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Minimum mass
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Title: Is there a black hole minimum mass?
Authors: Tomohiro Harada

Applying the first and generalised second laws of thermodynamics for a realistic process of near critical black hole formation, we derive an entropy bound, which is identical to Bekenstein's one for radiation. Relying upon this bound, we derive an absolute minimum mass ~ 0.04 sqrt{g*mPl, where g* and m Pl is the effective degrees of freedom for the initial temperature and the Planck mass, respectively. Since this minimum mass coincides with the lower bound on masses of which black holes can be regarded as classical against the Hawking evaporation, the thermodynamical argument will not prohibit the formation of the smallest classical black hole. For more general situations, we derive a minimum mass, which may depend on the initial value for entropy per particle. For primordial black holes, however, we show that this minimum mass can not be much greater than the Planck mass at any formation epoch of the Universe, as long as g* is within a reasonable range. We also derive a size-independent upper bound on the entropy density of a stiff fluid in terms of the energy density.

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Baby Universe
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A radical new project could permit human beings to create a "baby universe" in a laboratory in Japan. While it sounds like a dangerous undertaking, the physicists involved believe that if the project is successful, the space-time around a tiny point within our universe will be distorted in such a way that it will begin to form a new superfluid space, and eventually break off, separate in all respects from our experience of space and time, causing no harm to the fabric of our universe.

The project takes as its starting point two basic theories about the foundations of our universe: the big bang and inflation theory. The big bang theory, as many readers are well aware, observes that all objects in the known universe appear to be moving away from one another, suggesting that the universe was jump-started when all matter and energy were concentrated in an inconceivably tiny space, allowing them to overcome binding forces and causing a cosmic explosion.
It is well-tested and consistent with all currently accepted models for general cosmology, as tested against advanced theoretical and observational physics. But it is only one piece of the puzzle. Inflation is a key theory, developed in 1981, when MIT physicist Alan Guth observed that there appeared to have been a period immediately following the big bang when the universe "inflated" rapidly, allowing distinct regions of matter and energy to function comfortably free from any forces that might cause them to collapse against each other or disrupt each other's evolution.
This project is not exactly theoretical physics at work. It is closer to a physical application of observed phenomena, in combination, with the aim of achieving an as yet untested physical effect. Inflation theory helps provide the means of understanding how that effect might be brought about.

"Inflation theory, subsequently modified by Linde, relies on the fact that the 'vacuum' of empty space-time is not a boring, static place. Instead, it is subject to quantum fluctuations that cause strange bubbles to appear at random times. These bubbles of 'false vacuum' contain space-time with different —and very curious— properties" - New Scientist.

The space-time inside these false vacuums is organised and kept constant by a phenomenon known as the 'Higgs field'. It is believed that with the constant provided by the Higgs field, these bubbles of 'false vacuum' can be induced to withstand contact with the high pressure exterior vacuum and subsequently to expand through a kind of cosmic inflation like the one which followed the big bang at the beginning of our universe.
The key is a monopole, a unique spherical particle with only a north or south pole, only one charge. Adding mass and energy to this already extremely dense particle, could cause it to expand "eternally", providing the trigger needed to make the bubble of false vacuum into an ever-expanding universe, akin to our own, but entirely separate and likely to develop its own physical properties, laws and materials.
Here is the key to the "new universe" paradigm for the project. It would not be simply an extension of our own universe, a space where strange things happen. The New Scientist reports physicist Nobuyuki Sakai's discoveries regarding this process as follows:

"The baby universe has its own space-time and, as this inflates, the pressure from the true vacuum outside its walls continues to constrain it. As these forces compete, the growing baby universe is forced to bubble out from our space-time until its only connection to us is through a narrow space-time tunnel called a wormhole..."

Eventually, the "umbilical" connection between our space-time and the baby universe would be effectively cut, and the baby universe would enter into its own unique process of unending expansion. From our perspective, it would be lost inside a microscopic "black hole", which will not appear to expand into our space-time. Hawking radiation will be emitted and the tiny black hole will "evaporate", sealing the separation between the two universes.
Ultimately, this evaporation is what makes the project possible, but is also, perhaps, its most serious obstacle. It is expected that the separation between our space-time and the baby universe would occur so quickly, it might be impossible —within the limitations of our physical universe— to observe its having been created.

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The Graviton
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Albert Einstein said "I know why there are so many people who love chopping wood. In this activity one immediately sees the results".
Einstein spent the last thirty years of his life trying to find a theory that would unify electromagnetism with gravity, but success eluded him.

