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Our Hierarchical Universe

The Visible Cosmos

Humankind's gaze into the universe--aided by telescopes--has revealed a vast hierarchy of structure and motion. Planets moving around stars; stars grouped together, moving in a slow dance around the center of galaxies. Galaxies themselves--some 100 billion of them in the observable universe--form galaxy clusters bound by gravity as they journey through the void. But the largest structures of all are the superclusters, each containing thousands of galaxies and stretching many hundreds of millions of light years across space. These superclusters are arranged in filament or sheet-like structures, between which there are gigantic voids of seemingly empty space.

The Milky Way and Andromeda galaxies, along with about fifteen or sixteen smaller galaxies, form what's known as the Local Group of galaxies. The Local Group sits near the outer edge of an enormous supercluster, the Virgo cluster. What's more, the Milky Way and Andromeda are moving toward each other, the Local Group is falling into the middle of the Virgo cluster, and the entire Virgo cluster itself, along with a second supercluster, is speeding toward some unfathomable mass known only as "The Great Attractor."

These structures and their movements--which can't be explained as part of the expansion of the universe--must be guided by the gravitational pull of matter. And yet, scientists have not yet detected enough matter to account for this tremendous gravitational pull. And so we must add one more player into our hierarchical scenario: dark matter.

Looking Out, Looking Back

When cosmologists focus their telescopes on a galaxy billions of light years away, they are actually looking back in time, when the universe was very young. The light--or radio emissions--that we detect took billions of years to reach our instruments, so we are seeing the galaxy as it was then, not as it is now. These telescopes are therefore like time machines that help cosmologists learn more about the origins, and, ultimately, the fate of the universe.

Ever more powerful instruments have made it possible to look farther out and farther back than ever before possible, and what cosmologists are seeing is simply astounding. Theories about the origin and fate of the universe are being variously bolstered, refined, or turned on their heads based on these new observations.

Cosmologists hope that their observations will help answer these questions, some of which have dogged scientists for centuries:

How old is the universe?

Why was it so smooth?

How did giant superstructures emerge from an almost perfectly smooth universe?

How did galaxies form within these superstructures?

Will the universe expand forever?

Or will it collapse upon itself like a bubble that has reached its limits?

In the following sections, we'll examine what and how cosmologists know what they do know, and some of the "burning issues" in cosmology.

Expanding Universe

Footprints of Creation

Seeds of Structure

Mysterious Dark Matter

Walls within Voids

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Copyright, (c) 1995: Board of Trustees, University of Illinois

NCSA. Last modified 10/4/95.