Black Holes Destroy Dark Matter (and Emit Gamma Rays)

Black holes can cause dark matter to annihilate in their vicinity by concentrating the dark matter and enhancing the collision rate between dark matter particles. The best observational candidates are supermassive black holes, such as the 4 million solar mass black hole found at the center of our Milky Way galaxy. Some galaxies have much larger supermassive black holes, reaching as high as several billion or even tens of billions of solar masses. Most massive galaxies appear to have supermassive black holes in their centers.

Artist's conception of a supermassive black hole (public domain; courtesy NASA JPL)

Artist’s conception of a supermassive black hole (public domain; courtesy NASA JPL)

We infer the existence of supermassive black holes through their effect on nearby stellar or molecular cloud orbits. And we more directly detect supermassive black holes (SMBHs) by the radiation emitted from ordinary matter that is near the black hole (BH), but has not yet fallen into the BH’s event horizon (from which nothing, not even light, can escape). Such matter will often form a hot accretion disk around the SMBH. The disk or other infalling matter can be heated to millions of degrees by the strong gravitational potential of the BH as the kinetic energy of infall is converted to thermal energy by frictional processes. Ordinary matter (OM) heated to such high temperatures will give off X-rays.

Now if OM is being pulled into a SMBH, so is dark matter, which pervades every galaxy. Dark matter (DM) responds to the same gravitational potential from the SMBH. The difference is that OM is collisional since it easily interacts with other OM via the electromagnetic force, whereas DM is generally collisionless, since it does not interact via electromagnetism.

Nevertheless DM – DM collisions can occur, rarely, via a ‘direct hit’ (as if two bullets hit each other in mid-air) and this leads to annihilation. Two DM particles meet directly and their entire energy content, from their rest mass as well as their kinetic energy of motion, is converted into other particles. The cross-section strength is not known, but it must be small due to observational limits, yet is expected to be non-zero. The most likely candidates for decay products are expected to be photons, neutrinos, and electrons.

The leading candidate for DM is some sort of weakly interacting massive particle with a mass of perhaps 5 to 300 GeV; this is the range where DM searches from satellites and on Earth are focused. (The proton mass is a little less than 1 GeV = billion electron Volts.) So if two DM particles mutually annihilate, there is of order 10 GeV to 600 GeV of available rest mass energy to produce highly energetic gamma rays.

The likelihood of a direct hit is proportional to the square of the density of the DM. A SMBH’s gravitational potential acts as a concentrator for DM, allowing the density to be high enough that there could be a significant number of annihilation events, resulting in a detectable flux of escaping photons reaching Earth. Relativistic effects work to further increase the annihilation rate. And it is possible that the annihilation signal could scale as M³ (mass of the SMBH cubed), and thus the most massive SMBHs would be very strong gamma ray emitters. These would be highly energetic gamma rays with well over 1 GeV of energy.

Movie from NASA Goddard showing Dr. Jeremy Schnittman’s simulation

Dr. Jeremy Schnittman of the NASA Goddard Space Flight Center has investigated possible annihilation rates and the nature of the observable gamma ray spectrum for some simple dark matter models. He used a compute cluster to simulate hundreds of millions of DM particles moving in the general direction of a SMBH. One of his remarkable findings is that much higher gamma ray energies can be produced than previously believed, in the case of SMBHs which are rapidly spinning.

This is a result of something known as the Penrose process, which allows energy to be extracted from a rotating BH. There is a region called the ergosphere outside of the event horizon and when two DM particles annihilate in this region and produce two gamma rays, one gamma ray photon would fall into the event horizon (into the BH), and the other photon would escape to infinity, possibly in the direction of Earth. Dr. Schnittman’s simulation indicates that the energy boost can be as high as 6 times or more. The faster the SMBH is spinning, the greater the potential energy boost.

