Occam's Machete: Cosmology

One of the biggest stories in Cosmology over the last two decades has to be the discovery of the accelerating expansion of the universe. It left most scientists gob-smacked but has received no serious criticism since the evidence, mostly taken from type 1-A supernovas, continues to pile up. If the red shift of the 1-A flashes were the only evidence there might be other explanations for the data. Shouldn’t there be some other independent way to verify the expansion data, something that helps validate the explanation?

I love watching for these weird effects and anomalies because, once in a great while they pan out to be the real deal and lead to new science. Enter an international team of researchers led by Masamune Oguri at Kavli IPMU and Naohisa Inada at Nara National College of Technology who have conducted a unique survey of gravitational lensing effects. They calculate the probability of lensing at various times in the past to produce a model. But when the model is fit to the survey data it produces an acceleration very much consistent with the type 1-A red shift measurements. Woohoo, one more vote for Einstein’s Cosmological Constant.

Implicit in most of modern cosmology is the Cosmological Principal, which states the assumption that the universe is isotropic—operates with the same laws and same fundamental constants everywhere. Any observer anywhere in the universe would see, for instance, electromagnetism operate with the same strength. There is a lot of evidence that the principal holds true as far as we can measure it.

About decade ago a survey on one patch of the sky hinted that the measure of electromagnetism’s fine structure constant α may vary over time. Professor John Webb of UNSW and his colleges revisited the study recently with more data from a new source. They find that in one direction of the sky α is greater and in the other it is less than we observe on earth. They do this by studying the absorption spectra of quasars and supply the usual caveats about as yet unknown systemic biases.

If these observations prove out then it means the Cosmological Principal is only an approximation. Just as Newtonian Gravity works well for most calculations and must be replaced with Einstein’s Relativistic Gravity in others. Another implication, since the observations of α vary with direction and fit to a dipole, is that the laws of physics may depend on the observer’s location in space and time.

This study says nothing about other laws of physics but it does make finding a way to measure them across space and time a more interesting endeavor and there are competing theories of everything that would gain a boost from observation of these variations.

IMAGE CREDIT: Appears in published version of the paper, http://prl.aps.org/abstract/PRL/v107/i19/e191101

Much of modern cosmology is built on the idea that the universe is uniform everywhere and in all directions. If it wasn’t uniform that would be a pretty interesting discovery. So far there is precious little evidence that contradicts nearly perfect uniformity at large scales. But it may be very hard to detect non-uniformity. There are several theories about the cosmos beyond our universe that would seem to predict effects we could look for. The telltale signature of some of these theories would be circular anomalies in the CMB and a few have been posted about on this blog here, here, and here.

For example what if the cosmos is filled with many big bangs and our universe is only one bubble in a foam of universes? That is the central question in the idea of Eternal Inflation. A lot of people have thought about the implications of such a model and many have claimed to have seen evidence of such universe collisions. Usually when such claims are made established cosmologists come down hard on the observations as not supported by the data. But how do we decide if an observation is supported or not?

A team comprised of members of the University College London, the Imperial College London, and The Perimeter Institute have tackled the question. They start with the assumption that a hypothetical collision would result in some sort of disk-like structure in a 2D picture of the universe like the WMAP image of the CMB. They then simulate the structure and try to find the most conservative measure for detection. In other words they try to find the 2-sigma rule for detection of a disk. They also define a Bayesian parameter metric for the probability that a detected anomaly is real and not just a trick of chaos. The full paper is here.

This picture shows four of some 10 candidates for possible collision sites in the WMAP data that they found of interest. Two of these sites have been noticed before, one is the famous ‘cold spot’ visible to the naked eye (at least if the naked eye could see microwaves) and another, a hot spot, is described here. Their conclusion is that there is no 2-sigma observation of a disc but that there are some candidates that are never the less significant. Here is a picture of the candidates.

I can’t help thinking that the CMB is a snapshot taken at the time of inflation at the very beginning of the universe when it grew to almost its present size. There are other observations of the universe that reflect effects that have happened since inflation slowed down. For instance speed and direction of movement as in ‘dark flow’ or other observations of behavior seen since the universe emerged from the dark time with the formation of the first stars. I wonder if similar techniques could be applied to that data because if there were a collision during inflation, it would have continuing for the rest of the 13 some odd billion years the universe has been around and should be observable in other ways. Even without using observations of more recent events the authors look forward to applying their techniques to the coming Planck satellite CMB data.

