Dark matter and Dark energy

[DanishDynamite]

Observations of the expansion rate of the universe and the motions of the galaxies within it allow us to deduce the matter and energy content of the universe, using Einstein's theory of gravity.

Could you be a bit more specific? Which observations coupled with what theory assumptions lead us to which number?

Huh? If you mean "compared", through E=mc^2.

So somehow the energy of this "Dark Energy" was arrived at and then compared to matter via E = mc^2. How was this energy arrived at?

It's still a question. There are some theories which work, but they rely on untested assumptions which may or may not be correct.

So, still an unanswered question then, correct?

 

[arthwollipot]

arthwollipot - it'd be an understatement to say it's considered unlikely. There's very strong constraints on that 4% figure from the start of the thread. That's saying that only a small proportion of the total matter can be in the form of everyday atoms like stars or planets. We know that it's something we've not seen before that has essentially no interaction with light - it can't be merely everyday stuff that's not emitting light. It's got to be something else.

Fair enough.

It's also worth mentioning that there is a theory on the bleeding edge of theoretical physics that postulates that at least some of the gravitational effects of dark matter can be explained by gravity that originates in other universes. According to this theory, gravity is the only force that can travel between universes.

This may sound like science fiction, but it is mathematically consistent with the real world. You can occasionally read about this sort of stuff in Scientific American and books by Michio Kaku.

 

[robinson]

I view the current hypothesis, that there must exist an immense amount of an unknown and invisible form of both matter and energy, to be much like two other "baffling" problems in astronomy. Or astrophysics.

One of them isn't quite as baffling, but a year ago it certainly was.

These two "unknown" and "baffling" events, which occur almost all the time, led to scientist making up stuff to try and explain it. After looking at alternative theories, which claim to explain both problems, it was clear that the scientist in charge of explaining things were simply ignoring the laws of physics, which led to them making up new stuff, because they wanted to explain something, but the current theories didn't work.

In regards to dark matter, dark energy, it doesn't matter a bit if they, or something like them, exist or not. It matters a lot to scientist who postulate them, because it involves money, funding, research, but none of it matters to us, in a pragmatic sense.

The time and money being used on that research does matter, because it takes away resources from real problems, real research, that does matter. Things right here and now on our planet.

So in a way, it is like the same concern that some state about woo beliefs. Woo belief may interfere or detract from seeking real help, real solutions.

Worrying about dark matter/dark energy is like contemplating the nature of the Universe, while a bomb is ticking under your house.

It might be interesting to you, but most people would consider the bomb a more important issue.

 

[hammegk]

In regards to dark matter, dark energy, it doesn't matter a bit if they, or something like them, exist or not.

Regarding dark energy, should the research ever yield technology that gets more power out than in by tapping it, you would be wrong.

 

[robinson]

I'm sure that is used to secure funding. As well as a bunch of other woo reasons.

 

[sol invictus]

Are you talking about Mark matter, or Dark Energy? Because what you're describing fits Dark Matter, but both I and Sean Carroll are talking about Dark Energy. It's easy to get confused, because so many people in this thread are talking about them in the same breath.

Yes, sorry - MOND is invoked to explain galactic rotation curves without dark matter. However some fancier related ideas try to incorporate dark energy as well.

Could you be a bit more specific? Which observations coupled with what theory assumptions lead us to which number?

I'm not going to be very detailed - you can find that information on the web. There are many observations. A primary one is galactic rotation curves. There are more: the cosmic microwave power spectrum, supernova redshift surveys, weak lensing, and structure formation simulations. The most direct evidence for dark matter comes from the bullet cluster.

All of those are interpreted using Einstein's theory of gravity, general relativity.

So somehow the energy of this "Dark Energy" was arrived at and then compared to matter via E = mc^2. How was this energy arrived at?

Mostly supernova surveys and the CMB spectrum.

So, still an unanswered question then, correct?

Yes.

 

[Beth]

I view the current hypothesis, that there must exist an immense amount of an unknown and invisible form of both matter and energy, to be much like two other "baffling" problems in astronomy. Or astrophysics.

One of them isn't quite as baffling, but a year ago it certainly was.

