AST202S: Introduction to Modern Astrophysics

1
The Galactic Centre (revisited) We have already
explored motions in the Galactic centre from
which it has been deduced that the centre of the
galaxy hosts a supermassive black hole. How is
the centre of the Galaxy identified? A very
compact radio source lies at the centre of the
Galaxy this source has been called Sagittarius
(Sgr) A. Many HII regions, total extend is
about 90 by 260 parsecs the UV energy
output from OB stars needed to keep the region
ionized is at least 2 x 1033 (W) a few
million times the luminosity of the Sun.
Thermal emission Electromagnetic radiation
because a body is hot and in thermal
dynamic equilibrium often characterised by a
blackbody spectrum
A radio map of the overall Galactic Centre
region. Most of this emission is thermal.
2
The Galactic Centre (revisited) The multiple
components of Sgr A Sgr A West (thermal) Sgr A
East (non-thermal, most likely a supernova
remnant), and Sgr A (non-thermal, the core of
the Galaxy)
Sgr A West at radio wavelength (and at high
angular resolution)
3
The Galactic Centre (revisited) Strong emission
at the following wavelengths Infrared ? dense
group of old Population I stars density around
50 000 stars per parsec3 (a million time more
dense than the solar neighbourhood) X-ray ?
some point sources within a general halo of
extended emission (100 pc scale) the point
sources coincide with the infrared emission,
whereas the diffuse emission is due to hot
coronal gas. Gamma-ray ? a line with energy 511
keV has been observed and at a luminosity
of 1031 W (attributed to electron- positron
annihilation). Luminosity variable close to
the Galactic centre, but not at the centre.
Recall The core of the Galaxy is a super massive
black hole (a few million MSun).
4
EVOLUTION OF THE GALAXYS STRUCTURE I. The
stability of the spiral structure the wind-up
problem (spiral structure only expected to
last for a short time before being pulled
apart by differential rotation) Regular spiral
patterns observed in many other galaxies Lin
Shu in 1967 proposed a density-wave model ? the
spiral structure in a galaxy is regarded as a
wave pattern resulting from gravitational instabil
ities. The density wave rotates at a speed
less than that of the material speed of
galactic rotation ? the presence of a
density-wave implied a non-uniform distribution
of mass (a bar at the Centre of the Galaxy?) a
self-sustained and stable phenomenon
5
Gas motion is indicated by the red arrows and the
arm motion is indicated by the white arrows. The
gas enters the arms from behind, is
compressed and forms stars. The spiral pattern is
delineated by dust lanes, regions of high gas
density and newly formed O and B stars.
6
The hospital-bend analogue of a density wave. The
density wave (arm) moves slower as the traffic
(read speeding capetonian drivers in their 4x4s)
(the stellar material/gas), resulting in a
density peak.
7

• EVOLUTION OF THE GALAXYS STRUCTURE
• What drives the density wave?
• Gravitational interaction with satellite
  galaxies (the Magellanic clouds)
• There are certainly clear signs that the Galaxy
  and the LMC/SMC are
• gravitationally bound and interacting.
• Instabilities in the Galactic Bulge
• Asymmetries in the Galactic Bar
• The most accepted view is the
• interaction with satellite galaxies
• (see for example M51)

8

• EVOLUTION OF THE GALAXYS STRUCTURE
• How well does the density-wave model describe the
  observed spiral structure?
• it outlines the grand scheme of two and
  four-armed spiral patterns
• it explains the persistence of the spiral arms
  in the presence of galactic
• differential rotation
• it predicts the general features of a spiral arm
• it does not really explain the origin of the
  density waves
• it does not work out what keeps the density
  waves going
• as the density waves ripple through the
  interstellar
• medium, they lose energy and should dissipate
• in about 109 years.

