## Study: Our 3D Universe Could Have Originated From a 4D Black Hole 337

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Soulskill

from the useless-physics-trivia dept.

from the useless-physics-trivia dept.

New submitter TaleSlinger sends this quote from Nature:

*"Afshordi's team realized that if the bulk universe contained its own four-dimensional (4D) stars, some of them could collapse, forming 4D black holes in the same way that massive stars in our Universe do: they explode as supernovae, violently ejecting their outer layers, while their inner layers collapse into a black hole. In our Universe, a black hole is bounded by a spherical surface called an event horizon. Whereas in ordinary three-dimensional space it takes a two-dimensional object (a surface) to create a boundary inside a black hole, in the bulk universe the event horizon of a 4D black hole would be a 3D object — a shape called a hypersphere. When Afshordi's team modeled the death of a 4D star, they found that the ejected material would form a 3D brane surrounding that 3D event horizon, and slowly expand. The authors postulate that the 3D universe we live in might be just such a brane — and that we detect the brane's growth as cosmic expansion. 'Astronomers measured that expansion and extrapolated back that the Universe must have begun with a Big Bang — but that is just a mirage,' says Afshordi."*
## Re:Uhhh... what did he just say to us? (Score:5, Interesting)

If I understood it correctly they mean that on the other side is a universe with 4 spatial dimensions.

Think of it this way: in a universe with 3 spatial dimensions a black hole has a 2-d surface (shaped roughly like the surface of a sphere) as its event horizon. On the inside of the surface is the black hole. On the outside is the rest of the universe. Generalizing this to a hypothetical universe with 4 spatial dimension, a black hole in such a universe would have a 3-d "surface" surrounding it with the black hole inside of the surface and the rest of the universe outside of it.

By the way, there is already an idea floating around about how the edge of the visible universe seems be a bit like the event horizon of a black hole. Once something has passed the edge of the visible universe it is effectively lost to us, a bit like when something passes the event horizon of a black hole.

## Re:Uhhh... what did he just say to us? (Score:1, Interesting)

Technically there's nothing _inside_ the black hole. All the matter is on the surface. The circumference of a black hole is a function of the maximum amount of matter you can pack onto a two-dimensional sphere. A black hole *is* two-dimensional, it just appears three-dimensional from our perspective. Although that's pretty much what you said, minus semantics.

I'm not a physicist, but I think that's really cool. I suppose that phenomena could be a mere coincidence, but lots of people operate on the assumption that it's not coincidental. This phenomena is especially notable because it's observable and testable, unlike all the theories about 4th, 5th, etc dimensions, which work out on paper but aren't yet readily discernible (i.e. distinguishable) in actuality.

## Re:Uhhh... what did he just say to us? (Score:5, Interesting)

A common misconception is that black holes require singularities. Simple thought experiments show it differently.. for example, imagine living in a universe with a mass about that of a black hole that would have an event horizon that is just a little bit smaller than the universe. Now imagine that universe contracting. You can see that as it contracts it will eventually become small enough to form an event horizon without a singularity.

## Re:Uhhh... what did he just say to us? (Score:5, Interesting)

By the way, there is already an idea floating around about how the edge of the visible universe seems be a bit like the event horizon of a black hole. Once something has passed the edge of the visible universe it is effectively lost to usBecause we can only see things that have

sent light back towards us, ANDthat return light has already reached us. If something is further away from earth, than the distance that light could have possibly travelled back from the object towards earth from the time that the object was at that distance, then by induction: we cannot see the object yet.Because near the rim of the universe.... the universe is expanding faster than the speed of light; so it's far enough, that light would take longer to travel back to where earth is, than the duration the universe has existed.

Furthemore: since the universe can continue to expand at a rate faster than the speed of light --- the light travelling back towards earth, can never overtake the rate of the universe's expansion, and find its way back to us.

It is kind of like an infinite treadmkill ---- very similar to the concept of a gravitational well that is so deep not even light can escape.

We have an outer rim of our universe

expanding so quickly, that not even the very timespace; the spatial dimensions or the passage of time can escape it.## Re:Uhhh... what did he just say to us? (Score:5, Interesting)

Well, I'm not a physicist either and I could be wrong but I think that there are two equivalent views of what a black hole is. The holographic view is pretty strange...

The stuff that supposedly sits at the event horizon in the holographic view is not matter; it is information. My understanding is that the event horizon of a black hole can basically be though of as a data storage device that stores scrambled information about everything that the hole has swallowed, except for the information about the stuff that it has since spit out.

I imagine it works something like this: when the black hole swallows some matter the information content in that matter (that is the entropy) gets stored on the horizon and the horizon expands to make room for it. When the hole spits out a particle the horizon "erases" the information/entropy of that particle and the horizon contracts to make sure there isn't any empty "disk space".

