Evidence that the key assumption made in discovery of dark energy is in error • Hacker News Board

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

#151

Post by brylie » Tue, Jan 07, 2020, 9:09 AM UTC

CoryG89 wrote: post #62

brylie wrote: post #53

Earlier quoted context omitted.

I'm not an astronomer, but "tired light" makes more intuitive sense to me to explain the redshift. In effect, we already know that matter can affect light through gravitational interaction. Since light is travelling so fast and far it seems intuitive thst interstellar matter, as fleeting as it is, could act to slow light down over these great spacetime distances.

This doesn't make much sense to me. The speed of light is a constant, right? Gravitational interactions may change the directional component, but the speed would remain constant, no?

C is constant, which is the "speed of light in a vacuum." Light can, and does, travel slower than C, e.g. when traveling through gas or other material.

https://en.m.wikipedia.org/wiki/Speed_of_light

Scientists have even managed to freeze light for short periods of time.

https://io9.gizmodo.com/scientists-freeze-light-for-an-entir...

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

#152

Post by ash » Tue, Jan 07, 2020, 9:11 AM UTC

j-dr wrote: post #118

Even if this paper is correct, there is still overwhelming evidence for dark energy provided by other observations, in particular those of the Cosmic Microwave Background (CMB) and Baryon Acoustic Oscillations (BAO). This article provides some details on why Dark Energy is in no danger of being called in question: https://www.quantamagazine.org/no-dark-energy-no-chance-cosm...

As far as I can tell, Quanta Magazine article does mention the original post findings. Additionally, the Quanta article and the findings preprint were published just a week apart. Not enough time to formulate a good response:

* Quanta Magazine article: 17 Dec 2019

* Arxiv preprint: 10 Dec 2019 [1]

CMB and BAO are mentioned in the original post too:

> Other cosmological probes, such as the cosmic microwave background (CMB) and baryonic acoustic oscillations (BAO), are also known to provide some indirect and "circumstantial" evidence for dark energy, but it was recently suggested that CMB from Planck mission no longer supports the concordance cosmological model which may require new physics (Di Valentino, Melchiorri, & Silk 2019). Some investigators have also shown that BAO and other low-redshift cosmological probes can be consistent with a non-accelerating universe without dark energy (see, for example, Tutusaus et al. 2017). In this respect, the present result showing the luminosity evolution mimicking dark energy in SN cosmology is crucial and very timely.

[1]: https://arxiv.org/abs/1912.04903

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

#153

Post by mr_mitm » Tue, Jan 07, 2020, 9:12 AM UTC

mikorym wrote: post #133

As a mathematician, I will inherently be biased against dark matter and dark energy (if you're calculations don't fit, then your mathematics is not suitable, so change the math before you change the physical description). What is the relationship between dark matter and dark energy, and does this have any effect on the r^2 law assumptions and the introduction of dark matter? For example, does this mean that when we t…

> if you're calculations don't fit, then your mathematics is not suitable, so change the math before you change the physical description

We tried, but it doesn't fit the data nearly as well (e.g. MOND). Related xkcd: https://xkcd.com/1758/

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

#154

Post by osbertlancaster » Tue, Jan 07, 2020, 9:12 AM UTC

exabrial wrote: post #115

Can someone eli5 why if space itself is expanding, how can an observer detect that if they too are expanding at the same rate?

As I understand it:

We are not expanding. Size of e.g. a hydrogen atom is not changing.

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

#155

Post by mr_mitm » Tue, Jan 07, 2020, 9:16 AM UTC

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, most cosmologists believe we're heading for the Big Freeze.

The Big Rip would require something called "phantom energy", a special form of dark energy. While not entirely ruled out, our observations disfavor it and it would be problematic from a theoretic standpoint.

I can recommend this podcast on possible fates of the universe: https://www.preposterousuniverse.com/podcast/2019/10/28/70-k...

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

#156

Post by mr_mitm » Tue, Jan 07, 2020, 9:21 AM UTC

ltbarcly3 wrote: post #145

cthalupa wrote: post #131

Earlier quoted context omitted.