He also has looked at DM particles on bound orbits, which are likely to form into a (donut-shaped) corotating torus around the SMBH, aligned with its spin vector. The bound DM particle annihilations lead to lower energy gamma ray production, as compared to the unbound particles.

One of the important considerations is that the influence radius of the BH is very large. The size of the BH itself (event horizon or Schwarzschild radius) is small, even for SMBHs. The radius is proportional to the mass, via the relation 2GM/R = c² (G is the gravitational constant, c the speed of light and M and R are the BH mass and radius, respectively). A SMBH with a mass of 10 million solar  masses will have a radius of only around 30 million kilometers, or about 1/5 of the Earth-Sun distance (an AU, or astronomical unit).

But the gravitational influence is much greater, since DM particles are typically expected to be moving at around only a couple of hundred kilometers per second far away from the SMBH. Thus DM particles that are 1 million times further away than the SMBH will have their orbits in their galaxy perturbed by the SMBH. And the scale of influence is thus parsecs (1 parsec = 3.26 light-years) or tens of parsecs or even hundreds of parsecs, depending on the SMBH mass.

The most energetic gamma rays can be produced by unbound DM particles. These are on orbits which can approach near to the SMBH after falling from far away (a “swan dive” toward the SMBH) and these DM particles would then typically head out away from the SMBH in the opposite direction. But before they are able to, they have a direct hit with another DM particle and annihilate into gamma rays or some other decay products.

The search for gamma rays from annihilating DM around SMBHs is already underway. There is in fact a possible detection by the Fermi telescope at 130 GeV in our Milky Way galaxy, from the direction of the Sagittarius A* SMBH. Future more sensitive gamma ray surveys may lead to many detections, helping us to better understand both dark matter and black holes.

References

J.D. Schnittman, 2015. “The Distribution and Annihilattion of Dark Matter around Black Holes”, http://arxiv.org/abs/1506.06728

J.D. Schnittman, 2014. Phys. Rev. Letters 113, 261102,  “Revised Upper Limit to Energy Extraction from a Kerr Black Hole”


Dark Lenses Magnify Star Formation in Dusty Galaxies

Dusty star-forming galaxies (DSFGs) are found in abundance in the early universe. They are especially bright because they are experiencing a large burst of high-rate star formation. Since they are mainly at higher redshifts, we are seeing them well in the past; the high star formation rates occur typically during the early life of a galaxy.

The optical light from new and existing stars in such galaxies is heavily absorbed by interstellar dust interior to the galaxy. The dust is quite cold, normally well below 100 Kelvins. It reradiates the absorbed energy thermally at low temperatures. As a result the galaxy becomes bright in the infrared and far infrared portions of the spectrum.

Dark matter has two roles here. First of all, each dusty star-forming galaxy would have formed from a “halo” dominated by dark matter. Secondly, dark matter lenses magnify the DSFGs significantly, allowing us to observe them and get decent measurements in the first place.

An international team of 27 astronomers has observed half a dozen DSFGs at 3.6 micron and 4.5 micron infrared wavelengths with the space-borne Spitzer telescope. These objects were originally identified at far infrared wavelengths with the Herschel telescope. Combining the infrared and far infrared measurements allows the researchers to determine the galaxy stellar masses and the star formation rates.

The six DSFGs observed by the team have redshifts ranging from 1.0 to 3.3 (corresponding to  look back times of roughly 8 to 12 billion years). Each of the 6 DSFGs has been magnified by “Einstein” lenses. The lensing effect is due to intervening foreground galaxies, which are also dominated by dark matter, and thus possessing sufficient gravitational fields that are able to significantly deflect and magnify the DSFG images. Each of the 6 DSFGs is therefore magnified by a lens that is mostly dark.

The lenses can result in the images of the DSFGs appearing as ring-shaped or arc-shaped. Multiple images are also possible. The magnification factors are quite large, ranging from a factor of 4 to a factor of more than 16 times. (Without dark matter’s contribution the magnification would be very much less).

