Chapter 5: The Complicated Universe:
Dark Matter, Structure Formation, Inflation and Hidden Galaxies
IntroductionIn this final chapter we consider some of the more puzzling aspects of cosmology that have emerged in the last 15 years. New observations have raised considerable challenges to our understanding to the point where we realize that the Universe is far from simple. Here we will consider four relatively new ideas:
Dark MatterObjects which emit light, whether they are cigars, light bulbs, cows, stars or galaxies can be characterized by their emitted energy per unit mass. This is parameterized as the Mass to Luminosity ratio or M/L. For cosmological purposes, it is most convenient to express M/L in terms of solar masses and luminosities. For main sequence stars it can be shown that L is proportional to M3.5-4 . Thus, a 10 solar mass star star has M/L of approximately 0.001, a 1 solar mass star has M/L =1 and a 0.1 solar mass star has M/L of approximately 1000. The term "dark matter" refers to the the existence of objects which have extreme M/L. Identifying the nature and extent of the dark matter component in the Universe is, arguably, the most significant unsolved problem in all of cosmology.
The amount of dark matter directly impacts our measurements of W. The mass density of the Universe is insensitive to the actual nature of the mass. The only requirement is that this mass gravitates. This requirement can be met if the dark matter is composed of a small space density of very massive objects or a large space density of very low mass objects. It is the total integration of this mass-density over spacetime that determines W and the overall large scale geometry of the Universe. The dark matter may reside in galaxies or it may be distributed between the galaxies.
In principle, the dark matter content of the Universe could be extreme. While we don't know the exact form, distribution or amount of dark matter we do know the following: if the Universe is closed it is fairly simple to calculate the amount of mass density that must be present to close it. This depends on the critical density which was derived in Chapter 4 and that depends only on Ho. Since Ho is known to within a least a factor of 2, then the critical mass density is known to within a factor of 4. If we compare the required mass density to close the Universe, the closure density, to the total amount of stars and galaxies that are visible as a result of their emitted light we reach an astounding result:
If the Universe is closed then no more than 1% of its total mass is in the form of luminous stars and galaxies.
One percent! That means that, in principle, 99% of the Universe is composed of dark matter. Ninety-nine percent! That means we know next to nothing about the fundamental nature of the Universe. This is embarrassing but is it this correct? How do we know dark matter is really out there?
Evidence for Dark Matter
In general, the evidence for dark matter is a result of analyzing the motions of test particles on some scale and seeing if there is enough mass that can be accounted for by the light emitting objects on that scale to produce the observed motion. If there is not enough light, then some other form of matter must be providing the mass. An example of this process in which we can be sure there is no dark matter, is the case of our Solar System.
The first historical success of using perturbed motions to find matter was the discovery of Neptune in 1846 from the observed perturbations in the motion of Uranus. In the 20th century, a similar analysis of Neptune allowed for the discovery of Pluto in 1930. These successes spurred on efforts to discover a planet beyond Pluto, known as Planet 10 as the combined masses of Neptune and Pluto weren't quite enough to account for the observed perturbations in the orbits of the outer planets. Interestingly, the close passage of Voyager II near Neptune in 1989 allowed for a significant refinement of the mass of Neptune. This proper accounting of Neptune's mass removed the need for the existence of Planet X and indeed there is no longer any evidence for unaccounted mass in the Solar System. So, on the scale of the Solar System, there is no dark matter. All the mass that is needed to drive the orbital evolution of the Solar System is fully accounted for.
Similarly, if one analyzes the motions of nearby stars around us to constrain the local mass density of our place in the disk of our Galaxy, there is no strong evidence for dark matter. But, a census of the mass in our neighborhood is interesting. The total mass density that has been inferred is approximately 0.18 solar masses per cubic parsec. Testing for the presence of dark matter in the solar neighborhood now becomes an accounting problem. The possible sources of this mass in the solar neighborhood are 1) luminous stars, 2) interstellar gas, 3) stellar remnants (mostly white dwarfs) and 4) dark matter. Observations indicate that approximately 0.1 solar masses per cubic parsec of mass density is in the form of luminous stars (including the thick disk component of our Galaxy). This implies additional mass not in the form of luminous stars but it can readily be accounted for. Approximately 0.05 solar masses per cubic parsec is in the form of gas (gas gets turned into stars, of course), and the remaining 0.03 solar masses per cubic parsec is in the form of white dwarfs (stellar evolutionary end points of low mass stars).
