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It always depresses me (as an astrophysicist) how poor the communication is on dark matter, and this leads people to unfair conclusions. We have known for some

It always depresses me (as an astrophysicist) how poor the communication is on dark matter, and this leads people to unfair conclusions. We have known for some time about the limitations of our understanding of gravitational dynamics in extreme environments - the issue is not that "our theories must be right", it's that we have to square General Relativity (which is phenomenally well tested in other regimes) with the results we see. There are broadly 2 different ways of doing this: you can postulate that not all particles interact with electromagnetic radiation (after all, why should they?) or you can come how add extra terms into your gravitational laws. Both of these have been tried, but there are many reasons why the majority of astrophysicists prefer the dark matter approach. Firstly, there are observations which suggest large regions of mass that are not aligned with the visible mass (e.g. the bullet cluster). Secondly, dark matter can explain the statistical properties of the Cosmic Microwave Background radiation in a way that can not be achieved with ordinary matter alone. Thirdly, when we include dark matter in simulations, we get results that are a very good match for the universe as we observe it. etc. etc. If that weren't enough, we know that there are plenty of types of matter that are dark - e.g. neutrinos, early black holes etc. Whilst none of these have yet satisfied the conditions for dark matter as we understand it, it's perfectly possible that we have already identified some of the constituent parts, and there is a list of potential candidates as long as my arm that the particle physicists are working their way through. Finally, the biggest annoyance is the intimation that we are all just one big cabal. Let me put it like this: if you could provide indisputable evidence that dark matter was wrong, you would be a superstar in our field and everyone would be phenomenally excited, because it would demand the hunt for new physics. Scientists are a rational bunch, however, so any evidence must be weighed against the existing evidence already observed (which, as above, is substantial). Dark energy is slightly different. No one know's what this is, but models with a dark energy component do very well at modelling the universe. The question is, why? No one is pretending that this is a solved question, but what we do know is that whatever is driving the universe's accelerated expansion has properties that are very similar to what dark energy is. tldr: There are loads of great reasons for thinking dark matter exists, contrary to many posters here, and it is well accepted that dark energy is a mathematical term that represents something of unknown physical origin.

cygx 10y ago
Personally, I'm interested to see what happens to conformal gravity. The theory is able to fit galactic rotation curves without a need for dark matter[1], but supposedly leads to inconsistent results when trying to do the same for gravitational lensing[2]. However, that might have been an artifact of incorrect approximations and the calculations should be repeated using exact solutions[3]. [1] https://arxiv.org/abs/1011.3495 https://arxiv.org/abs/1011.3495 [2] https://arxiv.org/abs/1403.7930 https://arxiv.org/abs/1403.7930 [3] https://arxiv.org/abs/1609.07633 https://arxiv.org/abs/1609.07633
phkahler 10y ago
I keep seeing a couple problems in this whole discussion. Number one is the continued references to Keplers laws. Kepler only applies to 2-body systems where the mass of one is dramatically larger than the other. This works well for the solar system, where the sun is huge compared to the planets and the planets have little effect on each other. But do note that some planets were predicted to exist based on their effect on the orbits of other planets, so even in this sparse system the effects do make a difference. My point is that Kepler has no place in galactic dynamics but is often referenced - if that were the model that doesn't match observation then DUH! But the physicists are smarter than that. Second, I occasionally see mis-application of the divergence theorem and/or gauss's law. Some folks seem to treat a portion of a galaxy out to a radius R as a point mass by using this theorem. The problem of course is that to make that leap you must have a uniform spherical distribution of matter inside the spherical boundary. In other words, a disk-like distribution of matter does not have a uniform gravitational pull at all points of distance R from its center. You'll find it is stronger near the edge of the disk than at the same distance perpendicular to it. Third, this particular article keeps making reference to the "radial velocity" or speed of stars where I believe they are actually talking about the orbital or tangential velocity. And lastly, if the dark matter obeys the same laws of gravitation, there must be a reason for its distribution to be different than the other matter. At that point, I don't see any need for "strange dark matter" or new particles or physics. You really can't have it both ways - it's fundamentally different, yet it obeys the same rules. Having said that, I could see how exploding stars would spread non-visible matter (remnant material) in a way that might be very different from the stars themselves.

cygx 10y ago

Personally, I'm interested to see what happens to conformal gravity. The theory is able to fit galactic rotation curves without a need for dark matter[1], but supposedly leads to inconsistent results when trying to do the same for gravitational lensing[2]. However, that might have been an artifact of incorrect approximations and the calculations should be repeated using exact solutions[3]. [1] https://arxiv.org/abs/1011.3495 https://arxiv.org/abs/1011.3495 [2] https://arxiv.org/abs/1403.7930 https://arxiv.org/abs/1403.7930 [3] https://arxiv.org/abs/1609.07633 https://arxiv.org/abs/1609.07633

phkahler 10y ago

I keep seeing a couple problems in this whole discussion. Number one is the continued references to Keplers laws. Kepler only applies to 2-body systems where the mass of one is dramatically larger than the other. This works well for the solar system, where the sun is huge compared to the planets and the planets have little effect on each other. But do note that some planets were predicted to exist based on their effect on the orbits of other planets, so even in this sparse system the effects do make a difference. My point is that Kepler has no place in galactic dynamics but is often referenced - if that were the model that doesn't match observation then DUH! But the physicists are smarter than that. Second, I occasionally see mis-application of the divergence theorem and/or gauss's law. Some folks seem to treat a portion of a galaxy out to a radius R as a point mass by using this theorem. The problem of course is that to make that leap you must have a uniform spherical distribution of matter inside the spherical boundary. In other words, a disk-like distribution of matter does not have a uniform gravitational pull at all points of distance R from its center. You'll find it is stronger near the edge of the disk than at the same distance perpendicular to it. Third, this particular article keeps making reference to the "radial velocity" or speed of stars where I believe they are actually talking about the orbital or tangential velocity. And lastly, if the dark matter obeys the same laws of gravitation, there must be a reason for its distribution to be different than the other matter. At that point, I don't see any need for "strange dark matter" or new particles or physics. You really can't have it both ways - it's fundamentally different, yet it obeys the same rules. Having said that, I could see how exploding stars would spread non-visible matter (remnant material) in a way that might be very different from the stars themselves.