9
EVOLUTION OF THE GALAXYS STRUCTURE II. The
Milky Ways past Formed 10 15 billion
years ago. (Age estimate given by the oldest
observed stars in the Galaxy, although it could
well be that the first stars were formed in
smaller systems which later merged to create the
Milky Way). Contraction of cloud of
pre-galactic gas ? first Galactic stars and
globular clusters are formed before the gas has
settled into a thin disk. Rotation has flattened
the disk (similar to what happened during the
formation of the Solar system).
10

• EVOLUTION OF THE GALAXYS STRUCTURE II. The
  Milky Ways past
• Placing the Galaxys present structure into the
  context of its history
• the process of galactic evolution links the
  chemistry with the dynamics
• Recycling of the interstellar medium
• Pop. I and Pop. II stars not a discrete
  variation in chemical composition,
• but a continuous range of populations
• In general (but not for all objects), the lower
  the metal content the greater
• its height from the Galactic disk.
• 1. stars are born in the recycled interstellar
  medium
• Their atmospheric elemental abundances reflect
  that of the gas
• from which they are formed
• 2. the stars orbital motion about the Galaxy
  are inherited from their parent
• gas and dust clouds
• 3. massive stars evolve quickly and spew back
  into the interstellar medium
• material enriched with heavy elements

11

• EVOLUTION OF THE GALAXYS STRUCTURE II. The
  Milky Ways past
• An estimate of the collapse time scale of the
  initial gas cloud to form the galaxy
• can be made from the free-fall time (see your
  lecture notes on the formation of stars
• week 7)
• Where the density is given in kg/m3.
• For a cloud of 1012 MSun ( 1.99 x 1042 kg), and
  an initial radius of 100 kpc
• ( 3.086 x 1021 m), the density is 1.6 x 10-23
  kg/m3.
• The free-fall time is therefore around 5 x 108
  years

12
AST202S Introduction to Modern Astrophysics

• Brief historic overview
• Celestial mechanics
• Our solar system
• Formation of solar systems
• Electromagnetic radiation
• Stellar classification
• Star formation and stellar evolution
• Galaxies and Observational Cosmology
• Our Milky Way as a galaxy
• Galaxies and large-scale structures
• Galaxy formation and evolution
• The Early Universe

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(NORMAL) GALAXIES BEYOND THE MILKY WAY AND THEIR
SPATIAL DISTRIBUTION AIM To describe what kind
of galaxies exist in the local Universe
(elliptical, spiral and irregular galaxies),
describe their physical properties, and what
their distribution is in the Universe (groups of
galaxies, clusters, superclusters and
voids). Our Milky Way galaxy contains about
1011 stars and is regarded as a large
(giant) galaxy, though it is not the most massive
galaxy observed (the Andromeda galaxy is
slightly more massive). Smaller galaxies are
more numerous dwarf galaxies contain 1 (or
less) of the number of stars in the Milky
Way. Much of our understanding of galaxies has
traditionally been based on visible (optical)
observations with telescopes. Recall William
and John Herschel, Messier, Earl of Rosse, Hubble
etc. Recall The Great Debate only in 1924 was
the extragalactic nature of galaxies established
by Hubble.
14

• The Hubble Ultra
• Deep Field (HUDF)
• 3' x 3' in diameter
• million seconds of
• exposure time
• Ngal 10 000 in a
• 3' x 3' field of view

