Friday, December 16, 2011

Luke-warm dark matter: Bose-condensation of ultra-light particles

Galaxy Rotation Curves. Source:
What are ultra-light scalar particles? Particle physicists have already detected the Higgs boson. The Higgs is the first fundamental spin zero particle ever found! It weighs 120some GeV (more than 120 times heavier than a proton) and is produced by colliding particle beams at energies of several trillion electron volts in the largest particle accelerator on Earth.

However, there has been little exploration of the light side of the spectrum. The ultra-light scalar particles that I study should be comparable in size to a galaxy and have really light masses of about 10−23 eV. The mass of such ultralight dark matter particle is about 10−29 lighter than that of an electron!

Ultralight scalar field dark matter
 Agglomerations of ultra light scalar particles could form dark matter halos that prohibit structure formation on scales much smaller than the "size" of the particle. They thus avoid the overabundance of dwarf galaxies matching more closely to observations than other models. They also may be favored by observed dark matter distributions. Such particles would be photon-like if they roamed freely. However, given a large particle-antiparticle asymmetry, they Bose condense into dark matter halos. The condensate ends up being very pure with most particles in the ground state. The ones in excited states end up photon-like. The amount of hot dark matter is constrained more tightly by the latest Planck measurements.

Lukewarm dark matter
Ultralight scalar particles can Bose condense at finite temperatures when the temperature of the condensate is significantly larger than the mass of one boson (Urena-Lopez 2008). The light particles we know have a non-zero temperature. The temperature of CMB photons is 2.73 K, neutrinos decoupled at 1.9 K, and dark matter particles could also be lukewarm. We find that the temperature of these dark matter particles would be around 0.9 K if they decouple from regular matter before Standard model particles annihilate.

Constraints by Planck
Planck found that the effective number of neutrinos is 3.3±0.5. The standard deviation is still large and they have to combine their data with other surveys to obtain stricter constrains. More Planck data will also help. For our model, an Neffective of 3.3 corresponds to a T < 1.35 K for the scalar field, which is met by easily met since the natural temperature for scalar field dark matter is T ~ 0.9 K. If Neffective was 3.1, then the constraint would be tighter - T < 0.96 K but still within bounds. So, it not likely that this model will be ruled out by Planck observations or other similar experiments in the near future.

Why are ultralight scalar fields interesting in cosmology?
Ultralight scalar fields can be used to explain dark matter and dark energy with relatively simple mathematical models. Of course, they are also used in particle physics and string theory models. They are simple and they are seen pretty much everywhere in fundamental physics. The Higgs particle was recently detected, but there is no hint of other fundamental spin zero particles or of supersymmetry so far, which, in some sense, makes it easier for theorists .... why? ... well, because imaginary particles can have any properties whatsoever as long as they do not violate known experiments.

How do go about studying invisible matter? Well, direct studies can only be done on the Higgs particle because that is the one scalar particle we have found so far. There are experiments that search for dark matter particles on Earth looking for much heavier dark matter particles than the ones we consider, but their results have been null to date. Indirectly, we count galaxies, look at their sizes, mass distributions to try learn about dark matter.  We have looked at primordial light with WMAP and now with the Planck satellite, which tells us about the composition of the universe and provide limits on the amount of hot dark matter. Big Bang Nucleosynthesis measurements constrain the relic abundances of deuterium and 4He putting a bound on Neffective. There are also tests for violations of the Einstein's equivalence principle (EEP) on Earth and proposed tests of the EEP in space that could detect hints of dark matter or dark energy.

How to rule out ultralight scalar fields?
The direct way would be to somehow find dark matter particles and prove that they are heavier and that they are the dominant dark matter constituents. We could find "small" scale dark matter substructure, e.g., proof of the existence of a dark matter halo around the Sun or the Earth. It is also totally not clear how the huge particle-anti-particle asymmetry arose in the early universe. Without the asymmetry there would be noting to cause the particles to Bose condense to become halos. They could not all exist as hot dark matter because Planck and other experiments have not seen any excess of neutrinos, and so they would be ruled out. However, there is much more matter than antimatter in visible matter as well and we could not think of a good reason why there would not be an asymmetry for fundamental spin zero particles.
This post was motivated by an informal seminar by my brother, Mihai Bondarescu, on our  2010 Astrophysical Journal Letter at the University of Zurich. I was surprised at how much I forgot about this project.  Mihai gave a very  good talk that was followed by an interesting discussion. Since this discussion could result in potential collaborations, I thought I would summarize both our own work to remind myself of what we learned and why we did it and some of the work that has been done on this topic since 2010.

Note to self: I finished this post one year and 4 months after initially publishing it. I procrastinate too much to publish unfinished posts. Therefore the experiment should not be repeated.

Tuesday, December 13, 2011

LHC press conference: Have they found the Higgs particle?

