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'Dark fluid' with negative mass could dominate the universe

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Re: 'Dark fluid' with negative mass could dominate the universe

#221

Post by thaumasiotes »

deytempo wrote: post #163
thaumasiotes wrote: post #102

Earlier quoted context omitted.

> On the other hand, you can show that the inertial mass cannot be negative (else the Lagrangian would not be bounded from below and therefore you could not minimise it; the whole of physics then comes crumbling down). Here's a purely thought-experiment argument for inertial mass not being negative: If you had an object with negative inertial mass, then pushing on that object would cause it to accelerate into your ha…

How do you think Thor’s hammer works when it’s sitting on the toilet seat?

Obviously, toilets are capable of exerting more upward force than any mere mortal.

Re: 'Dark fluid' with negative mass could dominate the universe

#222

Post by acqq »

nonbel wrote: post #162
acqq wrote: post #156

Earlier quoted context omitted.

> Dark matter can be placed arbitrarily so it can match pretty much anything. Quite the opposite, and that statement is obviously uninformed. The science behind dark matter is very precise and exact. That's why the alternative theories can't come even near: once the same rigor which was applied to test dark matter is applied to the alternatives, the alternative theories simply break. > I'm afraid I have never seen th…

> "Quite the opposite, and that statement is obviously uninformed. The science behind dark matter is very precise and exact." From what I have seen they can get precise post-dictions but not precise pre-dictions because there are so many free parameters (basically you can put dark matter wherever you need it). > "I've given you the link to the complete articles and also highlighted the major points. Try to read and u…

> The claimed predictions from the 1970s and 1980s are not mentioned on that page.

They are mentioned, they just aren't elaborated: you have to do your own homework if you apparently "understand" the subject enough to claim your own opinion. A begin of that homework, however, as I've already written, you even have on the initial page that you chose to post, specifically:

"2013: ΛCDM provides an excellent fit to the improved CMB data"

As I've said, that's the "item 3" from the 5 item list, and it's ultimately confirmed by ESA's Planck satellite results in 2013. That is a strong confirmation.

Note: more details and the timeline are on the very page you've initially posted. Ad hominems don't change the facts.

Re: 'Dark fluid' with negative mass could dominate the universe

#223

Post by andrewshadura »

YaxelPerez wrote: post #95
debatem1 wrote: post #90

Stupid non-physicist question here: we know the universe is expanding, but where does it expand at? Are there physical areas which we can observe growing? And if so, shouldn't that mean that we observe distant objects as lighter than their interactions with closer objects would imply?

The explanation I've heard is that it's just space being created everywhere at once. There's no center from which everything expands. This VSauce video explains it better than I can (later in the video): https://youtu.be/3pAnRKD4raY

Imagine a flat ‘universe’ on the surface of a balloon which is being inflated. There would be no centre of expansion on the surface of the balloon, the space would appear everywhere equally.

Re: 'Dark fluid' with negative mass could dominate the universe

#224

Post by feanaro »

marcosdumay wrote: post #108
starbeast wrote: post #105

Earlier quoted context omitted.

Don't you have two things that can generate gravitational fields though? As distortions in spacetime can be caused by either mass or the momentum of massless particles, such as photons. Or do I have that wrong and photons merely follow the distortions and not cause any of their own? As presumably, you could have a negative gravitational field from a photon traveling backward in time, without having any issues with ch…

> Or do I have that wrong and photons merely follow the distortions and not cause any of their own? You have that wrong in the momentum of massless particle has mass, independently of its carrier. Massless particles do not actually exist, what exist are particles with zero "mass at rest" and that are never at rest.

You're talking about relativistic mass, but that isn't a commonly used concept anymore. Today it is indeed the norm to think of "mass" as being the mass at rest. Massless particles are really massless and have momentum without having mass.

Re: 'Dark fluid' with negative mass could dominate the universe

#225

Post by feanaro »

duality wrote: post #56
raverbashing wrote: post #19

Earlier quoted context omitted.

Given there have been no detections of individual gravitons, saying it's a spin-2 gauge boson is speculation at best (though it might be what the model predicts)

If it weren't, general relativity would be wrong as well and we know that (in its domain of applicability) it's a highly accurate model. GR models gravity as fluctuations in the metric tensor of spacetime, which is a symmetric rank-2 tensor. This is all you need to know to conclude that a graviton (a quantize fluctation of that tensor) must be spin-2. https://en.wikipedia.org/wiki/Graviton mentions "[I]t can be shown…

Well GR is potentially very wrong both at very large scales (seemingly requiring dark matter and dark energy) and very small scales (due to known present incompatibilities with quantum mechanics). It's only been shown to be a highly accurate model everywhere in between.

Re: 'Dark fluid' with negative mass could dominate the universe

#226

Post by feanaro »

andrepd wrote: post #50
galadran wrote: post #8

Paper: https://arxiv.org/pdf/1712.07962.pdf Not a physicist, but my summary of the author's point would be: If you allow for matter with a negative mass and plug it into a n-body simulation. You seem to get what looks like dark matter from first principles. Namely, a halo of non interacting matter around galaxies in just the right "non-cuspy" shape. Their conclusion is that this is worth considering, as almost all ma…

Here's an argument: If you think about it, "mass" actually means two different things. One one hand, there is mass as the "charge" of gravitational interaction, that is, the gravitational field generated by a body is proportional to its mass. Let's call this concept "gravitational mass". On the other hand, you have mass in the sense of "intertial mass", a body's resistance to change of velocity. Although we kinda tak…

Perhaps gravitational mass is indeed polarised and inertial mass corresponds to the absolute value of the gravitational mass?

Re: 'Dark fluid' with negative mass could dominate the universe

#229

Post by agapon »

agapon wrote: post #227

I recall reading article in a popular science magazine about hypothetical negative mass with exactly these properties. It was about 25 years ago and the magazine was Russian language. Unfortunately, I can't recall its name now.

Oh, thanks to the power of the internet search, I found a scan of that article: http://epizodsspace.airbase.ru/bibl/tm/1990/10/otrits-massa.... The journal is "Техника-молодежи" (a Soviet style name that I am incapable of translating to English) and the article is from 1990.

And that article refers to a publication by Robert Forward in a journal with a name like Aerospace Technology or Aerospace Engineering. But I found an article in on the same topic in NewScientist: https://www.newscientist.com/article/mg12517084-200-the-powe...

Wikipedia, of course, has an article on Negative mass as well.

Re: 'Dark fluid' with negative mass could dominate the universe

#230

Post by mrhappyunhappy »

“Negative masses are not a new idea in cosmology. Just like normal matter, negative mass particles would become more spread out as the universe expands – meaning that their repulsive force would become weaker over time. However, studies have shown that the force driving the accelerating expansion of the universe is relentlessly constant. ”

Is it though? How can you measure a universe-size force that takes billions of years to “move” on a human lifetime scale?

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