Dark Matter

[Malenfant]

As the name says, dark matter cannot be seen visually, at no wavelength! Yet since it is massive matter, it subtends gravitational interactions. The evidence for dark matter precisely came first from deviations of the rotation profiles of the luminous (i.e. visible) parts of galaxies from the known physics laws. The popular way out was to assume the presence of another form of matter, the mass of which affects the rotation profile but it is not lumninous, hence cannot be seen. There are various candidate particles (e.g. neutralinos in Supersymmetry) that could form dark matter.

There is still a small possibility that there is no dark matter at all, but rather that the laws of gravity have to be modified at such large distances ( typically from the galaxy centers to their periphery) . But this is NOT easy.

This always bugged me about Dark Matter - assuming its effects are real, is it possible that there's no dark matter at all and that what we're actually seeing is just extra curvature in spacetime from an unknown source?

I've read some serious ideas suggesting that gravity may not even originate from our own universe, but may originate in a 'nearby' extradimensional space and affect us across the boundary between universes. Or at least, that mass in other universes can affect our own through this method. It sounds somewhat sci-fi and wacky but given the conceptual strangeness of brane theory it's an intriguing idea...

And what exactly is "Dark Energy" as opposed to Dark Matter? I've heard a lot about that but haven't seen a clear explanation of it, all I know is that it has something to do with the expansion of the universe being faster than it should be?

[t00fri]

As the name says, dark matter cannot be seen visually, at no wavelength! Yet since it is massive matter, it subtends gravitational interactions. The evidence for dark matter precisely came first from deviations of the rotation profiles of the luminous (i.e. visible) parts of galaxies from the known physics laws. The popular way out was to assume the presence of another form of matter, the mass of which affects the rotation profile but it is not lumninous, hence cannot be seen. There are various candidate particles (e.g. neutralinos in Supersymmetry) that could form dark matter.

There is still a small possibility that there is no dark matter at all, but rather that the laws of gravity have to be modified at such large distances ( typically from the galaxy centers to their periphery) . But this is NOT easy.

This always bugged me about Dark Matter - assuming its effects are real, is it possible that there's no dark matter at all and that what we're actually seeing is just extra curvature in spacetime from an unknown source?

I've read some serious ideas suggesting that gravity may not even originate from our own universe, but may originate in a 'nearby' extradimensional space and affect us across the boundary between universes. Or at least, that mass in other universes can affect our own through this method. It sounds somewhat sci-fi and wacky but given the conceptual strangeness of brane theory it's an intriguing idea...

And what exactly is "Dark Energy" as opposed to Dark Matter? I've heard a lot about that but haven't seen a clear explanation of it, all I know is that it has something to do with the expansion of the universe being faster than it should be?

Malenfant,

first of all, I suggest for better readability that you might correct your typo ( "[/quiote]" as well as my username Toofri=> t00fri).
I went away from the Physics & Astronomy board since the things I read there recently had VERY little to do with either Physics or Astronomy .

Now...

To answer your first question: In principle, it is still POSSIBLE that at >= galactic distances, the laws of gravity get modified in some way, which then would also affect the conclusions about DarkMatter from the measured rotation data of galaxies. Yet, these required modifications look altogether much less attractive than the DarkMatter hypothesis. The main point being that we do have natural candidates (Neutralinos in SUSY) for DarkMatter and --at the same time-- would not have to modify a beautiful theory ad hoc in the large distance regime.

As you mentioned, there is considerable activity in theoretical particle physics, to consider extra space-time dimensions besides 3+1. This is notably suggested by string theory, being still a candidate theory for "everything". Naively on would expect extra dimensions to show up as a modification of Newton's law at SHORT distances, but it has been demonstrated that there are also LONG-distance modifications possible in such a setting!

So extra dimensions might indeed make DarkMatter unnecessary. There is however no concrete model available yet.

Another argument in favor of DarkMatter, I mentioned in another post, some time ago:

http://www.shatters.net/forum/viewtopic.php?t=7235

The evidence of an entire galaxy consisting of DarkMatter would be hard to simulate in terms of a modification of gravity...

Finally: Dark Energy... This is a long, exciting story.

