Milky Way Dark Matter Halo Loses ‘Weight’

640px-Milky_Way_ArchMilky Way Arch, CC BY 3.0Bruno Gilli/ESOhttp://www.eso.org/public/images/milkyway/

Mass estimates for our Milky Way vary widely, from less than 1 trillion, to as high as 4 trillion, times the mass of the Sun.

A recent paper by a group of astronomers in Australia argues for a mass that is very much at the low end of this range. Prajawal Kafle and collaborators present a kinematic analysis and build a model of the Milky Way that incorporates a disk, a bulge, and a dark matter halo. The analysis utilizes K giant and horizontal branch star catalogs.

The disk component – in which our Sun resides – contains stars and gas and active star formation from this gas. The spheroidal bulge contains the oldest stellar population of the galaxy, including globular clusters. The spherical halo, significantly larger and more massive than both the other components, is dominated by dark matter. It is chiefly responsible for the overall gravitational potential of the Milky Way, and is evidenced by the high rotational velocity of our galaxy in its outer regions.

The result of their analysis is that the dark matter halo “weighs in” at about 800 billion solar masses, the disk is about 100 billion solar masses, and the bulge is only about 10 billion solar masses. They also find a dark matter density in the solar neighborhood equivalent to about 1/3 of a proton per cubic centimeter, consistent with other estimates. (This number is important for calibrating Earth-bound direct detection experiments for dark matter.)

The relatively low mass they determine for the dark matter halo implies fewer satellite galaxies in close proximity to the Milky Way. We see only 3, the two Magellanic Clouds and the Sagittarius Dwarf Galaxy. In the past this has been seen as an issue for the favored Lambda – Cold Dark Matter (ΛCDM) Cosmology.

However their lower halo mass is actually consistent with the Milky Way gathering only 3 so-called sub-halos (satellite galaxies) and thus there may be no Missing Satellite Problem with ΛCDM. Some had suggested warm dark matter, rather than cold dark matter, may be necessary because of the putative missing satellite problem, but this may not be the case, with a lighter Milky Way dark matter halo.

Another team, Penarrubia and collaborators, has recently modeled the dynamics of the Local Group of galaxies. They are thus using a different methodology to determine the total mass of the Milky Way. They find a total mass for the Local Group of 2.3 trillion solar masses. The Local Group mass is almost entirely due to the Andromeda Galaxy and our Milky Way. They also determine a Milky Way to Amdromeda mass ratio of about 1/2. This implies a mass of about 0.8 trillion solar masses for our Milky Way, consistent with the Australian team’s result.

These two latest measurements of the Milky Way mass seem to indicate that the total mass of the Milky Way galaxy is less than 1 trillion solar masses. And these two results thus suggest that the ΛCDM cosmology is in fact consistent with the small number of satellite galaxies around our Milky Way. Another success for ΛCDM, it seems.

References:

http://article.wn.com/view/2014/10/10/Milky_Way_has_only_half_of_the_Dark_Matter_than_thought_earl/

http://arxiv.org/pdf/1408.1787v1.pdf – P. Kafle et al. 2014, “On the shoulders of giants: Properties of the stellar halo and Milky Way mass distribution”

J. Penarrubia et al. 2014, Monthly Notices of the Royal Astronomical Society, 443, 2204, “A dynamical model of the local cosmic expansion”


BICEP2 Apparently Detects Quantum Nature of Gravity and Supports Inflationary Big Bang

Can a single experiment do all of the following?

  1. Provide significant confirmation of the inflationary version of the Big Bang model (and help constrain which model of inflation is correct)
  2. Confirm the existence of gravitational waves
  3. Support the quantum nature of gravity (at very high energies)
  4. Provide the first direct insight into the highest energy levels imagined by physicists – 10^16 GeV (10,000 trillion GeV) – 12 orders of magnitude beyond the LHC

Apparently it can! BICEP2 is a radio telescope experiment located at the South Pole, taking advantage of the very cold, dry air at that remote location for greater sensitivity. It is focused on measuring polarization of the cosmic microwave background radiation that is a remnant of the hot Big Bang of the early universe. (BICEP is an abbreviation of Background Imaging of Cosmic Extragalactic Polarization; this is the second version of the experiment).