15
Galaxy classification scheme Pioneered by Edwin
Hubble (1924), classifying the galaxies simply
based on appearance. Hubble incorrectly thought
that the diagram was an evolutionary sequence
from left to right. This diagram is also known
as the tuning fork.
spiral galaxies
irregular galaxies
elliptical galaxies
En, where n10(a-b)/a a major axis, b minor
axis
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Ellipticals The elliptical galaxies (designated
by E) have the shape of an oblate
spheroid. Smooth density fall-off as log I(r)
r-1/4 These galaxies have no axis of
rotation their stars follow orbits with a
variety of inclinations. cD galaxies (at the
centre of clusters) superficially appear
as ellipticals but have extended halos. Galaxy
clusters rich in elliptical galaxies (in the Coma
cluster, one in two galaxies is elliptical)
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Elliptical galaxies contain little or no dust or
gas (although in some exceptional cases dust/gas
is observed accreted from interaction with
spiral?) Giant ellipticals (E) vs. dwarf
ellipticals (dE) big (100 kpc) small ( 1
kpc) massive lt 1012 MSun low-mass lt 106
MSun typical of glob.cluster dEs are 10
times more numerous than Es.
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Normal spiral galaxies
NGC1201 S0
NGC0488 Sab
NGC2811 Sa
NGC0628 / M74 Sc
NGC3031 / M81 Sb
NGC2841 Sb
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Barred spiral galaxies
NGC2859 SB0
NGC0175 SBab(s)
NGC1300 SBb(s)
Spiral galaxies are divided into the ordinary
spirals (designated S), or barred spirals
(SB). Both types have spiral-shaped arms, with
two arms generally placed symmetrical around the
centre of the axis of rotation. In ordinary
spirals, the arms emerge directly from the
nucleus in the barred spirals, a bar of material
cuts through the centre, and the arms originate
from the ends of the bar. Sa large nucleus,
ill-defined arms, tightly wound (early-type) Sb
more open arms, partly resolved into HII
regions Sc small nucleus, spiral arms extended
and resolved into clumps of stars (late-type) Sa
? Sab ? Sb ? Sbc ? Sc ? Scd ? Sd ? Sm
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Spiral galaxies Typical stars in the disks of
spiral galaxies A ? G (whitish colour) Typical
stars in the spiral arms O ? B (bluish
colour) Spiral galaxies are generally gas-rich
and have a large amount of dust ? ongoing star
formation. Both young and old populations coexist
in spiral galaxies, but the proportion of young
population I objects increases from Sa ? Sc Why
are there two types of spirals (barred and
normal)? (Ostriker Peebles) Dependence on halo
mass Low halo-mass galaxies develop bar-like
instabilities, massive halo galaxies do not show
bars ( again a piece in the puzzle of dark
matter) Spiral arm prominence depends on the
luminosity of a galaxy I (most luminous), II,
III, IV, V (least luminous)
NGC3370 Sc
21
Whirlpool galaxy (M51) SbcI-II
Black Eye galaxy SabII
22
Sombrero galaxy (M104) Sa/b
Sombrero galaxies at infrared wavelength (Spitzer
Space Telescope)
23
ESO510G13 A warped galaxy
24
S0 galaxies (lenticulars) Intermediate between
E7 and true spirals (Sa). Different from
ellipticals in the sense that S0s have a thin
disk (in addition to the spheroidal bulge) The
disk component has a slower intensity fall of
I(r) I0 e(-ar) Detections of S0 galaxies
depend on the quality of the observations and on
the orientation of the galaxy edge-on E vs. S0
(easy disk) Sa vs. S0 (difficult) face-on E
vs. S0 (difficult) Sa vs. S0 (easy spiral
structure) Formation of S0 galaxies still
uncertain Sa which has lost all its gas due to
tidal interactions with neighbouring
galaxies? Large programme with SALT
transformation of galaxies within dense
environments
NGC1201 S0
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Irregular galaxies Galaxies with no obvious
symmetrical or regular structure are classified
as irregular galaxies. Irr I contain resolved
OB stars and HII regions, and have clearly a
large Pop I component Irr II peculiar galaxies,
amorphous. dwarf irregulars dIrr There
are no dwarf spirals physical processes
that create spiral arms demand a system
with M gt 109 MSun
The Magellanic clouds are classified as Irr I
galaxies
26
M82 Irr II
27
Peculiar galaxies Most of the peculiar galaxies
obtain their peculiarities through interactions
with other galaxies.
28

• Morphological mix of galaxies
• Of all the observed galaxies,
• 77 are spirals
• 20 are ellipticals
• 3 are irregulars
• This morphological mix represents a selection
  bias this sample is dominated by
• luminous galaxies which we can see out to far
  distances!
• If one takes a volume-limited census (within 9.1
  megaparsec) ?
• 33 are spirals
• 13 are ellipticals
• 54 are irregulars
• Many of the irregulars are small galaxies of
  fairly low luminosity, as are dwarf ellipticals
• The morphological mix of galaxies also strongly
  depends on environment,

29

• Photometric characteristics of galaxies
• Integrated colours the light from a galaxy
  arises from all its stars. In a very general
• way, one can use a galaxys colour to infer the
  global stellar composition.
• Elliptical galaxies appear redder than spirals
• Old Population I dominates
• Spiral galaxies appear redder in their bulge,
  and blue in the outer disk region
• Mix of old Population I (nucleus) and young
  Population I (disk) the ratio
• of these depend on the galaxy type (Sa ? Sc)
• Irregular galaxies appear blue due to the
  presence of a young Population I component
• Sizes linear size derived from angular
  dimensions combined with the distance.
• How to define the edge of a galaxy? The slow
  (exponential and/or r-1/4) fall-off
• of the light intensity makes an unambiguous size
  difficult you can always look a little
• bit deeper and find a larger diameter ? isophotal
  diameter