It is a delicate process to subtract out the foreground galaxy, which is much brighter. The authors build a model for the foreground galaxy light profile and gravitational lensing effect in each case. They remove the light from the foreground galaxy computationally in order to reveal the residual light from the background DSFG. And they calculate the magnification factors so that they can determine the intrinsic luminosity of the DSFGs.

The stellar masses for these 6 DSFGs are found to be in the range of 80 to 400 billion solar masses, and their star formation rates are in the range of 100 to 500 solar masses per year.

One of the 6 galaxies, nicknamed HLock12, is shown in the Spitzer infrared image below, along with the foreground galaxy. The model of the foreground galaxy is subtracted out, such that in the rightmost panes, the DSFG image is more apparent. There are two rows of images, the top row shows measurements at 3.6 microns, and the bottom row is for observations at 4.5 microns.

This particular DSFG among the six was found to have a stellar mass of 300 billion solar masses and a total mass in dust of 3 billion solar masses. So the dust component is just about 1% of the stellar component. The estimated star formation rate is 500 solar masses per year, which is hundreds of times larger than the current star formation rate in our own Milky Way galaxy.

It is only because of the significant magnification through gravitational lensing (“dark lenses”) that researchers are able to obtain good measurements of these DSFGs. This lensing due to intervening dark matter allows astronomers to advance our understanding of galaxy formation and early evolution, much more quickly than would otherwise be possible.

HLock12

The figure 6 is from the paper referenced below. The top row shows (a) a Hubble telescope image of the field in the near infrared at 1.1 microns, and (b) the field at 3.6 microns from the Spitzer telescope. The arc is quite visible in the Hubble image in the upper right quadrant just adjacent to the foreground galaxy in the center. The model for the foreground galaxy is in column (c) and after subtraction the background galaxy image is in column (d), along with several other faint objects. The corresponding images in the bottom row are from Spitzer observations at 4.5 microns.

Reference

B. Ma et al. 2015, “Spitzer Imaging of Strongly-lensed Herschel-selected Dusty Star Forming Galaxies” http://arxiv.org/pdf/1504.05254v3.pdf


Dusty Star-Forming Galaxies Brightened by Dark Matter

The first galaxies were formed within the first billion years of the Universe’s history. Our Milky Way galaxy contains very old stars with ages indicating formation around 500 or 600 million years after the Big Bang.

Astronomers are very eager to study galaxies in the early universe, in order to understand galaxy formation and evolution. They can do this by looking at the most distant galaxies. With the expanding universe of the Big Bang, the farther away a galaxy is, the farther back in time we are looking. Astronomers often use redshift to measure the distance, and hence age, of a galaxy. The larger the redshift, z, the farther back in time, and the closer to a galaxy’s birth and the universe’s birth.

The interstellar medium of a galaxy consists of gas and dust. The gas can be hot or cold, and in atomic or molecular form. Atomic gas may be ionized by ultraviolet starlight, or X-radiation from neutron stars or black holes (not the black holes themselves, but hot matter near the black hole), from cosmic rays or from other astrophysical mechanisms. Our Milky Way galaxy is rich in gas and dust, and contains thousands of molecular clouds. These are very cold clouds composed mainly of molecular hydrogen but also many other molecular species. Molecular clouds are the primary sites of new star formation. The Horsehead Nebula is an example of a molecular cloud in the constellation of Orion.

"Hubble Sees a Horsehead of a Different Color" by ESA/Hubble. Licensed under CC BY 3.0 via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Hubble_Sees_a_Horsehead_of_a_Different_Color.jpg#/media/File:Hubble_Sees_a_Horsehead_of_a_Different_Color.jpg

“Hubble Sees a Horsehead of a Different Color” by ESA/Hubble. Licensed under CC BY 3.0 via Wikimedia Commons 

During their most active phase of star formation, a large galaxy might give birth to over 1000 solar masses worth of stars per year. By comparison, in the Milky Way galaxy, the new star formation rate is only of order 1 solar mass per year, the equivalent of 1 Sun, or, say, 2 stars with half the mass of our Sun, per annum. Over its entire 13 billion year life the Milky Way has formed many hundreds of billions of stars, so clearly the star formation rate was higher in the past.

