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Andrzej Marecki
N. Copernicus University, Toruń, Poland
The Island Universe of Immanuel Kant
- a Modern Perspective
Immanuel Kant (1724-1804)
The ancient Hebrew cosmological model is vastly dominated by religious content
whereas the astronomical ingredient is secondary (if not tertiary): the Sun, the Moon
and the stars are rather unimportant ornaments on the sky firmament. Note that planets
are not mentioned at all.
The medieval cosmology (as shown in the works of Dante) is still largely
religious but the astronomical component is much better pronounced. It reflects
the Ptolemaic, geocentric model of the Solar system. Each planet has its own orbit.
Thus, the distances to planets vary with each planet. However, the so-called fixed
stars, are equidistant and located on the outskirts of the Universe.
Claudius Ptolemaeus (Ptolemy)
Ptolemaic model of the Solar system was quite complicated. It allowed only
for circular orbits. Introduction of the so-called epicycles was necessary to
make it compatible with the observations. Sometimes the second order epicycles
i.e. the “epicycles on epicycles” were required to solve the discrepancies between
the model and the observations. Yet, it perhaps would be difficult if not impossible
for Ptolemy to explain completely the libration of the Moon (see the next image).
Ptolemaic model of the Solar system was in fact not only complicated but also
inaccurate. For many years Nicolaus Copernicus was carrying out detailed
observations that led him to a conclusion that the movements of planets would be
much better described assuming heliocentric orbits. De Revolutionibus (On the
revolutions) is perhaps one of the most important books ever written and printed.
It is to be noted, however, that the heliocentric model by Copernicus still posits
circular orbits. The old concept of the “sphere of fixed stars” is also present there.
Nicolaus Copernicus
Johannes Kepler
Isaac Newton
Only Johannes Kepler replaced circles with ellipses and thanks to Isaac Newton we
know why orbits are elliptical. His classical law of gravity, although now supplanted
by general relativity, is still sufficient and accurate enough to explain virtually all the
movements of the bodies in the Solar System.
Until 1610 astronomers (including Copernicus) have no telescopes and so they
could only see the Moon, the planets, meteors and, of course, (some) stars.
Occasionally, comets appeared on the sky. Out of these, only the members of the
latter class of objects were perceived as “nebulous”. Thanks to invention of the
telescope by Galileo Galilei not only could people notice that stars were point-like
whereas planets were not, but also they could see more nebulous objects.
Charles Messier – see his portrait in the next slide – who was a “comet hunter”,
set up a list of such objects that mimicked comets.
Charles Messier (1730 - 1817)
His list contained more than a hundred of objects. The patchwork made of their
state-of-the-art images is shown in the previous slide. Amazingly, Messier's list is
still useful today, namely the numbers assigned by him (preceded with “M”) are
common names of these objects. Thus, instead of “Andromeda galaxy” astronomers
just say/write “M31”.
Today we know that the objects in Messier list belong to different astrophysical
classes: globular clusters, open clusters, nebulae and galaxies.
At the end of the 18th century, William & Caroline Herschel
used the largest telescope of the era to study the shape of our Galaxy.
Milky Way Galaxy as seen by Herschel
Immanuel Kant (1724-1804)
Immanuel Kant was a philosopher but he was also interested in astronomy.
He heard about the “nebulae” and he postulated that they are separate “worlds”
similar to ours i.e. the Milky Way galaxy. He coined the concept of an “island
Universe”. At that time no observational evidence to support this model existed
but, surprisingly, Kant was right!
Our “island”: the Milky Way Galaxy
A contemporary model of the Galaxy
Edwin Hubble (1889-1953)
Only in the 20th century the idea of the “island Universe” gained firm observational
support thanks to work of Edwin Hubble. He discovered that the distances to some
“nebulae” are much greater than the sizes of the Galaxy. He named them
“extragalactic nebulae”. (Today this term has been replaced by a “galaxy”.)
Consequently, they are not parts of the Galaxy. For example, the distance to
Andromeda galaxy – see the next slide – is about 20 times the diameter of the
Galaxy. So, the Universe of Edwin Hubble appeared, indeed, as an ensemble
of galaxies – the “islands” on the “sea” called the Universe. Quite naturally, the
Milky Way Galaxy was by no means the “centre” of the Universe.
Hubble's discovery (announced in 1924) was truly revolutionary. He changed our
comprehension of the Universe in the same way Copernicus changed our
understanding of the Solar system.
Andromeda galaxy (M31)
The Local Group
Galaxies are often grouped. Our Galaxy is a member of a small group called “Local
Group”. There only two or three dozens of galaxies in a such a group. Clusters of
galaxies are much more numerous: there are thousands of them in a cluster.
The Local Group
Coma cluster
Hubble's greatest discovery
Five years later, in 1929, Edwin Hubble announced an even greater discovery. He
found that galaxies run away one from another. Their velocities are (seemingly)
proportional to their distances. This property of the Universe is known as the Hubble
law. The previous slide shows the genuine drawing by Hubble. Note that the proper
motions of the galaxies make the Hubble law apparently approximate. Consequently,
for many years the exact value of the velocity/distance ratio – the so-called Hubble
constant – was not known. This uncomfortable situation changed only in the end of
the 20th century thanks to... Hubble Space Telescope (HST) – see the next slide.
Hubble Space Telescope
Hubble law as established by Edwin Hubble (left) and by HST (right)
Thanks to the state-of-the-art observations carried out with HST, distances to
much farther galaxies could be measured. As can be easily noticed in the previous
slide (right panel), Hubble law works very well: velocity/distance ratio remains
constant in a wide range of these two quantities, particularly for more distant galaxies
where the proper motions velocities become negligible compared to the “Hubble
flow” velocity.
