Where is the universe expanding to? —A. KENNY, CANISBAY, SCOTLAND

Astrophysicist Alexander Kashlinsky of the NASA Goddard Space Flight Center offers this explanation:

The evolution of the universe is described by the physics of general relativity discovered by Albert Einstein during the early 20th century. In general relativity, space and time are merged into one continuum and the universe can be represented as a four-dimensional spacetime grid. When viewed from this perspective, the universe's expansion does not push it into new territory—rather the spacetime grid itself is expanding.

In prerelativistic, Newtonian physics (which describes celestial bodies as moving according to the laws discovered by Isaac Newton), space and time are absolute, with time no more than a parameter in the equations of motion. Gravity, meanwhile, is seen as a force of attraction between massive bodies, but its source is a mystery.

The physics of general relativity is conceptually distinct—even if its equations of motion can be reduced to Newtonian equations in many practical cases. In general relativity, the properties of the spacetime grid are uniquely specified (via gravity) by the bodies inhabiting it. Gravity curves the spacetime continuum, and general relativity thus describes gravitational interactions as manifestations of that curvature. Objects under gravity's influence “fall” from less curved parts of spacetime to more curved parts.

According to Einstein's general relativity equations, the spacetime containing matter cannot remain stationary and must either expand or contract. Galaxies, then, are not strictly moving away from one another but rather are attached to the fixed grid on the expanding fabric of spacetime, thereby giving the impression of moving away from one another. As an analogy, imagine placing dots on the surface of a balloon, then inflating it. The distances between the dots—which represent galaxies—will increase, so if you live in one of these dots, you will interpret the others as receding from you. In reality the dots remain in the same positions, with respect to the two coordinates (latitude and longitude) on the surface of the balloon, and it is the fabric of the balloon that is actually expanding.

In the framework of general relativity with only four dimensions, the question posed here does not have an answer, because it implies some other coordinate grid outside spacetime. Because spacetime is linked to matter, there is no outside to the surface of the balloon—it is all the spacetime that is available.

What is “junk” DNA, and what is it worth?

—A. Khajavinia, Isfahan, Iran

Wojciech Makalowski, a Pennsylvania State University biology professor and genomics researcher, replies:

All animals have a large excess of junk DNA—genetic material that does not code for the proteins used to build bodies and catalyze chemical reactions within cells. In our genetic blueprint, for instance, only about 2 percent of DNA actually codes for proteins.

In 1972 the late geneticist Susumu Ohno coined the term “junk DNA” to describe all noncoding DNA sections, most of which consist of repeated segments scattered randomly throughout the genome. Typically sections of junk DNA come about through transposition, or movement of sections of genetic material to different positions in the genome. As a result, most of these regions contain multiple copies of so-called transposons—sequences that literally copy or cut themselves out of one part of the genome and reinsert themselves somewhere else.

In the early 1990s interest in junk DNA, and especially in repetitive elements, began to grow; many biologists now regard such repetitions as genomic treasures. It appears that these transposable sequences increase the ability of a species to evolve by serving as hot spots for genetic recombination and by providing important signals for regulating gene expression. As such, repetitive elements are hardly “junk” but rather are integral components of our genomes.

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