Dark Energy/Matter

[BenSon]

Hi all, i'm not sure if this is the right forum for this but i think its more of an astrophysics question. Can somebody please give me the gist of dark matter/ dark energy? Is it like antimatter or what? Thanks in advance

 

~Scott

[elfstone]

The movement and rotation of galaxies cannot be explained by their visible mass. It would be required about 10 times more mass for the observations we have. Since we are currently unable to detect this matter, it is referred to as "dark matter". It's probably made up of subatomic particles we can't detect or brown dwarf stars that shed no light or other detectable waves.

[Severian]

Dark Matter and Dark energy are quite distinct. They are both needed to explain why the universe appears flat. The amount of mass and energy (density) in the universe would leave the universe very open, but other observatons indicate that it is flat. See http://map.gsfc.nasa.gov/m_mm/mr_content.html for more detail.

 

The data seems to indicate that Dark Matter is some type of particle, and since we haven't seen it yet it must be massive and weakly interacting. These are known as WIMPS (Weakly Interacting Massive Particles). One of the prime candidates is a 'neutralino' in models of supersymmetry. Basically the neutralino is weakly interacting and is massive but is also stable so it hangs around in the galaxy for a long time. This extra mass just hasn't been observed yet, but is there and provides enough mass to make up the shortfall.

 

Dark Energy is even weirder since it cannot be attributed to a particle. One possibility is that there is some sort or energy which is inherent to space-time itself (basically Einsteins cosmological constant). This is still not well understood.

[elfstone]

Why is dark energy needed?

Is the zero-point (vacuum) energy related to this at all?

[Severian]

Why is dark energy needed?

 

Observations of very distant supernovae tell us that the universe is actually accelerating away from us. This is pretty hard to explain since one usually expects mass in the universe to pull things together using gravity and thus any extra mass should be slowing down the expansion - not speeding it up. What you need is something which provides negative pressure. This is ecatly what Einstein's cosmological constant did - it pushes the universe outwards to stop it collapsing. Einstein said afterwards that this was has biggest mistake since a zero cosmological constant naturally leads one to postulate the Big Bang, but t is looking like it wasn't a mistake after all....

 

Is the zero-point (vacuum) energy related to this at all?

 

Yes, it is (almost) the same thing. However, vacuum energy gives the wrong sign and is far too large to be dark energy (by something like 120 orders of magnitude!).

[BlackHole]

QFT requires empty space to be filled with particles and anti-particles being continually created and annihilated. This could lead to a net density of the vacuum, which if present, would behave like a cosmological constant.

 

When Einstein was told by Hubble that the universe is expanding, he introduced a cosmological constant into his equations for General Relativity. This term acts to counteract the gravitational pull of matter, and so it has been described as an anti-gravity effect.

 

Astronomers plan to look at more distant high-redshift supernovae (far beyond the quasars)to precisely track how the universe’s expansion rate has changed, and this hopefully will narrow the dark energy alternatives. The Planck Surveyor and CLOVER array might also provide new results in 2007-2008. Hopefully LISA (in 2008) & Constellation-X will take place in 2016 or after.

 

For now, ΛCDM is the best model to explain 'dark energy'.

[BenSon]

Thanks for the replys, Ok so i get why we need Dark energy/matter to explain why the universe is the why it is but does any1 know what this stuff is? Can it interact with matter?

 

~Scott

[BlackHole]

Dark matter cannot interact with light (a phenomena of the electromagnetic force), therefore we can't see it. Dark matter is either baryonic (made up of protons, neutrons, etc) or non-baryonic (made up of exotic particles). Experts have suggested that dark matter could be made of WIMPs (Weakly interacting massive particles) or/and axions which interact only through the electroweak force and gravity. No one really knows what it is.

 

Now dark energy is even more complex than dark matter. It is believed to be the driving 'force' behind the speeding up of the expansion of the universe. "Dark energy" probably isn't an entirely satisfactory way to describe this phenomenon.

 

As of 2005, physicists have no other clue. Currently the Lambda-CDM model is the most consistent with all present observations.

[J.C.MacSwell]

 

When Einstein was told by Hubble that the universe is expanding' date=' he introduced a cosmological constant into his equations for General Relativity. This term acts to counteract the gravitational pull of matter, and so it has been described as an anti-gravity effect.

 

.[/quote']

 

I think he introduced it earlier, then realized (or thought) he'd made a mistake after receiving Hubble's data.

[BlackHole]

I think he introduced it earlier, then realized (or thought) he'd made a mistake after receiving Hubble's data.

 

Therefore it was not a blunder at all. Einstein's mistake was not mathematical. It was philosophical.

[BenSon]

Thanks for clearing that stuff up for me

 

~Scott

[Guest GA Douglass]

One way to put this is saying that our observations are correct but not complete (there's something we can't see) which is the "dark" side - on the "light" side the explaination is that for reasons we don't yet understand our observations are wrong.

 

Dark energy and dark matter are the darling theories of the news media which dislikes science to begin with (for lots of reasons) and loves cool sounding nomenclature.

 

Even on the "dark" side there are a large number of theories and explainations for our observations and not all of them require "forces unknown". Of all of them (including my own) my personal favorite is a dark matter theory occasionaly reffered to as the "bowl of soup" theory where the observable universe exisits as a kinda skin on the surface of the cosmos.

 

All and all from a purely scientific process point of view it would be better if we had other universes to compair this one to - a control and a few to experiement with... and maybe a vacation universe while I'm askin'.