30

• Photometric characteristics of galaxies
  continued
• Luminosities if we know distances along with
  fluxes (measured logarithmically by
• apparent magnitudes) for galaxies, one can
  calculate luminosities (absolute magnitudes).
• Considerations
• decide on the size of a galaxy (isophotal
  radius)
• correct for the extinction due to dust in the
  Milky Way
• correct for dust-extinction internal to the
  galaxy (very important!!) inclination
• correct for cosmological dimming (K-correction
  more later)
• Absolute magnitudes range from 8 (2x105 LSun)
  for dwarf ellipticals to 25 (1012 LSun)
• for supergiant ellipticals. Our galaxy, viewed
  from the outside, would have an
• absolute magnitude of roughly 21 (2.5x1010 LSun)

31
Masses of galaxies (round 1) a luminosity
approach Knowing the energy output of a galaxy
(its luminosity), one can do a simple calculation.
If the luminosity if 1011 LSun and if each star
contributes as much light for its mass as the Sun
does, then, on average, the mass of the galaxy
ought to be 1011 MSun. A better estimate would
require a (morphology-dependent) correction
for the amount of gas (up to 30) and dust (up to
5). Basic assumption, contents of external
galaxies produce 1 LSun for 1 MSun. Concept
application The stars that dominate a galaxys
light. A typical galaxy contains 1010 to 1011
stars. Most of these of course are located
on the main sequence in a continuous band with
increasing numbers towards the lowest masses and
luminosities the M dwarfs. Assuming a very
simplified view of a modestly sized galaxy with
1010 stars of uniform absolute magnitude MV 8
mag. How many supergiants of MV 4 mag would
it take for the light of the supergiant
population to be equal to the light of the
dwarf population?
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Concept application The stars that dominate a
galaxys light. The total brightness of the
dwarfs would be and the total brightness of
the supergiants would be where N is the number
we need to find out. 8 (4) 2.5 log
(I-4/I8) ? (I-4/I8) 6.3 x 104 In other
words, each supergiant contributes the same
amount of light as 63 000 dwarf stars. The
number of supergiants needed to produce the same
amount of light as the dwarfs is therefore 1.6 x
105 Assuming a uniform density (obviously
incorrect) and a diameter of 10 kpc for an E0
galaxy, the total volume is 4.2 x 1012
pc3. supergiants 3.8 x 10-8 stars pc-3, or 1
star every 300 pc dwarfs 2.4 x 10-2 stars
pc-3, or 1 star every 3.5 pc Note stars in the
disk of the Milky Way are about 1 pc apart on
average.
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The fundamental characteristics of
galaxies. Ellipticals Spirals Irregulars
I Mass (MSun) 105 to 1013 109 to 4x1011 108
to 3x1010 Absolute magnitude 9 to 23 15 to
21 13 to 18 Luminosity (LSun) 3x105 to
1011 108 to 2x1010 107 to 108 M/L (MSun/LSun
1) 100 2 to 20 1 Diameter (kpc) 1 to
200 5 to 50 1 to 10 Population content II
and old I I in arms, II and I, some II
old I overall Presence of dust Almost
none Yes Yes Sa Sb Sc,Sd Colour index
(B-V) 1.0 0.9 0.4 to 0.8 0.4 to 0.6 0.3 to
0.4 MHI / MT () Almost 0 2 2 5 2 10
2 22 4 Spectral type K K F to K A to F A to F
34
elliptical
spiral (Sc)
Spectroscopy of galaxies at optical wavelengths
Spectra give us information about the
galaxys redshift (see next section) and
the activity, dynamics and dominant stellar
component of the nuclear bulge. (the recession
velocity of NGC 7512 is 7056 km/s)
active galactic nuclei
star-forming galaxy
35

• A multi-wavelength view of galaxies
  (radio/infrared/X-ray)
• Galaxies at radio wavelengths
• radio continuum observations (synchrotron
  radiation ? magnetic field and
• highly energetic particles)
• mostly sensitive to late-type spirals (best
  candidates for producing relativistic
• electrons and ordered magnetic fields are young
  Pop I objects)
• radio galaxies special class of active galactic
  nuclei which produce
• more than 1033 J/s of radio power
• line radiation and neutral hydrogen content
• 21-cm observations of neutral hydrogen (again
  sensitive to gas-rich galaxies
• ? late-type spirals and irregular galaxies).

