Astrophysicists propose new twist on Big Bang | Hacker News Reader

Astrophysicists propose new twist on Big Bang

uwaterloo.ca

84 pointsblazar51 comments

>The researchers emphasize that this idea, though it may sound absurd, is grounded firmly in the mathematics describing space and time. Specifically they’ve used the tools of holography to “turn the big bang into a cosmic mirage.”

embedding into higher dimension spaces (or in general - into more "richer" structures) is a convenient mathematical tool and it works nice ... in mathematics. To work in physics that higher dimension space must really exist. Otherwsie, for all we know, out Universe can be just a 3d surface of a 4d nut that a small 4d squirrel puppy is about to bite into.

The next phrase mentions that this theory is testable

>Along the way, their model appears to address long-standing cosmological puzzles, and – crucially – produce testable predictions.

I'm very curious to know how.

From the fine paper[1]:

Finally, let us comment on potential testability of this model. As we pointed out, the simple model of cosmological perturbations, developed in Sec. 4 is already ruled out by cosmo- logical observations at > 5σ level, as it does not predict any deviations from scale-invariance. However, it is easy to imagine small corrections that could lead to a ∼ 4% deviation from scale-invariance, especially given that bulk temperature is so close (i.e. ∼ 20% of) the 5D Planck temperature. In the context of our model, the red tilt of the cosmological power spectrum implies that the amplitude of 5D bulk graviton propagator, which enters in Eq. (4.8), is getting stronger in the IR, suggesting gradual unfreezing of additional polarizations of graviton.

[1]:http://arxiv.org/pdf/1309.1487v2

So, if we look at parts of space further away from us (so the light reaching us tells us about older times), we should see fewer kinds of gravitons than we see around us now?

So all we have to do is detect, over time, more polarization states of the 5-D graviton. Seems simple enough.

I'm not sure of what a graviton is, but wikipedia tells me that it is not detectable by any current mean. I'd be tempted to say in light of this that their theory is not testable either.

http://en.wikipedia.org/wiki/Graviton

> any current mean[s]

But that's the point, innit. Who's to say we don't discover magic quantum carbon nanotubes or something thing that makes it feasible to setup a graviton detector at some point in the future. And who's to say it won't be useful, either.

I'm serious; the Higgs-Boson was theorized to exist in 1964, and was proven to exist only after great expense in 2012, utilizing many cutting edge technologies.

General Relativity was theorized in 1915, but not really tested in 1959, and now I use it almost every day in the form of GPS.

At the point it we can test it, it becomes testable. The authors, note, did not claim it was testable. They simply indicated the direction research should go in order to test it.

As per Einstein, it's true that accurate measurements of his GR effects didn't come along for decades, but he was answering known physical problems. And the crude measurements of his day aligned with his predictions, they just weren't accurate yet.

The graviton is the missing particule of the quantum mechanics. It is, simply put, a 'gravity unit' exchange between every corpse in the universe from which 'emerges' gravity.

I bet you there are a lot of efforts made right now to find a way to measure them!

Yup. You are correct.

We should distinguish "testable in principle" and "testable in practice". Simply because it is currently not testable in practice, does not mean that it's not testable in principle.

Saying a scientific hypothesis is untestable "in principle" is a deathknell while untestable "in practice" is not.

Ignorant question here, but is it not the case that the normal laws of physics don't apply at big bang time?

Do we expect the laws of mathematics to still apply?

Does anyone really have any clue about this stuff?

#BigBangAgnostic

It's always been my understanding that the normal laws of physics don't work at big bang time as well. One thing I've wondered about is how people are so adamantly confident in the theory of the Big Bang in light of this. Is it simply because they don't like the alternatives, because they've never been heard an alternative, or is there a better reason?

It's basically a belief system at that stage. It doesn't provide any more believable answers than religion for me.

The big bang, unlike theology, tells us why distant objects are more red than close objects, why we can't see anything further than ~13bil light years away, why there is microwave noise everywhere, etc. The mathematics that describe all those things imply certain things about earlier cosmological history.

