altDr. Alexander Kashlinsky is a principal investigator on several NASA and NSF grants studying topics related to cosmological bulk flows, cosmic microwave and infrared background radiation, and early stellar populations. Using the Wilkinson Microwave Anisotropy Probe, Kashlinsky recently discovered a phenomenon called “dark flow,” which are clusters of galaxies moving at a constant velocity toward a 20-degree patch of sky between the constellations of Centaurus and Vella.

NASA Tech Briefs: You have a PhD in astrophysics from Cambridge University in England and your area of expertise at NASA is observational cosmology. What prompted you to pursue a career in this field?

Dr. Alexander Kashlinsky: It actually started in my youth from reading too much science fiction, which I no longer do. I distinctly remember how it was triggered. I picked up a book from the shelf by Stanislaw Lem, called the “Magellanic Cloud,” which was about the first interstellar travel, and it conquered my mind at the time, but I ended up in astrophysics and not traveling to the stars. Later, when I was doing my PhD, I was very privileged to work with Martin Rees, who is a very inspirational scientist, and that triggered my interest in astronomy and particularly in cosmology. He was very open-minded, and very interested in completely different ideas, which I found very stimulating and very inspiring. The rest is history.

NTB: Several years ago you were part of a team that succeeded in isolating the energy radiated by the first stars formed after the Big Bang, called Population 3, from all other energy that makes up the cosmic infrared background. What did you learn from that breakthrough?

Kashlinsky: What we did at the time was, we analyzed very deep data available thanks to the Spitzer Infrared Telescope, and we were trying to find how much diffuse radiation is left after we removed the various contributions that we can isolate in the images. What we learned is that the residual diffuse background – the so-called cosmic infrared background radiation – has quite a bit of energy emitted from sources that are much too faint to be detected, even in deep Spitzer exposures. That most likely means that these sources are very far away, because we removed galaxies down to a very faint level, that is, very far away. They had very little time to radiate all this very substantial energy that we detected and, therefore, they had to be quite abundant and they had to be radiating at enormous rates compared to typical populations living today. That, in our opinion, meant these populations were dominated by very massive stars, or very massive black holes, that lived very short times, but each unit of their mass emitted so much more energy than present day stars – such as the sun – that they had to produce this signature.

What is important in this context is not only what we learned, but what we did not learn. With the current data we could not learn whether these sources were stars that emit their energy by converting hydrogen into helium, or they were massive black holes that existed in very early times and that emitted energy by accretion processes, by gas falling into them and emitting energy in the process.

NTB: More recently, using NASA’s Wilkinson Microwave Anisotropy Probe, you discovered a phenomenon you refer to as “dark flow.” What is dark flow?

Kashlinsky: What we set out to measure in that measurement was the so-called peculiar velocities of clusters of galaxies, which are deviations from the uniform expansion of the universe. We never expected to find what we found at the end. We designed a method several years ago to probe the expected – within the standard cosmological models – peculiar velocities. The trick was to use many, many clusters of galaxies whereby you detect a very faint signal by beating down the noise. So we teamed up with colleagues at the University of Hawaii who assembled this x-ray cluster catalogue, and applied that method to the Wilkinson Microwave Anisotropy Probe, and we were very surprised by the results.

We found a flow that does not decrease with distance as far as we could tell, and we could probe to several billion light years away from us. It roughly was a constant amplitude, whereas in the standard cosmological model you expect that it should’ve been decreasing linearly with increasing scale. That is, as you go from, say, a few hundred-million light years to a few billion light years, it should decrease by an order of magnitude. We did not find that. We found a more or less constant velocity all the way as far as we can probe. The reason we called it dark flow is because the matter distribution in the observed universe, which is very well-known from galaxy surveys and from cosmic microwave background anisotropy measurements, that matter distribution cannot account for this motion. So this is why we suggested that if this motion already extends so far, then it probably goes all the way across the observable universe to the so-called cosmological horizon, and it is caused by the matter inhomogenity, or, I should say, space-time inhomogenity, at very large distances well beyond the cosmological horizon, which is about 40 billion light years away from us.