"Space and time may have a structure as intricate as the fauna of a rich ecosystem, but on a scale far larger than the horizon of our observations." -Martin Rees
On the largest scales, dark matter has been undoubtedly the most successful theory in modern cosmology for explaining a huge variety of observations. From the motions of galaxies in clusters to the separation of mass and light when they collide, from the correlations between galactic positions to the fluctuations in the CMB, from the bending of starlight to the formation of large-scale structure, it’s clear that the Universe needs dark matter.
On the largest scales, the way galaxies cluster together observationally (blue and purple) cannot be matched by simulations (red) unless dark matter is included. Image credit: Gerard Lemson & the Virgo Consortium, with data from SDSS, 2dFGRS and the Millennium Simulation.
But individual galaxies have always been the most difficult test for dark matter. In particular, there have been empirical correlations -- or relationships between two different observables -- that have never had an underlying explanation successfully presented. One of the most difficult has been the Tully-Fisher relation, which relates the luminosity to the rotational speed of spiral galaxies. But a new simulation, at long last, has finally cracked that nut by incorporating not only gravitation and dark matter, but the relationship between baryons and dark matter.
The relationship between luminosity and rotational velocity for spiral galaxies, for both stars (left) and total normal matter (right). Simulation curves by the team are shown in solid lines, with data for individual galaxies indicated as points. This agreement is unprecedented. Image credit: A. Cattaneo et al., arXiv:1706.07106, submitted to MNRAS.
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Cosmological models are all based on the following simple hypothesis:
Hypothesis 1: Frequency shifts are due to wavelength shifts, never to speed-of-light shifts.
In a world different from Einstein's schizophrenic world, an alternative hypothesis would be conceivable:
Hypothesis 2: Frequency shifts are due to speed-of-light shifts, never to wavelength shifts.
Needless to say, cosmologists never think of Hypothesis 2:
George Orwell: "Crimestop means the faculty of stopping short, as though by instinct, at the threshold of any dangerous thought. It includes the power of not grasping analogies, of failing to perceive logical errors, of misunderstanding the simplest arguments if they are inimical to Ingsoc, and of being bored or repelled by any train of thought which is capable of leading in a heretical direction. Crimestop, in short, means protective stupidity."
There's one schizophrenic in this room, but his name isn't Albert.
Did a little back-trace on you Pentcho. Seems like you've been a cancer on most of the science forums since mid 90's... banned on most of them nowdays. Hell, some people even went so far as to create websites debunking all the bs you've been spreading... So what, you came here thinking no one would realize. Well.. your act lasted about 2 weeks. You are dictionary example of a misguided troll, if there ever was one.
Thank you for the fabulous article Ethan. I have a message for the modified gravity people: give up, you are fighting a battle that you cannot possibly win.
Hey hey, the image of the Baryon Oscillation Spectroscopic Survey (BOSS) looks like the space foam-model I am currently trying to develop: https://m.imgur.com/gallery/GdB8ZCP
"Hypothesis 2: Frequency shifts are due to speed-of-light shifts, never to wavelength shifts."
If this hypothesis is correct, the first conclusion will be that there is no expansion of the universe - it is STATIC.
Star light slows down as it travels through the space vacuum, an effect caused by a factor equivalent to vacuum friction. For not so distant stars this is expressed as Hubble redshift but beyond a certain distance the star light does not reach us at all (Olbers' paradox).
The idea that vacuum can slow down light is largely discussed, but only in a quantum gravity context:
Sabine Hossenfelder: "It's an old story: Quantum fluctuations of space-time might change the travel-time of light. Light of higher frequencies would be a little faster than that of lower frequencies. Or slower, depending on the sign of an unknown constant. Either way, the spectral colors of light would run apart, or 'disperse' as they say if they don't want you to understand what they say. Such quantum gravitational effects are miniscule, but added up over long distances they can become observable. Gamma ray bursts are therefore ideal to search for evidence of such an energy-dependent speed of light."
