Earth’s Dark Matter Sabers

hubble_lightsaber_image2.jpgOrion B Molecular Cloud Complex, a stellar nursery. Credits: NASA/ESA

This week NASA released a celestial light saber photo. It’s a stunning visualization of a ‘light saber’ apparition in the Orion B Molecular Cloud Complex. This is the universe of ordinary matter.

But in the world of dark matter which cohabits with ordinary matter in our universe, there are saber-like objects as well. In an earlier blog we talked about dark matter filaments and walls on the very large scale, at the scale of superclusters of galaxies and above. On the small scale, dark matter particles also align into structures as well.

Dark matter is a ‘collisionless’ fluid. That is, it interacts so rarely with either ordinary matter or itself, that it doesn’t thermalize and dissipate energy via radiation as does ordinary matter. It interacts through gravity alone, both with itself and with ordinary matter, but that can lead to structure as well, even at the very small scale. Regions of very enhanced density (gravitational clumping) are known as caustics.

If there were to be caustics in our solar system – regions of enhanced dark matter density – then it could ease the direct detection search for dark matter. There are many direct detection experiments underway on Earth, mostly with negative results, although for a number of years the DAMA/LIBRA experimenters have claimed direct detection; these results are highly disputed.

Our best understanding of dark matter is that it forms “fine-grained streams” or clumps that move together at the same velocity. These streams can be quite large and many streams should be found in our galactic neighborhood.

Now Gary Prézeau of NASA-JPL has noted that compact bodies, such as the Sun and planets, should act as lenses of sorts, focusing dark matter streams into strands of higher density. He refers to these as ‘dark matter hairs’ and calculates where the roots should lie relative to the center of the Sun or the center of the Earth or other planets.

In the case of the Earth the root of the ‘hair’ would be around 1 million kilometers behind the Earth – behind in this case meaning relative to our orbital motion around the galaxy’s center. The density enhancement might be as large as 1 billion times the normal density. In the solar neighborhood that average normal density is estimated to be 1/3 of a proton mass per cubic centimeter, so it’s not surprising that direct detection of dark matter is so difficult. But at hundreds of millions of proton masses per cc, or some tens of millions of dark matter particles per cc (depending on the dark matter mass) it could be much more feasible.

PIA20176_ipCredit: NASA/JPL – CalTech

I prefer to think of these as dark matter sabers emanating from the Earth.

It turns out that the James Webb Space Telescope, the successor to Hubble, will be placed at one of the Lagrange points (L2) in the Sun-Earth gravitational system, about 1.5 million kilometers out. Perhaps this would be a good place to put a dark matter detection experiment. The earthbound ones are getting to be quite massive, but with the potentially much greater sensitivity, a small scale detector could be quite effective. At the L2 point it would be aligned with the Sun’s orbit around our galaxy each year around the first of June. That could be the best time for detection, but there could be other hairs as well with density enhancements in the millions, and detectable at other times of the year.

References:

http://www.forbes.com/sites/startswithabang/2015/11/24/strange-but-true-dark-matter-grows-hair-around-stars-and-planets/ – Ethan Siegel article

http://www.jpl.nasa.gov/news/news.php?feature=4774 – NASA release on possible ‘dark matter hairs’ around Earth

http://arxiv.org/abs/1507.07009 – Gary Prezeau, 2015. “Dense Dark Matter Hairs Spreading out from Earth,  Jupiter, and other Compact Bodies”

http://darkmatterdarkenergy.com/2015/11/27/dark-matter-filaments-and-walls/ – dark matter structure at the very large scale


Eternal Inflation and the Multiverse

miniuniverses

Figure 1 from Andrei Linde’s paper “Brief History of the Multiverse”. Each blob represents a pocket universe, occupying a different region of space, and being born at a different time during eternal inflation. A particular pocket universe may be connected to its parent by some sort of bridge, or that connection may have broken or decayed. Different pocket universes will have different physics.

“Inflationary cosmology therefore suggests that, even though the observed universe is incredibly large, it is only an infinitesimal fraction of the entire universe” states Alan Guth, the original father of the inflationary Big Bang, in his article from 2007, “Eternal inflation and its implications”.

