Arquivo da tag: Cosmologia

Is the universe a hologram? (Science Daily)

Date:
April 27, 2015
Source:
Vienna University of Technology
Summary:
The ‘holographic principle,’ the idea that a universe with gravity can be described by a quantum field theory in fewer dimensions, has been used for years as a mathematical tool in strange curved spaces. New results suggest that the holographic principle also holds in flat spaces. Our own universe could in fact be two dimensional and only appear three dimensional — just like a hologram.

Is our universe a hologram? Credit: TU Wien 

At first glance, there is not the slightest doubt: to us, the universe looks three dimensional. But one of the most fruitful theories of theoretical physics in the last two decades is challenging this assumption. The “holographic principle” asserts that a mathematical description of the universe actually requires one fewer dimension than it seems. What we perceive as three dimensional may just be the image of two dimensional processes on a huge cosmic horizon.

Up until now, this principle has only been studied in exotic spaces with negative curvature. This is interesting from a theoretical point of view, but such spaces are quite different from the space in our own universe. Results obtained by scientists at TU Wien (Vienna) now suggest that the holographic principle even holds in a flat spacetime.

The Holographic Principle

Everybody knows holograms from credit cards or banknotes. They are two dimensional, but to us they appear three dimensional. Our universe could behave quite similarly: “In 1997, the physicist Juan Maldacena proposed the idea that there is a correspondence between gravitational theories in curved anti-de-sitter spaces on the one hand and quantum field theories in spaces with one fewer dimension on the other,” says Daniel Grumiller (TU Wien).

Gravitational phenomena are described in a theory with three spatial dimensions, the behaviour of quantum particles is calculated in a theory with just two spatial dimensions — and the results of both calculations can be mapped onto each other. Such a correspondence is quite surprising. It is like finding out that equations from an astronomy textbook can also be used to repair a CD-player. But this method has proven to be very successful. More than ten thousand scientific papers about Maldacena’s “AdS-CFT-correspondence” have been published to date.

Correspondence Even in Flat Spaces

For theoretical physics, this is extremely important, but it does not seem to have much to do with our own universe. Apparently, we do not live in such an anti-de-sitter-space. These spaces have quite peculiar properties. They are negatively curved, any object thrown away on a straight line will eventually return. “Our universe, in contrast, is quite flat — and on astronomic distances, it has positive curvature,” says Daniel Grumiller.

However, Grumiller has suspected for quite some time that a correspondence principle could also hold true for our real universe. To test this hypothesis, gravitational theories have to be constructed, which do not require exotic anti-de-sitter spaces, but live in a flat space. For three years, he and his team at TU Wien (Vienna) have been working on that, in cooperation with the University of Edinburgh, Harvard, IISER Pune, the MIT and the University of Kyoto. Now Grumiller and colleagues from India and Japan have published an article in the journal Physical Review Letters, confirming the validity of the correspondence principle in a flat universe.

Calculated Twice, Same Result

“If quantum gravity in a flat space allows for a holographic description by a standard quantum theory, then there must by physical quantities, which can be calculated in both theories — and the results must agree,” says Grumiller. Especially one key feature of quantum mechanics -quantum entanglement — has to appear in the gravitational theory.

When quantum particles are entangled, they cannot be described individually. They form a single quantum object, even if they are located far apart. There is a measure for the amount of entanglement in a quantum system, called “entropy of entanglement.” Together with Arjun Bagchi, Rudranil Basu and Max Riegler, Daniel Grumiller managed to show that this entropy of entanglement takes the same value in flat quantum gravity and in a low dimension quantum field theory.

“This calculation affirms our assumption that the holographic principle can also be realized in flat spaces. It is evidence for the validity of this correspondence in our universe,” says Max Riegler (TU Wien). “The fact that we can even talk about quantum information and entropy of entanglement in a theory of gravity is astounding in itself, and would hardly have been imaginable only a few years back. That we are now able to use this as a tool to test the validity of the holographic principle, and that this test works out, is quite remarkable,” says Daniel Grumiller.

This however, does not yet prove that we are indeed living in a hologram — but apparently there is growing evidence for the validity of the correspondence principle in our own universe.


Journal Reference:

  1. Arjun Bagchi, Rudranil Basu, Daniel Grumiller, Max Riegler. Entanglement Entropy in Galilean Conformal Field Theories and Flat HolographyPhysical Review Letters, 2015; 114 (11) DOI: 10.1103/PhysRevLett.114.111602
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How The Nature of Information Could Resolve One of The Great Paradoxes Of Cosmology (The Physics Arxiv Blog)

Feb 17, 2015

Stephen Hawking described it as the most spectacular failure of any physical theory in history. Can a new theory of information rescue cosmologists?