But still doesn't prove anything about where the universe "came from".

..and theology does?

What I said: It doesn't provide any more believable answers than religion for me.

Before there can be answers there must be questions. Theoretical physicists come up with new possible questions that are later narrowed down and answered by experimentation and observation.

The fact that there is an ongoing process of scientific discovery that has continued unabated, while theological knowledge remains stagnant or even decays, is alone enough reason to give credence to even the most far-fetched theory that might one day have a hope of being tested.

To summarize, hard science like theoretical physics is not about beliefs or answers or end goals, it's about a continual process of expanding knowledge, guided outward by the imaginations of theoreticians.

Yet. As we gather evidence ever closer to the actual moment of the big bang (such as the possible recent discovery of a particular kind of polarization in the CMB), we might figure out ways to prove or disprove theories about what might have preceded the big bang.

As I understand the process, theoreticians will invent mathematical constructs that explain everything we have measured already, plus make predictions about something else we can test. If all of the testable predictions are verified, then we are somewhat safer in accepting any untestable predictions of the same model. Then, an expanded mathematical theory will be devised with new physical tests, etc. ad infinitum.

It depends on what you mean by "big bang time". Standard physics started a few microseconds after the big bang, followed by what appears to be a rapid expansion of the early universe.

The Big Bang is widely adopted in light of this. Preceding the expansion there has to be a state of greater compression, but there is no explanation on how this state could be possible.

The big bang theory explains a lot of stuff, like the cosmic microwave background, and the redshift of far-away galaxies.

Its usually given as an answer to "where did the universe come from".

Only by ignorant people.. The more knowledgeable one will tell you that the big bang is the description of the state of the "young" universe but that we don't have the tools to describe/know what was before/what created it (yet).

NASA uses Newtonian mechanics to send spacecraft to the outer planets and beyond. Everybody knows that Newtonian mechanics stop being accurate in certain conditions, but that does not prevent it being an incredibly useful model outside those conditions.

The best experimentally-verified cosmological models we have point to the fact that the Universe was in a hot and dense state a few microseconds after a specific point of time about 13.7 billion years ago. Extrapolating those few microseconds back in time we get an unphysical result from our current models, but that simply means those models are inaccurate if applied to the beginning of time.

Contrary to the popular belief, The Big Bang model simply does not concern itself with what happened at the exact moment of Big Bang; what it does is describe the evolution of the Universe after that, and does it well.

>Contrary to the popular belief, The Big Bang model simply does not concern itself with what happened at the exact moment of Big Bang; what it does is describe the evolution of the Universe after that, and does it well.

That sounds as much of a cop out as the answers religious people give.

> It's always been my understanding that the normal laws of physics don't work at big bang time as well.

That's wrong. As soon as the universe's mass/energy expanded sufficiently for there to be a time dimension (IOW as soon as the universe was something other than a singularity), the present physical laws applied. Some of the physical constants with which we are familiar may have had different values, but the same set of rules have been in effect since there were four distinct dimensions. This is borne out by the existence and properties of the CMB, as well as other observations.

Your use of the term "big bang time" implies that there was, in fact, time, which means four dimensions existed, which means our present physical rules were in effect.

One idea has it that the natural forces we evaluate separately now, were unified by the conditions of the early universe, but this idea doesn't contradict the notion that basic physical principles were in any way different then.

One of the more interesting cosmological ideas is that the positive mass/energy of the universe is exactly balanced by negative gravitational potential energy, as the universe expands and continuing to the present, but only at one specific expansion velocity: what we call escape velocity. If this idea is true, it means the universe could have sprung into existence spontaneously, like a virtual particle pair, without violating the law of conservation of energy. Stephen Hawking explains this idea in his book "The Grand Design": "Because there is a law such as gravity, the universe can and will create itself from nothing. Spontaneous creation is the reason there is something rather than nothing, why the universe exists, why we exist. It is not necessary to invoke God to light the blue touch paper and set the universe going."

