Dark Energy and Matter

[Equilibrium]

I don't really understand dark energy or dark matter, can some one explain it to me a little? Also, why is dark energy causing universal expansion?

[Spyman]

Dark Matter are some cind of yet unknown matter that we can't see, it doesn't interact like normal matter so there is no light reflected or emitted from it. But we know it must be there because we can measure how it affects normal matter with its gravity.

 

Dark Energy is even less known, all we know is that the Universe is expanding at an accelerating rate and the best explanation for this is Dark Energy, but we don't know what it is or how it manages with such a feat.

 

Here are some links with better explanations and more details:

 

"Dark Matter

In astronomy and cosmology, dark matter is matter that is inferred to exist from gravitational effects on visible matter and background radiation, but is undetectable by emitted or scattered electromagnetic radiation."

http://en.wikipedia.org/wiki/Dark_matter

 

"Dark Energy

In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe.[1] Dark energy is the most popular way to explain recent observations and experiments that the universe appears to be expanding at an accelerating rate."

http://en.wikipedia.org/wiki/Dark_energy

 

"What Is Dark Energy?

More is unknown than is known. We know how much dark energy there is because we know how it affects the Universe's expansion. Other than that, it is a complete mystery."

"What Is Dark Matter?

We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see."

http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/

[timo]

First off: I am not an astrophysicist. I did some work on relativity and exotic matter as a physics student, but did not follow the developments in astrophysics for about the last 4 years. So take all below with a grain of salt.

 

I don't really understand dark energy [...], can some one explain it to me a little?

The cosmological constant: When you write down the equation for the dynamics of the gravitational field, the Einstein equation, this equation turns out not to be unique. Rather, there is a free real-valued parameter in there, i.e. the value of the parameter is not determined from the theory. This parameter is usually called the "cosmological constant [math]\lambda[/math]". What you can do about this parameter is that you compare the theory predictions for different values of this parameter to experimental results and fit this parameter. For most applications, [math]\lambda=0[/math] is an excellent approximation. For one case, however, it is expected that a tiny non-zero [math]\lambda[/math] is important. That is the measurement of the universe's expansion.

 

Now for your actual question: it can be considered somewhat inelegant to have a mathematical term in one of your most basic equations that is just a fit parameter without physical interpretation. So it is tempting to assume there is a physical reason why this term is there (assuming it is non-zero, of course). "Dark energy" is the idea that this term is the contribution to the energy density (**) which is caused by some unknown physical object (say an unknown form of matter). If you go beyond the explanation I just gave and analyze the structure of normal matter contributions to the Einstein equation and compare that to the contribution from the term with the cosmological constant, then you can also make some assumptions about the properties of this yet-unknown physical object. These properties are -assuming I recall that correctly- quite weird when compared to normal matter.

 

In essence: At the current state, depending on your personality you can consider dark energy to be either

- another name for the cosmological constant, or

- a yet-unknown physical object with very exotic properties.

 

 

(**): Strictly speaking not only the energy density, but I think my reply is already complicated enough.

Edited November 10, 2010 by timo

[Demonio]

I like to think of dark matter as liquid that the universe is suspended within and about why it causes the universe to expand? dark matter is expanding itself and our univere is expanding with it filling in the new space that is now available. The universe couldent exist outside the dark matter and also cannot leave its confines so basicly where dark matter goes so does the rest of the universe.

[imatfaal]

Demonio - but surely the dark matter is not causing expansion. dark matter was predicted to make up for the fact that there is too little apparent mass in orbiting galaxies within large clusters. dark matter is the unknown and unseen matter that makes up the difference between the measurable mass of the universe and the calculations. now dark energy is a different matter - that is an intellectual construct to stand in for the fact that we don't understand the large scale large time expansion rate of the universe

[Severian]

Dark matter is most probably a stable particle with only weak force couplings (that is, neutral and colorless) that has a mass about 100 times that of the proton.

 

Dark energy can be explained by just a Cosmological constant.

[timo]

In essence: At the current state, depending on your personality you can consider dark energy to be either

- another name for the cosmological constant, or

- a yet-unknown physical object with very exotic properties.

 

Dark energy can be explained by just a Cosmological constant.

 

Welcome to drawer number one.

[zapatos]

Is it known where dark matter resides? For example, does it seem to be concentrated in the center of galaxies, or is it perhaps evenly distributed?