One of the biggest puzzles in science is the cosmological constant paradox. This arises when physicists attempt to calculate the energy density of the universe from first principles. Using quantum mechanics, the number they come up with is 10^94 g/cm^3.

And yet the observed energy density, calculated from the density of mass in the cosmos and the way the universe is expanding, is about 10^-27 g/cm^3. In other words, our best theory of the universe misses the mark by 120 orders of magnitude.

That’s left cosmologists somewhat red-faced. Indeed, Stephen Hawking has famously described this as the most spectacular failure of any physical theory in history. This huge discrepancy is all the more puzzling because quantum mechanics makes such accurate predictions in other circumstances. Just why it goes so badly wrong here is unknown.

Today, Chris Fields, an independent researcher formerly with New Mexico State University in Las Cruces, puts forward a simple explanation. His idea is that the discrepancy arises because large objects, such as planets and stars, behave classically rather than demonstrating quantum properties. And he’s provided some simple calculations to make his case.

One of the key properties of quantum objects is that they can exist in a superposition of states until they are observed. When that happens, these many possibilities “collapse” and become one specific outcome, a process known as quantum decoherence.

For example, a photon can be in a superposition of states that allow it to be in several places at the same time. However, as soon as the photon is observed the superposition decoheres and the photon appears in one place.

This process of decoherence must apply to everything that has a specific position, says Fields. Even to large objects such as stars, whose position is known with respect to the cosmic microwave background, the echo of the big bang which fills the universe.

In fact, Fields argues that it is the interaction between the cosmic microwave background and all large objects in the universe that causes them to decohere giving them specific positions which astronomers observe.

But there is an important consequence from having a specific position — there must be some information associated with this location in 3D space. If a location is unknown, then the amount of information must be small. But if it is known with precision, the information content is much higher.

And given that there are some 10^25 stars in the universe, that’s a lot of information. Fields calculates that encoding the location of each star to within 10 cubic kilometres requires some 10^93 bits.

That immediately leads to an entirely new way of determining the energy density of the cosmos. Back in the 1960s, the physicist Rolf Landauer suggested that every bit of information had an energy associated with it, an idea that has gained considerable traction since then.

So Fields uses Landauer’s principle to calculate the energy associated with the locations of all the stars in the universe. This turns out to be about 10^-30 g /cm^3, very similar to the observed energy density of the universe.

But here’s the thing. That calculation requires the position of each star to be encoded only to within 10 km^3. Fields also asks how much information is required to encode the position of stars to the much higher resolution associated with the Planck length. “Encoding 10^25 stellar positions at [the Planck length] would incur a free-energy cost ∼ 10^117 larger than that found here,” he says.

That difference is remarkably similar to the 120 orders of magnitude discrepancy between the observed energy density and that calculated using quantum mechanics. Indeed, Fields says that the discrepancy arises because the positions of the stars can be accounted for using quantum mechanics. “It seems reasonable to suggest that the discrepancy between these numbers may be due to the assumption that encoding classical information at [the Planck scale] can be considered physically meaningful.”

That’s a fascinating result that raises important questions about the nature of reality. First, there is the hint in Fields’ ideas that information provides the ghostly bedrock on which the laws of physics are based. That’s an idea that has gained traction among other physicists too.

Then there is the role of energy. One important question is where this energy might have come from in the first place. The process of decoherence seems to create it from nothing.

Cosmologists generally overlook violations of the principle of conservation of energy. After all, the big bang itself is the biggest offender. So don’t expect much hand wringing over this. But Fields’ approach also implies that a purely quantum universe would have an energy density of zero, since nothing would have localised position. That’s bizarre.

Beyond this is the even deeper question of how the universe came to be classical at all, given that cosmologists would have us believe that the big bang was a quantum process. Fields suggests that it is the interaction between the cosmic microwave background and the rest of the universe that causes the quantum nature of the universe to decohere and become classical.

Perhaps. What is all too clear is that there are fundamental and fascinating problems in cosmology — and the role that information plays in reality.

Ref: arxiv.org/abs/1502.03424 : Is Dark Energy An Artifact Of Decoherence?

