Hot answers tagged

16

Hot dark matter would be made from very light, fast moving particles. Such particles could not possibly be gravitationally bound to any structure, but rather would be dispersed all across the universe. But dark matter is always "found" (or "inferred") either gravitationally bound to some visible structure (e.g. weak lensing detection of dark matter ...


16

Planets and stars, no. Globular clusters and galaxies, yes. Small scales To condense into such relatively compact objects as planets, stars, and even the more diffuse star-forming clouds, particles need to be able to dissipate their energy. If they don't do this, their velocities prohibit them from forming anything. "Normal" particles, i.e. atoms, do this ...


15

First of all I'll start with a few ideas: Baryonic Matter: Baryons are elementary particles made up of 3 quarks. This includes protons and neutrons, and the term baryonic matter refers to matter made of baryons, such as atoms. Examples of non-baryonic matter includes neutrinos, free electrons and other exotic matter. Things like planets, stars, dust, etc. ...


14

Some additions to the answer of MBR: In fact, we do not know that dark matter and dark energy do exist, but we have indirect clues. You will often see claims that dark matter and dark energy are two of the major problems of cosmology today, including by professional astronomers, but this is an epistemological misconception: you cannot call a hypothesis a ...


12

The problem with trying to form a black hole with dark matter is that dark matter can only weakly interact (if at all) with normal matter and itself, other than by gravity. This poses a problem. To get dark matter concentrated enough to form a black hole requires it to increase its (negative) gravitational binding energy without at the same time increasing ...


10

Dark matter and dark energy are two different things, accounting for different observations. Dark matter: Dark matter is needed to explain, among other things, the rotation curve of galaxies. One could expect these rotation curves to decrease at large radii (because one should expect keplerian rotation for galaxies), and it is not the case, the rotation ...


9

Dark matter, is just a name for something we know nothing of. It was named to account for an extra gravity source for which there have been indirect observations, but yet we cannot explain. The force of gravity exerted by light is negligibly small yet we have measured the gravitational pull of Dark Matter to be big enough to affect whole galaxies; it is ...


9

There are certainly people who study alternative (non-General Relativistic) theories of gravity. The most popular theories have so far been: Modified Newtonian Dynamics (MOND) - which essentially postulates that Newtonian Mechanics break down on some scale, leading to the rotation curves we see in galaxies. Tensor–vector–scalar gravity (TeVeS) - this is a ...


8

Dark Matter Your understanding of dark matter isn't bad, but here's a few clarifying details. Orbits: The speed of an object's orbit is related to 2 things: the radius of its orbit and the mass inside of it. In the solar system, over 99% of the mass is concentrated at the centre, so radius is the dominant effect on orbital speed. As we look at planets ...


7

No, the galactic magnetic field is very weak, about 0.1nT. It is able to bend the trajectory of highly-energetic charged particles and also to align dust grains across the magnetic field. However, is too weak to affect the rotation of a galaxy. Although the origin of galactic magnetic field is not clear yet, the supermassive black holes do not ...


7

Matter is the stuff you are made of. Antimatter is the same as matter in every way, looks the same, behaves the same, except its particles have electrical charges opposite to matter. E.g., our electrons are negatively charged, whereas a positron (an antimatter "electron") is positively charged. The positron is the "anti-particle" of the electron. When a ...


7

Dark matter attracts baryons not only by a "kind of gravity", but simply by gravity. Everything that has mass (even if it's only relativistic mass like light) attracts everything else by gravity, and thus baryons also attract dark matter. The reason dark matter "organizes" galaxies is just that there is much more of it, roughly 7 times more, to be specific. ...


6

You don't have to guesstimate to come up with the answer. What you do is look at the dynamics of stars with respect to the Galactic plane - in particular, the velocity dispersions of stars with known distances from the plane, combined with a reasonable assessment of where the Sun is with respect to the plane (close), yields an almost model-independent ...


6

What I ask myself is: Couldn't there be "negative" bundles of mass just the other way that pushes matter away instead of invisible dark matter that pulls it? The galaxy rotation curve indicates a (positively massed) dark matter distribution that is close to spherically symmetric; cf. dark matter halo. I take it that you are asking whether instead of ...


6

When you say "particle" candidates, I assume you're excluding MACHOs and RAMBOs. MACHOs are "dark" objects at the stellar scale like black holes, neutron stars, brown dwarfs, etc. RAMBOs are clusters of similar dark objects. MACHOs and RAMBOs are made of primarily baryonic matter (everyday stuff like protons and neutrons — electrons are not baryons but ...