The reason we discuss this here is to emphasize that simple stellar evolution ultimately produces objects with high M/L values. In a few billion years when star formation has ceased in our Galaxy, most of the stars will fade into their stellar remnants (black holes - a good form of dark matter(!), neutron stars or white dwarfs). In this sense galaxies are destined to end up "dark". It is interesting to speculate that some galaxies may have achieved this state already. Indeed, a possible detection of this population is discussed at the end of this chapter.
On scales of galaxies and clusters of galaxies, there is ample
evidence for dark matter. This comes from the form of galaxy
rotation curves. Figure 5.1 shows an image of a typical edge-on spiral galaxy, where the
thin disk and central spherical light concentration are evident.
If the distribution of light traces the distribution of mass, then we would expect a high mass concentration that corresponds with the bulge light. In that case, the galaxy is similar to the point mass approximation that governs Solar System orbits in which the orbital velocity decreases with distance from the point mass. The expected rotation curve would then look like the dashed line shown in Figure 5.2. Observations, however, reveal a significantly different picture as most all galaxy rotation curves have a rotational velocity that rises slowly and then flattens out. This is shown as the solid line in Figure 5.2.
In Figure 5.3 we show a typical rotation curve for a real spiral galaxy. These data show that the point mass approximation is invalid. The existence of flat rotation curves of galaxies can only be explained if the mass of the galaxy is increasing in direct proportion to the radius. We can understand this by referring to the derivation of Kepler's Third Law that was done in Chapter 1. Newton was able to show that for an object in circular orbit, the orbital velocity is determined by the total mass enclosed within the radius. In equation form, this is given by
Flat rotation curves can be reproduced in a model that assumes that most of the mass of the galaxy is distributed in a spherical halo around the disk. Since these halos contain very little light, they must be composed mostly of dark matter. Analysis of galaxy rotation curves suggests that approximately 90% of the mass of the galaxy is in the form of dark matter which is distributed in a spherical halo around the galaxy.
But why can't galaxy halos just be composed of stellar remnants? For instance, a galaxy halo that was composed of black holes would certainly be dark yet provide enough mass to account for the observed flat rotation curves. This can be ruled out by a very simple but powerful argument. A black hole is the endpoint evolution of a massive main sequence star. Thus, the object that is now the black hole wasn't always black. There was a period when it was producing light. Hence, if galaxy halos are now composed of black holes or other remnants, there would have been a time in the past when the halo was full of luminous stars. As we look out farther in distance we are looking back in time due to the finite speed of light. Hence, halos should be bright when galaxies are younger but observations of distant galaxies do not show any evidence for luminous halos as a function of look-back time. As a result we can rule out galactic halos as being composed of mostly stellar remnants.
There is another interesting possibility for the constituents of the halo that has recently been tested via a novel known as gravitational microlensing. In brief, when light passes very close to a star, General Relativity tells us that it must bend because it is traveling through curved space and photons are effected by gravity as they have an energy related mass. Einstein's theory predicts the existence of objects known as gravitational lenses. A gravitational lens acts just like a normal lens in that it can bend (refract) light as well as amplify the brightness of a distant light source. The unique character of gravitational lensing is that it's achromatic. There is no equivalent to the index of refraction for a normal lens as the behavior of light passing through curved spacetime is independent of its wavelength. Thus, a gravitational lens will effect both the blue and red portions of the spectrum equally and this is its unique signature.
If a significant fraction of the dark matter in the halo of our galaxy is composed of objects like brown dwarfs or planets then there will be occasional amplification of the light from extragalactic stars by the microlensing effect just described. These microlensing events, however, would be extremely rare. Thus, to detect them would require imaging a few million extragalactic stars a night to search for the one or two that might vary in brightness over the course of a year due to microlensing. Now of course, in any field of a million stars, many of the objects would be naturally variable. However, intrinsic stellar variability (novae, binary stars, pulsating stars) always shows wavelength dependent behavior. That is, the variation in the blue would be different than the variation in the red.