These two "unknown" and "baffling" events, which occur almost all the time, led to scientist making up stuff to try and explain it. After looking at alternative theories, which claim to explain both problems, it was clear that the scientist in charge of explaining things were simply ignoring the laws of physics, which led to them making up new stuff, because they wanted to explain something, but the current theories didn't work.

In regards to dark matter, dark energy, it doesn't matter a bit if they, or something like them, exist or not. It matters a lot to scientist who postulate them, because it involves money, funding, research, but none of it matters to us, in a pragmatic sense.

The time and money being used on that research does matter, because it takes away resources from real problems, real research, that does matter. Things right here and now on our planet.

So in a way, it is like the same concern that some state about woo beliefs. Woo belief may interfere or detract from seeking real help, real solutions.

Worrying about dark matter/dark energy is like contemplating the nature of the Universe, while a bomb is ticking under your house.

It might be interesting to you, but most people would consider the bomb a more important issue.

I must disagree with you Robinson. I think it's incredibly important. Either we only know about and understand a tiny percentage of the universe we live in OR some of our basic fundamental tenets of reality (i.e. our best physics models of matter) are unsound and providing us with wrong predictions that are corrected by adding in dark energy and dark matter. Either way, I think it is important to explore what is causing these poor predictions from our best models.

 

[robinson]

The thing is, nothing close to us, in any way, shows evidence for dark matter/dark energy. Which also brings up the obvious question. If 90% of the mass is invisible mythical dark "stuff", why doesn't it effect anything nearby? That much mass would be in our solar system, between us and the nearest stars, everywhere. Why no gravitational problems with those systems?

 

[godless dave]

It does effect things nearby - it effects the rotation of the galaxy we are in.

 

[sol invictus]

The thing is, nothing close to us, in any way, shows evidence for dark matter/dark energy. Which also brings up the obvious question. If 90% of the mass is invisible mythical dark "stuff", why doesn't it effect anything nearby? That much mass would be in our solar system, between us and the nearest stars, everywhere. Why no gravitational problems with those systems?

Because dark matter isn't distributed the same way visible matter is. Visible matter in most galaxies is confined to flat disks, and within those disks it's clumped into stars and planets. Dark matter (in the most plausible models, at least) is distributed more or less spherically (centered on the center of the disk) and with a much smoother density profile - it doesn't form compact objects.

So the density of it in the solar system is too low to be observable from its local gravitational effects. It might, however, be visible in its interactions with matter, and there are a number of experiments looking for it now.

 

[Beth]

Because dark matter isn't distributed the same way visible matter is. Visible matter in most galaxies is confined to flat disks, and within those disks it's clumped into stars and planets. Dark matter (in the most plausible models, at least) is distributed more or less spherically (centered on the center of the disk) and with a much smoother density profile - it doesn't form compact objects.

So the density of it in the solar system is too low to be observable from its local gravitational effects. It might, however, be visible in its interactions with matter, and there are a number of experiments looking for it now.

Is this why Cuddles wrote that MACHOs have been ruled out:

The first is MACHOs - MAssive Compact Halo Objects. This basically means big lumps of matter like planets, brown dwarfs, dark nebulae and so on. Perfectly normal things that we just can't see from here because they're too small or too dark. We know that such objects must exist because it is obvious that we can't actually see everything. However, as you say, these have pretty much been ruled out as a major source of dark matter. They certainly make up some of it, maybe even a few percent, but they just can't explain the amount of matter needed or, even more damning, the distribution that must exist.

 

[sol invictus]

Is this why Cuddles wrote that MACHOs have been ruled out:

I'm not an expert on those constraints, but I know that one good one comes from experiments that looked for microlensing events. The idea is you point a telescope at a nearby galaxy (I think they usd one of the Megellanic clouds) and look for events where one of the stars in it gets brighter for a moment. That happens when a massive object passes between you and the distant star, because the gravitational effect of the object lenses the light and focuses it a little.

Few or no (depending on which experiment you trust) events were observed, which translates (with reasonable assumptions, such as that the DM is distributed spherically) into the statement that at most only a little of dark matter can be machos.

 

[Beth]

I'm not an expert on those constraints, but I know that one good one comes from experiments that looked for microlensing events. The idea is you point a telescope at a nearby galaxy (I think they usd one of the Megellanic clouds) and look for events where one of the stars in it gets brighter for a moment. That happens when a massive object passes between you and the distant star, because the gravitational effect of the object lenses the light and focuses it a little.

