≡ Menu

Astronomy Without A Telescope – Could Dark Matter Not Matter?

Rotation curves of the Andromeda Galaxy. Actual rotational velocities of the outer stars are the white line, while velocities that would be expected from the estimated mass of the visible matter in the galaxy are the red line. Hence, we conclude that over 80% of the galaxy's mass must be dark matter. (Credit Queens Uni.).

You probably want to put on your skeptical goggles and set them to maximum for this one. An Italian mathematician has come up with some complex formulae that can, with remarkable similarity, mimic the rotation curves of spiral galaxies without the need for dark matter.

Currently, these galactic rotation curves represent key evidence for the existence of dark matter – since the outer stars of spinning galaxies often move around a galactic disk so fast that they should fly off into intergalactic space – unless there is an additional ‘invisible’ mass present in the galaxy to gravitationally hold them in their orbits.

The issue can be appreciated by considering the Keplerian motion of the planets in our Solar System. Mercury orbits the Sun at an orbital velocity of 48 kilometers a second – while Neptune orbits the Sun at an orbital velocity of 5 kilometers a second. In the Solar System, a planet’s proximity to the substantial mass of the Sun is a function of its orbital velocity. So, hypothetically, if the Sun’s mass was reduced somehow, Neptune’s existing orbital velocity would move it outwards from its current orbit – potentially flinging it off into interstellar space if the change was significant enough.

The physics of the Milky Way Galaxy is different from the Solar System, since its mass is distributed more evenly across the galactic disk, rather than 99% of its mass being concentrated centrally – the way it is in the Solar System.

Nonetheless, as this past Universe Today article explains, if we assume a similar relationship between the cumulative mass of the Milky Way and the orbital velocity of its outer stars, we must acknowledge that the visible objects within the Milky Way only have 10-20% of the mass that is required to contain the orbital velocity of stars in its outer disk. So we conclude that the rest of that galactic mass must be dark (invisible) matter.

This is the contemporary consensus view of how galaxies work – and a key component of the current standard model of the cosmology of the universe. But Carati has come along with a seemingly implausible idea that the rotational curves of spiral galaxies could be explained by the gravitational influence of faraway matter, without needing to appeal to dark matter at all.

Left image: the rotation curve of spiral galaxy NGC 3198 showing the actual velocities of its outer stars (plotted points), then the velocities that would be expected given the mass of visible matter in its disk - overlaid by the assumed contribution of the mass of a dark matter halo. Right image: Carati's theoretical curve calculated from the effect of faraway matter and its remarkable fit to observed values from NGC 3198.

Conceptually the idea makes little sense. Positioning gravitationally significant mass outside of the orbit of stars might draw them out into wider orbits, but it’s difficult to see why this would add to their orbital velocity. Drawing an object into a wider orbit should result in it taking longer to orbit the galaxy since it will have more circumference to cover. What we generally see in spiral galaxies is that the outer stars orbit the galaxy within much the same time period as more inward stars.

But although the proposed mechanism seems a little implausible, what is remarkable about Carati’s claim is that the math apparently deliver galactic rotation curves that closely fit the observed values of at least four known galaxies. Indeed, the math delivers an extraordinarily close fit.

With skeptical goggles firmly in place, the following conclusions might be drawn from this finding:
• There are so many galaxies out there that it’s not hard to find four galaxies that fit the math;
• The math has been retro-fitted to match already observed data;
• The math just doesn’t work; or
• While the author’s interpretation of the data may be up for discussion, the math really does work.

The math draws on principles established in the Einstein field equations, which is problematic as the field equations are based on the cosmological principle, which assumes that the effect of faraway matter is negligible – or at least that it evens out at a large scale.

Perplexingly, Carati’s paper also notes two further examples where the math can also fit galaxies with declining rotational velocities in their outer stars. This is achieved by switching the sign of one of the formulae components (which can be + or -). Thus, on the one hand the effect of faraway matter is to induce a positive pressure that contains the rapid rotation of stars, preventing them from flying off – and on the other hand, it can induce a negative pressure to encourage an atypical decay in a galaxy’s rotation curve.

As the saying goes, if something seems too good to be true – it probably isn’t true. All comments welcome.

Further reading:
Carati Gravitational effects of the faraway matter on the rotation curves of spiral galaxies.

Comments on this entry are closed.

  • George Papantonopoulos December 6, 2011, 9:54 AM

    What is the REALLY extraordinary conclusion we get from this article is the idea of physisits for “a close fit”!!!
    I quote: “Carati’s claim is that the math apparently deliver galactic rotation curves that closely fit the observed values of at least four known galaxies. Indeed, the math delivers an extraordinarily close fit.”
    Where you people see the “close fit” ?? I am not a physisist but I can read a graph… I can accept that for the the first 5-10 kpc the experimental line and the theoretical one do have a close fit… but further down they go light years apart…
    And that goes for both the DM model and the Carati’s one…
    And you know what is even more amuzing? people in the comments area fighting about the two models… LOL!

    • Paul Goldstein December 6, 2011, 5:58 PM

      @George P. Umm, don’t you see those little squares sitting right on the theoretical line?

