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New research reported by Royal Astronomical Society also backed by DESI shows no evidence of an accelerating universe.

"Remarkable" findings published today in Monthly Notices of the Royal Astronomical Society cast doubt on the long-standing theory that a mysterious force known as 'dark energy' is driving distant galaxies away increasingly faster.

The academic paper Strong progenitor age bias in supernova cosmology – II. Alignment with DESI BAO and signs of a non-accelerating universe seems fairly cryptic to me, but the claims ("translated" by RAS) sound very bold

For the past three decades, astronomers have widely believed that the universe is expanding at an ever-increasing rate, driven by an unseen phenomenon called dark energy [...] 2011 Nobel Prize in Physics.

If these results are confirmed, it would mark a major paradigm shift in cosmology since the discovery of dark energy 27 years ago.

This news looks huge to me but maybe I'm biased. Can someone explain why "there's still no real proof"?

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  • $\begingroup$ Didn't results from Webb also suggest it? Thing is there's still no real proof. $\endgroup$ Commented Nov 27 at 21:01
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    $\begingroup$ related astronomy.stackexchange.com/questions/54760/… $\endgroup$ Commented Nov 27 at 21:06
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    $\begingroup$ "How big is this news?" isn't really a suitable SE question. And asking someone to critique a paper that has just emerged is also rather premature. Perhaps you could just focus on what parts of the paper you don't follow. $\endgroup$ Commented 2 days ago
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    $\begingroup$ @Mithoron thanks for the links, TBH none seem to relate to the same idea of SN age, can you be more specific? Also not sure what you mean by "real proof". $\endgroup$ Commented 2 days ago
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    $\begingroup$ Keep in mind that by adding another free parameter (previously the $w_0w_a$ in DESI results, and now in this work the parametrization of how supernova brightness correlates with age of the universe), you will always get a better fit to any data set. The level of improvement can still be compelling of course. $\endgroup$ Commented yesterday

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This is the abstract of the linked paper:

Supernova (SN) cosmology is based on the key assumption that the luminosity standardization process of Type Ia SNe remains invariant with progenitor age. However, direct and extensive age measurements of SN host galaxies reveal a significant (⁠$5.5\sigma$⁠) correlation between standardized SN magnitude and progenitor age, which is expected to introduce a serious systematic bias with redshift in SN cosmology. This systematic bias is largely uncorrected by the commonly used mass-step correction, as progenitor age and host galaxy mass evolve very differently with redshift. After correcting for this age bias as a function of redshift, the SN data set aligns more closely with the $w_0w_a$ cold dark matter (CDM) model recently suggested by the Dark Energy Spectroscopic Instrument (DESI) baryon acoustic oscillations (BAO) project from a combined analysis using only BAO and cosmic microwave background (CMB) data. This result is further supported by an evolution-free test that uses only SNe from young, coeval host galaxies across the full redshift range. When the three cosmological probes (SNe, BAO, and CMB) are combined, we find a significantly stronger (⁠$>9\sigma$⁠) tension with the $\Lambda$CDM model than that reported in the DESI papers, suggesting a time-varying dark energy equation of state in a currently non-accelerating universe.

And here's an attempt to make a less-technical version:

One of the key lines of evidence for dark energy is supernovae data, where faraway supernovae appear to be receding faster than they should. This line of evidence relies on the idea that supernovae brightness remains constant throughout the universe's history. This assumption might not hold; measurements suggest that there is a $5.5\sigma$1 correlation between the supernova's brightness and the age of the universe. After correcting for this bias, the supernovae data appears to agree with the recent DESI $w_0w_a$ model2 of dark matter. Combining the supernovae data with other lines of evidence (BAO/CMB), there is a $9\sigma$3 disagreement with the standard model of cosmology ($\Lambda$CDM), suggesting that dark energy is changing, and the universe's expansion is currently not accelerating.

1 This is a pretty big disagreement.

2 This is a relatively recent measurement, by the DESI collaboration, suggesting that dark energy is changing.

3 As you might guess, if $5.5\sigma$ is big, $9\sigma$ is even bigger.

In other words, if the results are robust, then dark energy is not what it's commonly treated to be (a cosmological constant). Instead, it is becoming weaker over time - potentially to the point that the universe's expansion is no longer accelerating.

There is "no real proof", of course, because all of the above hinges on if the results are robust. Scientific results are often wrong (because of some unaccounted systemic effect, confounding variable, etc.). This result is still quite new, there might be some flaw in the analysis.

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  • $\begingroup$ So 5σ and especially 9σ is significant and more than enough to consider this "remarkable" and "major paradigm shift". Can you add a few words expanding on what you mean by robust and how that might be achieved and how long that might take. I understand that 1 month (since publication) is too soon - although DESI reports have been coming for some time, so perhaps this is not that much of a surprise for the astronomy community - is it just a matter of double-checking the data or you expect more data to become available? $\endgroup$ Commented yesterday
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    $\begingroup$ @adsp42 The large sigmas mean the result is significant enough to pay attention to. It doesn't mean the result is correct. There are a lot of possible confounding variables, for example, it could be that there is dust blocking our view of supernovae, or we are located in an emptier-than-average region of the universe, the distances to the supernovae is incorrectly estimated, etc. Ruling out all these other explanations take time & effort. $\endgroup$ Commented yesterday
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    $\begingroup$ @adsp42 As for how long it'd take for astronomers to agree, I can't say for sure, but an order of magnitude estimate is years. One of the most convincing things would be for another group of astronomers using another set of instruments arriving at the same result using another method. All that again takes time. $\endgroup$ Commented yesterday

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