NTB: It’s been theorized that this dark flow may somehow be related to inflation, the brief hyper-expansion of the universe that occurred shortly after the Big Bang. Can you explain that relationship to us?
Kashlinsky: Yes. What we suspect is happening is the following. Inflation was designed, if I’m not mistaken in the early 1980s, to explain why the universe we see around us is homogeneous and isotropic. It is homogeneous – it’s roughly the same on all scales – and isotropic – it’s roughly the same in every direction.
Now, the way inflation works is as follows. It says that at some very early time the universe, or the underlying space-time, was not homogeneous. What happened then was that there was some bubble, a very tiny bubble of space-time, which, by pure chance, happened to be homogeneous by purely casual process, and then because of the various high-energy processes in the early universe this bubble, along with the rest of the space-time, expanded by a huge amount. We, today, live inside a tiny part of that original homogeneous bubble and we, therefore, see the universe around us as homogeneous and isotropic because the scales of inhomogenities that are other bubbles have been pushed away very, very far. What it means, at the same time, is that the original space-time was not homogeneous. If we go sufficiently far away, we should see the remnants of the pre-inflationary structure of the universe, of the space-time. These remnants would cause a very long wavelength wave across our universe and because there would be a gradient in this wave from one edge of the universe to the other, or from one edge of the cosmological horizon to the other, we would see a certain tilt, or the matter would be flowing from one edge to the other.
The analogy I could think of is as follows. Suppose you are in the middle of a very quiet ocean and you see the horizon, which determines how far you can see. As far as you can see, the ocean is isotropic and homogeneous. You would then think, at first, that the entire universe is just like what you see locally, that it’s homogeneous and isotropic like your own horizon. But then, inside that ocean, you discover a very faint stream from one edge of the horizon to the other...a flow. From the existence of that flow, you could deduce that somewhere very far away there should be structures that are very different than what you see locally. There should be mountains for this flow to flow from, or some ravines for this flow to fall into. So that would give you a probe of what the underlying very large scale structure of what your universe, or space-time, or some today call it multiverse, is that it is not just like what you see locally, but that sufficiently far away your space-time is very different from what you see here. So, in that sense it’s very much in agreement with the underlying inflationary paradigm that the initial space-time was very inhomogeneous, and we just happen to live inside a very homogeneous and isotropic bubble, but if we were to go very, very far away, we should be able to see such inhomogenities.
NTB: The galaxy clusters that make up this dark flow are rapidly moving toward a 20-degree patch of sky between the constellations of Centaurus and Vella. Why there, and do we know what’s attracting them?
Kashlinsky: Our limit on the 20-degree patch is purely due to observational error. If we were to make this measurement with, say, an infinite sample of clusters of galaxies and infinitely noiseless cosmic microwave background data, we presumably would measure just one uniform direction measurement. Why there? It’s by pure chance. It just happens to flow in that particular direction. As for what’s attracting them, we know that such flow cannot be generated by the matter distribution inside the observable universe, inside the universe that we observe. So, we therefore concluded that it must be something else very, very far away from us that is attracting them.
NTB: What impact, if any, does the discovery of dark flow have on our understanding of the universe and how it works?
Kashlinsky: What it tells us is that what we call, today, the universe is part of the overall cosmos, the overall space-time, whose structure is very different than what we see locally. Today, various issues of terminology that, at first, what people would call universes essentially...people would think that this is all the space-time there is and the universe, by definition, is all that there is in it. Today, people start talking in terms of multiverse, and multiverse is then composed of the various universes such as our own — that is, our own cosmological horizon, or our own bubble in the terms of this inflationary language. But there could be various other universes in this multiverse, in this landscape in which we live.
So, in that sense, what these measurements may imply is that our universe is just one of many and others may be very different from ours, and that there is an underlying multiverse in which these universes exist. So, if you would, it could imply an ultimate Copernican principle. It could generalize it, ultimately, that not only is our planetary system one of many, and our planet one of many, our universe may be just one of many.