Before the first stars and galaxies form, the universe contains only hydrogen and helium, and no heavier elements. Those are produced by thermonuclear reactions in stellar interiors. This is a wonderful thing, because carbon, oxygen and other heavy elements are essential to life.

After a galaxy produces its first generation of massive stars, its interstellar medium will begin to contain carbon, nitrogen, oxygen and other heavy elements (heavy means anything above helium, in this context). Massive stars (above a few solar masses) evolve rapidly, with timescales in the millions of years, rather than billions, and explode as supernovae at the end of their lives. A large portion of their material, now containing heavy elements as well as hydrogen and helium, is expelled at high velocity and mixed into the interstellar medium. The carbon, nitrogen and oxygen which is then in the respective galaxy’s interstellar medium can be detected in atomic (including ionized) or molecular forms. The relative abundance of heavy elements grows with time as more stars are formed, evolve, and recycle matter into the interstellar medium.

High-redshift (z > 2) galaxies with active star formation are best observed in the infrared. The gas and dust in molecular clouds is quite cold, usually less than 100 K (100 degrees above absolute zero). And their radiation is shifted further toward the far infrared and sub-millimeter portions of the spectrum by the redshift factor of 1+z. So radiation emitted at 100 microns is detected at the Earth at 400 microns for a source at z = 3.

These are difficult measurements to make, because if the galaxy is very distant, it is also very faint. However the possibility of getting good measurements is helped by two things. One is that galaxies with very active star formation are intrinsically brighter.

And the other reason is that intervening clusters of galaxies are massive and contain mostly dark matter. As we look far back through the universe toward an early galaxy, there is a good chance that the line of sight passes through a cluster of galaxies. Clusters of galaxies contain hundreds or even thousands of galaxies, and are dominated by dark matter. Most of the infrared radiation can pass through the intracluster medium – the space between galaxies – without being absorbed; it does not interact with dark matter. The clusters are sufficiently massive to bend the light, however, according to general relativity. As the background galaxy’s light passes through the cluster during its multi-billion year journey to the Earth and our telescopes, the cluster’s gravitational potential modifies the light ray’s path. Actually the intervening cluster of galaxies does more than displace the light, it acts as a lens, causing the image to brighten by as much as 10 times or more. This makes it much easier to gather enough photons from the target galaxy to obtain good quality results.

An international research team with participants from Germany, the U.S., Chile, the U.K. and Canada has identified 20 high redshift “dusty star forming galaxies” at very high redshift (DSFG is a technical term for galaxies with high star formation rates and lots of dust) from the South Pole Telescope infrared galaxy survey. They have been able to further elucidate the nature of 17 of these early galaxies by measuring C II emission from singly ionized atomic carbon, and CO emission from carbon monoxide molecules for 11 of those. They have also determined the total far infrared luminosity for these target galaxies. Their results allow them to place constraints on the nature of the interstellar medium and the properties of molecular clouds.

The galaxies’ high redshifts, ranging from z = 2.1  to 5.7, actually makes it possible to make Earth-bound measurements in most cases. At lower redshifts the observations would not be possible from Earth because the Earth’s atmosphere is highly opaque at the observation frequencies. But it is much more transparent at longer wavelengths, so as the redshift exceeds z = 3, Earth-based observations are possible from favorable locations, in this case the Chilean desert. For three sources with redshifts around 2 the atmosphere prohibits ground-based observations and the team therefore made observations from the orbiting Herschel Space Telescope, designed for infrared work.

