Tag Archives: XMM-Newton

Does Dark Energy Vary with Time?

Einstein introduced the concept of dark energy 100 years ago.

The Concordance Lambda-Cold Dark Matter cosmology appears to fit observations of the cosmic microwave background and other cosmological observations including surveys of large-scale galaxy grouping exceedingly well.

In this model, Lambda is shorthand for the dark energy in the universe. It was introduced as the greek letter Λ into the equations of general relativity, by Albert Einstein, as an unvarying cosmological constant.

Measurements of Λ indicate that dark energy accounts for about 70% of the total energy content of the universe. The remainder is found in dark matter and ordinary matter, and about 5/6 of that is in the form of dark matter. 

Alternative models of gravity, with extra gravity in very low acceleration environments, may replace apparent dark matter with this extra gravity, perhaps due to interaction between dark energy and ordinary matter.

The key point about dark energy is that while it has a positive energy, it rather strangely has a negative pressure. In the tensor equations of general relativity the pressure terms act as a negative gravity, driving an accelerated expansion of the universe.

In fact our universe is headed toward a state of doubling in scale in each dimension every 11 or 12 billion years. In the next trillion years we are looking at 80 or 90 such repeated doublings.

That assumes that dark energy is constant per volume over time, with a value equivalent to two proton – antiproton pair annihilations per cubic meter (4 GeV / m³).

But is it?

The Dark Energy Survey results seem to say so. This experiment looked at 26 million galaxies for the clustering patterns, and also gravitational lensing (Einstein taught us that mass bends light paths).

They determined the parameter w for dark energy and found it to be consistent with -1.0 as expected for the cosmological constant model of unvarying dark energy. See this blog for details:

The pressure – energy density relation is:

P = w \cdot \rho \cdot c^2

The parameter w elucidates the relation between the energy density given by ρ and the pressure P. This is called the equation of state. Matter and radiation have w >= 0. In order to have dark energy with a negative pressure dominating, then w should be < -1/3. And w = -1 gives us the cosmological constant form.

EquationofStateImage credit:

Cosmologists seek to determine w, and whether it varies over time scales of billions of years.

The Concordance model is not very well tested at high redshifts with z > 1 (corresponding to epochs of the universe less than half the current age) other than with the cosmic microwave background data. Recently two Italian researchers, Risaliti and Lusso have examined datasets of high-redshift quasars to investigate whether the Concordance model fits.

Typically supernovae are employed for the redshift-distance relation, and cosmological models are tested against the observed relationship, known as the Hubble diagram. The authors use X-ray and ultraviolet fluxes of quasars to extend the diagram to high redshifts (greater distances, earlier epochs), and calibrate observed quasar luminosities with the supernovae data sets.

Their analysis drew from a sample of 1600 quasars with redshifts up to 5 and including a new sample of 30 high redshift z ~ 3 quasars, observed with the European XMM-Newton satellite.

They claim a 4 standard deviation variance for z > 2, a reasonably high significance.

Models with a varying w include quintessence models, with time-varying scalar fields. If w decreases below -1, it is known as phantom energy. Their results are suggestive of a value of w < -1, corresponding to a dark or phantom energy increasing with time.

For convenience cosmologists introduce a second parameter for possible evolution in w, writing as:

     w = w0 + wa*(1-a)   ,where a, the scale factor equals 1/(1+z) and a = 1 for present day.


The best fit results for their analysis are with w0 = -1.4 and wa ~ 1, but these results have large errors, as shown in Figure 4 above, from their paper. Their results are within the red (2 standard deviation, or σ) and orange (3σ) contours. The outer 3σ contours almost touch the cosmological constant point that has w0 = -1 and wa = 0.

These are intriguing results that require further investigation. They are antithetical to quintessence models, and apparently in tension with a simple cosmological constant.

The researchers plan on further analysis in future work by including Baryon Acoustic Oscillation (large scale galaxy clustering) measurements at z > 2.

References – Results from Dark Energy Survey of galaxies

Risaliti, G. and Lusso, E. 2018 Cosmological constraints from the Hubble diagram of quasars at high redshifts




Dark Matter: Made of Sterile Neutrinos?


Composite image of the Bullet Group showing galaxies, hot gas (shown in pink) and dark matter (indicated in blue). Credit: ESA / XMM-Newton / F. Gastaldello (INAF/IASF, Milano, Italy) / CFHTLS 

What’s more elusive than a neutrino? Why a sterile neutrino, of course. In the Standard Model of particle physics there are 3 types of “regular” neutrinos. The ghost-like neutrinos are electrically neutral particles with 1/2 integer spins and very small masses. Neutrinos are produced in weak interactions, for example when a neutron decays to a proton and an electron. The 3 types are paired with the electron and its heavier cousins, and are known as electron neutrinos, muon neutrinos, and tau neutrinos (νe, νμ, ντ).

