**Edwin Powell Hubble** was an American astronomer who played a cru...
#### From a Static to an Expanding Universe
Up until 1929, the vas...
The period/luminosity relation for Cepheid variables on which Hubbl...
Hubble uses standard candles in order to determine the relative dis...
This table shows Hubble's observations. It is important to notice t...
From his results Hubble notices that there seems to be a correlatio...
This table shows Hubble's measurements. Here too it is important to...
### Age of the Universe from Hubble constant:
One can easily fin...
### Hubble Parameter:
This plot is Hubble's most important observa...
The fact that we see other galaxies moving away from us does not im...
**Fun fact:** Milton Humason, began working as a janitor at the Mt....
If you want to learn more about the Hubble constant, its implicatio...
ASTRONOMY:
E.
HUBBLE
appearance
the
spectrum
is
very
much
like
spectra
of
the
Milky
Way
clouds
in
Sagittarius
and
Cygnus,
and
is
also
similar
to
spectra
of
binary
stars
of
the
W
Ursae
Majoris
type,
where
the
widening
and
depth
of
the
lines
are
affected
by
the
rapid
rotation
of
the
stars
involved.
The
wide
shallow
absorption
lines
observed
in
the
spectrum
of
N.
G.
C.
7619
have
been
noticed
in
the
spectra
of
other
extra-galactic
nebulae,
and
may
be
due
to
a
dispersion
in
velocity
and
a
blending
of
the
spectral
types
of
the
many
stars
which
presumably
exist
in
the
central
parts
of
these
nebulae.
The
lack
of
depth
in
the
absorption
lines
seems
to
be
more
pronounced
among
the
smaller
and
fainter
nebulae,
and
in
N.
G.
C.
7619
the
absorption
is
very
weak.
It
is
hoped
that
velocities
of
more
of
these
interesting
objects
will
soon
be
available.
A
RELATION
BETWEEN
DISTANCE
AND
RADIAL
VELOCITY
AMONG
EXTRA-GALACTIC
NEBULAE
By
EDWIN
HUBBLB
MOUNT
WILSON
OBSURVATORY,
CARNSGIS
INSTITUTION
OF
WASHINGTON
Communicated
January
17,
1929
Determinations
of
the
motion
of
the
sun
with
respect
to
the
extra-
galactic
nebulae
have
involved
a
K
term
of
several
hundred
kilometers
which
appears
to
be
variable.
Explanations
of
this
paradox
have
been
sought
in
a
correlation
between
apparent
radial
velocities
and
distances,
but
so
far
the
results
have
not
been
convincing.
The
present
paper
is
a
re-examination
of
the
question,
based
on
only
those
nebular
distances
which
are
believed
to
be
fairly
reliable.
Distances
of
extra-galactic
nebulae
depend
ultimately
upon
the
appli-
cation
of
absolute-luminosity
criteria
to
involved
stars
whose
types
can
be
recognized.
These
include,
among
others,
Cepheid
variables,
novae,
and
blue
stars
involved
in
emission
nebulosity.
Numerical
values
depend
upon
the
zero
point
of
the
period-luminosity
relation
among
Cepheids,
the
other
criteria
merely
check
the
order
of
the
distances.
This
method
is
restricted
to
the
few
nebulae
which
are
well
resolved
by
existing
instru-
ments.
A
study
of
these
nebulae,
together
with
those
in
which
any
stars
at
all
can
be
recognized,
indicates
the
probability
of
an
approximately
uniform
upper
limit
to
the
absolute
luminosity
of
stars,
in
the
late-type
spirals
and
irregular
nebulae
at
least,
of
the
order
of
M
(photographic)
=
-6.3.1
The
apparent
luminosities
of
the
brightest
stars
in
such
nebulae
are
thus
criteria
which,
although
rough
and
to
be
applied
with
caution,
168
PRoc.
N.
A.
S.
ASTRONOMY:
E.
HUBBLE
furnish
reasonable
estimates
of
the
distances
of
all
extra-galactic
systems
in
which
even
a
few
stars
can
be
detected.
