WIMPs or MACHOs or Primordial Black Holes

A decade or more ago, the debate about dark matter was, is it due to WIMPs (weakly interacting massive particles) or MACHOs (massive compact halo objects)? WIMPs would be new exotic particles, while MACHOs are objects formed from ordinary matter but very hard to detect due to their limited electromagnetic radiation emission.

Arnold_Schwarzenegger_2003

Schwarzenegger (MACHO), not Schwarzschild (Black Holes)

Image credit: Georges Biard, CC BY-SA 3.0

Candidates in the MACHO category such as white dwarf or brown dwarf stars have been ruled out by observational constraints. Black holes formed in the very early universe, dubbed primordial black holes, were thought by many to have been ruled out as well, at least across many mass ranges, such as between the mass of the Moon and the mass of the Sun.

The focus during recent years, and most of the experimental searches, has shifted to WIMPs or other exotic particles (axions or sterile neutrinos primarily). But the WIMPs, which were motivated by supersymmetric extensions to the Standard Model of particle physics, have remained elusive. Most experiments have only placed stricter and stricter limits on their possible abundance and interaction cross-sections. The Large Hadron Collider has not yet found any evidence for supersymmetric particles.

Have primordial black holes (PBHs) as the explanation for dark matter been given short shrift? The recent detections by the LIGO instruments of two gravitational wave events, well explained by black hole mergers, have sparked new interest. A previous blog entry addressed this possibility:

https://darkmatterdarkenergy.com/2016/06/17/primordial-black-holes-as-dark-matter/.

The black holes observed in these events have masses in a range from about 8 to about 36 solar masses, and they could well be primordial.

There are a number of mechanisms to create PBHs in the early universe, prior to the very first second and the beginning of Big Bang nucleosynthesis. At any era, if there is a total mass M confined within a radius R, such that

2*GM/R > c^2 ,

then a black hole will form. The above equation defines the Schwarzschild limit (G is the gravitational constant and c the speed of light). A PBH doesn’t even have to be formed from matter whether ordinary or exotic; if the energy and radiation density is high enough in a region, it can also result in collapse to a black hole.

 cosmicstrings.jpg

Cosmic Strings

Image credit: David Daverio, Université de Genève, CSCS supercomputer simulation data

The mechanisms for PBH creation include:

  1. Cosmic string loops – If string theory is correct the very early universe had very long strings and many short loops of strings. These topological defects intersect and form black holes due to the very high density at their intersection points. The black holes could have a broad range of masses.
  2. Bubble collisions from symmetry breaking – As the very early universe expanded and cooled, the strong force, weak force and electromagnetic force separated out. Bubbles would nucleate at the time of symmetry breaking as the phase of the universe changed, just as bubbles form in water as it boils to the surface. Collisions of bubbles could lead to high density regions and black hole formation. Symmetry breaking at the GUT scale (for the strong force separation) would yield BHs of mass around 100 kilograms. Symmetry breaking of the weak force from the electromagnetic force would yield BHs with a mass of around our Moon’s mass ~ 10^25 kilograms.
  3. Density perturbations – These would be a natural result of the mechanisms in #1 and #2, in any case. When observing the cosmic microwave background radiation, which dates from a time when the universe was only 380,000 years old, we see density perturbations at various scales, with amplitudes of only a few parts in a million. Nevertheless these serve as the seeds for the formation of the first galaxies when the universe was only a few hundred million years old. Some perturbations could be large enough on smaller distance scales to form PBHs ranging from above a solar mass to as high as 100,000 solar masses.

For a PBH to be an effective dark matter contributor, it must have a lifetime longer than the age of the universe. BHs radiate due to Hawking radiation, and thus have finite lifetimes. For stellar mass BHs, the lifetimes are incredibly long, but for smaller BHs the lifetimes are much shorter since the lifetime is proportional to the cube of the BH mass. Thus a minimum mass for PBHs surviving to the present epoch is around a trillion kilograms (a billion tons).

Carr et al. (paper referenced below) summarized the constraints on what fraction of the matter content of the universe could be in the form of black holes. Traditional black holes, of several solar masses, created by stellar collapse and detectable due to their accretion disks, do not provide enough matter density. Neither do supermassive black holes of over a million solar masses found at the centers of most galaxies. PBHs may be important in seeding the formation of the supermassive black holes, however.

