Since this is dated 2000 I thought I’d add a few updates:
For a long time the favorite theory of astrophysicists was “WIMPs”: Weakly-Interacting Massive Particles. This is because the Supersymmetry extension to the Standard Model of particle physics predicted the existence of very heavy particles that would only interact with conventional matter via the weak force, which would fit the bill for dark matter splendidly. The only problem is that the most likely candidates for supersymmetry particles have been ruled out- if they were there we ought to have detected them by now. This is still considered the least unlikely hypothesis, but it’s not the slam-dunk astrophysicists once thought.
Another problem is that recent measurements of galactic rotation seem to indicate that although the overall rotation doesn’t fit classical gravity, the differential between rates of rotation in galaxies scales exactly with the amount of observable mass. This has led some to reconsider Modified Newtonian Gravity as a possibility.
I’m reminded of Tom Weller’s Science Made Stupid (1985), a parody of high school science textbooks, which in the section on particle physics said:
To maybe add something a little more explicit: This covers why MOND is not a great alternative to dark matter.
Basically, MOND correctly accounts for galaxy rotation curves, which is not that exciting because it was designed to account for galaxy rotation curves. Dark matter accounts for galaxy rotation curves and a bunch of other observations as well.
I have a question … please excuse my ignorance if this is a stupid one …
Where is all the dark matter in our own solar system?
My understanding is that GR correctly describes the gravity here and in other solar system sized objects … the failure comes with galaxies and (more importantly) galactic clusters … if galaxies are made up of 90% dark matter … why isn’t our solar system made up of 90% dark matter? …
Because the regular matter in our solar system is a whole bunch more dense than the galactic average for either type of matter.
Regular matter* can stick together through electromagnetic interactions and form “clumps” of above average density, like stars and planets, and people. Dark matter, because it doesn’t interact electromagnetically at all, can’t stick together like that and can only form much looser clumps held together by gravity.
*(Although really, given the relative abundance, the term “regular matter” should refer to dark matter, and the stuff we are made of should be “weird matter”.)
Also the dark matter (as thin as it is) is apparently uniformly distributed in our neck of the galaxy. So we don’t see Pluto moving oddly since Pluto is feeling the same forces from dark matter that the Earth, the Sun, etc. are feeling.
Well, that’s if all of the dark matter is the same kind of stuff, at least. I’ve never seen any reason to assume that.
watchwolf49, if you draw the boundaries of solar systems in such a way that every point in the Galaxy is part of some solar system or another, then you’d have to say that our Solar System is a few lightyears across. Take ten times the mass of the Sun, and spread it uniformly over a volume that large, and you’ll get an extremely small density, small enough that it’d be quite plausible that we’d miss noticing it. There have still been some attempts to directly measure our own local dark matter, but nobody’s particularly surprised that they’ve all failed so far.
Back to the article, there are a number of other points that need updating (Karen really chose a poor time to write it, right before a lot of big discoveries). Probably the biggest is that, mostly due to results from the MAP satellite, cosmology now is a precision science: Our old factor-of-two error bars on most measurements have been replaced with 1% error bars. And of course, this precision has brought many new discoveries.
The biggest of those discoveries is that, although the Universe is in fact at or very close to critical density, most of it isn’t due to normal matter or dark matter. Most of the “stuff” in the Universe actually appears to be something even more mysterious, which we call “dark energy”. What little we know of dark energy is that it has a pressure which is both extremely large and negative, which causes it to have a repulsive gravitational effect. One implication of this is that the ultimate fate of the Universe does not depend on the total density: Even if the density is (or becomes) somewhat higher than the critical density, it’ll still not only expand forever, but at an ever-increasing rate. Or at least, until and unless something weird happens with the dark energy, which given our level of ignorance of it cannot be ruled out.
Another pair of new advances is that, first, we know that neutrinos do in fact have mass, but second, that we’ve learned enough about the structure of the dark matter to say that neutrinos still can’t account for more than a very small portion of it.
I found this which discusses the effect of dark matter on the orbits of planets in the solar system, and the orbits of stars in the galaxy.
Tl;dr: 18 orders of magnitude less effect than luminous matter in the solar system for Earth, but about on par with the luminous matter for stars at the Sun’s distance from the centre of the galaxy.
That’s a bit of an understatement. No one could even begin to imagine that the universe was actually accelerating in its expansion until we got that very high-resolution data. From my perspective as someone who tries to follow everything available on the topic that doesn’t require a physics degree, it seems to have led to quite a significant rethinking as to the nature of reality and its inception.
What a weird link. As long as the dark matter is locally uniform and its only interaction is via gravity, then the affect on the orbits of Solar System objects doesn’t matter. I.e., 18 orders of magnitude is a wee bit of an understatement. It’s more like a zillion orders of magnitude.
