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Our future, our universe, and other weighty topics


Tuesday, November 3, 2015

A Dubious Procedure for Trying to Get Signs of Another Universe

If you're a cosmologist looking to make a name for yourself, one pretty easy shortcut is to look for some unexplained cosmological feature (typically in the cosmic background radiation that pervades the universe), and to claim that this is evidence for some other universe beyond our own. Previous attempts along these lines have used what is called the Cold Dark Spot in the cosmic background radiation, an area of the sky in which the temperature of the cosmic background radiation is a little cooler than the average temperature. We have had cosmologists who used reasoning rather like this:

There's that little spot in the cosmic background radiation, and I don't understand that. My guess is that it is a sign that our universe bumped into another universe.

The idea of trying to find evidence for other universes by looking at features of the cosmic background radiation in our universe seems like a quixotic quest (I will avoid the less polite term “fool's errand.”) Even if we were to find a particularly striking feature in that radiation, it would merely tell us something about our universe or its history, rather than being an indication of some other universe. It should also be noted that the cosmic background radiation is essentially featureless, because it is uniform to 1 part in 100,000.

The latest attempt to get evidence of a “signal of another universe” by using the cosmic background radiation is a paper by Ranga-Ram Chary that has attracted some attention. New Scientist has done an article about the paper, with the headline “Mystery Bright Spots Could Be First Glimpse of Another Universe.” But I wouldn't get excited, because the technique used is extremely dubious and highly error-prone. The supposed “bright spots” are not actually things that were observed in a telescope, but just “leftovers” after some weird subtraction process.

Here is how New Scientist describes the technique used:

Instead of looking at the CMB itself, Chary subtracted a model of the CMB from Planck’s picture of the entire sky. Then he took away everything else, too: the stars, gas and dust. With our universe scrubbed away, nothing should be left except noise. But in a certain frequency range, scattered patches on the sky look far brighter than they should. If they check out, these anomalous clumps could be caused by cosmic fist-bumps: our universe colliding with another part of the multiverse.

But there is no simple and straightforward way to do this “subtraction” part, the part described by the phrase “he took away everything else, too: the stars, gas, and dust.” Doing the subtraction part gets a cosmologist into the very murky swamp known as “masking” (also called foreground subtraction). When cosmologists do masking, they take some existing image or radiation reading from the sky, and start subtracting things. But how to do such masking is always very debatable guesswork. It is exactly the difficulties and guesswork of masking that messed up the BICEP2 team that came up with that 2014 “proof of cosmic inflation” paper that turned out to be the biggest scientific false alarm of the decade.

On pages 4 and 5 of Chary's paper, he describes the ridiculously complicated process by which he did this subtraction. There was not one type of subtraction or masking, but many different types. In all such cases, a rather arbitrary decision and debatable decision was made about the exact way to do that particular type of masking or subtraction.  Doing the right subtraction or masking to get rid of the cosmic dust is a nightmare.

Cosmic dust messes up our swaggering cosmologists

Consequently, we can have little confidence in Chary's result. Chary's procedure seems rather like the following hypothetical procedure:

I started out with a complex picture showing Times Square in New York City. My idea was to subtract everything using “masking techniques.” So I subtracted out all the buildings, using a “building mask” procedure I have. Then I subtracted out all the people, using a “people mask” procedure I have. Then I subtracted out all the cars, using a “car mask” procedure I have. Then I subtracted out the sky and clouds, using a “sky and clouds mask” procedure I have. Then I subtracted out all the other things. I should have been left with black nothingness – but instead I see some mysterious light remaining. This light must be from some other universe.

If the procedure suggested above is rather ridiculous, how much more ridiculous is it to try something similar using the entire universe (where the uncertainties involving these subtractions is so much greater)? “Subtract your way to another universe” does not seem like a viable program of cosmological inquiry.

But is there any way in which we might get evidence of another universe? I can think of two ways in which you could get real evidence of another universe.

Method 1: We look up in our telescopes and see radically different physics going on in different parts of the sky. Perhaps in one part of the sky we see light traveling at, say, 50 times the speed of light on our planet; then in some other part of the sky, we see light traveling at one tenth the speed of light on our planet. This might be evidence that we are somehow peering into other universes with different physics from the physics of our universe. But no such observations have ever been reported. We have peered deep into the universe, to the observational limits of our telescopes, and found no such thing. Instead we have found great uniformity of the fundamental physical constants and laws wherever we look.

Method 2: We invent a machine capable of teleporting itself to some other universe. The machine could then make some observations to verify that the other universe had physics radically different from ours. For example, the machine might release a gram of matter on a flat windless plain. If the matter floated up rather than falling down, it might be evidence that the machine was in some weird other universe where gravity acted differently. The machine could then be teleported back to our universe to give us proof of its observations.

There are numerous reasons why it is extremely improbable that Method 2 will ever be applied successfully. The first is that no one has the slightest idea how to teleport a machine to a different universe (black holes won't do the job, since everything tells us that they will completely destroy any machine trying to pass through them). The second is that no one has the slightest idea how to teleport a machine back from another universe to our universe. The third is that if we assume that alternate universes would have a random collection of physics and fundamental constants, it would be more than 99.99999999% likely that a machine teleported to some other universe would be instantly destroyed as soon as it got there, because that universe would be very unlikely to have physics allowing for stable atoms and molecules. It requires a great deal of fine-tuning of various items of physics (such as the charge of the proton and the electron, and the strong nuclear force) for you to even have a universe in which objects as large as a small machine can exist without flying apart.

So much as cosmologists might like to get evidence of some other universe, it seems all but impossible that such evidence could ever be gathered.

 Postscript: The news reports about Chary's paper almost all have inaccurate titles. Almost all of these reports are entitled something like: Scientists report possible evidence of another universe.  But in this case, it is not scientists who are reporting such a thing -- it is only one scientist.