Wednesday, April 21, 2004

Bang! Goes the Universe

Your article concerning the ekpyrotic theory of the universe was fascinating [“Before the Big Bang,” February]. However, I had some questions. Is our sister “brane” a physical mirror of our own, or is it an entirely different universe? Furthermore, a trillion years from now, will I be sitting here writing Discover a letter? In other words, will the universes that are re-created at the moment of the singularity continue to repeat themselves? Or is the scattering of matter random? If it is random, and the process continues on into eternity, then I suppose the rule of infinity comes into play: Eventually, our exact universe will be re-created.


Boston, Massachusetts


Cosmologist Paul Steinhardt responds: The term universe is used to mean different things. The observable universe is the part we can see and interact with today. In most models, this is a tiny fraction of a much larger region of space to which it is connected. Our sister brane, our brane, and the space in between should all be viewed together as our universe, since the parts are connected and can interact with one another. For example, we may not be able to see particles on our sister brane because light from that brane cannot travel across the gap; however, we can feel the gravitational force of those particles. These particles and forces would not be identical to those of our own brane. The cycles are only statistically identical. You’ll have to wait many cycles before you write your letter to Discover again.

If I understand it correctly, the ekpyrotic theory allows multiple parallel universes, or branes, but does not specify where they come from. An attractive force exists between two branes, and when they eventually collide, the resulting energy is sufficient to create a new universe. Presumably, each new universe could develop a different set of physical laws. I am struck by the possible overlap between this picture of periodically colliding branes causing unrelated big bangs and that of a multiverse with ever-proliferating universes, as posited by the “many worlds” interpretation of quantum mechanics. Is there a link between the theory of parallel branes and that of the quantum multiverse? If so, could the colliding branes that produce recurring big bangs actually be split-offs from a common ancestor? 


Seattle, Washington

Steinhardt: The quantum multiverse concept and the ekpyrotic (and cyclic) model are independent ideas. In the ekpyrotic model, there is a single universe consisting of a pair of regularly colliding branes. Each cycle of evolution is connected to the ones that came before; the present structure of the universe is also a result of events that occurred in the previous cycle. If the ekpyrotic model were combined with the many worlds concept, then the pairs of branes would keep replicating to form totally disconnected pairs of branes that regularly collide. Both concepts could be correct.

“Before the Big Bang” left me buzzing with questions like this: Perhaps the distortion of the two parallel universes is not limited to the period immediately before collision. If so, could the attraction between universes create a warping of space similar to that produced by the gravity of massive objects? Could this explain observed gravitational anomalies without needing to invoke either dark matter or a sliding scale for the gravitational force? 


Lee’s Summit, Missouri

Steinhardt: The branes can stretch, wiggle, curve, and warp. The picture of nearly flat branes applies on average to our universe because the average distribution of matter is so uniform. However, high concentrations of matter can severely distort the branes. A black hole would, in fact, join the two branes together and severely warp them in regions close to the black hole horizon. The brane picture does not replace ideas like dark matter or dark energy but rather suggests specific ideas for what they might be, such as matter in our sister brane.

A Steady Diet

Three cheers for the Willett diet! Walter Willett’s conclusions in “What Does Science Say You Should Eat?” [February] should come as no surprise to those of us on diets crafted to prevent chronic disease. However, I arrived at exactly the same conclusions as Willett without ever having seen his research. Dr. [Mitchell] Gaynor’s Cancer Prevention Program and Dr. Charles Simone’s Cancer & Nutrition opened my eyes to the connection between disease and cancer. Surprisingly, my dietary changes yielded an unexpected weight loss of 40 pounds. Nevertheless, Willett’s book clearly puts a nice mainstream bow on a package of well-correlated, multiple-source data.


Springfield, Virginia

Rethinking the Spread of AIDS

Helen Epstein’s article about the spread of HIV in Africa [“Why Is AIDS Worse in Africa?” February] was interesting. Martina Morris seems to have looked only at the sexual behavior of different populations. I wonder whether Morris is aware of the genetic differences between African and European populations (including Americans of European descent) with regard to the CCR5 gene, which is a chemokine receptor on white blood cells. A mutant form of the gene, called delta 32, prevents the HIV virus from entering the cells. Having two copies of the recessive gene essentially makes you immune to the HIV virus, since it cannot enter the cells to replicate. Having one copy allows you to live longer if you are infected. There is a relatively high prevalence of this mutant gene in Europeans and their descendants, whereas the opposite is true of African and many Asian populations.


Marshfield, Wisconsin

Helen Epstein responds: The frequency of CCR5 mutations in non-African populations is far too low to explain the epidemiology of HIV. It is about 1 percent among Caucasians and nonexistent among Asians, African Americans, and other racial groups. Therefore, it cannot explain global patterns of HIV infection.


In “Gravity Measured, or Not” (The Year in Science, January) we implied that physicist Sergei Kopeikin had confirmed the existence of gravitons; his actual claim is to have measured the speed of gravity. In February’s Sky Lights, we misidentified the leader of the study using the Chandra X-ray Observatory. He is Andy Fabian, of the Institute of Astronomy at the University of Cambridge. In “Good-bye, Snowflake” (R&D, February) the last sentence should have read, “But if two albino-gene-carrying grandkids mate, their offspring will each have a 25 percent chance of turning out like their pale patriarch.”

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