The search is still on for a unifying theory of gravitational force and hopes are pinned on the location of the graviton - a hypothetical elementary particle that transmits the force of gravity. But the graviton is proving hard to find. Indeed, the next big research project which involves the largest earth-based laboratory in the world - a circular ring which goes underground for about twenty-seven miles and spans Switzerland, France and Germany - still won't allow us to detect gravitons per se, but might be able to prove their existence in other ways.

The idea of the graviton particle first emerged in the middle of the twentieth century, when the notion that particles as mediators of force was taken seriously. Physicists believed that it could be applicable to gravity and by the late 20th century the hunt was truly on for the ultimate theory, a theory of quantum gravity.

So why is the search for the graviton the major goal of theoretical physics? How will the measurement of gravitation waves help prove its existence? And how might the graviton unite the seemingly incompatible theories of general relativity and quantum mechanics?

Contributors
Roger Cashmore, Former Research Director at CERN and Principal of Brasenose College, Oxford
Jim Al-Khalili, Professor of Physics at the University of Surrey
Sheila Rowan, Reader in Physics in the Department of Physics and Astronomy at the University of Glasgow

Radio 4 Realplayer Stream of `In Our Time`.

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RE: Tiny black holes
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Tiny black holes could soon be made on demand in particle accelerators, but shortly after their birth, they might blink out of existence.
In the 14 October PRL, a team proposes a mechanism for this vanishing act: The space around these black holes could wrap upon itself and bud off, forming a new baby universe that is invisible to us. Such an event might signify the existence of extra dimensions beyond the three we are familiar with and might give clues to the properties of the extra dimensions.

An astronomical black hole forms when enough matter is squashed into a small enough space to reach a critical density. According to theory, the same critical density could be reached if two particles slam violently together, creating a tiny black hole. Just how violent the collision must be depends on the number of dimensions in our universe.
If there are only the three dimensions of space we're familiar with, then making black holes would require particle energies far greater than any known process can produce. But if the Universe has extra dimensions, as quantum theories of gravity predict, then gravity could get much stronger at very short distances and suck the two colliding particles together once they get close enough. Black hole creation could then be within reach of CERN's Large Hadron Collider (LHC), the new accelerator in Geneva due to start smashing protons in 2007.

In preparation for the LHC, theorists have been trying to predict the behaviour of such tiny black holes. A 2002 paper suggested that soon after being created, a black hole could disappear into the extra dimensions, but no one has fully described this process.
Now Antonino Flachi and Takahiro Tanaka at Kyoto University in Japan have fleshed out the picture.
In extra-dimensional theories, most particles would be trapped in our three-dimensional world, which physicists call the brane.
But gravitons, the carriers of gravitational forces, can travel out of the brane and into the extra dimensions. If the brane were a flat, stretchy sheet, a black hole could emit a graviton perpendicular to the sheet, and the black hole's recoil could distort the nearby brane region, creating a deep dimple.

To simulate the brane's warping, Flachi and Tanaka used a computer to crunch through general relativity equations. In their simulations, the events following the dimple creation depended upon the properties the team assumed for the brane--properties that remain unknown.
A "flexible" brane could close up around the dimple. This "brane bubble" could then pinch off from ours and break free, forming a so-called baby brane that is separate from and invisible to our own.
With a "stiffer" brane, tiny black holes would remain visible to us. So black holes briefly appearing at the LHC could give hints about the properties of the brane and the extra dimensions.
Dejan Stojkovic of Case Western Reserve University in Cleveland says the new study confirms his earlier paper suggesting that small black holes could leave our brane.
However, Greg Landsberg of Brown University in Providence, Rhode Island, is skeptical about the broad applicability of the results. He says there are many scenarios that include different assumptions that result in black holes being trapped on the brane.
Rather than spend a lot of time discussing various possibilities, he says, "I prefer to wait a few years until the LHC turns on."

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13 April 2005

A research group at Cambridge think that the universe might once have been packed full of tiny black holes. Dr Martin Haehnelt, a researcher in the group led by Astronomer Royal Martin Rees, will present new evidence to support this controversial idea at the Institute of Physics conference Physics 2005 in Warwick.
Most cosmologists believe that supermassive black holes grew up in big galaxies, accumulating mass as time went on. But Haehnelt says there is increasing evidence for a different view – that small black holes grew independently and merged to produce the giants which exist today.

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