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Cosmology - The Science Of Understanding The Universe

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PLEASE EXCUSE THE ANNOYING COMMERCIAL BREAKS IN THE CONVERSATION:

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And now, back to the regularly scheduled conversation...



Hello,

 

I'm pleased to report that I'm working on my next tutorial.

It will build on the first lesson about the Copernican Principle - this time going beyond the solar system, out into the stars and then beyond.  It will deal with our place in the Milky Way galaxy, our galaxy's place among the other galaxies and our place within the observable universe.  This will set the stage for a future tutorial about the Big Bang and the expansion of the universe - two very much misunderstood and misinterpreted aspects of cosmology.

 

I'm also pleased to report that Bhim will be helping us understand Einstein's theory of General Relativity.

He and I are working at our own speeds on these tutorials, so I cannot say who will be posting first.

 

Thanks,

 

BAA.

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Hello again.  smile.png

 

Welcome to the second tutorial on the subject of cosmology, the science of understanding the universe.

Before I go any further I strongly recommend that if you haven't read the first tutorial you should read it first, before going any further with this one.  Each tutorial builds upon the lessons of the preceding ones, so trying to grasp what's being taught partway thru this process will be more difficult than proceeding from the start.  Thank you.

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In the first tutorial I described how, beginning in the Renaissance period, the Copernican Revolution came to profoundly change the way we understand our place in the universe.  The old, Church-endorsed, Earth-centered (Geocentric) system was challenged by evidence showing that the Sun, the Moon and the planets did NOT revolve around the Earth.  The new, Sun-centered (Heliocentric) system thus removed the Earth from it's unique status, relegating it to that of just another of the planets.  

 

This shift in viewpoint is what cosmologists would refer to as a change in our, 'frame of reference'.

As we will see in this tutorial, whenever it's been assumed that the Earth, the Sun or even the Milky Way galaxy have some special frame of reference, the evidence has always disagreed.  Instead, the evidence seems to be telling us the opposite.   It tells us that nowhere in the universe is any more special than anywhere else and that all frames of reference are equal to each other.  Therefore, when trying to understand the universe, our starting point should be that no given location (like the Earth) is any more important, unique or special than any other.  

 

Cosmologists factor the Copernican principle into their work as a basic and fundamental assumption. 

Even though they observe the universe from just one location (the solar system) they always begin with the assumption that any observations of the universe, made from any location (in the entire universe) and by anyone/anything (not just humans) will always be just as valid as any other set of observations made elsewhere, by anyone/anything else.  They always assume that all observations are relative in value to each other and that none can be considered to be of any greater or lesser value than any other.  

 


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Using historical examples I'll now show how this relative frame of reference came to be adopted.  

How, after finding out that the Earth wasn't the center of the solar system, we also came to understand that the Sun isn't at the center of the Milky Way galaxy.  How we came to realize that the Milky Way isn't the entire universe.  And how we came to see that our place within the observable universe isn't a central one either.  Each step on this outward journey demonstrates the Copernican principle at work.  At every stage, as our frame of reference has widened, the Copernican principle has been confirmed with new data and new evidence.

 

The first example is William Herschel's 1785 model of the universe.  

(Please note that before the late 1920's, the Milky Way galaxy was considered to be the ENTIRE universe.)  

Hersch.gif

He was able to deduce that the Sun resides in a disk-shaped mass of stars, but he mistakenly assumed that it was at or very near the disk's center.  He placed the Sun above this pointer..................................^

 

By the early years of the 20th century a clearer picture of the universe was beginning to emerge.

The Dutch astronomer Jacobus Kapteyn produced this lenticular (lens-shaped) model, with the center being marked by a cross ( + ) and the Sun's location (the circled dot) being shown off-set by several thousand light years. 

kapteyn.gif

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A word of explanation..

The parsec is a type of measurement used by astronomers and is equal to 3.26 light years.  Therefore, a kiloparsec equals 3,260 light years.  So Kapteyn's model pegged the dimensions of the universe at 55,420 light years across and 9,780 light years thick.

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At about the same time an American astronomer called Harlow Shapley was taking a different approach in his studies.

He focused on globular clusters... http://en.wikipedia.org/wiki/Globular_cluster ...noting that their distribution wasn't centered on the Sun, but on a point in the direction of the constellation Sagittarius.  Applying Newton's laws of universal gravitation, he realized that the globular clusters must be orbiting a center of mass, which he concluded must be the true center of the universe.      

 

ShapleyGCs.gif

 

A count of the green dots (representing the globular clusters) shows that they are heavily concentrated to the right of the vertical axis, which passes thru the Sun.  There just six (6) to the Sun's left and eighty-one (81) to it's right.  The red X marks Shapley's inferred position of the center of mass.  This means that the Sun is tens of thousands of light years from Shapley's center of mass.  We now know that he was on the right track.  This Wikipedia page... http://en.wikipedia.org/wiki/Milky_Way ...gives us up to date information. This map shows our off-center location very well. http://en.wikipedia.org/wiki/Milky_Way#/media/File:Artist%27s_impression_of_the_Milky_Way_(updated_-_annotated).jpg

 

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A note to Henry, the EternalStudent.

 

Shapley's work is a classic example of scientific inference and deduction, Henry.

The globular clusters he was studying are tens to hundreds of thousands of light years from Earth.  This means that not only was it impossible for him to directly measure their distances by travelling to them, but that he was seeing them as there were, tens and hundreds of thousands of years ago.  That's how long their light takes to reach us.  

 

So Shapley couldn't know from direct experience anything about them.

He was obliged to work with what he could observe and then infer and deduce from his data that they were orbiting around something very massive indeed.   Back in the 1920's nobody could see thru the thick clouds of gas and dust that obscure the center of our galaxy.  But we now have Infrared telescopes that can look thru these clouds and see what's lurking at the very center of the Milky Way.  The object discovered there is called Sagittarius A* (pronounced A-Star).  http://en.wikipedia.org/wiki/Sagittarius_A*  

 

It's almost certainly a supermassive black hole, with a mass up to four million (4,000,000) times greater than our Sun.

Even though we can't directly see the black hole, we can do exactly what Shapley did with the globular clusters and infer it's presence by it's gravitational effects on nearby stars.  Here's a nice animation of those stars looping very rapidly (up to 1,000 km per second) around Sag A*, the yellow-white star symbol in the center.

 

gc_orbits_animfull.gif

 

Now Henry, if you also consider the following three points, you'll see that Shapley correctly inferred that the nucleus of our galaxy is also the center of it's mass and this is what the globular clusters are orbiting.  First, there's Sag A*, which is extremely massive.  Secondly, the nucleus is far more densely packed with stars, gas and dust than the much wider and more widely-spaced spiral arms that radiate out from it.  Lastly, the nucleus is home to several large clusters of supergiant stars, that weigh fifty or even a hundred times as much as our Sun.  The Arches cluster... http://en.wikipedia.org/wiki/Arches_Cluster ...and the Quintuplet Cluster... http://en.wikipedia.org/wiki/Quintuplet_cluster ...are two prominent examples.

 

Taking these three things together, we can see that the nucleus of the Milky Way galaxy is exactly what Shapley inferred it to be from his globular cluster data.  It's the massive hub around which the rest of the galaxy (the spiral arms) slowly revolve.  This elegantly demonstrates the very great power inference and deduction have when it comes to explaining the universe.

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Shortly afterward Shapley's discovery it was found that the universe itself was VERY much larger than we had ever imagined!

In 1925 Edwin Hubble published a scientific paper that changed our understanding of the cosmos forever.  In it he was able to show that Shapley, Kapteyn and Herschel were wrong to assume that the lens-shaped mass of stars where our Sun resides was the entire universe.  Instead, the concentration of stars we call the Milky Way was just a single galaxy, one of a multitude of other galaxies swarming in a truly vast universe.  

  


Using what was then the worlds largest telescope Hubble carefully studied what were then known as spiral nebulae (clouds) and he was able to gauge their true distance from us.  His findings indicated that these spirals couldn't be small whirlpools of gas and dust within our own galaxy.  They were so distant that they must be vast, spiral-shaped systems of stars - galaxies in their own right, just like the Milky Way.

