Occam's Machete: Cosmology

There is a spot in the sky at the edge of the observable universe where galaxies are moving towards something that cannot be observed and is probably beyond the edge of what we can observe. I’ve written several times (here and here) about dark flow since it was observed in 2008 by Kashlinsky and about one possible explanation for it here. In the time since dark flow was originally observed, the Kashlinsky data has been vetted pretty well.

But what is causing it? Kashlinsky thinks that a dense region of the universe exists just outside our observation horizon and imagines that this implies a non-uniformity to the universe, one that is great enough to call into question the Lambda Cold Dark Matter model. This model leads to a very uniform universe caused by early hyper inflation, just after the big bang. The CMB shows a pretty uniform universe in agreement with LCDM. So Kashlinsky is casting doubt on LCDM, basically saying that the universe may be very lumpy we just can’t see the lumps in the CMB from here.

Enter Dai et al who use a fundamentally different method to measure the flow of individual objects. They examine the type 1A supernova data to see if there is a relationship between the very far away novas (those with a high red shift) and their velocity towards the path of dark flow; they find no correlation, which means that the furthest away novas aren’t really moving faster than the closer ones in the path of the flow. Furthermore they find the likely velocity of the flow to be much smaller than the Kashlinsky suggests. Kashlinsky measured an aberrant bulk flow of more than 600 kilometers a second, while Dai et al found only 188 kilometers a second. Dai et al observe that this velocity is very close to that predicted by LCDM.

So what is one to conclude? Well Dai is measuring individual objects to see if there is a generalization to make about their movement and Kashlinsky is looking at the movement of great globs of matter. Also the methods they use are different. The main thing that the new data questions is whether the observations imply LCDM is wrong. Even within LCDM predictions something is going on out there that has yet to be explained very well. So we continue to observe and perchance to speculate…

Image courtesy of universe-review.ca which (though really slow) has an incredible array of photos and illustrations.

How big is the Universe and if there are many, what would happen if they collided? Not a new question and several answers have been proposed (here is one). Another interesting question is why does time move in a single direction? Why does entropy increase? Laura Mersini-Houghton proposes an idea that answers both questions saying that bubble universes form with different arrows of time, eventually adding up to no preferred direction. She even claims to have a way to test this by looking our into the dark for boundaries.

Some have proposed that the Dark Flow may be one of these collisions, and the (now controversial) Great Void may be another. Where bubble universes are pulling and pushing on ours in places. FQXi has given her a grant to pursue the idea.

Click to download the movie-all 18M of it.

There’s more news on that weird sucking from outside the visible universe.  This was first noted by Alexander Kashlinsky at Goddard Space Flight Center and I wrote about it here.  The team has now tracked the flow to twice the distance reported back in 2008—to 2.5 billion light years away.  This video shows the flow for different distances from the Earth.  Unfortunately they don’t know yet for sure whether the flow is away from us or towards us.  Everyone assumes it is away and there is some data to say that this assumption is true.  At least if it is flowing away from us we can blame it on something outside our light horizon, something really really big.  The galaxy clusters studied are travelling at a million miles per hour.  Read about the new data here.