MadSci Network: Astronomy

Re: Why are galaxies accelerating away,and not slowing down?

Date: Thu Jun 8 13:25:27 2000
Posted By: Denise Kaisler, Grad student, Astronomy, UCLA, Division of Astronomy
Area of science: Astronomy
ID: 959292896.As

First things first:

Distant galaxies aren't *accelerating* away from us.

However, more distant galaxies recede from us more quickly than nearby
galaxies. This happens because the universe is expanding in all directions
according to Hubble's famous equation:

	v = H*r

where v is the speed of a galaxy, H is a number, and r is the distance
of the galaxy from our galaxy, the Milky way. The number H is called
Hubble's constant and its value has produced some of the most heated
debates in all of modern astronomy. The most recent measurements tell us 
that the value of H is about 70 km/s/Mpc (where 1 Mpc or megaparsec is
about 3.3 million light years).

Hubble's law says that if galaxy B is twice as far away as galaxy A, B will
receed from us twice as quickly. 

But this explanation doesn't really answer your original question: why is
the universe expanding at all? Why isn't it slowing down or remaining
The energy that is driving the universe's expansion comes from the Big
Bang, a colossal explosion that happened about fifteen billion years ago.
The word "explosion" isn't really correct, but it's a good approximation.
Right now the universe is still expanding. Whether it continues to
expand forever, comes to a halt, or begins to contract depends on two
things: the amount of matter in the universe and the value of the
cosmological constant.

First, let's start with the amount of matter in the universe.

If there are enough stars, galaxies, and cold dark matter for their mutual
gravity to stop the expansion of the universe at some finite time, we live
in a closed universe. If there isn't enough stuff in our universe to stop
the expansion then we live in an open universe. In a closed universe, the
expansion will eventually stop and then reverse. All galaxies will then
begin to move towards each other -- the reverse of what we see now.

Seems pretty straightforward, right? But cosmologists aren't sure that the
picture is so simple. The complication has to do with a mistake that
Einstein made in 1915.

In the early part of the 1900s, astronomers believed that we lived in a
static universe where all galaxies stayed the same distance apart. But in
1915, when Einstein formulated his theory of general relativity, he was
dismayed to find that his theory predicted a dynamic universe -- one that
was either expanding or contracting. This was contrary to the the accepted
ideas of the time. No doubt it made him tear at his hair, perhaps giving
rise to the wild hairdo that's become one of his trademarks.

To make his new theory agree with the leading ideas of the time, Einstein
invented something called the cosmological constant -- a pressure that
would counteract the force of gravity and allow the universe to exist in a
steady state. But later on, astronomical observations by Edwin Hubble and
others showed that all galaxies except the ones very close to the Milky Way
were moving away from us. When Einstein heard of this, he abandoned his
cosmological constant, calling it the greatest mistake he ever made.

But recently, astronomers who study the origin and fate of the universe
have begun thinking that maybe there is something to Einstein's idea
after all. If the cosmological constant is not zero then it could have
big consequences about our understanging of the universe. For instance,
even if there is enough matter to close the universe, it might still keep
expanding forever. Another consequence is the "loitering universe" where
the young universe expanded for awhile but then loitered around in a steady
state for several billions of years before resuming its expansion.

A good resource for futher reading is chapter 28 of _Universe_ (5th ed.) by
Kaufmann and Freedman. 

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