The figure labelled Figure 3 below is taken from their paper. It indicates the redshift z on the x-axis (logarithmically) and the far infrared luminosity of the galaxy on the y-axis (as the log) as well. The 17 galaxies studied by the authors are indicated with red dots and labelled “SPT DSFGs”. Their very high luminosities are in the range of 10 to 100 trillion times the Sun’s luminosity. Note that the luminosities must be very high for detection at such a high redshift (distance from Earth). Also these luminosities are uncorrected for the lensing magnification, so the true luminosities are around an order of magnitude lower.

CIIEmissionDSFGfig3

The redshift range covered in this research corresponds to ages for the universe of around 1 billion years old (z = 5.7) to a little over 3 billion years old (z = 2.1). So the lookback time is roughly 11 to 13 billion years.

For those of us interested in dark matter, their findings regarding the degree of magnification by dark matter are also interesting. They find “strong lensing” or magnification in the range of 5 to 21 times for 4 sources that allowed for lens modeling. The other sources do not have magnifications measured, but they are presumed to be of the same order of magnitude of around 10 times or so, to within a factor of 2 either way.

It is only because the lensing is so substantial that they are able to measure these galaxies with sufficient fidelity to arrive at their results. So not only is dark matter key to galaxy formation and evolution, it is key to allowing us to study galaxies in the early universe. Dark matter forms galaxies and then helps us understand how they form!

Reference

B. Gullberg et al. 2015, ”The nature of the [CII] emission in dusty star-forming galaxies from the SPT survey” to be published, Monthly Notices of the Royal Astronomical Society, http://arxiv.org/pdf/1501.06909v2.pdf

C.M. Casey, D. Narayanan, A. Cooray 2015, “Dusty Star-Forming Galaxies at High Redshift”, http://arxiv.org/abs/1402.1456


Dark Globular Clusters

Globular clusters are highly compact star clusters containing hundreds of thousands or even millions of old stars. The stars found in a globular cluster can be up to almost 13 billion years old, and thus act as good tracers of the early history of our Milky Way Galaxy, or of other galaxies in which they are found.

Our Galaxy has about 150 or so globular clusters, and our neighbor the Andromeda Galaxy has over 500. Because they are so compact, they are tightly gravitationally bound, and tend to be very spherical, hence the description as globular. They also orbit our Galaxy in the halo and in various directions; they are not confined to the disk where most of the other stars of the Milky Way are found. This suggests a history for globulars (as they are also called) that predates the formation of the Milky Way’s disk and spiral arms. Many globulars may be remnants of dwarf galaxies that have been pulled into the Milky Way during its long lifetime, or have been captured by the Milky Way as it consumed another, smaller galaxy in its vicinity.

M13

M13 globular cluster

The large majority of globular clusters are thought to contain little dark matter. They contain no dust or gas, their matter appears to be all in stars. Astronomers are able to measure the mass of globulars by determining how fast stars are moving around in the core, and by measuring the size of the globulars. Assuming the globulars are gravitationally relaxed, which most appear to be, then the mass within a certain radius is proportional to the radius and the square of the velocities of stars relative to the globular’s center and within that radius. Some assumptions need to be made about the falloff of matter density, and projection effects corrected, but these can be calculated and compared with observations to provide a self-consistent model.

Going through these steps, and also measuring the absolute brightness of the globular (which requires a distance measurement) allows something called a mass-to-light ratio for the globular to be determined. This is stated in terms of solar units, i.e., using the mass in relation to the Sun’s mass and the absolute brightness in relation to the Sun’s luminosity.

Typical mass-to-light ratios for most globulars are of order unity, say 1 or 2 or 3. This indicates little dark matter is present (we know the dark matter content of ordinary stars such as the Sun is low).