It's amusing how confidently you state "wrong" and then make the same logical error in using a time-dependent conjunction ("As soon as...") in your own description.

The OP specified a time other than zero, so there are four dimensions. This means the usual physical rules apply. My use of "as soon as" is completely consistent with the era specified by the OP, i.e. when four dimensions are available and physics is "normal". And I made this perfectly clear in my reply.

So you're saying there was a time when time was zero? You might keep me up all night chuckling.

The big bang hypothesis could be stated (in a simplified way) by stating that the universe was once a lot more hot and dense than it is today. Using General Relativity, we can model the expansion of the universe. When we do the math, we find that out come correctly the relative abundance of light elements in the early universe as well as the spectrum for the cosmic microwave background, and many other quantities. It holds very well together.

If we extrapolate far enough back, we reach a point where the predicted density would be infinite. We call that point the beginning of time (or the big bang). This "prediction" of an infinite density is taken as a sign that our model breaks down at (or before) this point (by "before" I mean when we extrapolate backwards in time towards the infinite density time...).

So, it works extremely well for a time when the density was extremely large (but not infinite) until today. We just don't have enough information to know what would happen when densities larger than a certain value so as to make meaningful predictions.

To make a completely silly analogy: it would be like predicting the motion of a rocket aimed for a comet. Our knowledge of celestial mechanics is good enough to predict the trajectory up until the rocket "touches" the comet. We just do not have enough information about the comet composition to know if the rocket will just blow it apart or if it is the rocket itself that will splatter on the comet, or whatever.

Yes, we expect the laws of mathematics to still apply.

The laws of physics also apply, but we don't know all of them. For example Newtonian physical vs. relativity. Both are correct, but one only comes into play at high velocity or energy. Same with the big bang, there may be a more complete description of what happens that only comes into play at high energy.

This doesn't mean the current laws don't apply, but rather they are inaccurate, but we can't tell since the inaccuracy is too small at our energy level.

A much better mental model of this stuff is Newtonian mechanics is always wrong at every scale. At human masses, velocity's and time scales you can't detect the error but it's still there.

As such the physical laws are unchanged at the big bang, but the simpler aproximations in current use may be noticeably off at these scales. If they are, then they would also be wrong at all other timescales and energy levels etc. The trick is looking for edge cases we can detect that seperate our aproximations from the actual laws running reality.

Given all that; simpler aproximations continue to be useful as long as the errors are hard to detect.

>The laws of physics also apply, but we don't know all of them.

Classic. Making it up as you go along.....

Math is independent. If physics works different, it may not support humans, so there's no one around to think those thoughts, but if you accept the axioms of math everything else follows.

Granted, in some bizzaro physics 2+2 may actually equal 3, (along these lines http://lesswrong.com/lw/jr/how_to_convince_me_that_2_2_3/) but you should still be able to construct a successor function and mechanically evaluate 2+2 = 4 according to our rules, even though the answers would seem weird.

We may indeed be assuming stuff we don't know we're assuming. Still the other people would see, oh they're assuming X which is wrong. but the other people take X to be true, it all works out.

You apparently were flagged by a theoretical physicist.

A lot of "physicists" nowadays forget that we're supposed to be scientists, not dreamers. Dreams are nice, but experiment and observations valid theory.

I agree. Unless this has some modifications to the current model of the big bang which leads to _observable_effects_ , I don't know if this theory could even be validated as...well theory.

There are still theoretical physicists who rely on empirical validation of the hypotheses we create. That's rather the whole point, and I'd venture (hope?) that only a small fraction of theoretical physicist think otherwise.

You know theory means explanation, not imagination, right?

I use the definition for theory as a hypothesis that has received sufficient experimental validation. So, by definition, many of the frontier quantum-gravity stuff doesn't fit this, unfortunately as compelling and fascinating as they are.

Again, I'm just agreeing with what s/he said, that unless we have experiment/observation, we don't know whether this really describes our universe's origins and/or evolution.