Credit: CERN.
There was a press conference today at 2 p.m.  The range in which the Higgs particle could exist is narrowed to 115 to 130 GeV with 95% confidence. The mass of the Higgs appears to be around 125-126 GeV, but more data needs to be taken to declare a detection.

What is the Higgs boson?
The Higgs boson is a spin zero, scalar particle that is necessary to explain how most elementary particles obtain their mass in the Standard Model. The Higgs mechanism gives mass to every elementary particle that couples with the Higgs boson including the Higgs itself.

Thursday, December 8, 2011

The Most Massive Black Holes

Simulation of gas falling into a supermassive BH. Credit: NASA/JPL-Caltech/JHU/UCSC
Two ten billion solar mass black holes were found. The first one is in the brightest galaxy cluster at a distance of 98 Mpc from Earth, NGC 3842. It weighs about 9.7 billion solar masses. The second supermassive black hole is at the center of NGC 4889 at a distance of 104 Mpc from us. It weighs about 21 billion solar masses. Until now the record for the most massive black hole was held by a black hole in the center of the M87 galaxy that weighs 6.3 billion solar masses. This post is mainly a summary of the Nature article.

How do scientists estimate black hole masses?
Empirical scaling relations between the black hole mass, galaxy bulge dispersion and luminosity are used to estimate the black hole masses in galaxies where a direct measurement is not possible due to large distance from Earth or low central stellar density.  These two black holes are special because scientists were able to directly measure their masses, i.e., they measured the stellar velocities of the central regions of the host galaxy accurately enough to determine the black hole mass. The two black hole masses did not agree with the predictions, which were off by several standard deviations (about a factor of 10 in both cases).

A rapidly growing BH in Markarian 231. Credit: NASA/ESA Hubble Telescope
What instruments did they use? 
The line of sight stellar velocities were measured using the Gemini North and Keck 2 telescopes, in Hawaii. The stellar luminosity distribution of each galaxy is provided by surface photometry from Hubble and ground based telescopes.

Possible conclusions?
The most massive black holes appear to be in the brightest cluster elliptical galaxies and not in brightest field elliptical galaxies. Clusters contain more objects and so more mergers are likely to occur. Many predictions are contradictory. Direct measurements of more black holes masses will help revise these relations. So far it appears that the mass vs. velocity dispersion relation either disappears or steepens at high masses. The steepening can occur due to the accretion of residual gas after star formation stops, which is likely in these very massive galaxies.

What are the progenitors of these black holes?
Black holes heavier than 10 billion solar masses are observed as quasars in the early universe (a few billion years after the big bag). Quasars contain young, rapidly accreting black holes. However, as time passes, the accretion slows down and they become regular galaxies. Quasars are still not well understood. Much more research needs to be done.

Tuesday, December 6, 2011

Kepler Confirms Its First Planet in a Habitable Zone

Green area = habitable zone. Credit: NASA-JPL-Caltech
The Kepler mission confirmed the first Earth-like planet in its habitable zone. See the NASA briefing.  Its name is Kepler-22b and it is located about 600 light years away from us. This planet orbits a solar mass star (mass 0.97 times the mass of our sun) every 290 days. Its radii is 2.4 times bigger than the radius of our Earth. This means that the gravity on it will be stronger than on Earth. They say the near surface temperature of this planet is about 22 Celsius. The Kepler data so far resulted in over 2, 300 planet candidates. Out of these planets about 48 are believed to be in the habitable zone of their star.

How Does the Kepler Satellite Work?
The Kepler Satellite monitors the brightness of more than 100,000 stars for the life of the mission, which is expected to be extended beyond its current 3.5 years. The satellite was launched in March 2009. It observes the changes in brightness of these stars when planets pass in front of them. The size of a transiting planet is found from the size of its star and 'the deepness of the transit', which is the decrease in brightness of a star when a planet passes in front of it. Since Earth-like planets create 'small' dips in brightness, i.e., close to the noise level of the instrument, the Kepler mission requires three transits to declare a detection. This, of course, takes time and it is why we have to wait before even smaller Earth-like planets will be reported.

Friday, December 2, 2011

David's Green Science

My nephew, David, is 4 years old. He and my brother powered a clock using an apple, two wires connected to the clock and two electrodes. The clock was still working 24 hours later and showed the correct time. The electrodes in the potato battery kit can be replaced by other metals such as a nail and a coin. That's right! There is a kit for kids for building this type of battery for potatoes, but it works with most other fruits and vegetables.

Warning: The apples should not be eaten after they are used in this type of battery. They will contain zinc, which is toxic.

How does it work?  The zinc ions and copper ions are separated by the apple. Otherwise they would interact with each other and produce a little bit of heat. The electron transfer takes place over the copper wire, which powers our clock. Chemical energy is converted into electric energy by electron transfer. The electricity produced is not much (definitely not enough for a person to feel), but clocks need very little energy to run and this makes them ideal for this project.