Dark Matter Riches? Why Some Galaxies Have More Than Others (Synopsis)

"Motions of the stars tell you how much matter there is. They don’t care what form the matter is, they just tell you that it’s there." -Pieter van Dokkum

Everything in the Universe was born with the same ratios of dark matter to normal matter: approximately 5-to-1. It shows up in everything from the cosmic microwave background to galaxy clustering to internal motions of spiral and elliptical galaxies, including the Milky Way.

From simulations and inferred maps, dark matter (blue) may form some clumps, but overall exists in a massive, diffuse halo around the luminous, disk-like part of galaxies we're familiar with. Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI). From simulations and inferred maps, dark matter (blue) may form some clumps, but overall exists in a massive, diffuse halo around the luminous, disk-like part of galaxies we're familiar with. Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI).

But some galaxies, particularly dwarf galaxies and rapidly moving galaxies within clusters, show a significant enhancement of dark matter over what’s expected. Ratios of even hundreds-to-1 aren’t unheard of. This is a difficulty for theorists to explain, but it doesn’t mean that dark matter is wrong.

Dwarf galaxies, like the one imaged here, have a much greater than 5-to-1 dark matter to normal matter ratio, as bursts of star formation have expelled much of the normal matter. Image credit: ESO / Digitized Sky Survey 2. Dwarf galaxies, like the one imaged here, have a much greater than 5-to-1 dark matter to normal matter ratio, as bursts of star formation have expelled much of the normal matter. Image credit: ESO / Digitized Sky Survey 2.

On the contrary, it might mean that astrophysics, and how these galaxies evolve, explains exactly what the Universe gives us!

More like this

"Nothing in the standard cosmological model predicts this, and it is almost impossible to imagine how that model could be modified to explain it, without discarding the dark matter hypothesis completely." -David Merritt Dark matter is a hugely successful theory for explaining a whole slew of…
“Men occasionally stumble over the truth, but most of them pick themselves up and hurry off as if nothing ever happened.” -Winston Churchill When we look out at the Universe on the largest scales, from large-scale structure to the fluctuations in the CMB to lensed clusters of galaxies and to giant…
“[The black hole] teaches us that space can be crumpled like a piece of paper into an infinitesimal dot, that time can be extinguished like a blown-out flame, and that the laws of physics that we regard as ‘sacred,’ as immutable, are anything but.” -John Wheeler Dark matter is one of the biggest…
"All we know so far is what doesn't work." -Richard Feynman The past month has seen a slew of papers out highlighting the tension between modified gravity and dark matter. Both recognize the same puzzles and problems with the Universe, and both ideas recognize that either one could be valid. In…

If the normal matter was stripped away by (gas) ram pressure, it must have happened before many stars were formed, otherwise a decent faction of the mass would have been in the forms of stars,which aren't stripped out by fast travel through the intergalactic medium. So I think this new one is still an enigma.

By Omega Centauri (not verified) on 29 Aug 2016 #permalink

Re: Your article on the three scarce light elements.

Your description of the origin of Boron by spallation got me to thinking about the two main sources of boron on earth, Turkey and California.

If the early earth was struck by a cosmic ray stream big enough to penetrate and have an entry and exit point, on opposite sides of the earth, would that be a likely scenario for borons relative abundance and geographical localization ?

I cannot get credible scientific authorities to even examine this idea heuristically. You seem to be open to outside ideas and have credentials which would give the elaboration of this concept some heft.

What about it ?

Please let me know. Otherwise direct me to another font. I really don't want to ask this question on the "Quora" web site.

Bill Murphy
Retired Thoracic Surgeon
Physics major who studied under Clyde Cowan

By William Murphy (not verified) on 29 Aug 2016 #permalink

@William Murphy #2: No, it wouldn't work, and you should be able to work out for yourself why not. The most important word is "flux," the rate of incoming particles per unit area per second. The second most important word is "product," the number of atoms of boron (in this case) you want to produce.

For simplicity, you can assume perfect (100%) efficiency: every single incoming cosmic ray produces one boron atom. In reality, of course, the efficiency for an single species is tiny, as spallation produces a broad spectrum.

Start with an estimate of the total number of atoms of boron in each region, the approximate area of each region, and from those, work out what the flux would have to be. Also, estimate how _fast_ that flux would have to be delivered in order to produce all the boron in a limited area, before the Earth's rotation spread that area out too much.

This is an excellent homework problem requiring nothing but arithmetic, and an ability to do (or to look up) reasonable order of magnitude estimates. It's called a "Fermi problem," and I assign such problems to physics students all the time.

By Michael Kelsey (not verified) on 29 Aug 2016 #permalink

Once upon a time I had the notion that 5 additional branes, each with a Universe matching our own in mass-energy, (each with a unique set of particles/fields, but equivalent to ours from the perspective of observers made of the particular matter), was the source of Dark Matter. The hypothesis had a simple permutation principle to account for six, and only six, brane-Universes, thus explaining the 5 to 1 ratio of Dark Matter to Normal Matter in our Universe - the same applying to all the other 5 Universes.

But, alas, I read someplace that matter sequestered on another brane, or branes, was ruled out for some reason or another, as the source of Dark Matter.

By David Schroeder (not verified) on 05 Sep 2016 #permalink