TABLE
1
NSBULAx
WHoss
DisTANcEs
HAvE
BSN
ESTIMATSD
FROM
STARS
INVOIVXD
OR
FROM
MEAN
LumNosrrIns
IN
A
CLUSTER
OBJUCT
m,
r
V
M
S.
Mag.
..
0.032
+
170
1.5
-16.0
L.Mag.
..
0.034
+
290
0.5
17.2
N.
G.
C.
6822
..
0.214
-
130
9.0
12.7
598
..
0.263
-
70
7.0
15.1
221
..
0.275
-
185
8.8
13.4
224
..
0.275
-
220
5.0
17.2
5457
17.0
0.45
+
200
9.9
13.3
4736
17.3
0.5
+
290
8.4
15.1
5194
17.3
0.5
+
270
7.4
16.1
4449
17.8
0.63
+
200
9.5
14.5
4214
18.3
0.8
+
300
11.3
13.2
3031
18.5
0.9
-
30
8.3
16.4
3627
18.5
0.9
+
650
9.1
15.7
4826
18.5
0.9
+
150
9.0
15.7
5236
18.5
0.9
+
500
10.4
14.4
1068
18.7
1.0
+
920
9.1
15.9
5055
19.0
1.1
+
450 9.6
15.6
7331
19.0
1.1
+
500
10.4
14.8
4258
19.5
1.4
+
500
8.7
17.0
4151
20.0
1.7
+
960
12.0
14.2
4382
..
2.0
+
500
10.0
16.5
4472
..
2.0
+
850
8.8
17.7
4486
..
2.0
+
800
9.7
16.8
4649
..
2.0
+1090
9.5
17.0
Mean
-15.5
m,
=
photographic
magnitude
of
brightest
stars
involved.
r
=
distance
in
units
of
106
parsecs.
The
first
two
are
Shapley's
values.
v
=
measured
velocities
in
km./sec.
N.
G.
C.
6822,
221,
224
and
5457
are
recent
determinations
by
Humason.
m,
=
Holetschek's
visual
magnitude
as
corrected
by
Hopmann.
The
first
three
objects
were
not
measured
by
Holetschek,
and
the
values
of
me
represent
estimates
by
the
author
based
upon
such
data
as
are
available.
Mt
=
total
visual
absolute
magnitude
computed
from
mg
and
r.
Finally,
the
nebulae
themselves
appear
to
be
of
a
definite
order
of
absolute
luminosity,
exhibiting
a
range
of
four
or
five
magnitudes
about
an
average
value
M
(visual)
=
-
15.2.1
The
application
of
this
statistical
average
to
individual
cases
can
rarely
be
used
to
advantage,
but
where
considerable
numbers
are
involved,
and
especially
in
the
various
clusters
of
nebulae,
mean
apparent
luminosities
of
the
nebulae
themselves
offer
reliable
estimates
of
the
mean
distances.
Radial
velocities
of
46
extra-galactic
nebulae
are
now
available,
but
VOL.
15,
1929
169
170ASRNM:.HBBEPO.NA.S
individual
distances
are
estimated
for
only
24.
For
one
other,
N.
G.
C.
3521,
an
estimate
could
probably
be
made,
but
no
photographs
are
avail-
able
at
Mount
Wilson.
The
data
are
given
in
table
1.
The
first
seven
distances
are
the
most
reliable,
depending,
except
for
M
32
the
companion
of
M
31,
upon
extensive
investigations
of
many
stars
involved.
The
next
thirteen
distances,
depending
upon
the
criterion
of
a
uniform
upper
limit
of
stellar
luminosity,
are
subject
to
considerable
probable
errors
but
are
believed
to
be
the
most
reasonable
values
at
present
available.
The
last
four
objects
appear
to
be
in
the
Virgo
Cluster.
The
distance
assigned
to
the
cluster,
2
X
106
parsecs,
is
derived
from
the
distribution
of
nebular
luminosities,
together
with
luminosities
of
stars
in
some
of
the
later-type
spirals,
and
differs
somewhat
from
the
Harvard
estimate
of
ten
million
light
years.2
The
data
in
the
table
indicate
a
linear
correlation
between
distances
and
velocities,
whether
the
latter
are
used
directly
or
corrected
for
solar
motion,
according
to
the
older
solutions.