Limits on the PBH abundance in our galaxy and its halo (which is primarily composed of dark matter) are obtained from:

  1. Cosmic microwave background measurements
  2. Microlensing measurements (gravitational lensing)
  3. Gamma-ray background limits
  4. Neutral hydrogen clouds in the early universe
  5. Wide binaries (disruption limits)

Microlensing surveys such as MACHO and EROS have searched for objects in our galactic halo that act as gravitational lenses for light originating from background stars in the Magellanic Clouds or the Andromeda galaxy. The galactic halo is composed primarily of dark matter.

A couple of dozen of objects with less than a solar mass have been detected.  Based on these surveys the fraction of dark matter which can be PBHs with less than a solar mass is 10% at most. The constraints from 1 solar mass up to 30 solar masses are weaker, and a PBH explanation for most of the galactic halo mass remains possible.

Similar studies conducted toward distant quasars and compact radio sources address the constraint in the supermassive black hole domain, apparently ruling out an explanation due to PBHs with from 1 million to 100 million solar masses.

Lyman-alpha clouds are neutral hydrogen clouds (Lyman-alpha is an important ultraviolet absorption line for hydrogen) that are found in the early universe at redshifts above 4. Simulations of the effect of PBH number density fluctuations on the distribution of Lyman-alpha clouds appear to limit the PBH contribution to dark matter for a characteristic PBH mass above 10,000 solar masses.

Distortions in the cosmic microwave background are expected if PBHs above 10 solar masses contributed substantially to the dark matter component. However these limits assume that PBH masses do not change. Merging and accretion events after the recombination era, when the cosmic microwave background was emitted, can allow a spectrum of PBH masses that were initially less than a solar mass before recombination evolve to one dominated by PBHs of tens, hundreds and thousands of solar masses today. This could be a way around some of the limits that appear to be placed by the cosmic microwave background temperature fluctuations.

Thus it appears could be a window in the region 30 to several thousand solar masses for PBHs as an explanation of cold dark matter.

As the Advanced LIGO gravitational wave detectors come on line, we expect many more black hole merger discoveries that will help to elucidate the nature of primordial black holes and the possibility that they contribute substantially to the dark matter component of our Milky Way galaxy and the universe.

References

B. Carr, K. Kohri, Y. Sendouda, J. Yokoyama, 2010 arxiv.org/pdf/0912.5297v2 “New cosmological constraints on primordial black holes”

S. Cleese and J. Garcia-Bellido, 2015 arxiv.org/pdf/1501.07565v1.pdf “Massive Primordial Black Holes from Hybrid Inflation as Dark Matter and the Seeds of Galaxies”

P. Frampton, 2015 arxiv.org/pdf/1511.08801.pdf “The Primordial Black Hole Mass Range”

P. Frampton, 2016 arxiv.org/pdf/1510.00400v7.pdf “Searching for Dark Matter Constituents with Many Solar Masses”

Green, A., 2011 https://www.mpifr-bonn.mpg.de/1360865/3rd_WG_Green.pdf “Primordial Black Hole Formation”

P. Pani, and A. Loeb, 2014 http://xxx.lanl.gov/pdf/1401.3025v1.pdf “Exclusion of the remaining mass window for primordial black holes as the dominant constituent of dark matter”

S. Perrenod, 2016 https://darkmatterdarkenergy.com/2016/06/17/primordial-black-holes-as-dark-matter/

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Primordial Black Holes as Dark Matter?

LIGO Gravitational Wave Detection Postulated to be Due to Primordial Black Holes

Dark matter remains elusive, with overwhelming evidence for its gravitational effects, but no confirmed direct detection of exotic dark matter particles.

Another possibility which is being re-examined as an explanation for dark matter is that of black holes that formed in the very early universe, which in principle could be of very small mass, or quite large mass. And they may have initially formed at smaller masses and then aggregated gravitationally to form larger black holes.