Well... things that are mathematically equivalent turn out to have physical implications more often than one would expect. Not always, but often enough to be worth a look.

Does the mathematical equivalence mean anything more than "maybe somebody ought to look at that a bit more"? No, but that's still more than nothing.

>embedding into higher dimension spaces (or in general - into more "richer" structures) is a convenient mathematical tool and it works nice ... in mathematics. To work in physics that higher dimension space must really exist.

I'd put my money on those Phds in physics knowing that already.

They probably heard about Occam too.

Building a model based on specific reading on the current knowledge of physics, intuitions that hold true in light of this knowledge, and some mathematical tools, is a valid process, despite not being immediately empirically checkable. The "exists" part can be tested later.

That doesn't make their work equal to abitrary BS like "a 4d nut that a small 4d squirrel puppy is about to bite into".

>Otherwsie, for all we know, out Universe can be just a 3d surface of a 4d nut that a small 4d squirrel puppy is about to bite into.

absolutely. I know you are trying to portray such a scenario as incredulous, but it's entirely feasible.

Original paper title: Out of the White Hole: A Holographic Origin for the Big Bang

Arxive page (w/abstract): http://arxiv.org/abs/1309.1487

Direct link to paper: http://arxiv.org/pdf/1309.1487v2

This isn't a particularly new twist. The idea of our universe coming from a black hole has been around for quite some time, as this 1998 article discusses:

http://martinelli.org/rexpansion/bhole.htm

  "if we run time forwards, we can kind of visualize the beginning of
   our universe as a very rapid collapse (the inverse of "Inflation").
   A similar kind of event happens when a star (hot, spherical,
   massive, dense...) becomes so dense and massive it collapses in on
   itself and forms that mysterious black hole.  Here's an interesting
   question to explore... "Did our universe begin as a star that
   underwent a gravitational collapse?"  I.e., do we live in a black
   hole?"

Perhaps the novelty is a formal theory around the idea?

This tends to confirm Lee Smolin's embryo-theory of cosmological natural selection.

Source : http://en.m.wikipedia.org/wiki/Cosmological_natural_selectio...

Cosmological natural selection Smolin's hypothesis of cosmological natural selection, also called the fecund universes theory, suggests that a process analogous to biological natural selection applies at the grandest of scales. Smolin published the idea in 1992 and summarized it in a book aimed at a lay audience called The Life of the Cosmos.

The theory surmises that a collapsing[clarification needed] black hole causes the emergence of a new universe on the "other side", whose fundamental constant parameters (masses of elementary particles, Planck constant, elementary charge, and so forth) may differ slightly from those of the universe where the black hole collapsed. Each universe thus gives rise to as many new universes as it has black holes. The theory contains the evolutionary ideas of "reproduction" and "mutation" of universes, and so is formally analogous to models of population biology.

The resulting population of universes can be represented as a distribution of a landscape of parameters where the height of the landscape is proportional to the numbers of black holes that a universe with those parameters will have. Applying reasoning borrowed from the study of fitness landscapes in population biology, one can conclude that the population is dominated by universes whose parameters drive the production of black holes to a local peak in the landscape. This was the first use of the notion of a landscape of parameters in physics.

Leonard Susskind, who later promoted a similar string theory landscape, stated:

"I'm not sure why Smolin's idea didn't attract much attention. I actually think it deserved far more than it got."

It seems like the proposed cosmological natural selection is fundamentally different from biological natural selection in the sense that there is no actual pruning. That is to say that given a universe all of its children will survive and reproduce based solely on their laws of physics, independent of what all the other universes do. This would mean that all possible universes which could be reached through this mechanism will be reached, wheras in evolution as it happens on Earth, most of the potential organisms never exist because there ancestor got out-competed at some point.

OK,

The thing about this popular article is it seems to imply that the universe coming out of a black hole is different from it coming out from any kind of singularity.

