Why Dark Matter? (Synopsis)

“The Universe is made mostly of dark matter and dark energy, and we don’t know what either of them is.” -Saul Perlmutter

When we look out at the Solar System, the Sun dominates in terms of both light and mass. Responsible for nearly 100% of the light and for 99.8% of the mass, it stands to reason that stars would account for the vast majority of mass in the Universe. Yet when we apply what we observe of light and stars to structures like galaxies, clusters, and the large-scale structure of the Universe, not only do stars not get us there, but all the known forms of matter, including gas, dust, plasma and black holes, don’t get us there either.

Gas and dust in the nebula IC 2944, along with new stars. Image credit: NASA/ESA and The Hubble Heritage Team (STScI/AURA).

Gas and dust in the nebula IC 2944, along with new stars. Image credit: NASA/ESA and The Hubble Heritage Team (STScI/AURA).

In order to account for the full suite of observations as astronomers’ and cosmologists’ disposal, there has to be something more to the Universe, outmassing normal matter by a 5-to-1 ratio, than all forms of normal matter can explain. At this point in time, the only explanation that nabs them all is dark matter.

The Bullet Cluster, the first colliding galaxy clusters showing the separation between normal matter (pink, from the X-rays) and dark matter (blue, from gravitational lensing). Image credit: X-ray: NASA/CXC/CfA/M. Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U. Arizona/D. Clowe et al. Optical: NASA/STScI; Magellan/U. Arizona/D. Clowe et al.

The Bullet Cluster, the first colliding galaxy clusters showing the separation between normal matter (pink, from the X-rays) and dark matter (blue, from gravitational lensing). Image credit: X-ray: NASA/CXC/CfA/M. Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U. Arizona/D. Clowe et al. Optical: NASA/STScI; Magellan/U. Arizona/D. Clowe et al.

Come get the whole, succinct story on why dark matter is the only answer (for now) on Forbes today!

Comments

  1. #1 Denier
    United States
    June 8, 2016

    It doesn’t seem to be linked correctly. The Forbes article is here:

    http://www.forbes.com/sites/startswithabang/2016/06/08/why-dark-matter/#6e1daf197bc1

  2. #2 Denier
    United States
    June 8, 2016

    One of the phenomena attributed to dark mater is the uniform velocity of galactic rotation. Getting away from the ‘why’ for a moment and away from other observations requiring DM, how do we observe this spin velocity?

    For the sun, it takes 230 million years to complete an orbit of our galaxy. As compared to our human viewpoint, that seems so long. Are our telescopes able to see individual stars in other galaxies over enough time to clock their speed? Are they measuring some sort of red/blue shift from leading and trailing edges of galaxies? How do these galactic structure speed measurements work?

  3. #3 Ethan
    June 8, 2016

    Thank you, Denier @1. It has been eating links recently; ScienceBlogs recently updated their wordpress and I don’t like the new “feature” they’ve added as far as making links goes.

  4. #4 See Noevo
    June 8, 2016

    Whew! Lots of material here.

    1)
    “And by observing the motions of these individual galaxies — their relative redshifts and BLUEshifts — he was able to obtain a value for how much mass there is, inferred from gravitation, in the entire cluster.”

    In factual, not theoretical, terms, how would these blueshifts NOT contradict the theory that the universe is expanding with galaxies getting farther and farther apart from each other?
    ……………..
    2)
    “…the mass in stars gives you a number, and the mass from gravity gives you a greater number. Not a number greater by a little bit, either: one that was bigger by a factor of fifty.”

    Perhaps you’ll soon write a piece explaining the difference between
    a) mass in stars/planets/objects and
    b) “mass from gravity”.
    …………..
    3)
    “What she found was that unlike our Solar System, where the Sun’s mass dominates and Mercury zips around the Sun with nearly ten times the speed of the outermost planet, Neptune, the inner parts and the outer parts of the galaxies rotated at the same speeds. There must be more mass than the stars themselves indicated…but the leading explanation was the one that Zwicky came up with 40 years earlier: there must be some form of dark matter.”