The results announced by the BICEP2 team on March 17 at the Harvard-Smithsonian Center for Astrophysics, if they have been correctly interpreted, are the most important in cosmology in the 21st century to date. They are of such enormous significance that a Nobel Prize in Physics is highly likely, if the results and interpretation are confirmed.

We infer from a number of previous observations that there was likely an inflationary period very early on in the universes’s history. We are talking very, very, early – in the first billionth of a trillionth of a trillionth of a second. See this earlier post of mine here: http://darkmatterdarkenergy.com/2011/03/22/inflation/ This new result from BICEP2 is very supportive of inflationary Big Bang models, and that includes very simple models for inflation.

What is the observation? It is B-mode polarization in the cosmic microwave background radiation. The cosmic microwave background (CMB) is the thermal radiation left over from a time when the universe became transparent, at age 380,000 years, almost 14 billion years ago. There are two polarization modes for alignment of CMB radiation, the E-mode and the B-mode. The B-mode measures the amount of “curliness” in the alignment of CMB microwave photons (as you can easily see in the image below).

There are two known causes of B-mode polarization for the CMB. The first, also detected by the BICEP2 experiment, is due to intervening clusters of galaxies along the line of sight. These clusters bend the light paths due to their immense masses, in accordance with general relativity. These effects are seen at smaller spatial scales. At larger spatial scales, we have the more significant effect, whereby the gravitational waves generated during the inflation epoch imprint the polarization.

 

Image

 

You can easily see the curly B-mode polarization with a quick glance at BICEP2’s results. (bicepkeck.org)

 

What is being seen in today’s CMB is due to this second and more profound cause, which is nothing less than quantum fluctuations in space-time in the very, very early universe revealing themselves due to the gravitational waves that they generated. And these gravitational waves in turn caused a small curling effect on the cosmic microwave background, until the time of decoupling of radiation and matter. This is seen in the image at angular scales of a few degrees.

At age 380,000 years the universe became transparent to the CMB radiation and it traveled for another 13.8 billion years and underwent a redshift by a factor of 1500 as the universe expanded. So what was optical radiation at that time, becomes microwave radiation today, with a characteristic temperature of 2.7 Kelvins (degrees above absolute zero), while still retaining the curly pattern seen by BICEP2.

This is the first observation that provides some direct insight into extremely high energy scales within the context of a single experiment. We are talking here of approximately a trillion times higher energy than the Large Hadron Collider, the world’s most powerful particle accelerator, where the Higgs boson was discovered.

The BICEP2 results are a single experiment that for the first time apparently ties quantum mechanics and gravity together. It supports the quantum nature of gravity, which occurs at very high energy scales. The Planck scale at which space-time would be quantized corresponds to an energy level of 10^18 or 10^19 GeV (ten million trillion GeV), and inflation in many models begins when the universe has an energy level somewhat lower, at 10^16 GeV (ten thousand trillion GeV, where 1 GeV is a little more than the rest mass-energy of a proton).

And take a look at this interview of Sean Carroll by PBS’s Gwen Ifill to get some more context around this (hopefully correct!) universe-expanding discovery. Other astronomers are already racing to confirm it.

 

References:

http://www.cfa.harvard.edu/CMB/bicep2/ – BICEP2 web site at Harvard-Smithsonian Center for Astrophysics

http://www.theguardian.com/science/2014/mar/17/bicep2-how-hot-big-bang-science-dark-energy

http://www.scientificamerican.com/article/gravity-waves-cmb-b-mode-polarization/


Axions as Cold Dark Matter

WIMP (weakly interacting massive particle) searches have been getting more frustrating. The LHC, as of yet, has found no evidence for supersymmetric extensions to the Standard Model for particle physics. The least massive supersymmetric particle, if it exists, is a favored WIMP candidate. But detection limits on WIMPs from several experiments are becoming more severe, and are in conflict with other possible or claimed detections. One way out of this quandary may be that there is more than one type of WIMP particle responsible for dark matter (prior blog).

