I’ve seen a few videos. I am no smarter because of it. Just nothing clicking yet. Fourth dimensions, Brains, Bulks, Miniature black holes… I’ll keep trying.
OK, I’ll try and simplify it without doing injustice to the original papers. 
First of all, what is a “brane”? The term arose as a contraction of “membrane”, and is in effect, an extension of the ideas of string theory. Within which, a string is basically a one-dimensional entity moving through space - if you like, an actual physical realisation of a line segment (or, if the ends are joined, a circle). Of course, these entities are flexible, and can bend and twist in various ways, but I’ll deal with this in more detail shortly.
Now extend this idea to a two dimensional sheet, which again might simply be a sheet, or might fold back on itself to form a cylinder or, if you put a twist in it, a Möbius strip. Likewise, you can extend this to a three dimensional volume, which again might be open or closed, depending upon whether or not it doubles back on itself. You can extend this to entities of dimensions greater than 3, but visualising them starts to become a headache, and so you have to rely increasingly upon mathematics to make such entities tractable. A brane is simply any entity of this sort, in any number of dimensions, strings being a one-dimensional subset of the possible total set of branes.
Now there’s an important point to make at this juncture, namely, that these entities have to possess the properties of a manifold. A manifold is, in mathematics, a sort of generalisation of geometrical objects, which obeys one simple rule - namely, when viewed at sufficiently high magnifications, they approximate ever closer to a flat Euclidean space of the requisite dimension. Even extremely contorted manifolds have to obey this rule, though one has to view the contortions at very high magnifications before they approximate to a flat Euclidean space at the regions in question. The idea is similar to the way we produce flat maps for parts of the Earth, despite the Earth being a roughly spherical object - maps of small portions of the Earth don’t involve much distortion, but trying to map the entire planet as a single flat sheet involves huge distortions near the poles. Which is why precise maps of the Earth involve mapping small areas, and compiling all the small maps into an atlas.
Indeed, much of the terminology of cartography is carried over to manifolds - a mapping of one small part of a manifold onto a flat Euclidean space is a chart, and a collection of many charts covering the whole manifold is an atlas. And, just as there exist numerous different projections that can be used in cartography to produce charts of the Earth and atlases thereof, so any given manifold admits of a multiplicity of charts and atlases. Indeed, even a two-dimensional manifold may have, depending upon its shape, a veritable infinity of possible charts and atlases, limited only by mathematical ingenuity in their compilation. This abundance of mappings becomes even greater in three dimensions, and grows to a mid-boggling extent once you start increasing the number of dimensions further.
And thus, we have the key concept in place, namely that a brane is a physical realisation of a mathematical manifold.
Now, just as ideal mathematical manifolds can possess additional structure, such as a topology or the presence of embedded differentiable functions, branes can have additional features associated therewith, such as electrical charge or the presence of gravitation within their fabric, so to speak. Indeed, they can have any of a range of quantum fields tied thereto (including, of course, zero such additions, though such “bare branes” tend not to be particularly interesting).
Likewise, just as you can have a number of say, two dimensional sheets moving about in three dimensional space, you can have n-dimensional branes moving about in a space of (n+1) dimensions. Which is where Steinhardt & Turok’s work comes into play.
Their vision of what might be termed the “ultraverse” is one involving 10-dimensional branes moving about in the 11-dimensional space of M-theory. While this is hard for our intuition to grasp, the mathematics makes matters simple - you just add extra coordinates to the system, and apply the same rules to that space as you would to a more readily visualised 3D space, for example. That’s one of the key points - the same rules applying to a readily visualised 3D space carry over to these more exotic spaces, and all that’s involved is more computational tedium.
Now, these branes possess an interesting feature - namely, they have a quantum gravity field associated therewith, and which affects any particles (or lower-dimensional branes) moving about within their fabric. If this is beginning to look like a huge jumble of entities whirling about in a manner resembling some psychedelic fractal, then you’re not far off the mark, but I digress!
At this point, life becomes even more interesting, because the manner in which the gravity fields behave in each of those branes differs from brane to brane. Some of those branes have gravity behaving in the same manner as it does within our observable universe - namely, masses are attracted to each other, and such branes are labelled “positive tension” branes. Other branes have gravity acting as a repulsive force between masses, and are labelled “negative tension” branes.
Once the underlying mathematics had been worked out, the question Steinhardt & Turok asked was this - what would happen if two of these branes collided?
In particular, what would happen if a positive-tension brane collided with a negative-tension brane?
The answer they got was startling when they examined the mathematics - namely, the end result would be the formation of an entirely new brane, arising from the combination of the colliding branes, but with a very interesting geometrical structure - a geometrical structure identical to that of our observable universe. Even better, that structure would have donated to it, a huge amount of vacuum energy in the collision, facilitating matter synthesis within that structure. In short, they hit upon a possible mechanism for launching a Big Bang style universe from first principles.