Recently gravitational waves were discovered for the first time, by both of the LIGO instruments, located in Louisiana and in Washington State. The gravitational wave signal (GW150914) indicates that the source was a pair of black holes, of about 29 and 36 solar masses respectively, spiraling together into a single black hole of about 62 solar masses. A full 3 solar masses’ worth of gravitational energy was radiated way in the merger. Breaking news: LIGO has just this month announced gravitational waves from a second black hole binary of 22 solar masses total. One solar mass of energy was radiated away in the merger.

massrangescompactobjects.jpg

Image credit: NASA/JPL, http://www.nasa.gov/jpl/nustar/pia18842

Most of the black holes that we detect (indirectly, from their accretion disks) are stellar-sized in the range of 10 to 100 solar masses and are believed to be the evolutionary endpoints of massive stars. We detect them when they are surrounded by accretion disks of hot luminous matter outside of their event horizons. The other main category of black holes exceeds a million solar masses and can even be more than a billion solar masses, and are known as supermassive black holes.

It is possible that some of the stellar-sized and even elusive intermediate black holes were formed in the Big Bang. Such black holes are referred to as primordial black holes. There are a variety of theoretical formation mechanisms, such as cosmic strings whose loops in all dimensions are contained within the event horizon radius (Schwarzschild radius). In general such primordial black holes (PBHs) would be distributed in a galaxy’s halo, would interact rarely and not have accretion disks and thus would not be detectable due to electromagnetic radiation. That is, they would behave as dark matter.

Dr. Simon Bird and coauthors have recently proposed that the gravitational wave event (GW150914) could be due to two primordial black holes encountering each other in a galactic halo, radiate enough of their kinetic energy away in gravity waves to become bound to each other and inspiral to a single black hole with a final burst of gravitational radiation. The frequency of events is estimated to be of order a few per year per cubic Gigaparsec (a Gigaparsec is 3.26 billion light years), if the dark matter abundance is dominated by PBHs.

While low-mass PBHs have been ruled out for the most part, except of a window around one one-hundred millionth of a solar mass, the authors suggest a window also remains for PBHs in the range from 20 to 100 solar masses.

Dr. A. Kashlinsky has gone further to suggest that the cosmic infrared background (CIB) of unresolved 2 to 5 micron near-infrared sources is due to PBHs. In this case the PBHs would be the dominant dark matter component in galactic halos and would mediate early star and galaxy formation. Furthermore there is an unresolved soft cosmic X-ray background which appears to be correlated with the CIB.

This would be a trifecta, with PBHs explaining much or most of the dark matter, the CIB and the soft-X-Ray CXB! But at this point it’s all rather speculative.

The LIGO instruments are now upgraded to Advanced LIGO and as more gravitational wave events are detected due to black holes, we can gain further insight into this possible explanation for dark matter, in whole or in part. Improved satellite born experiments to further resolve the CIB and CXB will also help to explore this possibility of PBHs as a major component to dark matter.

References:

S. Bird et al. arXiv:1603.00464v2 “Did LIGO detect Dark Matter”

A. Kashlinksy arXiv:1605.04023v1 “LIGO gravitational wave detection, primordial black holes and the near-IR cosmic infrared background anisotropies”

http://www.space.com/26857-medium-size-black-hole-discovery-m82.html – “It’s Confirmed! Black Holes Do Come in Medium Sizes”

Video (artist’s representation) of inspiral and merger of binary black hole GW151226 (second gravitational wave detection): https://youtu.be/KwbXxzgAObU

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Galaxy Clusters Probe Dark Energy

Rich (large) clusters of galaxies are significant celestial X-ray sources. In fact, large clusters of galaxies typically contain around 10 times as much mass in the form of very hot gas as is contained in their constituent galaxies.

Moreover, the dark matter content of clusters is even greater than the gas content; typically it amounts to 80% to 90% of the cluster mass. In fact, the first detection of dark matter’s gravitational effects was made by Fritz Zwicky in the 1930s. His measurements indicated that the galaxies were moving around much faster than expected from the known galaxy masses within the cluster.

clusters_1280.abell1835.jpg

Image credit: X-ray: NASA/CXC/Univ. of Alabama/A. Morandi et al; Optical: SDSS, NASA/STScI (X-ray emission is shown in purple)

The dark matter’s gravitational field controls the evolution of a cluster. As a cluster forms via gravitational collapse, ordinary matter falling into the strong gravitational field interacts via frictional processes and shocks and thermalizes at a high temperature in the range of 10 to 100 million degrees (Kelvins). The gas is so hot, that it emits X-rays due to thermal bremsstrahlung.