To explain what it amounts to, would require something like a whole lecture...

In a nutshell: we meanwhile know, first from Supernova data and then also from cosmic microwave precision measurements (BOOMERANG, WMAP,...) that there is a new energetic component (Dark Energy) that accelerates the matter in our expanding universe at high redshift rather than decelerating it, as one would expect it from the conventional gravitational force. Termodynamically speaking, DarkEnergy corresponds to a NEGATIVE pressure in the equation of state! A highly puzzling affair. In addition, the natural scale associated with Dark Energy is such that it does not fit into ANY known particle physics scenario.

Bye Fridger

[Spaceman Spiff]

If Dark Matter is as t00fri says: something used to explain an apparent attractive force seen within galaxies, then Dark Energy is something used to explain an apparent repulsive force seen between galaxies.

What exactly each is is not really known, but we have to consider them because of what observational astronomers have observed. Any theory that might come up with predicting such effects would be of considerable interest to all concerned. Watch that space.

About what t00fri wrote: "There is still a small possibility that there is no dark matter at all, but rather that the laws of gravity have to be modified at such large distance"

There's the idea of MOND: MOdified Newtonian Dynamics. Here, a term proportional to inverse distance is added to the usual inverse square distance to give force. The constant to this extra term makes it significant only beyond about 70,000 light years.

Then there's the idea of a non-constant Gravitational constant.

There can be other alternatives: imagination is required to think them up, but it is usually most efficient to process them experimentally/observationally and write them off in order of increasing complexity.

Spiff.

[t00fri]

If Dark Matter is as t00fri says: something used to explain an apparent attractive force seen within galaxies, then Dark Energy is something used to explain an apparent repulsive force seen between galaxies.

What exactly each is is not really known, but we have to consider them because of what observational astronomers have observed. Any theory that might come up with predicting such effects would be of considerable interest to all concerned. Watch that space.

About what t00fri wrote: "There is still a small possibility that there is no dark matter at all, but rather that the laws of gravity have to be modified at such large distance"

There's the idea of MOND: MOdified Newtonian Dynamics. Here, a term proportional to inverse distance is added to the usual inverse square distance to give force. The constant to this extra term makes it significant only beyond about 70,000 light years.

Then there's the idea of a non-constant Gravitational constant.

There can be other alternatives: imagination is required to think them up, but it is usually most efficient to process them experimentally/observationally and write them off in or
der of increasing complexity.

Spiff.

Spiff,

I mentioned MOND of course, alas in another thread...
(the recent piling up of ignorant nonsense in our Physics & Astronomy board drove me away )
http://www.shatters.net/forum/viewtopic.php?t=7945&start=210

MOND is a fairly contrived...baroque construct which only very few theorists find attractive. But there is a modern "Harvard" version (Nima Arkani-Hamed et al) that might be more promising...

Bye Fridger

[Spaceman Spiff]

Hallo t00fri,

OK, I looked at that. I didn't expect MOND to be in the "New Celestia-1.4.0pre-FT1 (galaxies) for Testing" topic, but after mentioning the abundance of off-topic to Hunter Parasite, I should have known!

I have a photocopy of the first MOND paper somewhere (gathering dust). It was nice to see orthodoxy questioned, and it's an interesting read, but one is left wondering how this proposed solution fits everything else. Contrived, as you say, and not the least complex solution to hand yet (Occam's razor, bla bla).

(the recent piling up of ignorant nonsense in our Physics & Astronomy board drove me away )

That reminds me: I must follow up on that Special Relativity topic concerning travelling at the speed of light. Hmm, I'd also better start a thread on the scientific method.

Spiff.

[t00fri]

Hallo t00fri,

...

I have a photocopy of the first MOND paper somewhere (gathering dust). It was nice to see orthodoxy questioned, and it's an interesting read, but one is left wondering how this proposed solution fits everything else. Contrived, as you say, and not the least complex solution to hand yet (Occam's razor, bla bla).

...

Spiff.

Indeed, Spiff,

for quite a few years, I also enjoyed the MOND paradigm to some extent, just as an amusing illustration that besides mainstream folklore there might always be another explanation...

