Our universe may have started not with a big bang but with a big bounce—an implosion that triggered an explosion, all driven by exotic quantum-gravitational effects
By Martin Bojowald
The universe may not have started off with a bang after all.
Pat Rawlings/SAIC
KEY CONCEPTS
- Einstein’s general theory of relativity says that the universe began with the big bang singularity, a moment when all the matter we see was concentrated at a single point of infinite density. But the theory does not capture the fine, quantum structure of spacetime, which limits how tightly matter can be concentrated and how strong gravity can become. To figure out what really happened, physicists need a quantum theory of gravity.
- According to one candidate for such a theory, loop quantum gravity, space is subdivided into “atoms” of volume and has a finite capacity to store matter and energy, thereby preventing true singularities from existing.
- If so, time may have extended before the bang. The prebang universe may have undergone a catastrophic implosion that reached a point of maximum density and then reversed. In short, a big crunch may have led to a big bounce and then to the big bang.
Atoms are now such a commonplace idea that it is hard to remember how radical they used to seem. When scientists first hypothesized atoms centuries ago, they despaired of ever observing anything so small, and many questioned whether the concept of atoms could even be called scientific. Gradually, however, evidence for atoms accumulated and reached a tipping point with Albert Einstein’s 1905 analysis of Brownian motion, the random jittering of dust grains in a fluid. Even then, it took another 20 years for physicists to develop a theory explaining atoms—namely, quantum mechanics—and another 30 for physicist Erwin Müller to make the first microscope images of them. Today entire industries are based on the characteristic properties of atomic matter.
Physicists’ understanding of the composition of space and time is following a similar path, but several steps behind. Just as the behavior of materials indicates that they consist of atoms, the behavior of space and time suggests that they, too, have some fine-scale structure—either a mosaic of spacetime “atoms” or some other filigree work. Material atoms are the smallest indivisible units of chemical compounds; similarly, the putative space atoms are the smallest indivisible units of distance. They are generally thought to be about 10–35 meter in size, far too tiny to be seen by today’s most powerful instruments, which probe distances as short as 10–18 meter. Consequently, many scientists question whether the concept of atomic spacetime can even be called scientific. Undeterred, other researchers are coming up with possible ways to detect such atoms indirectly.
The most promising involve observations of the cosmos. If we imagine rewinding the expansion of the universe back in time, the galaxies we see all seem to converge on a single infinitesimal point: the big bang singularity. At this point, our current theory of gravity—Einstein’s general theory of relativity—predicts that the universe had an infinite density and temperature. This moment is sometimes sold as the beginning of the universe, the birth of matter, space and time. Such an interpretation, however, goes too far, because the infinite values indicate that general relativity itself breaks down. To explain what really happened at the big bang, physicists must transcend relativity. We must develop a theory of quantum gravity, which would capture the fine structure of spacetime to which relativity is blind.
- InsightsUsing Math to Explain How Life on Earth Began
- News ScanAugmented Reality Makes Commercial Headway
- Working KnowledgeHow Voting Machines Work
The details of that structure came into play under the dense conditions of the primordial universe, and traces of it may survive in the present-day arrangement of matter and radiation. In short, if spacetime atoms exist, it will not take centuries to find the evidence, as it did for material atoms. With some luck, we may know within the coming decade.
Pieces of Space
Physicists have devised several candidate theories of quantum gravity, each applying quantum principles to general relativity in a distinct way. My work focuses on the theory of loop quantum gravity (“loop gravity,” for short), which was developed in the 1990s using a two-step procedure. First, theorists mathematically reformulated general relativity to resemble the classical theory of electromagnetism; the eponymous “loops” of the theory are analogues of electric and magnetic field lines. Second, following innovative procedures, some that are akin to the mathematics of knots, they applied quantum principles to the loops. The resulting quantum gravity theory predicts the existence of spacetime atoms [see “Atoms of Space and Time,” by Lee Smolin; Scientific American, January 2004].
Other approaches, such as string theory and so-called causal dynamical triangulations, do not predict spacetime atoms per se but suggest other ways that sufficiently short distances might be indivisible [see “The Great Cosmic Roller-Coaster Ride,” by Cliff Burgess and Fernando Quevedo; Scientific American, November 2007, and “The Self-Organizing Quantum Universe,” by Jan Ambjørn, Jerzy Jurkiewicz and Renate Loll; Scientific American, July]. The differences among these theories have given rise to controversy, but to my mind the theories are not contradictory so much as complementary. String theory, for example, is very useful for a unified view of particle interactions, including gravity when it is weak. For the purpose of disentangling what happens at the singularity, where gravity is strong, the atomic constructions of loop gravity are more useful.
PAGE 1 | 2 | 3 | 4 | 5 | Next»
Read other pages in the link
http://www.sciam.com/article.cfm?id=big-bang-or-big-bounce&SID=mail&sc=emailfriend
No comments:
Post a Comment