https://www.forbes.com/sites/startswithabang/2019/10/18/dark-matters-biggest-problem-might-simply-be-a-numerical-error/

This problem, known as the core-cusp problem in cosmology, is one of the oldest and most controversial for dark matter. In theory, matter should fall into a gravitationally bound structure and undergo what’s known as violent relaxation, where a large number of interactions cause the heaviest-mass objects to fall towards the center (becoming more tightly bound) while the lower-mass ones get exiled to the outskirts (becoming more loosely bound) and can even get ejected entirely.

ChrispenEvan said:

https://www.forbes.com/sites/startswithabang/2019/10/18/dark-matters-biggest-problem-might-simply-be-a-numerical-error/

This problem, known as the core-cusp problem in cosmology, is one of the oldest and most controversial for dark matter. In theory, matter should fall into a gravitationally bound structure and undergo what’s known as violent relaxation, where a large number of interactions cause the heaviest-mass objects to fall towards the center (becoming more tightly bound) while the lower-mass ones get exiled to the outskirts (becoming more loosely bound) and can even get ejected entirely.

Very interesting.

Seems a bit unfair to blame it all on Cusp though.

I’d say there is more to be revealed.

The Rev Dodgson said:

ChrispenEvan said:

https://www.forbes.com/sites/startswithabang/2019/10/18/dark-matters-biggest-problem-might-simply-be-a-numerical-error/

This problem, known as the core-cusp problem in cosmology, is one of the oldest and most controversial for dark matter. In theory, matter should fall into a gravitationally bound structure and undergo what’s known as violent relaxation, where a large number of interactions cause the heaviest-mass objects to fall towards the center (becoming more tightly bound) while the lower-mass ones get exiled to the outskirts (becoming more loosely bound) and can even get ejected entirely.

Very interesting.

Seems a bit unfair to blame it all on Cusp though.

I’d say there is more to be revealed.

Extremely well written article. “Cusp” was probably named after this phenomenon.

I agree that the “core-cusp problem” is dark matter’s biggest problem. I fervently hope it’s just a numerical error.

“Violent relaxation” – should i investigate this further.

The article itself still says “might be”, “highly likely”. I want a confirmed “is”.

“In particular, the “core” of the halo that forms does so because of the specifics of the algorithm that approximates the gravitational force”. – i need to look up the paper to see the mathematics of where this appears. For small n in an n-body simulation there is no approximation necessary. For larger n, computer time requires one to ignore all but the closest neighbouring bodies. Is this the approximation tyey are talking of?

If so, i wonder if the method i worked out could help. I did some gravitational 2-body in gas simulations for my former csiro boss. I managed to speed his algorithm up by a factor by about 1000 by changing it from quadratic to cubic accuracy, and by relaxing the convergence criterion. Which allowed me to use a variable and much bigger timestep.

A bigger timestep is a pest for n-body simulations because individual bodies need really small timesteps when passing really close to each other. But it is a different sort of approximation to the standard near-neighbour one, so replacing one type of approximation by another could potentially solve the dark matter cusp problem.

mollwollfumble said:

The Rev Dodgson said:

ChrispenEvan said:

https://www.forbes.com/sites/startswithabang/2019/10/18/dark-matters-biggest-problem-might-simply-be-a-numerical-error/

This problem, known as the core-cusp problem in cosmology, is one of the oldest and most controversial for dark matter. In theory, matter should fall into a gravitationally bound structure and undergo what’s known as violent relaxation, where a large number of interactions cause the heaviest-mass objects to fall towards the center (becoming more tightly bound) while the lower-mass ones get exiled to the outskirts (becoming more loosely bound) and can even get ejected entirely.

Very interesting.

Seems a bit unfair to blame it all on Cusp though.

I’d say there is more to be revealed.

Extremely well written article. “Cusp” was probably named after this phenomenon.

I agree that the “core-cusp problem” is dark matter’s biggest problem. I fervently hope it’s just a numerical error.

“Violent relaxation” – should i investigate this further.

The article itself still says “might be”, “highly likely”. I want a confirmed “is”.

“In particular, the “core” of the halo that forms does so because of the specifics of the algorithm that approximates the gravitational force”. – i need to look up the paper to see the mathematics of where this appears. For small n in an n-body simulation there is no approximation necessary. For larger n, computer time requires one to ignore all but the closest neighbouring bodies. Is this the approximation tyey are talking of?

If so, i wonder if the method i worked out could help. I did some gravitational 2-body in gas simulations for my former csiro boss. I managed to speed his algorithm up by a factor by about 1000 by changing it from quadratic to cubic accuracy, and by relaxing the convergence criterion. Which allowed me to use a variable and much bigger timestep.

A bigger timestep is a pest for n-body simulations because individual bodies need really small timesteps when passing really close to each other. But it is a different sort of approximation to the standard near-neighbour one, so replacing one type of approximation by another could potentially solve the dark matter cusp problem.

> the heaviest-mass objects to fall towards the center (becoming more tightly bound) while the lower-mass ones get exiled to the outskirts (becoming more loosely bound) and can even get ejected entirely.

Then that tells us that if all dark matter particles have the same mass then the density distribultion in galaxies would be very different to all dark matter particles having the same mass?

A nice disproof of all dark matter “macho” theories is in the offer, and that also includes a disproof of mollwollfumble’s “ridiculous idea”. Good.

mollwollfumble said:

.

Then that tells us that if all dark matter particles have the same mass then the density distribultion in galaxies would be very different to all dark matter particles having the same mass?

I read that five times and I still can’t get it to make any sense.

The Rev Dodgson said:

mollwollfumble said:

.

Then that tells us that if all dark matter particles have the same mass then the density distribultion in galaxies would be very different to all dark matter particles having the same mass?

I read that five times and I still can’t get it to make any sense.

I see what you mean. I missed a “not”.
Try this:

Then that tells us that if all dark matter particles have the same mass then the density distribultion in galaxies would be very different to the density distribution if dark matter particles had masses that are not all the same.

Headline is misleading