After Hubble made his discovery, scientists speculated that cosmic expansion would eventually slow to a halt, resulting in a classical universe whose boundaries are static -- just taking billions of years to reach this condition. Then it was speculated that the expansion would eventually reverse, as gravity overcame the momentum of the Big Bang and drew the material of the firmament back toward its point of origin. Stephen Hawking predicted that the universe would someday implode upon itself in a Big Crunch, the opposite of a Big Bang. All such theories assumed that the expansion of the galaxies was caused by "coasting" on the energy of the Big Bang and could not last.
|At some far future point, the other galaxies may have fled so far away from the Milky Way that ours will truly be an "island universe," moving alone through a void of unimaginable expanse.|
Recent research suggests, however, that the expansion isn't slowing down--it is speeding up. The galaxies are moving faster all the time, and may continue to do so for eons, if not for eternity. The expansion could not be speeding up unless some force was adding energy to the equation, actively propelling the galaxies. So what in heck is going on?
Researchers aren't sure, but it now looks as if the universe contains an elusive, previously unrecognized fundamental force called the "cosmological constant," or lambda, which is roughly gravity-in-reverse. Gravity pulls things together, but becomes weaker with distance; lambda pushes things apart, and increases in strength over distance.
Thus, the farther apart the galaxies fly, the more powerful lambda grows, and the more it accelerates the galaxies further, endlessly speeding things up. Because lambda has no effect at short distances, it would be irrelevant to our daily affairs, in fact, nearly impossible to detect. But at cosmic distances it would be the greatest of forces, eternally speeding up the flight of the galaxies into deeper space.
Lambda theory is unproven, but being taken seriously by many researchers owing to the otherwise inexplicable evidence of accelerating cosmic expansion. This theory has many mind-bending implications. Let's consider just one:
Unknowing of other galaxies, the Greeks considered the Milky Way an "island universe," all that there was to creation. For centuries, this view dominated astronomy. Today we know instead that there are a least 40 billion other galaxies. But we also know they move ever-farther away from each other, and now it appears the speed of their flight may accelerate forever.
If so, at some far-future point the other galaxies may have fled so far from the Milky Way that ours will once again in effect be an "island universe," moving alone through a void of unimaginable expanse. The rest of creation will still be present but so inexpressibly distant that when our far descendants look beyond our galaxy, they may see only darkness. A vast universe seething with galaxies will have evolved into what is in effect billions of self-contained, isolated universes set against a backdrop whose expanse defies contemplation; a visitor from another plane of existence might not even guess that it had all once been connected. Such may be the scale of our cosmos, set in motion by - what?