In a spectacularly heroic experiment known as the MACHO project (MACHO stands for MAssive Compact Halo Objects), astronomers have monitored the brightnesses of millions of extragalactic stars in the LMC (the LMC is located directly behind our Galactic Halo) for the past 4 years and have now detected about a dozen events that can be reliably associated with microlensing. A signature of a lens event is shown in Figure 5.4 where it can be seen that the variation of the star in the blue and the red is identical. That means its light was amplified by microlensing. The duration over which the amplification event occurs (the event in Figure 5.4 lasted about 20 days) is related to the mass of the lens. The major result that the MACHO survey has produced to date is the total absence of short duration events which would be caused by lensing objects of a few Jupiter masses. This means that the halo of our Galaxy cannot be filled with sub-stellar mass objects. The bulk of the lensing events in the halo detected to date can be explained by regular stars (possibly some white dwarfs), of mass about 0.5 M solar masses.
Clusters of Galaxies
Clusters of galaxies represent a region of the Universe in which the mass density is sufficiently high that the expansion has been overcome. An example cluster is shown in Figure 5.5. This mass density serves to randomize the velocities of the cluster members as they assume some orbit in the cluster of galaxies. In that sense, galaxies in clusters are very much like the molecules inside a balloon of fixed volume. The galaxies can't escape and their overall relative velocities are determined by the mass of the cluster. The molecules in the balloon can't escape either and their overall velocity is determined by the temperature of the balloon. Thus, in a cluster of galaxies mass is analogous to temperature and indeed, this analogy explains why clusters of galaxies often emit X-rays. If the cluster is sufficiently massive it can heat the intracluster gas to high temperatures where it emits X-rays. This gives us an immediate diagnostic tool.
If we simply measure the relative velocities of the galaxies we can infer the total mass of the cluster. A small complication enters in that one needs to know the exact shape of the cluster; a spherical cluster is different than one that is shaped like a pancake - but the general principal applies. That dynamical mass estimate can then be compared to the mass estimate based on counting the number of galaxies in the cluster and adding up all their light. When this is done, it is typically found that the luminous galaxies in the clusters can only provide 5-10% of the total mass required for the cluster to exist. Taking into account the mass provided by the hot intracluster gas only raises this contribution by about a factor of 2.
Another means of estimating the mass of a cluster again appeals to
gravitational lensing. In this case, it is the entire mass of a
cluster that acts collectively as a lens.
Figure 5.6 shows a spectacular example of
this phenomena. Here, numerous arc-lets and rings can be seen. These are the distorted (lensed)
images of resolved galaxies located behind the cluster. The orientation
and degree of curvature of these features depends upon the cluster
mass distribution and the overall amount of mass. Again, when the
mass is inferred in this manner and compared to the luminous galaxies
in the cluster, a substantial mass discrepancy exists. When this
evidence is combined with the velocity data, there is
little doubt that in clusters of galaxies, a substantial amount of
dark matter must be present.
Intergalactic Dark MatterAn open question currently, yet a very important one, is whether or not there exists a substantial population of dark matter that is located in Intergalactic space - that is the space between galaxies and/or clusters of galaxies. If there is none, then dark matter is strictly confined to galaxies and clusters of galaxies. In that case we would expect W to be in the range 0.1 - 0.2 which is consistent with the determinations of W discussed in Chapter 4. However, when we discuss Inflation later on in this chapter we will see that it strongly prefers W = 1.0. Our dark matter census is well short of this value. Therefore, studies of the dark matter in galaxies and in clusters of galaxies has produced two important results: a) there is dark matter located there and b) there is not enough dark matter located there to close the Universe. Thus, if the Universe is closed (W > 1.0), then substantial amounts of dark matter has to exist in Intergalactic space where there are no galaxies. Thus, it is possible that there are two kinds of dark matter, that which is present in galaxies or in clusters of galaxies and that which is present outside of these regions.
