Archive for the ‘galaxies’ Category

Expanding space, or moving galaxies?

January 21, 2008

Consider a bunch of beads evenly spaced on a long elastic string.

Suppose that you position yourself next to one of the beads and look to an adjacent bead and you see it moving away from you.

What’s happening? There are two possibilities:

  1. The adjacent bead is moving along the elastic string, away from you
  2. The elastic is being pulled

How would you know which is the case?

It is well-known that the galaxies are moving outward, away from our galaxy.  Are they actually moving, or is space expanding?

Watching the history of the universe unfold

January 16, 2008

When astronomers peer at the heavens through telescopes, they see distant objects not as they are now, but as they were when the light reaching the telescopes embarked on its journey across space.  In this respect, a telescope is also a “timescope.”  For example, if a nearby star exploded yesterday, we would be blissfully unaware of this cataclysm for years, until the pulse of light announcing the star’s demise arrived on Earth.  Looking further afield, we see stars in the neighboring Andromeda galaxy as they looked about 2.5 million years ago.  More distant galaxies appear correspondingly older.  The Hubble Space Telescope routinely records images of galaxies as they appeared long before Earth even existed.  The oldest galaxies can actually be seen still in the process of formation, more that 12 billion years ago.  So by penetrating farther and farther into space, astronomers can watch the history of the universe unfolding in reverse.

Cosmic Jackpot by Paul Davies

The “big bang” was not an exploding ball of matter

January 13, 2008

Popular accounts of the big bang often describe it as the detonation of a compact ball of matter poised in a preexisting void, with the galaxies compared to fragments flying away from the center of the explosion.

Easy though this image may be to grasp, it is seriously misleading and the source of much confusion: people are inevitably prompted to ask, “Where is the center of the universe?”

If the big bang really had been an exploding ball of matter, then some galaxies would lie deep in the midst of the melee, surrounded on all sides, while others would be located near the edge of the assemblage.  Suppose this were so, and picture the view from a far-flung galaxy.  In one direction would lie the center of the universe; in the opposite direction there would be empty space.  The sky would appear dramatically different depending on which way an observer looked.

That is certainly not what we see from Earth: the universe looks very much the same in all directions.  As far as our telescopes can penetrate, which is about 13 billion light-years, encompassing roughly 100 billion galaxies, matter is distributed uniformly (strictly, it is clusters of galaxies that are distributed uniformly).  There is no evidence for any bunching up around some sort of center or, conversely, for any thinning out toward an edge.

How, then, should we describe the big bang and the expanding universe, given these observational facts?  Cosmologists have struggled to find ways to describe the expanding universe in simple language.  Here’s one attempt:

The big bang happened everywhere, not at one point in space.

A simple analogy that may help is to imagine a very long string of elastic with beads attached at regular intervals.  As the elastic is stretched, the beads move apart.  Every bead extends its separation from its neighbors, so the view from any given bead will be of other beads moving away.  All beads are equal: there is no center bead.

Cosmic Jackpot by Paul Davies