# Tag Info

-1

Tom Fangrow, thank you for your questioning mind and sharing your thoughts. Every single star and radiation source puts out a seemingly continuous and infinite supply of photons in every single direction for thousands of light years. You can see a star, move your head a few inches or a hundred miles and still see that star that might be thousands of light ...

2

OK, I'll give this a shot, cause . . . why not. The article you quoted covers a lot of ground - perhaps too much ground. And I'm not sure the quote of Matt Strassler is fair because he's answering a very specific question and while the source is given, it's not mentioned that it's a specific question that he's answering. but, lets jump to this part: ...

1

Let embrace your attitude here. If the atom is 'empty', all that we have is charge and mass. By Newton's third law we have that this 'empty' with charge and mass need to absorb energy and momentum too. Now we have an empty space with charge and mass that absorb momentum and energy. Furthermore, this empty are allowed to move through space, because is ...

5

Dark matter candidates are all "beyond the standard model" physics, which means that they represent an extension of the model into something more comprehensive. Some of these extensions are pretty minimal (I think both axions and massive, sterile neutrinos are in that category) others are quite comprehensive. Many theorists have favorite models and ...

-1

In my Physics Class we had to look into possible areas where Dark Matter could be "hiding". Where is all the dark matter? Actually, I think it's hiding in plain sight. If you've read up on relativity I think you can work it out. See Einstein's Leyden Address where he described a gravitational field as space which was "neither homogeneous nor isotropic", and ...

-9

There are two main misconceptions about dark matter. One is that dark matter is a clump of stuff traveling with the matter. The other is that matter does not interact with dark matter. Dark matter fills 'empty' space. 'Empty' space has mass. Spacetime has mass. Dark matter is displaced by matter. The Milky Way moves through and curves spacetime. The ...

10

There are several reasons to believe that dark matter is a particle. The most widely accepted alternative explanations for the different phenomena that led us to conjecture dark matter in the first place, can collectively be labeled "we don't understand gravity well enough". But no matter what, the effects of dark matter are sort of "localized". The ...

2

Dark matter is called "dark" precisely because it doesn't seem to interact with light, i.e. electromagnetism, at all, in the sense of being charged. Only it's gravitational influence can be seen (from which its existence is inferred).

3

I would think - but could not put my hands on any evidence to back this assertion up - that such neutrinos would be unavoidably thermally coupled to the rest of the universe prior to 1 second [before this, the neutrinos are sufficiently coupled to electrons and positrons through weak interactions that they follow their temperature]. So, even though cooled ...

8

It can't be solo neutrons, because they are unstable and decay into protons. So far as we know, there's not a stable configuration of mostly-neutrons that occurs in nature intermediate between heavy nuclei (uranium is roughly 3-to-2 parts neutrons) and neutron stars of 1-3 solar masses (which are about 90% neutrons). What you're describing would be the kind ...

1

As somebody said in the comments, physics has no "proofs". It has measurements/data and mathematical models that attempt to fit the data with specific assumptions. At most models are validated, not proven, if all known observations are consistent with the mathematical model. In the case of dark matter the first indications of its necessity in a ...

2

The answer to the first question is no. It is a basic but subtle point... Science is about adopting the simplest explanation of phenomena. This is usually referred to as Ocam's razor. With very exotic, high-energy theories like string theory or here a gravitino (I think you have a typo!) there is no single clear signature that can not be explained by ...

12

The answer comes from the virial theorem, which can be derived from the Jeans equations, which are the equivalent of the Euler equations of fluid dynamics for collisionless particles (i.e., dark matter). Incidentally, the virial theorem is also valid for an ideal fluid. For a derivation see Mo, van den Bosch & White 2010 (or I'm sure many other texts). ...

2

On average the density of dark matter in the universe is about the same as two hydrogen atoms per cubic metre. That's about 0.00000000000000000000001% of the density of air (a factor of $10^{-25}$). So even if it did interact with light, the effect of the relative permittivity and permeability would be negligable. But dark matter doesn't interact strongly ...

0

Concerning EM force: According to modern understanding, even if all matter could be removed from a volume, it would still not be "empty" due to vacuum fluctuations, dark energy, transiting gamma- and cosmic rays, neutrinos, along with other phenomena in quantum physics. Is the “quantum vacuum” contaminated given there is nothing that can block a magnetic ...

4

The permittivity and permeability of free space are non-zero in 'absolute vacuum' and they will be unaffected by dark matter which interacts extremely weakly (if at all) with the EM force. We might need to modify gravity to account for dark matter (hopefully not) but that's all.

2

Simple: people have tried to make modified gravity churn out the right numbers for all the observations we have, and nobody has managed to get results quite as good as General Relativity + Dark Matter produce. Here's a quote from Modified Newtonian dynamics, one of the modified gravity theories: MOND and its generalisations do not adequately account for ...

2

The first evidence for dark matter does not depend on General Relativity but on Newtonian mechanics: At large distances from the galactic centre the gravitational potential should be that produced by a central point mass and, in the absence of forces other than gravitation, it should be expected that GM/R2 = $\theta^{2}_{}$/R (G, universal gravitation ...

-5

In terms of dark matter, there are two notions which are incorrect. One is that dark matter is a clump of stuff traveling with the matter. The other is that dark matter does not interact with matter. Dark matter fills 'empty' space. Dark matter is displaced by matter. The Milky Way moves through and displaces the dark matter. The Milky Way's halo is the ...

3

The evidence for dark matter crops up in various places and on various scales - from the scale of fluctuations in the cosmic microwave background, to the formation of large scale structure in the Universe, through to the dynamics of galaxies in clusters (and gravitational lensing) and the dynamics of stars and gas within galaxies. Of critical importance are ...

3

Because General Relativity has an overwhelming amount of experimental evidence to support it. As a result, physicists look for dark matter, which works within GR, rather than to throw the baby out with the bath water, and thereby assume GR is wrong. When Albert Einstein introduced the world to GR, he proposed three tests that would support GR. Please ...

-1

"My question wasn't intended to challenge the standard interpretation, since I don't know nearly enough physics to do so. I'm just a curious sort. It was honestly an attempt to ascertain if there exist good theoretical reasons why it can't be curved spacetime, since that seems a logical possibility based on my laymen's knowledge, given the lack of any DM ...

3

Not quite likely, simply because of these experimental facts: a.) Distant Gravitational lensed: We observed the following picture, and it can't be by luck for galaxies to arrange themselves like that. One strong suggestion is there is something big (therefore it can't be black hole) and massive between. b.) Gravitational rotational curve: Classical ...

-9

Dark matter is not a clump of stuff traveling along with the matter. Matter moves through and displaces the mass which fills 'empty' space. The Milky Way is moving through and displacing the mass that fills 'empty' space. The Milky Way's halo is the state of displacement of the mass that fills 'empty' space. The state of displacement of the mass that ...

3

This would contradict general relativity. Because there are no static solutions of the einstein equation with localized curvature not being caused by some mass distribution.

4

This is yet another instance of taking the ubiquitous balloon analogy too far. See, while it's a wonderful way to express the expansion of the universe, there are some misconceptions that arise from it: We live in a universe of finite size (we don't know, but we think not) and non-zero curvature (according to WMAP, we don't, or at least we think we don't) ...

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