Planck stars, White Holes, Remnants and Planck-mass quasi-particles | Hacker News Reader

Planck stars, White Holes, Remnants and Planck-mass quasi-particles

arxiv.org

62 pointsngrilly32 comments

>A local dark matter density of the order of 0.01M⊙/pc3 corresponds to approximately one Planck-scale white-hole per each 10.000Km3. These objects are presumably moving fast with respect to our local frame, since we are rotating with the galaxy at hundreds of Km per second, while dark matter probably isn’t.

Anyone know why dark matter wouldn't be rotating along with the rest of the galaxy?

Dark matter is what should explain the anomalies we see in gravitational behavior of galaxies. So it's influencing gravity but it's not acting like regular matter and can't be detected apart from the gravitational influence it has on regular matter. So if dark matter would be rotating along with the galaxy then it would behave like regular matter so it wouldn't be dark matter.

It does not interact electromagnetically and does not have friction because of that.

Chandrasekhar dynamical friction

https://en.wikipedia.org/wiki/Dynamical_friction

Dark matter probably is.

Dark matter is matter. It gravitates. It obeys conservation of angular momentum. Unless the dark matter clump happened to form in a spherically symmetric contraction, it will be rotating, almost certainly coupled to the rotation of the galaxy.

The difference is that while the dark matter halo almost certainly has has net angular momentum, individual DM particles would not be coupled to the galaxy’s rotation to any meaningful degree, and would have essentially random velocities relative to a local corotating frame.

The entire reason that spiral galaxies exist is that baryonic matter interacts electromagnetically and can dissipate kinetic energy via heat (photons). That causes matter to eventually flatten into a disc rotating more-or-less uniformly.

In this sense a DM halo is similar to elliptical galaxies which certainly have net angular momentum, but their stars have essentially random orbital planes.

> The difference is that while the dark matter halo almost certainly has has net angular momentum, individual DM particles would not be coupled to the galaxy’s rotation to any meaningful degree

Does this take into account frame-dragging induced rotation?

Spacetime is essentially flat on a galactic level for any general-relativity purposes. I'm pretty sure you can safely ignore frame-dragging here.

> Anyone know why dark matter wouldn't be rotating along with the rest of the galaxy?

No one knows if dark matter exists and what it's made of, so you can make as many assumptions as you want to help your fancy research as long as it doesn't contradict other observations.

I want planet nine to be a primordial black hole so we can send a few probes out to orbit it and then chuck stuff into it to learn about quantum gravity.

This is the extreme stereotype of humanity: travel somewhere far away and continue to litter just like you do at home. No respect for the locals!

Worse, since it's far away it doesn't come back to make a bigger problem for you later.

Anything chucked into a black hole becomes either gamma rays or more black hole.

It's not littering, it's perfect composting!

or gravity waves

Wouldn't anything lobbed into a black hole with angular significant mass appear to be falling in forever?

From the reference frame of the object, it falls in real time. From the reference frame of us, all time would pass before it falls in.

In other words, I don't think it would be possible to get any meaningful data from a probe within the next trillion years or so?

Note there is an error in the paper going from equation (60) to (63): inequality (63) should read 10^-8 Hz < mu < 10^2 Hz. This is fairly important from an observational standpoint!

Lots of people seem to talk about things being a contributor to dark matter. Is the general vibe that it's made of more than one thing?

The general vibe is that (1) "dark matter" is absolutely required to explain observed velocity distributions in the universe at every scale larger than a few thousand stars and (2) nothing we've observed so far can be it. So... sure. Maybe all these ideas are right, though probably not. But something has to be it, so we might as well keep throwing darts until something lands.

In terms of theory it's easy to believe that the dark matter is a non-interacting particle of some kind such as a WIMP, right-handed neutrino, or axion.

The trouble is picking out which one is correct with all of the alternatives.

Physics as we know it got started when Newton developed a theory of gravity that explained what happens on the Earth and in the heavens. This success was continued when people linked the results of nuclear experiments to nucleosynthesis in stars and the big bang.

Dark matter seems to be AWOL however at the scale of the solar system and smaller so that link is broken right now -- we'd like to be able to observe the dark sector with a big detector, a particle accelerator or an axion telescope but so far we can't.

I think people phrase it that way because otherwise, it would be a much bolder claim to say that your pet theory forms all dark matter, thereby negating all the other possibilities. There's generally no need to make that much of a sweeping claim, and you probably don't have enough evidence to support it. Why not be cautious?

"This is a review of some recent developments on quantum gravity aspects of black hole physics. In particular, we focus on a scenario leading to the prediction of the existence of a Planck-mass quasi-stable object, that could form a component of dark matter."

— Carlo Rovelli & Francesca Vidotto (the authors)

As long as they aren't photino birds. -- Stephen Baxter, probably [1].

1. https://en.wikipedia.org/wiki/Ring_(Baxter_novel)

Between him and Alastair Reynolds I've become very tired of getting to the end of huge novel series and having the ending make me feel like none of the events mattered.

With Stephen Baxter, I always feel like it mattered, but I always feel sad. Forget happy endings - his books don't even have happy middles, or happy beginnings.

Read something more cheerful, like from Peter Watts.

Peter Watts novels play by anime rules half the time. "This biological creature can accelerate its whole body across a room and then decelerate to a complete stop in 40ms." "Vampire is super duper smert so she can stomp her feet to precisely manipulate extremely chaotic physical systems to achieve the exact outcome she wants". No other author keeps me so thoroughly engaged while making me constantly roll my eyes.

Jesus. Don't read Flood. I've never experienced existential dread with anything else.

Ha, you might enjoy The Drowned World

Try "The Killing Star".

Thanks, will put it on the reading list.

A.K.A. a "white hole."

He talks about this in a recent World Science Festival video: https://www.youtube.com/watch?v=WuaSoTyqrTA

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