|Even more interesting
||[Feb. 22nd, 2004|10:11 am]
String theory (taken from http://www.damtp.cam.ac.uk/user/gr/public/qg_ss.html)|
In String Theory, the myriad of particle types is replaced by a single fundamental building block, a `string'. These strings can be closed, like loops, or open, like a hair. As the string moves through time it traces out a tube or a sheet, according to whether it is closed or open. Furthermore, the string is free to vibrate, and different vibrational modes of the string represent the different particle types, since different modes are seen as different masses or spins.
One mode of vibration, or `note', makes the string appear as an electron, another as a photon. There is even a mode describing the graviton, the particle carrying the force of gravity, which is an important reason why String Theory has received so much attention. The point is that we can make sense of the interaction of two gravitons in String theory in a way we could not in QFT. There are no infinities! And gravity is not something we put in by hand. It has to be there in a theory of strings. So, the first great achievement of String Theory was to give a consistent theory of quantum gravity, which resembles GR at macroscopic distances. Moreover String Theory also possesses the necessary degrees of freedom to describe the other interactions! At this point a great hope was created that String Theory would be able to unify all the known forces and particles together into a single `Theory of Everything'.
From Strings to Superstrings
The particles known in nature are classified according to their spin into bosons (integer spin) or fermions (odd half integer spin). The former are the ones that carry forces, for example, the photon, which carries electromagnetic force, the gluon, which carries the strong nuclear force, and the graviton, which carries gravitational force. The latter make up the matter we are made of, like the electron or the quark. The original String Theory only described particles that were bosons, hence Bosonic String Theory. It did not describe Fermions. So quarks and electrons, for instance, were not included in Bosonic String Theory.
By introducing Supersymmetry to Bosonic String Theory, we can obtain a new theory that describes both the forces and the matter which make up the Universe. This is the theory of superstrings. There are three different superstring theories which make sense, i.e. display no mathematical inconsistencies. In two of them the fundamental object is a closed string, while in the third, open strings are the building blocks. Furthermore, mixing the best features of the bosonic string and the superstring, we can create two other consistent theories of strings, Heterotic String Theories.
However, this abundance of theories of strings was a puzzle: If we are searching for the theory of everything, to have five of them is an embarrassment of riches! Fortunately, M-theory came to save us.
One of the most remarkable predictions of String Theory is that space-time has ten dimensions! At first sight, this may be seen as a reason to dismiss the theory altogether, as we obviously have only three dimensions of space and one of time. However, if we assume that six of these dimensions are curled up very tightly, then we may never be aware of their existence. Furthermore, having these so-called compact dimensions is very beneficial if String Theory is to describe a Theory of Everything. The idea is that degrees of freedom like the electric charge of an electron will then arise simply as motion in the extra compact directions! The principle that compact dimensions may lead to unifying theories is not new, but dates from the 1920's, since the theory of Kaluza and Klein. In a sense, String Theory is the ultimate Kaluza-Klein theory.
For simplicity, it is usually assumed that the extra dimensions are wrapped up on six circles. For realistic results they are treated as being wrapped up on mathematical elaborations known as Calabi-Yau Manifolds and Orbifolds.
Apart from the fact that instead of one there are five different, healthy theories of strings (three superstrings and two heterotic strings) there was another difficulty in studying these theories: we did not have tools to explore the theory over all possible values of the parameters in the theory. Each theory was like a large planet of which we only knew a small island somewhere on the planet. But over the last four years, techniques were developed to explore the theories more thoroughly, in other words, to travel around the seas in each of those planets and find new islands. And only then it was realized that those five string theories are actually islands on the same planet, not different ones! Thus there is an underlying theory of which all string theories are only different aspects. This was called M-theory. The M might stand for Mother of all theories or Mystery, because the planet we call M-theory is still largely unexplored.
There is still a third possibility for the M in M-theory. One of the islands that was found on the M-theory planet corresponds to a theory that lives not in 10 but in 11 dimensions. This seems to be telling us that M-theory should be viewed as an 11 dimensional theory that looks 10 dimensional at some points in its space of parameters. Such a theory could have as a fundamental object a Membrane, as opposed to a string. Like a drinking straw seen at a distance, the membranes would look like strings when we curl the 11th dimension into a small circle.
Black Holes in M-theory
Black Holes have been studied for many years as configurations of spacetime in General Relativity, corresponding to very strong gravitational fields. But since we cannot build a consistent quantum theory from GR, several puzzles were raised concerning the microscopic physics of black holes. One of the most intriguing was related to the entropy of Black Holes. In thermodynamics, entropy is the quantity that measures the number of states of a system that look the same. A very untidy room has a large entropy, since one can move something on the floor from one side of the room to the other and no one will notice because of the mess - they are equivalent states. In a very tidy room, if you change anything it will be noticeable, since everything has its own place. So we associate entropy to disorder. Black Holes have a huge disorder. However, no one knew what the states associated to the entropy of the black hole were. The last four years brought great excitement in this area. Similar techniques to the ones used to find the islands of M-theory, allowed us to explain exactly what states correspond to the disorder of some black holes, and to explain using fundamental theory the thermodynamic properties that had been deduced previously using less direct arguments.