5

The current supernova is a supernova of type Ia. Supernovae of type Ia are used as standard candles for distance estimates, especially used to determine the Hubble constant. Hence by a better calibration of this kind of supernovae, more about the reliability and accuracy of distance estimates can be learned. The expansion rate (in relation to the distance) ...


5

This is a bit of a speculative question, but I can answer it. Dark matter has been observed in galaxies, and the distributions of dark matter in galaxies have also been measured. It seems clear that the matter is firmly in our universe - we just can't detect it with electromagnetic radiation. *However, there are ideas (extremely speculative) that dark ...


5

I don't see any reason why dark matter couldn't be considered a medium just as air or water are, but this does not mean that your conclusion is correct. Dark matter does not interact with photons, however the reason we know dark matter exists is because of the measurable effect of its gravity. Galaxies have a tendency to clump around it. Light travels ...


5

The question seems to be based on some misconceptions, so I'll try to clear them up. Dark matter is not the same as dark energy. Dark matter and dark energy are two completely separate and unrelated concepts. It's unfortunate that many articles trying to entice readers with tales of the unexplored frontiers of astronomy tend to use the phrase "dark matter ...


5

Two reasons. We know from looking at galaxy rotation curves and the motion of galaxies in clusters and from gravitational lensing, that the amount of "dark matter" is some 30% of the density of the universe. But on the other hand, estimates of the abundances of deuterium, helium, tritium and lithium produced in the big bang indicate that only 5% of the ...


5

As pointed out by Rob Jeffries, forming a black hole (BH) from dark matter (DM) is impossible (unless there is a [hypothetical] interaction by which dark-matter can lose energy that evades all detection). Accreting DM into an existing BH is still unlikely (since DM cannot lose its excess energy and angular momentum as easily as gas), but not impossible and a ...


5

The dark matter model that is used to explain the "missing mass" problem relating to our Galactic rotation curve, consists of a pseudo-spherical distribution that is much more extended than the visible stars and gas. Even though this "halo" contains more than ten times the mass of the visible matter, when you work out what it's density should be in the solar ...


5

Strange you should ask - I am currently working on a paper on NGC 2516 - "the southern Pleiades" and looking at the dynamical status of the stars compared with the distribution of visible mass. Our conclusion is that the radial velocities of the stars are in virial equilibrium, with velocity dispersions that are entirely consistent with the mass that is ...


4

We can't. That is an over simplification only used in elementary treatments simply to provide the the flavour of the argument. If you see it done somewhere in the refereed literature, it is probably incorrect. Of course it may be true that the mass is almost spherically symmetric, especially if it is dominated by a spherically symmetric dark matter ...


4

There are several types of supernova and ways that the core can collapse. Lets take an extreme case in which gamma-ray photodisintegration destroys all of the heavy elements (Si, Fe and Ni, etc) and breaks them all up into protons, neutrons and electrons. Each nucleus releases all of its binding energy, about 9 MeV per nucleon mass or 0.9% of the rest ...


4

The cause for the oscillations perpendicular to the galactic plane is the gravity of the non-spherical mass distribution (needed for a plane Kepler ellipse) in the Milky Way. Simplified, there is a dense galactic plane. The density is not exactly known; therefore there is some uncertainty (a few million years) about the precise oscillation period. Details ...


4

You cannot have dark without light. Not true in the case of dark matter (for the general case, see below). Dark matter is called "dark" because it appears that it doesn't absorb or emit electromagnetic radiation - light! It can interact with light via gravitational lensing, but dark matter particles have no electromagnetic charge (we think) and so are ...


4

Is it the paper David H pointed out, by Geringer-Sameth et al. (2015)? If so, then I should point out that they're not the first. Just using Wikipedia, I came across Weniger (2012) and Albert et al. (2008). Geringer-Sameth et al.'s abstract reads (in part) We present a search for gamma-ray emission from the direction of the newly discovered dwarf galaxy ...


4

Black holes are the result of mass so concentrated that gravity does not let anything out, including light. Just about the only things we know about dark matter is that it has mass and seems to only interact with ordinary matter through gravity. Since we do not know the physics of dark matter at all, it is impossible to say what processes might concentrate ...


3

Light may account for a small portion of dark matter, but it is unlikely to account for most/all of it. From a Wikipedia article on dark matter: http://en.wikipedia.org/wiki/Dark_matter the total mass–energy of the known universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[2][3] Thus, dark matter is estimated to constitute ...



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