This
suggests
a
new
solution
for
the
solar
motion
in
which
the
distances
are
introduced
as
coefficients
of
the
K
term,
i.
e.,
the
velocities
are
assumed
to
vary,
directly
with
the
distances,
and
hence
K
represents
the
velocity
at
unit
distance
due
to
this
effect.
The
equations
of
condition
then
take
the
form
rK
+
X
cos
a
cos
5
+
Y
sin
a
cos
5
+
Z
sin
5
=
v.
Two
solutions
have
been
made,
one
using
the
24
nebulae
individually,
the
other
combining
them
into
9
groups
according
to
proximity
in
direc-
tion
and
in
distance.
The
results
are
24
OBJuCeS
9
GROUPS
X
-65
50
+
3
70
Y
+226
95
+230
120
Z
-195
40
-133
70
K
+465
50
+513
60
km./sec.
per
106
parsecs.
A
2860
2690
D
+
40°
+
330
V.
306
km./sec.
247
km./sec.
For
such
scanty
material,
so
poorly
distributed,
the
results
are
fairly
definite.
Differences
between
the
two
solutions
are
due
largely
to
the
four
Virgo
nebulae,
which,
being
the
most
distant
objects
and
all
sharing
the
peculiar
motion
of
the
cluster,
unduly
influence
the
value
of
K
and
hence
of
Vo.
New
data
on
more
distant
objects
will
be
required
to
reduce
the
effect
of
such
peculiar
motion.
Meanwhile
round
numbers,
inter-
mediate
between
the
two
solutions,
will
represent
the
probable
order
of
the
values.
For
instance,
let
A
=
2770,
D
=
+36°
(Gal.
long.
=
320,
lat.
=
+180),
Vo
=
280
km./sec.,
K
=
+500
km./sec.
per
million
par-
170
ASTRONOMY:
E.
HUBBLE
PRoc.
N.
A.
So
ASTRONOMY:
E.
HUBBLE
secs.
Mr.
Stromberg
has
very
kindly
checked
the
general
order
of
these
values
by
independent
solutions
for
different
groupings
of
the
data.
A
constant
term,
introduced
into
the
equations,
was
found
to
be
small
and
negative.
This
seems
to
dispose
of
the
necessity
for
the
old
constant
K
term.
Solutions
of
this
sort
have
been
published
by
Lundmark,3
who
replaced
the
old
K
by
k
+
ir
+
mr2.
His
favored
solution
gave
k
=
513,
as
against
the
former
value
of
the
order
of
700,
and
hence
offered
little
advantage.
NZBULA"
WHOSE
DIsTNcus
ODJgCT
v
N.G.
C.
278
+
650
404
-
25
584
+1800
936
+1300
1023
+
300
1700
+
800
2681
+
700
2683
+
400
2841
+
600
3034
+
290
3115
+
600
3368
+
940
3379
+
810
3489
+
600
3521
+
730
3623
+
800
4111
+
800
4526
+
580
4565
+1100
4594
+1140
5005
+
900
5866
+
650
Mean
TABLE
2
ARS
ESTIMATUD
FROM
Vs
r
-110
1.52
-
65
+
75
3.45
+115
2.37
-
10
0.62
+220
1.16
-
10
1.42
+
65
0.67
-
20
1.24
-105
0.79
+105
1.00
+
70
1.74
+
65
1.49
+
50
1.10
+
95
1.27
+
35
1.53
-
95
1.79
-
20
1.20
-
75
2.35
+
25
2.23
-130
2.06
-215
1.73
RADIAL
VOLOCITMs
mg
Mt
12.0
-13.9
11.1
10.9
16.8
11.1
15.7
10.2
13.8
12.5
12.8
10.7
15.0
9.9
14.3
9.4
16.1
9.0
15.5
9.5
15.5
10.0
16.2
9.4
16.4
11.2
14.0
10.1
15.4
9.9
16.0
10.1
16.1
11.1
14.3
11.0
15.9
9.1
17.6
11.1
15.5
11.7
-14.5
10.5
-
15.3
The
residuals
for
the
two
solutions
given
above
average
150
and
110
km./sec.
and
should
represent
the
average
peculiar
motions
of
the
in-
dividual
nebulae
and
of
the
groups,
respectively.