Possible Kinds of Dark MatterIn considering the kinds of dark matter there are two broad possibilities. Normal dark matter is composed of baryons and is referred to as baryonic dark matter. An important feature concerning baryonic dark matter is that it wasn't necessarily always dark. A stellar mass black hole is the prime example as it once was a star radiating energy. Exotic dark matter would be non-baryonic in nature and is most probably in the form of some strange particle, hitherto undetected. In addition to important physical differences there may be an important philosophical difference. Astronomers want to build big telescopes in order to probe over vast distances and solve cosmological problems. But astronomers can only detect and measure baryons. If the mass of the Universe is mostly non-baryonic, then its fundamental nature will not be revealed through telescopic observation, but rather in some high energy accelerator experiment on the Earth. It is thus highly regrettable that continued funding of the Superconducting Super Collider was denied, as this might have been the grandest cosmological experiment ever done.
Some candidate forms of dark matter are listed below:
Non-baryonic Dark MatterBy any measure, if W = 1 then the Universe is dominated by non-baryonic dark matter. This form of dark matter would be a particle, like a neutrino or a WIMP (discussed earlier). Furthermore, that dark matter must be distributed more smoothly than the light and be distributed between the galaxies as well as in galaxies. In this scenario, the Universe is a sea of non-baryonic dark matter with little bits of baryonic material (e.g. galaxies) floating about in it (in spirit, similar to the cosmology of Thales in Chapter 1).
Non-baryonic dark matter comes in two basic forms: hot and cold. Hot dark matter (HDM) means the particle is relatively light so that when it was initially created in the early Universe it moved near the speed of light. There is only one strong candidate for HDM and that is a neutrino. In the late spring of 1998, new experimental data on the mass of the neutrino was released. That data is very discouraging in the cosmological context as the limits on the mass are far below that required to make the neutrino a major mass constituent of the Universe. This leaves us with cold dark matter (CDM). CDM consists of weakly interacting massive particles (WIMPs) that are able to cool and clump together even at relatively high temperatures. This requires the rest mass of most CDM WIMPs to be extremely large. In fact, a typical WIMP would have about 1016 times the rest mass energy of a proton. This is a heavy particle! As discussed in Chapter 3, some WIMPs could have been created very early on through quantum fluctuations and if they did not immediately annihilate with their respective anti-WIMPS, could survive as the dominant relic mass today. The basic problem is that no particle physics experiment on the earth has uncovered any evidence for these WIMPs or any other form of CDM despite searching for more than 20 years. If CDM exists, we are thus quite ignorant about its form. Paradoxically, however, the existence of galaxies may in fact require some form of CDM (see below).
Structure FormationThe distribution of galaxies (see Figure 4.8) is very complex. The basic mechanism that we can understand for the formation of structure in the Universe is known as Gravitational Instability. Gravitational Instability works by a relatively simple principle - if there is some density enhancement in a matter field, then that density enhancement will sweep up material and continue to grow. The problem with this scenario occurs if there is no dark matter in the Universe. As discussed in Chapter 3, baryonic material in the Universe would be subject to strong radiation pressure and drag and would have a difficult time clumping. In fact, it's not clear if galaxy formation would even ever occur if only baryons exist. Hence, one of the best physical arguments for dark matter is the very existence of galaxies. Baryonic density fluctuations will likely not produce galaxies. To produce galaxies you need a heavy particle that is not effected by radiation pressure so it can clump and form density enhancements very early on. If this is correct, it means that galaxies are surrounded by dark matter halos which trapped baryonic gas (e.g. hydrogen) and turned it into stars. This also means that there should be some halos which didn't trap any baryonic material to eventually form a luminous galaxy and therefore should remain dark.
The exact mechanism of how Gravitational Instability actually produced the complex patterns seen in galaxy clustering is unknown. This pattern of galaxy clustering is called large scale structure. By structure we mean the distribution of galaxies, clusters of galaxies and collections of clusters of galaxies known as superclusters. In Figure 4.8 we see a very complicated map of the galaxy distribution. Large filaments or walls are seen as are a number or prominent, quasi-spherical, voids. The overall distribution is quite cellular in nature and some of the observed structures are truly large. In particular, the band of points arcing across Figure 4.8 is known as the Great Wall of galaxies. It seems to be a coherent feature in the galaxy distribution which is hundreds of millions of light years long. The mechanisms that could create such huge, coherent features are quite mysterious.