I have no idea why a calculation on the total mass-equivalent inside Earth’s orbit was done. The effect the Earth feels from dark matter in the Solar System region is uniform in all directions. Something on the sunward side has just as much affect as something on the other side.
It’s only when you get non-uniform distributions of dark matter (which happens at a much larger scale) does things get interesting. E.g., the difference between the inside and outside the Solar System’s orbit around the galaxy does add up to something measurable.
It’s because we already know there is a concentration of mass at the centre of Earth’s orbit because it is all bright and glow-y, so the system is approximately spherically symmetric. Significant dark matter on the sunward side would make the sun seem more massive than it is (and the Earth would be moving faster than it “ought” to be to stay in orbit). But uniform dark matter outside the Earth’s orbit has no dynamic effect.
The calculation for the galaxy is a lower bound. We know the dark matter towards the centre of the galaxy is denser than it is here, but even if we assume that dark matter is distributed completely uniformly we still get a significant effect. The actual effect, accounting for the actual density distribution, must be even greater.
I understand what you’re saying here, and indeed if dark matter was spread evenly through the extended Solar System as you’ve described above … then it would be thin indeed, a vanishing small amount within the Earth’s orbit …
Unfortunately this doesn’t answer my question … if I may rephrase: why would dark matter be evenly distributed in the extended Solar System and not be evenly distributed in the extended Galaxy? … Spread ten times the mass of the Milky Way in a sphere over 1,000,000 ly in diameter … wouldn’t the amount within the Sun’s orbit (27,000 ly) again be vanishingly small?
I’m not trying to support MOND here … my limited and outdated understanding of this “theory” leads me to believe it is an exceptional poor alternative … however it does attempt to answer the question I’ve posed above … and suffers the same problem as epicycles, as it says nothing of the underlying physics involved …
Because electromagnetic interactions are critical to forming small-scale (by which I mean “sub-galactic-scale”) clumps of matter. Regular matter “sticks together” through electromagnetic interactions so if you have a small lump of regular matter, it can accrete more regular matter. Even if the end result is held together mostly through gravity, electromagnetic interactions help infalling matter to shed energy and stick.
E.g. if you fire an electron from space into the Earth, the electron will interact electromagnetically with a bunch of stuff and eventually it, or the end products of those reactions will “stick” to the Earth. But if you fire a neutrino into the Earth, it will most likely just keep going because there are (almost) no possible interactions to slow it down. Dark matter is more like the neutrinos than electrons.
Back in February 2000 the announcement of the accelerated expansion of the Universe in would’ve been pretty well publicized, so there was certainly scope to include it in the article. There are a few other things in the article that would’ve been considered out of date in terms of our understanding of cosmology and dark matter even by 2000, so I’d say a revision would be in order.
I knew someone would mistakenly think this was about the shell thing.
Check your link. That applies to a body. I.e., something with finite extent. Dark matter doesn’t have a finite extent.
Think about it. Why that sphere? Why not draw a 10AU sphere around Pluto and measure that sphere’s dark matter’s influence on the Earth’s orbit?
You can’t do a change of size or position thing on a spherical body because you have to stay centered. Where is the “center” of the dark matter “sphere” in our local area???
Dark matter does have a finite extent (at least, on galactic scales), but even if it didn’t, GR removes that requirement from the calculation. Dark matter outside the sphere of Earth’s orbit affects the Earth only in the same way that it affects everything else in the Solar System, so for purposes of the planets’ motion around the Sun, it can be ignored. Dark matter within that sphere, however, cannot be (except to the extent that it’s really really small).
I’m pretty sure dark matter is everywhere. The important distinction is the variation of dark matter. It varies enough on the scale of a galaxy to be important. On the scale of the Solar System, the local variation is far lower than the 10^18 ratio calculated in leahcim 's link.
(I find it interesting that scales on the size of star systems are amazingly insignificant in this context.)
Because presumably, IF it doesn’t react with itself via a “dark force” that doesn’t interact with ordinary matter, then dark matter has no way to slow down- a collapsing cloud of dark matter would simply pass through itself and reexpand again. Unlike a cloud of conventional matter, where the electric force (mainly by radiation) strongly counteracts gravity.
The basic difference is that normal matter has ways to lose momentum and angular momentum, mainly due to the electromagnetic interactions. For example if you drive a car into a wall the car will lose its momentum pretty quickly and will be at approximate rest to the surrounding normal matter; if you drive a similar mass of dark matter to the car into a similar mass of dark matter to the wall (or a normal wall) the ‘dark matter car’ will go straight on through the ‘wall’ without losing any momentum.