Or perhaps the dominant dark matter constituent is not a WIMP at all. Baryons (ordinary matter) only contribute 1/5 of the total mass, and only 5% of the total mass-energy of the universe. The dark matter component cannot be baryons – this is ruled out by the abundances of deuterium, helium, and lithium generated via nucleosynthesis during the first few minutes of the Big Bang.

Dark matter must be “cold” that is, moving at low to moderate speeds, based on the way galaxies are distributed and cluster together. If dark matter is not due to baryons and not due to WIMPs, what other alternative is there? Neutrinos and other known light particles are ruled out by observations.

But there is another light particle, the axion, which has not been observed, yet is a good candidate as a “weakly interacting light particle” explanation for dark matter. Axions do not require the existence of supersymmetry. They have a strong theoretical basis in the Standard Model as an outgrowth of the necessity to have charge conjugation plus parity conserved in the strong nuclear force (quantum chromodynamics of quarks, gluons). This conservation property is known as CP-invariance. (While CP-invariance holds for the strong force, the weak force is CP violating).

The neutron, composed of three quarks, is observed to have no electric dipole moment, to very high accuracy, indicating CP-invariance holds for the strong force. An additional mechanism (field) is required to “enforce” the invariance. Initial theoretical work was done by Nobel winning physicists Weinberg and Wilczek in the 1970s. The axion is the corresponding particle for this field, providing the favored explanation for allowing the preservation of CP-invariance.

The axion, if it exists, has a very low mass, in the range of over 1 micro-eV to as much as 10 milli-eV. (One eV, or electron-Volt is the energy from moving one electron through a potential of one Volt). By comparison, electron neutrino masses are less than 2 eV, but axions are probably a thousand or more times lighter than the elusive neutrino.

Image

Installation of the insert chamber into the magnet of the ADMX, October 4, 2013.

The Axion Dark Matter Experiment (ADMX) at the University of Washington is the best known experiment searching for axions as a principal component of dark matter. It relies on the prediction that axions can be converted to photons in the presence of a magnetic field. A well-designed experiment requires a large microwave cavity at very low temperature to minimize noise, and with a strong magnetic field. ADMX consists of a microwave cavity surrounded by an 8 Tesla magnet and cooled to liquid helium temperatures.

ADMX has not yet detected axions, but it has placed limits on their interaction cross-section for masses in the range of about 2 to 3 micro-eV. These limits are consistent with the two most popular theoretical models. The ADMX team is in the process of upgrading the apparatus to a sensitivity that would allow them to detect axions if they adhere to one of the two main axion dark matter models, and have mass less than 10 micro-eV. They are scheduled to be able to reach this goal by the end of 2015.

Another interesting possibility is that 110 micro-eV axions may explain an anomalous signal seen in an experiment with Josephson junctions, and thus are already detected. Josephson junctions consist of two superconducting elements separated by an insulating layer. If the axion mass is resonant with the junction frequency, a current spike would occur. An exceptionally high noise spike in one such prior experiment could conceivably be due to an axion resonance signal. We await further experiments to see if the behavior can be repeated at the same frequency.

And for fun, here’s an historical perspective on the axion, from the 1960s, featuring Arthur Godfrey:

 

References:

http://www.phys.washington.edu/groups/admx/home.html - ADMX web site

https://commons.lbl.gov/download/attachments/88868578/rybka_taup_2013.pdf?version=1&modificationDate=1378925423266 – talk by Gray Rybka from the ADMX team

http://arstechnica.com/science/2013/12/could-dark-matter-be-hiding-in-plain-sight-in-existing-experiments/ – Josephson junction anomaly

 

 


2013 in review

The WordPress.com stats helper monkeys prepared a 2013 annual report for this blog.

