Question 29-32 are based on the following passage.

    Since 1929, physicists have known that the universe is expanding. Most physicists hold that the entire observable universe was condensed into a tiny area of exceptional density and that this initial mass exploded outward in what is commonly referred to as the "Big Bang". The universe has been expanding steadily ever since this cataclysmic event and most physicists naturally assumed that this expansion must be gradually stowing down. Some theorists even predicted that the gravitational pull of the universe would cause the eventual contraction of the universe—the "Big Crunch." However, in 1998, a team of observers studying supernovae made a shocking discovery. Instead of slowing down, the expansion of the universe is accelerating. In fact, the acceleration appeared to have begun some 10 billion years ago. Later measurements of the cosmic microwave background and a survey of the distribution of galaxies seem to corroborate the initial finding that the expansion of the universe is indeed accelerating.

    Cosmologists believe the most likely explanation for the accelerate of the universe's expansion is an entity known as dark energy. According to theory, dark energy should account for approximately two-thirds of the total cosmic energy density. It also is believed to be gravitationally repulsive and diffused throughout the areas of space unoccupied by galaxies. Although theoretically possible, dark energy, if it were found to exist, would provide theorists with a number of difficulties and perhaps even require an extensive rethinking of fundamental physics. Some cosmologists believe that the existence of dark energy implies the existence of subatomic particles more than thirty orders of magnitude smaller than the electron. Others have tinkered with Einstein's theory of relativity in an attempt to account for the perceived effects of dark energy.

    Such drastic revisions to long-held tenets have encouraged some physicists to seek a more conventional solution to the accelerating universe enigma, one that does not invoke the specter of dark energy. These theories posit more mundane explanations, such as the aggregate gravitational effect of interstellar dust. Recently, fax independent groups have found evidence of the integrated Sachs-Wolfe effect, a phenomenon linked to the presence of dark energy. 

The passage is primarily concerned with ....