A postulated extension to the Standard Model would allow a new type of neutrino, known as a sterile neutrino. “Sterile” refers to the fact that this hypothetical particle would not feel the standard weak interaction, but would couple to regular neutrino oscillations (neutrinos oscillate among the 3 types, and until this was realized there was consternation around the low number of solar neutrinos detected). Sterile neutrinos are more ghostly than regular neutrinos! The sterile neutrino would be a neutral particle, like other neutrino types, and would be a fermion, with spin 1/2. The number of types, and the respective masses, of sterile neutrinos (assuming they exist) is unknown. Since they are electrically neutral and do not feel the standard weak interaction they are very difficult to detect. But the fact that they are very hard to detect is just what makes them candidates for dark matter, since they still interact gravitationally due to their mass.

What about regular neutrinos as the source of dark matter? The problem is that their masses are too low, less than 1/3 of an eV (electron-Volt) total for the three types. They are thus “too hot” (speeds and velocity dispersions too high, being relativistic) to explain the observed properties of galaxy formation and clumping into groups and clusters. The dark matter should be “cold” or non-relativistic, or at least no more than “warm”, to correctly reproduce the pattern of galaxy groups, filaments, and clusters we observe in our Universe.

Constraints can be placed on the minimum mass for a sterile neutrino to be a good dark matter candidate. Observations of the cosmic microwave background and of hydrogen Lyman-alpha emission in quasar spectra have been used to set a lower bound of 2 keV for the sterile neutrino’s mass, if it is the predominant component of dark matter. A sterile neutrino with this mass or larger is expected to have a decay channel into a photon with half of the rest-mass energy and a regular (active) neutrino with half the energy.

A recent suggestion is that an X-ray emission feature seen at 3.56 keV (kilo-electron Volts) from galaxy clusters is a result of the decay of sterile neutrinos into photons with that energy plus active (regular) neutrinos with similar energy. This X-ray emission line has been seen in a data set from the XMM-Newton satellite that stacks results from 73 clusters of galaxies together. The line was detected in 2 different instruments with around 4 or 5 standard deviations significance, so the existence of the line itself is on a rather strong footing. However, it is necessary to prove that the line is not from an atomic transition from argon or some other element. The researchers argue that an argon line should be much, much weaker than the feature that is detected.

In addition, a second team of researchers, also using XMM-Newton data have claimed detection of lines at the same 3.56 keV energy in the Perseus cluster of galaxies as well as our neighbor, the Andromeda galaxy.

There are no expected atomic transition lines at this energy, so the dark matter decay possibility has been suggested by both teams. An argon line around 3.62 KeV is a possible influence on the signal, but is expected to be very much weaker. Confirmation of these XMM-Newton results are required from other experiments in order to gain more confidence in the reality of the 3.56 keV feature, regardless of its cause, and to eliminate with certainty the possibility of an atomic transition origin. Analysis of stacked galaxy cluster data is currently underway for two other X-ray satellite missions, Chandra and Suzaku. In addition, the astrophysics community eagerly awaits the upcoming Astro-H mission, a Japanese X-ray astronomy satellite planned for launch in 2015. It should be able to not only confirm the 3.56 keV X-ray line (if indeed real), but also detect it within our own Milky Way galaxy.

Thus the hypothesis is for dark matter composed primarily of sterile neutrinos of a little over 7.1 keV in mass (in E = mc^2 terms), and that the sterile neutrino has a decay channel to an X-ray photon and regular neutrino. Each decay product would have an energy of about 3.56 keV. Such a 7 keV sterile neutrino is plausible with respect to the known density of dark matter and various cosmological and particle physics constraints. If the dark matter is primarily due to this sterile neutrino, then it falls into the “warm” dark matter domain, intermediate between “cold” dark matter due to very heavy particles, or “hot” dark matter due to very light particles.

The abundance of dwarf satellite galaxies found in the Milky Way’s neighborhood is lower than predicted from cold dark matter models. Warm dark matter could solve this problem. As Dr. Abazajian puts in in his recent paper “Resonantly Produced 7 keV Sterile Neutrino Dark Matter Models and the Properties of Milky Way Satellites”

the parameters necessary in these models to produce the full dark matter density fulfill previously determined requirements to successfully match the Milky Way galaxy’s total satellite abundance, the satellites’ radial distribution, and their mass density profile..