I am trying to understand the basic nature of these two concepts:

- Dark Energy
- Dark Matter

They both are used to explain phenomena that we do not fully understand:
- Dark Matter is assumed to exist because of the outer stars in far away
galaxies rotate around the center of their galaxy at a faster rate than can be
explained by the visible mass (the stars). Dark matter could be just planets,
asteroids, comments and interstellar gas. Or maybe it could be neutrinos,
the ones with mass.
- Dark Energy is assumed to exist to explain why the universe is expanding
at an accelerating rate. Is it true that there is no understanding of what
dark energy is?

This is my limited understanding. Is it correct? Am I missing something?

I also understand that the measurement and analysis of the background
microwaves somehow factor into this too. But, again I do not understand
what this connection is. Could someone explain this in layman's terms or
even better point me to an explanation that already exists.

It also seems like dark matter is something used to explain things that
happen on the scale of galaxies, while dark energy is used to explain
things on the scale of the entire universe. Is this correct?

Thanks in advance,

Doug

Well as far as i understand, it is true that the basic nature of Dark Energy and Dark Matter is not fully understood. And it is also correct that dark matter is something to explain things that happen on the scale of galaxies, and dark energy used to explain something that is propelling the universe apart. And as for measurement and analisis of the background microwaves
up to where i understand, are observations that provides a look at the young universe by measuring the residue of the Big Bang itself. If dark energy changes the way the universe expands, the distance between galaxies and the CMB and the time lapsed between the CMB and the era of galaxies must carry some clues about dark energy, left behind like tiny fingerprints on space time. I dont have much knowledge about this myself , I read this some time back and just cant remember the web page i read it on, but i am sure someone will give you a better understanding.....




Titana............

Dark energy only appears on scales of the universe, you are correct, but dark matter appears not only on scales of galaxies. If it's in galaxies, it is also imprinted on the expansion of the whole universe. In fact, there is a flavor of dark matter, called hot dark matter, that does not show up in galactic influences because it would not contract with galaxy clusters. It would only show up cosmologically. I'm not sure if dark energy eliminates the need for hot dark matter, however.

It's better to refer to as far as all scientists understand. dark energy and dark matter is still a mystery phenomeno and have different manner about what we have known about the matterials arround us.We can't explain their manners and conducts until we can't see and observe them.

djsykora wrote;
[Junior Member I am trying to understand the basic nature of these two concepts:

- Dark Energy
- Dark Matter]

You actually have to be really careful when reading anything that includes these two terms, because it depends on what they are talking about when they use them, and sometimes they are even switched incorrectly, or depending on ones concept of the universe.

Whoa.....It's a good thing I checked...I thought I was in the ATM!

I better let the mainstream answer.

There are actually several different things all tangled up here:
- there is certainly more mass in the universe, at just about all scales, than we can 'see'
- however, if you add up the mass of the stars we can see, the mass of the gas and dust which we can easily infer, etc, there are still several discrepancies
- the one which seems to get the most 'air time' is in spiral galaxies - we can observe the rotation curves, apply Newton's equations, and we get a mass much greater than we would expect from what we can 'see' (stars, dust, gas)
- many searches for the 'missing mass' have come up empty; certainly there are some dim stars, some rogue planets, some 'invisible gas', etc, but far less than what's needed to account for the missing mass. It is this 'non-baryonic' mass that is usually called Dark Matter
- other observations of 'galaxy halos' also 'show' there is lots of missing mass; perhaps the most interesting class of observation is 'weak gravitational lensing', which is, basically, that distant galaxies look distorted on lines of sight close to nearby galaxies (the closer the line of sight, the more distorted; the distortion is deflection of light by the mass of the galaxy)
- there are also some examples of strong lensing
- rich clusters of galaxies also seem to have lots of 'missing mass', which is not confined to galaxy halos; in fact, there seems to more mass in these clusters than all the mass in galaxies, combined (including in the halos); several techniques combine to give a consistent story
- models of the universe suggest that analysing the µK 'spots' in the cosmic microwave background (CMBR) can tell us about how much DM there is in the universe
- ditto, analyses of the way that galaxies, clusters, super-clusters, walls&voids, quasars, etc clump together (i.e. large scale structure - LSS)
- when you analyse the CMBR and LSS together, you get consistent numbers for the amount of DM; there is also consistency with estimates from DM in rich clusters
- very little of the DM can be 'hot' (which simply means it is zipping around at near the speed of light); most of it must be 'cold' (i.e. it doesn't move any faster than stars 'n gas 'n galaxies 'n stuff does); there are lots of reasons whyLater on the DE (and other parts I may not have addressed completely).

All of the following is from memory. Some of the memory may be faulty, but I think the gist aligns with current theory.

I believe that the relative abundances of light elements in the universe, particularly deuterium, are thought to constrain the amount of baryonic matter, so much so that most of the dark matter must be nonbaryonic (not made from protons and neutrons). Neutrinos are a form of nonbaryonic hot ("hot" meaning very fast moving) dark matter. The leading theory of something called the "strong CP problem" postulates the existence of particles called "axions," that would also be hot. Scientists at CERN and elsewhere are currently conducting axion searches, but the axions, if they exist, are probably too light for present-day technology to find. In any case, many scientists feel that there must also be a tremendous amount of cold nonbaryonic matter (much more than the total amount of baryonic matter) because hot matter wouldn't clump around galaxies and galaxy clusters enough to produce the observed effects.

The idea of the universe containing huge amounts of cold nonbaryonic matter is certainly intriguing. What could it consist of? One interesting set of candidates arises from the theory of supersymmetry, which is central to some unified field theories. According to supersymmetry, to each type of known particle of matter there corresponds a heavier type of force carrying particle, and to each known type of force carrying particle there corresponds a heavier type of particle of matter.

The large hadron collider, scheduled to begin operation in 2007, may discover some supersymmetric particles. In fact, if it doesn't, I believe that this failure will exclude some of the most staightforward unified field theories. A stable, electrically neutral supersymmetric particle might account for cold dark matter.

I find all this very exciting. I anticipate very interesting results from the large hadron collider. As a side note, the CDF collaboration at Fermilab may have just discovered a new resonance (particle) at 500 GEV. I saw this at PhysOrg.com

I think the CMB observations are relevant because, as I remember, they constrain the total mass density of the observable universe. I don't understand why, however.