Monthly Archives: August 2015

Dark Sector Experiments

A dark energy experiment was recently searching for a so-called scalar “chameleon field”. Chameleon particles could be an explanation for dark energy. They would have to make the field strength vanishingly small when they are in regions of significant matter density, coupling to matter more weakly than does gravity. But in low-density regions, say between the galaxies, the chameleon particle would exert a long range force.

Chameleons can decay to photons, so that provides a way to detect them, if they actually exist.

Chameleon particles were originally suggested by Justin Khoury of the University of Pennsylvania and another physicist around 2003. Now Khoury and Holger Muller and collaborators at UC Berkeley have performed an experiment which pushed millions of cesium atoms toward an aluminum sphere in a vacuum chamber. By changing the orientation in which the experiment is performed, the researchers can correct for the effects of gravity and compare the putative chameleon field strength to gravity.

If there were a chameleon field, then the cesium atoms should accelerate at different rates depending on the orientation, but no difference was found. The level of precision of this experiment is such that only chameleons that interact very strongly with matter have been ruled out. The team is looking to increase the precision of the experiment by additional orders of magnitude.

For now the simplest explanation for dark energy is the cosmological constant (or energy of the vacuum) as Einstein proposed almost 100 years ago.


The Large Underground Xenon experiment to detect dark matter (CC BY 3.0)

Dark matter search broadens

“Dark radiation” has been hypothesized for some time by some physicists. In this scenario there would be a “dark electromagnetic” force and dark matter particles could annihilate into dark photons or other dark sector particles when two dark matter particles collide with one another. This would happen infrequently, since dark matter is much more diffusely distributed than ordinary matter.

Ordinary matter clumps since it undergoes frictional and ordinary radiation processes, emitting photons. This allows it to cool it off and to become more dense under mutual gravitational forces. Dark matter rarely decays or interacts, and does not interact electromagnetically, thus no friction or ordinary radiation occurs. Essentially dark matter helps ordinary matter clump together initially since it dominates on the large scales, but on small scales ordinary matter will be dominant in certain regions. Thus the density of dark matter in the solar system is very low.

Earthbound dark matter detectors have focused on direct interaction of dark matter with atomic nuclei for the signal. John Cherry and co-authors have suggested that dark matter may not interact directly, but rather it first annihilates to light particles, which then scatter on the atomic nuclei used as targets in the direct detection experiments.

So in this scenario dark matter particles annihilate when they encounter each other, producing dark radiation, and then the dark radiation can be detected by currently existing direct detection experiments. If this is the main channel for detection, then much lower mass dark matter particles can be observed, down to of order 10 MeV (million electron-Volts), whereas current direct detection is focused on masses of several GeV (billion electron-Volts) to 100 GeV or more. (The proton rest mass is about 1 GeV)

A Nobel Prize awaits, most likely, the first unambiguous direct detection of either dark matter, or dark energy, if it is even possible.

References – Chameleon particle – dark energy experiment – dark photons – article on detecting dark matter generated dark radiation – Cherry et al. paper in Physical Review Letters


Most Distant Galaxy Known: over 95% of the way back to the origin

Recently, a team of astronomers from the U.S., U.K. and The Netherlands have confirmed the most distant galaxy known. This galaxy had previously been estimated to have a redshift of z = 8.57, from photometric methods, that is, from the general shape of the spectrum.


Image: Hubble Space Telescope, NASA/STScI

More accurate redshifts are obtained by measuring particular emission or absorption lines, which have precisely known laboratory (z = 0) wavelengths.

The team measured Lyman alpha line emission, and have determined the redshift to be z = 8.68, in good agreement with the photometric redshift. The Lyman alpha line is a main transition line in neutral hydrogen that occurs at 1216 Angstroms (.1216 microns) in the rest frame. The authors observed the line in the infrared and centered at 11,776 Angstroms (1.1776 microns) on 2 separate observing nights, detecting the Lyman alpha line each night. The redshift is given by 1 + z = 11,776/1216 = 9.68, thus z for this galaxy is 8.68.

The galaxy image is thought to be somewhat magnified by intervening dark matter gravitational lensing, but less than a factor of 2, and perhaps only around 20%.

The significance here is in the detection of Lyman alpha at such a high redshift, corresponding to a time when the universe was only 600 million years old, less than 5% of its current age. Not only does this result determine the age of this earliest known galaxy, but it also provides insight into the nature of the intergalactic medium.

The cosmic microwave background radiation is the most distant source we can see. It comes from all directions, filling the universe and reflects a time when the universe was only 380,000 years old and transitioned from ionized plasma to neutral hydrogen and helium.

Later on in the universe’s evolution, as the first galaxies and stars form, hot blue stars produce ionizing ultraviolet radiation, and the neutral gas is reionized – electrons are stripped from their atoms. This process has generally thought to have completed by redshift ~ 6, at a time when the universe was around 1 billion years old.

Lyman alpha emission is not expected in a region which is still neutral, that has not yet undergone the reionization process. So the implication here is that the surrounding intergalactic medium in the neighborhood of EGSY8p7 has already been reionized at a significantly higher redshift.

The universe does not become reionized in a uniform way, rather the process would be expected to happen in “bubbles” or regions surrounding energetic galaxies with hot blue stellar populations. Eventually all the ionized regions overlap and the intergalactic medium becomes fully ionized.

This detection helps astronomers to better understand how reionization occurred.

The team’s paper is submitted to the Astrophysical Journal Letters and can be found here: