<h2 style="text-align: center; "><span style="font-size: 1.714rem;">CLUSTERS AND SUPERCLUSTERS OF GALAXIES</span><br></h2><p class="MsoNormal"><span lang="">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 <b>cluster</b> 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.</span></p><p class="MsoNormal"><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal"><img src="https://res.cloudinary.com/drrz8xekm/image/upload/v1560603808/sdmp0eu2goxxowsmkwbh.jpg" data-filename="sdmp0eu2goxxowsmkwbh" style="width: 527.5px;"><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal" align="center" style="text-align:center"><span lang=""><a href="http://pixabay.com/illustrations/space-science-fiction-cosmos-blue-681635/">[Pixabay]</a><o:p></o:p></span></p><p class="MsoNormal"><span lang="">The <b>Virgo
Cluster</b> 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></span></p><p class="MsoNormal"><span lang="">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.</span></p><p class="MsoNormal"><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal" style="text-align: center; "><img src="https://res.cloudinary.com/drrz8xekm/image/upload/v1560603870/yva0zigvqbbhxsahldp5.jpg" data-filename="yva0zigvqbbhxsahldp5" style="width: 430px;"><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal" align="center" style="text-align: center; background-image: initial; background-position: initial; background-size: initial; background-repeat: initial; background-attachment: initial; background-origin: initial; background-clip: initial;"><span lang="" style="font-family:&quot;Arial&quot;,&quot;sans-serif&quot;;color:#222222">Abell 2744
galaxy cluster - Hubble Frontier Fields view (7 January 2014)<o:p></o:p></span></p><p class="MsoNormal" align="center" style="text-align: center; background-image: initial; background-position: initial; background-size: initial; background-repeat: initial; background-attachment: initial; background-origin: initial; background-clip: initial;"><a href="http://commons.m.wikimedia.org/wiki/File:Heic1401a-Abell2744-20140107.jpg#mw-jump-to-license">[NASA,ESA, and J. Lotz, M. Mountain, A. Public domain]</a><span lang="" style="font-family:&quot;Arial&quot;,&quot;sans-serif&quot;;color:#222222"><o:p></o:p></span></p><p class="MsoNormal"><span lang="">Clusters of galaxies are themselves in larger
groupings called <b>superclusters</b>. 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></span></p><p class="MsoNormal"><span lang="">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></span></p><h2 align="center" style="text-align:center"><span lang="">THE EXPANDING UNIVERSE<o:p></o:p></span></h2><h3><span lang="">RED SHIFT<o:p></o:p></span></h3><p class="MsoNormal"><span lang="">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 <i>D</i>
and the speed of recession <i>v</i>:<o:p></o:p></span></p><h4 align="center" style="text-align:center"><span lang="">v = HD<o:p></o:p></span></h4><p class="MsoNormal"><span lang="">Where H is the <b>Hubble constant</b>. The speed of recession <i>v </i>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 <i>v</i>.</span></p><p class="MsoNormal"><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal" style="text-align: center; "><img src="https://res.cloudinary.com/drrz8xekm/image/upload/v1560603935/cdsjrdxeq9x0qqsvmv1a.jpg" data-filename="cdsjrdxeq9x0qqsvmv1a" style="width: 527.5px;"><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal" align="center" style="text-align:center"><span lang="">Scatter
plot of fit of redshifts to Hubble's law<o:p></o:p></span></p><p class="MsoNormal" align="center" style="text-align:center"><a href="http://commons.m.wikimedia.org/wiki/File:Hubble_constant.JPG#mw-jump-to-license">[Brewsohare • CC BY-SA 3.0]</a><span lang=""><o:p></o:p></span></p><p class="MsoNormal"><span lang="">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 </span><i>Δλ</i><span lang="">. For a relative speed <i>v</i>, much smaller than the speed of light,
we have a good approximation:</span></p><p class="MsoNormal"></p><h4 style="text-align: center;"><i><span style="font-size: 14px; text-align: start;">Δλ/</span><span style="font-size: 1rem;">λ<sub style="">e</sub>= v/c&nbsp; &nbsp; &nbsp;</span></i><i style="font-size: 1.071rem;"><span style="font-size: 1rem;">OR&nbsp; &nbsp;&nbsp;</span></i><i style="font-size: 14px; text-align: start;">λ<sub>0</sub>/</i><i style="font-size: 1rem;">λ<sub>e</sub> - 1 = v/c</i><i style="font-size: 10.5px;">&nbsp;</i></h4><h4 style="text-align: center;"><p style="text-align: left; "><span style="font-weight: normal;">where&nbsp;</span><i style="font-size: 14px; font-weight: 400;">λ<sub>e</sub>&nbsp;</i><span lang="" style="font-weight: normal; font-size: 1rem;">is the wavelength of the radiation when <i>emitted</i>,&nbsp;</span><i style="font-size: 14px; font-weight: 400;">λ<sub>0</sub></i><span lang="" style="font-weight: normal; font-size: 1rem;">&nbsp;is the
wavelength observed on Earth, and <i>c</i>
is the speed of light. In cosmology, we are usually interested in light from
objects moving away from us so that&nbsp;</span><i style="font-size: 14px; font-weight: 400;">λ<sub style="font-size: 10.5px;">0&nbsp;</sub></i><span lang="" style="font-weight: normal; font-size: 1rem;">is greater than&nbsp;</span><i style="font-size: 14px; font-weight: 400;">λ<sub>e</sub></i><span style="font-size: 1rem; font-weight: normal;">, that is, the
wavelength has ‘shifted to the red’ end of the spectrum.</span></p></h4><p></p><h3><span lang="">The z factor</span></h3><p class="MsoNormal"><span lang="">Astronomers define a quantity <i>z</i> as the red shift for small recession
speeds <i>v </i>and hence small values of <i>z</i>:</span></p><h4 style="text-align: center; "><span lang="">z =&nbsp;</span><i style="font-size: 1rem;">Δλ/</i><i style="font-size: 1rem;">Δ<sub>e</sub> = v/c</i></h4><p class="MsoNormal"><span lang="">or, incorporating Hubble’s law:</span></p><h4 style="text-align: center; "><span lang="">z = H<sub>0</sub>D/c</span></h4><p class="MsoNormal"><span lang="">These relations are modified to incorporate
relativity theory for large values of <i>v</i>
and <i>z</i>.<o:p></o:p></span></p><h3><span lang="">The cosmological red shift<o:p></o:p></span></h3><p class="MsoNormal"><span lang="">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 <i>by the same
factor</i>.<o:p></o:p></span></p><p class="MsoNormal"><span lang="">The figure below gives the simple mathematics
of this effect, linking the expansion ratio to the <i>z</i> factor; where <i>z</i> is now
explained as a <b>cosmological</b> 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.</span></p><p class="MsoNormal" style="text-align: center; "><img src="https://res.cloudinary.com/drrz8xekm/image/upload/v1560604010/y3g7ropmkotcsbsoihhf.png" data-filename="y3g7ropmkotcsbsoihhf" style="width: 527.5px;"><span lang=""><br></span></p><p class="MsoNormal"><span lang=""> <o:p></o:p></span></p><p class="MsoNormal" align="center" style="text-align:center"><span lang="">Redshift
and blueshift<o:p></o:p></span></p><p class="MsoNormal" align="center" style="text-align:center"><a href="http://commons.m.wikimedia.org/wiki/File:Redshift_blueshift.svg#mw-jump-to-license">[AlešTošovský • CC-BY-SA-3.0]</a><span lang=""><o:p></o:p></span></p><h2 style="text-align: center; "><span lang="">EVERYTHING IS MOVING AWAY FROM EVERYTHING ELSE<o:p></o:p></span></h2><p class="MsoNormal"><span lang="">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. <i>Every large-scale
feature</i> 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></span></p><p class="MsoNormal"><span lang="">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></span></p><h2><span lang="">THE HUBBLE CONSTANT H<o:p></o:p></span></h2><p class="MsoNormal"><span lang="">The Hubble constant <i>H</i> is hard to measure accurately because distance measurements are
so inexact for the furthest galaxies. The units of the constant are usually
given as:</span></p><p class="MsoNormal"></p><h4 style="text-align: center;"><span style="font-size: 1rem;">speed in km s</span><sup>-1</sup><span style="font-size: 1rem;">/distance in Mpc</span></h4><span style="font-size: 1rem;">As at 2014, the currently accepted value for </span><i style="font-size: 1rem;">H</i><span style="font-size: 1rem;"> is 73.8 ±2.4 km s</span><sup>-1</sup><span style="font-size: 1rem;">Mpc</span><sup>-1</sup><span style="font-size: 1rem;">
</span><a href="https://wmap.gsfc.nasa.gov/universe/uni_expansion.html" style="background-color: rgb(255, 255, 255); font-size: 1rem;"><sup>source</sup></a><span style="font-size: 1rem;">.
The value as measured by astronomers has changed since Hubble’s day. This is
not because </span><i style="font-size: 1rem;">H</i><span style="font-size: 1rem;"> is changing physically
but because it is very hard to measure it accurately.</span><p></p><h2 style="text-align: center; "><span lang="">THE HUBBLE CONSTANT AND THE AGE OF THE UNIVERSE<o:p></o:p></span></h2><p class="MsoNormal"><span lang="">The Hubble constant gives us a rough measure
of the age of the Universe. First assume that <i>H</i> has actually been constant since the Universe formed, a time T<sub>H</sub>
ago. In that time, any two points have moved apart by a distance of Mpc of <i>D</i>, at a steady speed <i>v</i>, so:<o:p></o:p></span></p><h4 style="text-align: center; "><i>v = D/T<sub>H</sub></i></h4><p class="MsoNormal"><span lang="">But also:<o:p></o:p></span></p><h4 style="text-align: center; "><i>v = HD</i></h4><p class="MsoNormal"><span lang="">So we have:<o:p></o:p></span></p><h4 align="center" style="text-align:center"><i>H = 1/T<sub>H</sub></i></h4><p class="MsoNormal"><span lang="">This gives the age of the Universe as:<o:p></o:p></span></p><h4 align="center" style="text-align:center"><i>T<sub>H</sub> = 1/H = 1 Mpc/73.8 km s<sup>-1</sup></i></h4><p class="MsoNormal"><span lang="">A megaparsec is 3.1 </span><span lang="">×</span><span lang=""> 10<sup>19</sup> km, so T<sub>H</sub> comes to about 1.4 </span><span lang="">×</span><span lang="">10<sup>10</sup> years i.e 13.3 billion years (13 Gy or 13 <i>aeons</i>).<o:p></o:p></span></p><p class="MsoNormal"><span lang="">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<sup>-1</sup> Mpc<sup>-1</sup>, making the
Universe ten times younger, at 1.2 Gy, less than the age of the oldest rocks on
Earth!<o:p></o:p></span></p><p class="MsoNormal"><span lang="">The discovery that the expansion of the universe is <i>accelerating</i> – (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.</span></p><p class="MsoNormal" style="text-align: center; "><img src="https://res.cloudinary.com/drrz8xekm/image/upload/v1560604084/wyo3ml6ekljpbjaoswxh.jpg" data-filename="wyo3ml6ekljpbjaoswxh" style="width: 527.5px;"><br></p><p class="MsoNormal" align="center" style="text-align: center; background-image: initial; background-position: initial; background-size: initial; background-repeat: initial; background-attachment: initial; background-origin: initial; background-clip: initial;"><span lang="" style="font-family:&quot;Arial&quot;,&quot;sans-serif&quot;;color:#222222">This
illustration shows the three steps astronomers used to measure the expansion
rate of the Universe.<o:p></o:p></span></p><p class="MsoNormal" align="center" style="text-align: center; background-image: initial; background-position: initial; background-size: initial; background-repeat: initial; background-attachment: initial; background-origin: initial; background-clip: initial;"><a href="http://en.m.wikipedia.org/wiki/File:Three_steps_to_the_Hubble_constant.jpg">[NASA,ESA, A. Feild (STScI), and A. Riess (STScI/JHU) • CC BY 4.0]</a><span lang="" style="font-family:&quot;Arial&quot;,&quot;sans-serif&quot;;color:#222222"><o:p></o:p></span></p><h2><span lang="">THE FIRST 3 BILLION YEARS: A PUZZLE<o:p></o:p></span></h2><p class="MsoNormal"><span lang="">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 <i>non-homogeneous</i>
(uneven) appearance.<o:p></o:p></span></p><h3><span lang="">GUT and Guth<o:p></o:p></span></h3><p class="MsoNormal"><span lang="">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.</span></p><p class="MsoNormal"><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal" style="text-align: center; "><img src="https://res.cloudinary.com/drrz8xekm/image/upload/v1560604415/kuer0p0iwikmut5jk92k.png" data-filename="kuer0p0iwikmut5jk92k" style="width: 527.5px;"><span lang=""><o:p><br></o:p></span></p><p class="MsoNormal" align="center" style="text-align:center"><span lang="">SU(5)
fermions of standard model in 5+10 representations<o:p></o:p></span></p><p class="MsoNormal" align="center" style="text-align:center"><a href="http://commons.m.wikimedia.org/wiki/File:SU(5)_representation_of_fermions.png#mw-jump-to-license">[PaulBird • CC BY-SA 3.0]</a><span lang=""><o:p></o:p></span></p><p class="MsoNormal"><span lang="">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></span></p><h2><span lang="">Thanks for reading.</span></h2><h2><span lang="">REFERENCES<o:p></o:p></span></h2><p class="MsoNormal"><a href="https://wmap.gsfc.nasa.gov/universe/uni_expansion.html">How Fast is the Universe Expanding?</a><span lang=""><o:p></o:p></span></p><p class="MsoNormal"><span lang=""><a href="http://en.m.wikipedia.org/wiki/Supercluster">Supercluster</a><o:p></o:p></span></p><p class="MsoNormal"><a href="https://en.m.wikipedia.org/wiki/List_of_galaxy_groups_and_clusters">List of galaxy groups and clusters</a><span lang=""><o:p></o:p></span></p><p class="MsoNormal"><span lang=""><a href="https://en.m.wikipedia.org/wiki/Virgo_Supercluster" target="_blank">Virgo Supercluster</a><o:p></o:p></span></p><p class="MsoNormal"><span lang=""><a href="http:///en.m.wikipedia.org/wiki/Redshift#Distinguishing_between_cosmological_and_local_effects">Redshift</a><o:p></o:p></span></p><p class="MsoNormal"><span lang=""><a href="http:///en.m.wikipedia.org/wiki/Hubble%27s_law">Hubble's law</a><o:p></o:p></span></p><p class="MsoNormal"><span lang=""><a href="http://en.m.wikipedia.org/wiki/Alan_Guth">Alan Harvey Guth</a><o:p></o:p></span></p><p class="MsoNormal"><a href="http:///en.m.wikipedia.org/wiki/Grand_Unified_Theory">Grand Unified Theory</a><span lang=""><o:p></o:p></span></p><p class="MsoNormal"><a href="https://www.goodreads.com/book/show/173217.Life_on_a_Young_Planet">Life on a Young Planet: The First Three Billion Years of Evolution on ...</a><span lang=""><o:p></o:p></span></p><p class="MsoNormal"><a href="https://arxiv.org/abs/astro-ph/9611148">Clusters and superclusters of galaxies</a><span lang=""><o:p></o:p></span></p><p class="MsoNormal"><a href="http://www.astronomy.ohio-state.edu/~ryden/ast162_8/notes34.html">Lecture34: Clusters and Superclusters</a><span lang=""><o:p></o:p></span></p><p>

















































































































































</p><p class="MsoNormal"><a href="http://www.eniscuola.net/en/argomento/universe/the-birth-of-galaxies/cluster-and-supercluster-of-galaxies/">Clusterand supercluster of galaxies.....</a><span lang=""><o:p></o:p></span></p>