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I don't like Dark Matter and Dark Energy

by Illuminaughty
Tags: dark, energy, matter
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Illuminaughty
#1
May25-09, 01:05 PM
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Are there any theories of gravitation and spacetime that don't require the existence of dark matter to explain the peculiar rotation of galaxies and don't require dark energy to explain the expansion of space?

I would love some links.

These things just seem suspiciously like luminiferous ether to me. I would be absolutely fascinated by an explanation of the universe that doesn't require 95% of the universe to be invisible, because of course invisible things are often imaginary. It doesn't matter to me how crackpot the theories seem as long as they explain those two phenomenon without resorting to dark matter or dark energy.
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protonchain
#2
May25-09, 01:41 PM
P: 97
Quote Quote by Illuminaughty View Post
It doesn't matter to me how crackpot the theories seem as long as they explain those two phenomenon without resorting to dark matter or dark energy.
Hypocrisy is a cruel cruel mistress.

Fine, you want stuff without dark matter, here's some stuff straight from the wiki:

A proposed alternative to physical dark matter particles has been to suppose that the observed inconsistencies are due to an incomplete understanding of gravitation. To explain the observations, the gravitational force has to become stronger than the Newtonian approximation at great distances or in weak fields. One of the proposed models is Modified Newtonian Dynamics (MOND), which adjusts Newton's laws at small acceleration. However, constructing a relativistic MOND theory has been troublesome, and it is not clear how the theory can be reconciled with gravitational lensing measurements of the deflection of light around galaxies. The leading relativistic MOND theory, proposed by Jacob Bekenstein in 2004 is called TeVeS for Tensor-Vector-Scalar and solves many of the problems of earlier attempts. However, a study in August 2006 reported an observation of a pair of colliding galaxy clusters whose behavior, it was claimed, was not compatible with any current modified gravity theories.[27]
In 2007, John W. Moffatt proposed a theory of modified gravity (MOG) based on the Nonsymmetric Gravitational Theory (NGT) that claims to account for the behavior of colliding galaxies.[28]

In another class of theories one attempts to reconcile gravitation with quantum mechanics and obtains corrections to the conventional gravitational interaction. In scalar-tensor theories, scalar fields like the Higgs field couple to the curvature given through the Riemann tensor or its traces. In many of such theories, the scalar field equals the inflaton field, which is needed to explain the inflation of the universe after the Big Bang, as the dominating factor of the quintessence or Dark Energy. Using an approach based on the exact renormalization group, M. Reuter and H. Weyer have shown[29] that Newton's constant and the cosmological constant can be scalar functions on spacetime if one associates renormalization scales to the points of spacetime. Some M-Theory cosmologists also propose that multi-dimensional forces from outside the visible universe have gravitational effects on the visible universe meaning that dark matter is not necessary for a unified theory of cosmology.
Pengwuino
#3
May25-09, 05:10 PM
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Quote Quote by Illuminaughty View Post
Are there any theories of gravitation and spacetime that don't require the existence of dark matter to explain the peculiar rotation of galaxies and don't require dark energy to explain the expansion of space?

I would love some links.

These things just seem suspiciously like luminiferous ether to me. I would be absolutely fascinated by an explanation of the universe that doesn't require 95% of the universe to be invisible, because of course invisible things are often imaginary. It doesn't matter to me how crackpot the theories seem as long as they explain those two phenomenon without resorting to dark matter or dark energy.
Invisible things are often imaginary? For centuries atoms were "invisible" but it was simply a lack of technology and understanding that made them invisible. What can you think of that was thought of to be invisible but turned out to be imaginary? As for dark energy, it isn't "invisible", we simply don't know what it is. As for dark matter, we actually can "see" it. We see signs of a missing amount of matter all through the universe. Both appear "invisible" but in actuality, we know they're there and it's quite possible we just haven't figured out how to detect either, even though the gravity dark matter induces is quite a clear sign that most physicists have agreed upon.

To be honest, I'm sure the scientific community would love to believe that there's simple explanations that dont require things we can't detect yet or don't understand at all... but it's not that simple.