Few or no (depending on which experiment you trust) events were observed, which translates (with reasonable assumptions, such as that the DM is distributed spherically) into the statement that at most only a little of dark matter can be machos.

Thanks. When you say 'distributed spherically', do you mean distributed evenly within a sphere (a multivariate uniform distribution) or distributed with the heaviest concentrating at the center and tapering out in all dimensions evenly (a multivariate normal distribution)?

 

[69dodge]

Because dark matter isn't distributed the same way visible matter is. Visible matter in most galaxies is confined to flat disks, and within those disks it's clumped into stars and planets. Dark matter (in the most plausible models, at least) is distributed more or less spherically (centered on the center of the disk) and with a much smoother density profile - it doesn't form compact objects.

Do we know why the dark matter became distributed differently from visible matter, or just that it currently is distributed that way?

 

[BeAChooser]

It's also worth mentioning that there is a theory on the bleeding edge of theoretical physics that postulates that at least some of the gravitational effects of dark matter can be explained by gravity that originates in other universes. ... snip ... This may sound like science fiction, but it is mathematically consistent with the real world.

The lengths to which the mainstream will go to explain away the observations that don't fit their model get sillier and crazier all the time.

And did you folks miss this: http://www.telegraph.co.uk/earth/ma...&grid=&xml=/earth/2007/12/23/scicosmos123.xml

'Test tube universe' hints at unifying theory

By Roger Highfield, Science Editor

Last Updated: 12:01am*GMT*26/12/2007


A "universe in a test tube" that could be used to assess theories of everything has been created by physicists.

... snip ...

What is remarkable is that atoms in the liquid, at temperatures within a thousandth of a degree of absolute zero, form structures that, according to the team at Lancaster University, are similar those seen in the cosmos.

ROTFLOL!

 

[BeAChooser]

Originally Posted by DanishDynamite
Could you be a bit more specific? Which observations coupled with what theory assumptions lead us to which number?

I'm not going to be very detailed - you can find that information on the web. There are many observations. A primary one is galactic rotation curves.

In 1937 Hannes Alfven proposed that our galaxy contained a large-scale magnetic field and that charged particles moved in spiral orbits within it, owing to forces exerted by the field. He said plasma carried the electrical currents which create the magnetic field. There is now significant observational evidence that such large scale magnetic fields (and hence, electrical currents) exist in our galaxy. And that they have the geometry postulated by Alfven.

In the 1980s and early 90's, Anthony Peratt, an engineer at Los Alamos National Lab, used Aflven's model and the large particle in cell computer codes at LANL to simulate galaxies. Here is a paper by Peratt entitled "Advances in the Mathematical modelling of Astrophysical and Space Phenomena". http://public.lanl.gov/alp/plasma/downloads/AdvancesII.annotated.pdf . See Section 3.3 which focuses on galactic rotational velocities and the results from that modeling. Also check out Section 4. He shows that electrodynamic forces can account for the observed galactic rotation curves.

And here's another article by Dr Peratt on the subject of galactic rotation dynamics: http://www.cosmology.info/2005conference/wps/gallo_1.pdf Here's a portion of what he says in that article ... "When Plasma Physicists add known ElectroMagnetic Plasma effects into the Gravitational dynamics of Spiral Galaxies, they obtain the observed rotational dynamics of Spiral Galaxies. For scientifically published references, see the very extensive list below. Although EM Plasma Physics is well known and experimentally tested, the detailed calculations are very complex and require supercomputers that operate for months. There is no question that EM Plasma effects dominate the early formation of a Spiral Galaxy from an ionized plasma. As time progresses, matter is accreted into star formation. Then gravitational effects become stronger, as EM plasma effects become weaker as the inter-stellar plasma density decreases with time evolution. These effects are sufficiently complex that I can not describe them with simple arguments or simple mathematics. Supercomputers are necessary. ... snip ... PRIMARY REFERENCES. (1) “Physics of the Plasma Universe” by Anthony Peratt. (Springer-Verlag, 1992). ... snip ... (3) “Evolution of the Plasma Universe: I. Double Radio Galaxies, Quasars, and Extragalactic Jets”, A. L. Peratt, IEEE Trans. Plasma Sci. Vol. PS-14, N.6, pp.639-660, December 1986.(1.7M), (4) “Evolution of the Plasma Universe: II. The Formation of Systems of Galaxies”, A. L. Peratt, IEEE Trans. Plasma Sci. Vol. PS-14, N.6, pp.763-778, December 1986 (1.9M). In the above references, the evolution of galaxies from plasma inhomogeneities (which yield electric fields, currents and magnetic fields) is simulated. These calculations indicate a time evolution from Elliptical to Irregular to Spiral Galaxies."

