My interest is in dark matter mostly and its affects on our universe aswell as the galaxies that float around in it. What its gravitational properties do to these objects and what not? And on anti matter i have very little understanding of that too. Thank you.
Comments:
Antimatter is matter that has equal mass and opposite charge to normal matter. If it comes into contact with normal matter it causes mutual annihilation. Examples of antimatter are positrons, anti protons, anti neutrons etc. Here is antimatter (positrons) coming off a block of Potassium as protons decay into neutrons.
As for dark matter.
Dark matter is matter we don’t understand and can’t see. It is speculated the vast majority of the universe is made of Dark matter, which remember is a generic term for any matter we can’t see. We know it exists by the gravitational influence it has on matter we can see. As for the galaxies, we have observed that galaxies rotate in a strange way, namely the stars on the outer arms of the spirals orbit the centre of the galaxy roughly as fast as the stars close to the centre. This is not how gravity is supposed to work. Look at our solar system Mercury orbits the Sun every 88 days or so, Earth every 365 days and Neptune every 185 Earth years. So we expect the stars in the outer reaches of the galaxy to take longer to complete their orbits than those close to the centre. Since this is not the case there must be a heck of a lot more matter in this and every other galaxy than we can see with our telescopes.
Just remember dark matter simply means “we don’t know the heck it is”, the same goes for dark energy and dark flow, these are just words to describe something astronomers have observed that cosmologists and theoretical physicists can’t yet explain.
If dark matter makes up 96% of the universe, and visible matter only 4%, then is there dark matter around, on, or in the Earth or is it all out in space somewhere?
Comments:
George is close. Dark matter exists in large halos which envelope galaxies. The dark matter halos can have an arbitrary shape, in fact many people have made “maps” of the dark matter by looking at gravitational lenses. The size of the halo can be a few to many tens of times larger than the galaxy which resides in side of the halo. Many very large halos can host several galaxies (though rarely more than a few galaxies per halo).
The mistake George makes is when he says that it’s “near the Galactic center”. This is very false. The galactic center is *inside* a dark matter halo. In fact, the center of the halo and the galaxy need not be the same, such as in the case of this galaxy cluster:
http://en.wikipedia.org/wiki/Bullet_Cluster
Also, since the dark halos host galaxies, and earth is in a galaxy, then earth is in a dark halo. Therefore, dark matter *EXISTS* all around us, even here on Earth. There are several major experiments set up to detect dark matter and measure it’s properties. You can learn about a famous experiment at:
http://en.wikipedia.org/wiki/Cryogenic_Dark_Matter_Search
Dark matter is the molasses of the cosmos, as I understand it, and it can’t influence gravity. I understand that it is a bit like steam in an engine, keeping the train in motion. Correct me if I’m wrong.
Comments:
Dark matter is a lot like normal matter, but with one crucial exception: it doesn’t interact with the electromagnetic force. This means there is no electromagnetic attraction or repulsion, no interaction with light or any other form of EMR. Of course, the only reason we think this is because we can’t see it. Dark matter makes up for a huge percentage of the universe’s mass, and yet…we can’t see it. We can however, see it’s effects.
Dark matter still interacts with and exudes the gravitational force, and we can see this effect in a phenomenon known as Gravitational Lensing. Essentially, when we look into the sky and take pictures (with the Hubble Telescope), we can see that some galaxies are curved, or lensed. This is caused by the light from these galaxies being ‘bent’ by a strong gravitational field on the way to Earth. We can’t actually see what’s causing these fields, and they are presumed to be caused by the invisible dark matter.
Dark Energy is quite different from Dark Matter. It makes up for around 73 percent of the total mass-energy of the universe, and yet we still have no direct evidence of it. It is believed to not interact with any of the fundamental forces other than gravity however, and is also believed to possess a strong negative pressure. This presumed pressure is the reason we attribute the universe’s expansion to dark energy.
Also, whilst dark matter is known to form clumps and clusters throughout the universe, dark energy is spread evenly throughout space. In all honesty though, that’s about as much as we know about dark energy. It’s a very new concept in physics, and is very difficult to study. But it could answer a lot of questions that need to be answered (quantum gravitation, for one).
What do you think dark matter really is?
Why do you think it is important?
And does it REALLY matter?
Is that so, cosmo?
Then you can tell me what you think it is and why it we wouldn’t exist without it.
I assume you know exactly what it is after spending half your life studying?
Or have you no real answer?
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I like how you stated your question – very clever!
First – what is dark matter. Dark matter is dark because it is unseen and because it is unknown or more importantly unexplained. Lord Kelvin in the 19th Century estimated that if the Sun burned do to chemical reactions it would burn out in 30,000 years. But as it was discovered that the earth had a history much older than that, there must be something else that fuels the Sun and stars. It was not until nuclear energy was discovered and investigated in the 20th Century that we understood what happens within a star like the Sun.
Dark matter represents a similar mystery. If galaxies represent all the matter that matters, then as Zwicky first discovered we would expect a drop off in the velocities of stars as they orbit further from the center of the galaxy. Instead, Zwicky saw that galaxies have rotation curves as though they were orders of magnitude greater than what we see. The same holds for galaxy clusters of all sizes. So a mystery.
The other part of darkness – unseen – comes from the fact if we have unseen matter all over the place but yet can see back to the beginning of the universe, then the dark matter cannot interact strongly with light. Call that our first clue.
As to the second part of your question. It matters only if faced with a mystery you must find an answer. Of course it would be difficult to accept any of our current physical theories if they cannot explain this mystery, and in turn the validity of any scientific pursuit if mysteries as fundamental as where is matter can be left unanswered. Whether or not it is a big deal once the answer is found – don’t know, but maybe.
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Wouldn’t dark matter need to make up exactly 50% of the universes mass for the mirror matter hypothesis to be credible?
Yet dark matter makes up 96% of the Universes mass so this does not make sense. 4% and 96% seem no where near equal.
Comments:
Your argument would be correct if symmetry was preserved. However if mirror symmetry is broken then the mirror matter could be either heavier or lighter than ordinary matter and would interact only weakly if not at all via the electromagnetic, strong and weak forces and only via gravity. Hence if mirror matter existed it would be by definition be dark matter.
The 96% includes dark matter and dark energy which may or may not be coupled. The dark matter is about 6 times the ordinary matter we interact with (we see less than 25% of that). These numbers may become important numbers if mirror matter can be shown to exist or string theory will ever be confirmed.
Stay tuned.