But now some possible dark matter-containing globular clusters have been found. To be more precise, some globular clusters with quite high mass-to-light ratios have been found around Centaurus A.

eso1519a

Centaurus A galaxy and globular clusters observed by the Very Large Telescope (ESO)

Centaurus A is a peculiar galaxy about 10 million light-years or so away, and is the fifth brightest galaxy in the sky by apparent magnitude. It is classified as a giant elliptical, or lenticular, galaxy with peculiar characteristics. It has a very prominent dust lane through the center as seen in the photograph above, and also possesses a large relativistic jet visible at radio and X-ray wavelengths. There is a black hole at the center of Centaurus A with a mass of over 50 million solar masses. Centaurus A may have as many as 2000 globular clusters.

A study of 125 of the brighter globular clusters in the Centaurus A galaxy, by Matthew Taylor and co-authors, was made at the European Southern Observatory’s Very Large Telescope in Chile. The locations of the globulars are indicated in the green, blue and red circles superimposed on Centaurus A’s image. A certain fraction of the globulars with masses above a million solar masses showed the characteristic that the mass-to-light ratio was abnormally high, and becomes higher nearly in proportion to the mass of the globular. This set of globulars is denoted in red circles in the image. The most massive of the red circle globulars have the highest mass-to-light ratios.

The authors find, quoting from their paper, “a distinct group of objects which require significant dark gravitating components such as central massive black holes and/or exotically concentrated dark matter distributions.” These objects have mass-to-light ratios above 6, and half a dozen have mass-to-light ratios over 15, including one object with a very high ratio of 67.

Several explanations are proposed. One is rotation of the globulars in question, but the authors are able to rule this explanation out since the stability of the cluster would be destroyed. Another is massive black holes in the centers of these globulars. This would require black holes with masses in the range 40,000 to over 1 million solar masses, and would be quite an exciting finding in its own right. There is evidence for intermediate mass black holes in a few other globular clusters found in the Milky Way and other galaxies. An additional possibility is the accumulation of many smaller stellar-sized black holes and/or neutron stars in the center of the cluster that would modify the cluster’s dynamical properties.

Yet another possibility is the presence of significant dark matter. If verified there would be important implications for globular cluster formation histories. This study should lead to a rush for other observations to ferret out high mass-to-light ratio globular clusters and to allow astronomers to distinguish between the black hole, dark stellar remnant, and dark matter possible scenarios.

Already there is a Wikipedia entry for “dark globular clusters”.

References

http://www.eso.org/public/news/eso1519/

http://www.eso.org/public/archives/releases/sciencepapers/eso1519/eso1519a.pdf, submitted to the Astrophysical Journal


Blue Tides and the Milky Way

I recently wrote about the  largest high-redshift cosmological simulation of galaxy formation ever, which has been recently completed by a group of astrophysicists in the U.S. and the U.K. This tour-de-force simulation, named BlueTides, was performed on the Blue Waters Cray XE supercomputer at NCSA and employed 648,000 cores. The researchers utilized approximately 700 billion particles (!) to represent dark matter and ordinary matter and to create galaxies inside the supercomputer.

You can find the full article describing the simulation at insidehpc.com.

Galaxies are the fundamental building blocks of the large scale structure of the universe. Very early on, before the first galaxies formed, the universe was a highly uniform mix of dark matter and ordinary matter, but with about 5 times as much dark matter by mass relative to the ordinary matter (protons, neutrons, electrons) that makes up the visible parts of galaxies, including stars, gas and dust. Ares of higher dark matter density play a key role in gravitationally attracting ordinary matter that forms galaxies and stars.

When we think of the word galaxy we typically think of beautiful modern day spiral galaxies, such as the Andromeda Galaxy. Spiral galaxies are flattened, rotating disks; the spiral arms represent concentrations of matter and of star-forming regions. The most distant disk-shaped galaxies that have been detected are at redshifts of 2 to 3, so we are seeing them as they were when the universe was around 2 to 3 billion years old. (The higher the redshift the more distant the galaxy and also the farther back in time we are looking, toward the universe’s origin some 13.8 billion years ago).

ngc2207_hubble_960

Two spiral galaxies starting to collide. Image Credit: Debra Meloy Elmegreen (Vassar College) et al.,
& the Hubble Heritage Team (AURA/STScI/NASA)

The BlueTides simulation provides insight into what was going on when the universe was only around 1/2 a billion years old, with galaxy redshifts around 8 to 10. It does a good job of matching the limited observational data we have at such highredshifts, in particular the rest frame (before redshift) ultraviolet luminosities of the earliest detected galaxies from the Hubble Space Telescope surveys.