## Rubbish (Score:1, Interesting)

"Afshordi's team realized that..."Here. Let me correct that: "Afshordi's team

imaginedthat...""Thought experiments"

are NOT experiments, NOR are they "science".When Einstein used so-called "thought experiments" he was using them as a teaching tool to explain his theories, but we have now a large pool of people who live on research grants doing "theoretical" science and lots of "thought experiments" (which used to be called "day dreaming" and was a good way to avoid doing real work). I refuse to recognize any of this drivel as "science" until it actually involves experiments and data harvested from those experiments... until then it's just subsidized day dreaming with as much value to society as children putting on costumes and playing "make believe". Why four dimensions and not ninety four or perhaps three hundred four? Even if some wild imagination convinces you that at least four dimensions are needed and some razor tells you to keep it simple (and you therefore restrict your daydream to four) that does not mean the real universe conforms to your desires... there could be exactly 62 dimensions for reasons you will never even be able to imagine. The pseudo-intellectual garbage being peddled to the public as "science" these days is truly mind blowing... and meanwhile there are an uncounted number of real, solid scientific problems to be solved by means of actual science that would improve the lives of millions of people. IMHO Afshordi and team deserve no accolades and no particular attention; they'd benefit society more as day laborers picking fruit or as janitors mopping hospital floors. I love SCIENCE but I detest quasi-scientific junk being passed-off as something of value and tarnishing the reputation of real science. There's been so much of this junk showered upon the public that the public now has a lower opinion of completely valid and serious science and scientists than it had four or five decades ago. One big step backward for science, one grand faceplant for all mankind.Bah Humbug! (sad, weak smile)

## The trouble with mathematical models (Score:5, Interesting)

This is an illustration of where mathematical models can run amok.

Every kind of model has its limits. Bohr, for example, envisioned atoms as a nucleus of positively charged protons and neutral neutrons, with orbiting electrons. The model works well because it's something people can grasp. But the model has its limits, and there are many aspects of quantum behavior that cannot be explained by the Bohr model. The model is still useful because it does lead to many accurate scientific predictions.

A newer mathematical model, quantum mechanics, seeks to be even more accurate in its predictions than Bohr's model. It succeeded in many ways, and like the Bohr model, has led to many interesting discoveries. But it too has its limits.

In pure mathematics, exceeding three dimensions is effortless. Calculations involving four or more dimensions can easily be solved. But just because the mathematical model can do it, doesn't mean that the physical reality it attempts to model, can also do it. A model is designed to represent reality, but it is not itself reality. I suspect that all such mathematical models of the universe, which point to other dimensions, will eventually be shown to be purely mathematical.

## Re:Uhhh... what did he just say to us? (Score:3, Interesting)

It's not really the same because anything that collides with a black hole will cease to exist. There is no way for anyone with any sort of conceivable detector to observe what no longer exists. Even if the collapsed star's gravity did not stop the photons from exiting it would effectively vanish out of existence.

This is not true. Hawkings proved this already; Look up Hawking radiation. Black holes will eventually evaporate if it cannot attract enough matter to sustain its size. Highly charged particles are emitted at the poles of a black hole, and it's also been proven that not only does matter in the accretion disc accelerate to the speed of light before crossing the horizon, but that the black hole itself is also rotating at the speed of light creating relativistic frame dragging.

All of this would not be occurring if it "vanished out of existance", and thus violated the laws of thermodynamics. In fact, whether a 3D or 4D universe, matter and energy can be neither created nor destroyed. The

information, that is the quantum state, of mass and energy that is eaten by a blackhole is later ejected as what could be termed high energy 'noise'; x-rays and gamma rays. Black holes, it would seem, convert matter into energy, which is then re-emitted; They accelerate entropy locally, but they do not, in any way, "vanish" things. What goes into a black hole does eventually come back out... but what comes out, to the best of our knowledge, is a smear of particles which are emitted along a gaussian distribution with regard to energy state.It should also be noted that the standard model is known to be flawed in that it cannot accurately predict extremely high energy states -- this is one of the reasons why black holes are so interesting to astrophysicists; They are currently the only observable phenomenon where such high energy levels are. Unfortunately, because we are not directly aligned with the poles of very many black holes, which seem to align themselves to the galactic gravity plane for reasons not yet fully understood, there simply isn't enough observational data to say with confidence what the properties of such high energy particles would be.

Answering these questions is essential if we are to successfully create a grand unified theory. The standard model has already been proven to fall short of that; And quantum mechanics still can't even explain gravity... the Year of the Proven Quantum Gravity Particle comes about as often as the Year of the Linux Desktop... which is to say, we're still waiting.

## Re:Uhhh... what did he just say to us? (Score:5, Interesting)

This is not quite right: according to the Penrose singularity theorem, the existence of an event horizon implies that spacetime is singular (more precisely: geodesically incomplete).

You have overstepped the theorem. The theorem states that a singularity must eventually form if there is an event horizon, not that a singularity must exist at all points in time that the event horizon exists.

Remember than in a hollow sphere of any mass, gravity is neutral at all points that arent edge points. The sphere can be massive enough that the schwarzschild radius (aka the event horizon) can be outside the sphere, yet inside gravity is neutral and space-time remains flat. Entropy will eventually collapse the sphere, but thats eventually... not immediately.