In the study of large scale structure, theory and observations generally are not in good agreement. Several attempts to simulate large scale structure in Supercomputers have been done. Figure 5.7 shows an example of one such simulation. The patterns seen there are striking. There is much filamentary structure and there are large regions where there are no galaxies (these are called voids). The pattern is very much a cellular one in which structures seem to form at the interfaces between the voids. This void filled Universe is qualitatively similar to the real galaxy distribution, another example of which is shown in figure 5.8. Quantitatively however, these simulations generally produce too large of voids and too much filamentary structure. Hence, the details of structure formation remain relatively unknown to us. In simple terms we know that after recombination there were pockets of density enhancements around which material accumulated. What we don't know are the scales involved. In particular, there are two completely different mechanisms which could still produce the highly clustered, void-filled Universe that we observe.
The first mechanism suggests that extremely large objects formed first. This is known as the top-down scenario. These large objects have the mass of several galaxy clusters. Subsequent fragmentation of these very large regions into smaller regions then produced the hierarchical pattern we see today- galaxies in clusters, clusters within superclusters, etc. At the opposite extreme is the scenario which argues that sub-galactic size masses formed first. This is known as the bottom-up scenario. After these sub-galactic masses are formed, subsequent gravitational coalescence produce galaxies, which in turn gravitationally coalesce to produce clusters of galaxies. In some sense, the bottom-up scenario is similar to the accretion process that formed the planets, where tiny grains coalesced to make larger objects until eventually a planet formed.
Both the top-down and bottom-up scenarios can produce the
same kind of large scale structure in the Universe so,
even though the physics is much different in these scenarios, observations
of the nearby Universe and its associated patterns of galaxy
can't really tell us which one occurred. The main difference between
these two scenarios is that, in the top-down one we would expect
to see clusters of galaxies very early on in the Universe where as in
the bottom-up scenario we would expect to see galaxies before we
would see them arranged in clusters. Therefore, if we can look
farther and farther back in time, we may be able to observationally
map out the correct time sequence. Very recent deep
space observations with the Hubble Space Telescope (e.g. the Hubble
Deep Field) directly show the existence of sub-galactic masses which
seem to be coming together to form one galaxy. This evidence is
shown in Figure 5.9. This provides good support for the bottom-up
scenario but leaves open the general question of the high degree
of clustering seen in the Universe. Such clustering, if driven only
by gravitational processes, would have a difficult time producing the
largest coherent features (e.g. The Great Wall), we see in the
galaxy distribution in only a few billion years.
The Inflationary Paradigm and W = 1In the early days (1965-1975) of Big Bang cosmology, when there was little evidence for dark matter, the preferred model was a low density, baryon dominated Universe of age approximately 18 billion years. However, with the evidence presented above about the existence of dark matter, a fundamental alteration of this baryon-dominated cosmological model might be required. In addition, when looked at in detail, the Hot Big Bang model does not naturally predict some aspects of the large scale nature of the Universe. These predictive problems are enumerated below and can be ameliorated with a new twist on the physics of the very early Universe called inflation. Inflation is a very complicated physical theory but its essence is simple: it posits that the Universe we observe today resulted form a very strange phase in the very early Universe in which the the initial expansion epoch was exponential in nature. An exponential increase in the radius of the Universe produces a much larger Universe in a much shorter period of time, than uniform expansion does. This helps to resolve some problems with the standard Big Bang model.
There are basically three problems that inflation solves and they are known as the Flatness, Horizon and Smoothness problems:
Inflation makes a simple prediction that W = 1 and the Universe is spatially flat. This is because any initial curvature in the Universe was erased by the enormous expansion caused by inflation. Imagine taking a basketball and then exponentially inflating it so that its radius increased by a factor of approximately 1050 in a few millionths of a second. Any initial curvature is rapidly "inflated" out by the enormous increase in surface area of the basketball so that the final area is flat to essentially one part in 1050. In this manner, the inflationary theory makes a definite prediction about the geometry of the Universe; space must be flat.