I think it is time to start discussing the parallel idea: that slowing down light by vacuum produces the Hubble redshift (in a STATIC universe).
The epicycle model that was refined by 2nd century Claudius Ptolemy, could describe the (weird) observed motions of the planets very accurately. This model was upheld until 1610 when Galileo Galilei did observations that led to the conclusion that our solar system is not earth-centric but sun-centric. The sun-centric model could suddenly explain the observed planetary motions much better and the epicycle model was no longer necessary.
Currently an almost overwhelming amount of observations points towards the existence of dark matter. However, there could still be some other effect that produces exactly the (weird) motions of visible matter that we observe.
Although we cannot directly detect dark matter, we can calculate with every improving precision how much is needed where to produce the observed effects. These calculations can later be used to exactly explain what we see. But.. isn't that circular reasoning?
The epicycle model worked very well to describe and predict the motions of planets in the past, so why should we not keep to the well describing and predicting dark matter model today? It works.
Next question is who ordered dark matter and why?
Is there a new, falsifiable prediction that comes out of all of this? There are a number of ways dark matter could be falsified. For instance, it made the prediction that the SPARC acceleration law that relatively nearby galaxies seem to obey ought not to hold for very distant young galaxies, which seems to be getting validation from some careful observations recently. Dark matter also makes predictions on large scale structure formation, e.g. that galaxy clusters should start coming together at z=2.5 or so, which was borne out by the observation of CL J1001+0220. Any more?
So to what extent do the interplay of star formation, supernovas, and black holes create difficulties in understanding/simulating galactic evolution? I'm under the impression, these are able to heat and expel gas, and thereby stop or at least modulate star formation. I would think the luminosity to mass ratio would be sensitive to such details.
How many black holes havve you spawned today ?
https://vvattsupwiththat.blogspot.com/2017/06/tv-weathermans-fall-into-…
"The idea that vacuum can slow down light is largely discussed, but only in a quantum gravity context... [...] I think it is time to start discussing the parallel idea: that slowing down light by vacuum produces the Hubble redshift (in a STATIC universe)."
Here the crimestop is absolute (cosmologists lose everything if the universe is static) but still hints that vacuum damps down the energy/speed of traveling light do appear sometimes:
Nature: "As waves travel through a medium, they lose energy over time. This dampening effect would also happen to photons traveling through spacetime, the researchers found."
Quote: "Some physicists, however, suggest that there might be one other cosmic factor that could influence the speed of light: quantum vacuum fluctuation. This theory holds that so-called empty spaces in the Universe aren't actually empty - they're teeming with particles that are just constantly changing from existent to non-existent states. Quantum fluctuations, therefore, could slow down the speed of light."
Okay, Pentcho. It seems like you have a perfectly scientific idea. The only question is, is it right? You can now, please, provide evidence that the speed of light can change. Show us all a measurement of the speed of light in vacuum that is different from c. Put up or shut up.
BTW, your quantum fluctuations slow the speed of light thing does not qualify. The idea there is that interactions between photons and particles in the vacuum will slow the overall travel time of photons from point A to point B. That does not imply that the photons travel at a speed different from c when travelling between interactions with particles. (Of course, this effect has not been observed, so it's questionable anyway).
Just to add something to my previous comment: the situation regarding the quantum fluctuations slowing light is exactly the same as the fact that light travels at a slower speed in a medium. The slowing of light in a transparent medium in no way invalidates the relativity postulate that light speed is constant. That applies only to unhindered motion of light. The quantum fluctuations are postulated to hinder light in exactly the same way as interactions with real particles in a medium, and therefore the overall travel speed is diminished.
anneb,
You are correct, some other explanation that more simply accounts for observations could be provided at some point in the future. Until then, though, much as the epicycles were used to make accurate predictions of planetary motions, dark matter will continue to be the model used to make predictions about cosmological observations.