Recently, Drs. Morandi and Sun at the University of Alabama have implemented a new test of dark energy using the observed X-ray emission profiles of clusters of galaxies. Since clusters are dominated by the infall of primordial gas (ordinary matter) into dark matter dominated gravitational wells, then X-ray emission profiles – especially in the outer regions of clusters – are expected to be similar, after correcting for temperature variations and the redshift distance. Their analysis also considers variation in gas fraction with redshift; this is found to be minimal.

Because of the self similar nature of the X-ray emission profiles, X-ray clusters of galaxies can serve as cosmological probes, a type of ‘standard candle’. In particular, they can be used to probe dark energy, and to look at the possibility of the variation of the strength of dark energy over multi-billion year cosmological time scales.

The reason this works is that cluster development and mass growth, and corresponding temperature increase due to stronger gravitational potential wells, are essentially a tradeoff of dark matter and dark energy. While dark matter causes a cluster to grow, dark energy inhibits further growth.

This varies with the redshift of a cluster, since dark energy is constant per unit volume as the universe expands, but dark matter was denser in the past in proportion to (1 + z)^3, where z is the cluster redshift. In the early universe, dark matter thus dominated, as it had a much higher density, but in the last several billion years, dark energy has come to dominate and impede further growth of clusters.

The table below shows the percentage of the mass-energy of the universe which is in the form of dark energy and in the form of matter (both dark and ordinary) at a given redshift, assuming constant dark energy per unit volume. This is based on the best estimate from Planck of 68% of the total mass-energy density due to dark energy at present (z = 0). Higher redshift means looking farther back in time. At z = 0.5, around 5 billion years ago, matter still dominated over dark energy, but by around z = 0.3 the two are about equal and since then (for smaller z) dark energy has dominated. It is only since after the Sun and Earth formed that the universe has entered the current dark energy dominated era.

Table: Total Matter & Dark Energy Percentages vs. z 

Redshift

Dark Energy percent

Matter percent

0

68

32

0.25

52

48

0.5

39

61

0.75

28

72

1.0

21

79

1.5

12

88

The authors analyzed data from a large sample consisting of 320 clusters of galaxies observed with the Chandra X-ray Observatory. The clusters ranged in redshifts from 0.056 up to 1.24 (almost 9 billion years ago), and all of the selected clusters had temperatures measured to be equal to or greater than 3 keV (above 35 million Kelvins). For such hot clusters, non-gravitational astrophysical effects, are expected to be small.

Their analysis evaluated the equation of state parameter, w, of dark energy. If dark energy adheres to the simplest model, that of the cosmological constant (Λ) found in the equations of general relativity, then w = -1 is expected.

The equation of state governs the relationship between pressure and energy density; dark energy is observed to have a negative pressure, for which w < 0, unlike for matter.

Their resulting value for the equation of state parameter is

w = -1.02 +/- 0.058,

equal to -1 within the statistical errors.

The results from combining three other experiments, namely

  1. Planck satellite cosmic microwave background (CMB) measurements
  2. WMAP satellite CMB polarization measurements
  3. optical observations of Type 1a supernovae

yield a value

w = -1.09 +/- 0.19,

also consistent with a cosmological constant. And combining both the X-ray cluster results with the CMB and optical results yields a tight constraint of

w = -1.01 +/- 0.03.

Thus a simple cosmological constant explanation for dark energy appears to be a sufficient explanation to within a few percent accuracy.

The authors were also able to constrain the evolution in w and find, for a model with

w(z) = w(0) + wa * z / (1 + z), that the evolution parameter is zero within statistical errors:

wa = -0.12 +/- 0.4.

This is a powerful test of dark energy’s existence, equation of state, and evolution, using hundreds of X-ray clusters of galaxies. There is no evidence for evolution in dark energy with redshift back to around z = 1, and a simple cosmological constant model is supported by the data from this technique as well as from other methods.

References:

  1. Morandi, M. Sun arXiv:1601.03741v3 [astro-ph.CO] 4 Feb 2016, “Probing dark energy via galaxy cluster outskirts”
  2. http://chandra.harvard.edu/photo/2016/clusters/

Gamma Rays from Dark Matter at the Center of the Galaxy: Stronger Evidence

Evidence has been growing for the detection of dark matter more directly – at the center of the Milky Way Galaxy. Normally, we detect dark matter through its gravitational effects only, although there have been many attempts to detect it more directly, both through laboratory experiments here on Earth and from astronomical measurements. The Earthbound experiments are inconclusive at best, with some claims of detection being contradicted by other experiments.

