Download Hubble`s Law

Galaxy motions
At the start of the 20th century, the new technique of spectroscopy,
allied with the large telescopes which had begun to become available,
enabled observers to use the shifting of spectral lines to measure the
velocities of galaxies.
This was first done by Vesto
Slipher in 1912. He measured the
velocity of the Andromeda galaxy
(M31), and found it moving towards
us at 300 kilometres per second.
Hubble’s Law
Andromeda turned out to be
unusual in coming towards us.
Over the next 20 years Slipher
measured spectra of 40 more
galaxies, and found that the vast
majority were receding from us.
Edwin Hubble, working with
Milton Humason, was able to
measure many more spectra. He
discovered what we now know as
Hubble’s Law.
Edwin Powell Hubble
Hubble’s Law
What Hubble discovered was that the velocity of a galaxy was
proportional to its distance from us:
v=Hd
where H is a constant,
now known as Hubble’s
constant.
Hubble’s Law
What does it mean?
1) Galaxies are currently moving apart from one another.
Our Universe is expanding!
2) Almost certainly the Universe has a finite age, and
expanded to its present size from a highly dense initial state,
sometimes called the Big Bang.
3) It does not mean that there is a center to the “explosion”.
Hubble’s Law
Hubble’s Law has just the right form that every
observer sees all galaxies receding away such that
the Hubble Law is satisfied.
Hubble’s Constant
Hubble’s Law states that v = H d.
The proportionality constant is Hubble’s
constant, and its value tells us how fast the
Universe is expanding.
But what is that value?
Hubble’s Constant
Attempts over 70 years have been made to
measure the constant.
H = v/d
Typical velocities are measured in kilometres per second.
Typical galaxy separations are measured in megaparsecs.
So H should be measured in kilometres per second per megaparsec.
Hubble’s Constant
Hubble’s original measurement gave a Hubble
constant of
H = 500 km s -1 Mpc -1
That is, a galaxy one megaparsec away is
expected to be receding at 500 km s-1 Mpc-1.
We now know that this is far too high,
because Hubble drastically underestimated
the distances to his galaxies.
Present attempts
The correct answer is now believed to be between 50 and 100
km s-1 Mpc-1, though there is still controversy as to its exact
value.
•In 2001, a group led by Wendy Freedman arrived a a value
of H using the Hubble Space Telescope of 72 +/- 8
(km/s)/Mpc.
•Data from the CHANDRA x-ray satellite gave a value of
77 (km/s)/Mpc +/- 15%.
•In 2010, data from the WMAP satellite were used to
determine a value of 71 +/- 2.5 (km/s)/Mpc