Their simulation finds that, among the most massive galaxies in their simulation, “a significant fraction are visually disk-like, and surprisingly regular in shape”. In other words, they appear to be the progenitors of present-day spiral galaxies. They find that, at a redshift of 8, a full 70% of their virtual galaxies with masses above 10 billion solar masses are classifiable as disks, since a majority of stellar orbits lie in the plane of the disk.

Fig1.Fengetal.milkyway

Simulated high-redshift galaxies from BlueTides – Figure 1 from Feng et al.

They also find that mergers are not of major significance in the build-up of these early massive galaxies. Rather it appears that they grew primarily by cold gas arriving from preferred directions, namely along filaments in the density distribution of the background gas. It is well known that the universe has a web-like or filamentary structure of high density regions interspersed with voids (relatively empty regions). This filamentary structure is believed on the basis of many simulations of the large-scale universe to have begun at an early date.

A future infrared satellite known as WFIRST will have a field of view 200 times larger than the Wide Field Camera on the Hubble. Also, its design for infrared radiation detection makes it appropriate for studying the light from high-redshift galaxies. The authors predict that a survey of 2000 square degrees with WFIRST should find roughly 8000 massive disk-type galaxies at redshifts above 8. Future very large ground-based telescopes will be able to make follow-up observations of galaxies discovered by WFIRST. Such observations will provide further insight into the nature of galaxy formation, including accretion of material from the background and the details of dark matter’s role in the process.

Reference

Feng et al. 2015, “The Formation of Milky Way-Mass Disk Galaxies in the First 500 Million Years of a Cold Dark Matter Universe” http://arxiv.org/abs/1504.06618


Forming the First Galaxies: Blue Tides on Blue Waters

The largest high-redshift cosmological simulation of galaxy formation ever has been recently completed by a group of astrophysicists from the U.S. and the U.K. This tour-de-force simulation was performed on the Blue Waters Cray XE/XK system at NCSA and employed 648,000 CPU cores. They utilized approximately 700 billion particles (!) to represent dark matter and ordinary matter and to create virtual galaxies inside the supercomputer. The authors, who represent Carnegie Mellon University, UC Berkeley, Princeton University, and the University of Sussex, have given their simulation the moniker BlueTides.

The astrophysicists simulated galaxy formation in a “box” 2 billion light-years on a side. At redshifts of  z = 8 to 10,  which is the maximum for which we have observed real galaxies, their simulation matches the data from the Hubble Space Telescope. And yes, dark matter plays a key role – it is the main gravitational sink that pulls in ordinary matter that forms the stars, gas, and dust that are the primary visible components of galaxies. Without dark matter, our Milky Way would be much smaller, and you probably wouldn’t be here. You can learn more about the BlueTides simulation results and methodology at insidehpc.com.

  Hubble Ultra Deep Field; STScI


The Supervoid

The largest known structure in the universe goes by the name of the Supervoid. It is an enormously large under-dense region about 1.8 billion light-years in extent. Voids (actually low density regions) in galaxy and cluster density have been mapped over several decades.

The cosmic microwave background radiation map from the Planck satellite and earlier experiments is extremely uniform. The temperature is about 2.7 Kelvins everywhere in the universe at present. There are small microKelvin scale fluctuations due to primordial density perturbations. The over-dense regions grow over cosmic timescales to become galaxies, groups and clusters of galaxies, and superclusters made of multiple clusters. Under-dense regions have fewer galaxies and groups per unit volume than the average.