 

Since then, with the building of ever-bigger telescopes, we've found that galaxies like ours are... everywhere.

Anywhere we point our telescopes - we find thousands upon millions of galaxies.  Even though they are distributed in clusters of tens and hundreds and superclusters of thousands and tens of thousands, when we sum up the findings of our largest surveys we come to a sobering conclusion.  One that agrees with the Copernican principle.  Generally speaking, the universe looks the same in every direction.

 

This conclusion was put to the test in 1995 and 1998, using the Hubble Space Telescope. 

It stared for two ten-day periods at two apparently empty patches of sky - one in the constellation of Ursa Major (the Great Bear), near the celestial North pole and one in Tucana (the Toucan), not far from the celestial South Pole.  The resulting images are the now familiar and famous, Hubble Deep Fields, North and South.  

 


 


 

1024px-HubbleDeepField.800px.jpg

 


 

800px-Hubble_Deep_Field_South_full_mosai

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Please note that in both images the five or six 'spikey' objects are foreground stars from our own galaxy.

Every other object is a galaxy, made up of hundreds of millions of stars like our own, seen from millions and billions of light years further away.  Some are spirals, like our own Milky Way, some are elliptical galaxies (the fuzzy ovals) and some are irregularly-shaped galaxies caught in the act of interacting and merging with other galaxies.  Please also note the each Hubble field covers an area of the sky equivalent to... a grain of sand ...held at arms length.

 

Even though the spread of galaxies in each field looks different, statistically the contents of both images are equivalent.

So, even though the Hubble telescope looked at two regions on opposite sides of the sky - two regions separated by of billions of light years, the universe still looked generally the same.  There is no discernible center of the universe.  It is uniformly similar, wherever you go and wherever you look.  On the largest scales, no particular region of space seems to be any more special or important than any other.  

 

This finding agrees very well with the Copernican principle.

Therefore, we now confidently say that the universe itself has given us strong evidence that we should understand it from a relative and universal frame of reference, not an absolute and local one. 

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That concludes this tutorial.

As before, I look to Bhim and the RogueScholar to correct any mistakes or oversights I've made along the way.  We are also happy to field your questions in the Cosmology Peanut Gallery, here... http://www.ex-christian.net/topic/67152-cosmology-peanut-gallery/#.VRMOhPmsUuk

 

Thanks,

 

BAA.

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Hello again.  smile.png

 

This message is an update about where things stand in this thread.

Bhim and I have been in contact and have agreed on how to proceed.  Even though he's a very busy man, he's making time to put together a tutorial on the subject of Einstein's theory of General Relativity.  GR is fundamental to astronomy, astrophysics and cosmology.  Gaining a better understanding of what it is and how it works will 'open up' the universe to you in new and exciting ways.  I'm eagerly looking forward to what Bhim has to say and I hope you are too!  

 

I'll be continuing with my exploration of the role of Copernican Principle in cosmology.

Having steadily widened the scope of my tutorials (from our solar system, out to the intergalactic scale) I'll go on to explain how the CP is applied in the context of an e-x-p-a-n-d-I-n-g universe.  This will require a number of steps.

 

1.  A brief overview of how the universe was discovered to be expanding.

2.  An explanation of the kind of expansion we observe in the universe today.

3.  What was discovered when scientists ran this expansion backwards in time.

4.  An explanation of how the universe expanded in it's very first moments. (This is very different to # 2.)

5.  Dealing with the inevitable misunderstandings and misconceptions about the Big Bang.

 

Please note that # 5 is NOT a disparagement or indictment of anyone!

Given the bizarre and counter-intuitive nature of Big Bang cosmology I think it's inevitable that the Peanut Gallery will be FLOODED with questions and requests for further explanation and clarification.  

 

Bhim and I expect this, are reconciled to it and will do our best to help.

It's entirely possible that the forward progress of the tutorials will grind to a halt at this point - as puzzled and confused members get their queries answered at a pace that's good for them.  That's not a problem.  That's exactly what these tutorials are designed to do.  To provide understanding to anyone who wants it.

 

Please maintain a holding pattern while Bhim and I put together our respective tutorials.

 

Thanks,

 

BAA.

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