For those who are critical of dark matter (or any other theory in science for that matter), come up with an alternative. To supplant the accepted theory the alternative must:
1. Explain all known observations at least as well as the accepted theory.
2. Give at least one situation (and preferably many more than one) where the new theory would predict an observable result different from what the accepted theory would predict.
3. When the observation(s) described in 2. are actually made, they must agree with the new theory rather than with the accepted one.
If the alternative cannot do 1. above, then it's patently false. We know what already has been observed, so the new theory cannot contradict those observations. If it cannot do 2. above, then it really is not a different theory; it's just the accepted theory wrapped in a new package, so to speak. There is no way to test the new theory unless it predicts new results.
Good luck, critics. Your Nobel Prize awaits if you're successful. If you're really serious, though, I would suggest wasting less time commenting on internet science blogs and spending more time doing your actual scientific research.
"You can now, please, provide evidence that the speed of light can change."
In 1887 (prior to FitzGerald and Lorentz advancing the ad hoc length contraction hypothesis) the Michelson-Morley experiment was compatible with the variable (dependent on the speed of the light source) speed of light posited by Newton's emission theory of light, and accordingly incompatible with the constant (independent of the speed of the source) speed of light posited by the ether theory and later adopted by Einstein as his 1905 second postulate:
Wikipedia: "Emission theory, also called emitter theory or ballistic theory of light, was a competing theory for the special theory of relativity, explaining the results of the Michelson–Morley experiment of 1887. [...] The name most often associated with emission theory is Isaac Newton. In his corpuscular theory Newton visualized light "corpuscles" being thrown off from hot bodies at a nominal speed of c with respect to the emitting object, and obeying the usual laws of Newtonian mechanics, and we then expect light to be moving towards us with a speed that is offset by the speed of the distant emitter (c ± v)."
Banesh Hoffmann, Relativity and Its Roots, p.92: "Moreover, if light consists of particles, as Einstein had suggested in his paper submitted just thirteen weeks before this one, the second principle seems absurd: A stone thrown from a speeding train can do far more damage than one thrown from a train at rest; the speed of the particle is not independent of the motion of the object emitting it. And if we take light to consist of particles and assume that these particles obey Newton's laws, they will conform to Newtonian relativity and thus automatically account for the null result of the Michelson-Morley experiment without recourse to contracting lengths, local time, or Lorentz transformations. Yet, as we have seen, Einstein resisted the temptation to account for the null result in terms of particles of light and simple, familiar Newtonian ideas, and introduced as his second postulate something that was more or less obvious when thought of in terms of waves in an ether. If it was so obvious, though, why did he need to state it as a principle? Because, having taken from the idea of light waves in the ether the one aspect that he needed, he declared early in his paper, to quote his own words, that "the introduction of a 'luminiferous ether' will prove to be superfluous."
John Norton: "The Michelson-Morley experiment is fully compatible with an emission theory of light that CONTRADICTS THE LIGHT POSTULATE."
That is, in 1887 the Michelson-Morley experiment unequivocally DISPROVED the constancy of the speed of light.
Pentcho,
I said EVIDENCE, not dishonest quoting. You left out the end of the quote from Wikipedia, for instance, (I'm sure it was just an oversight, right?), where it says "...This theory is considered to be conclusively discredited by most scientists". Do better. Measure the speed of light from a stationary source, then measure the speed of light from a moving source. If you're right, you will get two different answers. You also seem to have left out the sections of the Wikipedia article that deal with just such measurements and have shown that the speed of light is indeed independent of the motion of the source.
Sure, the MM experiment can be accounted for by these emission theories, but other observations blatantly contradict them. It's not good enough to account for the results of a single experiment; a theory must account for ALL known results. Emission theory clearly doesn't.
