CLUSTERS AND SUPERCLUSTERS OF GALAXIES
Galaxies are clustered into groups, made of
galaxies quite close together compared with distances to neighbouring groups.
The Milky Way and the Andromeda Galaxy are in fact the two largest galaxies in
a cluster of 30 galaxies – the Local
Group occupying a region of space about 1 Mpc across and affecting each other
gravitationally. The largest two, the Milky Way and the Andromeda Galaxy, orbit
each other and have the greatest gravitational effect on the whole group.
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[Pixabay]<o:p></o:p>
The Virgo
Cluster is our nearest-neighbour cluster, about 18 Mpc away in the
direction of the constellation Virgo. It is a huge cluster, about 3 Mpc across,
spanning a region of the sky 14 times as large as the Moon but too faint to be
seen with the naked eye. It contains several thousand galaxies and its
gravitational field is large enough to affect the movement of our own Local
Group.<o:p></o:p>
Distance measurements show that nearly all galaxies
are grouped in such clusters. Clusters range in size from the very small with
just an isolated pair of galaxies to the very large. Large clusters may have
galaxies widely spread out, or close enough to be tightly bound by a common gravitational
field in which individual galaxies orbit.
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Abell 2744
galaxy cluster - Hubble Frontier Fields view (7 January 2014)<o:p></o:p>
[NASA,ESA, and J. Lotz, M. Mountain, A. Public domain]<o:p></o:p>
Clusters of galaxies are themselves in larger
groupings called superclusters. Our
Local Supercluster is 382 galaxy clusters about 700 Mpc across with the Local Group
at the centre, and the Virgo Cluster at the edge.<o:p></o:p>
There is a 100 Mpc region of empty space
before we reach the next supercluster (in the constellation of Hercules). This
pattern of superclusters and voids continues on an even larger scale. This may
resemble the way that matter arranged itself in the first moments of the early
Universe.<o:p></o:p>
THE EXPANDING UNIVERSE<o:p></o:p>
RED SHIFT<o:p></o:p>
In 1929, Hubble used the Doppler effect to
measure the speeds of 24 bright galaxies at different distances from Earth. He
found that all of them were moving away from us, and the further away the
galaxy was, the faster it was moving. He found a simple relation, called
Hubble’s law, between the distance D
and the speed of recession v:<o:p></o:p>
v = HD<o:p></o:p>
Where H is the Hubble constant. The speed of recession v is obtained by measuring the red shift of a known spectral line
due to the Doppler effect. Red shifts for galaxies in five comparatively close
clusters are seen in the figure below shows these shifts plotted on a graph to
calculate v.
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Scatter
plot of fit of redshifts to Hubble's law<o:p></o:p>
[Brewsohare • CC BY-SA 3.0]<o:p></o:p>
The Doppler effect is a shift in frequency due
to the relative motion between source and observer. In astronomy, it is usually
quantified in terms of a wavelength change Δλ. For a relative speed v, much smaller than the speed of light,
we have a good approximation:
Δλ/λe= v/c OR λ0/λe - 1 = v/c
where λe is the wavelength of the radiation when emitted, λ0 is the
wavelength observed on Earth, and c
is the speed of light. In cosmology, we are usually interested in light from
objects moving away from us so that λ0 is greater than λe, that is, the
wavelength has ‘shifted to the red’ end of the spectrum.
The z factor
Astronomers define a quantity z as the red shift for small recession
speeds v and hence small values of z:
z = Δλ/Δe = v/c
or, incorporating Hubble’s law:
z = H0D/c
These relations are modified to incorporate
relativity theory for large values of v
and z.<o:p></o:p>
The cosmological red shift<o:p></o:p>
It was observed that a line spectrum emitted from
a distant galaxy, a billion light years away, and found that the wavelengths of
the lines have increased. A cosmological explanation of this effect is as
follows. First, we must remember that the light was emitted a billion years ago.
Since then the space between us and that galaxy has increased due to the
expansion of the Universe. This means that the space occupied by the wavelength
of the light has increased by the same
factor.<o:p></o:p>
The figure below gives the simple mathematics
of this effect, linking the expansion ratio to the z factor; where z is now
explained as a cosmological red
shift The red shift can also be explained using the Doppler effect, which links
the speed of recession with the change in light frequency.
<o:p></o:p>
Redshift
and blueshift<o:p></o:p>
[AlešTošovský • CC-BY-SA-3.0]<o:p></o:p>
EVERYTHING IS MOVING AWAY FROM EVERYTHING ELSE<o:p></o:p>
It may seem that somehow the Earth is at the
centre of a Universe that is doing its best to recede as far from it as possible.
But this is an illusion. Every large-scale
feature of the Universe is moving away from every other one. This is
explained by general relativity theory as an expansion of space itself, carrying
with it the matter it creates and is created by.<o:p></o:p>
Here is a simple two-dimensional analogy. When
a marked balloon is blown up, all the markings move apart. The rate of
separation is the speed of recession and depends upon the separation, exactly as
in Hubble’s law.<o:p></o:p>
THE HUBBLE CONSTANT H<o:p></o:p>
The Hubble constant H is hard to measure accurately because distance measurements are
so inexact for the furthest galaxies. The units of the constant are usually
given as:
speed in km s-1/distance in Mpc
As at 2014, the currently accepted value for H is 73.8 ±2.4 km s-1Mpc-1source.
