Thursday, February 21, 2019

Distinguishing Between Dark Matter And Dark Energy

Sabine Hossenfelder does a good job of succinctly distinguishing between the properties of dark matter and dark energy in the lambdaCDM model, and also of explaining why the term "dark" in this context really means "transparent."

In particular, they differ in how they behave when contained in expanding volumes of space-time
Matter has specific behavior, which is that its energy-density dilutes with the inverse volume. Energy-density of radiation, in contrast, dilutes faster than the inverse volume, because the wavelengths of the radiation also stretch. . . . Dark Energy isn’t energy either. Instead, if you divide it by Newton’s constant, you get an energy density. In contrast to Dark Matter, however, this energy-density does not dilute with the inverse volume. Instead, it doesn’t dilute at all if the volume increases, at least not noticeably.
If the energy density remains entirely constant with the increase of volume, it’s called the “cosmological constant.” General types of Dark Energy can have a density that changes with time (or location), but we currently do not have any evidence that this is the case. The cosmological constant, for now, does just fine to explain observations.
With regard to the terminology "dark" which really means "transparent":
Dark Matter isn’t dark. “Dark” suggests that it adsorbs light, but really it doesn’t react with light at all. It would be better to call it transparent. Light just goes through. And in return, Dark Matter just goes through all normal matter, including planet Earth and you and me. Dark Matter interacts even less often than the already elusive neutrinos. . . . Dark Energy, too, is transparent rather than dark.
What phenomena do dark matter and dark energy, respectively, seek to explain?
Dark matter is what makes galaxies rotate faster and helps galactic structure formation to get started. . . . Dark Energy is what makes the expansion of the universe speed up.
It is also worth noting that the leading explanation for dark matter phenomena, which is used in the lambdaCDM Standard Model of Cosmology, is a beyond the Standard Model particle, with gravity modification to explain this phenomena serving as the insurgent alternative theory.

But, with respect to dark energy, the dominant lambdaCDM model is the cosmological constant (also known as "lambda") which is a modification of the equations of general relativity that explain gravity. The alternative and insurgent explanations of "dark energy" are substance based (such as "quintessence"). 

1 comment:

neo said...

Of course, some of the effects of dark matter could be due to MOND, and some theories that explain MOND observe it is on the same order as dark energy suggesting that MOND is a dark energy effect.

if correct, then there's only dark energy, with dark matter a product of MOND,