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Evidence that the key assumption made in discovery of dark energy is in error

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Re: Evidence that the key assumption made in discovery of dark energy is in error

#31

Post by crazygringo »

knzhou wrote: post #21
flick wrote: post #17

Not so fast. See [1] and [2] for an explanation. There are good reasons to suspect that there are errors made in this analysis. [1]: https://arxiv.org/abs/1912.02191 [2]: https://www.quantamagazine.org/no-dark-energy-no-chance-cosm...

Hold up, this is a different thing than what [1] and [2] are responding to.

Yes, I'm totally lost. Was there accidental confusion here? Or is the new paper (by completely different authors) somehow building on those other papers and flick is implying it suffers from the same criticisms?

Would love to know if this is genuinely newsworthy or not.

Re: Evidence that the key assumption made in discovery of dark energy is in error

#32

Post by unlinked_dll »

awb wrote: post #27
jshevek wrote: post #2

"...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."" Is he implying that other cosmologists…

> Is he implying that other cosmologists have proceeded as if dark energy is well evidenced? I thought dark matter and dark energy are simply placeholders for a discrepancy we see between our models of the universe and our observations. No?

it's my understanding that dark matter is another word for the indirect observation of matter that we can't otherwise detect, through gravity. I don't know if calling it a placeholder is the proper parlance.

Dark energy is something predicted by general relativity and iirc it's what is behind the expansion of space (and acceleration of expansion).

Re: Evidence that the key assumption made in discovery of dark energy is in error

#33

Post by dmix »

ncmncm wrote: post #29

Haha, called it! Feynman warned about this specific phenomenon: basing new physics on the points at the extreme end of your graph, where the measurements are least reliable. In this case, these points correspond to demolition of the most distant and youngest stars in the sample, which could differ in many ways from the common, much more recent events. Low metallicity is a difference that is obvious to every AP, but t…

Does this have any affect on the search for dark matter? Are the two interconnected at all?

Re: Evidence that the key assumption made in discovery of dark energy is in error

#34

Post by flashman »

To know if the universe is expanding, we need to know the following: how far away things are, and whether they're moving away from us. In an accelerating universe, there's evidence that things were moving away from us slower in the past.

The evidence that things are moving away from us is that as the universe expands, the wavelength of any light travelling through it also expands, shifting it towards the red end of the spectrum: redshift. (Think of the wavelength of a wavy line drawn on a piece of rubber that's then stretched out.) We've known for nearly 100 years that the universe is expanding. (Edit: not just that the objects in it are moving apart but that space itself is expanding.)

The evidence of accelerating expansion is that based on the redshift of nearby objects, we would expect distant objects to have a higher redshift than they actually do. That means they were moving away from us slower in the past, so something must be accelerating them.

We use the brightness of Type Ia supernovae to measure distance. Even though supernovae aren't identically bright, their brightness follows a curve which lets us calculate their peak brightness (they are standardizable).

This paper argues that the calculations cosmologists use to standardize supernovae brightness fail to take into account the age of the progenitor stars, as far as I can tell. If true, this means our distance measurements are inaccurate and these stars are actually closer than we thought, enough to restore to linear relationship between distance and redshift that one would expect in a universe expanding at a constant rate.

In other words, their redshift is lower not because the expansion rate of space was lower in the past, but because they're not as far away as we thought they were.

Re: Evidence that the key assumption made in discovery of dark energy is in error

#35

Post by lanstin »

dmix wrote: post #33
ncmncm wrote: post #29

Haha, called it! Feynman warned about this specific phenomenon: basing new physics on the points at the extreme end of your graph, where the measurements are least reliable. In this case, these points correspond to demolition of the most distant and youngest stars in the sample, which could differ in many ways from the common, much more recent events. Low metallicity is a difference that is obvious to every AP, but t…

Does this have any affect on the search for dark matter? Are the two interconnected at all?

No. Dark matter is assumed to exist from the excessively quick rotation of stars around the cores of galaxies. Dark energy is/was assumed to exist in anomalous expansion of the universe based on redshifts.

Re: Evidence that the key assumption made in discovery of dark energy is in error

#38

Post by pdonis »

unlinked_dll wrote: post #32
awb wrote: post #27

Earlier quoted context omitted.

> Is he implying that other cosmologists have proceeded as if dark energy is well evidenced? I thought dark matter and dark energy are simply placeholders for a discrepancy we see between our models of the universe and our observations. No?

it's my understanding that dark matter is another word for the indirect observation of matter that we can't otherwise detect, through gravity. I don't know if calling it a placeholder is the proper parlance. Dark energy is something predicted by general relativity and iirc it's what is behind the expansion of space (and acceleration of expansion).

> Dark energy is something predicted by general relativity

Not really. GR can accommodate it just fine, but GR did not predict it. In fact, Einstein, who originally introduced the cosmological constant (which was the term then for what we now call "dark energy") in order to allow a static solution to his equation, thereby missed the chance to predict that the universe was expanding, a decade before Hubble's observations showed that it was. Once it became clear that expanding models matched the data, and did not (then) require a cosmological constant to do so, Einstein dropped it, calling it "the biggest blunder of my life". Only in the 1990s, when evidence for accelerating expansion became available, was the cosmological constant (now called "dark energy") put back into the equations so they would match the data.

> it's what is behind the expansion of space (and acceleration of expansion)

The expansion itself, no. The acceleration, yes.

Re: Evidence that the key assumption made in discovery of dark energy is in error

#39

Post by pdonis »

philipov wrote: post #20
crazygringo wrote: post #18

Do I understand correctly that this is actually saying that accelerating expansion of the universe may be in error? That there is, instead, no accelerating at all, but merely constant expansion? Or can there still be acceleration without dark energy? I'm unclear if they are essentially the same hypothesis or if there are more complicated subtleties I'm missing.

There are very few ways to modify Einstein's Equations and still have them make sense. An easy one is to add a constant term. Such a term is known as the Cosmological Constant, and a positive value for it would result in a constant repulsive force between masses regardless of distance. This results in an accelerating expansion of the entire universe, but, with a supremely small value it only becomes noticeable at lar…

> Dark Energy is a hypothesis for explaining why such a very small cosmological constant might exist, suggesting that the universe is permeated with a constant negative energy field.

No, that's not what dark energy is. "Dark energy" is just a general term for "anything that acts like a cosmological constant in the Einstein Field Equation". It could be an actual physical constant, just an inherent property of spacetime, or it could be some kind of energy field. We don't know. (Btw, the energy associated with dark energy is positive, not negative; the pressure is what's negative.)

Re: Evidence that the key assumption made in discovery of dark energy is in error

#40

Post by pdonis »

6nf wrote: post #9
gbrown wrote: post #6

Earlier quoted context omitted.

I thought there was little reason to expect a big rip, because it requires assumptions about an increasing cosmological constant, which there’s no evidence for.

What do you mean? There's lots of evidence that the expansion of the universe is accelerating and that's the whole reason we believe that dark energy exists. Most cosmologists do believe we're heading for a big rip.

No, "big rip" and "accelerating expansion" are not the same thing. A big rip would require that the density of dark energy in the universe is increasing with time. We have no evidence that it is, and most cosmologists do not believe we are heading for a big rip.
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