No Big Bang? Quantum equation predicts universe has no beginning (Phys.org)

Feb 09, 2015 by Lisa Zyga

big bang

This is an artist’s concept of the metric expansion of space, where space (including hypothetical non-observable portions of the universe) is represented at each time by the circular sections. Note on the left the dramatic expansion (not to scale) occurring in the inflationary epoch, and at the center the expansion acceleration. The scheme is decorated with WMAP images on the left and with the representation of stars at the appropriate level of development. Credit: NASA

Read more at: http://phys.org/news/2015-02-big-quantum-equation-universe.html#jCp

(Phys.org) —The universe may have existed forever, according to a new model that applies quantum correction terms to complement Einstein’s theory of general relativity. The model may also account for dark matter and dark energy, resolving multiple problems at once.

The widely accepted age of the , as estimated by , is 13.8 billion years. In the beginning, everything in existence is thought to have occupied a single infinitely dense point, or . Only after this point began to expand in a “Big Bang” did the universe officially begin.

Although the Big Bang singularity arises directly and unavoidably from the mathematics of general relativity, some scientists see it as problematic because the math can explain only what happened immediately after—not at or before—the singularity.

“The Big Bang singularity is the most serious problem of general relativity because the laws of physics appear to break down there,” Ahmed Farag Ali at Benha University and the Zewail City of Science and Technology, both in Egypt, told Phys.org.

Ali and coauthor Saurya Das at the University of Lethbridge in Alberta, Canada, have shown in a paper published in Physics Letters B that the Big Bang singularity can be resolved by their  in which the universe has no beginning and no end.

Old ideas revisited

The physicists emphasize that their quantum correction terms are not applied ad hoc in an attempt to specifically eliminate the Big Bang singularity. Their work is based on ideas by the theoretical physicist David Bohm, who is also known for his contributions to the philosophy of physics. Starting in the 1950s, Bohm explored replacing classical geodesics (the shortest path between two points on a curved surface) with quantum trajectories.

In their paper, Ali and Das applied these Bohmian trajectories to an equation developed in the 1950s by physicist Amal Kumar Raychaudhuri at Presidency University in Kolkata, India. Raychaudhuri was also Das’s teacher when he was an undergraduate student of that institution in the ’90s.

Using the quantum-corrected Raychaudhuri equation, Ali and Das derived quantum-corrected Friedmann equations, which describe the expansion and evolution of universe (including the Big Bang) within the context of general relativity. Although it’s not a true theory of , the  does contain elements from both quantum theory and general relativity. Ali and Das also expect their results to hold even if and when a full theory of quantum gravity is formulated.

No singularities nor dark stuff

In addition to not predicting a Big Bang singularity, the new model does not predict a “big crunch” singularity, either. In general relativity, one possible fate of the universe is that it starts to shrink until it collapses in on itself in a big crunch and becomes an infinitely dense point once again.

Ali and Das explain in their paper that their model avoids singularities because of a key difference between classical geodesics and Bohmian trajectories. Classical geodesics eventually cross each other, and the points at which they converge are singularities. In contrast, Bohmian trajectories never cross each other, so singularities do not appear in the equations.

In cosmological terms, the scientists explain that the quantum corrections can be thought of as a cosmological constant term (without the need for dark energy) and a radiation term. These terms keep the universe at a finite size, and therefore give it an infinite age. The terms also make predictions that agree closely with current observations of the cosmological constant and density of the universe.

New gravity particle

In physical terms, the model describes the universe as being filled with a quantum fluid. The scientists propose that this fluid might be composed of gravitons—hypothetical massless particles that mediate the force of gravity. If they exist, gravitons are thought to play a key role in a theory of quantum gravity.

In a related paper, Das and another collaborator, Rajat Bhaduri of McMaster University, Canada, have lent further credence to this model. They show that gravitons can form a Bose-Einstein condensate (named after Einstein and another Indian physicist, Satyendranath Bose) at temperatures that were present in the universe at all epochs.

Motivated by the model’s potential to resolve the Big Bang singularity and account for  and , the physicists plan to analyze their model more rigorously in the future. Their future work includes redoing their study while taking into account small inhomogeneous and anisotropic perturbations, but they do not expect small perturbations to significantly affect the results.

“It is satisfying to note that such straightforward corrections can potentially resolve so many issues at once,” Das said.

More information: Ahmed Farag Ali and Saurya Das. “Cosmology from quantum potential.” Physics Letters B. Volume 741, 4 February 2015, Pages 276–279. DOI: 10.1016/j.physletb.2014.12.057. Also at: arXiv:1404.3093[gr-qc].

