The Island Universe of Immanuel Kant- a Modern Perspective

Immanuel Kant (1724-1804)

Andrzej MareckiN. Copernicus University, Toruń, Poland

The ancient Hebrew cosmological model is vastly dominated by religious contentwhereas the astronomical ingredient is secondary (if not tertiary): the Sun, the Moonand the stars are rather unimportant ornaments on the sky firmament. Note that planetsare not mentioned at all.

The medieval cosmology (as shown in the works of Dante) is still largelyreligious but the astronomical component is much better pronounced. It reflectsthe 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 fixedstars, are equidistant and located on the outskirts of the Universe.

Claudius Ptolemaeus (Ptolemy)85-165

Ptolemaic model of the Solar system was quite complicated. It allowed only for circular orbits. Introduction of the so-called epicycles was necessary tomake it compatible with the observations. Sometimes the second order epicyclesi.e. the “epicycles on epicycles” were required to solve the discrepancies betweenthe model and the observations. Yet, it perhaps would be difficult if not impossiblefor 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 alsoinaccurate. For many years Nicolaus Copernicus was carrying out detailedobservations that led him to a conclusion that the movements of planets would bemuch better described assuming heliocentric orbits. De Revolutionibus (On therevolutions) 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 positscircular 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 weknow why orbits are elliptical. His classical law of gravity, although now supplantedby general relativity, is still sufficient and accurate enough to explain virtually all themovements of the bodies in the Solar System.

Until 1610 astronomers (including Copernicus) have no telescopes and so theycould 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 thelatter class of objects were perceived as “nebulous”. Thanks to invention of thetelescope by Galileo Galilei not only could people notice that stars were point-likewhereas 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 theirstate-of-the-art images is shown in the previous slide. Amazingly, Messier's list isstill useful today, namely the numbers assigned by him (preceded with “M”) arecommon names of these objects. Thus, instead of “Andromeda galaxy” astronomersjust say/write “M31”.Today we know that the objects in Messier list belong to different astrophysicalclasses: 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 “islandUniverse”. At that time no observational evidence to support this model existedbut, 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 observationalsupport 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 toAndromeda galaxy – see the next slide – is about 20 times the diameter of theGalaxy. So, the Universe of Edwin Hubble appeared, indeed, as an ensembleof galaxies – the “islands” on the “sea” called the Universe. Quite naturally, theMilky Way Galaxy was by no means the “centre” of the Universe.

Hubble's discovery (announced in 1924) was truly revolutionary. He changed ourcomprehension of the Universe in the same way Copernicus changed ourunderstanding of the Solar system.

Andromeda galaxy (M31)

The Local Group

Galaxies are often grouped. Our Galaxy is a member of a small group called “LocalGroup”. There only two or three dozens of galaxies in a such a group. Clusters ofgalaxies 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. Hefound that galaxies run away one from another. Their velocities are (seemingly)proportional to their distances. This property of the Universe is known as the Hubblelaw. The previous slide shows the genuine drawing by Hubble. Note that the propermotions of the galaxies make the Hubble law apparently approximate. Consequently,for many years the exact value of the velocity/distance ratio – the so-called Hubbleconstant – was not known. This uncomfortable situation changed only in the end ofthe 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 tomuch farther galaxies could be measured. As can be easily noticed in the previousslide (right panel), Hubble law works very well: velocity/distance ratio remainsconstant in a wide range of these two quantities, particularly for more distant galaxieswhere the proper motions velocities become negligible compared to the “Hubbleflow” 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 the20th century.

The distances to the galaxies are enormous. They are normally expressed inmegaparsecs (Mpc). 300 Mpc is equivalent to a billion of light years. And thisis... quite a modest distance – the recession velocity of such a galaxy causes aredshift of less than 0.1. Can galaxies farther than that be observed? This is aso-called “good question”, i.e. a question that cannot be easily answered.To answer it we need special techniques of observations. One of such techniquesis 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 ofa big mass. Calculations show that to attain a measurable effect of that bendingeither the observer must be very close to the bending mass or the mass has to bevery, very large. Therefore, it is possible to observe bending of a ray of star lightby our Sun during a total eclipse if the star happens to lie close to the Sun/Moonlimb at the moment of the totality. Alternatively, huge masses of galaxies, or betteryet of clusters of galaxies, can cause distortions of the paths of rays of light emittedby far-away objects behind the deflector. Te next slide shows the details of thephenomenon of bending of light by a cluster of galaxies.

The previous slide shows clearly that galactic cluster acts here as a lens. We call ita gravitational lens. Gravitational lensing is quite similar to optical lensing exceptfor that the real (natural) gravitational lenses have very “irregular” shapes comparedto an optical lens of a camera. No wonder that the “images” created by gravitationallenses are very imperfect.If the alignment is nearly perfect i.e. if the observer, the lens and the object arealmost co-linear, the so-called Einstein ring develops. The Bull's eye galaxy isa unique example of a gravitationally lensed image where such a nearly idealalignment 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 virtueof such a lens is that it amplifies the light from the very distant object. To put itvery simply: thanks to a huge mass located between the observer and the objectwe receive the light that “originally” was directed to “someone else” in theUniverse. So, we receive much more light compared to the configuration devoidof a gravitational lens.

z~7 which translates to thedistance of 12.9 billion lightyears.(Kneib et al., Feb 15. 2004)

In the previous slide we can spot an arc-like patch of red light. The spectrum of thatlight contains the lines redshifted by a factor of 1+z=8. So, for example, if weassume that the wavelength we perceive amounts to, say, 640 nm, the originalemission in the reference frame of the emitter has only 80 nm. Thus, what we seeis 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 observableas 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, ofcourse, 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 observedfor many days! (Naturally, this “brute force” approach is very expensive and cannotbe 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 theepoch the content of the Universe is roughly the same: the Universe is populated bygalaxies! At last we got a firm evidence that Kant was right.

The galaxies are not distributed uniformly in the Universe. The large-scalestructure of the Universe has most likely a filamentary one and as such resemblesa “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 backwardsthe 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 theexpansion 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 ofCosmic Microwave Background (CMB) radiation – a cooled-down remnant of a muchhotter 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 pictureof 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 Universe.

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 theBig 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%.

Immanuel Kant (1724-1804)

The end