This paper is good for taking a step towards refining how to see order in the chaos and sheds light on how to measure the difference.

Left Handed Galaxy

One of the central ideas in modern cosmology is that the universe is pretty much uniform throughout. You have to look on the large scale to see the uniformity, like at the cosmic microwave background radiation or at the distribution of matter. There are still mysteries like, why is there so much more matter than anti-matter? Seems like they should have been created in equal portions at the Big Bang and proceeded to annihilate each other, leading to a universe with no…us.

In other posts we have seen that there is evidence that much of the matter in the universe is moving in a specific direction in what is called ‘Dark Flow’. Now another interesting question is being tackled. What if you looked at all of the galaxies out there and counted how many have right and left handed spin. The answer seems so obvious that no one really tried until recently and they found a non-obvious answer. There are about 7% more left handed galaxies that right handed, at least in the rather large portion of the sky they checked. Chances are a million to one that this is indeed a significant result and not a mere accident of observation.

The study (described at Physorg.com) was done by professor Michael Longo and a team of five undergraduates who catalogued the rotation direction of tens of thousands of spiral galaxies photographed in the Sloan Digital Sky Survey in an area north of the Milky Way’s pole. This leads to a different question, was the universe born with a spin? And this new data suggest the answer might be yes.

This is not just a cosmetic observation, black holes can have spin, particles can have spin. Spin can be used to extract energy from a system. The implications are only now being considered and all because someone thought to check the obvious answer. I have often thought that one of the greatest of human qualities is curiosity, and a close companion is healthy skepticism, they lead to good things.

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.

I love anomalies, the thrill of thinking that half of what we know is wrong and the other half is suspect. They hold the promise of new discoveries and a newer better understanding of reality. Unfortunately the vast majority of observations in cosmology, which appear to break the rules, do not stand the test of deep scrutiny.

I have written about the Cosmic Microwave Background (CMB) radiation several times, usually in the context of one anomaly or another. Most of them have since been deemed to be questionable. We are talking about tiny variations from the ideal random fluctuations and the sensitivity of the experiments is often pushed beyond its limits. It is not surprising that many conclusions drawn from the data are, well somewhat speculative. That said who would fail to find their possibilities intriguing?

Two new papers have been published describing ripples seen in the CMB. The most recent arXiv post by Stephen M. Feeney, et al, is based on some implications of Eternal Inflation. The model states that our universe banged then inflated quickly and so do other universes. As other universes (or false vacuum bubbles in the jargon) blow up they may slam against ours causing bruises in the CMB. They analyze data from WMAP with special software that looks for the telltale signs of these bruises.

In this image they show an idealized collision, the temperature modulation, a high needlet response, and results of edge detection in the CMB. Using these techniques they have found four candidates for primordial collisions. Check it out here: arxiv.org/abs/1012.1995

Another paper coauthored by the renowned Mathematical Physicist Sir Roger Penrose takes a different starting point for its analysis. Penrose is a proponent of Cyclic Inflation rather than Eternal Inflation. Cyclic Inflation starts from the question of why the beginning of the universe had such low entropy and postulates that at the end of the universe there are only black holes and that they evaporate, somehow removing the entropy from the universe and leaving it in an extremely low entropy state from which another big bang will start the whole rising entropy cycle anew.

He and his collaborator see evidence of concentric circles in the CBM which they imagine may have come about from the merger of ultra massive black holes that existed before the big bang. Check in out here: arxiv.org/abs/1011.3706

The first wave of anomalies (purportedly) seen in the CMB came from analyzing the data with little pre supposition about what the anomalies would look like. We now have at least two examples which start from an existing theory and try to see if there is evidence to support the theory. Both methods are valid but they have to be truly supported by the data and only time and those blessed second guessing trolls who bash through the data looking for mistakes will sort that out. And, of course those trolls will have the Planck data soon. In the meantime we have something to pique our imagination.

