Comments on Dark Matter and Dark Energy

A reader of my article Big bang fudge factors wrote the following comments:

Dark matter has been detected: neutrinos fit the definition of Weakly Interacting Massive Particles, as they have such small probabilities of interacting with atomic matter that it takes several moles of neutrinos to achieve the same probability of a single interaction as a single neutron or photon. Though individually nearly massless and invisible to matter, the sheer number of neutrinos surrounding us makes it possible to detect them, and makes their combined energy a significant component of the mass of the Universe.

Likewise, Massive Compact Halo Objects are quite ordinary matter. They are planetary and sub-planetary bodies, producing little or no light, and so hard to detect. To these, we add black holes, neutron stars, and brown dwarfs, which also emit little or no light, despite their mass.

Neither should the existence of dark energy be any surprise to Christians. After all, the Bible say, “The heavens are stretched out like a curtain.” Dark energy is the energy of the vacuum state, less than 1 microjoule per cubic meter, distributed uniformly. Only because of the vastness of space are we able to observe its effects. Even so, were this tremendous amount of energy somehow liberated, “the elements shall be consumed by fire.” The decay of the vacuum state, unleashing the tremendous amount of energy stored in it, could very well be the means by which the Lord transforms the Universe at the end of the age.

My responses are below.

On neutrinos

Neutrinos have been considered as a candidate dark matter particle. But because of their extremely small mass (< 1 eV), the three known neutrinos travel near the speed of light and are thus too “hot” (i.e. have too much energy) to comprise Cold Dark Matter (CDM). Cosmologists and astrophysicists need a cold dense form of matter that does not interact with electromagnetic energy but which can form into a halo around galaxies and in clusters to hold them together. It would be simply false to state categorically that neutrinos are cold dark matter. There is another alleged neutrino — the putative sterile neutrino — which some believe could be the source of dark radiation, but still that would not be dark matter. See Dark radiation in big bang cosmology. Besides it is more unobserved stuff from the dark sector of particle physics. See also Dark matter and the standard model of particle physics — a search in the dark.

On MACHOs

Very few were ever detected by the MACHO survey. On their own website they write:

“Our primary aim is to test the hypothesis that a significant fraction of the dark matter in the halo of the Milky Way is made up of objects like brown dwarfs or planets: these objects have come to be known as MACHOs, for MAssive Compact Halo Objects. The signature of these objects is the occasional amplification of the light from extragalactic stars by the gravitational lens effect. … We have taken ~27,000 images with this system since June 1992. Analysis of a subset of these data has yielded databases containing light curves in two colors for 8 million stars in the LMC and 10 million in the bulge of the Milky Way. A search for microlensing has turned up four candidates toward the Large Magellanic Cloud and 45 toward the Galactic Bulge.” (emphases added)

I personally know the leader of the MACHO project and he told me that MACHOs are no longer expected to comprise much of the dark matter content of the Galaxy.

On dark energy

The calculation of the vacuum energy content of the universe and that from big bang cosmology are different by 120 orders of magnitude. This is called the cosmological constant problem. It is the biggest discrepancy ever encountered in physics when theory was compared with an observational/experimental result. There currently is no agreement what dark energy is. Some think it is an evolving scalar field, others believe it to be a universal constant. But no one knows what it is. It behaves like nothing ever experienced in physics. It is only derived from cosmology and has no experimental laboratory support.

Some big bang cosmologists have shown that it is not even necessary if you allow for a non-uniform density of matter as a function of epoch in the universe. You can still have an accelerating expansion but dark energy is not needed. The Royal Astronomical Society press release states:

“The researchers believe that standard models of the universe fail to take account of its changing structure, but that once this is done the need for dark energy disappears.” (emphasis added)

Others do away with both acceleration and dark energy.

A recent study by Subir Sarkar of Oxford University (and I am sure he is a big bang believer) found that if you use all the currently available data no acceleration of the expansion of the universe is needed. Hence there is no need to assume dark energy. They used type Ia supernova data more than ten times the original sample size. They found that the evidence for acceleration may be flimsier than previously thought, with the data being consistent with a constant rate of expansion of the universe.  For more information on this see Now the expansion of the universe is not accelerating.

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