Slashdot Mirror
Astronomers Discover a Group of Quasars 4 Billion Light Years Across
New submitter mal0rd writes "NewScientist reports a 'collection of galaxies that is a whopping four billion light years long is the biggest cosmic structure ever seen. The group is roughly one-twentieth the diameter of the observable universe – big enough to challenge a principle dating back to Einstein, that, on large scales, the universe looks the same in every direction.' For reference, Andromeda is only 2.5 million light years away."
What exactly makes this "a structure"? All linked gravitationally or what?
Based on the map in the linked article, it appears that this Quasar has an angular diameter of about 10 degrees. The moon is about 0.5 degrees. So if the magnitude was high enough to be visible, this structure would be the size of a constellation. Of course, if it was that bright, it would have fried most of the observable universe.
Have gnu, will travel.
Consider all the entities [stars, galaxies, or whatnot] in your study as points in 3-space. The descriptive length of the data is the total number of bits that describes the location of all points in your study.
If all points are random and evenly distributed, then the total number of bits required is (number of points)x(number of bits for 1 location).
Suppose you notice a clumping of points. Is this a structure or random variation?
Rework your data description as follows: for any point, use the first bit to determine whether a point is a member of the clump or not, and subsequent bits to complete the description, depending on whether the point is in the clump.
For this description, the total number of bits required is 1x(total number of points) + (number of points in clump)x(number of bits for location relative to clump) + (number of points not in clump)x(number of bits for general location).
If the 2nd description is shorter than the 1st description, then by Occam's razor the second description is more likely correct.
In fact, the number of bits directly tells the probability that the 2nd description is correct: if the 2nd description requires 10 fewer bits (total) than the 1st, then the 2nd description is more likely to be correct by a factor of 1024. Alternately, there is a 1/1024 chance that the 2nd description is *not* the correct description of the data.
If you have lots of data, it's not unusual for a descriptive length to be thousands of bits shorter than the baseline description; meaning, that it's virtually certain that the new description is correct and that the new structure does not arise from random variation.
I haven't seen the data, but I assume that describing all galaxies in the universe using the newly described "clump" as a categorical structure gives a smaller descriptive entropy than describing all galaxies without the extra category of "clump".
All this observation really implies is that the true and full size of the universe is much larger than what has been documented so far.
Currently, we can observe a bubble of space around us to a radius of about 13.5 billion light years. That's as far as we can see. This may well be analogous to being at the center of a water balloon, submerged in a swimming pool of much greater volume.
We can currently see to the inner surface of that balloon, but the far greater mass of water outside of it remains hidden for now to our instrumentation.
Complex systems will always tend to appear homogenous, given enough subjective distance.
Fun fact: The rotational period of the Milky Way is approximately 200-250 million years.
The universe we currently observe is approximately 13.5 billion years old --- there is no way a spiral of such definition could form after only 50-odd rotations, and yet still be so topographically distinct from other such bodies.
That's simply not enough time.
2c
Years later, a doctor will tell me that I have an I.Q. of 48, and am what some people call "mentally retarded".
With a redshift of 1.3 this quasar group is probably close to the edge of the observable universe. What we see is from a time maybe some million years after the big bang. But at this time the universe was much smaller, so these quasars were much closer together than they are now. They are flying away from us since then into slightly different directions, and flying away from each other.
What I think this means is: We can not calculate the size of this group from the angular diameter and its distance, it has nothing to do with reality. The angular diameter comes from different directions that the individual quasars are flying away from us, not from actually being this large. We can only see this quasar group as it was billion years ago, and at that time it was much smaller. We don't know what it looks like now. Also our perception of the form of this group would be distorted if the directions that its components are flying is not just caused by a homogeneous expansion of the universe.
It's a good question. I think you've gotten things a little backwards, though, with regars to the problem of propagation - inflation is a proposed explanation for propagation in the sense that it allows otherwise separate regions of the sky to have been in causal contact in the past. But this certainly does have impact upon the current inflationary paradigm in the following sense:
If there were large structures or large inhomogeneities in the early universe (before inflation) then it would be hard to get inflation going. The basic models of inflation contain a field whose energy can be decomposed (and I'm playing very fast and loose here) into three parts: Potential, Kinetic and fluctuations. From these parts, we say that if the potential is large enough, the inflaton undergoes a "slow roll" down the potential during which our regular inflation happens. Fluctuations are treated as perturbations on this background, and it's from these that we expect to see the everyday structure in the universe. A warning though: We don't know the physics that causes these fluctuations to stop being quantum fluctuations and become classical perturbations in matter on this background.
Now, if the fluctuations are too big, this model breaks down - the inflaton can't be high enough up its potential, and so slow roll can't happen. Hence before inflation we have to assume that the universe is largely homogeneous and isotropic, and fluctuations begin very small (technically in the "Bunch-Davies vacuum state).
A big inhomogeneity AFTER inflation isn't too bad - it could well be that this is just the result of one of the longer wavelength fluctuations. Of course, one would then have to explain /why/ this wave in particular had such a large amplitude, but this really doesn't contravene inflationary models, it merely adds a new question about the initial conditions.
Now, if we had been dealing with a serious overdensity (tons of quasars in the same spot) rather than a large strung-out structure, we would certainly have a problem with inflation, but so far as I know this isn't too big of an issue.
Disclaimer: I work on the mathematical structure end of things, not the computation or observation, so there are certainly people more qualified than I, to whom I would happily defer if they want to post!
When Einstein said it looked the same in every direction, what he meant was that it's all governed by the same laws.
Actually it's more than that, it's also about the distribution of matter and energy on a large scale. It's assumed that matter is homogenous throughout the universe, homogenous literally means "no lumps" (above a certain size defined as "local" in your post). It's like an ideal gas, at the microscopic level you have all sorts of random "pressure" (kinetic energy of the individual atoms), at the macroscopic level there is just one pressure that is the same no matter what part of the gas you measure. This is because the macroscopic measurements are an average of all the individual microscopic pressures, the central limit theorem of statistics says that that the average of a big enough sample from a large population will always be very close to the real population average.
In other words the reason it's "odd" is that statistics says the observation can't be brushed aside as a fluke, if the distribution of quasars is lumpy then either the basic assumption of large scale homogeneity is wrong, or the observation is flawed. The OP's stupid question is by far the most insightful thing I've read about it so far, how are they defining the word "structure".
And did you exchange a walk on part in the war for a lead role in a cage? - Pink Floyd.