Timeline of the Universe (credit NASA WMAP)

It is a 3D timeline from the big bang to now. The diameter represents the size of the universe or more precisely the scale factor of the universe. It is hard to imagine but the universe did not start from a single point and explode into growing ball of gas. I know that’s what they imply when you listen to the TV pop science shows but the universe is everywhere, right? There is no center either. Its just that space happened all at once with all its bits in several dimensions and it started to expand in all those dimensions all at once.

The rate of expansion is illustrated in this picture, really fast at the beginning and then it slowed down. Now, as we have recently learned, it is starting to speed up again. So what does the diameter of the bell illustrate? In the beginning the bits of the universe were close together, now they are further apart. The diameter of the bell is proportional to how far each bit is from the next. This is called the scale factor. The outward expansion is just the scale factor growing at an accelerating rate. If nothing else changes things the bell will look like a trumpet pretty soon.

Something else cool about this picture is that it shows what we see in the WMAP and now the Planck images, the early microwave glow of the big bang. Using the satellites we capture photons from 13 billion years ago so they see what things looked like back then. The amazing thing about the image is that it is so uniform. In an explosion you’d expect to see billows, and clumps and gaps. But we don’t see much of that in the cosmic microwave background (CMB). This means the universe must have expanded very quickly indeed. Maybe faster than the speed of light (in fact almost certainly). That is why the first part of the bell gets big so quickly, the scale factor was growing so fast that each bit of the universe was moving away from every other at faster than the speed of light. The bits weren’t moving, it was space getting bigger, fluffier.

The interesting implication of this faster than light expansion, physicists call it inflation, is that some matter may be out there that we cannot see, past the edge of the observable universe. And that is the subject of the previous post, dark flow caused by ominous clumps of stuff outside our ability to see.

That is round about 390 words so there must be more in the picture but it is late and I will close with the other 610 left unwritten—for now.

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.

Since Einstein we have known that gravity was somehow special. Physicists have a unified standard model of forces and elementary particles but everyone knows that the big problem is gravity. String theory contains gravitons and explains gravity like any of the other forces. But String theory has a lot of things, in fact it is more a tool set or framework than a single theory.

But what if gravity is just a shadow on the cave wall? Or, to put it in terms a physicist would use, what if gravity is emergent—the holographic affect of entanglement entropy? I have written about it before because the idea seems to be gaining steam, at least among wannabe crackpots (like me perhaps).

Ted Jacobson (no crackpot) wrote the seminal paper in 1995 describing how to derive Eistein’s General Relativity equations from, “the form of black hole entropy together with the fundamental relation Q = TdS.” It is a short elegant paper that has been sited often of late. Back in 1995 it was pretty hard for an amateur to follow controversy in the physics world so I didn’t notice a big reaction to Jacobson.

Then came Verlinde who shook things up in January of this year. I’ve already written about the reaction.

Verlinde seems to have started a lot of people thinking because there have been several new papers jumping on the bandwagon (1,2). In a recent FQXi post Vlatko Vedral shills his own coverage of the territory in his book, Decoding Reality.

I adopted his (Jacobson’s) logic in my book to suggest, with a tongue in cheek, that gravity can in fact be derived from information theory (albeit with a little bit of help from quantum entanglement). Vlatko Vedral.

For whatever intuition is worth, the ideas of entropy (which is tied to the arrow of time) and entanglement (spooky action at a distance) do seem to be special too.  And wasn’t it that honored cosmologist David Brin who said our universe is just a gigantic simulation?

Update: FQXi covers several of the papers and posts you’ve seen here but you might like the writing better. Check it out.

Woit and Motl agree on something and that is that Verlinde is on the wrong track, maybe even a crackpot.  But neither seem to have torn apart the math with a critical eye and, well let’s just say they have strong pre-existing opinions about the proper way to think.  They are both smart but extreme enough that I sometimes question their opinions only because they hold them.  What’s an amateur to do to get a balanced analysis?

Enter Sabine Hossenfelder or Bee as she is known to her blog readers.  She has recently spent time examining Verlinde’s paper producing a blog post and arXiv paper.  She offers some thoughtful criticism and asks the question, “so what?”  Some sort of equivalence between entropy and gravity is only useful if it can be used as a bridge to apply gravity to the quantum world.  And she finds that part still lacking.