Many other problems are still open, but the application of string theory to the study of Black Holes promises to be one of the most interesting topics for the next few years.
Dark energy (taken from http://www.msnbc.msn.com/id/4327735)
Recent Hubble Space Telescope images of distant exploding stars add further confirmation to the permanence of a mysterious, repulsive force called dark energy that appears to dominate the universe.
While scientists are not ready to close the case, they said today that dark energy, which is thought to permeate the cosmos and work in opposition to gravity, does appear to be a constant presence as predicted.
The results bolster a theory that the universe won't end soon. But they leave researchers no more informed about the actual nature of dark energy.
"We still have almost no clue what it is," said study leader Adam Riess of the Space Telescope Science Institute (STScI) in Baltimore.
Dark energy was conjured to explain a phenomenal discovery in 1998: Nearly all galaxies in the universe are receding from each other at an ever-faster pace.
Gravity is losing some unknown battle, cosmologists admit. They theorize that about 70 percent of the universe is made up of dark energy, while most of the rest is another mysterious thing called dark matter and only a small fraction is real matter like stars, planets and living entities.
Albert Einstein was the first to consider something similar, which he called a cosmological constant. He said even the emptiest space would have some of this strange stuff in it.
But when Edwin Hubble discovered the expansion of the universe in the 1920s, Einstein called his cosmological constant his greatest blunder.
Einstein is back
With the more recent finding that the expansion is accelerating, Einstein's idea was revived.
The new findings support Einstein's cosmological constant, which modern cosmologists say implies that dark energy should not characteristically change over time. If that's right, the universe will continue to expand at an accelerating forever.
The new results suggest that even if Einstein and modern dark energy theory are both wrong, dark energy will not destroy the universe for at least 30 billion years, Riess and his colleagues say.
"Right now we're about twice as confident than before that Einstein's cosmological constant is real, or at least dark energy does not appear to be changing fast enough, if at all, to cause an end to the universe anytime soon," Riess said.
The universe is presently 13.7 billion years old.
Riess' team uses Hubble to find stars that exploded when the universe was about half its present age. A certain type of these supernovas, as they are called, shine with a known brightness. So examining the light that reaches Hubble tells astronomers how far away each one is and the rate at which the universe was expanding when that star exploded.
That rate of expansion has changed over time, other studies have shown. The initial expansion, after a theoretical Big Bang, was the most rapid and was called inflation. Then things leveled off before another round of acceleration, which is apparently underway now.
Riess' team has now observed 42 of the very distant supernovas -- including 16 in the new work -- in its Great Observatories Origins Deep Survey (GOODS) program.
The data was first presented last fall but has only now been fully analyzed. The results were discussed in a teleconference with reporters Friday and will be published in the Astrophysical Journal.
What's going on
There are two main ideas for the source of dark energy. It might percolate from empty space, as Einstein theorized, and is unchanging and of a fixed strength. The other holds that dark energy is associated with a changing energy field called "quintessence," something akin to a magnetic field. In that scenario, the field causes the current acceleration of the universe.
Another research team recently theorized that if the repulsion from dark energy gets stronger than Einstein's prediction, the universe could expand so incredibly that it would end in a Big Rip. All matter — galaxies, then stars, then planets, and everything right down to the atomic level — would be torn apart.
If dark energy can change, it might also one day reverse course and pull the universe back together in a Big Crunch. "This looks like the least likely scenario at present," Riess said.
There are two initial questions scientists are trying to answer: What is the strength of dark energy today, and does it grow or decay with time?
The new data show that if the repulsive force is changing, "it is not changing very rapidly," Riess said.
There is a lot of work ahead.
"Determining these two properties still leaves us very, very far from understanding what dark energy is," said Mario Livio, a theorist who heads the science division at the STScI. But until these first two parameters are determined, a fundamental understanding of the cosmos will remain elusive. It remains possible, for example, that our understanding of gravity "is completely lacking," Livio said.
There are other methods for probing dark energy, but none are as developed as the supernova observations. So in the near term, progress toward understanding dark energy will rely heavily on more observations of exploded stars that are even farther away and deeper in time.
Astronomers worry, however, that if Hubble stops working by around 2007, which would be the case under NASA's current plan, they would lose their primary tool in the hunt for distant supernovas.
And with NASA's new human spaceflight plans, other useful projects are in jeopardy. Many missions under NASA's Beyond Einstein initiative, including proposed missions to study dark energy, have taken a budgetary back seat to programs that will help get humans back on the Moon and on to Mars.
Whatever methods are applied, Anne Kinney, director of NASA's Astronomy and Physics Division, cautioned that final answers on the nature of dark energy will not likely come for a very long time. Science can sometimes be much like art, she said: "You approach, you don't arrive."