In the standard Big Bang model, this would require that the initial conditions of the Universe were homogeneous and that would be a very improbable state. Inflation, however, solves this directly. The initial conditions of the Universe could have been very heterogeneous but in each small domain, which was homogeneous, there was an inflationary event. Our observable Universe is just one of many possible domains. You can think about this as follows: look around the environment in which you are reading this book. This environment is very heterogeneous. Now imagine taking a cubic micron of that environment and inflating it by a huge factor so that if fills space and becomes the Universe. There is a high probability that the cubic micron you picked to inflate was homogeneous, even though you picked it out from a very heterogeneous initial state. Thus, the inflationary paradigm directly predicts that the observable Universe will be homogeneous. Without inflation, a special condition would had to exist early on to produce the homogeneous CMB that we observe. Overall, this horizon problem is a pretty strong argument for the inflationary theory.
In sum, the inflationary paradigm, while operating via some unknown
but clearly fundamental
physics, provides some elegant solutions to the problems encountered
in the standard Big Bang model which is baryon-mass dominated.
Lately, inflation has come under fire because most observations do
not indicate that W =1, as predicted. However, inflation
only predicts a spatially flat Universe (not a specific value for
W). Since the curvature of the Universe
is determined by both W and L then inflation predicts
that W + L = 1.0. Hence, if we believe
the observations that W < 1 and that the inflationary
paradigm must hold, we are again driven to considerations of a non-zero
L. Einstein's biggest blunder once again, may have instead
been another fundamental insight as dynamics of the Universe in this
case are now driven by vacuum energy. Resolution of the L
issue is one of the most exciting problems in current observational
cosmology. If the Cosmological Constant turns out to be real, the
Universe will have taken on an added degree of complexity.
Hidden Galaxies RevealedWe conclude this chapter by briefly considering another recent discovery that has significantly altered our view of the characteristics of galaxies. We begin by introducing a thought experiment. An important corollary to the Cosmological Principle (see Chapter 2) would assert that all observers in the Universe should construct the same catalogue of galaxies. If this were not the case, then different observers might have biased views and information about 1) the nature of the general galaxy population in the Universe, 2) the three dimensional distribution of galaxies, and 3) the amount of baryonic matter that is contained in galaxies. Thinking about the Cosmological Principle in terms of the homogeneity of observers' catalogues of galaxies raises an immediate and perhaps profound problem which can be stated very simply: If you see (detect) a galaxy you can catalog and classify it; if you don't see it you can't.
Since galaxies in general are quite diffuse and low contrast objects with respect to the noisy background of the night sky, which has finite brightness, one can easily conceive of observing environments that would make galaxy detection difficult. For instance, suppose that we lived on a planet that was located in the inner regions of an elliptical galaxy. The high stellar density would produce a night sky background that would be relatively bright and therefore not conducive for the discovery of galaxies. Or, consider the poor astronomer that lives on a planet which has two moderate size moons in orbit about it, at least one of which was in the night sky at all times. Such a planet would have no "dark" time and observers would be hard pressed to discover external galaxies.
Although these are extreme situations, they illustrate the basic point that for all observers, the finite brightness of their night sky acts as a visibility filter which, when convolved with the true population of galaxies, produces the population that appears in catalogs. Thus, we have no guarantee that our location in the outer regions of a spiral galaxy, on a planet with 50% dark time per lunar orbital cycle, allows us to detect and catalog a representative sample of galaxies.
Improvements in detector technology have now allowed for the discovery of galaxies that have low contrast with respect to the night sky. Some visual examples of surface brightness selection effects are shown below in Figure 5.10. Each of the galaxies here has the same total mass. The galaxy in the upper left part of the Figure (M51) has a very high contrast with respect to the background while the similar mass galaxy in the lower right panel is barely detectable above the noise of the night sky. It is galaxies like this that comprise this new population of hidden galaxies which have now been discovered by astronomers.
