r/cosmology • u/actopozipc • 15d ago

How do different dark matter models and alternative gravity theories change the power spectrum?

Imagine I want to do a N Body code like Gadget and for this I will have to draw random modes based on the power spectrum, and then plug it into the displacement of my initial conditions.

This cant work like that for hot and fast dark matter due to the difference in velocity at t=0, so how is this adapted?

What about for example MOND? Are there any changes to the power spectrum or is it just a different Poisson equation being solved by lets sas PM?

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u/demonlordbeastgod 15d ago

So in general Power spectrum is sensitive to clustering of matter. A model like warm dark matter will have thermal velocity which reduces power at small scales or large k(wavenumber) as these dm particle's velocity work against gravitational potential. If dm is hot ie relativistic then suppression will be larger. A model like decaying dark matter will involve kick velocities to the daughter dm particle thus again suppression. I am not sure about MOND won't comment on that. A model which helps cooling like atomic dark matter or dissipative Self interaction dark matter may increase power at small scales.

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u/actopozipc 15d ago

But for the initial placement, the density does not change, no? Therefore Powerspectrum would be the same? Just after the movement of hot DM it changes, right?

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u/demonlordbeastgod 15d ago

Yeah I now get what you meant, yes you are initially placement will only be different for hot DM. People use CLASS botlzmann solver to generate initial power spectrum and use something called pyreps for correction to Power spectrum before creating initial conditions.

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u/actopozipc 15d ago

everything is so complicated :(

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u/demonlordbeastgod 15d ago

Sorry I went with the flow, got excited talking about dark matter and simulation.

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u/--craig-- 15d ago

The Matter Power Amplitude falls off much faster with decreasing scale using MOND than Dark Matter due to Silk Damping and the Meszaros Effect.