Illuminaughty
#4
May25-09, 08:05 PM
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I don't like Dark Matter and Dark Energy

"Hypocrisy is a cruel mistress"? I'm going to choose to not understand that.

I'd heard of MOND but forgotten what it was called. Thanks.

Wikipedia alternatives all seem quite reasonable. I was hoping for something rather fringe, something that would inspire me to write a wacky sci-fi story or something.

"I'm sure the scientific community would love to believe that there's simple explanations that don't require things we can't detect yet or don't understand at all" If I were to write a sci-fi story that dealt with cosmological topics I'd rather go towards the "don't understand at all" rather than the "can't detect yet". 1 It would be more fun to write about some fringe area of cosmological investigation. 2 an invisible medium which drives cosmic expansion or an invisible medium which generates gravity sounds a lot like an invisible medium which propagates light waves, or invisible spheres on which the planets move around the Earth.

Ultimately I would be less embarrassed writing about something patently wacky, and more embarrassed if I wrote about dark matter and dark energy and 20 years from now they were proven to be artifacts of an incomplete understanding of gravity.
protonchain
#5
May25-09, 09:08 PM
P: 97
Quote Quote by Illuminaughty View Post
"Hypocrisy is a cruel mistress"? I'm going to choose to not understand that.
You stated that dark matter and dark energy theories were bogus and wanted other theories no matter how crackpot they were. That is hypocrisy right there: stating one theory is a crackpot, and wanting another one that is also a crackpot?

Whatever, as you choose not to understand what I mean, I'm going to choose not to understand your (lack of) logic.

Right now while I'm writing my term paper on the 21 cm Hydrogen emission lines, the reionization of the universe and the formation of the first stars and galaxies, I'm coming across references about the cold dark matter model. <sarcasm> I'm sure you must be deeply interested, as am I with providing you a plethora of references to CDM model research </sarcasm>
Phrak
#6
May25-09, 10:46 PM
P: 4,513
Illuminaughty,

I could be mistaken, and someone might correct me. On one side of Einstein's equation is momentum and energy due to the presence of matter. On the other side is spacetime curvature. So let's add dark matter and dark energy, then move it to the other side. Now it's a term added into the usual spacetime curvature-- it's not 'stuff' anymore but a property of spacetime.
Vanadium 50
#7
May25-09, 10:52 PM
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Quote Quote by Illuminaughty View Post
I don't like Dark Matter and Dark Energy
And I don't like brussels sprouts.

We don't get to pick how the universe is according to our likes and dislikes.
Illuminaughty
#8
May25-09, 11:24 PM
P: n/a
One last attempt to get answers from people who can have conversations like adults.

Let's refresh our memories about cosmological study.

1933 - Zwicky theorizes dark matter

42 years of the world thinking this must be wrong!

1975 - Rubin declares cold dark matter really is the best explanation we have for our observations

2009 - Illuminaughty asks, "While we're still waiting to find a WIMP, what were the bad explanations?"
Chronos
#9
May26-09, 01:09 AM
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The real evidence for dark matter originated with Zwicky, and noted by Illuminaughty. Viral theorem is powerful evidence. The case for dark energy is linked to the Perlmutter supernovae study. It too is powerful. We do not choose the universe we live in, it chooses us. We live in a mysterious universe.
Chalnoth
#10
May26-09, 02:40 AM
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It is quite clear by now that dark matter cannot reasonably be just a misunderstanding of gravity. All of the reasonable alternatives to dark matter have been ruled out quite conclusively.

As for dark energy, well, for that we just don't yet know. It may be a misunderstanding of gravity, or it may be some small cosmological constant, or it may be some sort of interesting matter field. We just don't yet know. Give it another 5-10 years, though, and we'll probably have a pretty good idea.
Chalnoth
#11
May26-09, 02:49 AM
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Quote Quote by Illuminaughty View Post
One last attempt to get answers from people who can have conversations like adults.

Let's refresh our memories about cosmological study.