The value as measured by astronomers has changed since Hubble’s day. This is
not because H is changing physically
but because it is very hard to measure it accurately.
THE HUBBLE CONSTANT AND THE AGE OF THE UNIVERSE<o:p></o:p>
The Hubble constant gives us a rough measure
of the age of the Universe. First assume that H has actually been constant since the Universe formed, a time TH
ago. In that time, any two points have moved apart by a distance of Mpc of D, at a steady speed v, so:<o:p></o:p>
v = D/TH
But also:<o:p></o:p>
v = HD
So we have:<o:p></o:p>
H = 1/TH
This gives the age of the Universe as:<o:p></o:p>
TH = 1/H = 1 Mpc/73.8 km s-1
A megaparsec is 3.1 × 1019 km, so TH comes to about 1.4 ×1010 years i.e 13.3 billion years (13 Gy or 13 aeons).<o:p></o:p>
Hubble’s own value for H was ten times larger
than the currently accepted value above because he got his distances wrong.
Hubble’s estimate was 800 km s-1 Mpc-1, making the
Universe ten times younger, at 1.2 Gy, less than the age of the oldest rocks on
Earth!<o:p></o:p>
The discovery that the expansion of the universe is accelerating – (which I
will discuss under “Dark Energy” in another post) – means that the Hubble
constant varies with time. The final story is yet to emerge because there is
even the possibility that the early Universe expanded more quickly than the
current rate, so giving a different value for its age.
This
illustration shows the three steps astronomers used to measure the expansion
rate of the Universe.<o:p></o:p>
[NASA,ESA, A. Feild (STScI), and A. Riess (STScI/JHU) • CC BY 4.0]<o:p></o:p>
THE FIRST 3 BILLION YEARS: A PUZZLE<o:p></o:p>
On the whole, we are more certain of what
happened in the first three minutes of the Universe than in the first 3 billion
years. Modern particle theory strongly supports the story of the Big Bang model.
But according to the simple Big Bang model, the Universe should be full of
galaxies spaced roughly equal distances apart, since originally the particles
should have been spaced evenly apart. It would not be too difficult to model
some instabilities which started the process of condensation into stars and
galaxies. But we don’t know how matter could have arranged itself into the
galactic superclusters we observe and that give the structure of matter in the
Universe the bubble-like non-homogeneous
(uneven) appearance.<o:p></o:p>
GUT and Guth<o:p></o:p>
A way out of this problem proposed in 1980 by
the American theoretical physicist Alan Guth. He developed the supergravity
theory, that at high enough energies, the fundamental interactions (forces) in
nature become one. This is the grand unification theory or GUT, which predicts
a very rapid inflation. Any small random inhomogeneities (unevennesses) in the
Universe just before the inflation are magnified as small bubbles of slightly
differing density. Then, as time goes on, the Universe grows too fast to smooth
itself out again, and the inhomogeneities remain as superclusters and clusters
of galaxies.
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SU(5)
fermions of standard model in 5+10 representations<o:p></o:p>
[PaulBird • CC BY-SA 3.0]<o:p></o:p>
In my next article, I’ll be discussing on The
Big Bang model of the universe and the Dark energy as promised above. Till
then, I remain my humble self @emperorhassy.<o:p></o:p>
Thanks for reading.
REFERENCES<o:p></o:p>
How Fast is the Universe Expanding?<o:p></o:p>
Supercluster<o:p></o:p>
List of galaxy groups and clusters<o:p></o:p>
Virgo Supercluster<o:p></o:p>
Redshift<o:p></o:p>
Hubble's law<o:p></o:p>
Alan Harvey Guth<o:p></o:p>
Grand Unified Theory<o:p></o:p>
Life on a Young Planet: The First Three Billion Years of Evolution on ...<o:p></o:p>
Clusters and superclusters of galaxies<o:p></o:p>
Lecture34: Clusters and Superclusters<o:p></o:p>
Very nice article!
Thanks for this!
Posted using Partiko Android
Thanks, @chappertron.
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Thanks, @holovision
Interesting article, I understood a surprising lot, despite not knowing a lot about astronomy. Just a quick question, though, what does MPC stands for? I imagine it measures something alike to light years, right?
Posted using Partiko Android
It is a mega-parsec, or one million of parsecs. It is a distance unit very practical in cosmology (to end with reasonable numbers):
1 Mpc = 3.26 light-years = 31 x 1012 km
Got it, I always thought that light years were the standard unit for measuring distance in cosmology, thanks for the explanation!
Posted using Partiko Android
You are welcome :)
The central value has changed but this central value is always coming with an uncertainty that is getting smaller and smaller.
Because it is simple and works extremely well. However, as you said, there are issues. But any other option also has issues. This is why research 9also on alternative cosmologies) is interesting.
U're always on point, @lemouth. Thanks for the great comments always.