Inflation is the very brief – yet extremely significant – period in our own universe’s history, perhaps of duration only a billionth of a trillionth of a trillionth of a second. During the inflation event, a very submicroscopic bubble of energy and space expanded tremendously, doubling in scale perhaps 100 times or more in each of the 3 spatial dimensions. That’s an increase in volume of around 90 factors of 10! This inflationary epoch drove the universe to become macroscopic in scale, and also to become highly homogeneous and topologically flat at large scales.

The inflationary Big Bang models solved a number of outstanding problems in cosmology, such as the horizon problem and the flatness problem. Basically at large scales we see a homogeneous and topologically flat universe in all directions. Without inflation, parts of the universe seen on opposite ends of the sky would never have been casually connected. However, with the inflation models, those regions were originally within each others’ casually connected ‘light cones’, prior to the inflation phase, before it pushed them out to much larger physical scale, at which point they become highly separated.

Andrei Linde is another one of the fathers of inflationary Big Bang theory, and the originator of the chaotic inflation models. Chaotic inflation, and another leading model, ‘new’ inflation, both appear to result in eternal inflation; this gives rise to the multiverse scenario. That is, inflation keeps going in most of space, while multiple universes form and separate from the inflation process.

The multiverse scenario states that our universe is only one of a very large number of universes, and in such a case, our particular universe may be referred to as a ‘mini-universe’ or ‘pocket universe’. Of course our universe is already enormously large, it’s just that the multiverse is giaganormously larger than that. With eternal inflation the multiverse keeps inflating in other regions, portions of which will later settle out into other ‘pocket universes’.

Linde has recently published a summary “A Brief History of the Multiverse” which describes the developments in inflationary Big Bang theory and models for the multiverse since 1982. I encourage those who are interested in multiverses to read his paper.

With this eternal inflation our universe was (most likely) not the first, it was just one of many and inflation has been going on for a very long time. Inflation would continue forever into the future. New mini-universes would continue to be spawned and settle out from the overall inflation. It appears that eternal inflation is not eternal into the past, however, just into the future (see Guth paper referenced below).

Each of these mini-universes could have different values of the fundamental physical parameters. This ties into string theory models which admit of a very large number of possibilities for physical parameters.

Some sets of these parameters are favorable to life, but many (most) would not be. In order to get life as we know it we need carbon and other heavy elements, formed in stars (and not during the Big Bang nucleosynthesis), and we need a long-lived mini-universe. Other mini-universes might have different values of dark matter and dark energy than in our own universe. This could lead to very short lifetimes with no chance to form galaxies and stars.

Sidebar: These models are motivated by string theory and inflationary cosmology. It makes more sense in this context to think of ‘mini-universes’ rather than ‘parallel universes’ that often get popularized in discussions of quantum physics e.g. the Many Worlds discussions. Sorry to break the news to you, but there is not another you in each of these other mini-universes, since, even though they are endless in number, they all have different physical conditions and different histories.

References

Guth, Alan 2007. “Eternal Inflation and its Implications” http://arxiv.org/abs/hep-th/0702178

Linde, Andrei 2015. “Brief History of the Multiverse” http://arxiv.org/pdf/1512.01203.pdf


Dark Matter Filaments (and Walls)

Dark matter is about 5 times as abundant as ordinary matter in the universe. We see its gravitational affects on large scales – within galaxies, in groups and clusters of galaxies, and in the overall way in which galaxies are spatially distributed on the largest scales.

Galaxies on the large scale are observed to be distributed in filaments and walls of excess galaxy density, along with voids of lower galaxy densities. These filaments and walls have been discovered during the past 30 years or so.

Since galaxies themselves are primarily composed of dark matter, it makes sense that the filaments and walls and voids are reflecting the large scale distribution of matter as well.

Basically, we can think of dark matter as the scaffolding upon which the ordinary matter clumps – the luminous matter seen from galaxies’ light emission and absorption and due to stars, gas, and dust within galaxies. And when we say light, we mean radiation at all frequencies including radio waves, infrared, visible, ultraviolet, X-rays and gamma rays. Radiation sources are due to ordinary matter, since dark matter couples very poorly to electromagnetic fields – that’s why it’s dark!

Nearsc

Map of our Neighborhood (you are in the center) http://www.atlasoftheuniverse.com/nearsc.htmlRichard Powell, CC-BY-SA-2.5 

So we know that filaments and walls that we see in the distribution of galaxies reflect (deliberate pun) the underlying distribution of the gravitationally dominant dark matter. It is the gravitational field of the dark matter which has controlled the clumping of ordinary matter into galaxies and clusters and superclusters of galaxies.