Again, from http://www.cosmology.info/2005conference/wps/gallo_1.pdf : "Following are the measured velocity profiles for four specific Spiral Galaxies from Ref 4, Fig 14. “Velocity Profile” means the rotational speed of the spiral galaxy as measured from the center of the spiral galaxy. The peculiarities are that the rotational speed is very low at the galactic center and rises quickly to an approximately constant rotational speed away from the center. This is completely different than expected from gravitational forces alone. For instance, in the simplest Solar System model, the planets closest to the center rotate at the very fastest speeds, and gradually decrease in speed at larger distances from the center. ... snip ... Following is a computer simulation of the velocity profile for a Spiral Galaxy from Ref 4, Fig 14 including ElectroMagnetic Plasma effects. Notice the similarity of the measured velocity profiles with the computer simulation including ElectroMagnetic Plasma effects for these Spiral Galaxies. The plasma core rotates very nearly as a solid body, while the spiral arms grow in length as they trail out along the magnetic isobars. ... snip ... The measured behavior is all very different than that obtained from gravitational effects alone, but the inclusion of ElectroMagnetic Plasma effects mimic the observed behavior. That is, the rotational speed is very low at the galactic center and rises very quickly to an approximately constant rotational speed at distances away from the center."

In short, dark matter isn't needed to explain the rotation curves of galaxies. And we've known it isn't needed for 30 years. Ordinary electromagnetic physics combined with the gravity from ordinary matter can account for the observations. Note that there are NO peer reviewed papers directly challenging the ones above. Mainstream science's approach has been to simply ignore the above. Therefore, inferring that dark matter MUST exist based on the rotational velocity observations is not good science. Never was. Never will be.

 

[Olowkow]

Therefore, inferring that dark matter MUST exist based on the rotational velocity observations is not good science. Never was. Never will be.

Isn't it the case that the term "dark matter" is just being used as a convenient term or "buzzword" in the interim until someone actually figures out what is really going on?

 

[BeAChooser]

Isn't it the case that the term "dark matter" is just being used as a convenient term or "buzzword" in the interim until someone actually figures out what is really going on?

Again, note that there are NO peer reviewed papers directly challenging the calculations of Peratt with regards to galactic rotation. NONE. Big Bang supporting, it has to be gravity, mainstream science's approach has been to simply ignore his PEER REVIEWED work. And instead rely on a gravity-related gnome to explain observations they can't otherwise explain. And that gnome is only one of many.

 

[Olowkow]

Well, I'm certainly no expert on these matters, but I just read a bunch of the hits for Peratt (only 388 on Google, by the way, for "Peratt galactic")

One, Wikipedia, contained this quote:

Examples of the highly speculative nature of Alfvén's conclusions include factually inaccurate explanations for star formation using Birkeland currents.[10] These plasma currents were held by Alfvén and his supporters to be responsible for many filamentary structures seen in astrophysical observations. However, there remains no direct observational evidence of such large scale plasma currents[citation needed] and mainstream astrophysical explanations for large-scale phenomena preclude plasma current mechanisms.

Alfvén seems to be problematic, and Peratt appears to base his ideas on his theories. My impression is that this is one of the fringe or non-mainstream ideas concerning galactic rotation.

As for no "peer reviewed" criticism of Peratt...so? Maybe no one takes him seriously?

 

[BeAChooser]

One, Wikipedia, contained this quote

I'll repeat again ... there are NO peer reviewed papers directly challenging the peer reviewed calculations of Peratt with regards to galactic rotation. NONE. And his peer reviewed articles were published in widely recognized scientific journals. Mainstream astrophysicists have simply ignored his work and the role that well understood electromagnetic phenomena might play in galactic rotation curves. That's not the way science is supposed to work. And by the way, who wrote that Wikipedia article? Hmmmmm?