Well, reading the real article, it seems this popular article got things garbled. The main problem they seem raise with the big bang is that it is a "naked" singularity - there's no barrier between the world and the singular point. So what they are doing is looking at a higher dimensional black hole and finding a non-naked singularity that would generate approximately the same effects as the big bang. There is still a singularity at work here, it is simply that the singularity is not directly spewing matter out. "No naked singularities" is a general principle I recall from scanning general relativity articles.

So how did this 4d universe come about? :D

I think this idea is almost certainly false, and it has it's genesis from a mostly discredited psuedo-physics theory called the holographic universe[1]. But at least there is a testable prediction! In my opinion, the fact that they offer a testable prediction merits a more thorough consideration of the claims.

First of all, it has been well understood for multiple decades that the General Relativity(GR) breaks down, and is no longer true, at mass/energy scales that are present inside of a black hole. This is known because physical quantities must be finite, but GR predicts the strength of a gravitational field inside of a black hole is infinite. If string theory is true (and I don't believe it is), then this would be a valid situation to apply it to, since we know that there is unknown physics at play inside of a black hole.

Furthermore, the standard big bang cosmology is based on the idea that the beginning of time is a "black hole in time", in the sense that as t->0, the strength of the gravitational field diverges to infinity. Thus, it has been well known for decades that GR is not a valid physical theory at very short time scales after the beginning of time (this would be the epoch of inflation[3]).

It is perhaps not apparent to the casual observer that this paper is based on a very bizarre sect within string theory, known as the "Holographic Universe", which according to Wikipedia "suggests that the entire universe can be seen as a two-dimensional information structure "painted" on the cosmological horizon". This is just a bunch of fancy words designed to blow smoke in your eyes, and the entire theory is sort of a joke as far as most physicists I know are concerned.

There are actually a number of strange cults within string theory, another one that is good for a laugh is the Anthropic Princple[4]. This theory claims that even though the 11 dimensional D-Brane that is folded into a Calubi-Yau manifold upon which string theory is based predicts trillions of possible universes, the universe we live in exists because it is the only possible one we could live in:

"the conditions happen to be just right for the existence of (intelligent) life on the earth at the present time. For if they were not just right, then we should not have found ourselves to be here now, but somewhere else, at some other appropriate time."

This kind of nonsense is why I left research in theoretical physics to purse computer science and programming, which truly is the cutting edge of science. There is a lot of valuable physics research going on in material science departments(for example), but the string theory department needs to constantly justify their funding by making extremely bold claims such as "what we perceive as the Big Bang could be the three-dimensional “mirage” of a collapsing star in a universe profoundly different than our own".

[1] http://en.wikipedia.org/wiki/Holographic_principle

[2] http://en.wikipedia.org/wiki/D-brane

[3] http://en.wikipedia.org/wiki/Inflation_(cosmology)

[4] http://en.wikipedia.org/wiki/Anthropic_principle

As far as I know, none of what you say is as cut-and-dried as you make it seem.

The Anthropic Principle is a common sense explanation to why our universe has a paradoxically perfect ("fine-tuned" by 120 orders of magnitude off what we would expect) cosmological constant that is just right for life and the universe to exist. It's somewhat horrifying, but I haven't seen a better explanation for fine-tuning. [1]

The holographic principle does not seem to be supported by a "very bizarre sect". In fact, it seems to be one of the better explanations for the black hole information paradox. [2]

Nima Arkani-Hamed dedicated a significant portion of his Messenger Lectures [3] (same lectures where Feynman delivered his famous series) to the principles that you dismiss out of hand as laughable above.

I'm a little depressed that the top comment is computer science > theoretical physics. Theoretical physics predicts quantum scale behavior to 11-digit accuracy. It's no joke.

[1] http://en.wikipedia.org/wiki/Cosmological_constant#Quantum_f... [2] http://en.wikipedia.org/wiki/Black_hole_information_paradox [3] http://www.cornell.edu/video/nima-arkani-hamed-quantum-mecha...

Reply on news.ycombinator.com