    So, these other solar systems have dark matter, but OUR solar system does NOT.
    I wonder what the reason would be.
    And I wonder how that reason would not contradict the assumption of the universe’s homogeneity.
    ………….
    4)
    “We went even further, and measured the large-scale structure of the Universe…”

    Ethan, you never responded to my query in the week #112 summary. I’ll repeat it:
    What IS this “large scale”, and specifically,
    WHAT IS THE VANTAGE POINT FROM WHICH ONE SURVEYS THIS SCALE?
    It seems to me this vantage point must be from far *outside the universe*, such that even the *one billion light year-wide* Boss Great Wall appears as just another flake in a tiny snow globe in one’s hand.
    ………….
    5)
    “But the most spectacular piece of evidence for dark matter came in 2005, when a team noticed evidence that two clusters of galaxies had collided together at tremendous speeds.”

    I wonder how galaxies colliding together does NOT contradict what we’re told about the universe expanding with galaxies getting farther and farther apart from each other.
    …………..
    6)
    “The individual galaxies themselves passed through each other mostly without interacting, similar to how two guns filled with birdshot, fired at one another, would have most of the bullets miss entirely. The gas and dust in the galaxies and clusters, however, would interact, heat up, slow down, and emit X-rays, somewhere in the middle.”

    Ah, X-rays.
    This brings to mind another point of mine which you didn’t respond to from the week #112 summary.
    You had said “… what the CMB actually is… this light was once, very early on, high-energy gamma rays. The EXPANDING UNIVERSE HAS REDSHIFTED IT DOWN into X-rays, UV, visible, infrared, and finally microwave/radio wavelengths, where it resides today.”

    And I said that this sounds like you’re saying that what appears as X-rays on earth would appear as CMB when viewed from a place *a long way from* earth.

    I’m still hoping you’ll respond.
    …………….
    7)
    In your pretty pink and blue pictures of the “colliding” galaxies, why isn’t the blue (supposedly representing dark matter) five or six times more prevalent than the pink?
    …………..
    8)
    “It’s very LIKELY that dark matter is the correct explanation, but perhaps the correct modification to Einstein’s General Relativity will come along that explains all of these observations as well… but these are some of the most compelling reasons, part of the full suite of evidence we must consider, when we evaluate WHETHER our Universe needs dark matter. At this point in time, it’s the only answer that works.”

    And here I was beginning to think you held that dark matter (along with dark energy) was a *fact*!
    But I guess you really don’t hold up dark matter as a fact.
    It’s just a theory.

    The other thing your words bring to my mind is this:
    Some are excoriated for advocating a “God of the gaps” to explain the as yet unexplained/undemonstrated,
    while the excoriators are advocating a “dark SOMETHING of the gaps” to explain the as yet unexplained/undemonstrated.

    Both are holding up something they can’t prove and haven’t seen.

    Perhaps the major distinction between the two is that one will accept *only* “natural” causes while the other accepts the cause of nature.

  5. #5 eric
    June 8, 2016

    @2: that’s an interesting question and I’d like to know the real answer. But my guess is that the observation of many spiral galaxies of very different ages leads to the conclusion that spirals are stable over billions of years. That in turn allows us to calculate how much mass would it take for that to be true.

  6. #6 eric
    June 8, 2016

    SN:
    1- because not all galaxies motion is uniform, of course.
    2 – see my #5
    3 -You misunderstood. The comparison is between our solar system (inner stuff moves faster) and galaxies (it doesn’t). Not our solar systems and other solar systems.
    4 – The vantage point is Earth. Unless you’ve built a telescope on Mars we don’t know about?
    5 – see 1.
    6 – Are you asking about a photon that is arriving at that place from Earth now, or departing from Earth now? The former will have a redshift dependent on the the (past) travel time. The latter will have a redshift dependent on the (future) travel time. Those will be different, and the answer is not always “it arrives at CMB energies.”
    7 & 8 LOL. I like how in 7 you complain you aren’t seeing enough dark matter in the picture, and then in 8 you complain that nobody’s seen it at all.

