Expert: Tom Whiting - 8/21/2007

Question
Hello,

I have a plethora of questions regarding astronomy so I would appreciate links to good web sites or good reference books (for "lay people").  Having studied the physical sciences until I was black-and-blue in the face, I actually have never delved into astrophysics or astronomy in general.  So, I no nothing about the subject (except when astrophysics/astronomical concepts coincide with general physics concepts like equations of motion, forces, relative motion, relativity, some quantum physics, etc).

How do we determine the distances to the various celestial bodies?  If the speed of light is truly constant, then how can we measure distances to bodies that are millions or even billions of light-years away?  In his theories of relativity, Einstein made the assumption that there was no "particle" (even though light is not a real particle and its not a real wave either) that could travel faster than c.  Is this now in error?  Can we "bounce" waves off of celestial bodies that are millions or billions of light-years away?  How do these measurement systems work?  What assumptions are made about them?  What error is inherent in these systems?  How was that determined?  I'm really looking for a lot of details here from a physical perspective that I can understand.

How do we know that the universe is spherical?  What evidence leads us to believe that?  If the universe is and has been expanding since its inception, wouldn't that sphere have to grow to accommodate that expansion?

I have many more questions, but I'll leave it at this for now.  Could you please annotate the answers as to what is observable (by what means) and what is theorical?

Answer
Hi Brian,
Wow!  A truly monumental question, and task for me; one that another engineer (myself, metallurgical engineer by degree)  will attempt to supply you.  On the outset, it would probably be more discerning for you to purchase and read a basic college astronomy textbook  (Astronomy 101) as these questions would mainly be answered by that simple textbook....with your engineering/math/physics background, you would have no problem reading and understanding such a textbook....one that is more descriptive and observational, as opposed to purely mathematical.  I recommend the author
Jay M. Pasachoff's "Astronomy:  From the Earth to the
Universe",  Saunders College Publishing Co. or one of his
more recent college 101 astronomy textbooks.  But there are other good ones out there too....just visit your local College
bookstore IF they offer a basic Astronomy 101 course.
They all pretty much cover the same information.

Distance determination is done by what astronomers refer
to as the "distance ladder" starting with radar "bounces"
off the nearby Solar System bodies, then trigonometric triangulation, followed by parallax measurements, then Cepheid Variable stars, the Novae, brightest galaxy method,  (presumed various  'standard candles')  then the moving cluster method, and finally red shift, which is kind of a form of very distant doppler (wavelength) shift interpretation.  Notice each rung of the distance ladder is slightly less accurate than the previous rung, but this is not a problem because once we get beyond a few thousand light years, the astronomer is only interested in relative, or proportionate distances, not absolute distances...  After all, we aren't going to travel 1000 light years on a weekend "walk in the park". ;-)
At the extreme distances using Red Shift, we'll gladly accept a built-in 10-20% error simply because there is, as yet,  no other known method to determine these large distances of billions of  light years.  (So we have no choice....in fact, perhaps we are fortunate that we have even one scientific-approved method!.....so don't knock it.) ;-)
{Also, there is good correlation between supernovae brightness observed in the external galaxies....good distance correlation relative to using supernovae as a standard candle
for distance determination. }

(Notice we do not "bounce" radar or laser waves off of anything except the very closest of bodies for distance determination...the Moon, Venus, Mercury, a few close passing asteroids...that's about it.  All other bodies' distances are determined by further up the distance ladder scale....first
by trigonometry, then parallax shift, then by analyzing the radiation from those bodies, the distant stars and galaxies...our ONLY source of  information from those bodies.
I'll assume from the tone and form of your question, that you are most interested in the highest rung of the distant ladder scale, mainly Red Shift for the most distant objects, since you mention Einstein, c, and Special Relativity.
The measurement system is relatively simple in practice...the spectrum of say a distant galaxy, is obtained with a very large
telescope, and this spectrum is compared to a continuous
spectrum at rest...the red  'shift' of the distant galaxy spectrum
is compared to the standard one at rest, the amount of red shift is measured, and then we have a direct readout of the
recessional velocity of that galaxy, and thus the relative
distance to that galaxy...relative distance, not absolute. And since we now know the most distant radiation, discovered in 1965 by Arno Pensias and Robert Wilson....the remnant radiation from the Big Bang  (also called the CBR-cosmic background radiation) which is red-shifted from visible light clear down to the microwave portion of the radio spectrum...we have an upper limit for all other vast distances. In fact, now astronomers refer not so much to distance, but age of distant bodies as a fraction or decimal equivalent...like for instance a distant galaxy may be only 0.1(one tenth)  the age of the Universe, the actual age of Universe being 13.7 billion years (+ or - 0.3 billion years).  No matter which way we point a telescope, we are always looking back in time...which lends credence to your mainly "spherical" Universe question.  We always look back in time because WE
are part of...the Big Bang sphere of influence.  We know of no
way to observe our Universe...from the 'outside' of it.