But the evidence for astronomical detection of dark matter is growing. Expected sources include dwarf galaxies https://darkmatterdarkenergy.com/tag/dwarf-galaxies/ that are found near our Milky Way. The low luminosity of dwarf galaxies due to stars and supernovae can make it easier to extract evidence of dark matter due to its self annihilation.

Our own Milky Way Galaxy has a higher concentration of normal matter at the center, and is expected to have a higher concentration of dark matter as well. For the past 5 years or so, there has been evidence for possible dark matter annihilation at the Galactic Center. See http://www.sciencedirect.com/science/article/pii/S0370269311001742.

The mechanism is dark matter self-annihilation, resulting in the creation of decay products of ordinary matter and gamma rays (highly energetic photons). See one of my prior blogs at: https://darkmatterdarkenergy.com/tag/dark-matter-annihilation/.

The leading dark matter candidate is some sort of WIMP (weakly interacting massive particle). WIMPs interact only via gravity and perhaps the weak nuclear force. WIMP self-annihilations can produce quarks, neutrinos, gamma rays and other ordinary matter particles.

There is a known gamma ray signal in the Galactic Center (the center of our Milky Way) that extends to 5 degrees away from the center, corresponding to roughly a kiloparsec in extent (a kiloparsec is 3260 light-years, and our Sun is 8 kiloparsecs from the Center). The major alternatives for this signal appear to be dark matter annihilation, cosmic ray interactions with interstellar gas, or emission from rapidly rotating neutron stars (millisecond pulsars).

A recent paper from T. Daylan and co-authors from Harvard, MIT, Princeton, the University of Chicago and the Fermi Laboratory is titled “The Characterization of the Gamma-Ray Signal from the Central Milky Way: A Compelling Case for Annihilating Dark Matter”. They have reanalyzed observations from the Fermi Gamma Ray Space Telescope and confirmed that the distribution of gamma rays in the Galactic Center (GC) is largely spherically symmetric and extended. This spatial distribution likely rules out neutron stars as the source, since these are preferentially found in the galactic disk. 

arxiv1402.6703v2.fig10.1to3GeV

1-3 GeV residual gamma ray image. From Fig. 10 of T. Daylan et al., this is corrected for galactic diffuse emission and has point sources subtracted. The image extends over a 10◦ by 10◦ region.

Dark matter, on the other hand, would be expected to have a roughly spherical distribution around the GC. Interstellar gas is also largely confined to the galactic disk, so this explanation is disfavored. Their study also confirms that the emission extends beyond the GC to what is known as the Inner Galaxy, further ruling out the two alternatives other than dark matter annihilation. The emission falls off in intensity away from the GC, in a manner consistent with a spherically symmetric dark matter density distribution that is in accordance with a Navarro-Frenk-White profile often used successfully in modeling dark matter halos. No evidence is found for any significant deviation from spherical symmetry for the GC and Inner Galaxy components, the latter extending out to around 2 kiloparsecs.

There are various possible annihilation channels for dark matter and the authors’ analysis appears to favor a dominant channel to primarily b quarks (and b antiquarks). In this scenario the WIMP mass appears to lie in the range of 36 to 51 GeV (by comparison a proton or neutron mass is about .94 GeV). Recall that there are 6 types of quarks, and protons and neutrons are composed of u and d (up and down) quarks. The others are b, t, c, s (bottom, top, charm and strange). The quarks other than u and d are unstable and will decay to u and d.

The spectral (energy) distribution peaks at gamma ray energies of around 1 to 3 GeV and is a good fit to the predictions for annihilation to a b quark pair (b and anti-b). In addition, the cross-section for annihilation calculated from the gamma ray intensity is consistent with that expected from the required rate of thermal production of dark matter particles in the early universe, of order 10^-26 cm^3/sec (actually a value of the cross-section multiplied by the average velocity). The observed dark matter abundance freezes out from thermal equilibrium in the early universe as it expanded and cooled, and implies a cross section of that order.

There is also the possibility for other decay channels, including decays to u, d, c, s and t quarks and to tau lepton particles. The spectral shapes disfavor decays to tau leptons and u, d quarks in particular. After decays to b quarks, the c (charm) and s (strange) quark channels are the most likely.  Either a c or s quark channel implies somewhat lower WIMP masses, around the 20 to 40 GeV range. Annihilations to other fermions appear less likely.