BTW, a couple of side notes: why should it matter that Newton was the name most associated with emission theory? Newtonian mechanics has been supplanted. Why should we still think Newton's ideas on light were right? Science doesn't work that way anyway. Sure Newton was a well-respected scientist who accomplished quite a lot. However, when the evidence indicated that his theories were incomplete, they were discarded in favor of relativity and quantum mechanics. Argument from authority really doesn't carry any weight in science, especially in the face of contradictory observational evidence.
Also, the quote about the introduction of the aether proving to be superfluous is dishonest if you're intending it to be an indication that Einstein needed aether to develop relativity. The concept of the aether came from Maxwell's work on electromagnetics. Maxwell showed that electromagnetic waves must necessarily travel at a very specific speed given by purely electrical and magnetic quantities. Being well versed in Galilean relativity, the physics community naturally asked the question "speed with respect to what?" Since there seemed to be no satisfactory answer, the aether was invented and it was with respect to the aether that light speed was to take on its proscribed value. The MM experiment was certainly not an attempt to disprove the existence of the aether and establish relativity; it was an attempt to measure the speed of the earth through the aether. The null result was quite a shock, and it helped to establish the principle that light needed no medium and that its speed was the same for all observers. The notion that light was a wave derived from Maxwell's equations. The notion of the aether was completely separate from the wave nature. Einstein recognized that his theory rendered the question of the aether moot.
"I said EVIDENCE, not dishonest quoting. You left out the end of the quote from Wikipedia, for instance, (I’m sure it was just an oversight, right?), where it says “…This theory is considered to be conclusively discredited by most scientists”."
I left this out because it is irrelevant - nothing to do with my claim which was:
"In 1887 (prior to FitzGerald and Lorentz advancing the ad hoc length contraction hypothesis) the Michelson-Morley experiment was compatible with the variable (dependent on the speed of the light source) speed of light posited by Newton’s emission theory of light, and accordingly incompatible with the constant (independent of the speed of the source) speed of light posited by the ether theory and later adopted by Einstein as his 1905 second postulate."
The silliest Einsteinians mercilessly reject my claim above and say that in 1887 the experiment was compatible with both the variable and the constant speed of light.
"I left this out because it is irrelevant"
ROFL... actual experiments which show your claim is false are irrelevant... that's pure gold. :D :D You really are delusional.
"actual experiments which show your claim is false"
My claim has two parts:
(1) In 1887 the Michelson-Morley experiment was compatible with the variable speed of light posited by Newton’s emission theory of light.
(2) In 1887 the Michelson-Morley experiment was incompatible with with the constant (independent of the speed of the source) speed of light posited by the ether theory and later adopted by Einstein as his 1905 second postulate.
Which part - (1) or (2) - have "actual experiments" shown to be false?
experiments showing that emission theory is wrong... you can't even remember what you are claiming? oh my.
as for which experiments.. i have shown you several in the previous posts about LIGO. None of which you were able to disprove. Unlike you, I won't repeat my posts on topic which is irrelevant to the post ethan made.. which is dark matter. You are free to read up on that comment under LIGO posts.
I'm honestly afraid that you will start claiming that earth is flat and it's the sun that revolves around earth, because there is a quote from 15th century by someone saying it is so.
Here's an idea for you.. instead of pasting quotes from 19th century... how about you show/explain why there is 0 experimental evidence for light traveling at speed other than 300.000km/s/s in vacuum regardless of velocity or direction of source. You know.. those things I remarked in that comment you're afraid to tackle.. Why don't we observe increase in data transfer when probes are moving towards earth... or decrease when they are moving away from earth? Very simple question, no?
"experiments showing that emission theory is wrong… you can’t even remember what you are claiming? oh my."
On the contrary, I repeated my claim a couple of times - you are just lying. Here is my claim again:
"In 1887 (prior to FitzGerald and Lorentz advancing the ad hoc length contraction hypothesis) the Michelson-Morley experiment was compatible with the variable (dependent on the speed of the light source) speed of light posited by Newton’s emission theory of light, and accordingly incompatible with the constant (independent of the speed of the source) speed of light posited by the ether theory and later adopted by Einstein as his 1905 second postulate."
