Astronomers have counted up the number of clusters in the cosmos and found a problem.
All is not quite right in the world of cosmology.
Last year, the Planck mission team released its first 15.5 months of temperature observations of the cosmic microwave background (CMB), the leftover radiation from the Big Bang. As I reported at the time, the observations were largely in phenomenal agreement with the status-quo cosmological model.
But the Planck data don’t entirely tie the universe up in a nice tidy bow. Some oddities arose. One of the lingering mysteries has to do with the number of galaxy clusters.
Galaxy clusters are essentially big lumps in the distribution of matter in the universe. Although clusters are relatively recent developments in the cosmos, the lumpiness they reveal should correspond to matter’s lumpiness in the primordial universe. The minute temperature fluctuations in the CMB reveal this primordial lumpiness, so with the right theoretical model, astronomers should be able to match up the two sets of observations, explains David Spergel (Princeton).
But as researchers reported at the winter American Astronomical Society meeting, it’s not proving that easy. James Bartlett (JPL and APC Université Paris Diderot-Paris 7, France) presented a map of 189 galaxy clusters based on Planck observations of what’s called the Sunyaev-Zeldovich (SZ) effect. The SZ effect happens when CMB photons steal energy from hot gas as they pass through it. This effect means that galaxy clusters (which are full of hot gas) basically leave “shadows” in the CMB. By looking for clusters’ fingerprints in microwave observations, astronomers can locate distant galaxy clusters and count how many existed at different times.
The number of galaxy clusters found using the SZ effect’s shadows match those tallied using X-ray and optical surveys. But it doesn’t match what researchers predict based on the clumpiness in the CMB. In fact, as Brad Benson (Fermilab/University of Chicago) explained in a subsequent talk, the CMB as seen by Planck suggests that the universe should have 2.5 times more galaxy clusters than astronomers actually observe. The problem isn’t going away, either: his team’s preliminary analysis of about 300 clusters, using the clusters’ SZ fingerprints in data from the South Pole Telescope, also show conflict with Planck’s CMB-based cosmology.
There are several potential explanations. One, the tension might not actually be there. Astronomers can calculate a cluster’s mass by extrapolating from X-ray observations, or by detecting its weak gravitational lensing effect when the cluster’s gravity slightly bends light that is passing by it en route to us. The assumptions that go into relating a cluster’s X-ray luminosity and its mass might be off. If all cluster masses are low by a factor of about 1.4, that could resolve the problem.
That might sound like an easy fix, but it’s not. Masses calculated using X-ray observations and weak lensing are consistent within about 10 to 15% of each other, Benson said. The chances of the measurements being off enough to match CMB predictions are less than 1 in 300 at best.
Neutrinos might also be to blame. Neutrinos are the second most common particle in the universe, after photons, and the majority was created during the Big Bang, Benson explains. Scientists know the particles should have some tiny mass (still undetermined). If neutrinos are 4 to 5 times heavier than the lower limit calculated from experiments, these relativistic particles could be the answer.
Or perhaps scientists’ analysis of the CMB temperature map is off, and the primordial matter fluctuations aren’t as strong as the Planck team proposes. An independent analysis by Spergel’s team suggests such is the case, which would ease the apparent discrepancy.
“This is, I think, a wonderful situation,” Spergel said at the meeting. In the next year or two, astronomers will either have evidence for new physics or have done away with the tension, he explained. “It’s an exciting moment.”
Part of the solution could come from the 5-year Dark Energy Survey (based in Chile), which has begun its study of the universe’s expansion and structure growth history. The effort includes studying how the number of galaxy clusters in a given volume has changed with time. At the moment, astronomers have observed about 1,300 clusters with the SZ effect; DES is expected to detect 100,000 (about one-hundredth the cosmos’s expected total), spread out over the latter half of the universe’s existence.
DES’s field of view overlaps that of the South Pole Telescope, which observes at the other end of the spectrum. If both sets of observations uphold the discrepancy, then it could mean cosmologists are indeed missing something.
AAS presentations (click here for meeting program):
J. G. Bartlett et al. "Planck Cluster Cosmology Results." Abstract #135.01
B. Benson et al. "The South Pole Telescope Cluster Survey." Abstract #135.02
Planck Collaboration. "Planck 2013 results. XX. Cosmology from Sunyaev-Zeldovich cluster counts."
Planck Collaboration. "Planck 2013 results. XXIX. Planck catalogue of Sunyaev-Zeldovich sources."