In
order
to
exhibit
the
results
in
a
graphical
form,
the
solar
motion
has
been
eliminated
from
the
observed
velocities
and
the
remainders,
the
distance
terms
plus
the
residuals,
have
been
plotted
against
the
distances.
The
run
of
the
re-
siduals
is
about
as
smooth
as
can
be
expected,
and
in
general
the
form
of
the
solutions
appears
to
be
adequate.
The
22
nebulae
for
which
distances
are
not
available
can
be
treated
in
two
ways.
First,
the
mean
distance
of
the
group
derived
from
the
mean
apparent
magnitudes
can
be
compared
with
the
mean
of
the
velocities
VOL.
15,
1929
171
ASTRONOMY:
E.
HUBBLE
corrected
for
solar
motion.
The
result,
745
km./sec.
for
a
distance
of
1.4
X
106
parsecs,
falls
between
the
two
previous
solutions
and
indicates
a
value
for
K
of
530
as
against
the
proposed
value,
500
km./sec.
Secondly,
the
scatter
of
the
individual
nebulae
can
be
examined
by
assuming
the
relation
between
distances
and
velocities
as
previously
determined.
Distances
can
then
be
calculated
from
the
velocities
cor-
rected
for
solar
motion,
and
absolute
magnitudes can
be
derived
from
the
apparent
magnitudes.
The
results
are
given
in
table
2
and
may
be
compared
with
the
distribution
of
absolute
magnitudes
among
the
nebulae
in
table
1,
whose
distances
are
derived
from
other
criteria.
N.
G.
C.
404
o~~~~~~~~~~~~~~~~
0.
S0OKM
0
DISTANCE
0
IDPARSEC
S
2
,10
PARSECS
FIGURE
1
Velocity-Distance
Relation
among
Extra-Galactic
Nebulae.
Radial
velocities,
corrected
for
solar
motion,
are
plotted
against
distances
estimated
from
involved
stars
and
mean
luminosities
of
nebulae
in
a
cluster.
The
black
discs
and
full
line
represent
the
solution
for
solar
motion
using
the
nebulae
individually;
the
circles
and
broken
line
represent
the
solution
combining
the
nebulae
into
groups;
the
cross
represents
the
mean
velocity
corresponding
to
the
mean
distance
of
22
nebulae
whose
distances
could
not
be
esti-
mated
individually.
can
be
excluded,
since
the
observed
velocity
is
so
small
that
the
peculiar
motion
must
be
large
in
comparison
with
the
distance
effect.
The
object
is
not
necessarily
an
exception,
however,
since
a
distance
can
be
assigned
for
which
the
peculiar
motion
and
the
absolute
magnitude
are
both
within
the
range
previously
determined.
The
two
mean
magnitudes,
-
15.3
and
-
15.5,
the
ranges,
4.9
and
5.0
mag.,
and
the
frequency
distributions
are
closely
similar
for
these
two
entirely
independent
sets
of
data;
and
even
the
slight
difference
in
mean
magnitudes
can
be
attributed
to
the
selected,
very
bright,
nebulae
in
the
Virgo
Cluster.
This
entirely
unforced
agreement
supports
the
validity
of
the
velocity-distance
relation
in
a
very
PRoc.
N.
A.
S.
172
ASTRONOMY:
E.
HUBBLE
evident
matter.
Finally,
it
is
worth
recording
that
the
frequency
distribu-
tion
of
absolute
magnitudes
in
the
two
tables
combined
is
comparable
with
those
found
in
the
various
clusters
of
nebulae.
The
results
establish
a
roughly
linear
relation
between
velocities
and
distances
among
nebulae
for
which
velocities
have
been
previously
pub-
lished,
and
the
relation
appears
to
dominate
the
distribution
of
velocities.
In
order
to
investigate
the
matter
on
a
much
larger
scale,
Mr.
Humason
at
Mount
Wilson
has
initiated
a
program
of
determining
velocities
of
the
most
distant
nebulae
that
can
be
observed
with
confidence.
These,
naturally,
are
the
brightest
nebulae
in
clusters
of
nebulae.
