at the time?

ps200306

People have been talking for a long time about possible systematic biases in Type Ia supernova brightnesses. Those are the "standard candles" used in the 1998 cosmic expansion measurements that led to the 2011 Nobel Prize for Perlmutter, Schmidt and Riess. They found the expansion of the universe was expanding due to so-called dark energy.

News just in suggests once again that the key assumption is in error. A research team has found that older (i.e. closer) stellar populations produce brighter Type Ia supernovae, with a 99.5% degree of confidence. The bombshell is that “when the luminosity evolution of SN is properly taken into account, the team found that the evidence for the existence of dark energy simply goes away”.

Commenting on the result, Prof. Young-Wook Lee (Yonsei Univ., Seoul), who led the project said, “Quoting Carl Sagan, extraordinary claims require extraordinary evidence, but I am not sure we have such extraordinary evidence for dark energy. Our result illustrates that dark energy from SN cosmology, which led to the 2011 Nobel Prize in Physics, might be an artifact of a fragile and false assumption.”

If this is accepted it’s going to be quite a shock to the system for the physics community. That’s not guaranteed, of course, but the latest result is based on nine years of work so can’t be lightly discarded. It’ll be published in this month’s Astrophysical Journal, but there's a preprint up on arxiv (arXiv:1912.04903):

Early-type Host Galaxies of Type Ia Supernovae. II. Evidence for Luminosity Evolution in Supernova Cosmology

Yijung Kang, Young-Wook Lee, Young-Lo Kim, Chul Chung, Chang Hee Ree

The most direct and strongest evidence for the presence of dark energy is provided by the measurement of galaxy distances using type Ia supernovae (SNe Ia). This result is based on the assumption that the corrected brightness of SN Ia through the empirical standardization would not evolve with look-back time. Recent studies have shown, however, that the standardized brightness of SN Ia is correlated with host morphology, host mass, and local star formation rate, suggesting a possible correlation with stellar population property. In order to understand the origin of these correlations, we have continued our spectroscopic observations to cover most of the reported nearby early-type host galaxies. From high-quality (signal-to-noise ratio ~175) spectra, we obtained the most direct and reliable estimates of population age and metallicity for these host galaxies. We find a significant correlation between SN luminosity (after the standardization) and stellar population age at a 99.5% confidence level. As such, this is the most direct and stringent test ever made for the luminosity evolution of SN Ia. Based on this result, we further show that the previously reported correlations with host morphology, host mass, and local star formation rate are most likely originated from the difference in population age. This indicates that the light-curve fitters used by the SNe Ia community are not quite capable of correcting for the population age effect, which would inevitably cause a serious systematic bias with look-back time. Notably, taken at face values, a significant fraction of the Hubble residual used in the discovery of the dark energy appears to be affected by the luminosity evolution. We argue, therefore, that this systematic bias must be considered in detail in SN cosmology before proceeding to the details of the dark energy.

Sabine Hossenfelder mentions this and similar doubts:

ps200306

ps200306 wrote: »

They found the expansion of the universe was expanding due to so-called dark energy.

Should have said: expansion was accelerating.

SophieLockhart

I presume the lads have spent their Nobel money by now Seriously though, there are several other lines of evidence for dark energy, not just supernovae. And there's no evidence that the SN connection is overturned by this paper, as they used a very small sample size and red shift range. The press release is more sensational than what the paper is actually saying, surprise surprise.

ps200306

Isn't it fair to say, though, that supernova data was supposed to the clincher? The other lines of evidence for dark energy are less compelling.

Some of it is just an aesthetical desire to make the universe flat. When I was a nipper the prevailing idea was that the universe was asymptotically flat -- no recollapse or runaway expansion -- just 'cos it sounded like a nice idea. Dark energy changed that, but it's suspiciously close to undetectable with a very small inflection point in the expansion just two billion years ago.

It's also well dodgy from the point of view of explaining it as "vacuum energy". Apparently it's 120 orders of magnitude weaker than expected on theoretical grounds. It's supposed to be commensurate with density measurements based on the CMB, but am I right in thinking there's now a significant discrepancy in the Hubble parameter as measured by the two approaches?

Just an interested amateur, so could be wrong on all this.

Fourier

ps200306 wrote: »

Isn't it fair to say, though, that supernova data was supposed to the clincher? The other lines of evidence for dark energy are less compelling.

They're not quite as strong but they are enough on their own to conclude that Dark Energy exists. They're only somewhat weaker.

ps200306 wrote: »

It's also well dodgy from the point of view of explaining it as "vacuum energy". Apparently it's 120 orders of magnitude weaker than expected on theoretical grounds.

That's if you do a naive simple calculation. In a more complex calculation the answer comes out much closer. The issue is we don't know what are the most relevant parts of the calculation for getting the correct answer and how far we have to carry the calculation for agreement. Computing things exactly in quantum field theory is usually impossible so we have a variety of approximation methods and it's an issue of how close the approximation needs to get for you to conclude that the "true" answer matches the evidence.

This is an increasing issue where our theories are getting so complicated it is becoming difficult to check what they predict.

ps200306 wrote: »

It's supposed to be commensurate with density measurements based on the CMB, but am I right in thinking there's now a significant discrepancy in the Hubble parameter as measured by the two approaches?

Yes that is correct. There is either some new physics out there or there is some systematic error.

Note that they come from two completely separate lines of reasoning. One is based on stellar physics and the other high energy and temperature particle physics. They come within about 5% of each other. So it's possible that we are making some kind of error in our approximations of stellar evolution or the early "plasma" near the time of the Big Bang. If we aren't then there is some new physics affecting one or both of the results.