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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
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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.
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 (1473-1543) Johannes Kepler (1571-1630) Isaac Newton (1642-1727)
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.
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.
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!
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.
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.
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 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.
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.