The
first
definite
result,4
v
=
+
3779
km./sec.
for
N.
G.
C.
7619,
is
thoroughly
consistent
with
the
present
conclusions.
Corrected
for
the
solar
motion,
this
velocity
is
+3910,
which,
with
K
=
500,
corresponds
to
a
distance
of
7.8
X
106
parsecs.
Since
the
apparent
magnitude
is
11.8,
the
absolute
magnitude
at
such
a
distance
is
-17.65,
which
is
of
the
right
order
for
the
brightest
nebulae
in
a
cluster.
A
preliminary
dis-
tance,
derived
independently
from
the
cluster
of
which
this
nebula
appears
to
be
a
member,
is
of
the
order
of
7
X
10"
parsecs.
New
data
to
be
expected
in
the
near
future
may
modify
the
significance
of
the
present
investigation
or,
if
confirmatory,
will
lead
to
a
solution
having
many
times
the
weight.
For
this
reason
it
is
thought
premature
to
discuss
in
detail
the
obvious
consequences
of
the
present
results.
For
example,
if
the
solar
motion
with
respect
to
the
clusters
represents
the
rotation
of
the
galactic
system,
this
motion
could
be
subtracted
from
the
results
for
the
nebulae
and
the
remainder
would
represent
the
motion
of
the
galactic
system
with
respect
to
the
extra-galactic
nebulae.
The
outstanding
feature,
however,
is
the
possibility
that
the
velocity-
distance
relation
may
represent
the
de
Sitter
effect,
and
hence
that
numer-
ical
data
may
be
introduced
into
discussions
of
the
general
curvature
of
space.
In
the
de
Sitter
cosmology,
displacements
of
the
spectra
arise
from
two
sources,
an
apparent
slowing
down
of
atomic
vibrations
and
a
general
tendency
of
material
particles
to
scatter.
The
latter
involves
an
acceleration
and
hence
introduces
the
element
of
time.
The
relative
im-
portance
of
these
two
effects
should
determine
the
form
of
the
relation
between
distances
and
observed
velocities;
and
in
this
connection
it
may
be
emphasized
that the
linear
relation
found
in
the
present
discussion
is
a
first
approximation
representing
a
restricted
range
in
distance.
Mt.
Wilson
Contr.,
No.
324;
Astroph.
J.,
Chicago,
Ill.,
64,
1926
(321).
2
Harvard
Coll.
Obs.
Circ.,
294,
1926.
'Mon.
Not.
R.
Astr.
Soc.,
85,
1925
(865-894).
'
hese
PROCUDINGS,
15,
1929
(167).
173
Vow.
15,
1929
If you want to learn more about the Hubble constant, its implications and evolution here is a very interesting review: [The Hubble Constant](http://www.astro.caltech.edu/~george/ay127/readings/FreedmanMadore2010.pdf)
From his results Hubble notices that there seems to be a correlation between the distance of nebulae and their velocities.
This table shows Hubble's observations. It is important to notice the **r** (distance to earth) and **v** (velocity) columns. There seems to be a linear correlation between **r** and **v**. When the value of **r** increase we see an increase in **v**.
The fact that we see other galaxies moving away from us does not imply that we are the center of the Universe! All galaxies will see other galaxies moving away from them in an expanding Universe unless the other galaxies are part of the same gravitationally bound group or cluster of galaxies.
**Fun fact:** Milton Humason, began working as a janitor at the Mt. Wilson observatory, then rose to become a night assistant and then an assistant astronomer. Humason observed spectra, while Hubble concentrated on finding distances to various objects.
The period/luminosity relation for Cepheid variables on which Hubble relies was discovered by [Henrietta Leavitt](https://en.wikipedia.org/wiki/Henrietta_Swan_Leavitt). [Her results](http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1912HarCi.173....1L&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf) were published by the Harvard Observatory in 1912 (but not under her name). Hubble said she should have received a Nobel Prize for her work.
This table shows Hubble's measurements. Here too it is important to notice the **r** (distance to earth) and **v** (velocity) columns. There seems to be a linear correlation between **r** and **v**. When the value of **r** increase we see an increase in **v**.