1933 - Zwicky theorizes dark matter

42 years of the world thinking this must be wrong!
Incorrect. It was 42 years of the astrophysicists not paying much attention because there were, at the time, bigger unanswered questions to be solved. Many just figured that it was a misunderstanding of clusters, which it partially was (because Zwicky was unaware of the hot x-ray gas that permeates clusters, and is much more massive than the galaxies themselves).

Quote Quote by Illuminaughty View Post
1975 - Rubin declares cold dark matter really is the best explanation we have for our observations
Incorrect. It was an independent observation that showed that the distribution of dark matter is completely different from that of the normal matter. When you have independent confirmation of results, the scientific community tends to stand up and take notice. Additionally, the large difference in the distribution was a really compelling reason to believe that there was something real here.

For many years after this, there was a large debate on whether this was dark matter or modified gravity. The correct position, for a long time, was, "I don't know which is correct." But now that we have data from the CMB anisotropies (dark matter leaves a very distinctive signature in the CMB anisotropies), as well as data from clusters like the Bullet Cluster, well, we're pretty certain that the correct answer is dark matter.

Quote Quote by Illuminaughty View Post
2009 - Illuminaughty asks, "While we're still waiting to find a WIMP, what were the bad explanations?"
You might want to first ask why they are considered bad explanations today.

Oh, and by the way, because of their large mass and relatively low velocities, with our current detector technology we wouldn't really have expected to detect most types of reasonable dark matter particles even if they interact just like neutrinos interact.
Allday
#12
May26-09, 03:02 AM
P: 163
Before WIMPs some people thought that massive compact halo objects (MACHOs) could be the source of the dark matter. This is the idea that the dark matter is just non luminous regular matter (Jupiters, dim stars, black holes ...) but has fallen out of favor since surveys looking for their signature as they pass in front of stars did not find enough. They were kind of given a death blow when the high quality CMB data came out and showed (through the ratio of peak heights) that whatever the unseen matter is, it cant interact with photons. Thats because these peaks are caused by oscillations in the photon-baryon fluid and are sensitive to the ratio of regular matter and "non-photon interacting matter".

Neutrinos were another early dark matter candidate that are no longer considered. The temperature of a relativistic neutrino background should be smaller than the CMB (this hasnt been observed but the neutrino cross sections are well understood and they would be in equilibrium with the cosmic plasma before they freeze out). The CMB gets an extra energy kick from electron-positron annhilations after the neutrinos freeze out which is why the neutrino background should be cooler. This implies an energy density which is less than the CMB and falls way short of the mass-energy in dark matter. Massive neutrinos would need to be fairly massive to be non-relativistic so they dont smooth out cosmic structure too much. This is not the case and actually their mass can now be better constrained using cosmological measurements than terrestrial experiments. People are still interested in families of neutrinos above and beyond the three of the standard model (called sterile neutrinos) but these (if they are discovered) would be weakly interacting massive particles (WIMPs) anyway.

Particle theorists like Lisa Randall are thinking about large extra dimensions that gravity can "leak" into, diluting it in our 3+1 world. I don't know if the Ekpyrotic model has anything different to say about dark matter and dark energy but it is a string theory inspired model in which the big bang was caused by the collision of two branes. If I remember correctly this process can occur an infinite number of times as our brane and neighboring branes collide over and over again. And finally, as people have mentioned above, there are a lot of people working on modifications to GR (or different theories of gravity all together).
fasterthanjoao
#13
May26-09, 04:26 AM
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Quote Quote by Illuminaughty View Post
These things just seem suspiciously like luminiferous ether to me. I would be absolutely fascinated by an explanation of the universe that doesn't require 95% of the universe to be invisible, because of course invisible things are often imaginary.
This isn't a fair statement, at all. Astronomy would also love to have a universe that is entirely visible, but we don't. We didn't make the universe, and it has no reason to pander to our desires. I'm know sure how much you actually know about cosmology, but dark matter and dark energy aren't as exotic (punpun) as you've probably been led to believe. Some of it comes as a natural-ish extension of GR, and some of it is just down to our lack of understanding about the universe. The need for dark matter has arisen as a conclusion - things have been measured and aren't responding as they should if everything else is well, dark matter is a possible explanation to give the desired results.

