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L2

Dark matter and galactic structure

Stars near the outer edges of spiral galaxies orbit far faster than Newtonian gravity applied to the galaxy's visible, luminous matter alone can account for — their rotation curves stay roughly flat with radius instead of falling off the way planetary orbits in the Solar System do — and the same mass discrepancy shows up independently in the dynamics of galaxy clusters and in gravitational lensing of light passing near them; the leading explanation is that galaxies are embedded in far larger haloes of non-luminous dark matter, which interacts gravitationally but, as far as every experiment to date can tell, not electromagnetically, and so neither emits, absorbs, nor scatters light, while outweighing all of the universe's ordinary matter roughly five to one.

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When astronomers measure how fast stars and gas orbit at different distances from the centre of a spiral galaxy — its rotation curve — Newtonian gravity applied to the visible, light-emitting matter makes a clear prediction: beyond the bulk of the visible mass, orbital speed should fall off with distance, the same way Mercury orbits faster than Neptune. Instead, observed rotation curves flatten out and stay roughly constant far beyond the edge of the visible disk. The straightforward reading of that discrepancy, using nothing but standard gravity, is that there is far more gravitating mass out there than telescopes can see — as much as five or six times more mass than all the stars, gas and dust combined.

This is not a one-off puzzle from a single measurement. The same missing-mass problem shows up independently in how fast galaxies move within clusters (they move too fast to stay gravitationally bound to each other unless there is much more mass present than is visible), and again in gravitational lensing, where the bending of light from background galaxies as it passes a foreground cluster reveals a mass distribution that matches the dark matter picture and far exceeds the visible matter alone. Because these three independent lines of evidence — rotation curves, cluster dynamics, and lensing — agree on both the amount and rough distribution of the missing mass, the case for some new form of matter, rather than an error in any one measurement, is strong.

What makes this matter 'dark' is specific: as far as every experiment run so far can tell, it does not interact via the electromagnetic force at all, which is why it neither emits, absorbs, nor scatters light and is invisible to every kind of telescope. It still interacts gravitationally, which is how its presence is inferred, and it appears to be distributed in extended, roughly spherical haloes that envelop galaxies well beyond their visible disks, rather than being concentrated in the disk the way ordinary matter is. It is worth being precise that dark matter is a different thing from dark energy: dark matter is extra gravitating mass, which pulls things together and would, on its own, tend to slow cosmic expansion; dark energy is a separate, repulsive effect responsible for the universe's expansion accelerating.

Key ideas

Requires: Newtonian gravitation

Common misconceptions

  • Dark matter is just ordinary matter that happens to be dark or hard to see — dust, planets, dim stars, black holes — rather than a form of matter that, as far as every search has found, does not interact electromagnetically at all; searches for enough non-luminous ordinary matter to explain the missing mass have come up far short.
  • Flat rotation curves mean the galaxy's outer stars are simply dragged along at a fixed speed by the visible disk, as though it were a rigid solid body, rather than each star's orbital speed being set — as in any Keplerian orbit — by the total gravitating mass enclosed within its own orbit, which keeps growing with radius because of the extended dark matter halo.
  • Dark matter and dark energy are the same thing, or interchangeable terms for 'the unexplained stuff,' rather than dark matter being extra gravitating mass that clumps into haloes and slows the universe's expansion, while dark energy is a repulsive effect driving the expansion's acceleration.

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Part of these UT courses

This concept is mapped as covering part of the following course(s) in the University of Tartu curriculum.