  7. #7 See Noevo
    June 8, 2016

    I’m humming some more of the cosmological blues (as in the blueshifts Ethan mentioned).

    If galaxies are not only receding from ours but are also spinning, then I’d expect that *virtually all* galaxies would present a blueshift edge as well as a redshift edge. (The rare exceptions would be galaxies with rotations exactly perpendicular to our point of view.)

    Yet, virtually all galaxies apparently do NOT present a blueshift, according to this Cornell PhD:
    “Almost all galaxies are redshifted because of the Hubble expansion of the universe. Only a handful of the most nearby galaxies are blue-shifted… There are in all about 100 known galaxies with blueshifts out of the billions of galaxies in the observable universe.”
    http://curious.astro.cornell.edu/about-us/97-the-universe/galaxies/cosmology/539-why-are-there-blue-shifted-galaxies-intermediate

  8. #8 See Noevo
    June 8, 2016

    One other cosmic blues note, regarding my 1) above.
    One reason I asked for the “factual, not theoretical” basis for the blues is that I have read
    “Are we to assume that these galaxies cast doubts on the expanding Universe theory ? Not necessarily so. Some THEORIES about the blueshifts are:” [followed by list of 5 theories].
    http://www.poyntsource.com/Richard/blueshifted_galaxies.htm

    (I was looking for facts supporting the theories, as opposed to theories supporting the theories.)

  9. #9 Ragtag Media
    United States
    June 8, 2016

    “Yet, virtually all galaxies apparently do NOT present a blueshift, according to this Cornell PhD:”

    Interesting.
    Is this true?
    Thanks SN for bringing this up.
    Anyone else have more information to add?

    TIA..

  10. #10 Ragtag Media
    June 8, 2016

    aight, found this:

    “Question
    Are there any galaxies that have a blue-shift?
    Asked by: Mike Cahill

    Answer
    Sure there are. In particular, the Andromeda galaxy exhibits a small blueshift.

    To give a bit of background, if you observe the light coming from distant galaxies, for the most part the light seems to be redshifted. In other words, we know what wavelengths of light to expect when we look at a galaxy, and the wavelengths we actually see end up being considerably longer.

    As you probably know, we interpret the redshifts of galaxies to mean that the universe is expanding. So if you could staple the galaxies to the ‘fabric’ of space, all of them would appear to be moving away from us — the farther away they are, the faster. This is Hubble’s Law.

    A useful analogy here is to take an empty balloon, draw dots all over it to represent galaxies, and pretend that we live on one of the dots. As you blow up the balloon, ALL of the dots move apart from each other. And the ones that are farthest away from us move the fastest.

    The difference between this analogy and the actual universe is that although the galaxies are being pulled away from each other by the universe’s expansion, they are not stapled down. Replacing the dots on the balloon by a bunch of ants give a feel for this idea. Astronomers refer to the velocity a stapled down galaxy would have as its ‘Hubble recessional velocity.’ Any deviation from this speed is its ‘peculiar velocity.’

    So, in a nutshell, if a galaxy’s peculiar velocity is toward us and larger than its Hubble recessional velocity, then its light will appear blueshifted. This is possible for galaxies that are nearby like Andromeda, but as galaxies get farther away, their Hubble velocities dwarf any peculiar velocities they might have. As such, it’s better to study far away galaxies when you’re interested in how the universe is expanding.
    Answered by: Leven Wadley, M.A., Physics Grad Student, Columbia University

    Almost all the galaxies are red shifted; they are moving away from us, due to the Hubble expansion of the Universe. There are a handful of the nearby galaxies that are blue shifted. In addition to the apparent motion due to Universal expansion, individual galaxies also have their own intrinsic or peculiar motions; i.e. each galaxy is in motion irrespective of the universe’s expansion and has its own unique velocity.