Saurya Das and Rajat K. Bhaduri, “Dark matter and dark energy from Bose-Einstein condensate”, preprint: arXiv:1411.0753[gr-qc].

The Paradoxes That Threaten To Tear Modern Cosmology Apart (The Physics Arxiv Blog)

Some simple observations about the universe seem to contradict basic physics. Solving these paradoxes could change the way we think about the cosmos

The Physics arXiv Blog on Jan 20

Revolutions in science often come from the study of seemingly unresolvable paradoxes. An intense focus on these paradoxes, and their eventual resolution, is a process that has leads to many important breakthroughs.

So an interesting exercise is to list the paradoxes associated with current ideas in science. It’s just possible that these paradoxes will lead to the next generation of ideas about the universe.

Today, Yurij Baryshev at St Petersburg State University in Russia does just this with modern cosmology. The result is a list of paradoxes associated with well-established ideas and observations about the structure and origin of the universe.

Perhaps the most dramatic, and potentially most important, of these paradoxes comes from the idea that the universe is expanding, one of the great successes of modern cosmology. It is based on a number of different observations.

The first is that other galaxies are all moving away from us. The evidence for this is that light from these galaxies is red-shifted. And the greater the distance, the bigger this red-shift.

Astrophysicists interpret this as evidence that more distant galaxies are travelling away from us more quickly. Indeed, the most recent evidence is that the expansion is accelerating.

What’s curious about this expansion is that space, and the vacuum associated with it, must somehow be created in this process. And yet how this can occur is not at all clear. “The creation of space is a new cosmological phenomenon, which has not been tested yet in physical laboratory,” says Baryshev.

What’s more, there is an energy associated with any given volume of the universe. If that volume increases, the inescapable conclusion is that this energy must increase as well. And yet physicists generally think that energy creation is forbidden.

Baryshev quotes the British cosmologist, Ted Harrison, on this topic: “The conclusion, whether we like it or not, is obvious: energy in the universe is not conserved,” says Harrison.

This is a problem that cosmologists are well aware of. And yet ask them about it and they shuffle their feet and stare at the ground. Clearly, any theorist who can solve this paradox will have a bright future in cosmology.

The nature of the energy associated with the vacuum is another puzzle. This is variously called the zero point energy or the energy of the Planck vacuum and quantum physicists have spent some time attempting to calculate it.

These calculations suggest that the energy density of the vacuum is huge, of the order of 10^94 g/cm^3. This energy, being equivalent to mass, ought to have a gravitational effect on the universe.

Cosmologists have looked for this gravitational effect and calculated its value from their observations (they call it the cosmological constant). These calculations suggest that the energy density of the vacuum is about 10^-29 g/cm3.

Those numbers are difficult to reconcile. Indeed, they differ by 120 orders of magnitude. How and why this discrepancy arises is not known and is the cause of much bemused embarrassment among cosmologists.

Then there is the cosmological red-shift itself, which is another mystery. Physicists often talk about the red-shift as a kind of Doppler effect, like the change in frequency of a police siren as it passes by.

The Doppler effect arises from the relative movement of different objects. But the cosmological red-shift is different because galaxies are stationary in space. Instead, it is space itself that cosmologists think is expanding.

The mathematics that describes these effects is correspondingly different as well, not least because any relative velocity must always be less than the speed of light in conventional physics. And yet the velocity of expanding space can take any value.

Interestingly, the nature of the cosmological red-shift leads to the possibility of observational tests in the next few years. One interesting idea is that the red-shifts of distant objects must increase as they get further away. For a distant quasar, this change may be as much as one centimetre per second per year, something that may be observable with the next generation of extremely large telescopes.

One final paradox is also worth mentioning. This comes from one of the fundamental assumptions behind Einstein’s theory of general relativity—that if you look at the universe on a large enough scale, it must be the same in all directions.

It seems clear that this assumption of homogeneity does not hold on the local scale. Our galaxy is part of a cluster known as the Local Group which is itself part of a bigger supercluster.

This suggests a kind of fractal structure to the universe. In other words, the universe is made up of clusters regardless of the scale at which you look at it.

The problem with this is that it contradicts one of the basic ideas of modern cosmology—the Hubble law. This is the observation that the cosmological red-shift of an object is linearly proportional to its distance from Earth.