Edwin Hubble is also famous because of his classification scheme of galaxies.
Hubble was truly a GREAT astronomer, one of the greatest discoverers of the
20th century.
The distances to the galaxies are enormous. They are normally expressed in
megaparsecs (Mpc). 300 Mpc is equivalent to a billion of light years. And this
is... quite a modest distance – the recession velocity of such a galaxy causes a
redshift of less than 0.1. Can galaxies farther than that be observed? This is a
so-called “good question”, i.e. a question that cannot be easily answered.
To answer it we need special techniques of observations. One of such techniques
is based on the phenomenon of gravitational lensing.
General relativity which, as we know very well now, is indeed a very “general”
theory describing the interplay of space and mass/energy, predicts that the space
is being curved by matter. Thus, the light ray is apparently bent in the vicinity of
a big mass. Calculations show that to attain a measurable effect of that bending
either the observer must be very close to the bending mass or the mass has to be
very, very large. Therefore, it is possible to observe bending of a ray of star light
by our Sun during a total eclipse if the star happens to lie close to the Sun/Moon
limb at the moment of the totality. Alternatively, huge masses of galaxies, or better
yet of clusters of galaxies, can cause distortions of the paths of rays of light emitted
by far-away objects behind the deflector. Te next slide shows the details of the
phenomenon of bending of light by a cluster of galaxies.
The previous slide shows clearly that galactic cluster acts here as a lens. We call it
a gravitational lens. Gravitational lensing is quite similar to optical lensing except
for that the real (natural) gravitational lenses have very “irregular” shapes compared
to an optical lens of a camera. No wonder that the “images” created by gravitational
lenses are very imperfect.
If the alignment is nearly perfect i.e. if the observer, the lens and the object are
almost co-linear, the so-called Einstein ring develops. The Bull's eye galaxy is
a unique example of a gravitationally lensed image where such a nearly ideal
alignment takes place.
“Bull's eye” gravitational lens system
In the case of a cluster acting as a lens the images take the shape of arcs
(or “arclets”). Although the “image” itself is not very useful, the great virtue
of such a lens is that it amplifies the light from the very distant object. To put it
very simply: thanks to a huge mass located between the observer and the object
we receive the light that “originally” was directed to “someone else” in the
Universe. So, we receive much more light compared to the configuration devoid
of a gravitational lens.
z~7 which translates to the
distance of 12.9 billion light
(Kneib et al., Feb 15. 2004)
In the previous slide we can spot an arc-like patch of red light. The spectrum of that
light contains the lines redshifted by a factor of 1+z=8. So, for example, if we
assume that the wavelength we perceive amounts to, say, 640 nm, the original
emission in the reference frame of the emitter has only 80 nm. Thus, what we see
is a far-UV radiation shifted to the optical domain! And, only thanks to the lens,
we can notice that light at all. Without it, the far-away galaxy would not be observable
as it is too faint.
So, it looks that very distant galaxies do exist. Can we observe them in a “normal”
way i.e. using a telescope? Not easily, because they are extremely faint. Well, of
course, we could but to this end the exposure time would have to be extremely long.
This was done with the HST. The co-called Hubble Deep Field (HDF) was observed
for many days! (Naturally, this “brute force” approach is very expensive and cannot
be widely used.) The result is shown in the next slide.
What we see in the HDF are very distant galaxies. The conclusion is that whatever the
epoch the content of the Universe is roughly the same: the Universe is populated by
galaxies! At last we got a firm evidence that Kant was right.
The galaxies are not distributed uniformly in the Universe. The large-scale
structure of the Universe has most likely a filamentary one and as such resembles
a “foam”. It means that there are huge voids with no galaxies inside.
So, we have learned today that galaxies are everywhere in the Universe.
Now let's ask two very fundamental questions. When and where did it all begin?
How old are galaxies? Modern cosmology knows the answers to these questions.
A preliminary answer can be deduced from Hubble law. If we extrapolate backwards
the Hubble flow we find that the Universe has its beginning. We call it the Big Bang.
Where did the Big Bang happen? Everywhere! The Big Bang is the beginning of the
expansion of the whole space. So we are “inside” the aftermath of the Big Bang.
The most important proof that the Big Bang happened indeed is the existence of
Cosmic Microwave Background (CMB) radiation – a cooled-down remnant of a much
hotter early Universe. The CMB is present everywhere.
CMB is a perfect black-body radiation with a temperature of 2.7 K
Full-sky image made from WMAP spacecraft data
The first detailed full sky picture
of the CMB, the oldest light in the Universe
The Wilkinson Microwave Anisotropy Probe (WMAP) has made the first detailed full-sky map
of the CMB. It is a “baby picture” of the Universe. Colours indicate “warmer” (red) and
“cooler” (blue) spots. The microwave light captured in this picture is from 379,000 years after
the Big Bang. At that epoch the Universe has a temperature of 3000 K.
Thanks to the WMAP probe, we could find that the geometry of the Universe is flat. This
means that the geometry we learned at school applies over the largest distances in the
The WMAP probe has made a precise determination of the age of the Universe possible:
the Universe is 13,700,000,000 years old. Most of the galaxies formed shortly after the
Big Bang. Thus, they are almost as old as the Universe itself.
The matter of which the galaxies are made is only a small portion of the Universe – 4%.
23% is an exotic type of material known as “cold dark matter” and 73% is an even more
exotic “dark energy”. The conclusion from this is rather pessimistic: having some
knowledge about the galaxies we know something about 4% of the Universe but we are
still largely ignorant about the remaining 96%.
The end
Immanuel Kant (1724-1804)