36

• A multi-wavelength view of galaxies
  (radio/infrared/X-ray)
• Galaxies at radio wavelengths (continued)
• Total amount of hydrogen is proportional to
• size of a galaxy, but the ratio of hydrogen mass
• to total mass (MHI/MT) depends on galaxy type!
• This relates to our conception of evolution
  occuring
• within galaxy systems. Galaxies with a lower
  fraction
• of HI mass must have already used up a lot of it
• in star-formation processes.
• Generally, the fraction of HI gas-mass to total
  mass
• is quite low (3 in lenticulars (SOs), 22 in
  irregulars).
• The present star formation rate (SFR) depends on
  the
• amount of hydrogen available and its density.

37
optical
HI (21 cm)
Infrared observations (continued) Primary
emission mechanism at IR wavelengths is thermal
radiation from interstellar dust grains heated by
starlight. IRAS observations give details of
stellar population and dust (composition and
distribution). X-ray emission from normal
galaxies X-ray emission in M31 (see
picture) comes from discrete sources ultra-lumino
us X-ray binaries (black hole and/or neutron
star binaries). The X-ray emission originates
from hot gas, possibly in accretion disks around
the compact objects.
IR
X-ray
Messier 31
38

• Some basic theoretical considerations
• Galaxies are the most striking constituents of
  the Universe, so what can we say
• about the physical processes which created these
  objects and which have
• governed their evolution?
• Implications of the classification scheme (from a
  physical perspective)
• (note that the classification scheme was derived
  from the optical appearance of a
• galaxy bulge/disk ratio and prominence of
  spiral arms)
• Three important observations
• The colour of a galaxy depends strongly on
  morphological type
• The integrated spectral type fo the nuclear
  region depends strongly on morphological
• type
• The spheroidal components of galaxies follow the
  r-1/4 law and the disk components
• follow an exponential distribution regardless
  of the specific morph. type
• Points (1) and (2) consistency in the
  evolutionary stage of stellar populations among

39
Angular momentum (per unit mass) probably mostly
decides the form of a galaxy the greater the
angular momentum, the more flattened the
galaxy. Gas dissipates its momentum in the
direction perpendicular to the plane as gravity
collapses gas across the galaxian poles, but in
the equatorial plane, gas is rotationally
supported. Elliptical galaxies efficient
(therefore rapid) condensation of gas into
stars Spiral galaxies slower rate of star
formation, and therefore later generations
of stars come from an increasingly flattened gas
source. What can we deduce from the
interrelationship among the infrared, radio and
X-ray observations for late-type spirals?
(relating relativistic electrons, thermal dust
sources, accretion disks around collapsed
objects) ? consider the role of star formation
rates in the energetics of disk
galaxies. Increase in SFR ? adding energy to
interstellar dust (reradiates the energy
from stellar photons into the IR) ? increase in
IR luminosity ? increase in number of
supernovae (origin of the synchrotron electrons
from supernovae? accelerated electrons to
relativistic velocities through shockwaves in
the ISM) ? larger number of collapses sources
40
Masses of galaxies (round 2) a dynamical
approach Method 1 The Virial Theorem (2ltKEgt
ltPEgt) fundamental assumption system is stable,
not collapsing, nor flying apart. ltv2gt 0.4
GM/rh where rh is the radius which encloses
half the mass, v is the velocity dispersion,
and M is the total mass of the galaxy. Method
2 Rotation curve (velocity as a function of
distance) a simplified view of this method
involves Keplers 3rd law (assuming that
the galaxy has a strong central concentration
of mass). This is true for the optical
light distribution ? also true for the
mass distribution? All the galaxies behave like
our own Galaxy, no Keplerian drop-off far from
the centre of each galaxy ? suggesting a large
fraction of each galaxys mass comes from its
outer regions.
41
Masses of galaxies (round 2) a dynamical
approach Method 3 Double galaxies and Newtons
form of Keplers 3rd law in order to determine
the mass of a (binary) galaxy, the distance,
angular size of the orbit, the period and the
position of the centre of mass must be
known. Galaxy revolve too slowly to determine
orbits, periods etc., so this only works on
a statistical basis and one can only determine
the sum of the mass of both galaxies. Results
for a statistical sample of 279 binary systems
average mass of a spiral is 1012
MSun Dynamical mass estimates considerably
higher than the luminosity-based mass estimates
(see round 1 of the discussion). ? Mass-to-light
ratio (M/L) in galaxies is 5 to 30 times larger
than the solar value consider mass-components
not adding to the luminosity of the system (dark
objects) dust, planets, comets, asteroids (in
our solar system, these have negligible masses).
What kind of object can add 5 to 30 times the
mass of all the stars in a galaxy
without producing any light? yes, Dark
matter What are galaxies? Self-gravitating
systems, in which stars move in
understandable orbits, many forms of
electromagnetic radiation coming from
understandable processes and understandable
evolutionary cycles of stellar births and deaths.
42