    The velocities are in the order of hundreds of kilometers per second and in regions close enough to our own galaxy where the Hubble expansion results in less outward expansion than this, the galaxies’ peculiar velocities (if they are large enough and sufficiently towards us) can overcome that expansion, resulting in a blue-shift.

    There are about 100 known galaxies with blueshifts out of the billions of galaxies in the observable universe. Most of these galaxies are in our own local group, and are all in orbit about each other. Most are dwarf galaxies among them include the Andromeda Galaxy, M31, etc. Click here for a list.

    The negative velocities in the z column are the blue shifted galaxies (moving towards us).
    Answered by: David Latchman, B.S., University of the West Indies, Trinidad”
    http://www.physlink.com/education/askexperts/ae384.cfm

  11. #11 See Noevo
    June 8, 2016

    “In your pretty pink and blue pictures of the “colliding” galaxies, why isn’t the blue (supposedly representing dark matter) five or six times more prevalent than the pink?”

    Speaking of the Pink (the real stuff), here’s a song that might be dedicated to the subject matter at hand.
    Along with the lyrics, pay particular attention to the VIDEO. It’s one of the best productions I’ve seen or heard on Dark Matter:

  12. #12 Michael Kelsey
    SLAC National Accelerator Laboratory
    June 8, 2016

    @Denier #2: Very good question! It only works for relatively nearby galaxies, where the angular size of the galaxy is large compared to the telescope resolution (the “point spread function” or PSF, typically a few arcseconds for ground-based telescopes).

    In such a situation, you can use a “long slit” spectrograph, which has an aperture limiter to pick out a narrow strip of the object of interest, and get a spectrum along that whole strip (see the Wikipedia article for a cool picture).

    To measure rotation, you don’t actually care about the _absolute_ red or blue shift, but rather the _difference_ between the shift at different points across a galaxy image. If the galaxy is rotating, then the left side will have a shift slightly different, and opposite in sign, compared to the right side, with respect to the average or middle value.

  13. #13 See Noevo
    June 9, 2016

    To eric,

    Perhaps you can do me favor.
    For a brief time today it appeared that Jason Rosenhouse had de-banned me, as I was able to post a short question about banning on his June 9 thread “A Delusional List Of Alleged Atheist Delusions”.

    However, after I took the time to compose a comment I found that I couldn’t post it. Apparently I’m still banned and Jason accidentally let me slip through the iron curtain earlier.

    But you could post this for me since 1) it involves you, and 2) it would perhaps give Jason another perspective to consider. You could preface “your” comment by saying “Some might argue that …”

    So, here’s the comment:
    ;;;;;;;;;;;;;
    To Jason Rosenhouse:

    You appear to be interested in what you call “nuanced discussions of abstract philosophy.”

    Maybe we can do some of that here, and in such a way that you don’t ban me again.

    You said “You can reasonably believe that morality is “nothing more than strongly felt subjective preference” while also believing that everyone SHOULD behave as though certain MORAL claims are objectively true. There’s just no contradiction there.”

    1)
    The first issue is your use, or misuse, of the word “moral”.
    The definition of the word “moral” is inseparable from the words and concepts of “right/good” and “wrong/evil”.

    Now, perhaps in a nod to modern thinking, Merriam-Webster adds to right/good what might be called consensus.
    M-W’s 3rd definition of “moral”: “considered right and good BY MOST people: agreeing with A STANDARD of right behavior.”

    Which is to say, and as I think YOU would say, there is no objective right/good but rather only
    a STANDARD for behavior, which is equivalent to
    what MOST people think, which is equivalent to
    the MORAL.

    Basically, the MAJORITY is MORAL,
    or what is moral is what the majority thinks, by your definition.

    I’d suggest that in the future you say what you really mean and refrain from using phrases like “moral beliefs.”
    You could use instead something like “majority beliefs”,
    or maybe even “group think.”

    2)
    The second and perhaps more central issue is your misuse of the word “should”, as in “everyone SHOULD behave as though certain moral claims are objectively true.”

    This is ridiculous. Because for an evolutionist, such as yourself, there is no should, only what is.