It is obvious that "experiments showing that emission theory is wrong" are irrelevant - my claim is true or false independently of what those experiments have shown.
so what is your claim?
Is your claim to the historical accuracy of state of affairs in 1887? No one is arguing about what people believed back then. Like I said earlier.. state of affairs in i.e. 1492 were something else again. So? It's 2017 now, not 1887, a lot of things happened since then.
p.s.
still waiting for your answer on #18 question.
@Sinsia Lazarek,
You have a gift for obnoxious hubris.
Why don't you do something else besides moo along with the rest of the herd. Ethan already has that job down.
.
Yes, here we are in 2017 blathering about dark matter and energy, the ultimate fudge factors in the universe to balance the failings of our equations that otherwise wouldn't explain what we are observing. Yes, things sure have changed. So much for the humility of science.
.
Snark away all you like, someday someone is going to look back on now with the same amount of arrogant contempt and ignorance you have for the past. You are a pip-squeak who happens to be standing on a lot of other people's accomplishments and crowing about how you know so much better. Plus ca change, plus c'est la meme chose.
@CFT
I'm trying to understand your position in all this? Are you a dedicated lawyer for Pentcho and MM? Do you share Pentcho's views that speed of light is variable? If so.. then my comments above apply to you too.
If not, then what's the point of your last comment? Can't carry a scientific debate, can't offer testable counter-arguments? Too bad. You think that by trying personal insults you're gonna change something and make me stop calling out trolls for what they are? Nope.
As for science progressing, sure it does. Thank you for making my point. It progresses, it doesn't digress. Even when someday we get better models and theories, they still won't change speed of light and rest. And yes, we all stand on the shoulders of giants before us. I'm not claiming any revolutionary idea or claiming anything for myself. Just defending the work of all those giants before me from arrogant trolls who seek to demolish them. If you don't like it, you don't have to read it. Or better yet, offer some science of your own.
Will someone please explain the difference, if any, between "dark matter" as a mysterious unknown and normal (known) baryonic matter which does not emit or reflect light, such as individual atoms (hydrogen, etc.), cosmic dust, and all non-luminous bodies.
Just asking.
@Michael Mooney #25: That is an excellent question!
Normal baryonic matter may or may not emit light, but it does absorb and scatter light! That's why dust clouds are dark; that's why we can look for absorption spectra as well as emission spectra. It also uses electromagnetic forces to interact with other normal matter (that's what friction and viscosity are!).
This stuff (whatever it is) was called "dark matter" because it doesn't appear to emit or absorb light (which normal baryonic matter does). But there's more to it than that. Whatever it is, dark matter doesn't interact with normal matter _at_all_. Galaxies have observable structure -- disks, arms, huge elliptical shapes, whatever. If the dark matter haloes enclosing them (by hypothesis) caused friction or viscosity because of interactions, we wouldn't see those structures at all. Instead of orbiting smoothly around a center, all of that galactic structure would have been slowed down and spiralled in to some small clump of stuff.
You don't _need_ dark matter to get that result for individual galaxies (c.f. MOND, or f(R) gravity, or whatever your favorite flavor is). But you do need dark matter (whatever it turns out to be made of) to get all of the other observations which we _see_, and which are inconsistent with just the luminous and absorbing matter we can observe.
@Michael Kelsey #26:
Thanks for your answer, but would enormous clouds of well scattered hydrogen atoms, for instance, "absorb and scatter light" or interact with other matter in an observable way?
@ MM #27
Yes, they are called nebula, and plenty of them around.
Visible nebulae are well known, but I was wondering, if the gas atoms were dispersed far enough apart, would they become undetectable and account for the gravity now attributed to an unknown form of (dark) matter.
(Question for MK, not SL, the basher.)