Since the beginning of history, human beings were very interested in the origin of the
universe and its future. Until the 20th century, many
theologians and philosophers had presented their ideas but these were
only ideas because there was no experimental evidence of any sort
that would form any basis of scientific theory. Early
philosophers thought that Earth is in the center while everything
else (sun and moon) revolves around the Earth.
The first of two great pieces of evidence were made by the astronomer Edwin Hubble in the year 1920.
before that astronomers thought the milky way is the only galaxy in
the universe. Hubble was studying a wispy object known as Nebulae. He
was studying the stars in the nebula. After eventually resolving
individual star in the nebulae. Hubble was able to show that
there are other galaxies out there other than our galaxy, composed of
hundreds of billions of stars. The galaxy Hubble was studying named as
Andromeda galaxy. Andromeda is the nearest galaxy to Milky Way
composed of over 100 billion stars. It is only 2.5 million light-years away from us. It is so close to as that can be seen from here
at the sky.
Hubble deduced that the
galaxies are moving away from each other and form us, and that the
greater their distance from us or from any other point of reference
in the universe, greater is their recessional speed. If d is the distance of the galaxy from any point of reference and v is the speed
of the galaxy with which it appears to be moving away from that point
then according to the Hubble’s law:
v
= Hd
Where H is the Hubble
parameter whose value can be measured if we know the speed and the
distance of the galaxy form that point in general from us. Hubble
parameter has dimensions of inverse time.
Hubble deduced that the
galaxies are moving away from each other and form us, and that the
greater their distance from us or from any other point of reference
in the universe, greater is their recessional speed. If d is the distance of the galaxy from any point of reference and v is the speed
of the galaxy with which it appears to be moving away from that point
then according to the Hubble’s law:
v
= Hd
Where H is the Hubble
parameter whose value can be measured if we know the speed and the
distance of the galaxy form that point in general from us. Hubble
parameter has dimensions of inverse time.
Hubble parameter:
in order to measure the Hubble parameter, we have to deduce the
distance and the relative speed of the galaxy independently. The relative speed of the galaxy can be determined by the Doppler shift
of the light coming from the galaxy. Doppler shift is the shift in
the wavelength or frequency of the light due to its motion. But the
distance is difficult to determine in fact Hubble’s estimates were
off by a factor of 10. The best set of current data gives a value of
Hubble parameter of:
Where Mpc is the ‘Mega
parsec’ it is the unit for the distance at cosmic scales. One
parsec is equal to 3.26 light-years. And one megaparsec is 106
pc.
1 Mpc = 106pc =
3.26 x 106 Light Years
due to uncertainties in the
distances, the Hubble parameter is uncertain. Its possible range is
between 65-80 km-1/Mpc.
If the universe has been
expanding forever with the same rate than the inverse of the Hubble parameter
H-1 will be the age of the universe and with the estimated
value the age of the universe is round about 14 x 109 years.
But with the range of uncertainty, the age of the universe is 12 – 15 x
109 years. But later we will see that the universe has not
been expanding at a constant rate.
Problem:
1) Can you think of any a possible explanation for the expanding of the universe other than Big
Bang?
2)
If Hubble law can be extrapolated to very large distances, at what the distance the recessional speed becomes equal to the speed of light?