    You wouldn’t say everyone SHOULD like vanilla more than chocolate, would you? Of course you wouldn’t.

    Which brings me to what I said on another thread to eric, who believes humans are just a confluence of chemicals:
    “What you really mean is that *morality is a matter of taste.*
    Some like vanilla, some like chocolate.
    Some like genocide, some don’t.
    Vive la difference. De gustibus non est disputandum.
    … And that’s a true statement.
    I never said matters of taste are *equally* important or *equally* trivial. They’re not.
    Choosing a $6 vanilla ice cream cone over a $6 chocolate one is trivial compared to
    choosing a $60,000 BMW over a $60,000 Audi, which is trivial compared to
    murdering 60,000 people over unjustly imprisoning 60,000 people.
    Nevertheless, they’re still just matters of taste, for you.”

    So, Jonathan, I’d suggest that in the future you say what you really mean and refrain from using the word “should”.
    There is no “should” in evolution or in its alleged results.
    ;;;;;;;;;;;;;;;;;;

    Thanks in advance, eric.

  14. #14 dean
    United States
    June 9, 2016

    sn, #13 is a pathetic plea for attention even for you.

  15. #15 eric
    June 9, 2016

    But you could post this for me since 1) it involves you, and 2) it would perhaps give Jason another perspective to consider

    Sorry, no. Jason’s blog, Jason’s rules. If you want back in, I suggest you write a polite letter to Jason outlining how you intend to change your posting behavior in the future.

    I would further suggest that the post you’ve asked me to forward – in essence, demanding he stop using the words “moral” and “good” and “evil” because he’s just a subjectivist and you demand they not be used for subjective criteria – is not the sort of behavior that’s going to pass his muster. Its whiny, its petulant, its arrogant, and its pedantic.

    But worse than all that, is boringly unoriginal. Do you have anything else to say about the subject other than the yawn-inducing fundie mantra that subjective morality is a contradiction in terms and only your God’s given morality is worthy of the name? Because we’ve heard it. And heard it. And heard it. And I gotta tell you, it rivals your ‘science can’t explain everythnig’ mantra in terms of rhetorical power to convince.

  16. #16 See Noevo
    June 9, 2016

    To eric #15:

    “I would further suggest that the post you’ve asked me to forward – in essence, demanding he stop using the words “moral” and “good” and “evil” because he’s just a subjectivist and you demand …”

    You forgot “should.”

    But demand?
    What kind of academic environment or training would lead you to say that “I’d SUGGEST” means “I DEMAND”?

    Suggest doesn’t mean demand, eric.
    And even if it did, which it does NOT, how would I enforce the alleged demand?

    So, I’d suggest that in the future you not say “suggest” means “demand.”
    It would save you from appearing to be an atheist prone to the liberal way of flipping the deception/distortion switch.

    “But worse than all that, is boringly unoriginal. Do you have anything else to say about the subject other than the yawn-inducing fundie mantra that subjective morality is a contradiction in terms…”

    2+2=4 might be considered “boringly unoriginal”, as well.
    And I wouldn’t normally say much if anything about it.

    But when some publicly insist that 2+2=5, the subject becomes less boring. And I’ll say somethings about it, even if I have to say repeatedly and boringly that “No, 2+2=4.”

  17. #17 Narad
    June 9, 2016

    For a brief time today it appeared that Jason Rosenhouse had de-banned me

    For the second time? Seems unlikely:

    “See Noevo–

    “I’ve suddenly remembered why I banned you the first time around. Bye!”

    as I was able to post a short question about banning on his June 9 thread “A Delusional List Of Alleged Atheist Delusions”.

    Where would that be?

    However, after I took the time to compose a comment I found that I couldn’t post it. Apparently I’m still banned and Jason accidentally let me slip through the iron curtain earlier.

    Lacking a screen shot or other confirmation, I’m going with your either lying or being too stupid to tell the difference.

  18. #18 eric
    June 9, 2016

    SN:

    But demand?
    What kind of academic environment or training would lead you to say that “I’d SUGGEST” means “I DEMAND”?

