Archivi tag: singularity

A Planck star instead of singularity inside black holes

E’ una idea proposta da due astrofisici, Carlo Rovelli e Francesca Vidotto, che in un articolo suggeriscono che un oggetto, noto come stella di Planck, possa esistere al centro dei buchi neri, una proposta che eliminerebbe perciò il concetto di singolarità facendo sì che l’informazione possa riemergere in qualche punto dello spazio nel nostro Universo.

The current thinking regarding  is that they have two very simple parts, an event horizon and a . Because a probe cannot be sent inside a black hole to see what is truly going on, researchers have to rely on theories. The singularity theory suffers from what has come to be known as the “information paradox“, black holes appear to destroy information, which would seem to violate the rules of general relativity, because they follow rules of quantum mechanics instead. This paradox has left deep thinking physicists such as Stephen Hawking uneasy, so much so that he and others have begun offering alternatives or amendments to existing theories. In this new effort, a pair of physicists suggest the idea of a Planck star. The idea of a Planck star has its origins with an argument to the Big Bang theory, this other idea holds that when the inevitable Big Crunch comes, instead of forming a singularity, something just a little more tangible will result, something on the Planck scale. And when that happens, a bounce will occur, causing the Universe to expand again, and then to collapse again and so on forever back and forth.

Rovelli and Vidotto wonder why this couldn’t be the case with black holes as well, instead of a singularity at its center, there could be a Planck structure, a star, which would allow for general relativity to come back into play.

If this were the case, then a black hole could slowly over time lose mass due to Hawking Radiation, as the black hole contracted, the Planck star inside would grow bigger as information was absorbed. Eventually, the star would meet the event horizon and the black hole would dematerialize in an instant as all the information it had ever sucked in was cast out into the Universe. This new idea by Rovelli and Vidotto will undoubtedly undergo close scrutiny in the astrophysicist community likely culminating in debate amongst those who find the idea of a Planck star an answer to the information paradox and those who find the entire idea implausible.

arXiv: Planck stars

A holographic origin for the Big Bang

Un gruppo di fisici teorici hanno pubblicato un articolo in cui propongono una nuova idea che spiegherebbe l’origine dell’Universo. Secondo gli scienziati, è possibile che lo spazio e il tempo vennero creati dal collasso quadridimensionale di una stella che spazzò i detriti nel cosmo per poi trasformarsi in un buco nero.

As it stands, the prevailing theory states the Universe was born from an infinitely dense singularity through some currently unknown mechanism. Actually, the entire big bang event itself is entirely unknown. Our equations have yet to be complete enough to describe the moment of creation, a revelation physicists think will follow the discovery of the theory of everything (which scientists might be one-step closer to doing). Until then, what happened “before the big bang,” the nature of the ‘singularity’ that caused the big bang, and the event itself will remain unknown without some major scientific breakthrough. At the moment, it’s anyone’s guess what happened. (Important side note: we have a lot of knowledge and experimental evidence talking about what happened immediately after the big bang, up to about 10-35 or so seconds after the event, so our timeline for cosmology is still preserved.) The standard big bang theory has some limitations and some serious problems. It’s limitations are mostly summed up by our inability to mathematically or practically study the big bang singularity, as mentioned before. On the flip side, the big bang theory doesn’t really explain why the Universe has a nearly uniform temperature (that’s where inflation theory comes in, which suggests that the Universe went through a period of rapid, faster-than-light expansion in its early history).

This new model is based on the slightly older idea that our Universe is basically a three-dimensional membrane floating in a fourth-dimensional “bulk universe.” That’s the basic idea that’s supporting this new model.

The tenets for the new theory are as follows:

  • The “bulk universe” has fourth-dimensional stars that go through the same life cycle that our three-dimensional stars go through.
  • Just as with our stars, the stars in the bulk universe could go supernova and collapse into a black hole.
  • This is where things start to get really cool. Just as our three-dimensional black holes have event horizons that appear two-dimensional, it’s plausible that the fourth-dimensional black holes have event horizons that appear three-dimensional.
  • This three-dimensional event horizon is knows as a hypersphere. This is the region of space in which our Universe exists.

This new way of looking at the Universe has some strong points in its favor. The model is able to explain the expansion of the Universe and is able to describe the Universe’s nearly uniform temperature, with one (rather large) limitation. The model disagrees with observations made by the Planck telescope, which recently created the most detailed map we have of the cosmic microwave background (post). The hypersphere model has about a four percent discrepancy, which means the hypersphere needs to be refined before it’ll gain any credence.

This new model could go a long way to helping us understand the nature of inflation.

Currently, the only thing we really know about inflation is that “it’s happening.” We don’t know why or how, but the named mechanism for it is known as dark energy. The model proposes that inflation is caused by the Universe’s motion through higher dimensions of space. It’s important to note that the paper where this study was published does not state whether the paper has been submitted to peer review. So, whereas the hypersphere idea is fantastic and fun, it has a long way to go before we can considered a viable hypothesis.