Here’s an excerpt:

A New York City subway train holds 1,200 people. This blog was viewed about 4,900 times in 2013. If it were a NYC subway train, it would take about 4 trips to carry that many people.

Click here to see the complete report.


Dark Matter Eludes LUX

The LUX (Large Underground Xenon) experiment has just announced results from their first run, which gathered data for 85 days between April and August of this year. LUX is located a mile underground (to shield from cosmic rays and other interference) in an old mine in South Dakota, and employs a liquid Xenon detector with total mass of 370 kg. LUX is searching for WIMP dark matter particles which recoil directly off the nucleus of Xenon atoms in the detector.

Image

Large Underground Xenon detector, Photo by Carlos Faham, CC Attribution 3.0 license

They have two important findings from this first swath of data. First, to within the sensitivity of their experiment, they detected no weakly interacting massive dark particles (WIMPs). And second, their experiment has much greater sensitivity than other experiments in the low mass range from about 5 GeV to 100 GeV (the proton rest mass is a bit less than 1 GeV). Thus it is placing much tighter constrains on the cross-section for dark matter to interact with a nucleus, and the density of dark matter at the Earth’s orbit. The previous largest Xenon-based experiment was XENON100 (for 100 kg total detector mass). With LUX, the sensitivity has improved over the results of that earlier experiment by around a factor of 20 for a possible 10 GeV WIMP mass, due to the larger target and better rejection of background events.

Image

This chart is the most interesting portion of Fig. 5 from the LUX first results paper (reference below). The blue line shows the upper limit on the cross section dropping from 10^-40 cm² to less than 10-44 cm² as a function of WIMP mass, as the mass increases from about 5 to 12 GeV (X-axis above the chart). Note the Y-axis is logarithmic, so the new limit is orders of magnitude below other limits (various colored curves) and claimed possible detections (shaded areas).

These new LUX results are in direct conflict with possible detections from CoGeNT (small red-shaded area on chart), CRESST (yellow-shaded), CDMS- II (green-shaded area), and DAMA/LIBRA (grey-shaded area), all of which were suggesting detections with a WIMP dark matter mass around 10 GeV. Now certain assumptions are made about the astrophysical parameters such as the density of dark matter at the Earth being the same as the average in our part of the galaxy. But other experimental results are based on similar assumptions, so this does not explain the discrepancy.

Both CoGeNT and CDMS-II sit in the same Soudan Laboratory in Minnesota, one state over from where the LUX experiment resides. However different experiments use different atoms as targets: CoGeNT uses germanium, CDMS uses silicon, CRESST uses calcium tungstate crystals and DAMA/LIBRA uses thallium doped sodium iodide. These latter two experiments are both located in Italy, in a mountain tunnel. All of these experiments are attempting to discern a very faint signal against significant backgrounds. And perhaps earn a Nobel Prize in Physics as well. So it’s natural for the researchers on the associated teams to lean toward optimism It remains quite possible, and now seems more and more probable, that the experiments other than LUX are observing some unexplained non-dark matter background effect, so this is a very significant result.

LUX is not finished, of course. It’s just getting going. So we await their further results, with either a possible WIMP detection in the future, or even tighter limits on the existence of lower mass WIMPs.

References:

http://luxdarkmatter.org – LUX consortium home page

http://t.co/1hyXRSiBsK – D.S. Akerib et. al., 2013,  “First results from the LUX dark matter experiment at the Sanford Underground Research Facility”

https://theconversation.com/dark-matter-experiment-finds-nothing-makes-news-19707 – LUX results are constraining WIMP parameter space

http://profmattstrassler.com/2013/10/30/breaking-news-two-great-new-measurements/ – on the LUX results


Higgs Boson and Dark Matter, Part 1

The discovery of the Higgs boson at the Large Hadron Collider (the LHC, at CERN, near Geneva) was announced on the 4th of July, 2012. This new particle was the important missing component of the Standard Model of particle physics; the Standard Model has had great success in describing the three non-gravitational forces: electromagnetism, the weak nuclear force, and the strong nuclear force.