    Oh I’m very happy to correct my post in this regard. Your request that he stop using the words “moral” and “good” and “evil” because he’s just a subjectivist and you demand they not be used for subjective criteria is whiny, petulant, arrogant, pedantic, and boringly unoriginal.

    Is that better?

  19. #19 eric
    June 9, 2016

    Please consider my second use of ‘demand’ to read ‘request’ too. There you go!

  20. #20 Narad
    June 9, 2016

    One might note a direct inference from S.N.’s story: he didn’t wait for a response to his putative “short question about banning.”

  21. #21 Narad
    June 10, 2016

    But when some publicly insist that 2+2=5, the subject becomes less boring. And I’ll say somethings about it, even if I have to say repeatedly and boringly that “No, 2+2=4.”

    The irony being that you can’t even define the meanings of the symbols.

  22. #22 Wow
    June 11, 2016

    “how do we observe this spin velocity? ”

    Doppler shift in the light of stars obliquely inclined toward us.

  23. #23 Wow
    June 11, 2016

    Wasn’t very clear, was it.

    The galaxy is oblique to us. Stars in that oblique galaxy are rotating. Toward and away from us on two opposite sides.

    We can therefore observe the doppler shift at a given radius.

  24. #24 Wow
    June 11, 2016

    “in essence, demanding he stop using the words “moral” and “good” and “evil” because he’s just a subjectivist and you demand they not be used for subjective criteria – ”

    But given that which god to believe in is a subjective choice, even religious morality is subjective.

  25. #25 Mickey Sloan
    Essex, England
    June 14, 2016

    Could Dark Matter be made up of Neutrons and Protons that didn’t fuse together to make Hydrogen in the Big Bang? Both particles have Mass and who knows how much of the stuff didn’t make it to the Basic Atom stage?

  26. #26 Denier
    United States
    June 14, 2016

    @Michael Kelsey #12

    I know I’m late with this as work got really busy but thank you for the explanation. I do appreciate it.

  27. #27 Naked Bunny with a Whip
    June 14, 2016

    @Mickey Sloan #25: Dark matter couldn’t be neutrons, since free neutrons decay into protons and electrons pretty quickly (half-life around 15 minutes). I don’t think it could be free protons, either, since they’d be electrically charged and affected by magnetic fields. I’d expect that to have a noticeable effect on the DM distribution. They’d also be detectable by scattering (and sometimes emitting?) light, presumably. We’d be talking about an awful lot of protons, after all.

  28. #28 Narad
    June 16, 2016

    One more time:

    In factual, not theoretical, terms, how would these blueshifts NOT contradict the theory that the universe is expanding with galaxies getting farther and farther apart from each other?

    Dumb, ingrate, or undiluted. troll, S.N.? Man up, johnnycakes.

  29. #29 philip coleman
    United States
    June 18, 2016

    Perhaps the under-layer or last negative fabric is this dark matter. All matter begins with a ghost structure which builds as the first positive layer we see today. Perhaps the dark matter arrives through worm holes and supernovae blasts. We know that the element chart we see today is just the beginning and will rise.

  30. #30 Chris Mannering
    July 17, 2016

    Michael Kelsey : “To measure rotation, you don’t actually care about the _absolute_ red or blue shift, but rather the _difference_ between the shift at different points across a galaxy image. If the galaxy is rotating, then the left side will have a shift slightly different, and opposite in sign, compared to the right side, with respect to the average or middle value.”

    That’s right..and I have the perception all measured spirals exhibit this differential, specifically in the arms. Is that correct, Michael? Anything more on this would be met with grovelingly pathetic gratitude baby!

  31. #31 Michael Kelsey
    SLAC National Accelerator Laboratory
    July 17, 2016

    @Chris Mannering #30: Yes. The differential is there whenever it is measured. Except for limiting cases like the Whirlpool Galaxy (M51a), where we see it almost perfectly face on, and therefore can’t get first-order Doppler differences from the two sides.

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