Tag Info

Hot answers tagged

13

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 ...


12

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 ...


9

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 ...


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 ...


8

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. ...


5

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 ...


5

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 ...


5

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 ...


5

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 ...


4

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) ...


3

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 ...


2

Another thing about dark energy/matter: People have a pretty good idea that dark energy exist because, when you chart the expansion of two objects in the universe over time, from its origin, there is a bell curve. Basically, the speed of the universe's expansion started out faster, than slowed down, and recently (or at least relatively), the expansion has ...


2

Yes, as mentioned elsewhere, it is possibly possible. Dark matter particles may be intrinsically unstable (though having long lifetimes, which are at least significantly longer than Hubble time). Check for more info here: http://arxiv.org/abs/1307.6434


2

The most favoured WIMPS at the moment are probably neutralinos, see http://en.wikipedia.org/wiki/Neutralino These particles are purely hypothetical at the moment. The mass estimates in the above Wikipedia article for the lightest neutralino range between 10 and 10,000 GeV, meaning that the production rates in SNs will be much lower than with an assumed 1 ...


2

Ultra-high energetic cosmic rays are thought to be caused by black holes as one option. Those energies should be sufficient for the formation of (hypothetical) heavy supersymmetric particles, which should decay to stable (hypothetical) neutralinos, candidates for (hypothetical) WIMPS. Assuming this theoretical framework, black holes (or similar dense ...


2

As Yashbhatt said, we can detect light: with our eyes (visible light only) and with special machines. We can also see the effects of some type of lights. Dark matter, however, cannot be detected for now. Also, light is energy, dark matter is matter. Why does your skin tan? It's because of the ultraviolet light. Why are you hot each summer? It's because of ...


1

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 ...


1

Negative energy should be gravitationally repulsive. But naturally occuring negative energy is too weak to be noticible on a large scale. Macroscopic known is the Casimir effect. From chemistry you'll certainly know van-der-Waals forces. There are several kinds of van-der-Waals forces. One of them are London-van der Waals forces, thought to be related to ...


1

Hydrogen clouds don't even make up a small component of dark matter, because hydrogen is not dark. The image below depicts the emission spectrum of hydrogen in the visible regime (a.k.a., the Balmer series). The emission spectrum of dark matter on the other hand would be completely black. Dark matter is gets its name from the fact that it doesn't absorb ...


1

Ok, it makes a lot of sense that the gravity of the disc of the Milky Way is pulling stars up and down as they go about their circum-galactic orbit. But this wouldn't explain why recent 3-D observations of the nearest stars using the FLAMES-GIRAFFE spectrograph on ESO’s Very Large Telescope and the IMACS spectrograph at the Las Campanas Observatory showed a ...


1

Essentially, the answer is Occam's razor: look for the simplest solution and avoid complicated and contrived solutions, unless observational evidence requires them. Yes, it is possible that two or more types of dark-matter particles exist. But any solution where not one species dominates requires fine-tuning and hence is unfavourable. So, unless there is a ...


1

It looks like Segue 2, found by the Keck Observatory, is a candidate for the lightest galaxy, but I don't know if its the smallest one.



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