  • Alton Moore December 7, 2011, 1:46 AM

    Another armchair observation here…. Given the number of dimensions proposed by most current theories, we know that many of them are wrapped up at the microscopic level. However, given our inability to explain the large-scale structure of the universe, there may be one or more hidden at the macroscopic level, too. Our current indications that space doesn’t seem to be curved may not even be testing the right things.

    So it could be that the relevant objects (things asserting what we regard as gravitational influence upon one another) are really “closer” than they appear to be on one level or another. While this might explain the galactic rotation conundrum, I don’t know how applicable it is to gravitational lensing observations, but I’m just throwing the idea out there. Until we pin down where all of the other dimensions are, I don’t know if it’s safe to assume that they’re all microscopic. Of course, they may all be microscopic and still exhibit macroscopic effects, possibly many more than we imagine, but I guess that remains to be seen.

  • Anonymous December 7, 2011, 4:12 PM

    Why would you start off your article with “You probably want to put on your skeptical goggles and set them to maximum for this one”? It makes me feel like you didn’t need to write the article at all if you don’t want to give your point a chance.

  • Iori Fujita December 9, 2011, 4:21 AM

    I proposed a new way of thinking.
    Two dimentional space needs no dark matter for the flat rotation curve of the galaxy.
    Iori Fujita

    • Torbjörn Larsson December 9, 2011, 9:48 AM

      The obvious problem with that is that space is 3D.

    • Anonymous December 10, 2011, 12:20 AM

      I’ve had look at this, a brief one though. It is very interesting.
      One thing I wonder which I would like to point out about the NGC 891 galaxy, where you mention that the dust could be dark matter… Dark matter is so called because it supposedly does not interact with electromagnetic radiation at all, I wonder if this is a translation problem or typing mistake??? In the same paragraph you mention that the condensed gravity field pushes light away. This doesn’t really comply with current physics! Could you expand on what you mean?
      Your view on the structure of black holes is remarkably similar to mine – the current, most widely accepted idea of jets being born from super hot plasma are surely inconceivable, as matter at these points (around the poles) of the black hole should certainly have less energy (most obviously less kinetic energy), it should fall in to the black hole more easily, and of course in this current model there would be a higher gravitational field at the poles (under the same process as gravity darkening, which you had mentioned).
      Good work!

  • Ivo Temelkov December 12, 2011, 11:44 AM

    so, we can either believe the preexisting hypothesis of dark matter, which is basically saying “well, we don’t have ANY bloody idea what’s going on, so we’re gonna add some invisible and hard to detect stuff to make the theories fit the observation”


    we can believe this guy with his clever mathematics, that actually eliminates the need for the ghost that is dark matter. I personally do not believe in ghosts.

  • Anonymous December 12, 2011, 3:27 PM

    Could this mystery of stellar motion in galaxies be so elusive because Science is viewing it through a strictly material lens (naturally). Therefore, Science insists(?) there MUST BE a material explanation (and there may very well be). What else would explain the outer stellar motions measured in galaxy-rims, or the dramatic gravitational lensing-effect of huge clusters – the inexplicable “weight of mass” behind the visible concentration of matter, the bending of light passing through its complex, or “lensed” around its vast enveloping halo(?).

    To risk an unwelcome view, could it be an invisible FORCE, one that upholds or maintains the great star systems in spiral form and galactic shape? A sustaining power having physical effects around the enormous Island Universe assemblages – manifestations on Matter (within) and Energy (passing by them), but undetectable otherwise – not directly observable beyond its powerful affects (so far)?

    There is “Dark Energy” : the expansion, acceleration of Space itself (if I understand). Is it an opposing agency to “Dark Matter” (poorly informed question, perhaps): that which reigns-in outer galaxy-disk star-fields, or influences passing “wavepaticles”(?) in space-time?

    Could it be that the two “Dark” agencies are related (apart from their mystery element). One works outward from within, another works inward from without. (If I am not badly inverted!)

    In the Microcosmos, the two Nuclear Forces, the strong and the weak, come to mind: they perfectly work together, so we exist in an ordered Cosmos. Are the two great “Dark” agencies separate and exclusive? Or are they complementary – working in perfect harmony in the Macrocosmos? Dual Forces working together in both the Atomic and Cosmic frontiers of our Universe – the morning study of which may be affording profoundly insightful reflections of our existence, from both ends of physical creation, which Science has only now begun to glimpse on the far horizon of its upward journey to a mountain-summit of vision (and astronomical observation).

    What would the Universe be like, if there was no no spreading expansionary force, and outer stars of a system were not held fast within, but sprayed off into the night (from rotating, dissipating galaxies void of “Dark Matter”)? What would space be like without the vast stretches of time’s separating distance? ….

    This interesting paragraph may be off-page, but seemed somehow in-line:

    “One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence[?!]. Then one version of Einstein’s gravity theory, the version that contains a cosmological constant, makes a second prediction: ’empty space’ can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the Universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the Universe.” — ( http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/ )

    Well, the author did say “All comments welcome.”