Another interesting fact: What does Hubble have in common with Lavoisier, Leibniz, Huygens, Fermat, Descartes and Copernicus? Scroll...
...
...
...
All of them were trained as lawyers, but (fortunately for us) found something more exciting to do. Hubble got his law degree from Oxford in 1913.
#### From a Static to an Expanding Universe
Up until 1929, the vast majority of scientists were convinced that the Universe was static. Einstein even added the famous cosmological constant $\Lambda$ to his equations to make them consistent with a static universe (which he later called the “greatest blunder” of his life). Before Hubble's observations physicists like A. Friedman and G. Lemaître had independently proposed expanding universe models but they had no data to support their theories and were largely ignored until after Hubble’s discovery. Hubble’s observations provided **the first observational support for an expanding Universe and for a time-scale idea of the Universe**. This lead to what later became known the big bang theory.
The image below depicts the Universe as we know it today, and the different phases of its evolution.
**Edwin Powell Hubble** was an American astronomer who played a crucial role in establishing the field of extragalactic astronomy. He is considered one of the most important observational cosmologists of the 20th century. Hubble is well known for "Hubble's law" that shows the recessional velocity of a galaxy increases with its distance from Earth (the purpose of this paper). His observations considered one of the most important Cosmological discoveries led to the conclusion that we live in an expanding Universe and also implies the existence of a timescale or age for the Universe. This forced cosmologists to start thinking about dynamic models of the Universe.
**Interesting fact:** He never received the Nobel prize because he was ineligible for it since astronomy was not then considered a branch of physics.
Hubble uses standard candles in order to determine the relative distance of "extragalactic nebulae". A standard candle is a class of astrophysical objects, such as supernovae, which have known luminosity due to some characteristic quality possessed by the entire class of objects. If an extremely distant object can be identified as a standard candle then the absolute magnitude M (luminosity) of that object is known. The absolute magnitude relates to the distance D (in cm) and to the apparent magnitude m as shown in the formula below:
$$M = m - 5 ((\log_{10}{D}) - 1)$$
Many objects that were classified as "nebulae" were actually galaxies beyond the Milky Way. Vesto Slipher and American astronomer had provided the first evidence for this argument almost a decade before but Hubble's observations played a crucial role in characterizing these objects.
### Hubble Parameter:
This plot is Hubble's most important observation. It shows that there is a relation between the speed of the nebulae and their distance to earth. As the distance increases the speed increases too. This seems to indicate that objects that are far away from earth are receding at a faster pace.
Hubble's initial value for the expansion rate, Hubble Constant:
$$H_0 = 500 km/s/Mpc.$$ The Hubble Constant is the unit of measurement used to describe the expansion of the Universe.
In recent years, the value of the Hubble's parameter has been considerably refined, and the current value given by the WMAP mission is 72 km/s per megaparsec.
**Interesting fact:** Hubble's parameter is always given with a subscript 0, $H_0$ because it is time dependent and 0 simply means that it is the value at this given point in time.
The figure below shows the results obtained by the Hubble Space Telescope Key Project (Freedman et al. 2001).
### Age of the Universe from Hubble constant:
One can easily find the age of the Universe ($T_U$) from Hubble's constant:
\begin{eqnarray*}
T_U &=& 1/H_0. \\
\end{eqnarray*}
Taking into account the most recent value of the Hubble parameter $H_0 = 72 km/s/Mpc$, and taking into account that $1Mpc = 3.0857\cdot10^{19} km$ one can then write:
\begin{eqnarray*}
T_U &=& 1/H_0 \\
&=& \frac{3.0857\cdot10^{19}}{72} s \\
&=& 4.346\cdot 10^{17} s \\
&=& 13,800,000,000\,\, years \\
\end{eqnarray*}
**Fun Fact:** If the Universe was expanding as fast as Hubble predicted and you would calculate it's age using Hubble's results: $$H_0=500km/s/Mpc$$ you would find that it to be **1.5 billion years old**. But by 1929, we already know that the earth was older than 1.5 billion years.
Short video of Lawrence Krauss discussing Hubble's results:
[](https://www.youtube.com/watch?v=EIpEzZqkd9c&feature=youtu.be&t=562 "L Krauss")