|Figure 5.10 Digital images of galaxies with different contrasts with respect to the sky background. Galaxies like those shown in the upper left are easy to detect while galaxies like those in the lower right are considerably more difficult to detect. As the total mass in these galaxies is nearly the same, its clear that hidden, diffuse galaxies can still contain substantial amounts of material.|
These hidden galaxies, despite being as large as our own Galaxy, have escaped detection for many decades because their signal effectively competes with the noise of the night sky background. We are just now beginning to detect these new objects and to study their properties in some detail. While these objects have significantly different properties from more normal spiral and elliptical galaxies, their main interest to us here is in a cosmological context as they provide use additional sites where mass has accumulated but has gone unnoticed. As such, their recovery has essentially increased the mass of the Universe that is contained in galaxies. In fact, the data are consistent with as much mass as in normal galaxies being contained in this newly discovered population. Moreover, there is data which indicates that the M/L ratio of these LSB galaxies is significantly higher than it is for more normal galaxies. If this is indeed the case, then it argues that the majority of the baryonic matter in the Universe in fact, is contained in a population of galaxies that has just been recently discovered.
While the discovery of LSB galaxies does not solve our overall missing mass problem (there is still ample room for non-baryonic dark matter), it very much serves as a reminder that observational cosmology is still in its infancy and that there are still new things to discover and learn about the Universe. Furthermore, this serves as a reminder about the important role of bias in observational cosmology. For instance, prior to the discovery of this new population of galaxies, galaxy evolutionary theory was thought to be secure. Now we know that we have missed approximately 50% of the general galaxy population so that our previous view was both biased and incomplete. Even now, for instance, most introductory textbooks don't even mention this new population of galaxies (things change slowly ...). Hence, prior to the discovery of LSB galaxies we had a very biased view of the general galaxy population. Its not possible to construct a general evolutionary model based on a biased sample. With the detection of LSBs we have a more representative sample of galaxies and with that sample we have new challenges to our theories. In particular, the properties of LSB galaxies suggest they have very low mass densities compared to normal galaxies and this raises serious questions about how they could have formed in the first place.
Overall, however, the discovery of LSB
galaxies should be gratifying for it is a signature that the Universe still has not revealed
all to us and there are still likely to be many more mysteries to
encounter in the coming years. Because of that, we should fully expect
the cosmology that we have specified here will be subject to great change,
perhaps in the very near future.
If there is dark matter we are rather unsure of its nature. However, if the Universe is closed, then it's very likely that the dark matter is non-baryonic and in the form of some particle we have not yet discovered. Furthermore, there must be a substantial amount of dark matter distributed between the galaxies. On the other hand, the observational determination that W is in the range 0.1-0.3 is consistent with all the dark matter being located where there are galaxies (i.e in galaxies or in clusters of galaxies). But the form of this dark matter remains unknown. The MACHO observations of our Galactic Halo strongly limits the amount of sub-stellar mass objects that are located there. Similarly, the lack of halo brightening in distant galaxies limits the parent population of luminous stars which could evolve to become stellar remnants.
Another argument for the existence of dark matter is the complexity observed in the galaxy distribution. Currently the formation of large scale structure in the Universe is not well understood. However, this formation is greatly facilitated if there is some form of dark matter in the early Universe that is able to clump together during the radiation dominated era.
This chapter also introduced the complex concept of inflation, which is a new twist to the standard Hot Big Bang Theory. The great advantage of the inflationary theory is that the Universe is predicted to be homogeneous. In the standard Big Bang model, homogeneity would have to be an improbable initial condition. Inflation also predicts that space is flat. If inflation is indeed the mechanism that has produced the observed homogeneous Universe and if we believe the data which suggests that W is in the range 0.1 -- 0.3, then the spatial flatness requires L to dominate the energy field in the Universe today. If this is true, then Einstein's "biggest blunder" in reality was one of his greatest triumphs.
We closed this chapter with a brief discussion about a new population of galaxies that has remained hidden from our view due to their extremely diffuse nature. The discovery of this population is important because it shows that not only has our view of the general galaxy population been severely biased, but that it's possible for the Universe to offer us new objects, never before seen. As a consequence we should expect the cosmological model which has been presented here to be subject to considerable change in just the next few years. As intelligent beings we will continue to be curious and that curiosity will drive innovation in the kinds of instruments and observations that can be made. With new observations comes new insight unless we are encumbered by bias. In many respects, the study of Cosmology is a humbling experience which reminds us that of all that we know, the amount that remains unknown is vast by comparison.