• Hubbles law and the distance scale
• Dramatic leaps in our understanding of the
  Universe have come throughout history
• from gauging the 3rd dimension (distance)
• Copernicus and Kepler Scale and dynamics of the
  Solar system
• Bessel and Henderson First trigonometric
  parallax, increasing the scale of the
• Universe beyond the Solar system
• Hubble The use of Cepheid variables in the
  Andromeda galaxy ?
• galaxies beyond the Milky Way
• Hubble Hubbles law, the expansion of the
  Universe
• These laws have allowed us to appreciate fully
  the immense scale and size of the Universe
• Vesto Slipher in 1912 found Doppler shifts in
  spiral nebulae exceeding the 300 km/s
• velocities found in the Milky Way.
• Hubbles interpretation of Sliphers (and
  Lundmarks) observations of the Doppler
• shifts of spiral nebulae led to Hubbles law in
  1929.

43
Name of the cluster of galaxies
The distances used to calibrate the Hubble law
(plotted here along the abscissa) come from
various methods (e.g. Cepheids, novae,
supernovae). Note that the slope is the Hubble
constant (is now known to be 70 km/sMpc)
only the galaxies shown on the right are plotted.
44
The expansion of the Universe Hubble's Law
v H x d, where H 70 km/s/Mpc
Milky Way
45
REDSHIFT, DISTANCE AND THE AGE OF THE
UNIVERSE Once the Hubble law is calibrated (with
Cepheids for instance), it can be inverted to
give a distance estimate based on a galaxys
redshift Note only valid for
nearby (z lt 0.8) galaxies. The second
important aspect of Hubbles law is that almost
all galaxies have redshifted spectra (some
blueshifts occur for very nearby galaxies). ?
galaxies are flying away from each other ?
Universe is expanding Thirdly (related to the
second point), galaxies at greater distances move
faster. This effect is not an acceleration, but
rather uniform expansion! Prior to Hubbles
discovery, almost all philosophical thought about
the Universe was centred on a static Universe
neither expanding nor contracting. Einsteins GR
theory was doctored with a cosmological
constant to save the theory. In fact, we now
think that the cosmological constant is a very
important component of the Universe (supported
only by recent observations)
46

• REDSHIFT, DISTANCE AND THE AGE OF THE UNIVERSE
• For higher-redshift galaxies (z gt 0.8),
  cosmological effects become important
• and one has to take the geometry of the Universe
  into account (more on that in
• the cosmology section of this course).
• For a flat Universe, the proper relationship
  is
• It is possible to estimate (somewhat simply) the
  time since expansion started
• this is called the Hubble time.
• The argument is as follows
• the distance-time relationship for constant
  speed is
• comparing this with Hubbles law
• implies that

47
REDSHIFT, DISTANCE AND THE AGE OF THE
UNIVERSE This simple time estimate is remarkably
close to the age of the oldest stars, but it is
not quite correct. Proper calculations of Hubble
time will need to take more accurate cosmological
models into account. The Hubble time calculated
here represents the time in the past when all
galaxies if no acceleration occurred in their
motions were jammed together at the
beginning of the expansion we call this event
the Big Bang. The Big Bang took place at a
certain time, not at a certain place (all of
space was packed together). Since then, the
Universe has been expanding not into empty
space, but space itself expands as time passes
the galaxies are simply luminous markers of this
expansion. The redshifts of galaxies arise from
the different distances of our cosmic markers, at
different times in the history of the Universe.
The observed expansion does not imply that we
are at the centre of the Universe if the
expansion is uniform, an observer in another
galaxy observes the same Hubble Law. H (Hubble
constant) is not really a constant it is
affected by the gravitational effects that
galaxies have on one another (H decreases as the
Universe age). The value of H mentioned before is
H0, for the current epoch!