The largest inhomogeneous region detected in the cosmic microwave background map is known as the Cold Spot and has a very slightly lower temperature by about 70 microKelvins (a microKelvin being only a millionth of a degree). It may be partly explained by a supervoid of radius 320 Megaparsecs, or around 1 billion light-years radius.

Superclusters heat cosmic microwave background photons slightly when they pass through, if there is significant dark energy in the universe. Supervoids cool the microwave background photons slightly. The reason is that, once dark energy becomes significant, during the second half of the universe’s expansion to date, it begins to smooth out superclusters and supervoids. It pushes the universe back towards greater uniformity while accelerating the overall expansion.

A photon will gain energy (blueshift) when it heads into a supercluster on its way to the Earth. This is an effect of general relativity. And as it leaves the other side of the supercluster as it continues its journey, it will lose energy (redshift) as it climbs out of the gravitational potential well. But while it is passing through the supercluster, that structure is spreading out due to the Big Bang overall expansion, and its gravitational potential is weakening. So the redshift or energy loss is smaller than the original energy gain or blueshift. So net-net, photons gain energy passing through a supercluster.

The opposite happens with a supervoid. Photons lose energy on the way in. They gain  energy on the way out, but less than they lost. Net-net photons lose energy, become colder, when passing through supervoids. Now all of this is relative to the overall redshift that all photons experience as they travel from the Big Bang last scattering surface to the Earth. During each period that the universe doubles in size, the Big Bang radiation doubles in wavelength, or halves in temperature.

In a newly published paper titled “Detection of a Supervoid aligned with the Cold Spot in the Cosmic Microwave Background”, astronomers looked at the distribution of galaxies in the direction of the well-established Cold Spot. The supervoid core redshift distance is in the range z = 0.15 to z = 0.25, corresponding to a distance of roughly 2 to 3 billion light-years from Earth.

They find a reduction in galaxy density of about 20%, and of dark matter around 14%, in the supervoid, relative to the overall average density values in the universe. The significance of the detection is high, around 5 standard deviations. The center of the low density region is well aligned with the position of the Cold Spot in the galactic Southern Hemisphere.

Both the existence of this supervoid and its alignment with the Cold Spot are highly significant. The chance of the two being closely aligned to this degree is calculated as just 1 chance in 20,000. The image below is Figure 2 from the authors’ paper and maps the density of galaxies in the left panel and the temperature differential of the microwave background radiation in the right panel. The white dot in the middle of each panel marks the center of the Cold Spot in the cosmic microwave background.

Supervoid.F2.large

A lower density of galaxies is indicated by a blue color in the left panel. Red and orange colors denote a higher density of galaxies. The right panel shows slightly lower temperature of the cosmic microwave background in blue, and slightly higher temperature in red.

The authors have calculated the expected temperature reduction due to the supervoid; using a first-order model it is about 20 microKelvins. While this is not sufficient to explain the entire Cold Spot temperature decrease, it is a significant portion of the overall 70 microKelvin reduction.

Dark Energy is gradually smearing out the distinction between superclusters and supervoids. Dark Energy has come to dominate the universe’s mass-energy balance fairly recently, since about 5 billion years ago. If there is no change in the Dark Energy density, over many billions of years it will push all the galaxies so far apart from one another that no other galaxies will be detectable from our Milky Way.

References

I. Szapudi et al, 2015 M.N.R.A.S., Volume 450, Issue 1, p. 288, “Detection of a supervoid aligned with the cold spot of the cosmic microwave background” – http://mnras.oxfordjournals.org/content/450/1/288.full

S. Perrenod and M. Lesser, 1980, P.A.S.P. 91:764, “A Redshift Survey of a High-Multiplicity Supercluster” http://www.jstor.org/discover/10.2307/40677683?uid=2&uid=4&sid=21106121183081

  


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