# Tag Info

## Hot answers tagged dark-matter

30

Dark matter galaxies are possible but very speculative. On a theoretical level, they are hard to form because dark matter interacts only gravitationally (see Anders Sandberg's answer), which makes it hard to lose energy and become bound structures. On an observational level, they would be hard to detect. Gravitational lensing can do something, but since one ...

27

All Conselice et al. (2016) appear to suggest is that when you look at something like the Hubble deep field, there are many faint (and presumably low mass) galaxies that are not seen. This has absolutely no effect on the need for dark matter. The main results are: (i) as you look back in time, the overall (co-moving) density of galaxies (more massive than a ...

27

Probably not. Dark matter should really be called "transparent matter" since it does not interact with light. This has an important consequence: it is hard for dark matter - whatever it is - to lose energy by radiating. This is why normal matter can form clouds that accrete into dense regions that in turn become galaxies and stars: energy is ...

25

The local dark matter density is actually quite tiny, on the order of $\rho\sim10^{-19}\text{ g/cm}^3$ (see e.g. Bovy & Tremaine (2012)). This means that there is roughly $0.001$-$0.01M_{\odot}$ of dark matter per cubic parsec - a staggeringly small amount. 1000 cubic parsecs would contain about one solar mass of dark matter - and that's a cube 10 ...

24

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 ...

23

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 ...

23

Multiple theories and hypotheses have been proposed as an alternative to dark matter (DM). The most popular are, arguably, MOND (MOdified Newtonian Dynamics):A term for various theories where the gravitational force falls off less steeply than $1/r^2$ at large distances. TeVeS (Tensor–vector–scalar):A relativistic generalization of MOND. f(R) gravity:A ...

21

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 ...

19

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 ...

18

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 ...

18

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. ...

18

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, ...

14

We lack the precision to say that there aren't regions where there is matter without dark matter or vice-versa. But what is clear is that the ratio of dark matter to normal matter, which is (or needs to be) around 5 on average to explain the flatness of the universe, varies by orders of magnitude from place to place. The reason for this is that matter ...

14

The idea that belts or spheres of dust might be responsible for (some) microwave emission is not crazy. Indeed we know that dust does emit microwaves and indeed the contribution of such dust has to be removed from the CMB signal before it can be interpreted cosmologically. There is some debate about some of the larger scale anisotropies (particularly the &...

13

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 ...

13

(Short answer: No, scroll to the last point.) It is irrelevant to an external observer whether the matter that fell into the black hole was dark matter or baryonic, by the no hair theorem. The only properties of a black hole from our point of view are mass, electric charge and angular momentum. (But of course we don't understand quantum gravity.) From the ...

13

Yes, neutron stars might actually accumulate weakly interacting dark matter and this allows some observational constraints on its nature. Basically, the temperature and continued existence of neutron stars places bounds on the density and interaction cross-section of dark matter. A dark matter particle that does not interact with matter will just have its ...

12

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 ...

12

Let me see if I can answer at least some of this. Yes, there is dark matter between galaxies. This is demonstrated by the fact that in galaxy groups and clusters, you need more dark matter than is found in the galaxies themselves to explain what's going on: in terms of why the groups or clusters are gravitationally bound in spite of the extreme velocities ...

12

Cosmological parameters are measured in a variety of ways, and their values will depend on which measurements you trust the most. The paper you link to (Planck Collaboration et al. 2016) with the 2015 results from the Planck observations of the cosmic microwave background is probably the one that most people will accept, but even in that paper you will find ...

11

Yes, it is a thing. Weakly Interacting Massive Particles (aka WIMPS) are thought to come in matter and anti-matter forms and have a self-annihilation cross-section in order to produce the "correct" amount of dark matter (in relation to photons) that we measure today. As the opening paragraph of the wikipedia article on WIMPS states - there are a number of ...

11

I'm going to answer this question, not because I believe you want a thoughtful, coherent answer, but rather for others who may come across this question and look at the propaganda you've posted. I don't believe anything I can say will change your mind, but hopefully I can inform other, unsuspecting viewers The Context The video you've posted is from ...

11

These neutrinos would have to be really cold. The cosmic neutrino background is at 1.9K, and they are considered hot dark matter, because they would have been highly relativistic at the epoch of structure formation. To be considered cold dark matter, and also to be captured in orbits in galaxies, the neutrinos would have to be much colder - totally non-...

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 ...

10

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 ...

10

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 ...

10

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 ...

10

Black holes formed after the epoch of primordial nucleoynthesis would be part of the baryonic matter, since they would be mostly formed of baryons. It is possible or even probable that every black hole must contain some non-baryonic matter too, since there is nothing to prevent it being accreted, in small amounts, directly into black holes. However, the ...

9

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 ...

9

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 ...

Only top voted, non community-wiki answers of a minimum length are eligible