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Some months ago there were several articles like this one (https://bigthink.com/starts-with-a-bang/physicists-question-fate-universe/) claiming that the universe's accelerated expansion was found to be slowing down

These articles say that the findings are preliminary and more studies would be needed to confirm it. But is this right or just pop-sci hype? Are professional cosmologists and astrophysicists beginning to see that the universe expansion has been slowing down in the "recent" past of the universe?

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The DESI Collaboration recently reported a significant statistical preference for dark energy with a nonconstant equation of state, such that it is more like a cosmological constant in the past and becoming (slightly) more like matter as time goes on. Specifically, taking dark energy to have pressure $P=w\rho$ with $$w(a) = w_0 + w_a(1-a)$$ (where $\rho$ is the dark energy density and $a$ is the scale factor), their data favor $w_0>-1$ and $w_a<0$ (see DESI 2024 VI). A cosmological constant has $w_0=-1$ and $w_a=0$. Matter has $w=0$.

Directly interpreting this as "the acceleration is slowing down" is subtle because that also involves the comparison between dark energy and matter density. A more precise interpretation of what the data suggest is that the expansion rate was slightly higher around redshift 0.2-0.5 (2.5-5 billion years ago) than it would be if dark energy were a cosmological constant; see figure 7 of the paper. Another interpretation is that the dark energy density was higher in the past than it is today.

You are correct that this result is not at discovery level, and it may or may not persist with more data.

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  • $\begingroup$ It's also difficulty to take seriously the model DESI used of the dark energy equation of state because their 'best fit' has $w<-1$ in the recent past, $z\sim 1$ of something. And it has been shown that this behavior cannot appear in a causal, Lorentz-invariant quantum field theory Hartman, Kundu, Tajdini (2016), the framework of which we know describes our world quite well. Perhaps more precise data will still show some deviation from the behavior of a cosmological constant, but it won't end up being like this. $\endgroup$
    – SethK
    Commented Aug 3 at 17:27
  • $\begingroup$ @SethK You are correct that the $w_0$ $w_a$ parametrization should not be taken literally. It is physically motivated, however -- not because a realistic dark energy model would have that equation of state, but because it yields expansion histories that match those predicted by realistic models (see arxiv.org/abs/astro-ph/0208512 and arxiv.org/abs/0808.0189). $\endgroup$
    – Sten
    Commented Aug 3 at 18:29
  • $\begingroup$ (Basically the expansion history is insensitive to the dark energy equation of state at $z\gtrsim 1$ because dark energy is subdominant then.) $\endgroup$
    – Sten
    Commented Aug 3 at 18:38
  • $\begingroup$ Do you know somewhere it's shown that the DESI preference for their $w_0 w_a$ model is driven by the lowest redshift bins? I would think the preference for a rapidly changing equation of state must come from bins where that deviation would be felt. If indeed that preference is driven by $z\sim 0$ I'd feel better about it. $\endgroup$
    – SethK
    Commented Aug 3 at 19:02
  • $\begingroup$ Note also that the 'best fit' model of DESI is way outside the region of parameter space considered in the Linder papers you referenced, or seen in their 'realistic' models. So I think it's hard to call it physically motivated still. $\endgroup$
    – SethK
    Commented Aug 3 at 19:06

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