It isn't the only thing people are looking at just now, there are alternatives out there but the work that's been done so far all points in the direction of dark matter existing - it's worth thinking that this has come around as a result of much work in the field, and people are trying as hard as they can to find tests to find out what the truth is.

Also, 'invisible things are often imaginary', what? If you close your eyes does it mean the universe no longer exists? no? why not? There are many things that exist that you can't see. 'Invisible' means only that we're unable to see it, but then we're biased. The human eye can only see a minute fraction of the electromagnetic spectrum, and there's never been any complaints with that - we see other parts of the spectrum through detectors, and though there is no electromagnetic interaction, the fact that we can't 'see' dark matter means only that we haven't yet built the appropriate detector :)
Vanadium 50
#14
May26-09, 06:22 AM
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Quote Quote by Illuminaughty View Post
One last attempt to get answers from people who can have conversations like adults.
You call us childish, when at the same time you specifically ask for crackpot theories? Where did I leave my irony meter?

Your description of dark matter history is completely erroneous. Zwicky's observation of galactic clusters and the inference of their mass obtained by the virial theorem was a single uncorroborated point. The conclusion is "clusters weigh more than you expect from just counting the stars", and it certainly wasn't the case that the world thought this was wrong. It's more that this doesn't point you anywhere: is it dust? Gas? Without another measurement you have no information on the cause of this.

It wasn't until the Rubin measurement (which doesn't have anything to do with whether DM is hot or cold) that you had anything else to add to the collected data.
Chalnoth
#15
May26-09, 07:12 AM
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Just to add a little bit, here's a link to an excellent blog post by Sean Carroll on (mostly) dark matter: http://blogs.discovermagazine.com/co...matter-exists/

He correctly notes, by the way, that modified gravity is still alive and well as an explanation for the accelerated expansion of the universe. Dark energy is what most people talk about not because cosmologists are really confident that the correct explanation is some unknown type of matter/energy, but instead because it's much easier to say than, "dark energy or modified gravity," or "the cause of the accelerated expansion." Unless the cosmologist is going into particulars, then, you can usually expect that when a cosmologist says, "dark energy," they're probably including modified gravity into that.
Amanheis
#16
May26-09, 04:27 PM
P: 66
In my opinion it is totally legit to doubt the existence of dark matter and dark energy. It definitely is currently the best model so it definitely has to be taken seriously, to put it mildly, but that doesn't mean it's right. There could be other possibilities. MOND was already mentioned, but it also needs some form of dark matter to explain mass discrepencies in clusters and I think they haven't found a relativistic version of it yet, which is discouraging. Also, my roommate investigated inhomogenities in the universe which could explain the obvservations without dark matter. Remember that any cosmology lecture starts with "Assuming that the universe is homogenous and isotropic...". Now that is a very, very reasonable assumption, but we don't know that it is right. If it's wrong, everything that follows in this lecture is also wrong.
And I strongly believe that the last word in the "dark matter" matter (no pun intended) hasn't been spoken yet.
cristo
#17
May26-09, 04:45 PM
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Quote Quote by Amanheis View Post
Remember that any cosmology lecture starts with "Assuming that the universe is homogenous and isotropic...". Now that is a very, very reasonable assumption, but we don't know that it is right.
Erm.. well the fact that we are here tells us that an homogeneous assumption is incorrect.
Amanheis
#18
May26-09, 04:59 PM
P: 66
Could you elobarate? When we say the universe is homogenous, it is implied that we mean on large scales, in terms of energy distribution. The Friedmann equations, on which everything in modern cosmology is based on, are derived on the assumption of a homogenous and isotropic universe. Our existence doesn't have anything to with that in the slightest, thus I don't think you are making any sense.
Obviously the universe is not homogenous in the sense that space is much less dense 10000 miles from here, but that is splitting hairs and not the point. If you have taken any cosmology class, you should know that.


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