What is dark matter, and how is it affecting the universe?

Robert Caldwell, a cosmologist at Dartmouth College, offers this explanation:

Dark matter is a proposed solution to an as yet unresolved phenomenon—the mismatch between measurements of the gravitational mass and the luminous mass (the mass contributed by light-emitting matter) of galaxies and clusters, gravitationally bound groups of galaxies. This disparity suggests the presence of matter in the universe that does not efficiently produce light—hence, it is invisible, or “dark.”

We can determine the gravitational mass of an object, such as a star, by measuring the velocity and radius of the orbits of its satellites. To find the luminous mass of a galaxy, we use the known relations between stellar mass, color and luminosity to translate the observed colors and intensity of light from the galaxy into the total mass of its constituent stars. This mass-to-light comparison indicates that the gravitational mass of galaxies and clusters far exceeds the luminous mass.

Thus, more matter exists than we can see. Other indicators, including recent NASA measurements of the cosmic microwave background radiation (which provides a glimpse of the universe at an early age), give us further information: dark matter outweighs normal matter by a factor of 6 to 1.

What could dark matter be? Many physicists and astronomers suspect it is a type of particle that they have not yet been able to detect. The prototypical dark matter candidate is something like a neutrino—a particle that is similar to an electron but has a much smaller mass and no electric charge. All known types of neutrinos, however, are too light and too rare to fit the theoretical description of dark matter.

How does dark matter affect the universe? It must be the basic building block of the largest structures in the universe: galaxies and clusters. And dark matter does not just explain the behavior of distant bodies in the cosmos; it must be abundant within our galaxy as well. Estimates of the Milky Way's makeup predict that our solar system is immersed in a fine sea of dark matter with a density as high as roughly 105 particles per cubic meter. As Earth travels around the sun, moreover, we experience dark “seasons” as we move with or against the flow of this dark sea.

Does the moon also have a tidal effect on Earth's atmosphere?

—T. DIGENTI, SOMERVILLE, MASS.

Rashid Akmaev, a research scientist at the University of Colorado at Boulder, replies:

The short answer is yes. At various times this question has occupied such famous scientists as English physicist Isaac Newton and French mathematician Pierre-Simon Laplace, whose theory describing the behaviors of oceans predicted the existence of atmospheric tides two centuries ago.

First, let us consider how ocean tides occur. At the point on the ocean's surface closest to the moon, the moon's gravitational force is strongest, pulling the ocean toward it. On the opposite side of Earth the moon's attractive force is weakest, which allows the ocean to bulge outward again, in this case away from the moon.

Now think of the atmosphere as an ocean whose seafloor is Earth's surface. Laplace's theory predicts two atmospheric pressure maxima—peaks in the amount of atmospheric material overhead—per lunar day corresponding to the two ocean bulges. As the ocean swells, so does the atmosphere above it.

Surprisingly, observations show that the sun causes much stronger semidaily atmospheric tides, although the solar gravitational influence is less than half that of the moon. Laplace suggested that the strong solar tide was primarily generated by solar heating and not by solar gravity—a hypothesis that scientists finally confirmed in the 1960s.

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