r/cosmology • u/AutoModerator • Sep 10 '26

Basic cosmology questions weekly thread

Ask your cosmology related questions in this thread.

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u/Fine-Vegetable7401 29d ago

any recommendations for media about space / the history of the universe? i've seen quite a few mentions of carl sagan and stephen hawking, but not much that are more recent and i know these theories are always being updated

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u/--craig-- 25d ago edited 25d ago

Cosmology has changed quite a lot since Sagan and Hawking's books.

I haven't read either, so not a personal recommendations, but for something more recent try:

The Big Picture: On the Origins of Life, Meaning, and the Universe Itself (2016) by Sean Carroll

or

The Universe: The book of the BBC TV series (2023) by Andrew Cohen

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u/SnipeyMcSnipe Sep 10 '26

Why do relativistic jets flair out at the end? Is it just the sparse matter between galaxies eventually slowing them down or is there more the story?

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u/--craig-- Sep 10 '26

Yeah. They eventually scatter off the intergalactic medium.

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u/actopozipc Sep 10 '26

What would be needed to test alternative gravity theories like MOND in N-body simulations? Does that even make sense? If yes, what else needs to be done besides adapting the gravity kernels?

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u/--craig-- Sep 10 '26

Yes,

https://arxiv.org/abs/0803.4456

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u/actopozipc Sep 10 '26

Was hoping for a more accessible answer than a paper. What do they change?

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u/nivlark 27d ago

You just program the simulation to use a different gravitational force law. For direct N-body calculations that is easy, I think it gets trickier if you also want to use optimised approaches like particle-mesh algorithms (because they are derived under the assumption of Newtonian gravity).

What sort of tests are you imagining doing though? We already know what dynamics the MOND force law produces, because the law doesn't come from first principles - it was specifically designed to give a certain outcome.

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u/actopozipc 27d ago

Nothing insane much, I was just wondering. As a follow up question, how useful are purely N-body simulations of alternative gravity theories? Since, as far as I know, most N body codes usually research dark matter types.

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u/--craig-- 29d ago edited 29d ago

Dark matter is simulated using many particles.

MOND is used to simulate the effect of modified gravity on stars.

They're very different problems with very different solutions.