It is so profoundly embedded in modern cosmology that most currently accepted theories of universal expansion depend on its linear nature. That’s all okay if the universe is homogeneous (and therefore linear) on the largest scales.

But the evidence is paradoxical. Astrophysicists have measured the linear nature of the Hubble law at distances of a few hundred megaparsecs. And yet the clusters visible on those scales indicate the universe is not homogeneous on the scales.

And so the argument that the Hubble law’s linearity is a result of the homogeneity of the universe (or vice versa) does not stand up to scrutiny. Once again this is an embarrassing failure for modern cosmology.

It is sometimes tempting to think that astrophysicists have cosmology more or less sewn up, that the Big Bang model, and all that it implies, accounts for everything we see in the cosmos.

Not even close. Cosmologists may have successfully papered over the cracks in their theories in a way that keeps scientists happy for the time being. This sense of success is surely an illusion.

And that is how it should be. If scientists really think they are coming close to a final and complete description of reality, then a simple list of paradoxes can do a remarkable job of putting feet firmly back on the ground.

Ref: arxiv.org/abs/1501.01919 : Paradoxes Of Cosmological Physics In The Beginning Of The 21-St Century

World’s oldest temple built to worship the dog star (New Scientist)

16 August 2013 by Anil Ananthaswamy
Magazine issue 2930

THE world’s oldest temple, Göbekli Tepe in southern Turkey, may have been built to worship the dog star, Sirius.

The original star sign? <i>(Image: Vincent J. Musi/ National Geographic Stock)</i>

The original star sign? (Image: Vincent J. Musi/ National Geographic Stock)

The 11,000-year-old site consists of a series of at least 20 circular enclosures, although only a few have been uncovered since excavations began in the mid-1990s. Each one is surrounded by a ring of huge, T-shaped stone pillars, some of which are decorated with carvings of fierce animals. Two more megaliths stand parallel to each other at the centre of each ring (see illustration).

Göbekli Tepe put a dent in the idea of the Neolithic revolution, which said that the invention of agriculture spurred humans to build settlements and develop civilisation, art and religion. There is no evidence of agriculture near the temple, hinting that religion came first in this instance.

“We have a lot of contemporaneous sites which are settlements of hunter-gatherers. Göbekli Tepe was a sanctuary site for people living in these settlements,” says Klaus Schmidt, chief archaeologist for the project at the German Archaeological Institute (DAI) in Berlin.

But it is still anybody’s guess what type of religion the temple served. Giulio Magli, an archaeoastronomer at the Polytechnic University of Milan in Italy, looked to the night sky for an answer. After all, the arrangement of the pillars at Stonehenge in the UK suggests it could have been built as an astronomical observatory, maybe even to worship the moon.

Magli simulated what the sky would have looked like from Turkey when Göbekli Tepe was built. Over millennia, the positions of the stars change due to Earth wobbling as it spins on its axis. Stars that are near the horizon will rise and set at different points, and they can even disappear completely, only to reappear thousands of years later.

Today, Sirius can be seen almost worldwide as the brightest star in the sky – excluding the sun – and the fourth brightest night-sky object after the moon, Venus and Jupiter. Sirius is so noticeable that its rising and setting was used as the basis for the ancient Egyptian calendar, says Magli. At the latitude of Göbekli Tepe, Sirius would have been below the horizon until around 9300 BC, when it would have suddenly popped into view.

“I propose that the temple was built to follow the ‘birth’ of this star,” says Magli. “You can imagine that the appearance of a new object in the sky could even have triggered a new religion.”

Using existing maps of Göbekli Tepe and satellite images of the region, Magli drew an imaginary line running between and parallel to the two megaliths inside each enclosure. Three of the excavated rings seem to be aligned with the points on the horizon where Sirius would have risen in 9100 BC, 8750 BC and 8300 BC, respectively (arxiv.org/abs/1307.8397).

The results are preliminary, Magli stresses. More accurate calculations will need a full survey using instruments such as a theodolite, a device for measuring horizontal and vertical angles. Also, the sequence in which the structures were built is unclear, so it is hard to say if rings were built to follow Sirius as it rose at different points along the horizon.

Ongoing excavations might rule out any astronomical significance, says Jens Notroff, also at DAI. “We are still discussing whether the monumental enclosures at Göbekli Tepe were open or roofed,” he says. “In the latter case, any activity regarding monitoring the sky would, of course, have been rather difficult.”

This article appeared in print under the headline “Stone Age temple tracked the dog star”