Bye Fridger

[WildMoon]

Okay, before I go and read the MOND thing (and probably end up making my head selfdestruct in the act) I wanna say:

If Dark Energy is a repulsive force, this would make White Holes possible, though they would no longer be thought to exert anti-gravity, they would just have alot of Dark Energy. For, white holes would not be possible is they exerted anti-gravity, because unless they spontaneously appear, they could not form (no star).

Also, Dark Energy could hurt the Big Bang theory since it says due to the galaxy expanding, the galaxies and other stuff move away from each other like when you stick stickers on a balloon and blow up that ballon (but wait, this ought to mean they don't move away from each other, just the universe's center, or where ever the universe began, wouldn't it?). So, as long as objects move away from a certain point and not each other, then Dark Energy would not put a damper on the Big Bang theory (even though there is no other theory to take the Big Bang's place, so it won't matter anway, will it? Besides, there's the cosmic background radiation)

Also, to be a bit unscientific, I like the idea of dark matter other than the other thing with another dimension.

[t00fri]

Okay, before I go and read the MOND
thing (and probably end up making my head selfdestruct in
the act) I wanna say:

If Dark Energy is a repulsive force, this would make
White Holes possible, though they would no longer be
thought to exert anti-gravity, they would just have alot of
Dark Energy. For, white holes would not be possible is they
exerted anti-gravity, because unless they spontaneously
appear, they could not form (no star).

Also, Dark Energy could hurt the Big Bang theory since it
says due to the galaxy expanding, the galaxies and other
stuff move away from each other like when you stick
stickers on a balloon and blow up that ballon (but wait,
this ought to mean they don't move away from each other,
just the universe's center, or where ever the universe
began, wouldn't it?). So, as long as objects move away
from a certain point and not each other, then Dark Energy
would not put a damper on the Big Bang theory (even
though there is no other theory to take the Big Bang's
place, so it won't matter anway, will it? Besides, there's the
cosmic background radiation)


Also, to be a bit unscientific, I like the idea of dark matter
ther than the other thing with another dimension.

Wildmoon (and other DarkEnergy fans),


the MOND paper (1983-84) was written long before you
were born and thus might need some "modernization" . It
was notably written before DarkEnergy was known to
exist...


DarkEnergy may be viewed as a potential (energy)
that just makes the galaxies recede from each other faster
than what is expected from the recession movement due
to the BigBang alone. Or in terms of that familiar
expanding balloon picture with galaxies distributed on its
surface, DarkEnergy would contribute a NEGATIVE pressure
making the balloon expansion faster...

The original evidence came from the following clever
observation (Perlmutter et al).

It was known that like Cepheids act as "standard candles",
Supernovae of Type Ia act as "standard BOMBS " ,
meaning that the emitted light intensity after a SN Ia
explosion appears to be universal!
Hence from observing the /apparent magnitudes/ of such
supernovae, we may calculate their distances!

That in turn allows to compare the measured
distance-redshift (<-> z) relation of SN Ia observations
with that predicted from standard cosmology. The form of
the comoving distance as function of the redshift z
parametrically depends on possible fractions of energy
density due to e.g. luminous and dark matter in the
universe.

To everybody's GREAT surprise, it was found from this
comparison at high redshift z, that a significant
accelerating component ("dark" energy fraction ~ 0.7)
must be added in order to make cosmology fit the SN Ia
data!

At this stage, many scientists were still sceptical, since the
SN Ia effect is pretty small after all and might also be due
to systematic uncertainties. However, the subsequent
precision experiments about the cosmic microwave
background (CMB) radiation (BOOMERANG,WMAP,...)
produced INDEPENDENT and convincing evidence for
DarkEnergy. The required magnitude of DarkEnergy from
CMB measurements was about equal to the earlier required
strength from SN Ia data! That eliminated remaining
doubt...

Another way of putting the notion of DarkEnergy is that
--if probed by the gravitational force-- "empty"
space appears to have a tiny but nonvanishing energy
density! Actually, calling it "cosmological constant",
Einstein thought about this possibility already in his
equations, long ago...