The mass of the Higgs is about 126 GeV (giga electron-Volts) and by way of comparison the proton mass is a bit under 1 GeV.  The Higgs particle is highly unstable, with a decay lifetime of only about one tenth of one billionth of one trillionth of a second (10^-22 seconds). While the  Higgs field pervades all of space, the particle requires very high energy conditions to “pop out” of the field for a very short while. The only place where these conditions exist on Earth is at the LHC.

The Higgs boson is not detected directly at LHC, but inferred (with high confidence) through the detection of its decay products. The main decay channels are shown in the pie chart below, and include bottom quarks, W vector bosons (which mediate the weak force), gluons (which mediate the strong force holding quarks inside protons and neutrons), tau leptons (very heavy members of the electron family), Z vector bosons (also weak force mediators), and even some photon decay channels. While the two photon channel is rare, much less than 1% of the decays, it is the most important channel used for the Higgs detection because of the “clean” signal provided.

Image

What is the relationship between the Higgs and dark matter? In an earlier blog, http://wp.me/p1mZmr-3K , I discussed why the Higgs particle itself cannot be the explanation for dark matter. Dark matter must be stable; it must persist over the nearly 14 billion year lifetime of the universe. In today’s universe it’s very difficult and expensive to create a Higgs particle and it vanishes immediately.

But in the very early universe, at a tiny fraction of a second after its creation (less than the present-day Higgs boson lifetime!), the “temperature” and energy levels were so high that the Higgs particle (or more than one type of Higgs particle) would have been abundant, and as today it would have decayed to many other, lighter, particles. Could it be the source of dark matter? It’s quite plausible, if dark matter is due to WIMPs – an undiscovered, stable, weakly interacting massive particle. That dark matter is due to some type of WIMP is currently a favored explanation among physicists and cosmologists. WIMPs are expected from extensions to the Standard Model, especially supersymmetry models.

One possible decay channel would be for the Higgs boson to decay to two dark matter WIMPs. In such a decay to two particles (a WIMP of some sort and its anti-particle), each would have to have a rest mass energy equivalent of less than half of the 126 GeV Higgs boson mass; that is, the dark matter particle mass would have to be 63 GeV or less.

There may be more than one type of Higgs boson, and another Higgs family particle could be the main source of decays to dark matter. In supersymmetric extensions to the Standard Model, there is more than one Higgs boson expected to exist. In fact the simplest supersymmetric model has 5 Higgs particles! 

Interestingly, there are 3 experiments which are claiming statistically significant detections of dark matter, these are DAMA/LIBRA, COGENT,  and CDMS-II. And they are all suggesting a dark matter particle mass in the neighborhood of just 10 GeV. Heavy, compared to a proton, but quite acceptable in mass to be decay products from the Higgs in the early universe. It’s not a problem that such a mass might be much less than 63 GeV as the energy in the decay could also be carried off by additional particles, or as kinetic energy (momentum) of the dark matter decay products.

At the LHC the search is underway for dark matter as a result of Higgs boson decays, but none has been found. The limits on the cross-section for production of dark matter from Higgs decay do not conflict with the possible direct detection experiments mentioned above.

The search for dark matter at the LHC will actively continue in 2015, after the collision energy for the accelerator is upgraded from 8 TeV to 14 TeV (trillion electron-Volts). The hope is that the chances of detecting dark matter will increase as well. It’s a very difficult search because dark matter would not interact with the detectors directly. Rather its presence must be inferred from extra energy and momentum not accounted for by known particles seen in the LHC’s detectors.

 


Multi-component Dark Matter: Let there be Heavy and Let there be Light

Around 27% of the universe’s mass-energy is due to dark matter, according to the latest Planck satellite results; this finding is consistent with a number of other experiments as well. This is approximately 6 times more than the total mass due to ordinary matter (dominated by protons and neutrons) that makes up the visible parts of galaxies, stars, planets, and all of our familiar world.
 