What assumptions are made? Well, they are not so much assumptions as "extensions"....When Edwin Hubble first realized that those spiral nebula we see in the night sky were
really extra-Galactic Island Universes as he called them in 1924, he also noticed that the more distant a galaxy, the faster it was receding from us.  Then in 1929 he made a startling mathematical discovery which you can appreciate....the results were linear.  That is,  a galaxy 10 times
away from us is receding at 10 times the speed of a nearby galaxy...and one 100 times farther was receding at 100 times faster, and so on.  Thus the introduction of the Hubble
Constant, today's modern day value is right around 71 Km/sec per megaparsec (3,260,000 light years).

{Picture a raison bread in the oven rapidly rising, expanding, and each galaxy is a raison, and it doesn't matter which raison you pick for the Milky Way, the most distant raison is receding the fastest from your 'raison'....as the bread expands.}
So we simply correlate red shift to the Hubble constant for
an actual (but not absolute) distance.  Quite simple in practice.  The hard part is the hours of gathering light from a
dim, distant galaxy thru a 10 meter telescope to collect enough photons on a detector to get a spectrum...that's the really hard part!!  So one of our assumptions is that the Hubble constant pretty much holds throughout the observable Universe as the Universe expands at that rate.  And yes, we also assume that c is a constant in a vacuum.  Actually a photon of light (radio, UV, X-Ray, Gamma, whatever) exhibits both wave and massless particle properties...but no matter, we assume the velocity of light is a constant throughout the short age of the known Universe. Even IF these extensions turn out to be not as accurate as we assume, it would not make a whole lot of difference in the results because, again, we are dealing only with proportionate distances, not absolute.
What errors are inherent?  As stated earlier, there is probably
a built in 10-20% error in our most distant objects, but unless
one comes up with a better method of long distance measurement, we have to live with it. How is error determined? By proportionality to the lower ladder rungs that are more accurate distance determinators.  (Or at least they are thought to be)...again, we know of no other form of distance measurement other than the above stated  ladder" .....no one is going to stretch a mechanical tape measure from the sun to the nearest star, Alpha Centauri at 4.3 light years  (25 trillion miles).  ;-)
How do we know the Universe is spherical (or roughly so)?
Well, I guess we can eliminate one and two dimension systems, and since there are only 3 spacial dimensions, I guess that's an inherent assumption.  (I guess one could
argue that perhaps oblate spheroid -football shaped-
is another possibility, but again, it would not make much
difference on vast distance determinations on the proportionate scale.   Yes, the sphere has been growing since
its inception....in fact, latest indications are that the current
known Universe is actually accelerating it's expansion for the past 5 or 6 billion years, based upon recent studies of
Type Ia supernovae (another assumed 'standard candle'.)

Realize it's the SPACE expanding, not the matter and energy in it....when we say the Universe is expanding, we mean the space-time continuum....the galaxies are just pretty much going along for the ride, similar to an empty canoe moving downstream at an ever increasing rate through the rapids.
(The galaxies themselves are not expanding- there are not
'large rockets' attached to the galaxies themselves!!).  We need to clear that up, right away.  It's just the space expanding.

I personally don't think Einstein is in error at all...in fact, the
one time he thought he was wrong...the establishment of
a Cosmological Constant....may turn out that he was right
after all....the man is correct even when HE thought HE
was wrong!!  He stated that it was the biggest mistake he
ever made...and yet, he may have been right after all.
So he was wrong to say he was wrong!  The rest of us should
be so fortunate.
So no, I don't think Einstein  is in error about the light speed constant in a vacuum.  Besides, using the Lorentz transformation equation which you are probably familiar with,
time becomes zero at light speed, and
one would have to go backward in time at super-light speeds,
and that's a no-no in physics, even relavistic physics.   (We can't take the square root of a negative number in real practice).
Oh, one last thought...above I stated that the Universe now
seems quite young at 13.7 billion years....I base that upon
the presumed lifespans of the little red dwarf stars which are
measured in trillions of years...we have yet to observe a little
red dwarf star (Like Barnard's runaway star, or Wolf 359 in
Leo, and they are thought to constitute over 80% of the stellar population-the most common star in fact) move to the
next step of it's stellar evolution.  So in my opinion, we, the
known Universe, have only just begun "last month"..the
Solar System formed up about a week ago, and us humans
have been on the scene only for one second.  PLUS, this
Universe is still 97% hydrogen and helium from the Big Bang...the heavier elements (Lithium thru Uranium) are only up to a lowly 3%.  And it's the star's nuclear processing that
produces the heavy (Li to U) elements!!
That information alone, tells me that we've only.....just
begun last month. The known visible Universe has a lifespan
measured in hundreds of trillions of years before it suffers
a heat or energy production death ! Actually, our Universe 13.7 billion year history is beginning to look quite...short!   IMHO.
Hope all this helps answer your questions....
If you would like any more info, please contact me directly
at bwhiting@velocity.net
And thanks for the intriguing questions,
Clear Skies,
Tom Whiting
Erie, PA