[Spyman]

Is it known where dark matter resides? For example, does it seem to be concentrated in the center of galaxies, or is it perhaps evenly distributed?

"Galaxies show signs of being composed largely of a roughly spherically symmetric, centrally concentrated halo of dark matter with the visible matter concentrated in a disc at the center."

http://en.wikipedia.org/wiki/Dark_matter

[lemur]

Dark matter is most probably a stable particle with only weak force couplings (that is, neutral and colorless) that has a mass about 100 times that of the proton.

 

Dark energy can be explained by just a Cosmological constant.

I find it plausible that protons/neutrons could somehow fuse into larger particles under high gravity conditions. It also seems plausible that such particles could resist absorption and re-emission of radiation because of their large inertia. Light might simply bend around them, no? My question would be how such particles form and under what conditions they would break down into smaller particles and/or energy. Is it possible that they might only be sustainable within certain high gravity/pressure situations and fission into smaller particles as gravity/pressure decreases, and that would explain why they're not observed locally? Would this mean that they could be produced and disseminated by very massive stars, and then circulate within very dense regions of spacetime/gravitation?

[alpha2cen]

How to detect Dark Matter?

Big problem is where the Dark Matter is.

And which concentration the Dark Matter exist?

If they were condensated like water, it would be more easy to detect.

At that time detection method is a problem.

If they were distributed very tinn like interstellar medium, it would be more difficult.

what we can observe it from Galaxy by telescope is more easy, because light can pass through many dark matter components.

But when we detect it directly nearby, concentraion is important factor.

We only want to detect WIMP, but problem is their concentraion and detectors sesitivity.

Recent method is based on indirect interaction effect.

WIMP and matter rare interaction.

Our assumption is "we are in the Dark Matter pool, but we cant detect."

We need more big instrument, and more sensitive detector.

[zapatos]

An interesting article on Space.com. They also have a very good map.

 

http://www.space.com/scienceastronomy/new-dark-matter-map-hubble-telescope-101112.html

 

"Astronomers have created a new, incredibly detailed map of dark matter by using the Hubble Space Telescope to peer through a huge cluster of galaxies as if it were a cosmic magnifying glass."

 

"Astronomer Dan Coe of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and Edward Fuselier of the United States Military Academy at West Point teamed up to apply a new mathematical formulation to Hubble observations of Abell 1689. The result is the most accurate, detailed calculation so far of the cluster's mass distribution, including the mass that can't be accounted for by the visible matter – meaning, the dark matter."

 

"Because the universe has been continually expanding since its birth, it was once much denser than it is now. The extra heavy cores of galaxy clusters suggest they were born during these early stages when such dense conglomerations of matter were around."

 

"But if galaxy clusters did get an early start in forming, that presents a quandary, because scientists would expect to see a lot more of them around today.

 

That indicates that perhaps the force of dark energy was stronger in the young universe than scientists have thought. Because dark energy works against gravity, pulling matter apart, its force would have suppressed the formation of galaxy clusters, and could have counteracted the head start clusters had in forming, scientists think."

[lemur]

An interesting article on Space.com. They also have a very good map.

 

http://www.space.com...ope-101112.html

 

"Astronomers have created a new, incredibly detailed map of dark matter by using the Hubble Space Telescope to peer through a huge cluster of galaxies as if it were a cosmic magnifying glass."

 

"Astronomer Dan Coe of NASA's Jet Propulsion Laboratory in Pasadena, Calif., and Edward Fuselier of the United States Military Academy at West Point teamed up to apply a new mathematical formulation to Hubble observations of Abell 1689. The result is the most accurate, detailed calculation so far of the cluster's mass distribution, including the mass that can't be accounted for by the visible matter – meaning, the dark matter."

 

"Because the universe has been continually expanding since its birth, it was once much denser than it is now. The extra heavy cores of galaxy clusters suggest they were born during these early stages when such dense conglomerations of matter were around."

 

"But if galaxy clusters did get an early start in forming, that presents a quandary, because scientists would expect to see a lot more of them around today.

 

That indicates that perhaps the force of dark energy was stronger in the young universe than scientists have thought. Because dark energy works against gravity, pulling matter apart, its force would have suppressed the formation of galaxy clusters, and could have counteracted the head start clusters had in forming, scientists think."

So did the early dark matter decay into visible matter or energy? What happened to it and why/how?