Here are some of the most important known filaments discovered over the past 3 decades:

  • the Coma Filament
  • the Perseus-Pegasus Filament
  • the Ursa Major Filament
  • the Lynx-Ursa Major Filament (LUM)
  • the CIG J2143-4423 Filament

Now these are mostly named after constellations, but of course they are behind the constellations and external to our Milky Way galaxy, and at large distances. The most distant known appears to be at redshift z = 2.38, which means that the light comes from a time when the universe was less than 3 billion years old. It has a linear scale of 350 million light-years.

Galaxy filaments include also structures known as walls. And here are the major walls that have been discovered:

  • the CfA2 Great Wall
  • the Sloan Great Wall
  • the Sculptor Wall
  • the Grus Wall
  • the Fornax Wall
  • the Hercules-Corona Borealis Great Wall

The 3-D perspective map above shows superclusters and voids in our neighborhood. It is centered on the Milky Way and extends out to 500 million light-years. Superclusters are highlighted in blue, and 3 walls are highlighted in yellow: the Coma Wall, the Centaurus Wall, and the Sculptor Wall. A number of Void regions are highlighted in red.

The CfA2 Great Wall (also known as the Coma Wall) extends from the Hercules Supercluster to the Coma Supercluster to the Leo Supercluster on the right hand side of the map. It was the first wall discovered, in 1989, by Margaret Geller and John Huchra of the Havard-Smithsonian Center for Astrophysics (CfA).

This Hercules-Corona Borealis Great Wall is much further away, at a redshift of around z = 2, which means that we are seeing it from a time when the universe was a little over 3 billion years old (more than 10 billion years ago). It has an enormous length of around 10 billion light-years. Which is incredible, since the comoving distance  to the Hercules-Corona Borealis Great Wall is 17 billion light-years.

Remember, when you look at these maps, you are also in effect seeing the distribution of the underlying dark matter.

In the next blog, we will talk about possible dark matter filaments on a much, much smaller scale, on the scale of the Sun and the planets within our Solar System, including Earth.

References:

https://en.wikipedia.org/wiki/Galaxy_filament

https://en.wikipedia.org/wiki/CfA2_Great_Wall


Dark Axion Stars

In a post from 4 years ago I discussed “Dark Matter Powered Stars”.

The context here was neutralino dark matter, which is a possible explanation for very massive stars in the early universe. The idea is that the very first stars could be thousands of solar masses, much greater than is possible with ordinary matter dominated stars. They would be powered by dark matter annihilation in their cores during the early part of their life. They would eventually collapse to black holes and could be candidates to seed supermassive black holes found at the center of many galaxies.

Sirius_A_and_B_Hubble_photo

Hubble Space Telescope image of Sirius A and Sirius B (lower left) 

NASA, ESA, H. Bond (STScI), and M. Barstow (University of Leicester)

Another dark matter candidate apart from the neutralino is the axion. While the neutralino is expected to have masses in the several to tens of GeV (Giga-electron-Volts), the axion mass is a tiny fraction of an eV, at least a trillion times smaller than the expected neutralino mass. So there would be many more of them, of course, to explain the amount of dark matter we detect gravitationally.

Neither neutralinos nor axions have been discovered to date. The axion does not require supersymmetry beyond the Standard Model of particle physics, so in that sense it is a more conservative proposed candidate.

Currently we detect dark matter only through its gravitational effects – in galaxies, in clusters of galaxies, and at the very large scale by looking at thermal variations in the cosmic microwave background radiation.

In addition there are three main direct methods to try to ‘see’ these elusive particles. One is to directly detect dark matter (e.g. neutralinos) here on Earth when it collides with ordinary matter – or in the case of axions – generates photons in the presence of a magnetic field. Another is to attempt to create it at the Large Hadron Collider, and the third is to look in space for astrophysical signals resulting from dark matter. These could include gamma rays produced in the galactic center when dark matter mutually annihilates.

In a paper recently published in the journal Physical Review Letters and titled “Accretion of dark matter by stars”, Richard Brito, Vitor Cardoso and Hirotada Okawa discuss a different kind of dark star, one whose dark matter component is axions. The paper is available here.

There are two formation scenarios envisaged. The first is that dark matter (axion) stellar cores form and then these accrete additional dark matter and ordinary matter. In the second scenario, a star forms primarily from ordinary matter, but then accretes a significant amount of dark matter.