The most likely explanation for dark matter is some kind of WIMP – Weakly Interacting Massive Particle. The amount of deuterium (heavy hydrogen) produced in the very early universe rules out ordinary matter as the primary explanation for the various “excess” gravitational effects we see in galaxies, groups and clusters of galaxies and at the largest distance scales in our observable universe.

WIMPs are thought to be a class of new, exotic particles, beyond the Standard Model. A favored candidate for dark matter is the lightest supersymmetric particle, although supersymmetry is as of yet unproven. The lightest such particle would be very stable, having nothing into which it could decay readily. A mass range of 5 to 300 GeV or so is the range in which most of the direct detection experiments looking for dark matter are focused (1 GeV = 1 giga-volt is a little more than the mass of a proton).

But there would certainly be more than one kind of such WIMP formed in the very early universe. These would be heavier particles, and most would quickly decay into lighter products, but there might still be some heavier WIMPs remaining, provided their lifetime was sufficiently long. Particle physicists are actively working on models with more than one component for dark matter, typically two-component models with a heavy particle above 100 GeV mass, and a lighter component with mass of order 10 GeV (more or less).

In fact there are observational hints of the possibility of both a light dark matter particle and a heavier one. The SuperCDMS team has announced a possible detection around 8 or 9 GeV. The COGENT experiment has a possible detection in the neighborhood of 10 GeV, and in general agreement with the SuperCDMS results.

ImageFermi satellite payload, photo credit: NASA/Kim Shiflett

At the heavier end, the Fermi LAT gamma-ray experiment has made a possible detection of an emission line at 130 GeV which might result from dark matter decay. A pair of such gamma rays could be decay products from annihilation of a heavier dark matter particle with mass around 260 GeV. Or a particle of that mass could decay into some other particle plus a photon with about half of the heavy dark matter mass, e.g. 130 GeV.

Kajiyama, Okada and Toda have built one such model with a light particle around 10 GeV and a heavier one in the 100 to 1000 GeV region. They claim their model is consistent with the observational limits on dark matter placed by the XENON 100 experiment.

Gu has developed a model with 2 dark matter components with magnetic moments. Representative values of the heavy dark matter particle mass at 262 GeV and the lighter particle mass at 20 GeV can reproduce the observed overall dark matter density. He also finds the decay lifetime for the heavier particle state to be very long, in excess of 10^20 years, or 10 billion times the age of the universe, thus in this model the heavy dark matter particle easily persists to the present day. In fact, he finds that the heavy particle dominates with over 99% of the total mass contribution to dark matter. And a heavy dark matter particle with the chosen mass can decay into the lighter dark matter particle plus an energetic 130 GeV gamma ray (photon). This might explain the Fermi LAT results.

Furthermore, the existence of two highly stable dark matter particles of different masses would help explain some astrophysical issues related to the formation of galaxies and their observed density profiles. Mikhail Medvedev has numerically simulated galaxy formation using supercomputers, with a model incorporating two dark matter components. This model results in a better fit to the observed velocities of dwarf galaxies in the Local Group than does a model with a single dark matter component. (The Local Group includes our Milky Way, the Andromeda galaxy, the Magellanic Clouds and a number of dwarf galaxies.) His model also helps to explain the more flattened density profiles in galaxies and the smaller numbers of dwarf galaxies actually observed relative to what would be predicted by single dark matter component models.

Occam’s razor says one should adopt the simplest model that can explain observational results, suggesting one should add a second component to dark matter only if needed. It seems like a second dark matter component might be necessary to fully explain all the results, but we will require more observations to know if this is the case.

References:

http://prd.aps.org/abstract/PRD/v88/i1/e015029 Y. Kajiyama, H. Okada and T. Toda 2013, “Multicomponent dark matter particles in a two-loop neutrino model”

http://www.sciencedirect.com/science/article/pii/S2212686413000058
P. Gu 2013, “Multi-component dark matter with magnetic moments for Fermi-LAT gamma-ray line”

http://arxiv.org/pdf/1305.1307v1.pdf M. Medvedev 2013, “Cosmological Simulations of Multi-Component Cold Dark Matter”

 

 

 


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