From Quarks to Quasars: Revising the Big Bang? New Theory on Creation.
arXiv: Out of the White Hole: A Holographic Origin for the Big Bang

Towards understanding the Big Bang

Le leggi della fisica non sono in grado di descrivere cosa accadde durante il Big Bang. Infatti, sia la teoria dei quanti che la relatività generale non permettono di spiegare lo stato fisico singolare, infinitamente denso e caldo che caratterizzava le fasi iniziali della storia dell’Universo. Forse un giorno, la formulazione di una teoria che permetta di descrivere la gravità su scale quantistiche potrebbe fornirci una risposta (vedasi Idee sull’Universo). Oggi, alcuni scienziati del  Max Planck Institute for Gravitational Physics (Albert Einstein Institute, AEI) a Golm/Potsdam e del Perimeter Institute in Canada hanno fatto una scoperta importante in questo contesto. La loro idea è quella di assumere che lo spazio consista di piccolissime unità chiamate “mattoni fondamentali”. Partendo da questo concetto, gli scienziati arrivano alla formulazione delle equazioni più importanti della cosmologia, e cioè le equazioni di Friedmann, che permettono di descrivere l’Universo. Il risultato è che questo processo mostra, in definitiva, che la meccanica quantistica e la relatività possono essere effettivamente unificate.

For almost a century, the two major theories of physics have coexisted but have been irreconcilable: while Einstein’s General Theory of Relativity describes gravity and thus the world at large, quantum physics describes the world of atoms and elementary particles. Both theories work extremely well within their own boundaries; however, they break down, as currently formulated, in certain extreme regions, at extremely tiny distances, the so-called Planck scale, for example. Space and time thus have no meaning in black holes or, most notably, during the Big Bang. Daniele Oriti from the Albert Einstein Institute uses a fluid to illustrate this situation: “We can describe the behaviour of flowing water with the long-known classical theory of hydrodynamics. But if we advance to smaller and smaller scales and eventually come across individual atoms, it no longer applies. Then we need quantum physics“. Just as a liquid consists of atoms, Oriti imagines space to be made up of tiny cells or “atoms of space”, and a new theory is required to describe them: quantum gravity.

In Einstein’s relativity theory, space is a continuum. Oriti now breaks down this space into tiny elementary cells and applies the principles of quantum physics to them, thus to space itself and to the theory of relativity describing it. This is the unification idea.

A fundamental problem of all approaches to quantum gravity consists in bridging the huge dimensional scales from the space atoms to the dimensions of the Universe. This is where Oriti, his colleague Lorenzo Sindoni and Steffen Gielen, a former postdoc at the AEI who is now a researcher at the Perimeter Institute in Canada, have succeeded. Their approach is based on so-called group field theory. This is closely related to loop quantum gravity, which the AEI has been developing for some time. The task now consisted in describing how the space of the Universe evolves from the elementary cells. Staying with the idea of fluids: How can the hydrodynamics for the flowing water be derived from a theory for the atoms? This extremely demanding mathematical task recently led to a surprising success. “Under special assumptions, space is created from these building blocks, and evolves like an expanding Universe“, explains Oriti. “For the first time, we were thus able to derive the Friedmann equation directly as part of our complete theory of the structure of space“, he adds. This fundamental equation, which describes the expanding Universe, was derived by the Russian mathematician Alexander Friedmann in the 1920s on the basis of the General Theory of Relativity. The scientists have therefore succeeded in bridging the gap from the microworld to the macroworld, and thus from quantum mechanics to the General Theory of Relativity: they show that space emerges as the condensate of these elementary cells and evolves into a Universe which resembles our own. Oriti and his colleagues thus see themselves at the start of a difficult but promising journey. Their current solution is valid only for a homogeneous Universe, but our real world is much more complex. It contains inhomogeneities, such as planets, stars and galaxies. The physicists are currently working on including them in their theory. And they have planned something really big as their ultimate goal.

On the one hand, they want to investigate whether it is possible to describe space even during the Big Bang.

A few years ago, former AEI researcher Martin Bojowald found clues, as part of a simplified version of loop quantum gravity, that time and space can possibly be traced back through the Big Bang. With their theory, Oriti and his colleagues are hoping to confirm or improve this result. If it continues to prove successful, the researchers could perhaps use it to explain also the assumed inflationary expansion of the Universe shortly after the Big Bang as well, and the nature of the mysterious dark energy. This energy field causes the Universe to expand at an ever-increasing rate. Oriti’s colleague Lorenzo Sindoni therefore adds: “We will only be able to really understand the evolution of the Universe when we have a theory of quantum gravity“. The AEI researchers are in good company here: Einstein and his successors, who have been searching for this for almost one hundred years.

Max Planck Institute: Quantum steps towards the Big Bang

arXiv: Cosmology from Group Field Theory Formalism for Quantum Gravity