Our physics color commentator Rob Sheldon kindly writes to offer his view of the recent claims around dark matter:
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For those who want more than headlines, here’s the dark matter scoop.
For the last 30 years or so, we have been told that the best model of the beginning of the universe is the Big Bang model with several “improvements”. 1) Inflation was tacked to the very very beginning, 2) Dark Matter to the very beginning, and 3) Dark Energy to the very ending.
1) Inflation was invented to explain why the speed of the explosion (kinetic energy) perfectly matches gravity’s pull (gravitation potential energy) to 60 decimal places.
2) Dark Matter was added, at first just to balance the spinning of galaxies and keep their stars from being flung out, but later was given the added job of providing the lumpiness to birth the galaxies out of butter-smooth matter.
3) Dark Energy was added much much later to explain the vast bubbles of nothingness between the galaxy clusters. The 2011 Nobel prize for Dark Energy was some handwaving concerning 75 unusually dim supernovae, but has since been challenged by an improved data-set of over 1000 supernovae.
The term for 2+3 is “Lambda-CDM” where Lambda (an upside-down V) represents Einstein’s “cosmological constant” or “dark energy”. CDM stands for “cold dark matter”, as distinguished from “hot dark matter”, meaning the dark matter is cold enough to make lumps, whereas hot DM doesn’t make lumps.
In brief, inflation smooths out the universe, DM makes it lumpy again, and DE unlumps it by adding bubbles. If you notice how each dial undoes the previous one, you are starting to see the problem with the “Lambda-CDM” model.
This latest result attempts to measure the “lumpiness” of our Milky Way galaxy by comparing pulsar signals with models. Pulsars are spinning stars that send out a beacon of radio waves like a lighthouse, and the arrival times can be monitored to parts per billion. This makes them extremely sensitive to accelerations and/or bending of the radio waves by DM lumps as they arrive at Earth. By comparing modelled beacons to actual beacons, one can see which “lumpiness” models work best.
And the answer?
None of them are performing particularly well. Those that haven’t been disproven yet are the ones that are the least lumpy. Let’s just say that the data are consistent with there being no dark matter lumps at all. Which is not the same thing at all as saying the Milky Way has less dark matter than thought because that is determined by the spinning galaxies, rather, what has been determined is that the Milky Way is not as lumpy as modelled.
This suits me just fine, because I think we already have found the dark matter. It’s called “comets”.
Rob Sheldon is the author of Genesis: The Long Ascent babd The Long Ascent, Volume II
See also: Researchers: Less dark matter than thought? What if there isn’t any? Maybe we are pursing a phantom? Just a thought.