We are talking about dark matter fractions which may be say 5% or 20% of the total mass of the star.

The authors find that stable configurations seem to be possible and that the axion dark matter may lead to stellar oscillations in the microwave band. So looking for stellar oscillations in the Gigahertz range may be another astrophysical detection method for dark matter. They intend to explore the idea more deeply in future research.


TAIPAN: A Million Galaxy Survey

Taipan is an ambitious survey planned for southern hemisphere galaxies, with the goal of mapping and measuring as many as one million galaxies in our Milky Way’s neighborhood. This will provide a deeper understanding of cosmology and galaxy evolution in the relatively nearby region of our universe.

There are more than a hundred billion galaxies in our visible universe. In order to refine our understanding of galaxies, their distribution and evolution, and of the overall cosmological properties of the universe, we want to sample a very large number of galaxies.

It is naturally easier to detect galaxies that are relatively nearby, and those that are more luminous.

Since the universe is expanding in an isotropic and homogeneous manner, galaxies are in general receding away from one another – in accordance with the Hubble relation below. The Taipan survey will explore our local neighborhood, with redshifts up to about 0.3.

For nearby galaxies,

V = cz = H*d

where V is the recession velocity, c is the speed of light, z is the redshift, H is the Hubble constant, and d is the galaxy’s distance. If we evaluate for z = 0.3 and the best estimate of the Hubble constant of 68 kilometers/second/Megaparsec, this implies a survey depth of 1300 Megaparsecs, or over 4 billion light-years.

The Taipan galaxy survey will begin next year and run for four years, using the UK Schmidt telescope, which is actually in Australia at the Siding Springs Observatory. Up to 150 galaxies in the field of view will be observed simultaneously with a fibre optic array. Of course the positions of galaxies is different in each field to be observed, so the fibers are robotically placed in the the proper positions. Many thousands of galaxies can thus be observed each night.

Short video of a Starbug fiber robot

One expected result will be refinement of the value of the Hubble constant, now uncertain to a few percent, reducing its uncertainty to only 1%.

The Taipan galaxy survey will also provide a better constraint on the growth rate of structure in the universe, decreasing the uncertainty down to about 5% for the low-redshift data points. This is a factor of 3 improvement and will provide a stricter test on general relativity.

The Taipan survey will also look at galaxies’ peculiar velocities, which are the deviations away from the general Hubble flow described in the equation above. These peculiar velocities reflect the details of the gravitational field – that is dominated by the distribution of dark matter primarily, and ordinary matter secondarily. On average galaxies are moving according to the Hubble equation, but in regions where the density of matter (dark and ordinary both) is higher than average they are pulled away from the Hubble flow toward any concentrations of matter. Bound galaxy groups and clusters form in such regions.

427249main_hicksoncompact31-labeled

The mapping of peculiar velocities and the details of local variations in the gravitation field will enable fundamental tests of gravity on large scales.

Another of the important areas that Taipan will explore is how galaxies evolve from young active star-forming blue galaxies to older reddish, less active galaxies. Ordinary matter cycles through stars and the interstellar medium of a given galaxy. As stars die they shed matter which ends up in molecular clouds that are the sites of new star formation. Taipan will help to increase our understanding of this cycle, and of galaxy aging in general. Star formation slows down as more and more gas is tied up in lower mass, longer-lived stars, and the recycling rate drops. It also can be quenched by active galactic nuclei events (AGN are powered by supermassive black holes found at galactic centers).

Taipan will be the definitive survey of galaxies in the southern hemisphere, and is expected to significantly add to our understanding of galaxy evolution and cosmology. We look forward to their early results beginning in 2016.

Reference:

taipan-survey.org


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.

Large_Underground_Xenon_detector_inside_watertank

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

https://en.wikipedia.org/wiki/Chameleon_particle – Chameleon particle

http://news.sciencemag.org/physics/2015/08/tiny-fountain-atoms-sparks-big-insights-dark-energy?rss=1 – dark energy experiment

http://www.preposterousuniverse.com/blog/2008/10/29/dark-photons/ – dark photons

http://scitechdaily.com/physicists-work-on-new-approach-to-detect-dark-matter/ – article on detecting dark matter generated dark radiation

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.231303 – 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.

EGSY8p7-a

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:

http://arxiv.org/pdf/1507.02679v2.pdf


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