In summary, quoting from their paper:

“This signal consists of a very large number of events, and has been detected with overwhelming statistical significance. The the excess consists of ∼ 10,000 gamma rays per square meter, per year above 1 GeV (from within 10◦ of the Galactic Center). Not only does this large number of events enable us to conclude with confidence that the signal is present, but it also allows us to determine its spectrum and morphology in some detail. And as shown, the measured spectrum, angular distribution, and normalization of this emission does indeed match well with that expected from annihilating dark matter particles.”

“There is no reason to expect that any diffuse astrophysical emission processes would exhibit either the spectrum or the morphology of the observed signal. In particular, the spherical symmetry of the observed emission with respect to the Galactic Center does not trace any combination of astrophysical components (i.e. radiation, gas, dust, star formation, etc.), but does follow the square of the anticipated dark matter density.”

There are also possible detections, marginally significant, of gamma ray emission due to dark matter in nearby dwarf galaxies, and in the direction of the Virgo cluster. We look forward to additional observations and theoretical work to confirm dark matter annihilation signals in our own galaxy and nearby galaxies.

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Gravitational Waves and Dark Matter, Dark Energy

What does the discovery of gravitational waves imply about dark matter and dark energy?

The first detection of gravitational waves results from a pair of merging black holes, and is yet another magnificent confirmation of the theory of general relativity. Einstein’s theory of general relativity has passed every test thrown at it during the last 100 years.

While the existence of gravitational waves was fully expected to be confirmed, the discovery took several decades and represents a technological tour de force. Detected at the two LIGO sites, one in Louisiana and one in Washington State, the main event lasted only 0.2 seconds, and was seen as a change of length in the “arms” of the detector (laser interferometers) of only one part in a thousand billion billion.

LIGO signal 2

The LIGO detection of gravitational waves. The blue curve is from the Louisiana site and the red curve from the Washington state site. The two curves are shifted by 7 milliseconds to account for the speed-of-light delay between the two sites. Note that most of the power in the signal occurs within less than 0.2 seconds. The strain is a measure of proportional change in length of the detector arm and is less than 1 part in 10²¹.

Nevertheless, this is the most energetic event ever seen by mankind. The merger of two large black holes totaling over 60 times the Sun’s mass resulted in the conversion of 3 solar masses of material into gravitational wave energy. Imagine, there were 3 Suns worth of matter obliterated in the blink of an eye. During this brief period, the generated power was greater than that from the light of all of the stars of all of the galaxies in our known universe.

What the discovery of gravitational waves has to say about dark matter and dark energy is essentially that it further confirms their existence.

Although there is as of now no direct detection of dark matter, we infer the existence of dark matter by using the equations of general relativity (GR), in a number of cases, including:

  1. Gravitational lensing – Typically, a foreground cluster of galaxies distorts and magnifies the image of a background galaxy. GR is used to calculate the bending and magnification, primarily caused by the dark matter in the foreground cluster.
  2. Cosmic microwave background radiation (CMBR) – The CMBR has spatial fluctuation peaks (harmonics) and the first peak tells us about ordinary matter and the third peak about the density of dark matter. A GR-based cosmological model is used to determine the dark matter average density.

Dark matter is also inferred from the way in which galaxies rotate and from the velocities of galaxies within galaxy clusters, but general relativity is not needed to calculate the dark matter densities in such cases. However, results from these methods are consistent with results from the methods listed above.

In the case of dark energy, it turns out to be a parameter in the equations of general relativity as first formulated by Einstein. The parameter, lambda, (Λ) is known as the cosmological constant, and represents the minimum energy of the vacuum. For many years astronomers and cosmologists thought it might take the value of zero. However in 1998 multiple teams confirmed that the value is positive and not zero, and it turns out that dark energy has more than twice the energy content of dark matter. Its non-zero value is actually another stunning success for general relativity.

Thus the detection of gravitational waves indirectly provides further support for the canonical cosmological model ΛCDM, with both dark matter and dark energy, and fully consistent with general relativity.

References

http://www.sciencemag.org/news/2016/02/gravitational-waves-einstein-s-ripples-spacetime-spotted-first-time – ScienceMag article

B. P. Abbott et al. (LIGO Scientific Collaboration and Virgo Collaboration), Phys. Rev. Lett. 116, 061102 – Published 11 February 2016 – http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102

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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”

https://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


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