This is a false dichotomy. We have known energy is not (always) conserved since long before we began discussing dark energy. This is such a misunderstood topic vis a vis dark energy that Discovery specifically has an article on it: https://www.discovermagazine.com/the-sciences/energy-is-not-...

Nope. We have no actual evidence of a violation of the conservation of energy. What has happened here is that astrophysicists got way out on a branch. They were quite excited and came up with a theory that seemed to explain why there appeared to be increasing redshift with distance, and later that theory was shown to violate conservation of energy. Doubling down, they waved their hands and said that conservation of e…

Please read this and let me know if you are still convinced that energy must be conserved in General Relativity: https://www.preposterousuniverse.com/blog/2010/02/22/energy-... (edit: Nevermind, this is the same content as the Discovery article linked above, except with better formatting).

In short: conservation of energy is a result from the Noether theorem, which says it must be conserved iff the laws of physics are invariant under time translations. That's the case in Newtonian physics where we also have plenty of experiments supporting it, but not in GR. Precisely because of the expansion of the universe, the metric is time dependent and thus there can not be an associated conserved quantity. This has been known before we knew about dark energy or even the expansion of the universe.

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

#157

Post by cygx » Tue, Jan 07, 2020, 9:35 AM UTC

exabrial wrote: post #115

Can someone eli5 why if space itself is expanding, how can an observer detect that if they too are expanding at the same rate?

Spatial expansion is a great heuristic explanation, but it can be misleading: Locally, the cosmological constant will manifest as just another pseudo-force, easily overcome by nuclear and electromagnetic forces holding us together.

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

#158

Post by tsimionescu » Tue, Jan 07, 2020, 9:55 AM UTC

briantakita wrote: post #134

_bohm wrote: post #124

Earlier quoted context omitted.

It should be noted that among cosmologists, electric universe is widely considered to be bunk science.

And Plasma Physicists think that "dark matter", "dark energy", & the assertion of gravity (ignoring plasma) being the dominant force in the Univese is mathematics without physical evidence. Thus "pathological science". Irving Langmuir, discoverer of the double layer effect in plasmas & coined the term "plasma" physics, came up with the definition of "Pathological Science". He was also around during the birth of the "…

They may feel that way, but the difference is that standard cosmology is a highly-successful predictive model consistent with all popular physical theories, while plasma cosmology is proposing physically unlikely phenomena that have never been observed, interpreting prehistoric art for 'observations', and fails to account for many modern observations. It's perhaps not as bad as the so-called electric universe, but it is certainly a much more likely example of pathological science than standard cosmology is.

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

#159

Post by mikorym » Tue, Jan 07, 2020, 10:00 AM UTC

has2k1 wrote: post #141

mikorym wrote: post #133

As a mathematician, I will inherently be biased against dark matter and dark energy (if you're calculations don't fit, then your mathematics is not suitable, so change the math before you change the physical description). What is the relationship between dark matter and dark energy, and does this have any effect on the r^2 law assumptions and the introduction of dark matter? For example, does this mean that when we t…

> if you're calculations don't fit, then your mathematics is not suitable, so change the math before you change the physical description Physics is littered with mathematical formulations predicting new phenomenon (forces and particles). > What is the relationship between dark matter and dark energy There is no known relationship between dark matter and dark energy. They could have been called unknown matter and unkn…

> Physics is littered with mathematical formulations predicting new phenomenon (forces and particles).

Sorry, I should have clarified: I mean I would spend all my days doubting the mathematics, rather than doubting the physical universe. If that makes sense? Of course all of mathematics can be argued to have a physical sort of root.

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

#160

Post by lorepieri » Tue, Jan 07, 2020, 10:00 AM UTC

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…

Well, actually there are infinite ways of modifying the equations and still have them compatible with current observations. Check out for instance https://en.wikipedia.org/wiki/F(R)_gravity but there are many more examples with additional fields.