Through a Universe, Darkly

Martin Rees’s field trip through our universe [“A Field Guide to the Invisible Universe,” December] seems to be led by people playing blindman’s bluff: “The excess mass must be dark matter. (That conclusion assumes gravity behaves the same on cosmic scales as it does on Earth—a logical but untested article of faith….)” It’s time astronomers looked more closely at gravity and its properties instead of trying to make old ideas fit recent observations. Why not follow Einstein’s thinking about ways to modify the effects of gravity, such as by adding a cosmological constant? It seems the only physicist to propose this was Moti Milgrom in the 1980s, who was ignored until observation fit his model of gravity’s acceleration at great distances [see “Nailing Down Gravity,” Discover, October]. The work being done by such gravity wave– detection groups as those at the Laser Interferometer Gravitational Wave Observatory should bring up more questions about what gravity is. If astrophysicists concentrate more on answering some of these questions, they may not need hidden matter and energy to explain the workings of the  universe.

ROBERT ROUDEBUSH

Carpentersville, Illinois

If the preponderance of gravitational force in a galaxy is due to dark matter, can dark matter collapse into something similar to a “dark hole”? Or is dark matter somehow immune to the effects that apply to regular matter in a gravitational system?

REGINALD POOLE

Sacramento, California

Senior editor Corey S. Powell replies: Mr. Poole raises an interesting question: Does dark matter form the same kind of structures as visible matter? On intergalactic scales, dark matter seems roughly to follow the distribution of visible matter. Within galaxies, the two should accumulate quite differently because dark particles cannot interact through radiation or chemistry. Even taking that into account, dark matter doesn’t seem to clump together as intensely as some models predict. Perhaps dark-matter particles scatter or annihilate when they meet, preventing them from collapsing to form a “dark hole.” It is also possible, as Mr. Roudebush says, that the answer to the dark-matter riddle lies in our (mis)understanding of gravity. For now, however, there is significantly more evidence for dark matter than there is for nonstandard models of gravity.

A Question of Health

When I read “Why Is Asthma Raging Out of Control?” from “The 8 Greatest Unanswered Questions of Medical Science” in the December issue, I thought I might be able to add some insight. As an ecotoxicologist, my goal is to understand the interactions among contaminants, the physical environment, and life. In discussing the issue of farm children being less likely to develop asthma than city children, author Brad Lemley left out the issue of air pollution. Our cities are polluted in extremis in comparison with farms; the particulates released from burning diesel fuel have been linked to asthma outbreaks in several cities. Cities are quagmires of pollution from the exhaust of incinerators; fumes from vehicle-fueling operations and industrial facilities; particulates from paint chips, generator and alternator operations, and asbestos; acidified vapors caused by acid-producing compounds; and volatile organic compounds from such things as paint, furniture, carpeting, and cleaning solvent. The accumulation of all these things may overwhelm less-robust immune systems, thus leading to our current outbreak.

GREGG MORRIS

Hood River, Oregon

As I read the section “Can We Conquer Obesity?” in “The 8 Greatest Unanswered Questions of Medical Science,” I became increasingly angry. The quotes from Richard Atkinson, president of the American Obesity Association—“Drugs are the future of treatment for obesity” and “Diet and exercise haven’t worked for 50 years”—were too much to let go without comment. Certainly there are cases where medical intervention is warranted, but they are the distinct minority. Our society suffers from a multitude of ills that contribute to the problem of obesity. We live in a culture that encourages a sedentary lifestyle. We live in a world of supersize portions and increasingly poor diets, a world that does not hold people responsible for their own actions. Even if viruses cause obesity in rare cases, it is an insignificant factor compared with the known cause of obesity in the preponderance of cases: eating too much and exercising too little. Drugs, indeed! Try advocating responsible behavior.

GARY SAWYER

Eldersburg, Maryland

Rewards of a Lifetime

Karen Wright’s article on the limits on human longevity [“Staying Alive,” November] was refreshing in that it examined senescence from an evolutionary viewpoint. However, the evolutionary logic of gerontologists Steven Austad and George M. Martin is deeply flawed. The luxuries of the developed world do allow women to live longer and delay childbirth until late in life. But this is not likely to produce a population with a genetic disposition to live longer. For that to happen would require that those older mothers bear considerably more children than younger mothers, who with their shorter generation time have a considerable reproductive advantage. And in reality, nations with access to modern medicine and other technologies tend to have low rates of population growth.

ZACHARY HANDLER

Minneapolis, Minnesota

Steven Austad replies: You make some good points. However, the most critical thing to remember is that natural selection does not favor longevity per se but only genes and individuals who leave the most offspring. Our new longevity, brought about by sanitation and medicine, means that more individuals survive and thus have the potential to reproduce later in life. In fact, this is what women now seem to be doing. It’s not the delayed reproduction that matters but that the number of children women are having later in life make up a larger fraction of our children than ever before. This will delay the gradual fading of natural selection’s power with age, thus eventually favoring genetically increased longevity.