Clearly, there is then the eminent challenge to theorists
working on fundamental physics (like myself ), to
CALCULATE the magnitude of that "cosmological constant"
<=> Dark Energy in terms of other characteristic known
energy scales!

The puzzle is that the value of the "cosmological constant"
is MANY orders of magnitude smaller than our current
understanding of fundamental physics seems to suggest.
On the other hand, the fractional DarkEnergy is large
enough to dominate the energy density of the Universe on
cosmological scales!

Actually, the energy fraction due to matter is only ~ 0.3,
with most of it from DarkMatter around galaxies!. The
needed DarkEnergy fraction is ~ 0.7, however, with their
sum adding up to 1! Note that DarkMatter and DarkEnergy
are qualitatively very different, though.


Clearly an outstanding question is about a possible
(dynamical) relation between DarkMatter and DarkEnergy!


Please, don't suggest E_dark = m_dark * c^2


Bye Fridger

[t00fri]

Hi again,

let me point out an important "addendum" to what I wrote
in my previous post.

In Newton's theory mass is the source of the gravitational field
and thus gravity is always attractive. In general relativity, both energy and pressure source the
gravitational field. This fact is reflected in the second Friedmann equation:
[code]
1/R(t)* d^2 R(t)/dt^2 =4*Pi/3*G*(?

[t00fri]

Hi again,

here is a quite comprehensive site devoted to MOND, with
lots of links and discussion of the pros and cons...

MOND-site

Have a look into the face of the MOND inventor
Milgrom

Here is a copy of Milgrom's original MOND articles in PDF format
MOND1
MOND2
MOND3

Here is a more recent review together with Jacob Bekenstein

Review1

and finally a most recent conference review(2005) by Jacob Bekenstein:
Review2
This latter review is presumably a very well readable entry point...

Bye Fridger

[jdou]

Hi Fridger,

Thanks to help us to try to have a thin (very thin) understanding of our universe. I speak for me
When I read your explains and equations I 'm feel like a "brain dead" (Hi Bob )
So, I searched some others infos about galaxies to well understand and I found the "Millenium Run" project on the site of "Max Planck Institute".
There are some very interesting articles (but some in German) about dark matter, universe gravity etc... (even if sometime I understand just the upper case letters )
There are also some nice video to illustrate these articles.
But I'm sure you already know that

Euh..., what you want to say when you write :

...and discussion of the pros and cons...

Because je ne suis pas un pro , donc...

Bye jdou

[t00fri]

Hey jdou ,

...
Euh..., what you want to say when you write :

...and discussion of the pros and cons...

Because je ne suis pas un pro , donc...

Bye jdou

This was certainly great French "humeur" again!

Cheers,
Bye Fridger

[jdou]

Yes it's just for french

I forgot the link for the others guys

http://www.mpa-garching.mpg.de/galform/data_vis/

I recommend you the "millenium_flythru_fast" video.

[t00fri]

Yes it's just for french

I forgot the link for the others guys

http://www.mpa-garching.mpg.de/galform/data_vis/

I recommend you the "millenium_flythru_fast" video.

Many thanks for that link, jdou!

these animations about "large scale structure" filiaments are just GREAT. It's just as I was dreaming since a long time, how the display of my proposed Cosmo-Celestia mode should be...


Bye Fridger

[jdou]

I dont know for the others, but for the beotien I am this representation of our universe looks like an internal view of our brain with synapes, neurons and electrical signals traffic...

So, in this case, are we only a thought of "God" ? (or somebody else)

Perharps I read too much books of science-fiction
(or drink not enough german biers )

[WildMoon]

Perharps I read too much books of science-fiction
(or drink not enough german biers )

Is this the secret to t00fri's smartness? Well then, time to secretly get german beer and commit an act of underage drinking

So, in this case, are we only a thought of "God"? (or somebody else)

[qoute]I think, therefore I am.

Because I am, I am my own self, and control my own life and my own destiny.

Sorry if that part offends any religious people. I'm not religious so I wouldn't know. If it is offensive I'll remove it immediatly and we'll all forget it.

[jdou]

It's was just a reflection without no particular religious meaning
Don't forget the " "