Why is there a massive void in space?

Most of us have heard the terms before; interplanetary, interstellar (great movie by the way), intergalactic… but I’m going to go one step higher: Interuniversal. The space between universes… Yes, I’m implying there is more than one, and I’m going to tell you about an observable phenomena that might just prove it.

It’s called the Cosmic Microwave Background (CMB) Cold Spot, dubbed a “Supervoid” by some theories. Sure, the Supervoid sounds like the ideal place I want to be when it’s a Sunday evening and I haven’t even begun to do any of my tutorial work due on Monday at 10… but really, it’s the largest structure in space that scientists have ever discovered. It’s SO big in fact; it doesn’t fit into our current model of the universe. The Big Bang theory of creation does allow for regions of space hotter/colder than the average, but not regions this big. Simply put, it is too big to exist, yet it does. But what is it?

(1) A diagram of the CMB, where the circled area is the location of the cold spot

The cold spot is an area of space 1-2.5 billion light years across (I know, huge error bar) – which is enough space to hold around 10000 galaxies… but it’s empty, and has an estimated 20% less matter than other regions in the universe. Our best estimate is that the known universe is around 100 billion light years across, which means this spot, a single structure, takes up 1-2% of the entire universe. It was actually this emptiness that helped us discover this area in space. It is colder than the average temperature of the universe (2.7 K) by 70 micro-Kelvin, and this is exactly what made it stand out in the CMB. Now, scientists have a few explanations for this, the most interesting of which is the suggestion that this spot is the imprint of a parallel universe, one that had either been entangled with our own before being separated by cosmic inflation, or one that had collided with ours in this spot.



(2) Diagram depicting how a ‘brane’ universe might look

This was, and still is, a very controversial idea, but if it were true it would provide the first empirical evidence for parallel universes outside of our own! How can this be explained? One possible way is M-theory, the extension of the very well-known String Theory. It requires reality to have a total of 11 space-time dimensions, where our universe exists on a 3 dimensional ‘brane’ (+ time). This leaves the idea that there are other such branes that can support other universes, all contained within the ‘bulk’, the name for the multiverse in this model. Another idea that stems from this model is that the Big Bang itself came from the collision of two branes. This theory also gives the added bonus of explaining why gravity, when compared to the 3 other fundamental forces, is so weak. It states that gravity is non localised across the multiverse, while the strong, weak and electromagnetic forces are localised in our universe only.

Another really exciting idea about what this massive gap in space could be is actually a ‘larger than supermassive’ black hole. If you haven’t read Kate Gould’s brilliant article on Black Holes, I recommend you do so for some general background info. But a black hole would do a decent job at explaining the void; mainly because there is nothing inside it (I stress this again because I simply find it beyond fascinating). The only problem with this is that black holes are still a part of the cosmic microwave background, or CMB, and this gap in the universe is not. However, it has been suggested that due to the enormous size of the void, if it were a black hole, it would actually be able to swallow parts of the CMB itself, which would explain why it’s colder, too.

(3) Could we all be living inside a black hole?

On the other hand concerning black holes, there is black hole cosmology – the idea that our entire universe might be on the inside of a black hole in a bigger, and perhaps higher dimensional universe, and every black hole in our universe contains a universe within it. This would make sense (at least to me) because the universe is expanding, and a black hole ‘sucks in’ everything it encounters. This brings in the idea of a ‘white hole’ which is just the opposite of a black hole – it ‘spits out’ matter and light. It is speculated that the Big Bang began from a singularity, so perhaps said singularity was indeed inside a black hole, and a white hole was formed, giving birth to our own universe. Going even further, perhaps dark energy, the ingredient of our universe that keeps the expansion going (also the ingredient we have very little idea about), is in fact matter that has been swallowed up by the black hole that encloses our universe. This is one of those ideas that borders a little too close to the realm of science fiction (unless we prove it… I mean, until), but I feel like it has its place here nonetheless.

I digress. When considering many complex ideas such as we have done here, I have to consider Occam’s razor. Occam’s razor is basically the idea that the simplest explanation must be the correct one. And the simplest explanation in this case is the same theory that gave the cold spot the name of ‘Supervoid’ – that the area itself is just a Supervoid: a vast area of empty space. The really interesting part of this theory is that it suggests that photons passing through the exceptionally large void would have to lose energy, because of the temperature difference; they would become colder, due to what is known as the integrated Sachs-Wolfe (ISW) effect. The Sachs-Wolfe effect is a property of the CMB, in which photons from the CMB are redshifted due to the effect of gravity. This is why the CMB spectrum appears uneven – The ISW comes into consideration when we use a model of a universe that is dominated by something other than matter, e.g. Dark Energy. By measuring the ISW effect due to the Supervoid, physicists may be able to detect the influence of dark energy, thus increasing our understanding about this mysterious part of our universe.

To anyone who isn’t satisfied by some gigantic void being nothing more than some texture anomaly, I completely agree. As far as I’m concerned, anything (the universe) that exists together in an enclosed area (the multiverse) is bound to collide at some point, and thus, as far as I’m concerned, when we look at a void, we are looking at the craters left by other universes. But for now, I’m going to go start my tutorial questions.


(1) ESA Planck Collaboration

(2) http://www.leslievcarlson.com/contact/

(3) PBS (http://www.pbs.org/wgbh/nova/next/physics/are-black-holes-real/)








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