But it gets even better.
That launching of a Big Bang universe would involve the generation, within that universe, of gravitational waves.
This is the key starting point for the idea being testable. Like more familiar waves, gravitational waves have a wavelength associated therewith, and if you have a suitable gravitational wave detector, you can measure that wavelength. Of course, more recent events within our observable universe clutter the picture, with gravitational waves from such sources as neutron star collisions or black hole mergers. But those have well-defined signatures, and can be subtracted from the background once you have sufficient data to recognise said signatures. We’re not there yet, but progress is being made in this direction.
Now, once you have the ability to distinguish between primordial gravitational waves, originating from the Big Bang, and gravitational waves of more recent origin, you’re in a position to test Steinhardt & Turok’s ideas.
How?
Welcome to the concept of a power spectrum.
This concept originated in the study of light sources, and works as follows. You measure your incoming waves, and record how many you receive at a given wavelength. In the case of light, you split the incoming light into a spectrum, and measure how bright different parts of the spectrum are. Plot a graph of that brightness against wavelength, and you have a power spectrum. For sunlight, this tends to be biased toward yellow and green wavelengths, because of interference from the atmosphere, but measurements taken from space produce a somewhat flatter power spectrum, with a more equal distribution across the wavelengths. Artificial light sources have very characteristic power spectra - incandescent bulbs are strongly biased toward long wavelengths (red to yellow), while fluorescent lights have “lumpy” spectra with gaps in them, unless they’re specially constructed with special (and sometimes expensive) fluorophores to fill in the gaps. Aquarium lighting is expensive because it’s designed to produce particular power spectra - some units are optimised for photosynthesis of freshwater plants in freshwater fish tanks, others are optimised to stimulate coral growth in a marine aquarium, etc.
The same principle carries over to gravitational waves. Once you have the ability to detect primordial gravitational waves and measure the power spectrum, you can then plot it on a graph. And here’s the prediction made by Steinhardt & Turok - namely, that the power spectrum of primordial gravitational waves should be biased preferentially toward short wavelengths, if their ideas are correct, with the graph taking a form that slopes downwards as you move toward longer wavelengths.
The beauty of this prediction is that no other cosmological model (to my knowledge, at least - I’m prepared to be corrected on this!) makes a prediction about gravitational wave power spectra. As such, this prediction is a unique test of Steinhardt & Turok’s ideas.
This test, of course, will be performed, once scientists are confident enough that they can perform the test in an unequivocal manner. Quite simply, if the experimental data from said test produces a power spectrum curve matching Steinhardt & Turok’s prediction, they pick up a Nobel Prize. Of course, a curve differing from that prediction will be “Game Over” for their model, but at the moment, with respect to experimental testability, their model is the only real game in town. It’s the only model with a testable prediction that’s within our remit to perform with existing technology.
And if the light bulb comes on over your head after reading this, my work is done. 
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Thanks for the explanation.
Goes back to watching Star Trek Generations… (seriously, I’m re-watching it)
Several flashing lights and a headache…great explanation. Thanks Calli.
I think that the origin of the Universe is a lot simpler than is currently believed.
I believe that the 1st and 2nd Laws of Thermodynamics are a function of probability, and I created a “thought experiment” (shown in another thread) that shows this.
So, I think that the Big Bang is a statistical anomaly that happens once in a very great while in an eternal Universe that is in a state of maximum entropy (“heat death”) most of the time.
My idea eliminates the need for God, and it gives us infinite regression into the past and infinite progression into the future without the need for modifying the basic laws of physics.
There are problems with my argument. First, cosmic inflation seems to contradict some parts of my argument. Also, my arguments don’t account for dark matter and don’t explain the accellerating expansion of the Universe.
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My favorite speculation for the origin of the universe is a quantum fluctuation, perhaps one of many in a much larger cosmos.
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I think we will never know and theorist can postulate all they like and no one will ever discover the truth.
Just my opinion.
At T=0 not only can we not see past this point, or study the properties before hand… I don’t think as moderately evolved primates we can even comprehend the scale of time, let alone the possible events at the moment and/or prior to it.
Personally I stick to a very simplistic explanation to it.
Whatever happened at T=0 and possibly before, WAS a naturally occurring phenomena.
Why? Because everything is and it doesn’t require “but meh holy book!” Or celestial Bjorn Borg to explain it.
Everything as we regress back from humans, to our shared primate ancestor, to the earth forming from an accretion disc, our solar system developing, to its elements coming from various dead stars and all the way back to the big bang… follow naturally explained phenomena and natural causation.
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It is being reported that several “natural” tau neutrinos have been detected. By natural I mean from outer space, not out of a particle accelerator built by a slightly less hairy ape. Someone like me might get carried away and say it is the first crude neutrino telescope.