Comment Re:Hubble constant == 1/age-of-universe !! (Score 1) 182
No, it is not a coincidence that the age of the universe is 1/H (H=Hubble
Constant). The Hubble Law can be written as:
v = Hd where v=velocity of galaxy, and d=distance of galaxy. this is
also where the funky units for the Hubble Constant come from.
Astronomers like to measure distances in Mpc, so simply multiplying H
by the distance in Mpc gives the velocity in km/s. In actuality it is
much easier to measure the velocity, so astronomers usually divide H by
the velocity to get an idea of the distance.
Here is a quick estimate for the age of the universe: Consider a huge
explosion of galaxies at time t=0. After a while when we look out at the
galaxies, the faster ones will have moved further. A specific galaxy
will have moved:
d = v t where d is the distance, v is the velocity, and t is the time
since the explosion.
This can be rewritten as v = d / t. Now compare this equation to the
Hubble law -- H must equal 1/t, or t (time since explosion) must equal
1/H.
This is the Astro 101 explaination. The correct derivation requires
differential equations and a many assumptions about the flatness and
density of the universe. The correct values for the age of the universe
are:
0 t
t = 2/3H^-1 for a just dense enough universe
2/3H^-1
What always got my goat is that H is called the Hubble constant, but it
changes with time.
-=[doug]=- who has never even pretended to be a cosmologist
Constant). The Hubble Law can be written as:
v = Hd where v=velocity of galaxy, and d=distance of galaxy. this is
also where the funky units for the Hubble Constant come from.
Astronomers like to measure distances in Mpc, so simply multiplying H
by the distance in Mpc gives the velocity in km/s. In actuality it is
much easier to measure the velocity, so astronomers usually divide H by
the velocity to get an idea of the distance.
Here is a quick estimate for the age of the universe: Consider a huge
explosion of galaxies at time t=0. After a while when we look out at the
galaxies, the faster ones will have moved further. A specific galaxy
will have moved:
d = v t where d is the distance, v is the velocity, and t is the time
since the explosion.
This can be rewritten as v = d / t. Now compare this equation to the
Hubble law -- H must equal 1/t, or t (time since explosion) must equal
1/H.
This is the Astro 101 explaination. The correct derivation requires
differential equations and a many assumptions about the flatness and
density of the universe. The correct values for the age of the universe
are:
0 t
t = 2/3H^-1 for a just dense enough universe
2/3H^-1
What always got my goat is that H is called the Hubble constant, but it
changes with time.
-=[doug]=- who has never even pretended to be a cosmologist
